BRCA1 exon 11 is one of the biggest human exons, spanning 3426 bases. This gene is potentially involved in DNA repair as well as cell growth and cell cycle control. Exon 11 is regulated at the splicing level producing three main different combinations of BRCA1 mature transcripts; one including the whole of exon 11 (full isoform), one skipping the entire exon (D11 isoform), and one including only 117 base pairs of exon 11 (D11q isoform). Using minigene and deletion analyses, we have previously described important splicing regulatory sequences located at the beginning of this exon (5' end). We have now found additional important sequences located at its 3' end. In particular, we describe the presence of a strong splicing enhancer adjacent to the downstream 5' splice site, which minimizes competition from an upstream 5' splice site and so ensures long exon inclusion. Analyses of the proteins binding these RNA sequences have revealed that Tra2beta and hnRNP L are involved in the regulation of BRCA1 exon 11 by influencing the recognition of donor sites. Interestingly, BRCA1 exon 11 carrying deletion of the regulatory sequences bound by these factors also showed unexpected responses to up- or downregulation of these regulatory proteins, suggesting that they can also bind elsewhere in this large exon and elicit different effects on its recognition. The identification of sequences and proteins relevant for the regulation of BRCA1 exon 11 now provides better knowledge on how this exon is recognized and may represent an important step toward understanding how large exons are regulated.
Keywords: BRCA; BRCA1; Splicing; Tra2beta; alternative splicing; donor site; exon11; hnRNPL; tra2B.