Rejuvenation of Meiotic Cohesion in Oocytes during Prophase I Is Required for Chiasma Maintenance and Accurate Chromosome Segregation

摘要

Chromosome segregation errors in human oocytes are the leading cause of birth defects, and the risk of aneuploid pregnancy increases dramatically as women age. Accurate segregation demands that sister chromatid cohesion remain intact for decades in human oocytes, and gradual loss of the original cohesive linkages established in fetal oocytes is proposed to be a major cause of age-dependent segregation errors. Here we demonstrate that maintenance of meiotic cohesion in Drosophila oocytes during prophase I requires an active rejuvenation program, and provide mechanistic insight into the molecular events that underlie rejuvenation. Gal4/UAS inducible knockdown of the cohesion establishment factor Eco after meiotic S phase, but before oocyte maturation, causes premature loss of meiotic cohesion, resulting in destabilization of chiasmata and subsequent missegregation of recombinant homologs. Reduction of individual cohesin subunits or the cohesin loader Nipped B during prophase I leads to similar defects. These data indicate that loading of newly synthesized replacement cohesin rings by Nipped B and establishment of new cohesive linkages by the acetyltransferase Eco must occur during prophase I to maintain cohesion in oocytes. Moreover, we show that rejuvenation of meiotic cohesion does not depend on the programmed induction of meiotic double strand breaks that occurs during early prophase I, and is therefore mechanistically distinct from the DNA damage cohesion re-establishment pathway identified in G2 vegetative yeast cells. Our work provides the first evidence that new cohesive linkages are established in Drosophila oocytes after meiotic S phase, and that these are required for accurate chromosome segregation. If such a pathway also operates in human oocytes, meiotic cohesion defects may become pronounced in a woman's thirties, not because the original cohesive linkages finally give out, but because the rejuvenation program can no longer supply new cohesive linkages at the same rate at which they are lost. Author Summary Meiosis is a specialized type of cell division that gives rise to sperm and eggs. In a woman's thirties, errors in meiotic chromosome segregation rise exponentially, significantly increasing the probability that she will conceive a fetus with Down Syndrome (Trisomy 21). Accurate chromosome segregation during meiosis depends on protein linkages (cohesion) that hold sister chromatids together. The widely held view is that under normal conditions, cohesion can only be established during DNA replication, and the original cohesive linkages formed in fetal oocytes are gradually lost as a woman ages. However, it seems unlikely that the same cohesion proteins could survive for even five years, much less 25 years. Here we show that Drosophila oocytes possess an active rejuvenation program that is required to load newly synthesized cohesion proteins and to establish new cohesive linkages after meiotic DNA replication. When we reduce the proteins responsible for rejuvenation after meiotic S phase, cohesion is lost and meiotic chromosomes missegregate. If such a rejuvenation pathway also exists in human oocytes and becomes less efficient with age, oocytes of older women may no longer be able to replace cohesive linkages at the same rate that they are lost.