HELLS and PRDM9 from a pioneer complex to open chromatin at meiotic recombination hotspots

Chromatin barriers prevent spurious interactions between regulatory elements and DNA-binding proteins. One such barrier, whose mechanism for overcoming is poorly understood, is access to recombination hotspots during meiosis. Here we identify that the DNA-binding protein PRDM9 and chromatin remodeler HELLS function together to open chromatin at hotspots providing access to the DNA double-strand break (DSB) machinery. Recombination hotspots are decorated by a unique combination of histone modifications, not found at other regulatory elements. HELLS is recruited by PRDM9, and is necessary for both histone modification and DNA accessibility at hotspots. In male mice lacking HELLS, DSBs are retargeted to other sites of open chromatin, leading to germ cell death and sterility. Together, these data provide a model for hotspot activation where HELLS and PRDM9 function as a pioneer complex to create a unique epigenomic environment to open chromatin in preparation for proper placement and repair of DSBs. Measure histone modifications and chromatin accessibility in male germ cells collected from various mouse backgrounds