Exclusion of breast cancer as an integral tumor of hereditary nonpolyposis colorectal cancer.

Hereditary nonpolyposis colorectal cancer (HNPCC) is an autosomal dominant genetic predisposition syndrome that accounts for 2-7% of all colorectal cancers. Diagnosis of HNPCC is based on family history (defined by Amsterdam or Bethesda Criteria), which often includes a history of multiple synchronous or metachronous cancers. The majority of HNPCC results from germ-line mutations in the DNA mismatch repair (MMR) genes hMSH2 and hMLH1 with rare alterations in hMSH6 and hPMS2 in atypical families. Both HNPCC and sporadic MMR-deficient tumors invariably display high microsatellite instability (MSI-H). Two types of HNPCC families can be distinguished: type I (Lynch I) with tumors exclusively located in the colon; and type II (Lynch II) with tumors found in the endometrium, stomach, ovary, and upper urinary tract in addition to the colon. A proposed association of breast cancer with type II HNPCC is controversial. To address this important clinical question, we examined MSI in a series of 27 female patients who presented with synchronous or metachronous breast plus colorectal cancer. Although MSI-H was found in 5 of 27 (18.5%) of the colon cancers, in all cases the matched breast cancer was microsatellite stable. We also examined the breast tumors from three women who were carriers of MMR gene mutations from HNPCC families. None of these three breast tumors displayed MSI nor was the expression of MMR proteins altered in these tumors. We conclude that breast cancer largely arises sporadically in HNPCC patients and is rarely associated with the HNPCC syndrome.