In animal models, especially in the cow and the mouse, resumption of the meiotic process and progression to a condition of meiotic maturity can be achieved in vitro very efficiently by fully grown-oocytes collected from graafian follicles, with rates of maturation exceeding 80-90%. On the contrary, in our species only 50-60% of immature oocytes attain the mature stage in vitro. The reduced rate by which oocyte meiotic maturation is achieved in the human species could be due to inadequate conditions of in vitro maturation (IVM) systems. Alternatively, it could also be due to the fact that some of immature oocytes are intrinsically incompetent to support meiotic resumption and progression as a result, for example, of activation of checkpoints that regulate the cell cycle. Oocyte meiotic failure has significant clinical implications not only for assisted reproduction treatments in which oocyte are matured in vitro, but also for conventionally stimulated IVF cycles, in which up to 30% of all retrieved oocytes do not mature in vivo despite full exposure to gonadotropins.
In a recent study (1), Biogenesi (Italy) explored the possibility that human immature oocytes might be affected by DNA damage and that such a condition, or inability to trigger a repair action, is associated with the inability to initiate the meiotic process.
DNA damage is a very common insult to which all cells are exposed during their lifetime following exposure to mutagens, or as a consequence of pathologies or aging. Mammalian oocytes are particularly exposed to DNA damage. In fact, at the time of primordial follicle formation, breaks of the DNA double helix – double strand breaks (DSBs) – have physiological relevance in the context of meiotic recombination, by allowing the exchange of portions of DNA between homologous chromosomes. In addition, DNA damage can potentially accumulate during the long period of oocyte dormancy (lasting up to more than 40 years in the human) that precedes growth and maturation. To test the above hypothesis, the study assessed the presence of DSBs sites and the occurrence of a repair response by detecting the two markers gH2AX and Rad51, respectively, by adopting immunofluorescence confocal microscopy as an investigation tool. gH2AX is an effector that modifies and makes accessible the lesion site, acting also as a catalyst for the recruitment of the necessary DNA repair factors. Rad51 is the central enzymatic component of the mechanism that ensures homologous recombination and error-free repair of DSBs.
In the study conducted by the Biogenesi team, single or multiple sites of DNA DSB were observed in almost half (47%) of immature oocytes recovered from IVF cycles. In oocytes of the same source that were unable to resume the meiotic process in vitro, DNA DSBs were much more represented (81%). In the same oocyte groups, occurrence of a DNA repair response, as suggested by positivity to Rad51, was observed with rates (42.1% and 74.1%, respectively) mirroring those expressing the presence of DNA damage. These findings are consistent with the hypothesis that the presence of DNA damage triggers an arrest of the meiotic process to allow the timely expression of a repair response. Although significant per se, the present study involved only immature oocytes. In future studies, it will be important to established whether mature oocytes are also affected by DNA damage. In fact, the possible presence of damaged DNA in mature oocytes bears potentially important implications. A recent study carried out in the mouse model indicates that DNA damage may not be detected or repaired during oocyte maturation and therefore can be passed on to the embryo.
The study extends previous data on the occurrence of DNA DSBs and the existence of a signal of DNA repair response in human fully-grown immature oocytes (21). It also suggests for the first time that DNA damage may be a cause of meiotic arrest. This opens new avenues for future research on genetic integrity as a novel biomarker of oocyte quality.
1) Coticchio G, Dal Canto M, Guglielmo M-C, Albertini DF, Renzini MM, Merola M, Lain M, Sottocornola M, De Ponti E, Fadini R. Double-strand DNA breaks and repair response in human immature oocytes and their relevance to meiotic resumption. J Assist Reprod Genet. In press http://www.ncbi.nlm.nih.gov/pubmed/26238391
Dr. Giovanni Coticchio
Biogenesi Scientific Coordinator