Functional Impact of Sequence Alterations Found in BRCA1 Promoter/5'UTR Region in Breast/Ovarian Cancer Families from Upper Silesia, Poland

Abstract

The 5' region of BRCA1 contains multiple regulatory sequences flanking the two alternative promoters alpha and beta and two alternative, non-coding exons, 1a and 1b. Aberrations within the 5' region BRCA1 (encompassing two alternative promoters alpha and beta and exons 1a and 1b) may be associated with an increased risk of breast and ovarian cancer. In this study we screened 150 patients for polymorphism and mutations in this region of BRCA1. All probands came from familial breast and/or ovarian cancer that had been found to be mutation-negative in a previous search for founder mutations in BRCA1 (185delAG, C61G, 4153delA, 5382insC) or BRCA2 (6174delT, 9631delC). In our study we found several sequence alterations within the non-coding region of BRCA1 by using direct DNA sequencing and allele-specific PCR amplification. Three families with a polymorphic deletion in BRCA1 exon 1b (2223delAAAAA, Acc. U37574) were found. Moreover, two linked nucleotide substitutions (2642A>T, 2743T>C, Acc. U37574) in BRCA1 intron 1 were detected in 16 patients. In order to assess the functional significance of these two sequence variants, we constructed a reporter vector encoding firefly luciferase under the transcriptional and translational control of wild type and altered BRCA1 promoter region. The reporter assay was performed using a lung cancer cell line (NCI-H1299) and a breast cancer cell line (MCF7). We have demonstrated that the analysed sequence variants have no functional significance in our experimental system. However, we have found that the BRCA1 promoter has lower relative activity in the breast cancer cell line compared with the lung cancer cell line. Based on the results of our functional experiments we conclude that the polymorphic deletion 2223delAAAAA and two linked substitutions 2642A>T and 2743T>C do not significantly alter BRCA1 expression and are probably not disease-causing mutations.

Figure 1

Schematic representation of the 5' region of the BRCA1 gene and location of sequence alterations found in this gene fragment -1a and 1b represent two alternative first exons, 2 represents the exon 2 with the translation start site.

Figure 2

The pGL3 - BRCA1 - prom plasmid map. The localization of sequence alterations and the recognition sequences of the restriction enzymes used for the cloning are shown. The thick arrow represents the coding region of the firefly luciferase gene (luc)

Figure 3

The normalized activity of the reporter gene (luc) under transcriptional and translational control of the wild-type and polymorphic forms BRCA1 promoter/5'UTR region in NCI-H1299 (grey) and MCF7 (dark grey) cell lines