SETD5-coordinated chromatin reprogramming regulates adaptive resistance to targeted pancreatic cancer therapy

Oncogenic KRAS signaling is a hallmark of pancreatic ductal adenocarcinoma (PDAC), a lethal malignancy with few treatment options. A major roadblock in deploying therapies targeting the KRAS-MAPK pathway is the rapid emergence of resistant cancer cells. However, molecular mechanisms underlying adaptive targeted therapy resistance in PDAC are poorly understood. Here we find SETD5 (SET domain containing 5) as a major epigenetic driver of PDAC resistance to MEK1/2 inhibition (MEKi). SETD5 is identified in a genetic screen of annotated methyltransferases that mediate MEKi toxicity in PDAC cells. SETD5 expression is induced upon PDAC cells and tumors acquring MEKi-resistance and SETD5 deletion restores vulnerability of refractory PDAC to MEKi therapy in mouse models and patient-derived xenografts (PDX). SETD5 lacks intrinsic histone methyltransferase activity and rather scaffolds a distinct corepressor complex that regulates HDAC3 and G9a catalytic behaviors to couple selective deacetylation with methylation of histone H3 at lysine 9 (H3K9). Gene silencing by the SETD5 complex regulates a network of known drug resistance pathways to reprogram cellular responses to MEKi, a resistance program disrupted by G9a and HDAC3 blockade. Combinatorial pharmacological targeting of MEK1/2, G9a, and HDAC3 results in sustained tumor growth inhibition in murine and human models of PDAC. Together, our work uncovers SETD5 as a master epigenetic regulator of acquired MAPK-pathway therapy resistance and suggests an efficacious clinical path for MEKi in PDAC. Overall design: mRNA profiles of KPCR cells (n = 3) (sgControl + Trametinib 0.2µM, sgSETD5 + Trametinib 0.2µM, or DMSO + Trametinib 0.2µM, UNC0638 0.6 µM + RGFP966 0.6µM + Trametinib 0.2µM).