DNA mismatch repair gene variant classification: evaluating the utility of somatic mutations and mismatch repair deficient colonic crypts and endometrial glands
Authors: Walker Romy , Mahmood Khalid , Como Julia , Clendenning Mark , Joo Jihoon E. , Georgeson Peter , Joseland Sharelle , Preston Susan G. , Pope Bernard J. , Chan James M. , Austin Rachel , Bojadzieva Jasmina , Campbell Ainsley , Edwards Emma , Gleeson Margaret , Goodwin Annabel , Harris Marion T. , Ip Emilia , Kirk Judy , Mansour Julia , Marfan Helen , Nichols Cassandra , Pachter Nicholas , Ragunathan Abiramy , Spigelman Allan , Susman Rachel , Christie Michael , Jenkins Mark A. , Pai Rish K. , Rosty Christophe , Macrae Finlay A. , Winship Ingrid M. , Buchanan Daniel D. ,   the ANGELS study
Published: September 2023

Germline pathogenic variants in the DNA mismatch repair (MMR) genes (Lynch syndrome) predispose to colorectal (CRC) and endometrial (EC) cancer. Lynch syndrome specific tumor features were evaluated for their ability to support the ACMG/InSiGHT framework in classifying variants of uncertain clinical significance (VUS) in the MMR genes. Twenty-eight CRC or EC tumors from 25 VUS carriers (6xMLH1, 9xMSH2, 6xMSH6, 4xPMS2), underwent targeted tumor sequencing for the presence of microsatellite instability/MMR-deficiency (MSI-H/dMMR) status and identification of a somatic MMR mutation (second hit). Immunohistochemical testing for the presence of dMMR crypts/glands in normal tissue was also performed. The ACMG/InSiGHT framework reclassified 7/25 (28%) VUS to likely pathogenic (LP), three (12%) to benign/likely benign, and 15 (60%) VUS remained unchanged. For the seven re-classified LP variants comprising nine tumors, tumor sequencing confirmed MSI-H/dMMR (8/9, 88.9%) and a second hit (7/9, 77.8%). Of these LP reclassified variants where normal tissue was available, the presence of a dMMR crypt/gland was found in 2/4 (50%). Furthermore, a dMMR endometrial gland in a carrier of anMSH2exon 1-6 duplication provided further support for upgrade of this VUS to LP. Our study confirmed that identifying these Lynch syndrome features can improve MMR variant classification, enabling optimal clinical care.

Simple Summary

Lynch syndrome is caused by germline pathogenic variants in the DNA mismatch repair (MMR) genes predisposing carriers to colorectal and endometrial cancer. Genetic testing for Lynch syndrome, in the form of multigene panel testing, frequently identifies variants of uncertain clinical significance (VUS). These VUS have limited clinical actionability and create uncertainty for patients and clinicians regarding their risk of cancer. In this study, we tested carriers of germline VUS for features consistent with Lynch syndrome, namely 1) tumor microsatellite instability/MMR-deficiency, 2) the presence of a somatic second hit in the MMR gene harboring the VUS by tumor sequencing and 3) the presence of MMR-deficiency in normal colonic mucosa crypts or normal endometrial glands. Our findings showed that microsatellite instability/MMR-deficiency status and somatic second hits were consistent with MMR variant classifications as determined by the ACMG/InSiGHT framework. In addition to this, the presence of MMR-deficient crypts/glands were consistent with pathogenic variant classification.