Promoter hypermethylation and post-transcriptional mechanisms for reduced BRCA1 immunoreactivity in sporadic human breast cancers

Background: Germline mutation of BRCA1 is well known to cause familial breast cancer. Although somatic mutations of the BRCA1 gene are rare in sporadic breast cancers, a high incidence of reduced BRCA1 immunareactivity has been demonstrated. As one of the mechanisms for this, gene silencing by hypermethylation of the BRCA1 promoter region has been reported. Here, we show the presence of a post-transcriptional mechanism by examining promoter hypermethylation, mRNA expression levels and immunoreactivity of BRCA1 in sporadic human breast cancers. Methods: Paired samples of 20 invasive ductal carcinomas and one invasive lobular carcinoma were obtained from sporadic breast cancer cases. The BRCA1 protein expression levels were determined by immunohistochemistry using a well-characterized antibody. The methylation status of the BRCA1 promoter region was determined by sequencing after bisulfite modification. The mRNA expression levels were determined by semi-quantitative reverse transcription polymerase chain reaction (PCR). Mutations in the entire BRCA1 coding region were analyzed by PCR single-strand conformation polymorphism analysis. Results: Reduced immuno reactivity was observed in 13 of the 21 cancers. Hypermethylation was observed in five of the 13 cancers with reduced immunoreactivity and mRNA expression was almost absent in these five cancers. In the remaining eight cancers, mRNA expression was not decreased. None of the 21 cancers examined harbored BRCA1 mutations. Conclusion: These findings showed that post-transcriptional mechanisms, such as low efficiency of translation or reduced stability of BRCA1 protein, are also involved in reduced BRCA1 immunoreactivity in sporadic breast cancers.