A characterization of genetic variants in BRCA1 intron 8 identifies a mutation and a polymorphism

The biochemical and genetic characterizations of two variants that occur in BRCA1 intron 8 are presented. The variant IVS8+2T→C induces an aberrant transcript that deletes exon 8. This exon-skipping deletion disrupts the open reading frame by juxtaposing exon 7 and exon 9 in the aberrant splice product. Theoretically, 50 abnormal residues from reading frame 2 are translated following exon 7 before a stop codon is encountered. The chromosomal contribution to the relevant RNA species was tracked using a silent polymorphism at codon 694 (serine AGC or AGT). Nucleotide sequencing of this polymorphic codon demonstrated that the aberrant transcript was derived solely from the chromosome encoding AGT. The normally spliced productive transcript also displayed loss of heterozygosity and was derived solely from the chromosome encoding AGC at codon 694. Also, a haplotype analysis using a breast cancer patient database showed that the chromosome bearing serine 694-AGT carried IVS8+2T→C. A second more common variant, IVS8-58delT, was characterized as a polymorphism. Analysis of RNA from patient samples used the same silent polymorphism at codon 694 and showed that the normal message was derived from both chromosomes.