PTPS-29: UBIQUITIN CARBOXYL TERMINAL HYDROLASE-L1 (UCH-L1) IS A TUMOR SUPPRESSOR IN ATYPICAL TERATOID RHABDOID TUMORS (ATRTs)

BACKGROUND: Atypical teratoid rhabdoid tumor (ATRTs) constitute 20% of brain tumors in young children (<3 yrs of age) and is characterized by bi-allelic mutations of the INI-1 gene. The cell of origin in this tumor is not known and the mechanism by which INI-1 causes tumor is still not clear. Ubiquitin carboxyl-terminal hydrolase isoenzyme 1 (UCHL1), is an abundant neuronal de-ubiquitinating enzyme which acts as a tumor suppressor gene that is inactivated by promoter methylation / gene deletion in several cancers as well as an oncogene with overexpression in other cancers. The role of UCHL1 in brain tumors has not yet been studied. METHODS/RESULTS: We identified UCHL1 is silenced in ATRT cell lines (BT-12, BT-16) along with absent basal expression of p53 and NOXA by western blotting. UCHL1 silencing is mediated by promoter hyper-methylation (detected by using methylation specific PCR (MSP)); and pharmacologic de-methylation reactivated UCHL1 expression in these cell lines along with expression of p53 and NOXA. Ectopic expression of UCHL1 dramatically inhibited the growth of ATRT cell lines (using colony forming assay and MTT assay) and increased sensitivity to radiation. The growth inhibition was mediated by promoting tumor cell apoptosis resulting from activating the p53 tumor suppressor pathway. CONCLUSIONS: We have shown that UCHL1 is silenced in ATRT cell lines and acts as a tumor suppressor gene. We hypothesize that loss of p53 function due to silencing of UCHL1 drives tumor formation in the background of bi-allelic INI1 mutation in ATRT. We will further test this hypothesis using patient derived primary cell cultures and animal models (using genetically engineered mouse models INI1 +/- / UCHL1-/- mice).