Do old eggs really stink? On oocyte epigenetic aging and its connection to chromosomal aging

Oocytes aging is characterized by non-disjunction of chromosomes and infertile eggs. Possible mechanisms for this early aging include the loss of Cohesin from chromosomes, which is not re-loaded during adulthood. We hypothesize that epigenetic aging contributes to the process of Cohesin loss and meiotic dysfunction. To establish that we immuno-stained mouse oocytes for heterochromatin markers and compared young and old females for the staining pattern. In addition, we used surplus human GV oocytes from the IVF clinic and correlated maternal age with the heterochromatin staining. In all cases studied- both in mouse and human, heterochromatin stained significantly weaker in older females, for different heterochromatin related histone modifications (H3k9me, H4k20me, H3k27me). Moreover, staining for dsRNA and Dicer reveals a parallel increase in RNA processing in the older ooctes cytoplasm. These results were confirmed by a small RNA-seq method to capture RNAs from a small number of oocytes. In human oocytes we also observed a reduction in H3K9me2 staining and a correlation between the level of H3K9me and Rec8. To examine whether Cohesin loss is an upstream event to heterochromatin loss we removed all meiotic Cohesin from young oocytes and immune-stained these oocytes for heterochromatin markers. From our results, it seems that Cohesin removal does not reduce heterochromatin in the oocyte, but in fact the opposite is true. However, treatment with Chaetocin to inhibit H3K9me2 deposition causes Cohesin loss. These results suggest that Cohesin loss is not the cause for heterochromatin loss in oocytes, but cohesion is dependent on epigenetic integrity.