Cooperative super-enhancer inactivation caused by heterozygous loss of CREBBP and KMT2D skews B-cell fate decisions and accelerates development of T-cell depleted lymphomas [ATAC-seq]

Mutations in chromatin modifiers are a hallmark of many tumors, especially lymphomas arising from germinal center (GC) B cells. Given that these lymphoma mutations all induce aberrant gene repression, it is surprising that they often co-occur in individual patients. The most common pairing are mutations affecting CREBBP and KMT2D. Both impair enhancer activity in overlapping pathways to facilitate lymphomagenesis, hence their co-occurrence is especially puzzling. Herein, we report that combined haploinsufficiency of CREBBP and KMT2D (C+K) do indeed accelerate lymphomagenesis (vs either allele alone) and confer a more malignant phenotype.  Single cell RNA-seq analysis of GC reaction showed that C+K haploinsufficiency induced aberrant GC hyperplasia by altering cell fate decisions, skewing B cells away from memory B and plasma cell differentiation and favored instead expansion of centroblasts.  Integrative epigenomic studies in murine and human B cells showed that C+K deficiency particularly impairs enhancer activation for immune synapse genes involved in exiting the GC reaction. This effect was especially severe at super-enhancers for genes governing cell fate decisions induced by T cell help, pointing to a particular dependency for both co-activators at these specialized regulatory elements. Mechanistically, CREBBP and KMT2D formed a complex, were highly co-localized on chromatin, and were required for each-other’s stable recruitment to enhancers.  Given the impaired expression of immune synapse genes, it was notable that C+K lymphomas in mice and humans manifested significantly reduced CD8+ T cell abundance. This suggests that deficiency of the two chromatin modifiers cooperatively induced an immune evasive phenotype due to failure to activate key immune synapse super-enhancers, associated with altered immune cell fate decisions. These findings point to the potential need for epigenetic adjuvant therapy to augment reactivity with immunotherapy approaches in patients with C+K deficiency. ATAC-seq-based chromatin accessibility profiling for mouse GC B cells sorted from sheep red blood cell (SRBC)-immunized genetically engineered mouse models (GEMMs), which include Cg1Cre+/- (WT); Cg1Cre+/-, Crebbp fl/+ (C); Cg1Cre+/-, Kmt2d fl/+ (K); Cg1Cre+/-, Crebbp fl/+, Kmt2d fl/+ (CK); n = 4-5 mice for each genotype.