Chromosome segregation in mammalian meiosis

We are trying to identify mechanisms that ensure each egg receives a single copy of every chromosome. The inheritance of too many or too few chromosomes can result in spontaneous miscarriages or severe birth defects as in Down’s Syndrome or trisomy of chromosome 21.

We know that cells in the body regulate the equal distribution of chromosomes by entrapping sister chromosomes within protein rings. These rings are only destroyed when cells are ready to divide.

In women, the rings must hold together sister chromosomes in eggs from birth until fertilization, which can be separated by decades. We are investigating how these rings are stably maintained. Loss of rings over time could be responsible for maternal age-related infertility.

We are using animal models to study chromosome segregation in eggs with the aim to understand the equivalent processes in humans. We are studying eggs lacking specific proteins that are known to be important for chromosome segregation in other organisms. We are taking movies of mutant eggs carrying fluorescently labelled chromosomes to identify which aspects of chromosome segregation are defective due to loss of a particular protein. We also hope to identify new regulators of the egg divisions by studying infertile mutants.

Our objective is to gain a greater understanding of the molecular mechanisms regulating chromosome segregation during mammalian meiosis. Meiosis consists of two rounds of chromosome segregation, in which bivalent chromosomes are first converted to univalents and then univalents to single chromatids. We are taking two approaches to study these mechanisms in the mouse, with the ultimate goal of understanding better the equivalent processes in women. We have adopted a genetic approach in which oocytes with specific gene deletions are observed in real time using time lapse microscopy as they undergo the first and second meiotic divisions. Our first approach recognizes that many if not most of the proteins that drive meiosis are also involved in mitosis and are therefore essential for development and oogenesis. To create oocytes that lack these proteins, we have developed a method that removes the proteins specifically from oocytes that have undergone recombination but before they undergo chromosome segregation. Briefly, we use the Zp3-cre transgene to delete both copies of floxed alleles of genes known to be important for mitosis during the growing stage of oocytes. Since the generation of conditional alleles is relatively labour-intensive, this approach is only applied to a few specific genes about which we have posed very specific questions as to their function during meiosis. The approach is hypothesis-driven and its limitation is that new unknown players will be not be discovered. We therefore plan to complement this approach by one that could identify genes required for the meiotic divisions for which there is no prior knowledge of their function. Our second approach relies on identifying sterile or sub-fertile mutants in a large-scale screen of novel knockout mice carried out by the Mouse Genetics Programme at the Wellcome Trust Sanger Institute. We test whether these mutants still produce oocytes and, if so, analyse them using live-cell imaging technology and chromosome spreads. Mutants displaying defects in chromosome segregation provide candidates for proteins that might deteriorate in aging oocytes and could explain the maternal age effect. To date, we lack an understanding of the phenomenon that the risk of producing aneuploid oocytes increases by more than two logs in human females between the ages of 24 and 45. Our research aimed at identifying novel regulators of meiotic chromosome segregation will eventually provide insight into chromosome non-disjunction in older women.