Two identical triplet sisters carrying a germline BRCA1 gene mutation acquire very similar breast cancer somatic mutations at multiple other sites throughout the genome.

Monozygotic twins, each of whom has breast cancer, offer a natural study population for gene-environmental interactions as causation of cancer, because they are genetically identical. If heritable factors play a large role in the origin of a neoplasm, disease concordance should be significant in monozygotic twins. Two monozygotic triplet sisters carrying a germline BRCA1 gene mutation (5382insC) who both developed breast cancer at early ages were studied for loss of heterozygosity (LOH) in their microdissected, paraffin-embedded tumors along with control blood and stromal breast tissue at 19 chromosomal arms using 161 microsatellite markers. Microdissected areas of normal lobular and ductal epithelium and ductal in situ carcinoma were also studied for LOH using a subset of microsatellite markers. The mother's DNA (extracted from peripheral blood lymphocytes) was analyzed to determine the parental allele under LOH in each case. Both tumors demonstrated similar histologic features suggestive of a secretory variant of ductal carcinoma. The tumors from both sisters had similar overall LOH frequency expressed by the fractional allelic loss (FAL) indices (0.56 vs. 0.60) and demonstrated concordance for loss or retention at 82 of 97 informative markers (85% correlation). In addition, detailed mapping analysis of several chromosomal arms revealed that identical breakpoints were detected in both tumors at several chromosome regions. Finally, in both sisters' tumors, when a chromosome exhibited allelic loss, all of the markers exhibited LOH of the same parental allele even when there were intervening regions of retention of heterozygosity. In contrast, 17 archival sporadic breast carcinomas demonstrated a wide range of FAL indexes and highly individual patterns of LOH. Our findings support the hypothesis that inherited factors play a role in the development of the multiple somatic deletions occurring in breast carcinomas. Whether one of these factors is the mutant BRCA1 allele or some other gene(s) remains to be determined. Genes Chromosomes Cancer 28:359-369, 2000.