PCDH9 acts as a tumor suppressor inducing tumor cell arrest at G(0)/G(1) phase and is frequently methylated in hepatocellular carcinoma

Tumor suppressor genes (TSGs) are frequently involved in the pathogenesis of hepatocellular carcinoma (HCC). The epigenetic and genetic alterations of a novel TSG-protocadherin 9 (PCDH9) and its functions in the pathogenesis of HCC were investigated. The methylation status of the PCDH9 promoter was quantitatively analyzed, and the PCDH9 expression was analyzed in HCC cell lines treated with 5-azacytidine. The effects of PCDH9 re-expression and knockdown on growth, proliferation and tumorigenic potential were determined. The results indicated that expression of PCDH9 mRNA was restored in hypermethylation HCC cells following treatment with the DNA de-methylation reagent 5′-Aza. Methylation of the PCDH9 promoter was observed in 22% primary HCC tissues (24/111 tumors). Among the primary HCC cases, the methylated PCDH9 appeared to be associated with a larger tumor size (≥5 cm; P=0.0139) and a more pronounced intrahepatic dissemination (P=0.0312). In addition, it was observed that restored PCDH9 expression could inhibit tumor cell proliferation and xenograft tumor formation. Furthermore, restored PCDH9 expression could inhibit cell proliferation of HCC cell lines via inducing cell cycle arrest at G(0)/G(1) phase. Thus, it is suggested that PCDH9 may act as a novel tumor suppressor candidate gene in HCC pathogenesis.