Immunohistochemical analysis using a BRAF V600E mutation specific antibody is highly sensitive and specific for the diagnosis of hairy cell leukemia.

Hairy cell leukemia (HCL) is usually diagnosed by morphology and flow cytometry studies. However, it is challenging sometimes to distinguish HCL from its mimics. Recently, the BRAF V600E mutation has been described as a disease-defining molecular marker for HCL which is present in nearly all cases of HCL but virtually absent in mimics of HCL. In this study, we investigated the possibility of using immunohistochemical detection of the BRAF V600E mutant protein to differentiate HCL from its mimics. A total of twenty-eight FFPE tissue specimens were studied, including HCL (n=12), HCL variant (HCL-v, n=3), splenic marginal zone lymphoma (SMZL, n=6), and other marginal zone lymphomas (MZL, n=7). Immunohistochemical studies were performed using a mouse monoclonal antibody (clone VE1, Spring Bioscience, CA) specific for BRAF V600E mutation. Molecularly confirmed BRAF V600E mutation positive and negative cases were used as the positive and negative controls respectively. All 12 cases of HCL showed cytoplasmic BRAF V600E protein expression in leukemia cells by immunohistochemical study regardless of tumor burden, whereas all cases of HCL mimics including HCL-v, SMZL, and MZL were negative for BRAF V600E protein. Using this BRAF V600E mutation specific antibody, this immunohistochemical study has 100% sensitivity and 100% specificity for the diagnosis of HCL in our cohort. In conclusion, immunohistochemical detection of the BRAF V600E mutant protein is highly sensitive and specific for the diagnosis of HCL. Compared to PCR or sequencing-based methodologies, immunohistochemistry is a relatively rapid and inexpensive alternative for the differential diagnosis between HCL and its mimics.