The kinetics of donor cell mtDNA in embryonic and somatic donor cell-derived bovine embryos

Mitochondrial Inheritance Following Nuclear Transfer: From Cloned Animals to Patients with Mitochondrial Disease

Mitochondria are indispensable power plants of eukaryotic cells that also act as a major biochemical hub. As such, mitochondrial dysfunction, which can originate from mutations in the mitochondrial genome (mtDNA), may impair organism fitness and lead to severe diseases in humans. MtDNA is a multi-copy, highly polymorphic genome that is uniparentally transmitted through the maternal line. Several mechanisms act in the germline to counteract heteroplasmy (i.e., coexistence of two or more mtDNA variants) and prevent expansion of mtDNA mutations. However, reproductive biotechnologies such as cloning by nuclear transfer can disrupt mtDNA inheritance, resulting in new genetic combinations that may be unstable and have physiological consequences. Here, we review the current understanding of mitochondrial inheritance, with emphasis on its pattern in animals and human embryos generated by nuclear transfer.

Mitochondrial enrichment in infertile patients: a review of different mitochondrial replacement therapies

Poor ovarian responders exhibit a quantitative reduction in their follicular pool, and most cases are also associated with poor oocyte quality due to patient’s age, which leads to impaired in vitro fertilisation outcomes. In particular, poor oocyte quality has been related to mitochondrial dysfunction and/or low mitochondrial count as these organelles are crucial in many essential oocyte processes. Therefore, mitochondrial enrichment has been proposed as a potential therapy option in infertile patients to improve oocyte quality and subsequent in vitro fertilisation outcomes. Nowadays, different options are available for mitochondrial enrichment treatments that are encompassed in two main approaches: heterologous and autologous. In the heterologous approach, mitochondria come from an external source, which is an oocyte donor. These techniques include transferring either a portion of the donor’s oocyte cytoplasm to the recipient oocyte or nuclear material from the patient to the donor’s oocyte. In any case, this approach entails many ethical and safety concerns that mainly arise from the uncertain degree of mitochondrial heteroplasmy deriving from it. Thus the autologous approach is considered a suitable potential tool to improve oocyte quality by overcoming the heteroplasmy issue. Autologous mitochondrial transfer, however, has not yielded as many beneficial outcomes as initially expected. Proposed mitochondrial autologous sources include immature oocytes, granulosa cells, germline stem cells, and adipose-derived stem cells. Presently, it would seem that these autologous techniques do not improve clinical outcomes in human infertile patients. However, further trials still need to be performed to confirm these results. Besides these two main categories, new strategies have arisen for oocyte rejuvenation by improving patient’s own mitochondrial function and avoiding the unknown consequences of third-party genetic material. This is the case of antioxidants, which may enhance mitochondrial activity by counteracting and/or preventing oxidative stress damage. Among others, coenzyme-Q10 and melatonin have shown promising results in low-prognosis infertile patients, although further randomised clinical trials are still necessary.

Autologous mitochondrial microinjection; a strategy to improve the oocyte quality and subsequent reproductive outcome during aging

Along with the decline in oocyte quality, numerous defects such as mitochondrial insufficiency and the increase of mutation and deletion have been reported in oocyte mitochondrial DNA (mtDNA) following aging. Any impairments in oocyte mitochondrial function have negative effects on the reproduction and pregnancy outcome. It has been stated that infertility problems caused by poor quality oocytes in women with in vitro fertilization (IVF) and repeated pregnancy failures are associated with aging and could be overcome by transferring large amounts of healthy mitochondria. Hence, researches on biology, disease, and the therapeutic use of mitochondria continue to introduce some clinical approaches such as autologous mitochondrial transfer techniques. Following mitochondrial transfer, the amount of ATP required for aged-oocyte during fertilization, blastocyst formation, and subsequent embryonic development could be an alternative modality. These modulations improve the pregnancy outcome in women of high reproductive aging as well. In addition to overview the clinical studies using mitochondrial microinjection, this study provides a framework for future approaches to develop effective treatments and preventions of congenital transmission of mitochondrial DNA mutations/diseases to offspring. Mitochondrial transfer from ovarian cells and healthy oocytes could lead to improved fertility outcome in low-quality oocytes. The modulation of mitochondrial bioactivity seems to regulate basal metabolism inside target oocytes and thereby potentiate physiological activity of these cells while overcoming age-related infertility in female germ cells.

Transmission of Dysfunctional Mitochondrial DNA and Its Implications for Mammalian Reproduction

Mitochondrial DNA (mtDNA) encodes proteins for the electron transport chain which produces the vast majority of cellular energy. MtDNA has its own replication and transcription machinery that relies on nuclear-encoded transcription and replication factors. MtDNA is inherited in a non-Mendelian fashion as maternal-only mtDNA is passed onto the next generation. Mutation to mtDNA can cause mitochondrial dysfunction, which affects energy production and tissue and organ function. In somatic cell nuclear transfer (SCNT), there is an issue with the mixing of two populations of mtDNA, namely from the donor cell and recipient oocyte. This review focuses on the transmission of mtDNA in SCNT embryos and offspring. The transmission of donor cell mtDNA can be prevented by depleting the donor cell of its mtDNA using mtDNA depletion agents prior to SCNT. As a result, SCNT embryos harbour oocyte-only mtDNA. Moreover, culturing SCNT embryos derived from mtDNA depleted cells in media supplemented with a nuclear reprograming agent can increase the levels of expression of genes related to embryo development when compared with non-depleted cell-derived embryos. Furthermore, we have reviewed how mitochondrial supplementation in oocytes can have beneficial effects for SCNT embryos by increasing mtDNA copy number and the levels of expression of genes involved in energy production and decreasing the levels of expression of genes involved in embryonic cell death. Notably, there are beneficial effects of mtDNA supplementation over the use of nuclear reprograming agents in terms of regulating gene expression in embryos. Taken together, manipulating mtDNA in donor cells and/or oocytes prior to SCNT could enhance embryo production efficiency.

Mitochondria and reproduction: possibilities for testing and treatment

Mitochondria, known as the energy factories in all cells, are key regulators of multiple vital cellular processes and affect all aspects of mammalian reproduction, being essential for oocyte maturation, fertilization and embryonic development. Mitochondrial dysfunction is consequently implicated in disease as well as age-related infertility. Since mitochondria are inherited exclusively from the mother, the female gamete is central to reproductive outcome and therapeutic interventions, such as mitochondrial replacement therapy (MRT), and development of new diagnostic tools. The primary purpose of MRT is to improve oocyte quality, embryogenesis and fetal development by correcting the imbalance between mutant and wild-type mitochondrial DNA (mtDNA) in the oocyte or zygote, either by replacing mutant mtDNA or supplementing with wild-type counterparts from heterologous or autologous sources. However, the efficacy and safety of these new technologies have not yet been tested in clinical trials, and various concerns exist. Nonetheless, the perspectives for such procedures are intriguing and include two distinct patient populations that could potentially benefit from the clinical implementation of MRT; 1) patients with mtDNA-disease transmission risk; 2) patients undergoing IVF with recurrent poor embryo outcomes due to advanced maternal age. In this review, we outline the intrinsic roles of mitochondria during oogenesis and early embryogenesis in relation to disease and infertility, and discuss the progress in MRT with the developments in reproductive technologies and the related concerns. In addition, we assess the use of mtDNA as a potential biomarker for embryo viability in assisted reproduction.

Novel reproductive technologies to prevent mitochondrial disease

BACKGROUND

The use of nuclear transfer (NT) has been proposed as a novel reproductive treatment to overcome the transmission of maternally-inherited mitochondrial DNA (mtDNA) mutations. Pathogenic mutations in mtDNA can cause a wide-spectrum of life-limiting disorders, collectively known as mtDNA disease, for which there are currently few effective treatments and no known cures. The many unique features of mtDNA make genetic counselling challenging for women harbouring pathogenic mtDNA mutations but reproductive options that involve medical intervention are available that will minimize the risk of mtDNA disease in their offspring. This includes PGD, which is currently offered as a clinical treatment but will not be suitable for all. The potential for NT to reduce transmission of mtDNA mutations has been demonstrated in both animal and human models, and has recently been clinically applied not only to prevent mtDNA disease but also for some infertility cases. In this review, we will interrogate the different NT techniques, including a discussion on the available safety and efficacy data of these technologies for mtDNA disease prevention. In addition, we appraise the evidence for the translational use of NT technologies in infertility.

OBJECTIVE AND RATIONALE

We propose to review the current scientific evidence regarding the clinical use of NT to prevent mitochondrial disease.

SEARCH METHODS

The scientific literature was investigated by searching PubMed database until Jan 2017. Relevant documents from Human Fertilisation and Embryology Authority as well as reports from both the scientific and popular media were also implemented. The above searches were based on the following key words: 'mitochondria', 'mitochondrial DNA'; 'mitochondrial DNA disease', 'fertility'; 'preimplantation genetic diagnosis', 'nuclear transfer', 'mitochondrial replacement' and 'mitochondrial donation'.

OUTCOMES

While NT techniques have been shown to effectively reduce the transmission of heteroplasmic mtDNA variants in animal models, and increasing evidence supports their use to prevent the transmission of human mtDNA disease, the need for robust, long-term evaluation is still warranted. Moreover, prenatal screening would still be strongly advocated in combination with the use of these IVF-based technologies. Scientific evidence to support the use of NT and other novel reproductive techniques for infertility is currently lacking.

WIDER IMPLICATIONS

It is mandatory that any new ART treatments are first adequately assessed in both animal and human models before the cautious implementation of these new therapeutic approaches is clinically undertaken. There is growing evidence to suggest that the translation of these innovative technologies into clinical practice should be cautiously adopted only in highly selected patients. Indeed, given the limited safety and efficacy data, close monitoring of any offspring remains paramount.

Mitochondrial transfer: Implications for assisted reproductive technologies

The use of mitochondrial transfer as a clinic procedure is drawing closer to reality. Here we provide a detailed overview of mitochondrial transfer techniques – both established and recent – including pronuclear, spindle, ooplasmic and blastomere transfer. Reasons as to why some techniques are more suitable for the prevention of mitochondrial DNA disease than others, as well as the advantages and disadvantages of each methodology, are discussed. The possible clinical introduction of these techniques has raised concerns about the adverse effects they may have on resultant embryos and offspring. Success rates of each technique, embryo viability and developmental consequences post mitochondrial transfer are addressed through analysis of evidence obtained from both animal and human studies. Counterarguments against potential mitochondrial-nuclear genome incompatibility are also provided. Additional clinical applications of mitochondrial transfer techniques are discussed. These include the rescue or enhancement of fertility in women of advanced maternal age or those suffering from diabetes. An alternative to using mitochondrial DNA transfer for germ line therapies is the therapeutic use of somatic cell nuclear transfer for the generation of personalised stem cells. Although ethically challenging, this method could offer patients already suffering from mitochondrial DNA diseases a novel treatment option.

Experimental strategies towards increasing intracellular mitochondrial activity in oocytes: A systematic review

PURPOSE

The mitochondrial complement is critical in sustaining the earliest stages of life. To improve the Assisted Reproductive Technology (ART), current methods of interest were evaluated for increasing the activity and copy number of mitochondria in the oocyte cell.

METHODS

This covered the researches from 1966 to September 2015.

RESULTS

The results provided ten methods that can be studied individually or simultaneously.

CONCLUSION

Though the use of these techniques generated great concern about heteroplasmy observation in humans, it seems that with study on these suggested methods there is real hope for effective treatments of old oocyte or oocytes containing mitochondrial problems in the near future.

Xenooplasmic transfer between buffalo and bovine enables development of homoplasmic offspring

Nuclear-mitochondrial incompatibilities may be responsible for the development failure reported in embryos and fetuses produced by interspecies somatic cell nuclear transfer (iSCNT). Herein we performed xenooplasmic transfer (XOT) by introducing 10 to 15% of buffalo ooplasm into bovine zygotes to assess its effect on the persistence of buffalo mitochondrial DNA (mtDNA). Blastocyst rates were not compromised by XOT in comparison to both in vitro fertilized embryos and embryos produced by transfer of bovine ooplasm into bovine zygotes. Moreover, offspring were born after transfer of XOT embryos to recipient cows. Buffalo mtDNA introduced in zygotes was still present at the blastocyst stage (8.3 vs. 9.3%, p = 0.11), indicating unaltered heteroplasmy during early development. Nonetheless, no vestige of buffalo mtDNA was found in offspring, indicating a drift to homoplasmy during later stages of development. In conclusion, we show that the buffalo mtDNA introduced by XOT into a bovine zygote do not compromise embryo development. On the other hand, buffalo mtDNA was not inherited by offspring indicating a possible failure in the process of interspecies mtDNA replication.

Nuclear transfer to prevent mitochondrial DNA disorders: revisiting the debate on reproductive cloning

Preclinical experiments are currently performed to examine the feasibility of several types of nuclear transfer to prevent mitochondrial DNA (mtDNA) disorders. Whereas the two most promising types of nuclear transfer to prevent mtDNA disorders, spindle transfer and pronuclear transfer, do not amount to reproductive cloning, one theoretical variant, blastomere transfer does. This seems the most challenging both technically and ethically. It is prohibited by many jurisdictions and also the scientific community seems to avoid it. Nevertheless, this paper examines the moral acceptability of blastomere transfer as a method to prevent mtDNA disorders. The reason for doing so is that most objections against reproductive cloning refer to reproductive adult cloning, while blastomere transfer would amount to reproductive embryo cloning. After clarifying this conceptual difference, this paper examines whether the main non-safety objections brought forward against reproductive cloning also apply in the context of blastomere transfer. The conclusion is that if this variant were to become safe and effective, dismissing it because it would involve reproductive cloning is unjustified. Nevertheless, as it may lead to more complex ethical appraisals than the other variants, researchers should initially focus on the development of the other types of nuclear transfer to prevent mtDNA disorders.

Ooplast-mediated developmental rescue of bovine oocytes exposed to ethidium bromide

Ooplasm transfer has been used successfully to treat infertility in women with ooplasmic insufficiency and has culminated in the birth of healthy babies. To investigate whether mitochondrial dysfunction is a factor in ooplasmic insufficiency, bovine oocytes were exposed to ethidium bromide, an inhibitor of mitochondrial DNA replication and transcription, during in-vitro maturation (IVM). Exposure of immature oocytes to ethidium bromide for 24h during IVM hampered meiotic resumption and the migration of cortical granules. However, a briefer treatment with ethidium bromide during the last 4h of IVM led to partial arrest of preimplantation development without affecting oocyte maturation. Ooplasm transfer was then performed to rescue the oocytes with impaired development. In spite of this developmental hindrance, transfer of normal ooplasm into ethidium bromide-treated oocytes resulted in a complete rescue of embryonic development and the birth of heteroplasmic calves. Although this study unable to determine whether developmental rescue occurred exclusively through introduction of unaffected mitochondria into ethidium bromide-damaged oocytes, e.g. ethidium bromide may also affect other ooplasm components, these results clearly demonstrate that ooplasm transfer can completely rescue developmentally compromised oocytes, supporting the potential use of ooplasm transfer in therapeutic applications.

Analysis of heterogeneous mitochondria distribution in somatic cell nuclear transfer porcine embryos

We previously reported that translocation of mitochondria from the oocyte cortex to the perinuclear area indicates positive developmental potential that was reduced in porcine somatic cell nuclear transfer (SCNT) embryos compared to in vitro220.). The present study is focused on distribution of donor cell mitochondria in intraspecies (pig oocytes; pig fetal fibroblast cells) and interspecies (pig oocytes; mouse fibroblast cells) reconstructed embryos by using either pig fibroblasts with mitochondria-stained MitoTracker CMXRos or YFP-mitochondria 3T3 cells (pPhi-Yellow-mito) as donor cells. Transmission electron microscopy was employed for ultrastructural analysis of pig oocyte and donor cell mitochondria. Our results revealed donor cell mitochondrial clusters around the donor nucleus that gradually dispersed into the ooplasm at 3 h after SCNT. Donor-derived mitochondria distributed into daughter blastomeres equally (82.8%) or unequally (17.2%) at first cleavage. Mitochondrial morphology was clearly different between donor cells and oocytes in which various complex shapes and configurations were seen. These data indicate that (1) unequal donor cell mitochondria distribution is observed in 17.2% of embryos, which may negatively influence development; and (2) complex mitochondrial morphologies are observed in IVF and SCNT embryos, which may influence mitochondrial translocation and affect development.