Methylation of CpG loci in 5 '-flanking region alters steady-state expression of adenomatous polyposis coli gene in colon cancer cell lines

The APC genetic locus has been linked to the tumorigenesis and progression of colorectal cancer, although the precise mechanism of its involvement in this disease remains unknown. We used high sensitivity mapping of the methy lated cytosine, Northern blot analysis and immunocytochemical staining in s ix colorectal cancer cell lines (DLD-1, SW480, Colo320, HT29, WiDr, and Col o201) to examine the relationship between the methylation status of the CpG loci in the 5'-flanking region of the APC gene and its expression. APC mRN A expression levels determined by Northern blot analysis correlated well wi th APC protein levels visualized by immunocytochemistry. In these colorecta l cancer cell lines, no major genetic alterations of the APC gene, such as amplification or deletion, were detected. Analysis of the epigenetic contro l of APC gene expression in these lines revealed that methylation of the Cp G loci in the 5'-untranslated region of APC mRNA repressed steady-state exp ression of the gene. Furthermore, epigenetic alteration of the APC gene was independent of the APC protein truncation and CpG methylation of the hMLH1 promoter. Although less eminent than protein truncation by point mutation within the coding region of the APC gene, epigenetic alteration suppressing APC gene expression may significantly contribute to oncogenesis and the pr ogression of colorectal cancer. (C) 2001 Wiley-Liss, Inc.