The use of BRAF V600E mutation-specific immunohistochemistry in pediatric Langerhans cell histiocytosis

BRAF p.V600E mutations are detected in greater than 50% of pediatric Langerhans cell histiocytosis (LCH) lesions. However, the use of mutation-specific BRAF V600E immunohistochemistry (IHC) as a surrogate for molecular testing in pediatric LCH is unknown. We tested the mutation-specific BRAF V600E monoclonal antibody (clone VE1) in formalin-fixed, paraffin-embedded LCH samples from 26 pediatric patients (14 males and 12 females, ages 7mo-17y) using allele-specific real-time polymerase chain reaction (PCR) with a limit of detection of 0.5% as the comparative gold standard. BRAF VE1 staining was scored for both intensity (0-3+) and percentage of immunoreactive tumor cells (0%-100%). BRAF VE1 immunoreactivity was determined using both lenient (1+, 1%) and stringent (2+, 10%) scoring criteria. Using lenient-scoring criteria, we found that the sensitivity and specificity of IHC compared with allele-specific real-time PCR were 100.0% and 18.2%, respectively. The poor specificity of lenient IHC analysis was attributable to weak, 1+ staining in both BRAF-mutated and wild-type LCH. Using stringent-scoring criteria, we found that specificity improved to 100.0% at the expense of sensitivity that decreased to 80.0%. Stringent scoring generated 3 false-negative results, but in all cases, neoplastic tissue comprised less than 5% of the stained section and/or the specimen was decalcified. In conclusion, highly sensitive molecular assays remain the gold standard for BRAF mutation analysis in LCH paraffin-embedded lesions. To avoid false-positive results, unequivocal VE1 staining of 2+ intensity in greater than or equal to 10% neoplastic histiocytes is required. However, negative VE1 results require additional studies to exclude false-negatives, and stringent-scoring criteria may not be optimal for scant or decalcified specimens.