Distinct transcription start sites generate two forms of BRCA1 mRNA.

Using primer extension and 5' RACE, we have mapped the 5' end of the BRCA1 gene and identified a new 5' exon. Two distinct BRCA1 transcripts differing by the first exons were found; these transcripts were generated by the alternative use of dual promoters and alternative splicing. The expression of the distinct transcripts was examined in four primary tissues (placenta, mammary gland, testis and thymus), six normal or cancer cell lines, four primary breast tumor tissues and four primary ovary tumour tissues. Both transcripts were detected in all the samples studied, with the exon 1a transcript being the major expressed form in mammary gland and the exon 1b transcript in placenta. This suggests that the two transcripts may be expressed in a tissue-specific fashion. The 5' flanking regions of both BRCA1 transcripts were analysed, neither contains a TATA box. Initiator elements, which have been proposed to mediate transcription in TATA-less promoters, were found at the transcription initiation sites. Transcription factor binding sites such as Sp1, PEA3, C/EBP, CREB, E4F1 and Pu boxes were identified in the 5' flanking regions of the exon 1a transcript, and Sp1, NF-kB and PEA3 binding sites in the 5' flanking region of the exon 1b transcript. The interactions of these DNA elements with trans-acting factors are likely to modulate the alternative use of the distinct transcription start sites and the expression of the BRCA1 gene.