Transcriptomic profile of germinal center B cells from conditional GC-specific (Cg1-Cre) Crebbp-HET, Kmt2d-HET, and compound Crebbp-HET/Kmt2d-HET mice, with littermate controls

Inactivating mutations of the KMT2D methyltransferase and the CREBBP acetyltransferase are among the most common genetic alterations in B-cell lymphoma and co-occur in 40-60% of follicular lymphoma (FL) and diffuse large B cell lymphoma (DLBCL) cases, suggesting they may be co-selected. Here we show that combined germinal center (GC)-specific haploinsufficiency of Crebbp and Kmt2d synergize in vivo to promote the expansion of abnormal GCs, a common pre-neoplastic event. These enzymes form a biochemical complex on selected enhancers/super-enhancers that are critical for the delivery of signals in the GC light-zone and are only corrupted upon dual Crebbp/Kmt2d loss, both in mouse GC B cells and in human DLBCLs. Moreover, we find that CREBBP directly acetylates KMT2D in GC-derived B cells. Accordingly, inactivation of CREBBP by FL/DLBCL-associated truncating and/or missense mutations abrogates its ability to catalyze KMT2D acetylation. Importantly, genetic and pharmacologic loss of CREBBP and the consequent decrease in KMT2D acetylation leads to reduced levels of H3K4me1, supporting a role for acetylation in modulating KMT2D activity. These data identify a direct biochemical and functional interaction between CREBBP and KMT2D, with implications for their role as tumor suppressors in FL/DLBCL and for the development of combinatorial therapies targeting enhancer defects induced by their loss. Conditional Crebbp fl/+Cg1Cre, Kmt2d fl/+Cg1Cre, Crebbp fl/+Kmt2d fl/+Cg1Cre and wild type mice were immunized with sheep red blood cells at day 0 and day 5, and sacrificed 7 days after the second immunization. Germinal center B cells (B220+CD95+PNA+) were sorted from splenic B cells. Total RNA was extracted from single cell suspensions, assessed for quality on a Bioanalyzer, and used for library construction and next generation sequencing on an Illumina NovaSeq 6000 instrument