Error-Prone Chromosome-Mediated Spindle Assembly Favors Chromosome Segregation Defects in Human Oocytes

Abstract

Chromosome segregation errors occurring during the meiotic divisions of a human oocyte are the leading cause of pregnancy loss and several genetic disorders. When chromosomes fail to split into perfect halves during meiosis, the embryo cannot survive or will have a genetic defect, such as Down syndrome. Despite the importance of meiosis in human eggs for fertility and human development, the basis for error-prone chromosome segregation is not known.The authors developed an experimental system for ex vivo high-resolution fluorescence microscopy that allowed them to examine human oocytes freshly harvested from women undergoing gonadotropin-stimulated, in vitro fertilization cycles. Through examination of meiosis in more than 100 live human oocytes, an error-prone, chromosome-mediated, spindle assembly mechanism was identified as a major contributor to chromosome segregation defects. Human oocyte spindle assembly was mediated primarily by chromosomes and the small guanosine triphosphatase Ran independent of centrosomes or other microtubule organizing centers in a process requiring about 16 hours. This unusually slow process is in sharp contrast to mitotic spindles and meiotic spindles in mouse oocytes and other species, which rarely become unstable upon establishment of a bipolar spindle, thus rendering meiosis more efficient and less prone to segregation errors.Spindles assembled during meiosis in human oocytes display a high proportion of abnormal kinetochore-microtubule attachments and are intrinsically unstable. Progression into anaphase with abnormal attachments put human oocytes at risk of chromosome segregation errors, providing 1 mechanism for the high rates of aneuploidy.

Mechanisms of kinetochore‐microtubule attachment errors in mammalian oocytes

“…A key difference between human and mouse oocytes is the absence of MTOC clusters in human oocytes. This spindle instability in human oocytes, which is more pronounced than that observed in mouse oocytes, may be attributable to the absence of MTOC clusters (Holubcov a et al 2015). The polymerized microtubules self-assemble initially an apolar spindle, which is slowly converted into a bipolar spindle.…”

“…In human oocytes, the bipolar spindle assembly requires an even longer period (approximately 16 h) (Holubcov a et al 2015). A key difference between human and mouse oocytes is the absence of MTOC clusters in human oocytes.…”

Meeting the meiotic challenge: Specializations in mammalian oocyte spindle formation

“…However, like the MI spindle, MII spindles forming after loss of the RanGTP gradient do not display defects in spindle localization. In human oocytes, RanGTP gradient inhibition during meiosis I severely impairs microtubule nucleation, and MI spindle formation (Holubcova, Blayney, Elder, & Schuh, ). The effects of RanGTP gradient loss on human MII spindle formation remains unexplored, and given the different RanGTP MI and MII susceptibilities observed in other species, this exciting inquiry should be completed for human MII spindles.…”

“…Several studies have reported the presence of MTOCs in human oocytes (Battaglia, Goodwin, Klein, & Soules, ; Battaglia, Klein, & Soules, ; Pickering, Johnson, Braude, & Houliston, ). But, this idea has been challenged by a large human oocyte study that detected no γ‐tubulin or pericentrin positive MTOCs in human oocytes (Holubcova et al, ). Determining whether human oocytes possess MTOCs, and the similarity of their protein makeup could indicate which model organism(s) most closely model microtubule dynamics of the human meiotic spindle.…”

Parthenogenesis and developmental constraints

“…There is a universal constraint in metazoans against having more than one centrosome in a cell (Buss, 1987;Galis, Metz, & van Alphen, 2018). As an aside, another consequence of the constraint is lowered fidelity of meiotic divisions, as the acentriolar spindle formation in the ovum is associated with increased aneuploidy rates, which are probably involved in the high rates of miscarriage in humans (Bennabi, Terret, & Verlhac, 2016;Holubcova, Blayney, Elder, & Schuh, 2015; see also Koehler et al, 1996). The centrosome is duplicated exactly once during the cell cycle, during the S phase, so that during mitosis there are two centrosomes that each form one of the two spindle poles that segregate the chromosomes, producing an equal distribution of chromosomes between daughter cells (e.g., Sir et al, 2013;Meraldi, 2016).…”