In human eggs, aneuploidy increases with age and can result in infertility and genetic diseases. Studies in mouse oocytes suggest that reduced centromere cohesion and spindle assembly checkpoint (SAC) activity could be at the origin of chromosome missegregation. Little is known about these two features in humans. Here, we show that in human eggs, inter-kinetochore distances of bivalent chromosomes strongly increase with age. This results in the formation of univalent chromosomes during metaphase I (MI) and of single chromatids in metaphase II (MII). We also investigated SAC activity by checking the localization of BUB1 and BUBR1. We found that they localize at the kinetochore with a similar temporal timing than in mitotic cells and in a MPS1-dependent manner, suggesting that the SAC signalling pathway is active in human oocytes. Moreover, our data also suggest that this checkpoint is inactivated when centromere cohesion is lost in MI and consequently cannot inhibit premature sister chromatid separation. Finally, we show that the kinetochore localization of BUB1 and BUBR1 decreases with the age of the oocyte donors. This could contribute to oocyte aneuploidy.

Robertsonian translocation (RT) carriers are phenotypically normal, but they are known to be at increased risk of repeated miscarriages compared with the general population estimated at about 15% of pregnancies, and also resulting in the birth of a child with a mental retardation or congenital anomalies. Preimplantation Genetic Testing (PGT) is therefore a solution for RT carriers. An appropriate probe strategy allows to differentiate balanced embryos, unbalanced embryos, and mosaic embryos. We performed the first comparative analysis between two or three probes FISH strategies to analyze if a probe strategy choice for PGT-SR studies of Robertsonian translocations (RT) influences the fate of embryos? Our investigations present 13 years of experience of PGT for Robertsonian translocation carriers to improve the accuracy of abnormality detections. A deeper analysis of 283 PGT-SR attempts by comparing two strategies of probes highlighted the irrelevance of using a third probe for FISH diagnosis and above all a significant difference of mosaic embryo rates between probe strategies. These findings could be used as new recommendations of Robertsonian translocation management in many laboratories to improve their practices. It could be readily run, less expensive, reliable and accurate. Furthermore, the propounded strategy of mosaic embryo transfer should be considered after a detailed genetic counseling.

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