Histone Monoubiquitination in Chromatin Remodelling: Focus on the Histone H2B Interactome and Cancer

SIMPLE SUMMARY: Post-translational modifications (PTM) of histone tails represent epigenomic regulation of the chromatin landscape, influencing gene expression and the response to DNA damage. This review focusses on cancer-associated roles of ubiquitin as a histone PTM, specifically in conjunction with an E3 ubiquitin ligase cascade that results in the addition of a single ubiquitin (monoubiquitination) to histone H2B at lysine 120 (H2Bub1). H2Bub1 has roles in chromatin accessibility important for transcriptional elongation, the DNA damage response, cellular proliferation and developmental transitions, including in stem cell plasticity. It has been implicated in inflammation and tumour progression, with examples of its loss associated with a worse prognosis for patients with some cancers. Many factors involved in the H2Bub1 interactome are well known cancer-associated proteins, including p53, BRCA1 and components of the SWI/SNF remodelling complex. Increased knowledge of H2Bub1 and its interactome offers new opportunities for therapeutic targeting of malignancy. ABSTRACT: Chromatin remodelling is a major mechanism by which cells control fundamental processes including gene expression, the DNA damage response (DDR) and ensuring the genomic plasticity required by stem cells to enable differentiation. The post-translational modification of histone H2B resulting in addition of a single ubiquitin, in humans at lysine 120 (K120; H2Bub1) and in yeast at K123, has key roles in transcriptional elongation associated with the RNA polymerase II-associated factor 1 complex (PAF1C) and in the DDR. H2Bub1 itself has been described as having tumour suppressive roles and a number of cancer-related proteins and/or complexes are recognised as part of the H2Bub1 interactome. These include the RING finger E3 ubiquitin ligases RNF20, RNF40 and BRCA1, the guardian of the genome p53, the PAF1C member CDC73, subunits of the switch/sucrose non-fermenting (SWI/SNF) chromatin remodelling complex and histone methyltransferase complexes DOT1L and COMPASS, as well as multiple deubiquitinases including USP22 and USP44. While globally depleted in many primary human malignancies, including breast, lung and colorectal cancer, H2Bub1 is selectively enriched at the coding region of certain highly expressed genes, including at p53 target genes in response to DNA damage, functioning to exercise transcriptional control of these loci. This review draws together extensive literature to cement a significant role for H2Bub1 in a range of human malignancies and discusses the interplay between key cancer-related proteins and H2Bub1-associated chromatin remodelling.