Epigenetic landscape of testis specific histone H2B variant and its influence on sperm function

Background- The biological relevance of the major histone variant TH2B in sperm is not fully understood. Studies in TH2A/TH2B double knockout male mice indicate its role in chromatin compaction and male fertility. Additionally, presence of TH2B and TH2A reportedly generates more dynamic nucleosome leading to open chromatin structure characteristic of a transcriptionally active genome. Given that mature human sperm are transcriptionally and translationally inactive, the presence of TH2B in mature sperm is intriguing. To address its role in sperm, we investigated the genomewide localization of TH2B in sperm of fertile men. This is important to know as most of the canonical H2B is replaced by TH2B during spermiogenesis.Results- We have identified by ChIP-seq, the genomic loci associated with TH2B in fertile human sperm. Bioinformatics analysis revealed approx. 5% sperm genome and 5527 genes to be associated with TH2B. Out of these 105 (1.9%) and 144 (2.6%) genes showed a direct involvement in sperm function and early embryogenesis, respectively. Chromosome wide analysis for TH2B distribution indicated its least distribution on X and Y chromosomes and varied distribution on autosomes. Relatively higher percentage of genes on chromosome 4, 18, 3 and 2 were occupied by TH2B. TH2B enrichment was more in promoter and gene body region. Gene Ontology analysis revealed signal transduction and associated kinase activity as most enriched biological and molecular function respectively. We observed enrichment of TH2B at developmentally important loci like HOXA and HOXD and on genes required for normal sperm function, few of which were validated by ChIP-qPCR. Relative expression of these genes was found to be altered in particular subgroup of infertile men showing abnormal chromatin packaging. Chromatin compaction positively correlated with sperm motility, concentration, viability and with transcript levels of PRKAG2 and CATSPER B.Conclusion- ChIP-seq analysis of TH2B revealed a putative role of TH2B in sperm function and embryo development. Altered expression of TH2B associated genes in infertile individuals with sperm chromatin compaction defects, indicates involvement of TH2B in transcriptional regulation of these genes in post meiotic male germ cells. This altered transcriptome may be a consequence or cause of abnormal nuclear remodeling during spermiogenesis.