The control of meiotic DNA break formation by key chromosome axis components, IHO1 and HORMAD1, in mammals.

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Generation of haploid gametes requires that homologous chromosomes (homologues) of diploid germ cells segregate during the first division of meiosis. Segregation of homologues requires physical links between them, which occur via crossovers. Crossovers are generated by a specialized recombination process that starts with programmed generation of DNA double strand breaks (DSBs) in the first meiotic prophase. Multiple DSBs are generated along the proteinaceous core/axis of each chromosome. DSBs drive the pairing of homologues, which results in synapsis between homologues. The repair of DSBs also generates the inter-homologue crossovers. Given the importance of crossovers and the potential genotoxicity of DSBs, tight control is exerted on DSB formation. Hence, DSBs are allowed to form only on unsynpased axes, where DSBs are needed to promote homologue synapsis. Our recent work identified IHO1 as a protein that associates with unsynapsed axis and is crucial for DSB formation. According to our model, IHO1 recruitment to axes largely depends on IHO1´s interaction with an unsynapsed-axis-sensor, HORMAD1. Thus, we hypothesize that IHO1-HORMAD1 interaction is key to the mechanism that limits DSB formation to unsynapsed axes. Our data suggest that regulation of IHO1-HORMAD1 interaction and the resulting modulation of IHO1 axis-localization contribute to the inhibition of DSB formation (1) on synapsed chromosomes, (2) in the vicinity of existing DSB breaks and (3) in advanced prophase stages where homology search is terminated. Thus, IHO1 appears central to mechanisms that protect the germ line from excessive DNA break formation.We will test our model of DSB formation and IHO1 function within it, and we will address the molecular basis of the regulatory mechanisms that target IHO1. One of the key aims is to address the importance of IHO1-HORMAD1 interaction and IHO1 axis association in DSB formation. Hence, we used CRISPR/Cas9 to generate mice with mutant IHO1 versions that are selectively defective in IHO1-HORMAD1 interaction and IHO1 axis association. I propose phenotypic analysis of these mice and that of additional meiotic mutants affecting IHO1 and HORMAD1 functions and regulation. We will combine this with interaction studies of IHO1 to gain deeper understanding of IHO1 functions and regulation.The proposed experiments will provide major new insights into the logic and the molecular basis of the spatiotemporal control of DSB formation. Given that correct control of recombination is needed to maintain genome integrity in the germ line, and that impaired recombination causes human aneuploidies, the expected results will have high impact with a clear relevance to human reproductive health.