Transposon mutagenesis identifies chromatin modifiers cooperating with <i>Ras</i> in thyroid tumorigenesis and detects <i>ATXN7</i> as a cancer gene.

Abstract

Oncogenic <i>RAS</i> mutations are present in 15-30% of thyroid carcinomas. Endogenous expression of mutant Ras is insufficient to initiate thyroid tumorigenesis in murine models, indicating that additional genetic alterations are required. We used Sleeping Beauty (SB) transposon mutagenesis to identify events that cooperate with Hras^G12V in thyroid tumor development. Random genomic integration of SB transposons primarily generated loss-of-function events that significantly increased thyroid tumor penetrance in <i>Tpo-Cre/homozygous FR-Hras</i>^<i>G12V</i> mice. The thyroid tumors closely phenocopied the histological features of human RAS-driven, poorly differentiated thyroid cancers. Characterization of transposon insertion sites in the SB-induced tumors identified 45 recurrently mutated candidate cancer genes. These mutation profiles were remarkably concordant with mutated cancer genes identified in a large series of human poorly differentiated and anaplastic thyroid cancers screened by next-generation sequencing using the MSK-IMPACT panel of cancer genes, which we modified to include all SB candidates. The disrupted genes primarily clustered in chromatin remodeling functional nodes and in the PI3K pathway. <i>ATXN7</i>, a component of a multiprotein complex with histone acetylase activity, scored as a significant SB hit. It was recurrently mutated in advanced human cancers and significantly co-occurred with <i>RAS</i> or <i>NF1</i> mutations. Expression of <i>ATXN7</i> mutants cooperated with oncogenic RAS to induce thyroid cell proliferation, pointing to <i>ATXN7</i> as a previously unrecognized cancer gene.