Insights into the Role of Telomeres in Human Embryological Parameters. Opinions Regarding IVF

Potential effects of assisted reproductive technology on telomere length and telomerase activity in human oocytes and early embryos

Telomeres are repetitive DNA sequences at eukaryotic chromosome ends and function in maintaining genome integrity and stability. These unique structures undergo shortening due to various factors including biological aging, consecutive DNA replication, oxidative stress, and genotoxic agents. Shortened telomeres can be lengthened by the enzyme telomerase and alternative lengthening of telomeres in germ cells, early embryos, stem cells, and activated lymphocytes. If telomeres reach to critical length, it may lead to genomic instability, chromosome segregation defects, aneuploidy, and apoptosis. These phenotypes also occur in the oocytes and early embryos, produced using assisted reproductive technologies (ARTs). Thus, a number of studies have examined the potential effects of ART applications such as ovarian stimulation, culture conditions, and cryopreservation procedures on telomeres. Herein, we comprehensively reviewed impacts of these applications on telomere length and telomerase activity in ART-derived oocytes and embryos. Further, we discussed use of these parameters in ART centers as a biomarker in determining oocyte and embryo quality.

Pathomechanisms of Prenatally Programmed Adult Diseases

Based on epidemiological observations Barker et al. put forward the hypothesis/concept that an adverse intrauterine environment (involving an insufficient nutrient supply, chronic hypoxia, stress, and toxic substances) is an important risk factor for the development of chronic diseases later in life. The fetus responds to the unfavorable environment with adaptive reactions, which ensure survival in the short run, but at the expense of initiating pathological processes leading to adult diseases. In this review, the major mechanisms (including telomere dysfunction, epigenetic modifications, and cardiovascular-renal-endocrine-metabolic reactions) will be outlined, with a particular emphasis on the role of oxidative stress in the fetal origin of adult diseases.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

Functional association between telomeres, oxidation and mitochondria

Prior research has substantiated the vital role of telomeres in human fertility. Telomeres are prerequisites for maintaining the integrity of chromosomes by preventing the loss of genetic material following replication events. Little is known about the association between sperm telomere length and mitochondrial capacity involving its structure and functions. Mitochondria are structurally and functionally distinct organelles that are located on the spermatozoon's midpiece. Mitochondria produce adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS), which is necessary for sperm motility and generate reactive oxygen species (ROS). While a moderate concentration of ROS is critical for egg-sperm fusion, and fertilization, excessive ROS generation is primarily related to telomere shortening, sperm DNA fragmentation, and alterations in the methylation pattern leading to male infertility. This review aims to highlight the functional connection between mitochondria biogenesis and telomere length in male infertility, as mitochondrial lesions have a damaging impact on telomere length, leading both to telomere lengthening and reprogramming of mitochondrial biosynthesis. Furthermore, it aims to shed light on how both inositol and antioxidants can positively affect male fertility.

Telomere Length Regulation

The number of (TTAGGG)_n repeats at the ends of chromosomes is highly variable between individual chromosomes, between different cells and between species. Progressive loss of telomere repeats limits the proliferation of pre-malignant human cells but also contributes to aging by inducing apoptosis and senescence in normal cells. Despite enormous progress in understanding distinct pathways that result in loss and gain of telomeric DNA in different cell types, many questions remain. Further studies are needed to delineate the role of damage to telomeric DNA, replication errors, chromatin structure, liquid-liquid phase transition, telomeric transcripts (TERRA) and secondary DNA structures such as guanine quadruplex structures, R-loops and T-loops in inducing gains and losses of telomere repeats in different cell types. Limitations of current telomere length measurements techniques and differences in telomere biology between species and different cell types complicate generalizations about the role of telomeres in aging and cancer. Here some of the factors regulating the telomere length in embryonic and adult cells in mammals are discussed from a mechanistic and evolutionary perspective.

Impact of superovulation and in vitro fertilization on LINE-1 copy number and telomere length in C57BL/6 J mice blastocysts

OBJECTIVE

Millions of babies have been conceived by IVF, yet debate about its safety to offspring continues. We hypothesized that superovulation and in vitro fertilization (IVF) promote genomic changes, including altered telomere length (TL) and activation of the retrotransposon LINE-1 (L1), and tested this hypothesis in a mouse model.

MATERIAL AND METHODS

Experimental study analyzing TL and L1 copy number in C57BL/6 J mouse blastocysts in vivo produced from natural mating cycles (N), in vivo produced following superovulation (S), or in vitro produced following superovulation (IVF). We also examined the effects of prolonged culture on TL and L1 copy number in the IVF group comparing blastocysts cultured 96 h versus blastocysts cultured 120 h. TL and L1 copy number were measured by Real Time PCR.

RESULTS

TL in S (n = 77; Mean: 1.50 ± 1.15; p = 0.0007) and IVF (n = 82; Mean: 1.72 ± 1.44; p < 0.0001) exceeded that in N (n = 16; Mean: 0.61 ± 0.27). TL of blastocysts cultured 120 h (n = 15, Mean: 2.14 ± 1.05) was significantly longer than that of embryos cultured for 96 h (n = 67, Mean: 1.63 ± 1.50; p = 0.0414). L1 copy number of blastocysts cultured for 120 h (n = 15, Mean: 1.71 ± 1.49) exceeded that of embryos cultured for 96 h (n = 67, Mean: 0.95 ± 1.03; p = 0.0162).

CONCLUSIONS

Intriguingly ovarian stimulation, alone or followed by IVF, produced embryos with significantly longer telomeres compared to in vivo, natural cycle-produced embryos. The significance of this enriched telomere endowment for the health and longevity of offspring born from IVF merit future studies.

Reduced telomere length in amniocytes: an early biomarker of abnormal fetal development?

Telomeres protect chromosome ends and control cell division and senescence. During organogenesis, telomeres need to be long enough to ensure the cell proliferation necessary at this stage of development. Previous studies have shown that telomere shortening is associated with growth retardation and congenital malformations. However, these studies were performed in newborns or postnatally, and data on telomere length (TL) during the prenatal period are still very limited. We measured TL using quantitative PCR in amniotic fluid (AF) and chorionic villi (CV) samples from 69 control fetuses with normal ultrasound (52 AF and 17 CV) and 213 fetuses (165 AF and 48 CV) with intrauterine growth retardation (IUGR) or congenital malformations diagnosed by ultrasound. The samples were collected by amniocentesis at the gestational age of 25.0 ± 5.4 weeks and by CV biopsy at 18.1 ± 6.3 weeks. In neither sample type was TL influenced by gestational age or fetal sex. In AF, a comparison of abnormal versus normal fetuses showed a significant telomere shortening in cases of IUGR (reduction of 34%, P < 10-6), single (29%, P < 10-6) and multiple (44%, P < 10-6) malformations. Similar TL shortening was also observed in CV from abnormal fetuses but to a lesser extent (25%, P = 0.0002; 18%, P = 0.016; 20%, P = 0.004, respectively). Telomere shortening was more pronounced in cases of multiple congenital anomalies than in fetuses with a single malformation, suggesting a correlation between TL and the severity of fetal phenotype. Thus, TL measurement in fetal samples during pregnancy could provide a novel predictive marker of pathological development.

Leukocyte Telomere Length Correlates with Extended Female Fertility

Current social trends of delayed reproduction to the fourth and fifth decade of life call for a better understanding of reproductive aging. Demographic studies correlated late reproduction with general health and longevity. Telomeres, the protective ends of eukaryotic chromosomes, were implicated in various aging-associated pathologies and longevity. To examine whether telomeres are also associated with reproductive aging, we measured by Southern analysis the terminal restriction fragments (TRF) in leukocytes of women delivering a healthy infant following a spontaneous pregnancy at 43–48 years of age. We compared them to age-matched previously fertile women who failed to conceive above age 41. The average TRF length in the extended fertility group (9350 bp) was significantly longer than in the normal fertility group (8850 bp; p-value = 0.03). Strikingly, excluding women with nine or more children increased the difference between the groups to over 1000 bp (9920 and 8880 bp; p-value = 0.0009). Nevertheless, we observed no apparent effects of pregnancy, delivery, or parity on telomere length. We propose that longer leukocyte telomere length reflects higher oocyte quality, which can compensate for other limiting physiological and behavioral factors and enable successful reproduction. Leukocyte telomere length should be further explored as a novel biomarker of oocyte quality for assessing reproductive potential and integrating family planning with demanding women’s careers.

Sperm telomere length as a novel biomarker of male infertility and embryonic development: A systematic review and meta-analysis

BACKGROUND

Telomeres have an essential role in maintaining the integrity and stability of the human chromosomal genome and preserving essential DNA biological functions. Several articles have been published on the association of STL with male semen parameters and clinical pregnancy. The results, however, are either inconclusive or inconsistent. Therefore, this meta-analysis aimed to systematically assess the accuracy and clinical value of sperm telomere length (STL) as a new marker for diagnosing male infertility and predicting the quality of embryonic development.

METHODS

We performed a comprehensive systematic search for relevant publications in PubMed, the Cochrane Library, Web of Science, Embase, Scopus, and Ovid, from database build to August 2022. All experimental studies exploring the association of STL with male semen quality, male infertility, or embryonic development were included.

RESULTS

Overall, Twelve prospective observational cohort studies (1700 patients) were eligible for inclusion in the meta-analysis. The meta-analysis showed a positive linear correlation between STL and semen parameters. The optimal cut-off value for STL diagnosing male infertility was 1.0, with a sensitivity and specificity of 80%. Regarding STL and embryonic development, the clinical pregnancy rate was associated with longer STL, and there was no significant difference between the two groups regarding fertilization rate.

CONCLUSION

Our study showed that STL has good diagnostic and predictive value for male fertility and clinical pregnancy and could be used as a new biomarker for diagnosing male infertility and predicting embryonic development.

SYSTEMATIC REVIEW REGISTRATION

https://www.crd.york.ac.uk/PROSPERO/, identifier CRD42022303333.