Breast cancer in an MSH2 gene mutation carrier.

A 49-year-old woman presented with breast cancer. She is a member of a family with the hereditary nonpolyposis colorectal cancer syndrome for which a 2-base pair deletion in exon 11 of the mismatch repair gene MSH2 (c1705_1706 delGA) had been identified. Breast cancer is rare in the hereditary nonpolyposis colorectal cancer syndrome. Microsatellite analysis of the tumor showed a microsatellite instable pattern for markers Bat25, Bat26, and Bat40, and no changes for markers D2S123 and D5S346, a so-called microsatellite instability-high pattern. Immunohistochemical staining for the mismatch repair enzymes MSH2 and MSH6 was negative, whereas the tumor cells were positive for MLH1, a pattern suggestive for biallelic MSH2 gene inactivation. We tested the tumor for loss of heterozygosity of the MSH2 gene and found loss of the wild-type MSH2 allele. These data strongly suggest that the MSH2 gene was involved in the development of this breast tumor.