A Study of H2BE76K: A Nucleosome Destabilizing Histone Mutation in Breast Cancer
Student thesis: Doctoral Thesis
Related Research Unit(s)
In recent years, oncohistone mutations have garnered a huge amount of interest in the epigenetic cancer field, ever since mutations in histone H3 were identified and found to be the primary drivers of brain and bone cancers. Mutations in other core histone genes have also been recently identified in cancer, although their roles in cancer remain unclear. In this study, the E76K mutation in histone H2B (H2BE76K) was identified as one of the most frequent H2B mutations in breast cancer and various other cancers. Initial immunoprecipitation experiments showed that H2BE76K mutation destabilizes the nucleosome by weakening intranucleosomal H2A-H2B dimer-H3-H4 tetramer interactions. Since the nucleosome is refractory to transcription, I therefore hypothesized that the H2BE76K mutation promotes transcription by destabilizing the nucleosome. The introduction of the H2BE76K mutation by CRISPR/Cas9 in the MDA-MB-231 human breast cancer cell line induces significant gene expression changes, as well as enhances the colony formation ability of MDA-MB-231 cells. Interestingly, genome-wide mapping of the H2BE76K mutant histone by CUT&RUN-seq showed that H2BE76K preferentially localizes in genic regions. The local H2BE76K genic enrichment appears to enhance transcription through H2BE76K’s nucleosome destabilization effect, resulting in the increased gene expression of a subset of genes in the H2BE76K mutant cells. The direct transcriptional changes induced by H2BE76K enhances the expression of the H2BE76K enriched gene ADAM19, which when depleted, reverses the enhanced colony formation ability of the H2BE76K mutant cells. Overall, this suggests the direct contribution of a nucleosome destabilizing histone mutation in breast cancer development. Lastly, I propose that H2BE76K induces indirect gene expression changes through its enhanced interactions with the histone chaperone NAP1L1. In summary, my findings confirm the role of the H2BE76K mutation in promoting breast cancer development, and explain the molecular mechanism underlying the effect of a nucleosome destabilizing mutation in breast cancer.