Meiosis and causes of aneuploidy in human oocytes

Most of our knowledge about aneuploidy in mammalian oocytes stems from studies in mouse oocytes. However, the relevance of this work for human aneuploidy is often unclear because we still know very little about meiosis in human oocytes. Studies of meiosis in live human oocytes for instance that could reveal the causes of aneuploidy were completely missing. Our lab has pioneered methods that facilitated the first studies of meiosis and causes of aneuploidy directly in live human oocytes.



Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes

We have been able to record videos of spindle assembly and chromosome segregation in more than 100 human oocytes. These videos allowed us for the first time to establish the different stages through which meiosis progresses in human oocytes (Fig. 1). They also provided exciting new insights into the causes of chromosome segregation errors in human oocytes. We found that human oocytes often assemble a bipolar spindle by progressing through a prolonged multipolar spindle stage. Oocytes progressing through this stage are particularly likely to have lagging chromosomes in anaphase and abnormal kinetochore microtubule attachments. Thus, our data suggest that spindle instability and transient multipolarity contribute to the high frequency of chromosome segregation errors in human oocytes in comparison to mitotic cells, even in young women (Holubcova et al., 2015).

Figure 1. Stages and timing of meiosis in human oocytes.(A) Stages of meiosis in human oocytes determined from live human oocytes expressing EGFP-MAP4 (microtubules) and H2B-mRFP1 (chromosomes). A schematic representation of each stage (scheme; microtubules in green; chromosomes in magenta) and stage-specific time-lapse images (z-projections, 4 sections, every 5 μm) merged with differential interference contrast [DIC] are shown (bottom row). Outlined regions are magnified above (middle row). Scale bar, 20 μm. Time displayed in hours: minutes. (B) Quantification of timing of meiotic progression from live oocytes expressing EGFP-MAP4 (microtubules) and H2B-mRFP1 (chromosomes) as shown in (A). The box plot shows median (line), mean (small square), and 25th and 75th (boxes), 5th and 95th percentile (whiskers) of time after NEBD. The number of oocytes is specified in italics.


 

Sister kinetochore splitting and precocious disintegration of bivalents could explain the maternal age effect

Our work also shed light on why aneuploidy in human oocytes increasing with maternal age. We found that many sister kinetochores in human oocytes are separated and do not behave as a single functional unit during the first meiotic division. Having separated sister kinetochores allows bivalents, the unit of two homolougs chromosomes linked to each other by recombination, to rotate by 90 degrees on the spindle and increased the risk of merotelic kinetochore-microtubule attachments. Advanced maternal age led to an increase in sister kinetochore separation, rotated bivalents and merotelic attachments. Chromosome arm cohesion was weakened, and the fraction of bivalents that precociously dissociated into univalents was increased. Together, these data suggest that multiple age-related changes in chromosome architecture contribute to the increase of oocyte aneuploidy increases with advanced maternal age (Zielinska et al., 2015).

Figure 2. Changes of chromosome architecture with advancing maternal age in human oocytes.

  


References

Holubcova, Z., Blaney, M., Elder, K., and Schuh, M. (2015). Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes. Science 348:1143-7.

Zielinska, A., Holubcova, Z., Blayney, M., Elder, K., and Schuh, M. (2015) Sister kinetochore splitting and precocious disintegration of bivalents could explain the maternal age effect. eLife 2015;10.7554/eLife.11389. 

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