High frequency of hypermethylation at the 14-3-3 sigma locus leads to genesilencing in breast cancer

Expression of 14-3-3 sigma (sigma) is induced in response to DNA damage, an d causes cells to arrest in G(2), By SAGE (serial analysis of gene expressi on) analysis, we identified sigma as a gene whose expression is 7-fold lowe r in breast carcinoma cells than in normal breast epithelium. We verified t his finding by Northern blot analysis. Remarkably, sigma mRNA was undetecta ble in 45 of 48 primary breast carcinomas. Genetic alterations at sigma suc h as loss of heterozygosity were rare (1/20 informative cases), and no muta tions were detected (0/34), On the other hand, hypermethylation of CpG isla nds in the sigma gene was detected in 91% (75/82) of breast tumors and was associated with lack of gene expression, Hypermethylation of sigma is funct ionally important, because treatment of sigma-non-expressing breast cancer cell lines with the drug 5-aza-2'-deoxycytidine resulted in demethylation o f the gene and synthesis of a mRNA. Breast cancer cells lacking sigma expre ssion showed increased number of chromosomal breaks and gaps when exposed t o gamma-irradiation, Therefore, it is possible that loss of sigma expressio n contributes to malignant transformation by impairing the G(2) cell cycle checkpoint function, thus allowing an accumulation of genetic defects. Hype rmethylation and loss of sigma expression are the most consistent molecular alterations in breast cancer identified so far.