Reproductive and epigenetic outcomes associated with aging mouse oocytes

Female aging entails a decline in fertility in mammals, manifested by reduced oocyte reserves and poor oocyte quality accompanied by chromosomal anomalies and reduced litter size. In addition to compromised genetic integrity, recent studies suggest that epigenetic mechanisms may be altered in aging oocytes, with age affecting the expression of DNA methyltransferases, which catalyze the important epigenetic modification, DNA methylation. Loss of DNA methylation patterns, most notably for imprinted genes, is lethal to mouse embryos. To investigate how maternal age affects embryonic development and underlying DNA methylation patterns, young and aged C57BL/6 females were mated with C57BL/6 or C57BL/6(CAST7) males to allow for the identification of parental alleles; resulting blastocysts and mid-gestation embryos and placentas were evaluated. Although pregnancy, ovulation and implantation rates were similar between age groups, an age-related increase in resorption sites, morphological abnormalities and delayed development was found. Interestingly, placental morphology was also perturbed by aging, with elevated numbers of trophoblast giant cells in aged pregnancies. Normal monoallelic expression of the imprinted genes H19 and Snrpn was unaltered in blastocysts from aged females. We failed to observe any age-related changes in methylation of the differentially methylated regions of imprinted genes Snrpn, Kcnq1ot1, U2af1-rs1, Peg1, Igf2r and H19. Restriction Landmark Genome Scanning showed no significant differences in genome-wide DNA methylation in embryos and placentas, regardless of maternal age. Our findings demonstrate that maternal age affects post-implantation embryo and placental development; however embryos capable of developing to mid-gestation appear to undergo normal acquisition and maintenance of DNA methylation patterning.