Blunted CD40-responsive enhancer activation in <i>CREBBP</i>-mutant lymphomas can be restored by enforced CD4 T-cell engagement

The CREBBP lysine acetyltransferase (KAT) is frequently mutated in follicular lymphoma and diffuse large B-cell lymphoma and has been studied using gene knockout in murine and human cells. However, most CREBBP mutations encode amino acid substitutions within the catalytic KAT domain (CREBBP KAT-PM) that retain an inactive protein and have not been extensively characterized. Using CRISPR gene editing and extensive epigenomic characterization of lymphoma cell lines, we found that CREBBP KAT-PM lead to unloading of CREBBP from chromatin, loss of enhancer acetylation, and prevention of EP300 compensation. These enhancers were enriched for those that are dynamically loaded by CREBBP in the normal centroblast-to-centrocyte transition in the germinal center, including enhancers activated in response to CD40 signaling, leading to blunted molecular response to CD40 ligand in lymphoma cells. We provide evidence that CREBBP KAT-PM inhibits EP300 function by binding limiting quantities nuclear transcription factor (TF), thereby preventing its compensatory activity. This effect can be experimentally overcome by expressing saturating quantities of TF or biologically attenuated by strong stimulation of CD40 signaling that increases nuclear TF abundance. Importantly, epigenetic responses to CD40 signaling can be induced by enforcing CD4 T-cell engagement using a bispecific antibody, leading to CD40-dependent restoration of antigen presentation machinery in CREBBP KAT-PM cells and cell death. Therefore, we provide a mechanistic basis for enhancer deregulation by CREBBP KAT-PM and highlight enforced CD4 T-cell engagement as a potential approach for overcoming these effects.