Disordered DNA methylation leads to targetable transcriptional plasticity in ATRT

Atypical teratoid rhabdoid tumor (ATRT) is a highly aggressive but genetically simple pediatric central nervous system tumor, defined by biallelic inactivation of the chromatin regulator SMARCB1 with remarkably few other cooperating mutations. Despite its genetic homogeneity, ATRT exhibits profound clinical and epigenetic heterogeneity, with three major subgroups (ATRT-TYR, ATRT-MYC, and ATRT-SHH) defined by DNA methylation and transcriptional signatures. Beyond these subgroup-defining features, we aimed to investigate epigenetic variability within tumors by applying whole-genome bisulfite sequencing and probabilistic modeling to quantify stochastic DNA methylation in primary ATRT samples encompassing all three subgroups. We show that ATRT exhibits a destabilized and increasingly stochastic methylome. While ATRT global methylation patterns diverge according to subgroup, some methylation perturbations, such as hypermethylation and increased methylation entropy over bivalent promoters, are consistent across subgroups. We find that methylation stochasticity alterations map onto potential drivers of ATRT, such as LIN28a, the HOXD cluster for ATRT-MYC, and OTX2 for ATRT-TYR, and identify actionable targets, such as hypermethylation of the tumor suppressor CDKN2a across all subgroups. We investigate the sensitivity of the aberrant DNA methylation landscape of ATRT to pharmacologic DNA methyltransferase inhibition (DNMTi) and histone deacetylase inhibition (HDACi). We show that decitabine leads to profound demethylation of patient-derived ATRT cell lines, including reversal of hypermethylation at bivalent promoters and the CDKN2a locus. The addition of HDACi leads to dramatic gene expression changes, including upregulation of innate immune signaling pathways, such as STING/interferon signaling, genes under the regulation of bivalent promoters, and reactivation of the tumor suppressor CDKN2A. The combination of DNMTi and HDACi synergistically reduces cell viability. Taken together, we show that ATRT has a highly stochastic methylome sensitive to epigenetic manipulation.