ELUCIDATING THE ONCOGENIC ROLE OF ATRX DEFICIENCY IN GLIOMA

We and others recently identified loss-of-function mutations in the ATRX gene as core components of a highly recurrent molecular signature characterizing adult diffuse astrocytoma. ATRX deficiency has also been implicated in large subsets of pediatric glioma, neuroblastoma, and pancreatic neuroendocrine tumor. ATRX is a SWI/SNF chromatin remodeling factor, which appears to play a crucial role in heterochromatin maintenance, particularly at telomeres. Its dysfunction has been correlated with genomic instability and DNA replications stress, along with abnormal telomere elongation. Interestingly, germline mutations in ATRX do not cause cancer, but are instead associated with neurodegeneration and mental retardation. To further elucidate the pathogenic role of ATRX in glioma, we first assessed its expression level across different glioma subtypes. As expected, ATRX expression was lost in virtually all IDH-mutant gliomas without 1p/19q codeletion (primarily astrocytomas), but was universally maintained in IDH-wild type tumors (primarily glioblastomas), and 1p/19q co-deleted tumors (oligodendrogliomas). Interestingly, we found that ATRX-mutant gliomas were not universally negative for ATRX, with some tumor cells retaining expression. This finding, which we have confirmed by deep sequencing, suggests that ATRX mutations do not confer a strong proliferative advantage per se, but instead predispose cellular subclones to transformative events downstream, perhaps through genomic destabilization and DNA copy number alteration. To investigate issues of clonality and DNA copy number further, we have performed single cell sequencing on a number of ATRX-mutant gliomas, identifying several distinct subclones within each sample, and are in the process of integrating focused genotyping data for ATRX and other relevant genes. We are also in the process of a detailed investigation into the epigenomic and transformative consequences of ATRX deficiency using disease-relevant in vitro and in vivo modeling systems. Characterizing the pathogenic cascade induced by ATRX deficiency will provide crucial insights into therapeutic development for a number of tumor entities, including diffuse astrocytoma.