Clinical significance and regulation of cohesin depletion in mammalian oocytes

Abstract

The likelihood of chromosome segregation errors in human oocytes increases dramatically from the age of 35. This results in an increased chance of pregnancies from older women ending in miscarriage. Oocytes are particularly vulnerable to ageing, as it can take five decades for them to undergo growth, from their formation during embryogenesis as prophase arrested primordial-stage oocytes. Chromosomal stability in oocytes is provided by cohesin. This is loaded onto oocyte chromosomes during embryogenesis and removed during anaphase after oocyte growth. Recent work in our laboratory indicates that cohesin is predominantly lost at the primordial stage. However, questions remain around the timing and mechanism of loss. I will seek to address this as well as investigate whether loss of cohesin can explain age-related segregation errors in human oocytes. My findings indicate that cohesin loss occurs gradually during ageing, and that the meiosis-specific cohesin subunit REC8 appears more affected by age than the universal sub-unit SMC3. I find that RAD21 is expressed at the primordial-oocyte stage, suggesting a potential turnover of RAD21-containing complexes could explain this difference. The protease separase, which cleaves cohesin during anaphase, is also detectable in the oocyte cytoplasm at the primordial stage. Using an oocytespecific separase null mouse, I find that cohesin depletion occurs in the absence of separase. Finally, I use human oocytes to test the clinical significance of cohesin depletion. My findings indicate that ageing is associated with loss of REC8 from between chromatids. However, due to diffuse association of cohesin with chromatin in oocytes from older women, no overall reduction in chromosome-associated REC8 occurs. Thus, in contrast to mice, ageing in human oocytes appears to manifest as mislocalisation, rather than reduction of REC8. These findings indicate that ageing impacts on cohesin in both mouse and human oocytes but that the outcome manifests differently between organisms.