Accurate quantitation of mitochondrial DNA reveals uniform levels in human blastocysts irrespective of ploidy, age, or implantation potential

Unraveling the mysteries of early embryonic arrest: genetic factors and molecular mechanisms

Early embryonic arrest (EEA) is a critical impediment in assisted reproductive technology (ART), affecting 40% of infertile patients by halting the development of early embryos from the zygote to blastocyst stage, resulting in a lack of viable embryos for successful pregnancy. Despite its prevalence, the molecular mechanism underlying EEA remains elusive. This review synthesizes the latest research on the genetic and molecular factors contributing to EEA, with a focus on maternal, paternal, and embryonic factors. Maternal factors such as irregularities in follicular development and endometrial environment, along with mutations in genes like NLRP5, PADI6, KPNA7, IGF2, and TUBB8, have been implicated in EEA. Specifically, PATL2 mutations are hypothesized to disrupt the maternal-zygotic transition, impairing embryo development. Paternal contributions to EEA are linked to chromosomal variations, epigenetic modifications, and mutations in genes such as CFAP69, ACTL7A, and M1AP, which interfere with sperm development and lead to infertility. Aneuploidy may disrupt spindle assembly checkpoints and pathways including Wnt, MAPK, and Hippo signaling, thereby contributing to EEA. Additionally, key genes involved in embryonic genome activation-such as ZSCAN4, DUXB, DUXA, NANOGNB, DPPA4, GATA6, ARGFX, RBP7, and KLF5-alongside functional disruptions in epigenetic modifications, mitochondrial DNA, and small non-coding RNAs, play critical roles in the onset of EEA. This review provides a comprehensive understanding of the genetic and molecular underpinnings of EEA, offering a theoretical foundation for the diagnosis and potential therapeutic strategies aimed at improving pregnancy outcomes.

Investigating developmental characteristics of biopsied blastocysts stratified by mitochondrial copy numbers using time-lapse monitoring

BACKGROUND

For in vitro fertilization (IVF), mitochondrial DNA (mtDNA) levels in the trophectodermal (TE) cells of biopsied blastocysts have been suggested to be associated with the cells' developmental potential. However, scholars have reached differing opinions regarding the use of mtDNA levels as a reliable biomarker for predicting IVF outcomes. Therefore, this study aims to assess the association of mitochondrial copy number measured by mitoscore associated with embryonic developmental characteristics and ploidy.

METHODS

This retrospective study analyzed the developmental characteristics of embryos and mtDNA levels in biopsied trophectodermal cells. The analysis was carried out using time-lapse monitoring and next-generation sequencing from September 2021 to September 2022. Five hundred and fifteen blastocysts were biopsied from 88 patients undergoing IVF who met the inclusion criteria. Embryonic morphokinetics and morphology were evaluated at 118 h after insemination using all recorded images. Blastocysts with appropriate morphology on day 5 or 6 underwent TE biopsy and preimplantation genetic testing for aneuploidy (PGT-A). Statistical analysis involved generalized estimating equations, Pearson's chi-squared test, Fisher's exact test, and Kruskal-Wallis test, with a significance level set at P < 0.05.

RESULTS

To examine differences in embryonic characteristics between blastocysts with low versus high mitoscores, the blastocysts were divided into quartiles based on their mitoscore. Regarding morphokinetic characteristics, no significant differences in most developmental kinetics and observed cleavage dysmorphisms were discovered. However, blastocysts in mitoscore group 1 had a longer time for reaching 3-cell stage after tPNf (t3; median: 14.4 h) than did those in mitoscore group 2 (median: 13.8 h) and a longer second cell cycle (CC2; median: 11.7 h) than did blastocysts in mitoscore groups 2 (median: 11.3 h) and 4 (median: 11.4 h; P < 0.05). Moreover, blastocysts in mitoscore group 4 had a lower euploid rate (22.6%) and a higher aneuploid rate (59.1%) than did those in the other mitoscore groups (39.6-49.3% and 30.3-43.2%; P < 0.05). The rate of whole-chromosomal alterations in mitoscore group 4 (63.4%) was higher than that in mitoscore groups 1 (47.3%) and 2 (40.1%; P < 0.05). A multivariate logistic regression model was used to analyze associations between the mitoscore and euploidy of elective blastocysts. After accounting for factors that could potentially affect the outcome, the mitoscore still exhibited a negative association with the likelihood of euploidy (adjusted OR = 0.581, 95% CI: 0.396-0.854; P = 0.006).

CONCLUSIONS

Blastocysts with varying levels of mitochondrial DNA, identified through biopsies, displayed similar characteristics in their early preimplantation development as observed through time-lapse imaging. However, the mitochondrial DNA level determined by the mitoscore can be used as a standalone predictor of euploidy.

Mitochondria as determinants of reproductive senescence and competence: implications for diagnosis of embryo competence in assisted reproduction

Mitochondria are commonly recognized as the powerhouses of the cell, primarily responsible for energy production through oxidative phosphorylation. Alongside this vital function, they also play crucial roles in regulating calcium signaling, maintaining membrane potential, and modulating apoptosis. Their involvement in various cellular pathways becomes particularly evident during oogenesis and embryogenesis, where mitochondrial quantity, morphology, and distribution are tightly controlled. The efficiency of the mitochondrial network is maintained through multiple quality control mechanisms that are essential for reproductive success. These include mitochondrial unfolded protein response, mitochondrial dynamics, and mitophagy. Not surprisingly, mitochondrial dysfunction has been implicated in infertility and ovarian aging, prompting investigation into mitochondria as diagnostic and therapeutic targets in assisted reproduction. To date, mitochondrial DNA copy number in oocytes, cumulus cells, and trophectoderm biopsies, and fluorescent lifetime imaging microscopy-based assessment of NADH and flavin adenine dinucleotide content have been explored as potential predictors of embryo competence, yielding limited success. Despite challenges in the clinical application of mitochondrial diagnostic strategies, these enigmatic organelles have a significant impact on reproduction, and their potential role as diagnostic targets in assisted reproduction is likely to remain an active area of investigation in the foreseeable future.

Dependency of mitochondrial quantity on blastocyst timeline obscures its actual effect to pregnancy outcomes

Objectives

To explore the correlation between mitochondrial quantity and the blastocyst development timeline as well as their respective contributions to early pregnancy.

Methods

A retrospective study was conducted using a dataset comprising 2,633 embryos that underwent preimplantation genetic testing for aneuploidy (PGT-A) between January 2016 and December 2023. The study was divided into three subsets to address distinct aspects: the representativeness of a single trophectoderm (TE) biopsy for mitochondrial quantity (n=43), the correlation between morphokinetic features and mitochondrial quantity (n=307), and the association analysis among mitochondrial quantity, blastocyst timeline factor, and reproductive outcomes (n=2,283). Distribution assessment of mitochondrial quantity across an individual blastocyst involved the identification within multiple biopsies and spent culture media. Timeline evaluation included correlating mitochondrial quantity with time-lapse datasets. Finally, multivariate logistic regression models, incorporating potential effectors alongside mitochondrial quantity, were employed to analyze their respective contributions to early pregnancy endpoints.

Results

Of distribution assessment, mitochondrial quantity exhibited an even distribution across the entire trophectoderm (Spearman's ρ=0.82), while no detectable mtDNAs in the corresponding spent culture media. Then the timeline correlation study revealed significant association between mitochondrial quantity and blastocyst features of both the day of expanded blastocyst formation (95% Confidence intervals, CIs: 0.27~4.89, p=0.03) and the timing of expanded blastocyst formation (tEB) (95% CIs: -0.24~-0.01, p=0.04) in the regression model, indicating a strong dependency between mitochondrial quantity and the blastocyst development timeline. For the contribution to early pregnancy, multivariate logistic regression models showed that the day of expanded blastocyst formation contributed to four endpoints persistently: positive for HCG (odd ratio, OR: 0.71, p=0.006), gestational sac (OR: 0.78, p=0.04), fetal heartbeat (OR: 0.71, p=0.004), and progression to 14 weeks (OR: 0.69, p=0.002). Contrastingly, no notable correlation was observed between the mitochondrial quantity and these endpoints.

Conclusions

Strong interaction was observed between mitochondrial quantity and the blastocyst timeline, particularly the timing of expanded blastocyst formation. It suggests that the primary determinant influencing pregnancy outcomes lies in the time-dependent parameter of blastocyst rather than in the specific mitochondrial quantity.

The role of mitochondrial dynamics in oocyte and early embryo development

Mitochondrial dysfunction is widely implicated in various human diseases, through mechanisms that go beyond mitochondria's well-established role in energy generation. These dynamic organelles exert vital control over numerous cellular processes, including calcium regulation, phospholipid synthesis, innate immunity, and apoptosis. While mitochondria's importance is acknowledged in all cell types, research has revealed the exceptionally dynamic nature of the mitochondrial network in oocytes and embryos, finely tuned to meet unique needs during gamete and pre-implantation embryo development. Within oocytes, both the quantity and morphology of mitochondria can significantly change during maturation and post-fertilization. These changes are orchestrated by fusion and fission processes (collectively known as mitochondrial dynamics), crucial for energy production, content exchange, and quality control as mitochondria adjust to the shifting energy demands of oocytes and embryos. The roles of proteins that regulate mitochondrial dynamics in reproductive processes have been primarily elucidated through targeted deletion studies in animal models. Notably, impaired mitochondrial dynamics have been linked to female reproductive health, affecting oocyte quality, fertilization, and embryo development. Dysfunctional mitochondria can lead to fertility problems and can have an impact on the success of pregnancy, particularly in older reproductive age women.

Dual-Activated H2O2-Responsive AIE Probes for Oocyte Quality Assessment

Nonobstructive azoospermia (NOA) is an important cause of infertility, and intracytoplasmic sperm injection (ICSI) is the mainstay of treatment for these patients. In cases where a sufficient number of sperm (usually 1-2) is not available, the selection of oocytes for ICSI is a difficult problem that must be solved. Here, we constructed a dual-activated oxidative stress-responsive AIE probe, b-PyTPA. The strong donor-acceptor configuration of b-PyTPA leads to twisted intramolecular charge transfer (TICT) effect that quenches the fluorescence of the probe, however, H2O2 would specifically remove the boronatebenzyl unit and release a much weaker acceptor, which inhibits TICT and restores the fluorescence. In addition, the presence of a pyridine salt makes b-PyTPA more hydrophilic, whereas removal of the pyridine salt increases the hydrophobicity of PyTPA, which triggers aggregation and further enhances fluorescence. Thus, the higher the intracellular level of oxidative stress, the stronger the fluorescence. In vitro, this dual-activated fluorescent probe is capable of accurately detecting senescent cells (high oxidative stress). More importantly, b-PyTPA was able to characterize senescent oocytes, as assessed by the level of oxidative stress. It is also possible to identify high quality oocytes from those obtained for subsequent ICSI. In conclusion, this dual-activated oxidative stress-assessment probe enables the quality assessment of oocytes and has potential application in ICSI.

How should the best human embryo in vitro be? Current and future challenges for embryo selection

In-vitro fertilization (IVF) aims at overcoming the causes of infertility and lead to a healthy live birth. To maximize IVF efficiency, it is critical to identify and transfer the most competent embryo within a cohort produced by a couple during a cycle. Conventional static embryo morphological assessment involves sequential observations under a light microscope at specific timepoints. The introduction of time-lapse technology enhanced morphological evaluation via the continuous monitoring of embryo preimplantation in vitro development, thereby unveiling features otherwise undetectable via multiple static assessments. Although an association exists, blastocyst morphology poorly predicts chromosomal competence. In fact, the only reliable approach currently available to diagnose the embryonic karyotype is trophectoderm biopsy and comprehensive chromosome testing to assess non-mosaic aneuploidies, namely preimplantation genetic testing for aneuploidies (PGT-A). Lately, the focus is shifting towards the fine-tuning of non-invasive technologies, such as "omic" analyses of waste products of IVF (e.g., spent culture media) and/or artificial intelligence-powered morphologic/morphodynamic evaluations. This review summarizes the main tools currently available to assess (or predict) embryo developmental, chromosomal, and reproductive competence, their strengths, the limitations, and the most probable future challenges.

When the Embryo Meets the Endometrium: Identifying the Features Required for Successful Embryo Implantation

Evaluation of the optimal number of embryos, their quality, and the precise timing for transfer are critical determinants in reproductive success, although still remaining one of the main challenges in assisted reproduction technologies (ART). Indeed, the success of in vitro fertilization (IVF) treatments relies on a multitude of events and factors involving both the endometrium and the embryo. Despite concerted efforts on both fronts, the overall success rates of IVF techniques continue to range between 25% and 30%. The role of the endometrium in implantation has been recently recognized, leading to the hypothesis that both the "soil" and the "seed" play a central role in a successful pregnancy. In this respect, identification of the molecular signature of endometrial receptivity together with the selection of the best embryo for transfer become crucial in ART. Currently, efforts have been made to develop accurate, predictive, and personalized tests to identify the window of implantation and the best quality embryo. However, the value of these tests is still debated, as conflicting results are reported in the literature. The purpose of this review is to summarize and critically report the available criteria to optimize the success of embryo transfer and to better understand current limitations and potential areas for improvement.

Good practice recommendations on add-ons in reproductive medicine†

STUDY QUESTION

Which add-ons are safe and effective to be used in ART treatment?

SUMMARY ANSWER

Forty-two recommendations were formulated on the use of add-ons in the diagnosis of fertility problems, the IVF laboratory and clinical management of IVF treatment.

WHAT IS KNOWN ALREADY

The innovative nature of ART combined with the extremely high motivation of the patients has opened the door to the wide application of what has become known as 'add-ons' in reproductive medicine. These supplementary options are available to patients in addition to standard fertility procedures, typically incurring an additional cost. A diverse array of supplementary options is made available, encompassing tests, drugs, equipment, complementary or alternative therapies, laboratory procedures, and surgical interventions. These options share the common aim of stating to enhance pregnancy or live birth rates, mitigate the risk of miscarriage, or expedite the time to achieving pregnancy.

STUDY DESIGN, SIZE, DURATION

ESHRE aimed to develop clinically relevant and evidence-based recommendations focusing on the safety and efficacy of add-ons currently used in fertility procedures in order to improve the quality of care for patients with infertility.

PARTICIPANTS/MATERIALS, SETTING, METHODS

ESHRE appointed a European multidisciplinary working group consisting of practising clinicians, embryologists, and researchers who have demonstrated leadership and expertise in the care and research of infertility. Patient representatives were included in the working group. To ensure that the guidelines are evidence-based, the literature identified from a systematic search was reviewed and critically appraised. In the absence of any clear scientific evidence, recommendations were based on the professional experience and consensus of the working group. The guidelines are thus based on the best available evidence and expert agreement. Prior to publication, the guidelines were reviewed by 46 independent international reviewers. A total of 272 comments were received and incorporated where relevant.

MAIN RESULTS AND THE ROLE OF CHANCE

The multidisciplinary working group formulated 42 recommendations in three sections; diagnosis and diagnostic tests, laboratory tests and interventions, and clinical management.

LIMITATIONS, REASONS FOR CAUTION

Of the 42 recommendations, none could be based on high-quality evidence and only four could be based on moderate-quality evidence, implicating that 95% of the recommendations are supported only by low-quality randomized controlled trials, observational data, professional experience, or consensus of the development group.

WIDER IMPLICATIONS OF THE FINDINGS

These guidelines offer valuable direction for healthcare professionals who are responsible for the care of patients undergoing ART treatment for infertility. Their purpose is to promote safe and effective ART treatment, enabling patients to make informed decisions based on realistic expectations. The guidelines aim to ensure that patients are fully informed about the various treatment options available to them and the likelihood of any additional treatment or test to improve the chance of achieving a live birth.

STUDY FUNDING/COMPETING INTEREST(S)

All costs relating to the development process were covered from ESHRE funds. There was no external funding of the development process or manuscript production. K.L. reports speakers fees from Merck and was part of a research study by Vitrolife (unpaid). T.E. reports consulting fees from Gynemed, speakers fees from Gynemed and is part of the scientific advisory board of Hamilton Thorne. N.P.P. reports grants from Merck Serono, Ferring Pharmaceutical, Theramex, Gedeon Richter, Organon, Roche, IBSA and Besins Healthcare, speakers fees from Merck Serono, Ferring Pharmaceutical, Theramex, Gedeon Richter, Organon, Roche, IBSA and Besins Healthcare. S.R.H. declares being managing director of Fertility Europe, a not-for-profit organization receiving financial support from ESHRE. I.S. is a scientific advisor for and has stock options from Alife Health, is co-founder of IVFvision LTD (unpaid) and received speakers' fee from the 2023 ART Young Leader Prestige workshop in China. A.P. reports grants from Gedeon Richter, Ferring Pharmaceuticals and Merck A/S, consulting fees from Preglem, Novo Nordisk, Ferring Pharmaceuticals, Gedeon Richter, Cryos and Merck A/S, speakers fees from Gedeon Richter, Ferring Pharmaceuticals, Merck A/S, Theramex and Organon, travel fees from Gedeon Richter. The other authors disclosed no conflicts of interest.

DISCLAIMER

This Good Practice Recommendations (GPRs) document represents the views of ESHRE, which are the result of consensus between the relevant ESHRE stakeholders and are based on the scientific evidence available at the time of preparation.ESHRE GPRs should be used for information and educational purposes. They should not be interpreted as setting a standard of care or bedeemedinclusive of all proper methods of care, or be exclusive of other methods of care reasonably directed to obtaining the same results.Theydo not replace the need for application of clinical judgement to each individual presentation, or variations based on locality and facility type.Furthermore, ESHRE GPRs do not constitute or imply the endorsement, or favouring, of any of the included technologies by ESHRE.

The effect of vitrification on blastocyst mitochondrial DNA dynamics and gene expression profiles

PURPOSE

Does vitrification/warming affect the mitochondrial DNA (mtDNA) content and the gene expression profile of blastocysts?

METHODS

Prospective cohort study in which 89 blastocysts were obtained from 50 patients between July 2017 and August 2018. mtDNA was measured in a total of 71 aneuploid blastocysts by means of real-time polymerase chain reaction (RT-PCR). Transcriptomic analysis was performed by RNA sequencing (RNA-seq) in an additional 8 aneuploid blastocysts cultured for 0 h after warming, and 10 aneuploid blastocysts cultured for 4-5 h after warming.

RESULTS

A significant decrease in mtDNA content just during the first hour after the warming process in blastocysts was found (P < 0.05). However, mtDNA content experimented a significantly increased along the later culture hours achieving the original mtDNA levels before vitrification after 4-5 h of culture (P < 0.05). Gene expression analysis and functional enrichment analysis revealed that such recovery was accompanied by upregulation of pathways associated with embryo developmental capacity and uterine embryo development. Interestingly, the significant increase in mtDNA content observed in blastocysts just after warming also coincided with the differential expression of several cellular stress response-related pathways, such as apoptosis, DNA damage, humoral immune responses, and cancer.

CONCLUSION

To our knowledge, this is the first study demonstrating in humans, a modulation in blastocysts mtDNA content in response to vitrification and warming. These results will be useful in understanding which pathways and mechanisms may be activated in human blastocysts following vitrification and warming before a transfer.

Opening the black box: why do euploid blastocysts fail to implant? A systematic review and meta-analysis

BACKGROUND

A normal chromosomal constitution defined through PGT-A assessing all chromosomes on trophectoderm (TE) biopsies represents the strongest predictor of embryo implantation. Yet, its positive predictive value is not higher than 50-60%. This gap of knowledge on the causes of euploid blastocysts' reproductive failure is known as 'the black box of implantation'.

OBJECTIVE AND RATIONALE

Several embryonic, maternal, paternal, clinical, and IVF laboratory features were scrutinized for their putative association with reproductive success or implantation failure of euploid blastocysts.

SEARCH METHODS

A systematic bibliographical search was conducted without temporal limits up to August 2021. The keywords were '(blastocyst OR day5 embryo OR day6 embryo OR day7 embryo) AND (euploid OR chromosomally normal OR preimplantation genetic testing) AND (implantation OR implantation failure OR miscarriage OR abortion OR live birth OR biochemical pregnancy OR recurrent implantation failure)'. Overall, 1608 items were identified and screened. We included all prospective or retrospective clinical studies and randomized-controlled-trials (RCTs) that assessed any feature associated with live-birth rates (LBR) and/or miscarriage rates (MR) among non-mosaic euploid blastocyst transfer after TE biopsy and PGT-A. In total, 41 reviews and 372 papers were selected, clustered according to a common focus, and thoroughly reviewed. The PRISMA guideline was followed, the PICO model was adopted, and ROBINS-I and ROB 2.0 scoring were used to assess putative bias. Bias across studies regarding the LBR was also assessed using visual inspection of funnel plots and the trim and fill method. Categorical data were combined with a pooled-OR. The random-effect model was used to conduct the meta-analysis. Between-study heterogeneity was addressed using I2. Whenever not suitable for the meta-analysis, the included studies were simply described for their results. The study protocol was registered at http://www.crd.york.ac.uk/PROSPERO/ (registration number CRD42021275329).

OUTCOMES

We included 372 original papers (335 retrospective studies, 30 prospective studies and 7 RCTs) and 41 reviews. However, most of the studies were retrospective, or characterized by small sample sizes, thus prone to bias, which reduces the quality of the evidence to low or very low. Reduced inner cell mass (7 studies, OR: 0.37, 95% CI: 0.27-0.52, I2 = 53%), or TE quality (9 studies, OR: 0.53, 95% CI: 0.43-0.67, I2 = 70%), overall blastocyst quality worse than Gardner's BB-grade (8 studies, OR: 0.40, 95% CI: 0.24-0.67, I2 = 83%), developmental delay (18 studies, OR: 0.56, 95% CI: 0.49-0.63, I2 = 47%), and (by qualitative analysis) some morphodynamic abnormalities pinpointed through time-lapse microscopy (abnormal cleavage patterns, spontaneous blastocyst collapse, longer time of morula formation I, time of blastulation (tB), and duration of blastulation) were all associated with poorer reproductive outcomes. Slightly lower LBR, even in the context of PGT-A, was reported among women ≥38 years (7 studies, OR: 0.87, 95% CI: 0.75-1.00, I2 = 31%), while obesity was associated with both lower LBR (2 studies, OR: 0.66, 95% CI: 0.55-0.79, I2 = 0%) and higher MR (2 studies, OR: 1.8, 95% CI: 1.08-2.99, I2 = 52%). The experience of previous repeated implantation failures (RIF) was also associated with lower LBR (3 studies, OR: 0.72, 95% CI: 0.55-0.93, I2 = 0%). By qualitative analysis, among hormonal assessments, only abnormal progesterone levels prior to transfer were associated with LBR and MR after PGT-A. Among the clinical protocols used, vitrified-warmed embryo transfer was more effective than fresh transfer (2 studies, OR: 1.56, 95% CI: 1.05-2.33, I2 = 23%) after PGT-A. Lastly, multiple vitrification-warming cycles (2 studies, OR: 0.41, 95% CI: 0.22-0.77, I2 = 50%) or (by qualitative analysis) a high number of cells biopsied may slightly reduce the LBR, while simultaneous zona-pellucida opening and TE biopsy allowed better results than the Day 3 hatching-based protocol (3 studies, OR: 1.41, 95% CI: 1.18-1.69, I2 = 0%).

WIDER IMPLICATIONS

Embryo selection aims at shortening the time-to-pregnancy, while minimizing the reproductive risks. Knowing which features are associated with the reproductive competence of euploid blastocysts is therefore critical to define, implement, and validate safer and more efficient clinical workflows. Future research should be directed towards: (i) systematic investigations of the mechanisms involved in reproductive aging beyond de novo chromosomal abnormalities, and how lifestyle and nutrition may accelerate or exacerbate their consequences; (ii) improved evaluation of the uterine and blastocyst-endometrial dialogue, both of which represent black boxes themselves; (iii) standardization/automation of embryo assessment and IVF protocols; (iv) additional invasive or preferably non-invasive tools for embryo selection. Only by filling these gaps we may finally crack the riddle behind 'the black box of implantation'.

The prenatal environment and its influence on maternal and child mitochondrial DNA copy number and methylation: A review of the literature

Mitochondrial DNA (mtDNA) is sensitive to environmental stressors and associated with human health. We reviewed epidemiological literature examining associations between prenatal environmental, dietary, and social exposures and alterations in maternal/child mtDNA copy number (mtDNAcn) and mtDNA methylation. Evidence exists that prenatal maternal exposures are associated with alterations in mtDNAcn for air pollution, chemicals (e.g. metals), cigarette smoke, human immunodeficiency virus (HIV) infection and treatment. Evidence for their associations with mtDNA methylation was limited. Given its potential implications as a disease pathway biomarker, studies with sufficient biological specificity should examine the long-term implications of prenatal and early-life mtDNA alterations in response to prenatal exposures.

Mitochondrial DNA quantification correlates with the developmental potential of human euploid blastocysts but not with that of mosaic blastocysts

PURPOSE

We aimed to study the association between adjusted mtDNA levels in human trophectoderm biopsy samples and the developmental potential of euploid and mosaic blastocysts.

METHODS

We analyzed relative mtDNA levels in 2,814 blastocysts obtained from 576 couples undergoing preimplantation genetic testing for aneuploidy from June 2018 to June 2021. All patients underwent in vitro fertilization in a single clinic; the study was blinded-mtDNA content was unknown at the time of single embryo transfer. The fate of the euploid or mosaic embryos transferred was compared with mtDNA levels.

RESULTS

Euploid embryos had lower mtDNA than aneuploid and mosaic embryos. Embryos biopsied on Day 5 had higher mtDNA than those biopsied on Day 6. No difference was detected in mtDNA scores between embryos derived from oocytes of different maternal ages. Linear mixed model suggested that blastulation rate was associated with mtDNA score. Moreover, the specific next-generation sequencing platform used have a significant effect on the observed mtDNA content. Euploid embryos with higher mtDNA content presented significantly higher miscarriage rates and lower live birth rates, while no significant difference was observed in the mosaic cohort.

CONCLUSION

Our results will aid in improving methods for analyzing the association between mtDNA level and blastocyst viability.

ESHRE good practice recommendations on recurrent implantation failure

STUDY QUESTION

How should recurrent implantation failure (RIF) in patients undergoing ART be defined and managed?

SUMMARY ANSWER

This is the first ESHRE good practice recommendations paper providing a definition for RIF together with recommendations on how to investigate causes and contributing factors, and how to improve the chances of a pregnancy.

WHAT IS KNOWN ALREADY

RIF is a challenge in the ART clinic, with a multitude of investigations and interventions offered and applied in clinical practice, often without biological rationale or with unequivocal evidence of benefit.

STUDY DESIGN SIZE DURATION

This document was developed according to a predefined methodology for ESHRE good practice recommendations. Recommendations are supported by data from the literature, if available, and the results of a previously published survey on clinical practice in RIF and the expertise of the working group. A literature search was performed in PubMed and Cochrane focussing on 'recurrent reproductive failure', 'recurrent implantation failure', and 'repeated implantation failure'.

PARTICIPANTS/MATERIALS SETTING METHODS

The ESHRE Working Group on Recurrent Implantation Failure included eight members representing the ESHRE Special Interest Groups for Implantation and Early Pregnancy, Reproductive Endocrinology, and Embryology, with an independent chair and an expert in statistics. The recommendations for clinical practice were formulated based on the expert opinion of the working group, while taking into consideration the published data and results of the survey on uptake in clinical practice. The draft document was then open to ESHRE members for online peer review and was revised in light of the comments received.

MAIN RESULTS AND THE ROLE OF CHANCE

The working group recommends considering RIF as a secondary phenomenon of ART, as it can only be observed in patients undergoing IVF, and that the following description of RIF be adopted: 'RIF describes the scenario in which the transfer of embryos considered to be viable has failed to result in a positive pregnancy test sufficiently often in a specific patient to warrant consideration of further investigations and/or interventions'. It was agreed that the recommended threshold for the cumulative predicted chance of implantation to identify RIF for the purposes of initiating further investigation is 60%. When a couple have not had a successful implantation by a certain number of embryo transfers and the cumulative predicted chance of implantation associated with that number is greater than 60%, then they should be counselled on further investigation and/or treatment options. This term defines clinical RIF for which further actions should be considered. Nineteen recommendations were formulated on investigations when RIF is suspected, and 13 on interventions. Recommendations were colour-coded based on whether the investigations/interventions were recommended (green), to be considered (orange), or not recommended, i.e. not to be offered routinely (red).

LIMITATIONS REASONS FOR CAUTION

While awaiting the results of further studies and trials, the ESHRE Working Group on Recurrent Implantation Failure recommends identifying RIF based on the chance of successful implantation for the individual patient or couple and to restrict investigations and treatments to those supported by a clear rationale and data indicating their likely benefit.

WIDER IMPLICATIONS OF THE FINDINGS

This article provides not only good practice advice but also highlights the investigations and interventions that need further research. This research, when well-conducted, will be key to making progress in the clinical management of RIF.

STUDY FUNDING/COMPETING INTERESTS

The meetings and technical support for this project were funded by ESHRE. N.M. declared consulting fees from ArtPRED (The Netherlands) and Freya Biosciences (Denmark); Honoraria for lectures from Gedeon Richter, Merck, Abbott, and IBSA; being co-founder of Verso Biosense. He is Co-Chief Editor of Reproductive Biomedicine Online (RBMO). D.C. declared being an Associate Editor of Human Reproduction Update, and declared honoraria for lectures from Merck, Organon, IBSA, and Fairtility; support for attending meetings from Cooper Surgical, Fujifilm Irvine Scientific. G.G. declared that he or his institution received financial or non-financial support for research, lectures, workshops, advisory roles, or travelling from Ferring, Merck, Gedeon-Richter, PregLem, Abbott, Vifor, Organon, MSD, Coopersurgical, ObsEVA, and ReprodWissen. He is an Editor of the journals Archives of Obstetrics and Gynecology and Reproductive Biomedicine Online, and Editor in Chief of Journal Gynäkologische Endokrinologie. He is involved in guideline developments and quality control on national and international level. G.L. declared he or his institution received honoraria for lectures from Merck, Ferring, Vianex/Organon, and MSD. He is an Associate Editor of Human Reproduction Update, immediate past Coordinator of Special Interest Group for Reproductive Endocrinology of ESHRE and has been involved in Guideline Development Groups of ESHRE and national fertility authorities. D.J.M. declared being an Associate Editor for Human Reproduction Open and statistical Advisor for Reproductive Biomedicine Online. B.T. declared being shareholder of Reprognostics and she or her institution received financial or non-financial support for research, clinical trials, lectures, workshops, advisory roles or travelling from support for attending meetings from Ferring, MSD, Exeltis, Merck Serono, Bayer, Teva, Theramex and Novartis, Astropharm, Ferring. The other authors had nothing to disclose.

DISCLAIMER

This Good Practice Recommendations (GPR) document represents the views of ESHRE, which are the result of consensus between the relevant ESHRE stakeholders and are based on the scientific evidence available at the time of preparation. ESHRE GPRs should be used for information and educational purposes. They should not be interpreted as setting a standard of care or be deemed inclusive of all proper methods of care, or be exclusive of other methods of care reasonably directed to obtaining the same results. They do not replace the need for application of clinical judgement to each individual presentation, or variations based on locality and facility type. Furthermore, ESHRE GPRs do not constitute or imply the endorsement, or favouring, of any of the included technologies by ESHRE.

Reduced mitochondrial DNA content correlate with poor clinical outcomes in cryotransfers with day 6 single euploid embryos

Objective

To investigate whether the mitochondrial DNA (mtDNA) content of a single biopsy at trophoblast correlates with the developmental potential and reproductive outcomes of blastocyst.

Methods

A retrospective analysis applied the dataset of 1,675 embryos with preimplantation genetic testing for aneuploidy (PGT-A) from 1,305 individuals, and 1,383 embryos involved cryotransfers of single euploid embryo between January 2015 and December 2019. The studied cohort was divided for algorithm establishment on the NGS platform (n=40), correlation of biological features (n=1,635), and correlation of reproductive outcomes (n=1,340). Of the algorithm derived from the NGS platform, the reliability and repeatability were validated via qPCR assay and inter-run controls, respectively. Of the correlation across biological features, stratification analyses were applied to evaluate the effect from a single contributor. Eventually, the correlation between the mtDNA ratios and reproductive outcomes was adjusted according to the significant effector(s).

Results

The mtDNA ratios showed statistically different between embryos with different days of blastocyst formation ([Day 5]: 1.06 vs. [Day 6]: 0.66, p=0.021), and between embryos with different expansion stages ([Expansion 5]: 1.05 vs. [Expansion 6]: 0.49, p=0.012). None or weakly correlated with the maternal age, morphology, ploidy, and gender. Analyzed by the different days of blastocyst formation with fixed expansion score as 5 in the euploid single embryo transfers (eSET), the day 6 eSET showed significantly lower reduced mtDNA ratio (n=139) in failure groups of fetal heartbeat (p=0.004), ongoing pregnancy (p=0.007), and live birth (p=0.01); however, no correlation between mtDNA ratios and pregnancy outcomes was observed in the day 5 eSET (n=1,201).

Conclusions

The study first demonstrated that mtDNA ratio was dependent on the days of blastocyst formation while expansion stage was fixed. Lower mtDNA ratios were observed in the day 6 eSET with adverse outcomes. The present stratification analyses reveal that the timeline of embryo is an important covariate to the mtDNA content.

Maternal spindle transfer for mitochondrial disease: lessons to be learnt before extending the method to other conditions?

Mitochondrial diseases are a group of conditions attributed to mutations of specific genes that regulate mitochondrial function. Maternal spindle transfer (MST) has been proposed as a method to prevent the transmission of these diseases and utilisation of the technique resulted in the birth of a baby free of disease in 2017 in Mexico. Potential flaws in research governance and the associated criticism emerged from the expansion of MST to provide a potentially new assisted reproductive technique to overcome infertility problems characterised by repeated in vitro embryo development arrest caused by mitochondrial dysfunction and cytoplasmic deficiencies of the oocyte. This applied technique represents a good example of the need to strike "a balance between taking appropriate precautions and hampering innovation". The purpose of this article is to explore, through a comprehensive literature search, whether and how this process can evolve from an experimental method to treat a medical condition to a standard of care solution for certain types of infertility. We argue that a number of key issues should be considered before applying the technique more broadly. These include regulatory oversight, safety and efficacy, cost, implications for research, essential laboratory skills and oversight, as well as the care needs of patients and egg donors.

Mitochondrial DNA copy number in cumulus granulosa cells as a predictor for embryo morphokinetics and chromosome status

While morphokinetic evaluation of embryos has become the most commonly used technique in IVF to select embryos for transfer, studies have demonstrated that mitochondrial DNA (mtDNA) copy number is correlated with embryo viability and transfer outcomes. Correspondingly, this cohort study aims to evaluate the association between the mtDNA copy number in cumulus granulosa cells (CGCs) with embryo morphokinetic parameters and chromosomal status. Real-time PCR was employed to measure the mtDNA copy number of the 129 CGCs in samples obtained from 30 patients undergoing the IVF-IMSI program at Morula IVF Jakarta between July and October 2020. Bivariate and multiple analyses were utilized to determine its relationship with embryo morphokinetics, blastocyst yield, and chromosomal status. According to the analysis, there was a significant correlation between the mtDNA copy number and the blastocyst status after adjusting for the maternal age and sperm morphology (coefficient 0.832, p value = 0.032, RR value 2.299). Moreover, a significant link was observed between mtDNA copy number in CGC and early embryo developmental phase M1 (t2-t8), using the equation of M1 is 5.702-0.271 mtDNA copy number of CGCs + 0.017 maternal age + 0.013 sperm motility -0.115 sperm morphology (p value = 0.032). However, no correlation was found between the mtDNA copy number in CGCs with the other morphokinetic parameters (M2: tC-tEB, M3: t2-tEB, DC, RC, MN with p > 0.05), or the chromosomal status of the embryos (euploid: 139.44 ± 133.12, aneuploid: 142.40 ± 111.30, p = 0.806). In conclusion, our study suggests that mtDNA copy number in CGCs can serve as a useful biomarker for blastocyst status and early embryo developmental phase but not for chromosomal status.

Age-related changes in Folliculogenesis and potential modifiers to improve fertility outcomes - A narrative review

Reproductive aging is characterized by a decline in oocyte quantity and quality, which is directly associated with a decline in reproductive potential, as well as poorer reproductive success and obstetrical outcomes. As women delay childbearing, understanding the mechanisms of ovarian aging and follicular depletion have become increasingly more relevant. Age-related meiotic errors in oocytes are well established. In addition, it is also important to understand how intraovarian regulators change with aging and how certain treatments can mitigate the impact of aging. Individual studies have demonstrated that reproductive pathways involving antimullerian hormone (AMH), vascular endothelial growth factor (VEGF), neurotropins, insulin-like growth factor 1 (IGF1), and mitochondrial function are pivotal for healthy oocyte and cumulus cell development and are altered with increasing age. We provide a comprehensive review of these individual studies and explain how these factors change in oocytes, cumulus cells, and follicular fluid. We also summarize how modifiers of folliculogenesis, such as vitamin D, coenzyme Q, and dehydroepiandrosterone (DHEA) may be used to potentially overcome age-related changes and enhance fertility outcomes of aged follicles, as evidenced by human and rodent studies.

Mitochondrial DNA content and developmental competence of blastocysts derived from pre-pubertal heifer oocytes

In the cattle-breeding industry, there is an increasing demand for in vitro embryo production from pre-pubertal heifers. In this study, we evaluated the differences in mitochondrial DNA content, oxidative stress, and developmental competence in blastocysts derived from pre-pubertal and pubertal heifers. We found higher mitochondrial DNA copy numbers in blastocysts produced from pre-pubertal heifers than from pubertal heifers. In the group of pre-pubertal animals, there was a significantly lower number of blastocysts produced in vitro from the same number of collected oocytes, and these blastocysts did not differ from those obtained from pubertal oocytes in terms of their morphological quality. The morphologically appropriate blastocysts derived from pre-pubertal heifers had higher concentrations of reactive oxygen species and glutathione. In blastocysts derived from pre-pubertal heifers, we found alterations in the expression of gene markers for developmental competence, which correlated with higher mitochondrial DNA content, suggesting a lower quality of blastocysts derived from pre-pubertal animals than from pubertal animals. The inadequate redox balance in blastocysts obtained from pre-pubertal females, along with higher mitochondrial DNA copy number, as well as differential gene expression of markers of developmental competence, elucidate the low quality of blastocysts derived from pre-pubertal animals, despite their unaltered morphology.

Assessment of mitochondrial DNA viability ratio in day-4 biopsied embryos as an add-in to select euploid embryos for single embryo transfer

The aim of this study was to assess mitochondrial DNA analysis as a predictor of the pregnancy potential of biopsied preimplantation embryos. The study included 78 blastomeres biopsied from day 4 cleavage stage euploid embryos. The embryo karyotype was confirmed by 24-chromosome preimplantation genetic testing for aneuploidies using the Illumina Next-Generation Sequencing (NGS) system. Mitochondria viability ratios (mtV) were determined from BAM files subjected to the web-based genome-analysis tool Galaxy. From this cohort of patients, 30.4% of patients (n = 34) failed to establish pregnancy. The mean mtV ratio [mean = 1.51 ± 1.25-1.77 (95% CI)] for this group was significantly (P < 0.01) lower compared with the embryo population that resulted in established pregnancies [mean = 2.5 ± 1.82-2.68 (95% CI)]. mtV multiple of mean (MoM) values were similarly significantly (P < 0.01) lower in blastocysts failing to establish pregnancy. At a 0.5 MoM cut-off, the sensitivity of mtV quantitation was 35.3% and specificity was 78.2%. The positive predictive value for an mtV value > 0.5 MoM was 41.4%. This study demonstrates the clinical utility of preimplantation quantification of viable mitochondrial DNA in biopsied blastomeres as a prognosticator of pregnancy potential.

SARS-CoV-2 can infect human embryos

The spread of SARS-CoV-2 has led to a devastating pandemic, with infections resulting in a range of symptoms collectively known as COVID-19. The full repertoire of human tissues and organs susceptible to infection is an area of active investigation, and some studies have implicated the reproductive system. The effects of COVID-19 on human reproduction remain poorly understood, and particularly the impact on early embryogenesis and establishment of a pregnancy are not known. In this work, we explore the susceptibility of early human embryos to SARS-CoV-2 infection. By using RNA-seq and immunofluorescence, we note that ACE2 and TMPRSS2, two canonical cell entry factors for SARS-CoV-2, are co-expressed in cells of the trophectoderm in blastocyst-stage preimplantation embryos. For the purpose of viral entry studies, we used fluorescent reporter virions pseudotyped with Spike (S) glycoprotein from SARS-CoV-2, and we observe robust infection of trophectoderm cells. This permissiveness could be attenuated with blocking antibodies targeting S or ACE2. When exposing human blastocysts to the live, fully infectious SARS-CoV-2, we detected cases of infection that compromised embryo health. Therefore, we identify a new human target tissue for SARS-CoV-2 with potential medical implications for reproductive health during the COVID-19 pandemic and its aftermath.

Quantitative proteome profiling reveals molecular hallmarks of egg quality in Atlantic halibut: impairments of transcription and protein folding impede protein and energy homeostasis during early development

Background

Tandem mass tag spectrometry (TMT labeling-LC-MS/MS) was utilized to examine the global proteomes of Atlantic halibut eggs at the 1-cell-stage post fertilization. Comparisons were made between eggs judged to be of good quality (GQ) versus poor quality (BQ) as evidenced by their subsequent rates of survival for 12 days. Altered abundance of selected proteins in BQ eggs was confirmed by parallel reaction monitoring spectrometry (PRM-LC-MS/MS). Correspondence of protein levels to expression of related gene transcripts was examined via qPCR. Potential mitochondrial differences between GQ and BQ eggs were assessed by transmission electron microscopy (TEM) and measurements of mitochondrial DNA (mtDNA) levels.

Results

A total of 115 proteins were found to be differentially abundant between GQ and BQ eggs. Frequency distributions of these proteins indicated higher protein folding activity in GQ eggs compared to higher transcription and protein degradation activities in BQ eggs. BQ eggs were also significantly enriched with proteins related to mitochondrial structure and biogenesis. Quantitative differences in abundance of several proteins with parallel differences in their transcript levels were confirmed in egg samples obtained over three consecutive reproductive seasons. The observed disparities in global proteome profiles suggest impairment of protein and energy homeostasis related to unfolded protein response and mitochondrial stress in BQ eggs. TEM revealed BQ eggs to contain significantly higher numbers of mitochondria, but differences in corresponding genomic mtDNA (mt-nd5 and mt-atp6) levels were not significant. Mitochondria from BQ eggs were significantly smaller with a more irregular shape and a higher number of cristae than those from GQ eggs.

Conclusion

The results of this study indicate that BQ Atlantic halibut eggs are impaired at both transcription and translation levels leading to endoplasmic reticulum and mitochondrial disorders. Observation of these irregularities over three consecutive reproductive seasons in BQ eggs from females of diverse background, age and reproductive experience indicates that they are a hallmark of poor egg quality. Additional research is needed to discover when in oogenesis and under what circumstances these defects may arise. The prevalence of this suite of markers in BQ eggs of diverse vertebrate species also begs investigation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08859-0.

Association Between MitoScore, BMI, and Body Fat Percentage as a Predictive Marker for the Outcome of In-Vitro Fertilization (IVF)

Background

Infertility is defined as the inability to establish a pregnancy within 12 months of regular and unprotected sexual intercourse. In response to these problems, assisted reproductive techniques (ARTs) have made profound impacts on the therapeutic management of infertility. However, in-vitro fertilization (IVF) success rates are confounded by several internal and external factors. A relatively new approach to embryo assessment is known as MitoScore (Igenomix, Miami, USA). As a result, we sough to evaluate whether MitoScore can help in predicting in IVF outcomes, and to assess the relationship between MitoScore, BMI, and body fat percentage in determining the success of ARTs.

Methods

Using retrospective cohort, a study population consisting of 166 women aged 26-43 who were undergoing ART with pre-implantation genetic testing for aneuploidy (PGT-A) was assessed to determine if MitoScore, BMI, and body fat percentage impacted IVF outcomes.

Results

MitoScore, BMI, and body fat percentage were significantly lower in pregnant women as compared to non-pregnant women. Furthermore, MitoScore was correlated with subclasses of IVF outcomes (delivery, biochemical pregnancy, and spontaneous abortion) and was found to be positively correlated with BMI in patients with biochemical pregnancies.

Conclusion

Our findings suggest that MitoScore, BMI, and body fat percentage could act as critical parameters in determining the success of ART. However, the association between MitoScore, BMI, and body fat percentage does not appear to be a significant confounding factor to determine pregnancy outcome at this stage. Still, many factors need to be considered to establish the correlation reliably.

mtDNA content in cumulus cells does not predict development to blastocyst or implantation

STUDY QUESTION

Is relative mitochondrial DNA (mtDNA) content in cumulus cells (CCs) related to embryo developmental competence in humans and/or the bovine model?

SUMMARY ANSWER

mtDNA content in CCs provides a poor predictive value of oocyte developmental potential, both in vitro and following embryo transfer.

WHAT IS KNOWN ALREADY

CCs are closely connected to the oocyte through transzonal projections, serving essential metabolic functions during folliculogenesis. These oocyte-supporting cells are removed and discarded prior to ICSI, thereby providing interesting biological material on which to perform molecular analyses designed to identify markers that predict oocyte developmental competence. Previous studies have positively associated oocyte mtDNA content with developmental potential in animal models and women. However, it remains debatable whether mtDNA content in CCs could be used as a proxy to infer oocyte developmental potential.

STUDY DESIGN SIZE DURATION

mtDNA content was analyzed in CCs obtained from 109 human oocytes unable to develop to blastocyst, able to develop to blastocyst but failing to establish pregnancy or able to develop to blastocyst and to establish pregnancy. mtDNA analysis was also performed on bovine cumulus samples collected from 120 oocytes unable to cleave, oocytes developing into cleaved embryos but arresting development prior to the blastocyst stage or oocytes developing to blastocysts.

PARTICIPANTS/MATERIALS SETTING METHODS

Human CCs samples were obtained from women undergoing IVF. Only unfrozen oocytes and embryos not submitted to preimplantation genetic testing were included in the analysis. Bovine samples were obtained from slaughtered cattle and individually matured, fertilized and cultured in vitro. Relative mtDNA was assessed by quantitative PCR analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

mtDNA content in human and bovine CCs did not differ according to the developmental potential of their enclosed oocyte. Moreover, mtDNA content in bovine oocytes did not correlate with that of their corresponding CCs.

LARGE SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

The lack of correlation found between mtDNA content in human CCs and oocytes was also assessed in bovine samples. Although bovine folliculogenesis, mono-ovulatory ovulation and early embryo development exhibit considerable similarities with that of humans, they may not be fully comparable.

WIDER IMPLICATIONS OF THE FINDINGS

The use of molecular markers for oocyte developmental potential in CCs could be used to enhance success rates following single embryo transfer. However, our data indicate that mtDNA in CCs is not a good proxy for oocyte quality.

STUDY FUNDING/COMPETING INTERESTS

This research was supported by the Industrial Doctorate Project IND2017/BIO-7748 funded by the Madrid Region Government. The authors declare no competing interests.

Does Trophectoderm Mitochondrial DNA Content Affect Embryo Developmental and Implantation Potential?

A retrospective case control study was undertaken at the molecular biology department of a private center for reproductive medicine in order to determine whether any correlation exists between the mitochondrial DNA (mtDNA) content of trophectoderm and embryo developmental potential. A total of 275 couples underwent IVF treatment, producing a total of 716 embryos. The trophectoderm was biopsied from each embryo at the blastocyst stage (day 5 or day 6 post-fertilization) subjected to low-pass next-generation sequencing (NGS), for the purpose of detecting aneuploidy. For each sample, the number of mtDNA reads obtained after analysis using NGS was divided by the number of reads attributable to the nuclear genome. The mtDNA copy number was found to be higher in aneuploid embryos than in those that were euploid (mean mtDNA ratio ± SD: 1.13 ± 1.37 versus 1.45 ± 1.78, p = 0.02) and in day 5 biopsies compared to day 6 biopsies (1.41 ± 1.66 vs. 1.19 ± 1.27, p = 0.001), whereas no statistically significant differences in mtDNA content were seen in relation to embryo morphology (1.58 ± 2.44 vs. 2.19 ± 2.89, p = 0.12), genetic sex (1.27 ± 1.29 vs. 1.27 ± 1.18, p = 0.99), maternal age (1.31 ± 1.41 vs. 1.33 ± 1.29, p = 0.43), or its ability to implant (1.14 ± 0.88 vs. 1.21 ± 1.16, p = 0.39). mtDNA has small potential to serve as an additional, independent biomarker for embryo selection.

The Role of Mitochondria in Human Fertility and Early Embryo Development: What Can We Learn for Clinical Application of Assessing and Improving Mitochondrial DNA?

Mitochondria are well known as 'the powerhouses of the cell'. Indeed, their major role is cellular energy production driven by both mitochondrial and nuclear DNA. Such a feature makes these organelles essential for successful fertilisation and proper embryo implantation and development. Generally, mitochondrial DNA is exclusively maternally inherited; oocyte's mitochondrial DNA level is crucial to provide sufficient ATP content for the developing embryo until the blastocyst stage of development. Additionally, human fertility and early embryogenesis may be affected by either point mutations or deletions in mitochondrial DNA. It was suggested that their accumulation may be associated with ovarian ageing. If so, is mitochondrial dysfunction the cause or consequence of ovarian ageing? Moreover, such an obvious relationship of mitochondria and mitochondrial genome with human fertility and early embryo development gives the field of mitochondrial research a great potential to be of use in clinical application. However, even now, the area of assessing and improving DNA quantity and function in reproductive medicine drives many questions and uncertainties. This review summarises the role of mitochondria and mitochondrial DNA in human reproduction and gives an insight into the utility of their clinical use.

Role of Granulosa Cells in the Aging Ovarian Landscape: A Focus on Mitochondrial and Metabolic Function

Mitochondria are at the intersection of aging and fertility, with research efforts centered largely on the role that these specialized organelles play in the relatively rapid decline in oocyte quality that occurs as females approach reproductive senescence. In addition to various roles in oocyte maturation, fertilization, and embryogenesis, mitochondria are critical to granulosa cell function. Herein, we provide a review of the literature pertaining to the role of mitochondria in granulosa cell function, with emphasis on how mitochondrial aging in granulosa cells may impact reproduction in female mammals.

Mitochondrial DNA Copy Number in Cleavage Stage Human Embryos-Impact on Infertility Outcome

A retrospective case control study was undertaken at the molecular biology department of a private center for reproductive medicine in order to determine whether any correlation exists between mitochondrial DNA (mtDNA) content of cleavage-stage preimplantation embryos and their developmental potential. A total of 69 couples underwent IVF treatment (averaged women age: 36.5, SD 4.9) and produced a total of 314 embryos. A single blastomere was biopsied from each embryo at the cleavage stage (day-3 post-fertilization) subjected to low-pass next generation sequencing (NGS), for the purpose of detecting aneuploidy. For each sample, the number of mtDNA reads obtained after analysis using NGS was divided by the number of reads attributable to the nuclear genome. The mtDNA copy number amount was found to be higher in aneuploid embryos than in those that were euploid (mean mtDNA ratio ± SD: 6.3 ± 7.5 versus 7.1 ± 5.8, p < 0.004; U Mann−Whitney test), whereas no statistically significant differences in mtDNA content were seen in relation to embryo morphology (6.6 ± 4.8 vs. 8.5 ± 13.6, p 0.09), sex (6.6 ± 4.1 vs. 6.2 ± 6.8, p 0.16), maternal age (6.9 ± 7.8 vs. 6.7 ± 4.5, p 0.14) or its ability to implant (7.4 ± 6.6 vs. 5.1 ± 4.6, p 0.18). The mtDNA content cannot serve as a useful biomarker at this point in development. However, further studies investigating both quantitative and qualitative aspects of mtDNA are still required to fully evaluate the relationship between mitochondrial DNA and human reproduction.

Mitochondrial DNA levels in trophectodermal cells show no association with blastocyst development and pregnancy outcomes

Background:

In patients undergoing assisted reproduction, levels of mitochondrial DNA (mtDNA) in the trophectodermal cells of the developing blastocyst are suggested to be associated with its ability to implant. However, discrepancies exist regarding the use of mtDNA levels as a reliable biomarker to predict outcomes of assisted reproduction.

Aims:

The aim of the study is to explore the association of trophectodermal mtDNA levels to determine blastocyst quality, implantation potential of blastocyst and clinical outcomes in couples who have undergone pre-implantation genetic testing for aneuploidy (PGT-A).

Study Setting:

Private fertility centre.

Study Design:

Retrospective analysis.

Materials and Methods:

We analysed mtDNA levels in the trophectodermal cells of 287 blastocysts from 61 couples undergoing PGT-A. The levels of mtDNA were estimated by next-generation sequencing method. mtDNA levels were correlated with maternal age, blastocyst morphology, ploidy status, implantation rates, miscarriage rate and live birth rate.

Statistical Analysis Used:

Linear regression and one-way ANOVA with Tukey's all column comparison test.

Results:

The trophectodermal mtDNA levels did not correlate with maternal age. There were no significant differences in their levels in grade 1 and grade 2 blastocysts. No significant differences were seen between mtDNA levels of implanted and non-implanted blastocysts or those blastocysts that resulted in miscarriage or live birth. However, significantly lower amounts of mtDNA were seen in euploid blastocysts as compared to that in aneuploid blastocysts.

Conclusion:

mtDNA levels in the trophectodermal cells of the blastocyst do not associate with blastocyst quality (grade 1 and grade 2), implantation potential and clinical outcomes but can differentiate between aneuploid and euploid blastocysts. Our study does not support the use of trophectodermal mtDNA levels as a biomarker for blastocyst quality and predictor of clinical outcomes.

Mitochondrial function of human embryo: Decline in their quality with maternal aging

Background

Female fertility declines with age, due to increased chromosomal aneuploidy and possible reduced mitochondrial function in the embryo.

Methods

This review outlines how mitochondrial function in human embryos, as predicted from oxygen consumption rate (OCR) measurements, changes in preimplantation stage, and what factors, particularly maternal age, affect mitochondrial function in embryos.

Main findings

The structure of the mitochondrial inner membrane and its respiratory function developed with embryo development, while the copy number of mitochondrial DNA per specimen was transiently reduced compared with that of the oocyte. The undifferentiated state of the inner cell mass cells appears to be associated with a low OCR. In contrast, the copy number of mitochondrial DNA increased in trophoblast cells and mitochondrial aerobic metabolism increased.The OCRs at morulae stage decreased with maternal age, but there was no relationship between maternal age and the copy number of mitochondrial DNA at any stages. The higher oxygen spent at the morula stage; the shorter time was needed for development to the mid-stage blastocyst.

Conclusions

The mitochondrial respiratory function of human embryos developed along with embryonic growth. Mitochondrial function at morula stage declined with their maternal age and reduced mitochondrial function decreased the rate of development from morula to blastocyst.

Extensive analysis of mitochondrial DNA quantity and sequence variation in human cumulus cells and assisted reproduction outcomes

STUDY QUESTION

Are relative mitochondrial DNA (mtDNA) content and mitochondrial genome (mtGenome) variants in human cumulus cells (CCs) associated with oocyte reproductive potential and assisted reproductive technology (ART) outcomes?

SUMMARY ANSWER

Neither the CC mtDNA quantity nor the presence of specific mtDNA genetic variants was associated with ART outcomes, although associations with patient body mass index (BMI) were detected, and the total number of oocytes retrieved differed between major mitochondrial haplogroups.

WHAT IS KNOWN ALREADY

CCs fulfil a vital role in the support of oocyte developmental competence. As with other cell types, appropriate cellular function is likely to rely upon adequate energy production, which in turn depends on the quantity and genetic competence of the mitochondria. mtDNA mutations can be inherited or they can accumulate in somatic cells over time, potentially contributing to aging. Such mutations may be homoplasmic (affecting all mtDNA in a cell) or they may display varying levels of heteroplasmy (affecting a proportion of the mtDNA). Currently, little is known concerning variation in CC mitochondrial genetics and how this might influence the reproductive potential of the associated oocyte.

STUDY DESIGN, SIZE, DURATION

This was a prospective observational study involving human CCs collected with 541 oocytes from 177 IVF patients. mtDNA quantity was measured in all the samples with a validated quantitative PCR method and the entire mtGenome was sequenced in a subset of 138 samples using a high-depth massively parallel sequencing approach. Associations between relative mtDNA quantity and mtGenome variants in CCs and patient age, BMI (kg/m2), infertility diagnosis and ART outcomes were investigated.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Massively parallel sequencing permitted not only the accurate detection of mutations but also the precise quantification of levels of mutations in cases of heteroplasmy. Sequence variants in the mtDNA were evaluated using Mitomaster and HmtVar to predict their potential impact.

MAIN RESULTS AND THE ROLE OF CHANCE

The relative mtDNA CC content was significantly associated with BMI. No significant associations were observed between CC mtDNA quantity and patient age, female infertility diagnosis or any ART outcome variable. mtGenome sequencing revealed 4181 genetic variants with respect to a reference genome. The COXI locus contained the least number of coding sequence variants, whereas ATPase8 had the most. The number of variants predicted to affect the ATP production differed significantly between mitochondrial macrohaplogroups. The total number of retrieved oocytes was different between the H-V and J-T as well as the U-K and J-T macrohaplogroups. There was a non-significant increase in mtDNA levels in CCs with heteroplasmic mitochondrial mutations.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Although a large number of samples were analysed in this study, it was not possible to analyse all the CCs from every patient. Also, the results obtained with respect to specific clinical outcomes and macrohaplogroups should be interpreted with caution due to the smaller sample sizes when subdividing the dataset.

WIDER IMPLICATIONS OF THE FINDINGS

These findings suggest that the analysis of mtDNA in CCs is unlikely to provide an advantage in terms of improved embryo selection during assisted reproduction cycles. Nonetheless, our data raise interesting biological questions, particularly regarding the interplay of metabolism and BMI and the association of mtDNA haplogroup with oocyte yield in ovarian stimulation cycles.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by National Institutes of Health grant 5R01HD092550-02. D.J.N. and C.R. co-hold patent US20150346100A1 and D.J.N. holds US20170039415A1, both for metabolic imaging methods. D.W. receives support from the NIHR Oxford Biomedical Research Centre. The remaining authors have no conflicts of interest to declare.

Molecular Drivers of Developmental Arrest in the Human Preimplantation Embryo: A Systematic Review and Critical Analysis Leading to Mapping Future Research

Developmental arrest of the preimplantation embryo is a multifactorial condition, characterized by lack of cellular division for at least 24 hours, hindering the in vitro fertilization cycle outcome. This systematic review aims to present the molecular drivers of developmental arrest, focusing on embryonic and parental factors. A systematic search in PubMed/Medline, Embase and Cochrane-Central-Database was performed in January 2021. A total of 76 studies were included. The identified embryonic factors associated with arrest included gene variations, mitochondrial DNA copy number, methylation patterns, chromosomal abnormalities, metabolic profile and morphological features. Parental factors included, gene variation, protein expression levels and infertility etiology. A valuable conclusion emerging through critical analysis indicated that genetic origins of developmental arrest analyzed from the perspective of parental infertility etiology and the embryo itself, share common ground. This is a unique and long-overdue contribution to literature that for the first time presents an all-inclusive methodological report on the molecular drivers leading to preimplantation embryos’ arrested development. The variety and heterogeneity of developmental arrest drivers, along with their inevitable intertwining relationships does not allow for prioritization on the factors playing a more definitive role in arrested development. This systematic review provides the basis for further research in the field.

Novel Approaches in Addressing Ovarian Insufficiency in 2019: Are We There Yet?

Ovarian insufficiency is described as a multifaceted issue typically encountered in the field of assisted reproduction. The three main identified diagnoses of ovarian insufficiency include premature ovarian failure (POF), poor ovarian response (POR), and advanced maternal age (AMA). Patient heterogeneity in the era of individualized medicine drives research forward leading to the emergence of novel approaches. This plethora of innovative treatments in the service of adequately managing ovarian insufficiency is called to undertake the challenge of addressing infertile patients exploring their reproductive options. This review provides an all-inclusive presentation and critical analysis on novel treatments that have not achieved routine clinical practice status yet, but have recently emerged as promising. In light of the lack of randomized controlled trials conveying safety and efficiency, clinicians are left puzzled in addressing the "how" and "for whom" these approaches may be beneficial. From ovarian injection employing platelet-rich plasma (PRP) or stem cells to artificial gametes and ovaries, ovarian transplantation, and mitochondrial replacement therapy, this descriptive review provides insight toward assisting the practitioner in decision making regarding these cutting-edge treatments. Biological mechanisms, invasiveness levels, efficiency, as well as possible complications, the current status along with bioethical concerns are discussed in the context of identifying future optimal treatment.

The Molecular Regulation in the Pathophysiology in Ovarian Aging

The female reproductive system is of great significance to women’s health. Aging of the female reproductive system occurs approximately 10 years prior to the natural age-associated functional decline of other organ systems. With an increase in life expectancy worldwide, reproductive aging has gradually become a key health issue among women. Therefore, an adequate understanding of the causes and molecular mechanisms of ovarian aging is essential towards the inhibition of age-related diseases and the promotion of health and longevity in women. In general, women begin to experience a decline in ovarian function around the age of 35 years, which is mainly manifested as a decrease in the number of ovarian follicles and the quality of oocytes. Studies have revealed the occurrence of mitochondrial dysfunction, reduced DNA repair, epigenetic changes, and metabolic alterations in the cells within the ovaries as age increases. In the present work, we reviewed the possible factors of aging-induced ovarian insufficiency based on its clinical diagnosis and performed an in-depth investigation of the relevant molecular mechanisms and potential targets to provide novel approaches for the effective improvement of ovarian function in older women.

Trophectoderm Mitochondrial DNA Content Associated with Embryo Quality and Day-5 Euploid Blastocyst Transfer Outcomes

Mitochondria play a critical role in cell function and embryo development. Recently, increasing studies have investigated whether mitochondrial DNA (mtDNA) can be used as a predictive biomarker of embryo implantation. However, the results of its effect on implantation are still controversial. To further understand the clinical application value of mtDNA content for reproductive potential, we analyzed the influence of relative mtDNA quantity on embryo quality and transfer outcomes based on the results of second-generation sequencing of preimplantation genetic testing patients in our center. Biopsied trophectoderm (TE) from aneuploid blastocysts contained much larger amounts of mtDNA than those from euploid blastocysts (p < 0.000). In an analysis of only euploid blastocysts (n = 769), female age had no effect on mtDNA content (p = 0.216). TE cells biopsied on day 5 (n = 355) contained significantly higher amounts of mtDNA compared to those biopsied on day 6 (n = 388) or day 7 (n = 26) (p < 0.000). Higher quality trophoblast was associated with lower mtDNA content (p = 0.026), but quality of inner cell mass was not correlated with quantity of mtDNA (p = 0.112). For transferred embryos, the biopsied date and mtDNA content were significantly associated with embryo implantation and live birth outcomes. Day-5 euploid blastocysts with lower quantities of mtDNA exhibited higher implantation rate and live birth rate. However, our data indicated that mtDNA content may not be considered an independent predictive marker, it may be a useful reference for the selection of day-5 transferred euploid blastocysts.

An overview of the current and emerging platforms for preimplantation genetic testing for aneuploidies (PGT-A) in in vitro fertilization programs

Preimplantation genetic testing for aneuploidies (PGT-A) and PGT for monogenic disorders (PGT-M) have currently been used widely, aiming to improve IVF outcomes. Although with many years of unsatisfactory results, PGT-A has been revived because new technologies have been adopted, such as platforms to examine all 24 types of chromosomes in blastocysts. This report compiles current knowledge regarding the available PGT platforms, including quantitative PCR, array CGH, and next-generation sequencing. The diagnostic capabilities of are compared and respective advantages/disadvantages outlined. We also address the limitations of current technologies, such as assignment of embryos with balanced translocation. We also discuss the emerging novel PGT technologies that likely will change our future practice, such as non-invasive PGT examining spent culture medium. Current literature suggest that most platforms can effectively reach concordant results regarding whole-chromosome ploidy status of all 24 types of chromosomes. However, different platforms have different resolutions and experimental complexities; leading to different turnaround time, throughput and differential capabilities of detecting mosaicism, segmental mutations, unbalanced translocations, concurrent PGT-A and PGT-M etc. Based on these information, IVF staff can more appropriately interpret PGT data and counsel patients, and select suitable platforms to meet personalized needs. The present report also concisely discusses some crucial clinical outcomes by PGT, which can clarify the role of applying PGT in daily IVF programs. Finally the up-to-date information about the novel use of current technologies and the newly emerging technologies will also help identify the focus areas for the design of new platforms for PGT in the future.

Euploidy rates are not affected when embryos are cultured in a continuous (CCM) or sequential culture medium (SCM): a sibling oocyte study

PURPOSE

To determine if euploidy rates and embryo development differ when blastocysts are cultured in CCM or SCM.

METHOD

A single-center retrospective observational study was performed from September 2018 to March 2019. Patients [23-46 years] with at least four fresh mature oocytes (MII) without severe male factor infertility were included. Sibling MII were injected and cultured in Global®Total®LP (CCM) or Sage Quinn's Advantage® Cleavage and Blastocyst media (SCM) under 6% CO₂, 5% O₂, and 89% N₂. Fertilization, cleavage, day (D) 5 blastulation, usable blastocyst (blastocysts biopsied/normally fertilized oocytes), and euploidy rates were recorded. Blastocysts were graded prior to trophectoderm (TE) biopsy on D5, 6, or 7 for genetic testing and mitochondrial DNA (mtDNA) quantification.

RESULTS

According to clinical practice, 1452 MII were randomly distributed: 751 in CCM and 701 in SCM. No differences were observed in fertilization and cleavages rates for CCM and SCM (77.4% vs 75.5%, p = 0.429 and 97.6% vs 99.1%, p = 0.094, respectively). Blastulation rate on D5 was higher in CCM (70.6% vs 62.2, p = 0.009); however, usable blastocyst rates were comparable (CCM: 58.3% vs SCM: 56.7%, p = 0.625). From a Poisson regression model adjusted for confounding factors, euploidy rates were not different between media (aOR = 1.18, [0.94-1.48], p = 0.157). Euploid blastocyst's mtDNA values were similar (CCM: 32.2, [30.5, 34.1] and SCM: 33.5, [31.8, 35.2], p = 0.345) and top-quality blastocysts (AA/BA) were increased in SCM (OR=1.04, [1.00-1.09], p = 0.037).

CONCLUSION

Under controlled in vitro conditions, euploidy rates and embryo development are comparable when embryos are cultured in CCM or SCM.

Ultrastructure of mitochondria of human oocytes in different clinical conditions during assisted reproduction

Infertility affects around 8% of couples with a slight change in percentage in the last years. Despite the significant efforts made in Assisted Reproductive Technologies (ARTs) in handling this disorder, oocyte quality remains a crucial factor for a positive outcome. A better understanding of the dynamics underlying oocyte maturation, fertilization, and embryo development remains one of the main areas for progress in the ARTs field. Mitochondria are believed to play an essential role in these processes. Mitochondria have a crucial part in producing energy for oocyte maturation and embryo development throughout precise cellular functions comprising Ca²⁺ homeostasis regulation, glycolysis, amino acid and fatty acid metabolism, and regulation of apoptosis. Recent studies suggest that mitochondrial structure, content, and function may be related to oocyte competence, embryo viability, and implantation success during ARTs. Their defects could lead to low fertilization rates and embryonic development failure. This review aimed to provide an overview of the available literature data surrounding the correlation between changes at ultrastructural level of mitochondria or correlated-mitochondrial aggregates and oocyte quality and ARTs treatments. Our reported data demonstrated that oocyte mitochondrial ultrastructural alterations could be partial or complete recovery during the early embryo stages. However, these changes could persist as quiescent during the pre-implantation embryo development, causing abnormalities that become evident only during fetal and postnatal life. These factors led to consider the mitochondria as a crucial marker of oocyte and embryo quality, as well as a strategic target for further prospective therapeutical approaches.

The Ratio of Mitochondrial DNA to Genomic DNA Copy Number in Cumulus Cell May Serve as a Biomarker of Embryo Quality in IVF Cycles

Previous studies have reported that the mitochondrial DNA (mtDNA) contents of cumulus cells (CCs) in ovarian follicular fluid are correlated with embryo quality. Quantification of mtDNA CCs has been suggested as a biomarker of embryo viability. The aim of this study was to determine the relationship between mitochondrial DNA (mtDNA)/genomic DNA (gDNA) ratio in CCs and IVF outcomes such as fertilization rates and embryo quality in infertile women. This is an observational study on 144 cumulus-oocyte complexes obtained from 144 patients undergoing IVF-intracytoplasmic sperm injection (ICSI) at a single fertility center. The CCs in ovarian follicular fluid from patients undergoing IVF-ICSI were collected by ovum pick-up. A relative copy number quantification was used to determine mtDNA/gDNA ratio. Quantitative real-time PCR for various markers (β2M and mtMinArc gene) was used to determine average mtDNA/gDNA ratio of CCs. Investigation of the correlation between mtDNA/gDNA ratio in CCs and IVF outcomes showed no statistically significant correlation between the mtDNA/gDNA ratio in CCs and fertilization rates. However, mtDNA/gDNA ratio and embryo quality showed a statistically significant positive correlation. A significantly higher mtDNA/gDNA ratio was observed in the good quality embryo group compared with the poor quality embryo group (P < 0.05). In addition, the mtDNA/gDNA ratio showed negative correlation with the patient's age (correlation coefficient= -0.228, P < 0.05). Results of this study demonstrate a negative correlation of mtDNA/gDNA ratio in CCs with patient's age, and a low copy number of mtDNA in CCs may have adverse effects on embryo quality in IVF cycles. These results suggest that the ratio of mtDNA/gDNA in CCs may serve as a biomarker in predicting IVF outcomes.

Mitochondrial DNA Copy Number in Human Blastocyst: A Novel Biomarker for the Prediction of Implantation Potential

The relationship between mitochondrial DNA (mtDNA) copy number and the outcome of embryo transfer is under debate. Our aim was to explore the relationship between mtDNA copy number in human blastocysts and embryonic development to determine whether mtDNA represents a novel biomarker for the prediction of implantation potential. A total of 246 blastocysts were analyzed by next-generation sequencing. There was no correlation between mtDNA copy number and maternal age in all blastocyst groups and euploid blastocyst groups. Additionally, the mtDNA copy number was not significantly higher in aneuploid blastocysts. Subsequently, no relationship was observed between mtDNA copy number and blastocyst quality. The assessment of clinical pregnancy outcome after the transfer of euploid blastocysts to the uterus indicated that the mtDNA copy number was significantly lower in the clinical pregnancy group than in those who failed implantation. The cut-off value of mtDNA copy number was 320.5, which was a highly predictive value. Blastocysts with an increased mtDNA copy number had lower implantation potential, and mtDNA copy number was largely equal in terms of maternal age, chromosome ploidy, and quality of blastocysts.

Does blastocyst mitochondrial DNA content affect miscarriage rate in patients undergoing single euploid frozen embryo transfer?

PURPOSE

To determine whether the blastocyst mitochondrial DNA (mtDNA) content is related to the miscarriage rate in patients undergoing single euploid frozen embryo transfer (SEFET).

METHODS

A total of 355 single euploid frozen embryo transfer cycles were studied retrospectively between April 2017 and December 2018. A trophectoderm biopsy was performed on day 5/6 blastocysts. Post next-generation sequencing (NGS), the mtDNA content was calculated as the ratio of mitochondrial DNA over nuclear DNA, and the association between blastocyst mtDNA content and miscarriage rate was evaluated.

RESULT(S)

Three hundred fifty-five euploid blastocysts were selected for SEFET in 314 patients with an average age of 33.7 ± 5.6 years; 255 were biopsied on day 5 (71.8%) and 100 on day 6 (28.2%). Frozen embryo transfer (FET) was performed either in a hormone replacement therapy (HRT) cycle (71.8%; n = 255) or in a natural cycle (NC) (28.2%; n = 100). A pregnancy rate of 66.2% (235/355) was obtained with clinical pregnancy and miscarriage rates of 52.4% (n = 186) and 5.6% (n = 20), respectively. There was no significant difference neither between the blastocyst mtDNA content of pregnant and nonpregnant patients (27.7 ± 9.2 vs. 29.4 ± 8.6, P = 0.095) nor between patients with a clinical pregnancy and miscarriage (30.5 ± 9.3 vs. 27.3 ± 9.2, P = 0.136). Multivariate logistic regression analysis showed the same nonsignificant relationship, except for the miscarriage rate and BMI (OR 1.149, 95% CI 1.03-1.28; P = 0.012).

CONCLUSION(S)

Mitochondrial DNA content is unable to predict the miscarriage of implanted human euploid blastocysts.

Mitochondrial DNA copy number of cumulus cells is not linked to embryo implantation in good prognosis IVF patients

RESEARCH QUESTION

Could the mitochondrial DNA (mtDNA) copy number of cumulus cells be used as a biomarker of the potential of embryo implantation in good prognosis IVF patients?

DESIGN

A prospective cohort study on good prognosis IVF patients from a large reproductive medicine centre. A total of 392 embryos from 61 cycles (including 31 implanted and 30 non-implanted cycles) were enrolled in the study. The corresponding cumulus cell mtDNA copy number of embryos was tested by real-time quantitative polymerase chain reaction. The corresponding cumulus cell mtDNA copy numbers were compared between implanted and non-implanted embryos and also compared between high quality and poor quality embryos. Then, a mitochondrial function assay including mitochondrial membrane potentials, concentration of reactive oxygen species (ROS) and ATP content of the corresponding cumulus cells were compared between high quality and poor quality embryos to verify the above experimental findings.

RESULTS

For the same population, the mean cumulus cell mtDNA copy numbers for implanted and non-implanted embryos were 255.61 ± 81.02 and 254.50 ± 73.29 (P = 0.47), and those for high quality and poor quality embryos were 266.02 ± 98.56 and 295.71 ± 70.64 (P = 0.99), respectively. There was no significant difference in cumulus cell mtDNA copy number between implanted and non-implanted embryos or between high quality and poor quality embryos. The mitochondrial membrane potential, ROS levels and ATP content of the corresponding cumulus cells did not differ significantly between high quality and poor quality groups.

CONCLUSIONS

Measurement of cumulus cell mtDNA copy number might not provide any advantage to embryo prioritization in good prognosis IVF patients. Any suggested link between cumulus cell mtDNA copy number and embryo implantation requires further validation.

Embryo and Its Mitochondria

The mitochondria, present in almost all eukaryotic cells, produce energy but also contribute to many other essential cellular functions. One of the unique characteristics of the mitochondria is that they have their own genome, which is only maternally transmitted via highly specific mechanisms that occur during gametogenesis and embryogenesis. The mature oocyte has the highest mitochondrial DNA copy number of any cell. This high mitochondrial mass is directly correlated to the capacity of the oocyte to support the early stages of embryo development in many species. Indeed, the subtle energetic and metabolic modifications that are necessary for each of the key steps of early embryonic development rely heavily on the oocyte’s mitochondrial load and activity. For example, epigenetic reprogramming depends on the metabolic cofactors produced by the mitochondrial metabolism, and the reactive oxygen species derived from the mitochondrial respiratory chain are essential for the regulation of cell signaling in the embryo. All these elements have also led scientists to consider the mitochondria as a potential biomarker of oocyte competence and embryo viability, as well as a key target for future potential therapies. However, more studies are needed to confirm these findings. This review article summarizes the past two decades of research that have led to the current understanding of mitochondrial functions in reproduction

Suboptimal trophectoderm mitochondrial DNA level is associated with delayed blastocyst development

PURPOSE

To provide a comprehensive analysis of mtDNA quantity in D5 and D6 blastocysts, as well as a further insight to the origin of delayed blastocyst development.

METHODS

A retrospective cohort analysis of 829 D5 and 472 D6 blastocysts from 460 patients who underwent in vitro fertilization (IVF) with next-generation sequencing (NGS)-based preimplantation genetic testing for aneuploidy (PGT-A). The quantity of trophectoderm mtDNA was extrapolated from the NGS data, followed by the analysis of mean mtDNA levels between D5 and D6 blastocysts of the same ploidy (aneuploid/euploid) and transfer outcomes (positive/negative clinical pregnancy).

RESULTS

D5 blastocysts had significantly higher euploidy rate and clinical pregnancy rate when compared with D6 blastocysts. The proportion of blastocysts derived from patients ≧ 40 years old were similar between the D5 and D6 cohorts. When blastocysts with identical ploidy were analyzed, the D5 cohorts all had significantly higher mean mtDNA levels than their D6 counterparts. Similarly, when embryo transfers with identical outcome were analyzed, the D5 cohorts also had significantly higher mean mtDNA levels than the D6 cohorts. Trophectoderm mtDNA level was independent of maternal age and blastocyst morphology grades.

CONCLUSIONS

Our data provided further evidence D5 blastocysts contained significantly greater mtDNA quantity than D6 blastocysts, and mtDNA quantity could be a key factor that affects the development rate of blastocysts. Furthermore, one must avoid using an arbitrary threshold when incorporating mtDNA quantity into the embryo selection criteria, as the observed value may have vastly different clinical implication when blastulation rate is also considered.

Normalized Mitochondrial DNA Copy Number Can Optimize Pregnancy Outcome Prediction in IVF

The aim of this study is to explore the relationship between mitochondrial DNA (mtDNA) copy number and embryo implantation potential in in vitro fertilization (IVF). A retrospective study of 319 blastocysts from patients undergoing preimplantation genetic testing (PGT) at Reproductive Medicine Center in Tongji Hospital from January 2016 to February 2018 was conducted. We used multiple annealing- and looping-based amplification cycles (MALBAC) technology to amplify the genetic materials from the trophectoderm cells of blastocysts, and next-generation sequencing (NGS) technology to test mitochondrial DNA copy number. Box-Cox transformation was introduced to eliminate the skewness distribution of mtDNA copy number, and the transformed data were defined as adjusted mtDNA. Subsequently, associations between adjusted mtDNA and the clinical characteristics of patients were assessed by univariate analysis and multiple linear regression. In addition, Gaussian Naive Bayes classifier was also used to predict pregnancy outcomes. We observed that only antral follicle count (AFC) was significantly associated with adjusted mtDNA without the influence of multicollinearity. What's more, the distribution of the adjusted mtDNA of blastocysts resulting in live birth was more concentrated than that of others. The area under the curve (AUC) of the prediction model that combined adjusted mtDNA with other clinical characteristics of patients was up to 0.81, higher than that excluded adjusted mtDNA. Among patient clinical characteristics, AFC was significantly associated with adjusted mtDNA. Mitochondrial DNA copy number may help to optimize the pregnancy outcome prediction in IVF.

Qualitative and Quantitative Ovarian and Peripheral Blood Mitochondrial DNA (mtDNA) Alterations: Mechanisms and Implications for Female Fertility

The reduction of female fertility over time is considered as a natural consequence of ovarian aging. The exact mechanism underlying this process is not fully elucidated. However, it is becoming increasingly evident that qualitative and quantitative mitochondrial genome alterations might play a relevant role. The former include mitochondrial DNA (mtDNA) damage caused by oxidative stress, the accumulation of acquired mtDNA mutations, the effects of inherited mtDNA mutations, and alterations in the mitochondrial stress response mechanism. The latter refer to alterations in the oocytes, granuolosa cells, and embryonic cells mtDNA content. The present review aims to investigate the evidence about: (1) the effect of qualitative and quantitative mtDNA alterations on female fertility, paying particular attention to those with a pathophysiology characterized by a relevant role of oxidative stress; (2) the use of oocytes, granulosa cells (GCs), embryonic cells, and peripheral blood cells mtDNA copy number as a female fertility surrogate biomarker; (3) experimental therapies tested to try to subvert the ovarian aging process with particular reference to antioxidant treatments.

Mitochondrial DNA Content May Not Be a Reliable Screening Biomarker for Live Birth After Single Euploid Blastocyst Transfer

An increasing number of studies have related the mitochondrial DNA (mtDNA) content to embryo viability and transfer outcomes. However, previous studies have focused more on the relationship between mtDNA and embryo implantation, few studies have studied the effect of the mtDNA content on live birth. In the study, we investigated whether mtDNA content is a reliable screening biomarker for live birth after single blastocyst transfer. A total of 233 couples with 316 blastocyst stage embryos undergoing in vitro fertilization treatment and pre-implantation genetic testing analysis were included in the study. All embryos were chromosomally normal and had undergone single-embryo transfers. There was no significant difference observed in the blastocyst mtDNA content among the live birth, miscarriage and non-implanted groups (p=0.999), and the mtDNA content in blastocysts from the miscarriage and live birth groups was similar [median (interquartile range), 1.00*10⁸(7.59*10⁷- 1.39*10⁸) vs 1.01*10⁸ (7.37*10⁷- 1.32*10⁸)]. Similarly, no significant association was observed between mtDNA content and embryo implantation potential (p=0.965). After adjusting for multiple confounders in a logistic regression analysis with generalized estimating equations, no associations between mtDNA content and live birth were observed in all blastocysts, Day-5 and Day-6 blastocysts (p=0.567, p=0.673, p=0.165, respectively). The live birth rate was not significantly different between blastocysts with an elevated mtDNA content and blastocysts with a normal mtDNA content (26.7% vs 33.6% p=0.780). Additionally, there was no linear correlation between the mtDNA content and maternal age (p=0.570). In conclusion, the mtDNA content does not seem to be a potential biomarker for embryo transfer outcomes (i.e., implantation and live birth) based on the existing testing tools. Embryos with an elevated mtDNA content also have development potential for successful live birth.

Comparative analysis of cell-free DNA content in culture medium and mitochondrial DNA copy number in porcine parthenogenetically activated embryos

We examined the effect of ploidy on mitochondrial DNA (mtDNA) copy number in embryos and the amount of cell-free mitochondrial and nucleic DNA content (cf-mtDNA and cf-nDNA) in spent culture medium (SCM). Oocytes collected from the ovaries were matured, activated, incubated in medium containing cycloheximide (CHX) or CHX and cytochalasin B (CB) for 4.5 h to produce haploid or diploid embryos (H-group and D-group embryos). These embryos were cultured for 7 days, and the blastocysts and SCM were examined. The amount of mtDNA and nDNA was determined by real-time PCR. The rate of development to the blastocyst stage was higher for the D-group than for the H-group. Moreover, D-group blastocysts had less mtDNA compared to the H-group blastocysts. After activation, the mitochondrial content was constant before the blastocyst stage in D-group embryos, but increased earlier in H-group embryos. The amount of cf-mtDNA in the SCM of D-group blastocysts was greater than that of H-group blastocysts. However, when the cf-mtDNA in the SCM of 2 cell-stage embryos (day 2 post-activation) was examined, the amount of cf-mtDNA was greater in the H-group than in the D-group embryos. When D-group embryos were cultured for 7 days, a significant correlation was observed between the total cell number of blastocysts and cf-nDNA content in the SCM. Hence, although careful consideration is needed regarding the time point for evaluating mtDNA content in the embryos and SCM, this study demonstrates that mtDNA in the embryos and SCM was affected by the ploidy of the embryos.

Mitochondrial DNA content decreases during in vitro human embryo development: insights into mitochondrial DNA variation in preimplantation embryos donated for research

OBJECTIVE

To assess the mitochondrial DNA (mtDNA) load and variation in human oocytes and during preimplantation embryo development using specimens donated for research.

DESIGN

Prospective cohort study.

SETTING

Not applicable.

PATIENTS

A total of 50 in vitro fertilization patients and 11 oocyte donors whose specimens were obtained between July 2017 and July 2018.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

All specimens were separately collected. Quantitative polymerase chain reaction was performed with SurePlex DNA Amplification System (Illumina). Primers for the adenosine triphosphate 8 mitochondrial gene and the β-actin were used. Data were statistically analyzed by analysis of variance with the Scheffé multiple pairwise comparison for categorical variables and by linear regression for numerical variables.

RESULTS

Human metaphase II (MII) oocytes had significantly more total mtDNA copy number than day 3 embryos, and day 3 embryos had more total and per-cell mtDNA copy number than aneuploid blastocysts. There was a significant decrease in mtDNA content associated with failed-fertilized oocytes compared to noninseminated metaphase II oocytes.

CONCLUSIONS

During preimplantation development, before implantation, human embryos undergo a significant decrease in total mtDNA content and no increase in mtDNA content at the blastocyst stage. Oocytes need to carry a correct threshold of mitochondrial load in the oocyte in order to successfully fertilize. An active degradation of mtDNA before implantation occurs after fertilization takes place. These findings could be used to improve knowledge about the best embryo culture conditions and would serve as a basis for further studies addressing again the use of mtDNA content as an embryo viability marker.

Mitochondrial oxygen consumption rate of human embryos declines with maternal age

PURPOSE

The fertility of women decreases with age because of factors such as an increased incidence of aneuploidies and-possibly-decreased mitochondrial activity in oocytes. However, the relationship between maternal aging and mitochondrial function of their embryos remains unknown. Here, we assessed the relationship between maternal age and mitochondrial functions in their oocytes and embryos METHODS: The relationships between maternal age and oxygen consumption rates (OCRs), mitochondrial DNA (mtDNA) copy numbers, or blastocyst development was investigated using 81 embryos donated from 63 infertility couples. The developmental rates from morulae to blastocysts were retrospectively analyzed using data of 105 patients.

RESULTS

The OCRs of morulae decreased with maternal age (r² = 0.48, P < 0.05) although there were no relationships between maternal age and mtDNA copy number in any stages. The more oxygen consumed at the morula stage, the shorter time was required for embryo development to the mid-stage blastocyst (r² = 0.236, P < 0.05). According to the clinical data analysis, the developmental rate from morulae to blastocysts decreased with maternal age (P < 0.05, < 37 years, 81.1%, vs. ≥ 37 years, 64.1%).

CONCLUSIONS

The data of the present study revealed that mitochondrial function at the morula stage of human embryos decreased with their maternal age and a decrease of mitochondrial function led to slow-paced development and impaired developmental rate from morulae to blastocysts.