Appointment period: 8/16/2010 to 8/15/2012
Understanding how chromatin architecture shapes sex-specific germ line checkpoint regulation to maintain genome integrity.
Meiosis is a specialized form of cellular division that involves the faithful segregation of chromosomes to produce gametes with the correct ploidy. To accomplish this, a conserved set of checkpoint proteins function to detect improperly paired chromosomes and to subsequently activate a signaling cascade that prevents the formation of aneuploid gametes. Indeed, asynapsis of a homologous chromosome pair elicits a checkpoint response that can in turn trigger germ-line apoptosis. In a heterogametic germ line, however, the sex chromosomes proceed through meiosis without a homologous pairing partner and are not recognized by checkpoint machinery as being unpaired. The molecular mechanisms by which the unpaired X chromosome is kept shielded from checkpoint machinery are poorly understood, and to investigate this process, I have conducted a directed RNAi screen in C. elegans to identify regulatory factors that block checkpoint signaling in a heterogametic germ line. I have uncovered a role for a number of highly conserved methyltransferases in mediating this process and allowing meiosis to occur with high fidelity in a heterogametic germ line. Elimination of the X-specific repressive histone modifications histone H3 lysine 9 dimethylation (H3K9me2) or H3 lysine 27 trimethylation (H3K27me3) results in a significant increase in apoptotic nuclei that is specific to a heterogametic germ line; absence of these repressive marks appears to be dispensable for maintaining germ cell homeostasis in a wild-type XX germ line. Furthermore, while the repressive chromatin mark H3K9me2 is also recruited to an asynapsed X chromosome pair (e.g. in the him-8 XX germ line), my data demonstrate that this process is functionally distinct from the mechanism by which the single X chromosome is targeted and silenced in a heterogametic germ line. In summary, my data imply a direct role for the establishment and maintenance of a repressive chromatin architecture of the single X chromosome to prohibit access of checkpoint proteins and shield the lone X from spurious activation of an apoptotic program. Currently, I am using both biochemical and genetic assays to determine how repressive HMTs function within the recombination repair pathway to influence sex chromosome-specific checkpoint signaling and to maintain germ line homeostasis.