H2BFWT nucleosome structure reveals its nucleosome-destabilizing mechanisms through reductions of specific DNA-histone interactions and explains the mechanistic impact of its infertility-related H100R SNP during early spermatogenesis in humans

Spermatogenesis is the process by which spermatogonial stem cells differentiate into haploid spermatozoa. During this process, multiple testis-specific histone variants are involved in the dynamic chromatin transitions. H2BFWT is a testis-specific H2B variant only found in primates, and several reports showed that single-nucleotide polymorphisms (SNPs) of H2BFWT are associated with male non-obstructive infertility. Different from another testis-specific variant TH2B, H2BFWT is preferentially localized in sub-telomeric regions and the promoter regions of testicular high expression genes during the transition from differentiated spermatogonia to early spermatocytesspermatogenesis. Cryo-EM structural analysis shows that the H2BFWT nucleosome is defined by weakened interactions between H2A-H2BFWT dimer and H4, and between the histone octamer and DNA. WIn addition, we found that the shape of the H2A L1 loops was also changed by the presence of H2BFWT. Furthermore, our structural model shows that H2BFWTH100R, which is one of the SNPs associated with infertility, further destabilizes the nucleosome and increases the nucleosome unwrapping rate by interfering with the interaction with H4K91. Taken together, our results suggest that H2BFWT may be necessary for the regulation of spermatogenesis-related genes transcription in spermatogonia by decreasing RNA polymerase II transcription barriers, and that the H100R mutation overdrives its nucleosome-destabilizing effects which likely explain how it causes infertility.