Characterization of colorectal cancer (CRC) in Lynch syndrome (LS) patients with MSH6 or PMS2 mutations.

1555 Background: The majority of LS patients harbor germline mutations in the MLH1 or MSH2 genes. However, ~10% have MSH6 and ~<5% PMS2 mutations. An attenuated form of LS has been suggested in MSH6/PMS2 carriers with decreased CRC risk and older age of disease onset. As recent guidelines suggest that initiation of CRC screening may be delayed in such patients, we characterized our patients with MSH6/PMS2-associated CRC. Methods: We obtained an IRB waiver to identify all LS patients with CRC, defined as the presence of a deleterious germline mutation in a MMR gene, from the Clinical Genetics database at MSKCC. Clinical, pathologic, and genetic features were extracted from medical records and Progeny software. Results: Of 147 LS patients with CRC, 23 had mutations in the MSH6 (n=16, 11%) or PMS2 (n=7, 5%) genes. Mean age at CRC diagnosis was 48.5 yrs (range 32-70) in MSH6 and 40.7 (range 22-57) in PMS2 carriers. 16 (70%) and 5 (22%) were diagnosed at age ≤50 or ≤35, respectively. 4 (17%) had metachronous a...