SRA

The frequency of egg aneuploidy and trisomic pregnancies increases with maternal age. To what extent individual approaches can delay the “maternal age effect” is unclear since multiple causes contribute to chromosomal abnormalities in mammalian eggs. We propose that ovulation frequency determines the physiological ageing of oocytes, a key aspect of which is the ability to accurately segregate chromosomes and produce euploid eggs. To test this hypothesis, ovulations were reduced using successive pregnancies, hormonal contraception and a pre-pubertal knockout mouse model, and the effects on chromosome segregation and egg ploidy were examined. We show that each intervention reduces chromosomal abnormalities in eggs of aged mice, suggesting that ovulation reduction delays oocyte ageing. The protective effect can be partly explained by retention of chromosomal Rec8-cohesin that maintains sister chromatid cohesion in meiosis. In addition, single-nucleus Hi-C (snHi-C) revealed a deterioration in 3D chromatin structure including an increase in extruded loop sizes in long-lived oocytes. Artificial cleavage of Rec8 is sufficient to increase extruded loop sizes, suggesting that cohesin complexes maintaining cohesion restrict loop extrusion. These findings suggest that ovulation suppression protects against Rec8 loss, thereby maintaining both sister chromatid cohesion and 3D chromatin structure and promoting production of euploid eggs. We conclude that the “maternal age effect” can be delayed in mice. An implication of this work is that long-term ovulation-suppressing conditions can potentially reduce the risk of Down's syndrome pregnancies at advanced maternal age. Overall design: In total, 126 snHi-C libraries were generated. We generated 20 snHi-C libraries from GV oocytes of C57BL/6J 2-2.5 month old (C57BL/6J_young) and 20 snHi-C libraries of C57BL/6J 14-18 month aged (C57BL/6J_aged) mice. We generated 12 snHi-C libraries from GV oocytes of C57BL/ICRFat 14-15 month aged virgin (ICRF_aged.virgin), 14 snHi-C libraries from GV oocytes of C57BL/ICRFat 14-15 month aged constant-mated (ICRF_aged.mated) mice and 11 snHi-C libraries from C57BL/ICRFat GV oocytes of 2-2.5 month old (young). We generated 49 snHi-C libraries from Rec8TEV/TEVWaplfl/fl (Tg)Zp3-Cre GV oocytes (n(-TEV) = 21 and n(+TEV protease) = 28).