Initiation of translation from a downstream in-frame AUG codon on BRCA1 can generate the novel isoform protein ΔBRCA1(17aa)

Expression of the breast and ovarian cancer gene BRCA1 is regulated at both the transcriptional and post-transcriptional levels. We found that the expression of the BRCA1 protein may also be regulated at the translational level. In addition to an AUG start codon at position 1, BRCA1 mRNA has a second in-frame AUG (+17) that acts as an alternative start codon to generate a novel BRCA1 protein that lacks the first 17 amino acids (ΔBRCA1(17aa)). We fused cDNAs encoding the second exon of BRCA1 of the wild-type BRCA1 gene (wt-BRCA1) and a mutated BRCA1 gene (mt-BRCA1), in which the first initiation site and its Kozak consensus sequence were abolished, with the nucleophosmin (NPM) reporter gene and used them for in vitro and in vivo translation assays. In both systems, the wt-BRCA1-NPM constructs produced two distinct proteins (18 and 16 kD) begun from the first and second AUGs. The mt-BRCA1-NPM constructs produced only the shorter 16-kD protein lacking the first 17 amino acids of the BRCA1 gene. Next, we analysed the N-terminal protein sequence of purified BRCA1 protein from normal thymocytes and found two different BRCA1 proteins, derived from translation of the first and second in-frame AUGs. Thus, BRCA1 protein expression can be regulated at the translation level in normal cells. Characterization of ΔBRCA1(17aa) may shed light on the function and regulation of BRCA1 in normal cells as well as the pathogenesis of breast and ovarian cancers.