Age-dependent loss of cohesion protection in human oocytes

Aneuploid human eggs (oocytes) are a major cause of infertility, miscarriage and chromosomal disorders. Such aneuploidies increase greatly as women age, originating from defective linkages between sister-chromatids (cohesion) in meiosis. We found evidence that loss of a specific pool of the cohesin protector protein, shugoshin 2 (Sgo2) contributes to this phenomenon. Our data indicate that Sgo2 preserves sister chromatid cohesion in meiosis by protecting a cohesin bridge between sister chromatids. In human oocytes, Sgo2 localizes to both sub-centromere cups and the pericentromeric bridge which spans the sister chromatid junction. Sgo2 normally colocalizes with cohesin, however, in oocytes from older women, Sgo2 is frequently lost specifically from the pericentromeric bridge and sister chromatid cohesion is weakened. Mps1 and Bub1 kinase activities maintain Sgo2 at sub-centromeres and the pericentromeric bridge. Removal of Sgo2 throughout meiosis I by Mps1 inhibition reduces cohesion protection, increasing the incidence of single chromatids at meiosis II. Therefore, Sgo2 deficiency in human oocytes can exacerbate the effects of maternal age by rendering residual cohesin at pericentromeres vulnerable to loss in anaphase I. Our data show that maternal age-dependent loss of Sgo2 at the pericentromere bridge in human oocytes impairs cohesion integrity and contributes to the increased incidence of aneuploidy observed in human oocytes with advanced maternal age.