Immunohistochemical detection of the BRAF V600E mutant protein in colorectal neoplasms.

Reliable assessment of the BRAF mutation status is becoming increasingly important in the clinical management of colorectal carcinomas (CRC). The aim of this study was to investigate the use of a recently developed mutation-specific antibody (VE1; SpringBio, Pleasanton, CA) to detect the BRAF V600E protein in paraffin tissue. We analyzed by immunohistochemistry (IHC) 117 cases that had been evaluated for BRAF mutation using a MALDI-TOF mass spectrometry-based assay. Immunohistochemical staining was evaluated without the knowledge of the genetic data and was considered positive when there was distinct homogenous cytoplasmic staining in the tumor cells. The analyzed cases included 4 polyps, 63 primary CRC, and 50 metastatic CRC. Forty-five of the 46 (97.8%) cases that were positive by IHC had a BRAF V600E mutation by genetic analysis; the 1 discordant case was notably of signet ring cell type. Similarly, 66 of the 67 (98.5%) cases that were negative by IHC were also negative by genetic analysis. Four cases that showed weak cytoplasmic staining and/or nuclear staining in the tumor cells were considered to be IHC equivocal; by genetic analysis, 2 of the 4 were positive and 2 were negative. The overall sensitivity and specificity of IHC for the detection of a BRAF V600E mutant tumor was 93.7% and 95.6%, respectively. Our results support the use of VE1 IHC for identification of colorectal neoplasms harboring the BRAF V600E mutation. Difficulties in immunohistochemical interpretation may arise in a small number of cases and in those cases molecular testing is required.