Loss of Centromere Cohesion in Aneuploid Human Oocytes Correlates with Decreased Kinetochore Localization of the Sac Proteins Bub1 and Bubr1

Abstract

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.

Sirt2‐BubR1 acetylation pathway mediates the effects of advanced maternal age on oocyte quality

“…High‐level expression of BubR1 extends mice lifespan and delays age‐related deterioration and aneuploidy (Baker et al., 2013). Similarly, a marked reduction of BubR1 levels was detected in oocytes from old women and aged mice (Lagirand‐Cantaloube et al., 2017; Pan, Ma, Zhu, & Schultz, 2008; Riris et al., 2014). Moreover, increasing evidence suggests that BubR1 insufficiency is closely associated with high frequency of spindle/chromosome defects and resultant aneuploidy in oocytes (Baker et al., 2004; Wei et al., 2010).…”

Segregating Chromosomes in the Mammalian Oocyte

“…Importantly, unlike in mouse, sister kinetochores in human oocytes are easily distinguishable as distinct units, even in those from younger (<30 year-old) donors [53,125]. Nonetheless, distance between sister kinetochores increases further with age in human oocytes both in meiosis-I and meiosis-II [87,53,125,126]. Thus, following aging, oocytes possess bivalents with dramatically separated sister kinetochores.…”

“…Thus, although SAC signalling is in operation and serves to limit segregation error by extending meiosis, thus allowing more time for correct attachments to be achieved, it is not the exquisitely sensitive aneuploidy-preventing checkpoint exhibited in mitosis. Similarly in human oocytes, though there is some evidence of SAC presence, mechanistic experiments are few, and anaphase in the presence of misaligned chromosomes is common [45,53,87]. Although the full explanation for this apparent paradox remains mysterious, two very recent lines of investigation provided intriguing clues.…”

Age-Related Loss of Cohesion: Causes and Effects

“…This reduction was apparent even at 5–7 h after germinal vesicle breakdown after a 10 µM nocodazole treatment [ 60 ]. Furthermore, the kinetochore localization of both Bub1 and Bubr1 proteins decreases with age in human oocytes [ 61 ]. A recent study indicates that Smc1β is essential for activation of SAC activity during mouse oocyte meiosis [ 62 ].…”