H3K27Ac mapping in DLBCL cell lines

Diffuse large B-cell lymphoma (DLBCL) is the most common B-cell non-Hodgkin lymphoma and remains incurable in ~40% of patients. Coding-genome sequencing efforts identified several genes/pathways altered in this disease, including new potential therapeutic targets. However, the non-coding genome of DLBCL remains unexplored. Here we show that active superenhancers (SEs) are highly and specifically hypermutated in 99% of DLBCL samples. Such aberrant somatic hypermutation (ASHM) displays signatures of Activation Induced Deaminase activity and is linked to genes encoding B cell developmental regulators and oncogenes. To elucidate the functional consequences of SE-ASHM, we explored two recurrent mutational hotspots identified in the SEs linked to the BCL6 and BCL2 proto-oncogenes, but unmutated in normal GC B cells. These mutations prevent the binding and transcriptional downregulation by the transcriptional repressors BLIMP1 and NR3C1, respectively. Genetic correction of selected mutations restored repressor DNA binding as well as target gene transcriptional repression. Notably, this led to counter-selection of cells harboring corrected alleles, indicating oncogenic dependency from the SE mutations. This pervasive SE mutational mechanism reveals a new major set of genetic lesions deregulating gene expression, which expands the involvement of known oncogenes in DLBCL pathogenesis and identifies new deregulated gene targets of therapeutic relevance. Overall design: CHIP-Seq analysis of H3K27 was performed on 29 DLBCL cell lines using two independent antibodies