Genetic alteration and misexpression of Polycomb group genes in hepatocellular carcinoma.

Although the abnormal expression of Polycomb-group (PcG) proteins is closely associated with carcinogenesis and the clinicopathological features of hepatocellular carcinoma (HCC), the genetic mutation profile of PcG genes has not been well established. In this study of human HCC specimens, we firstly discovered a highly conserved mutation site, G553C, in the Polycomb Repressive Complex 2 (PRC2) gene enhancer of zeste homolog 2 (EZH2). This site also harbors a single nucleotide polymorphism (SNP), rs2302427, which plays an important antagonistic role in HCC. Kaplan-Meier survival curves showed that the tumor-free and overall survival of patients with EZH2 G553C were superior to those without the mutation. The G allele frequencies in patients and healthy subjects were 0.2% and 0.122%, respectively, with significant differences in distribution. The individuals carrying the GG and the GC genotypes at rs2302427 showed 3.083-fold and 1.827-fold higher risks of HCC, respectively, compared with individuals carrying the wild-type allele. Furthermore, Immunohistochemical staining revealed that the expression levels of CBX8 (in 53/123 samples) and BMI1 (in 60/130 samples) were markedly increased in human HCC specimens. Importantly, the overall and tumor-free survival rates were significantly reduced in the group of patients who simultaneously expressed PRC1 and PRC2. These results argue that a combination of PRC1 and PRC2 expression has a significant predictive/prognostic value for HCC patients. Taken together, our results indicate the abnormal expression and genetic mutation of PcG members are two independent events; cumulative genetic and epigenetic alterations act synergistically in liver carcinogenesis.

EZH2 represses target genes through H3K27-dependent and H3K27-independent mechanisms in hepatocellular carcinoma.

UNLABELLED: Alterations of polycomb group (PcG) genes directly modulate the trimethylation of histone H3 lysine 27 (H3K27me3) and may thus affect the epigenome of hepatocellular carcinoma (HCC), which is crucial for controlling the HCC cell phenotype. However, the extent of downstream regulation by PcGs in HCC is not well defined. Using cDNA microarray analysis, we found that the target gene network of PcGs contains well-established genes, such as cyclin-dependent kinase inhibitors (CDKN2A), and genes that were previously undescribed for their regulation by PcG, including E2F1, NOTCH2, and TP53. Using chromatin immunoprecipitation assays, we demonstrated that EZH2 occupancy coincides with H3K27me3 at E2F1 and NOTCH2 promoters. Interestingly, PcG repress the expression of the typical tumor suppressor TP53 in human HCC cells, and an increased level of PcG was correlated with the downregulation of TP53 in certain HCC specimens. Unexpectedly, we did not find obvious H3K27me3 modification or an EZH2 binding signal at the TP53 promoters, suggesting that PcG regulates TP53 expression in an H3K27me3-independent manner. Finally, the reduced expression of PcGs effectively blocked the aggressive signature of liver cancer cells in vitro and in vivo. IMPLICATIONS: Taken together, our results establish the functional and mechanistic significance of certain gene regulatory networks that are regulated by PcGs in HCC.

The functional and mechanistic relatedness of EZH2 and menin in hepatocellular carcinoma.

BACKGROUND & AIMS: The alterations of histone modification may serve as a promising diagnostic biomarker of hepatocellular carcinoma (HCC), but the clinical and mechanistic relatedness of the histone H3 lysine 27 and 4 trimethylation (H3K27me3 and H3K4me3) in HCC remains poorly understood. Here we propose that the combination of H3K27me3 and H3K4me3 is a more precise predictive/prognostic value for outcome of HCC patients. METHODS: We used chromatin immunoprecipitation (ChIP) assays and a ChIP-on-chip screen to analyse HCC. RESULTS: We found that the EZH2 occupancy coincides with the H3K27me3 at promoters and directly silences the transcription of target genes in HCC. The H3K27me3-related gene network of EZH2 contains well-established genes, such as CDKN2A, as well as previously unappreciated genes, including FOXO3, E2F1, and NOTCH2, among others. We further observed independently increasing profiles of H3K27me3 and H3K4me3 at the promoters of certain target genes in HCC specimens. Importantly, Kaplan-Meier analysis reveals that 3-year overall and tumour-free survival rates are dramatically reduced in patients that simultaneously express EZH2 and menin, compared to rates in the EZH2 or menin under expressing patients. Furthermore, an inhibitor of H3K27me3 alone, or in combination with an H3K4me3 inhibitor, effectively blocked the aggressive phenotype of HCC cells. CONCLUSIONS: Our results indicate that a combined analysis of both H3K27me3 and H3K4me3 may serve as powerful diagnostic biomarkers of HCC, and targeting both might benefit anti-HCC therapy.