Frequent epigenetic inactivation ofhSRBCin gastric cancer and its implication in attenuated p53 response to stresses

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hSRBCis a putative tumor suppressor located at 11p15.4, at which frequent genomic loss has been observed in several human malignancies. To explore the candidacy ofhSRBCas a suppressor of gastric tumorigenesis, we analyzed the expression and mutation status ofhSRBCin gastric tissues and cell lines.hSRBCtranscript was expressed in all normal and benign tumor tissues examined, but undetectable or very low in 73% (11/15) cancer cell lines and 41% (46/111) primary tumors. Loss or reduction ofhSRBCexpression was tumor-specific and correlated with stage and grade of tumors. While allelic loss or somatic mutations of the gene were infrequent, its expression was restored in tumor cells by 5-aza-2′-deoxycytidine treatment and aberrant hypermethylation of 23 CpG sites in the promoter region showed a tight association with altered expression. Transient or stable expression of hSRBC led to a G1 cell cycle arrest and apoptosis of tumor cells, and strongly suppresses colony forming ability and xenograft tumor growth. In addition, hSRBC elevated apoptotic sensitivity of tumor cells to genotoxic agents, such as 5-FU, etoposide and ultraviolet. Interestingly, hSRBC increased the protein stability of p53 and expression of p53 target genes, such asp21Waf1,PUMAandNOXA, while hSRBC-mediated cell cycle arrest and apoptosis were abolished by blockade of p53 function. Our findings suggest thathSRBCis a novel tumor suppressor whose epigenetic inactivation contributes to the malignant progression of gastric tumors, in part, through attenuated p53 response to stresses.

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