BRAF V600E-specific immunohistochemistry for the exclusion of Lynch syndrome in MSI-H colorectal cancer

Abstract

The differentiation between hereditary and sporadic microsatellite-unstable (MSI-H) colorectal cancer is a crucial step in Lynch syndrome diagnostics. Within MSI-H colorectal cancers, the BRAF V600E mutation is strongly associated with sporadic origin.
We here asked whether BRAF V600E-specific immunohistochemistry (clone VE1) is helpful in separating sporadic from Lynch syndrome-associated MSI-H colorectal cancers. To that end, we performed VE1 immunohistochemistry and BRAF sequencing in a series of 91 MSI-H colorectal cancer specimens from patients tested for Lynch syndrome. Concordance of VE1 immunohistochemistry and molecular BRAF mutation status was observed in 90 out of 91 (98.9%) MSI-H samples. All eleven tumors classified as BRAF V600E mutation-positive by Sanger sequencing were immuno-positive, and 79 (98.8%) out of 80 tumors classified as BRAF wild type showed negative staining. All VE1-positive tumors were MLH1 and PMS2-negative by immunohistochemistry. None of the tumors from MMR gene germline mutation carriers (n=28) displayed positive VE1 staining, indicating that BRAF V600E mutation-specific immunostaining has a low risk of excluding Lynch syndrome patients from germline mutation analysis. In conclusion, implementation of VE1 immunohistochemistry was able to detect BRAF mutated MSI-H colorectal cancers with a sensitivity of 100% and a specificity of 98.7%. Among MLH1-negative colorectal cancers, the rate of VE1-positive lesions was 21%, offering the exclusion of these patients from MMR germ line testing. We therefore suggest the integration of VE1 immunohistochemistry into the diagnostic panel of Lynch syndrome.