Characterization of oncohistone H2B variants in <i>Schizosaccharomyces pombe</i> reveals a key role of H2B monoubiquitination deficiency in genomic instability by altering gene expression

Various amino acid substitutions commonly occur at one residue of a histone in human cancers, but it remains unclear whether these histone variants have distinct oncogenic effects and mechanisms. Our previous modeling study in the fission yeast Schizosaccharomyces pombe demonstrated that the oncohistone mutants H2BG52D, H2BD67N, and H2BP102L cause the homologous recombination defects and genomic instability by compromising H2B monoubiquitination (H2Bub). However, it is unknown whether other amino acid changes at the H2B-Gly52/Asp67/Pro102 residues influence H2Bub levels and whether they cause genomic instability by altering H2Bub-regulated gene expression. Here, we construct diverse oncomutants at the sole H2B gene htb1-Gly52/Asp67/Pro102 sites in S. pombe and study their impacts on genotoxic response, H2Bub levels, and gene expression. Interestingly, the oncomutants htb1-G52D, htb1-D67N, and htb1-P102L exclusively exhibit significant genotoxic sensitivity, reduced H2Bub levels, and altered gene expression. These defects can be rescued by restoring H2Bub levels with the deletion of the H2B deubiquitinase ubp8+. These strong genetic correlations suggest that H2Bub deficiency plays a determinant role in the genomic instability of htb1-Gly52/Asp67/Pro102 oncomutants and that the alteration of gene expression due to reduced H2Bub levels is a novel mechanism underlying the genomic instability caused by htb1-G52D, htb1-D67N, and htb1-P102L oncomutations.