Fig. 1

Screening algorithm for mismatch repair deficiency and Lynch syndrome workup. All primary colorectal cancers (CRC) are immunostained for MLH1, MSH2, MSH6, and PMS2. The presence of all four proteins indicates intact expression and no further workup is necessary unless clinically suspicious. When MLH1 and PMS2 are absent, BRAF V600E mutation analysis is performed. BRAF mutation essentially excludes Lynch syndrome. If no BRAF mutation is detected, the subsequent step depends on the degree of suspicion of Lynch syndrome based on the patient’s age and history; if high suspicion, genetic sequencing of MLH1 or PMS2 are done; if low suspicion, MLH1 promoter hypermethylation is then analyzed. Non-MLH1 hypermethylated cases undergo germline sequencing of MLH1 or PMS2; those with methylation are presumed to represent sporadic tumors and Lynch workup is not indicated. Some laboratories use MLH1 methylation testing rather than BRAF mutational analysis after immunohistochemistry in those tumors found to have absence of MLH1 and PMS2. Cases with absence of MSH2/MSH6, or isolated loss of MSH6, or isolated loss of PMS2, may represent Lynch syndrome and should undergo germline sequencing. Finally, if Lynch germline mutation is not identified in deficient MMR cases, tumor sequencing for double somatic mutation should be considered. In any patient diagnosed with CRC under the age of 50, a comprehensive germline panel should be considered