Drug addiction mutations unveil a repressive methylation ceiling in EZH2-mutant lymphoma

Abstract

Drug addiction, a phenomenon where cancer cells paradoxically depend on continuous drug treatment for survival, has uncovered cell signaling mechanisms and cancer co-dependencies. Here, we discover mutations that confer drug addiction to inhibitors of the transcriptional repressor Polycomb Repressive Complex 2 (PRC2) in diffuse large B-cell lymphoma (DLBCL). Drug addiction is mediated by hypermorphic mutations in the CXC domain of the catalytic subunit EZH2, which maintain H3K27me3 levels even in the presence of PRC2 inhibitors. Drug discontinuation leads to overspreading of H3K27me3, surpassing a repressive methylation ceiling compatible with lymphoma cell survival. Exploiting this vulnerability, we show that inhibition of SETD2 induces H3K27me3 spreading and blocks lymphoma growth. Collectively, our findings demonstrate that fundamental constraints on chromatin landscapes can yield biphasic dependencies in epigenetic signaling in cancer cells. More broadly, we highlight how approaches to identify drug addiction mutations can be leveraged to discover cancer vulnerabilities and cell signaling thresholds.