Studies in the Cohen lab have focused on the role of the DNA mismatch repair proteins in meiotic recombination. Most notably, we focus on two heterodimers of MutS and MutL homologs: MSH4/MSH5 and MLH1/MLH3, respectively (see Figure 1). These two heterodimers are essential for crossover formation via the CLASS I crossover pathway, which is the predominant pathway for producing reciprocal recombination events from the excessive numbers of double strand breaks (DSBs) formed during early prophase I. Our studies to date have demonstrated that Class I events account for 90-95% of crossovers in male mice and that MLH1/MLH3 are the final markers for such crossovers towards the end of prophase I.
Class II crossover events, which account for 5-10% of crossovers, are regulated by the MUS81 endonuclease and, interestingly, we have shown that loss of Mus81 results in a compensatory rise in class I crossovers. These crossovers occur without a rise in the numbers of MSH4/MSH5 foci, which suggests that the compensation occurs in pachynema of prophase I, somewhat later than originally suspected. This observation suggests that an integrated regulatory event occurs between the crossover pathways at/after pachynema. Studies in the lab are aimed at elucidating this late-stage regulatory interaction, and we hypothesize that one of three molecular regulators may participate in such events:
Studies utilizing mouse knockouts for all these genes will help to elucidate these mechanisms and are ongoing in the lab. We are also interested in understanding how such events are differentially regulated in males and females, since evidence is accumulating that such processes may be sexually dimorphic in mammals.
Figure 1. Localization of key mismatch repair proteins, MSH5, MLH1, and MLH3, on meiotic chromsomes during prophase I in male mice.
Figure 2. Summary of key events of prophase I. A larger version can be accessed by clicking on the image below.
Meiosis is the process by which diploid precursors replicate their DNA (to become 4n) and then undergo two successive divisions to reduce their chromosome content to haploid numbers (n) for subsequent sexual reproduction. In most species studied, meiosis is highly conserved at the molecular level and is very stringently controlled to ensure that chromosomal errors are not transmitted to subsequent generations.
Prophase I is the defining event of meiosis, since this stage is the one that is unique to this type of cell division. During prophase I, two key events happen: firstly, homologous chromosomes pair and become intimately tethered together in a process known as synapsis (depicted here), and secondly, they then exchange DNA in order to further tether themselves together via DNA:DNA interactions (Figure 2). Thus, synapsis, which is mediated by a protein structure called the synaptonemal complex, and DNA tethering, mediated by the process of reciprocal recombination (or crossing over), both serve to keep homologous chromosomes together until the first meiotic division, at which time homologous chromosomes segregate to two distinct daughter cells.
During meiosis II, these homologous chromosomes, which each consist of a replicated pair of sister chromatids, undergo a second division which is reminiscent of a mitotic division, in which the sister chromatids separate into daughter cells. In males, this yields four haploid gametes, while in females, only one haploid gamete is produce, while the “waste” chromosomes get expelled into two polar bodies.
Sun X., Wu X., Cohen P.E. Non-Human primates exhibit errors in meiosis I. Molecular Reproduction and Development. In press (2012) DOWNLOAD
Sun X., Cohen P.E. Studying recombination in mouse oocytes. Methods in Molecular Biology. In Press (2012)
Holloway J.K., Mohan S., Balmus G., Sun X., Modzelewski A., Borst P.L., Freire R. Weiss R.S., Cohen P.E. Mammalian BTBD12 (SLX4) protects against genomic instability during mammalian spermatogenesis. PLoS Genetics 7(6): e1002094 (2011) DOWNLOAD
Holloway J.K., Morelli M.A., Cohen P.E. The Mouse Ortholog of the Human Bloom's Syndrome Mutated (BLM) is Critical for Integrating Multiple Pathways of Recombination during Meiosis. Journal of Cell Biology 188: 779-789 (2010). PMID: 20308424 DOWNLOAD