title: Promoter hypermethylation and post-transcriptional mechanisms for reduced BRCA1 immunoreactivity in sporadic human breast cancers.
abstract: 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 immunoreactivity 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 immunoreactivity 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Carcinogenesis Division, National Cancer Center Research Institute, Tokyo, Japan.
authors.forename: Kazuaki
authors.initials: K
authors.lastname: Miyamoto
authors.forename: Takashi
authors.initials: T
authors.lastname: Fukutomi
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Asada
authors.forename: Kuniko
authors.initials: K
authors.lastname: Wakazono
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Tsuda
authors.forename: Toshimasa
authors.initials: T
authors.lastname: Asahara
authors.forename: Takashi
authors.initials: T
authors.lastname: Sugimura
authors.forename: Toshikazu
authors.initials: T
authors.lastname: Ushijima
mesh_terms: D000328:Adult; D000368:Aged; D019313:BRCA1 Protein / Q000235:genetics / Q000378:metabolism*; D001943:Breast Neoplasms / Q000235:genetics* / Q000378:metabolism; D018270:Carcinoma, Ductal, Breast / Q000235:genetics* / Q000378:metabolism; D019175:DNA Methylation; D005260:Female; D020868:Gene Silencing; D019398:Genes, BRCA1; D006801:Humans; D008875:Middle Aged; D011401:Promoter Regions, Genetic; D011499:Protein Processing, Post-Translational; D012333:RNA, Messenger / Q000378:metabolism
chemical_list: D019313:BRCA1 Protein; D012333:RNA, Messenger