Infrequent occurrence of microsatellite instability in sporadic and familial breast cancer.

Microsatellite instability was analysed in 93 primary breast tumours at 13 chromosomal loci frequently altered in breast cancer. RER (replication errors) were observed at a low (5%) frequency in sporadic, familial and hereditary breast tumours, as well as in breast tumours from patients with multiple primary cancers. Our study suggests that the RER+ phenotype is rare in breast tumours, and that breast cancer is not included in the hereditary non-polyposis colon cancer (HNPCC) syndrome. Moreover, the RER+ tumours revealed an atypical pattern of microsatellite alteration as compared with those usually seen in HNPCC tumours. In agreement with the findings in HNPCC tumours, all RER+ breast tumours were diploid, although having a similar frequency of allelic imbalance as RER- tumours. Thus, mismatch repair deficiency is rare in breast cancer, is most likely caused by somatic mutations, and possibly in a set of DNA repair genes different from that involved in the HNPCC syndrome.