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

Abstract

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 and 3 (13%) synchronous primary CRCs, and 5 (22%) had additional LS-associated cancer. Although all 23 LS patients met Revised Bethesda guidelines, only 50% of MSH6 and 0 of the PMS2 carriers met Amsterdam I/II criteria (AC). Of 32 independent primary CRCs, 18 (56%) were stage I/II, 9 (28%) stage III, and 1 (3%) stage IV. 9/9 MSH6 and 4/4 PMS2 CRCs had high-frequency microsatellite instability. In MSH6 carriers, 11/13 had absence of MSH6protein expression only on IHC, 1 had inconclusive MSH6 staining, and 1 had absence of both MSH6 & MSH2 proteins. In PMS2carriers, 7/7 had absence of PMS2 on IHC, 2 also had equivocal/focal MLH1 staining. Right-sided CRC was present in 50% and at least 40% had mucinous features. 26/29 (90%) of tumors underwent segmental resection. 6/11 stage II patients received adjuvant chemotherapy including 2 with pT4N0 tumors. With a mean follow-up of 5.6 yrs to date, 1 patient is known to have developed recurrent CRC. Conclusions: Although the majority of MSH6/PMS2 CRC patients do not meet AC, 70% of CRCs were diagnosed at age ≤50, 22% at age ≤35, and 30% had synchronous/metachronous CRCs. These findings have important implications for CRC surveillance and may not support delaying colonoscopy initiation in MSH6/PMS2 LS families.