Mutation-specific antibody detects mutant BRAFV600E protein expression in human colon carcinomas.

BACKGROUND: A point mutation (V600E) in the BRAF oncogene is a prognostic biomarker and may predict for nonresponse to anti-EGFR antibody therapy in patients with colorectal carcinoma. BRAFV600E mutations are frequently detected in tumors with microsatellite instability and indicate a sporadic origin. We used a mutation-specific antibody to examine mutant BRAFV600E protein expression and its concordance with BRAFV600E mutation data.

METHODS: Primary stage III colon carcinomas were analyzed for BRAFV600E mutations in exon 15, and 50 BRAFV600E mutation carriers and 25 wild-type tumors were selected for analysis of BRAF proteins by immunohistochemistry (IHC). IHC was performed in archival tissue specimens using a pan-BRAF antibody and a mutation-specific antibody against BRAFV600E proteins. Staining was scored by 2 pathologists who were blinded to clinical and mutation data.

RESULTS: Using a pan-BRAF antibody, total BRAF protein expression was observed in the tumor cell cytoplasm in 74 of 75 colon carcinomas. A mutation-specific antibody identified diffuse cytoplasmic staining of mutant BRAFV600E proteins in 49 of 74 cancers. Analysis using a polymerase chain reaction-based assay revealed that all 49 of these cancers carried BRAFV600E mutations. In contrast, BRAFV600E staining was absent in all 25 tumors that carried wild-type copies of BRAF.

CONCLUSIONS: A BRAF mutation-specific (V600E) antibody detected tumors with BRAFV600E mutations and exhibited complete concordance with a DNA-based method. These results support the use of IHC as a simplified strategy to screen colorectal cancers for BRAFV600E mutations in clinical practice.