Promoter CpG island hypermethylation during breast cancer progression

This study was designed to evaluate the changes in promoter CpG islands hypermethylation during breast cancer progression from pre-invasive lesions [flat epithelial atypia (FEA), atypical ductal hyperplasia (ADH), and ductal carcinoma in situ (DCIS)] to invasive ductal carcinoma (IDC). We performed MethyLight analysis for the methylation status of 57 promoter CpG island loci in 20 IDCs and their paired normal breast tissues. After selecting 15 CpG island loci showing breast cancer-specific DNA methylation, another set of normal breast tissue (n = 10), ADH/FEA (n = 30), DCIS (n = 35), and IDC (n = 30) of the breast were analyzed for these loci. We found six new methylation markers of breast cancer, namely DLEC1, GRIN2B, HOXA1, MT1G, SFRP4, and TMEFF2, in addition to APC, GSTP1, HOXA10, IGF2, RARB, RASSF1A, RUNX3, SCGB3A1 (HIN-1), and SFRP1. The number of methylated genes increased stepwise from normal breast to ADH/FEA and DCIS, while IDC did not differ from DCIS. Methylation levels and frequencies of APC, DLEC1, HOXA1, and RASSF1A promoter CpG islands were significantly higher in ADH/FEA than in normal breast tissue. GRIN2B, GSTP1, HOXA1, RARB, RUNX3, SFRP1, and TMEFF2 showed higher methylation levels and frequencies in DCIS than in ADH/FEA. DICS and IDC did not differ in the methylation levels or frequencies for most CpG island loci except SFRP1 and HOXA10. Our findings showed that promoter CpG island methylation changed significantly in pre-invasive lesions, and was similar in IDC and DCIS, suggesting that CpG island methylation of tumor-related genes is an early event in breast cancer progression.