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Telomere length and aneuploidy: clinical and biological insights into human preimplantation embryos

Reproductive BioMedicine Online, Volume 28, Issue 5, Pages 531–532, May 2014

Telomeres consist of a repeated TTAGGG DNA sequence, which typically extends over several thousand base pairs and serves to cap the ends of each chromosome, stabilizing them and protecting against degradation. It has long been appreciated that telomere length declines with age. This phenomenon is traditionally explained by a failure to replicate sequences at the very end of the chromosome during S-phase of each cell cycle, causing telomeres to progressively shorten with every mitosis. This inexorable process can be mitigated by telomerase, a reverse transcriptase that facilitates the addition of new TTAGGG repeats to the 3′ end of DNA strands, thereby increasing telomere length. For our purposes, it is important to appreciate that telomerase is inactive in somatic cells, its expression restricted to germ cells and certain stages of preimplantation development.

The association of telomere shortening and advancing age naturally generates intriguing hypotheses concerning whether altered telomere length could play a role in age-related aneuploidy. Could vicissitudes of telomere length have an impact on the homologous chromosome pairing necessary for appropriate segregation? Might the popular ‘production line’ hypothesis ( Henderson and Edwards, 1968 ) be of relevance? The hypothesis proposes that the oocytes ovulated later in life are those that entered meiosis last, derived from oogonia that underwent a greater number of mitotic divisions than those ovulated earlier. The increased number of cell divisions would be expected to result in a relative shortening of telomeres. A more complete comprehension of the factors predisposing to aneuploidy, both meiotic (principally of oocyte origin) and mitotic (arising in the first few cell divisions following fertilization), is highly desirable and overdue. Telomeres may represent a useful focus for research in this context. Putting matters of biology to one side for a moment, another important question is to ask whether measurement of telomere length in preimplantation embryos can provide information of clinical value?

Mania and colleagues (2014) compared the telomeres of aneuploid and euploid cells, both derived from disaggregated cleavage-stage embryos previously diagnosed as aneuploid based on fluorescence in situ hybridization (FISH) analysis of a single blastomere biopsy. Telomere length was found to be slightly shorter in aneuploid cells versus those that were chromosomally normal (∼3.9 kb versus ∼4.0 kb). The difference seems subtle, but suggests that there might be an influence of telomere length on aneuploidy predisposition. One could speculate that the impact of this variation might have more dramatic consequences for embryos with telomere lengths at the extreme-low end of the range. In the study by Mania et al., telomere length was significantly shorter in the cells of cleavage-stage embryos when the mother was of advanced reproductive age, or when the couple had a history of miscarriages. These results suggest that there may be certain types of patients for whom telomeres play a particularly important role, significantly contributing to the risk of aneuploidy and consequent negative effects in terms of reproductive outcomes.

There is evidence that telomere lengths are reset between the cleavage and blastocyst stages of development ( Schaetzlein et al., 2004 ), but it remains possible that differences in telomere size experienced during the first five days following fertilization might have long-lasting consequences for the embryo, fetus and perhaps even after birth. After all, the reason for the increased frequency of preterm birth and intrauterine growth retardation in ART pregnancies remains fundamentally unexplained. Could the small but definite increase in frequency of birth defects (odds ratio 1.2–1.3) have a cellular correlate and be the result of issues such as telomere shortening? One looks forward to additional studies comparing non-mosaic aneuploid and non-mosaic euploid embryos, taking into account maternal age and prior fertility. Likewise, the application of newer technologies that replace FISH, measuring telomere length more robustly and providing a comprehensive aneuploidy assessment, is eagerly anticipated.

Studies that indicate links between measurable aspects of embryo biology and viability are of great clinical interest, offering the tantalizing possibility of improved embryo selection and with it the prospect of high-efficiency elective single embryo transfer. The work of Mania and colleagues begs the question of whether to pursue assessment of telomere length to identify embryos most suitable for transfer? This approach would be analogous to other attempts to increase pregnancy rates, or even avoid transfer of genetically abnormal embryos, using indirect approaches such as monitoring of cell-cycle timing.

In terms of aneuploidy, which is believed to be the single biggest cause of embryo implantation failure and miscarriage, what advantages can indirect methods of detection offer? In some cases, such strategies might allow embryo biopsy to be avoided, thereby reducing the risk to the embryo and potentially detecting chromosomally abnormal embryos at lower cost than current ‘direct’ approaches. However, trophectoderm biopsy with laser assistance has diminished, although certainly not eliminated altogether, the technological prowess needed for safe and effective sampling of genetic material from embryos. Furthermore, new genetic technologies such as next-generation sequencing (NGS) are likely to do much to reduce the costs of aneuploidy detection ( Wells, in press ). Direct methods of aneuploidy detection are associated with sensitivity and specificity rates that exceed 95%, vastly superior to equivalent rates for any indirect method reported thus far.

It seems that direct methods, utilizing embryo biopsy, must still, at least for the time being, be considered the strategy of choice for aneuploidy diagnosis. However, it may well be the case that assessment of telomeres or other aspects of the embryo are able to provide valuable information unrelated to aneuploidy that makes their quantification worthwhile. Although four separate randomized controlled trials have now shown that ART outcomes can be significantly improved if comprehensive chromosome analysis is used to assist embryo selection (Schoolcraft et al, 2012, Yang et al, 2012, Forman et al, 2013, and Scott et al, 2013), it remains the case that even transfer of a blastocyst that appears to be both morphologically perfect and chromosomally normal cannot guarantee a pregnancy. Clearly other cellular and/or embryological factors also have importance in terms of implantation and pregnancy potential. Once identified, the measurement of such elements may prove extremely valuable for embryo selection.

From a genetic perspective, ever-evolving technologies are refining the types of analyses that can be undertaken, permitting increased resolution and detection of defects at the sub-chromosomal, or even gene sequence, level. In the coming years the power of methods such as NGS will revolutionize embryo screening and diagnosis. There remain perhaps 15,000 human genes the function of which is either poorly defined or entirely unknown. There is no doubt that many will prove to encode developmental genes that must be intact and functional for proper embryogenesis. In the future, interrogation of selected genes will further confirm normalcy of a given embryo, taking genetic screening above and beyond the simple detection of aneuploidy. Given the increased power and decreasing costs of genetic analyses, one can envision trophectoderm biopsy on all blastocysts, vitrification while awaiting results of genetic testing, and in a subsequent cycle, single embryo transfer. It seems likely that the vexing problem of multiple gestations can be addressed in large part by the simple qualitative analysis that genetics will deliver. Measurement of quantitative factors including telomere length, mitochondrial number, secretion/utilization of components detectable in culture medium and cell cycle lengths provide supplementary information, which is less definitive, but may nonetheless be of some value for assessing viability. Analysis of these aspects of embryo biology might also lead to increased understanding of normal and abnormal developmental processes. An exciting possibility for the future is that improvements in knowledge could eventually help us to move away from passive screening and selection strategies and towards interventions with the capacity to actually improve embryo viability. After all, once techniques for the identification of the most viable embryo have been perfected, the only way for ART success rates to improve further is to increase the potential of the gametes and embryos produced.


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