Molecular mechanisms of recombination hotspots in humans

Meiotic recombination involves the exchange of DNA between two homologous chromosomes, forming cross-overs and gene conversion events. The cross-over process is important for the proper segregation of chromosomes during meiosis, and drives genetic diversity. Human hotspots are enriched for a 13-bp motif, CCNCCNTNNCCNC; a close match to this motif occurs in about 40% of our cross-over hotspots. A DNA binding protein called PRDM9, having histone trimethyltransferase (H3K4me3) activity, binds the motif and is becoming established as a major determinant of recombination hotspots (narrow regions with high cross-over activity). This research aimed to understand the mechanisms involved in promoting PRDM9 binding to its target sites, and subsequently, initiating cross-over hotspot activity. We first explored the relationship between PRDM9 binding and DNA sequence, to directly confirm whether PRDM9 binds to the 13-bp hotspot motif using in-vitro gel-shift assays, and found that it does bind sequence specifically to the canonical 13-mer motif. PRDM9 is able to bind the motif in a highly selective manner, with certain single base pair changes abolishing binding. However, we observe that it is also able to tolerate degeneracy in its binding sites, as demonstrated by strong in-vitro binding to degenerate versions of the 13-bp motif. Hence, these results confirmed that PRDM9 is able to directly bind to the 13-bp hotspot motifs, and given that it can also tolerate degeneracy, this raised the question of why PRDM9 is able to bind only a subset of all such potential binding sites in the genome. To address this, a ChIP-seq analysis was performed to identify genome wide binding sites for PRDM9. This information also helped us to characterise binding sites and investigate if factors such as the local chromatin environment play a role in specifying PRDM9 binding tar- gets and hotspot formation. We were able to identify over 170,000 PRDM9 binding sites in the genome. Surprisingly, these binding sites were also enriched in promoter regions, however, bound sites in these regulatory regions showed low recombination activity. We found that PRDM9 is able to confer the H3K4me3 mark on all bound sites, even those without a pre-existing H3K4me2 mark. We also investigated the role of other chromatin related marks on PRDM9 binding and found that binding occurs in chromatin accessible, but nucleosome rich regions, whereas heterochromatin regions tend to inhibit binding. Further, for hotspot formation, it was seen that less chromatin accessible, nucleosome dense regions away from transcribed sites, are preferred. Hotspots tend to avoid regions marked by transcription activating histone modifications, however, these regions do not appear to inhibit PRDM9 binding itself. These results show how PRDM9 binding in the genome is dependent on both primary DNA sequence and the surrounding epigenetic factors. Together these factors promote binding and, with additional downstream factors, positioning of hotspot locations in the human genome.