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To study genetic changes associated with the development of breast cancer and the extent of its hereditary predisposition, paraffin-embedded tissue samples were obtained from monozygotic twin pairs concordant for breast cancer through the linked Swedish Twin and Cancer Registries. DNA samples extracted from the matched tumour and normal tissues of nine twin pairs were analysed for allelic imbalance using a series of microsatellite markers on chromosomes 13 and 17, containing loci with known tumour suppressor genes. Multiple losses of constitutional heterozygosity (LOH), consistent with a loss of large genomic region, the whole chromosome or chromosome arm, was found in at least three pairs of twins. One double mitotic crossover was identified in one tumour sample in a pair concordant for LOH at multiple loci on both chromosomes. Recombination breakpoints were mapped to regions delineated by D13S218 and D13S263, and D13S155 and D13S279, respectively. In general, no genetic effect of losing the same allele within a twin pair was found. However, for one marker at chromosome 13 (D13S328, between the  BRCA2  and the  Rb -1 loci) and two markers on chromosome 17 (D17S786, distal to the  p53  locus, and D17S855, an intragenic  BRCA1  marker) the proportion of twin pairs with the same LOH was significantly higher than expected. These regions may reflect hereditary genomic changes in our sample set. In addition, tumour DNA samples from a subset of 12 twin pairs were analysed for  BRCA1  and  BRCA2  mutations using exon-by-exon single-strand conformation polymorphism analysis. Two unclassified  BRCA2  variants, with a putative pathogenic effect, were identified, but no pathogenic alterations were found in the  BRCA1  gene.

Allelic imbalance on chromosomes 13 and 17 and mutation analysis of BRCA1 and BRCA2 genes in monozygotic twins concordant for breast cancer