IHC refers to staining tumor tissue for protein expression of the four MMR genes known to be mutated in LS (MLH1, MSH2, MSH6, and PMS2)
A normal IHC test implies that all four MMR proteins are normally expressed (retained).
Loss (absence) of expression of one or more of the four DNA MMR proteins is often reported as abnormal or positive IHC.
When IHC is reported as positive, caution should be taken to ensure that positive refers to absence of mismatch expression and not presence of expression.
Normal is the presence of positive protein staining (retained/intact) and abnormal is negative or loss of staining of protein.
Loss of protein expression by IHC in any one of the MMR genes guides further genetic testing (mutation detection to the genes where the protein expression is not observed)
Abnormal MLH1 IHC should be followed by tumor testing for BRAF V600E mutation or MLH1 promoter methylation.
The presence of BRAF V600E mutation or MLH1 promoter methylation is consistent with sporadic cancer
However, caution should be exercised in excluding cases from germline screening based on BRAF V600E mutations in the setting of a strong family history
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