Transcriptional silencing of Dickkopf gene family by CpG island hypermethylation in human gastrointestinal cancer

AIM: To clarify alterations of Dickkopfs (Dkks) and Kremen2 (Krm2) in gastrointestinal cancer. METHODS: We investigated the expression profiles and epigenetic alterations of Dkks and Krm2 genes in gastrointestinal cancer using RT-PCR, tissue microarray analysis, and methylation specific PCR (MSP). Cancer cells were treated with the demethylating agent and/or histone deacetylase inhibitor. WST-8 assays and in vitro invasion assays after treatment with specific siRNA for those genes were performed. RESULTS: Dkks and Krm2 expression levels were reduced in a certain subset of the gastrointestinal cancer cell lines and cancer tissues. This was correlated with promoter hypermethylation. There were significant correlations between Dkks over-expression levels and beta-catenin over-expression in colorectal cancer. In colorectal cancers with beta-catenin over-expression, Dkk-1 expression levels were significantly lower in those with lymph node metastases than in those without. Down-regulation of Dkks expression by siRNA resulted in a significant increase in cancer cell growth and invasiveness in vitro.CONCLUSION: Down-regulation of the Dkks associated to promoter hypermethylation appears to be frequently involved in gastrointestinal tumorigenesis.