Deletion of the Importin-alpha Gene in the Breast Cancer Cell

ABSTRACT

BACKGROUND: BRCA1 (breast-cancer gene 1) is a tumor suppressor gene that accounts for nearly all families of both early onset breast and ovarian cancer and about 45% of families with breast cancer only. Sporadic nonhereditary breast cancer is recognized as the most common form of this malignancy. However, presence of germ-line mutations in the BRCA1 gene of these tumors is an infrequent event. The BRCA1 protein includes a ring domain and an acidic domain, both of which are characteristics of certain transcription factors, as well as two putative nuclear localization signals (NLS) that interact with importin-alpha. The normal BRCA1 protein is located in the nucleus of most breast-cell types whereas the BRCA1 protein of breast cancer cells is aberrantly localized in the cytoplasm. This mislocation of the BRCA1 protein in breast cancer cells may be due to defects in the NLS receptor-mediated pathway for the nuclear import of the BRCA1 gene product. Identification of importin-alpha mutations as a cellular protein responsible for the nuclear import of BRCA1 in breast-cancer cell lines and primary breast cancers is the focus of this investigation. METHODS: A series of 15 surgical samples of breast cancer and 3 samples of breast-cancer cell lines (Hs578T, ZR75-1, MCF-7) was assayed for the presence of the deletion mutant in importin-alpha by using both RT-PCR amplification of importin-alpha transcripts and sequencing analysis. RESULTS: Three of the 15 primary breast cancers and 1 of the 3 breast-cancer cell lines showing deletions in importin-alpha transcripts produced two different truncated transcripts. 1208 bp deletions were observed in transcripts from breast cancer (T-1, T-3) and ZR75-1, which is specified by the nucleotide 251-1458 of the transcript. Another transcript encoded by primary breast cancer (T-2) included a 1312 bp deletion in the nucleotide 61-1372 of the transcript. CONCLUSIONS: The deletions eliminated part of the importin-alpha transcript segment encoding the putative NLS-binding domain but not the importin-beta binding domain, suggesting that these deletion mutants could not bind to NLS of the BRCA1 protein. These results suggest that the composite effects of mislocationof the BRCA1 protein by deletion of the NLS-binding domain in importin-alpha may contribute to tumorigenesis in sporadic breast cancer.