High frequency of hypermethylation at the 14-3-3 σ locus leads to gene silencing in breast cancer

Expression of 14-3-3 sigma (sigma) is induced in response to DNA damage, and causes cells to arrest in G(2), By SAGE (serial analysis of gene expression) analysis, we identified sigma as a gene whose expression is 7-fold lower in breast carcinoma cells than in normal breast epithelium. We verified this finding by Northern blot analysis. Remarkably, sigma mRNA was undetectable in 45 of 48 primary breast carcinomas. Genetic alterations at sigma such as loss of heterozygosity were rare (1/20 informative cases), and no mutations were detected (0/34), On the other hand, hypermethylation of CpG islands 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 functionally important, because treatment of sigma-non-expressing breast cancer cell lines with the drug 5-aza-2'-deoxycytidine resulted in demethylation of the gene and synthesis of a mRNA. Breast cancer cells lacking sigma expression showed increased number of chromosomal breaks and gaps when exposed to gamma-irradiation, Therefore, it is possible that loss of sigma expression contributes to malignant transformation by impairing the G(2) cell cycle checkpoint function, thus allowing an accumulation of genetic defects. Hypermethylation and loss of sigma expression are the most consistent molecular alterations in breast cancer identified so far.