DETECTION OF GERMLINE BRCA1 MUTATIONS IN BREAST-CANCER PATIENTS BY QUANTITATIVE MESSENGER-RNA IN-SITU HYBRIDIZATION

Mutations in the breast cancer susceptibility gene 1 (BRCA1) may accou nt for one half of all familial breast cancers. Because of the wide sp ectrum of different germline mutations, identification of BRCA1 mutati on carriers using current techniques is laborious and difficult. The m ajority of the identified mutations, however, lead to aberrant express ion of the gene product in tumor tissue, potentially allowing the dete ction of BRCA1-linked breast cancers using simple histochemical techni ques. We performed quantitative mRNA in situ hybridization analysis on archival paraffin-embedded tumor specimens from 25 patients with char acterized germline BRCA1 mutations or linkage and from 29 patients wit h sporadic breast cancers. BRCA1 mRNA levels were invariably low in tu mors from BRCA1 mutation carriers. Normal breast epithelium surroundin g the BRCA1 tumors showed higher mRNA levels than the tumor tissue, in dicating that the low mRNA levels were due to somatic inactivation of the wild-type BRCA1 allele in the tumor tissue. The expression levels in the sporadic tumors were, on average, six times higher than in the BRCA1 tumors (P < 0.0001). The difference allowed identification of BR CA1-mutated and sporadic tumors with more than 95% specificity and sen sitivity. We conclude that the analysis of BRCA1 gene expression by mR NA in situ hybridization may be useful in screening for patients with BRCA1-linked breast cancer.