The protean phenotype of MSH6 pathogenic variants (PV) in Lynch syndrome (LS) patients (pts).
Authors: McSweeny Michelle J , Montgomery Susan , Whitaker Kristen Danielle , Daly Mary Beryl , Hall Michael J. 
Published: June 2021

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Background: LS is among the most common hereditary cancer (CA) syndromes. PVs in MSH6 are 2-4 fold more common in the population (1/758) than those in MLH1 (1/1946) or MSH2 (1/2841), and are increasingly regarded as lower penetrance for CRC due to published data supporting later mean age of CRC onset and lower CRC risk. Unlike for MLH1/MSH2, NCCN 2020 CA risk estimates recognize only endometrial CA (EC) and CRC risks in MSH6+ carriers as clearly above SEER population estimates. Further, risks of other LS manifestations such as skin disease/Muir-Torre, ovarian CA (OC), and possible rare tumors in LS like sarcoma, have been minimally characterized in MSH6+ carriers. Methods: Pedigree data for 44 MSH6+ index (first-evaluated family member by our program) pts consecutively ascertained since 2009 at Fox Chase (FCCC) were reviewed. 1 pt w/a rare MSH6 uncertain variant w/personal history (PHx) of MSH6-expression deficient EC (age 50) and MSH6-deficient sebaceous skin CA (age 50) and a strong family history (FHx) c/w LS is also included here. 34% (15/44) index pts were referred to FCCC for cascade testing due to a known MSH6 PV in the family. Of the remaining 29 index pts, ascertainment included: 14% w/positive universal LS tumor screening, 21% w/early-onset or synchronous LS CA, 14% w/multi-gene panel for PHx of OC, 10% w/incidental MSH6+ result (2 had testing for PHx breast CA, 1 tumor genomic profiling), and 28% w/PHx and/or FHx of LS CA warranting genetic testing. Age of CA onset and path data were verified in > 90% index pts. Results: Index pts had a mean age of 55.5 yrs, and 77% were female. Overall, 11% (5/44) of MSH6+ index pts were found to have LS at diagnosis of synchronous primary CAs (3 EC/OC, 1 CRC/CRC, 1 CRC/EC), and 4/5 of these occurred <50 yrs. An additional 20% (9/44) index pts reported PHx of >2 metachronous LS CAs. OC was the presenting CA in 14% (6/44) female index pts; 2 additional index pts had rarer OC variants (Mullerian duct @ 41, primary peritoneal CA @ 50). Skin manifestations of LS were documented in 9.1% (4/44) index pts (3 sebaceous, 1 SCC in-situ/Bowen’s disease); 1 other family had documented sebaceous CAs in an FDR (father) but the 2 daughters seen @FCCC (both 30s) had yet to develop skin lesions. 2 index pts were found to have LS after developing early-onset breast CA (age 39) and contralateral breast CA (ages 50 and 54). Finally, 7% (3/44) index pts had a PHx of sarcoma: 2 were liposarcomas (ages 57 and 67), and 1 was a dermatofibrosarcoma. 2 other index pts had siblings w/childhood sarcomas. Conclusions: Our data, encompassing 44 MSH6+ pts evaluated in our clinic and consecutively ascertained, suggest MSH6 PV carriers develop synchronous primaries (11%), common and rare OC histologic types (18%), sarcomas (7%) and skin disease/Muir-Torre (9%). While common in the population and lower penetrance for CRC, MSH6 PV can behave in uncommon ways and may have significant extra-colonic CA risks such as OC, sarcoma and skin manifestations.