Histology of colorectal adenocarcinoma with double somatic mismatch repair mutations is indistinguishable from those caused by lynch syndrome

Lynch syndrome (LS) is the most common form of hereditary colon cancer (CRC). Germline mutations in the mismatch repair (MMR) genes MLH1, MSH2 (EPCAM), MSH6, and PMS2, followed by a second hit to the remaining allele leads to cancer development. Universal tumor screening for LS is routinely performed on CRC, and screening has identified patients with unexplained MMR deficiency that lack MLH1 methylation and a germline mutation. Tumor sequencing has since identified double somatic (DS) mutations in the MMR gene corresponding with the absent protein in 69% of these patients.

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