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Mechanisms of chromosome segregation in mammalian oocytes

Final Report Summary - CHROMOOCYTE (Mechanisms of chromosome segregation in mammalian oocytes)

Depending on the age of the woman, 10-50% of eggs are chromosomally abnormal. This high percentage of abnormal eggs results from chromosome segregation errors during oocyte maturation, the process by which a diploid oocyte matures into a fertilisable egg. Thus, errors during meiosis in human oocytes are the most common cause of pregnancy loss and contribute to approximately 95% of human aneuploidy, such as Down’s syndrome. The project ChromOocyte aimed to identify and characterise key mechanisms that lead to the formation of aneuploid eggs in mammals. Over the funding period, we have made significant progress towards this aim, including the following main achievements:

We carried out the first large scale RNAi screen for meiotic genes in mammals. We analysed the function of 774 genes by high-resolution imaging of chromosomes and microtubules during meiosis in live mouse oocytes and identified several genes that are essential for meiosis (Pfender et al., Nature 2015). For instance, we identified the phosphatase Dusp7 as an essential regulator of multiple steps of meiosis (Tischer and Schuh, Cell Reports 2016). We also studied the function of Btg4 in keeping eggs arrested in metaphase II before fertilisation (Pasternak et al., Open Biology 2016). In addition, we found that Myosin Vb helps to protect mouse eggs against polyspermy by driving the excocytosis of cortical granules (Cheeseman et al., Nature Commun. 2016).

We also developed Trim-Away, a technique to degrade endogenous proteins acutely in mammalian cells without prior modification of the genome or mRNA making it applicable even for non-dividing cells including oocytes (Dean et al., Cell 2017; Dean et al., Nature Protoc. 2018). We also showed that Trim-Away is suitable to selectively degrade disease-causing variants of protein, opening new avenues for the development of therapeutics for treating human diseases.

We carried out the first studies of meiosis and causes of aneuploidy directly in live human oocytes and established the different stages through which meiosis progresses in these cells. We also identified multiple age-related changes of chromosome architecture that contribute to the increase of aneuploidy with advanced maternal age. For example, human oocytes often assemble a bipolar spindle by progressing through a prolonged multipolar spindle stage, which increases the likelihood of lagging chromosomes in anaphase. This phenomenon may be facilitated by a large number of abnormal kinetochore-microtubule attachments. Our data suggest that spindle instability and merotelic attachments contribute to the high frequency of chromosome segregation errors in human oocytes, even in young women (Holubcová et al., Science 2015).

We also observed that many sister kinetochores in human oocytes were separated and did not behave as a single functional unit during the first meiotic division. This allowed bivalents, the unit of two homologous 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 (Zielinska et al., eLife 2015).
We investigated the function of an F-actin spindle and found an unexpected actin-dependent mechanism that drives the accurate alignment and segregation of chromosomes in mammalian eggs. Actin filaments permeated the microtubule spindle in eggs of several mammalian species, including humans. Disrupting actin in mouse eggs led to significantly increased numbers of misaligned chromosomes as well as lagging chromosomes during meiosis I and II. We found that actin drives accurate chromosome segregation by promoting the formation of functional kinetochore fibers, the microtubule bundles that align and segregate chromosomes. Thus, actin is essential to prevent chromosome segregation errors in eggs (Mogessie and Schuh, Science 2017).

Together, the results of this project significantly advanced our understanding the process of meiosis and the causes of aneuploidy in mammalian eggs, including humans. In the future, we will continue to investigate the mechanisms underlying chromosome segregation errors in mammalian oocytes, making use of the methods and tools developed over the course of the funding period.