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AIM:To identify the relationship between DNA hyper- methylation and histone modification at a hyperme- thylated,silenced tumor suppressor gene promoter in human gastric cancer cell lines and to elucidate whether alteration of DNA methylation could affect histone modification. METHODS:We used chromatin immunoprecipitation (CHIP) assay to assess the status of histone acetylation and methylation in promoter regions of the p16 and mutL homolog 1 (MLH1) genes in 2 gastric cancer cell lines,SGC-7901 and MGC-803.We used methylation- specific PCR (MSP) to evaluate the effect of 5-Aza-2’- deoxycytidine (5-Aza-dC),trichostatin A (TSA) or their combination treatment on DNA methylation status. We used RT-PCR to determine whether alterations of histone modification status after 5-Aza-dC and TSA treatment are reflected in gene expression. RESULTS:For the p16 and MLH1 genes in two cell lines, silenced loci associated with DNA hypermethylation were characterized by histone H3-K9 hypoacetylation and hypermethylation and histone H3-K4 hypomethylation. Treatment with TSA resulted in moderately increased histone H3-K9 acetylation at the silenced loci with no effect on histone H3-K9 methylation and minimal effects on gene expression.In contrast,treatment with 5-Aza- dC rapidly reduced histone H3-K9 methylation at the silenced loci and resulted in reactivation of the two genes.Combined treatment with 5-Aza-dC and TSA was synergistic in reactivating gene expression at the loci showing DNA hypermethylation.Similarly,histone H3-K4 methylation was not affected after TSA treatment,and increased moderately at the silenced loci after 5-Aza-dC treatment. CONCLUSION:Hypermethylation of DNA in promoter CpG islands is related to transcriptional silencing of tumor suppressor genes.Histone H3-K9 methylation in different regions of the promoters studied correlates with DNA methylation status of each gene in gastric cancer cells.However,histone H3-K9 acetylation and H3-K4 methylation inversely correlate with DNA methylation status of each gene in gastric cancer cells.Alteration of DNA methylation affects histone modification.