[762] Immunohistochemical Detection of the BRAF V600E Mutant Protein in Colorectal Adenocarcinoma

Efsevia Vakiani, Ying Zhou, David S Klimstra, Jinru Shia. Memorial Sloan-Kettering Cancer Center, New York, NY

Background: Reliable assessment of the BRAF mutation status is becoming increasingly important in the clinical management of colorectal carcinomas (CRC). BRAF mutations occur in approximately 10% of CRC and the vast majority are characterized by a missense substitution of valine by glutamic acid at amino acid position 600 (V600E). The presence of BRAF V600E mutation can help guide targeted therapy as it is associated with resistance to EGFR inhibitors, while in microsatellite unstable cases it can differentiate sporadic from hereditary CRC. The aim of this study was to investigate the use of a recently developed mutation-specific antibody to detect expression of the BRAF V600E protein in routinely processed formalin-fixed paraffin-embedded (FFPE) tissue.
Design: We analyzed 49 FFPE cases (44 resections, 5 biopsies) that had been previously evaluated for the presence of BRAF mutations using a MALDI-TOF mass-spectrometry based genotyping assay. For the immunohistochemical detection of the mutant protein we used the BRAF V600E mutation-specific antibody VE1. Cases were evaluated by two pathologists who were blinded to the genetic data and were considered positive when there was distinct homogeneous cytoplasmic staining in the tumor cells.
Results: The analyzed cases included 1 sessile serrated polyp (SSP), 1 adenoma with intramucosal carcinoma, 24 primary invasive CRC and 23 metastases. Twenty three cases (47%), including the SSP, were scored as positive and all these cases were found to have the BRAF V600E mutation by genetic analysis. Twenty six (53%) cases including all biopsies were scored as negative; of those cases, 3 (11.5%) showed a BRAF V600E mutation based on DNA sequencing. The three discrepant cases included the adenoma with intramucosal carcinoma and two resections of metastases; all three cases showed weak cytoplasmic staining that was not clearly different from a weak background staining. Two cases that showed a BRAF D594G mutation by genetic analysis were negative immunohistochemically, as expected.
Conclusions: Distinct and diffuse cytoplasmic staining with the BRAF V600E specific antibody VE1 appears to reliably predict for the presence of the BRAF V600E mutation in the tumor cells. Some cases, however, may show a weak staining reaction leading to false negative results. A larger number of cases needs to be analyzed to confirm these findings and to further explore the utility of immunohistochemistry in small specimens.
Category: Gastrointestinal

Monday, March 4, 2013 1:00 PM

Poster Session II # 126, Monday Afternoon

 

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