LAUSR

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that transduces signals from the cell membrane through the mitogen-activated protein kinase (MAPK) phosphorylation cascade to promote global cellular responses including growth, proliferation, survival, and migration (Fig. 1). Oncogenic activating mutations in core components of the MAPK pathway (RAS, RAF, MEK, and MAPK/ERK proteins) frequently are found in common human tumors. BRAF is a serine/threonine kinase consisting of 766 amino acids (Reference Sequence [RefSeq] database NM_004333) that is encoded by the v-Raf murine sarcoma viral oncogene homolog B (BRAF ) gene on chromosome 7 (7q34). Activated BRAF phosphorylates MEK, which leads to MAPK/ERK activation and downstream transcriptional changes. An estimated 80% of BRAF mutations involve a thymine-to-adenine transversion at nucleotide 1799 of the coding sequence (BRAF c.1799T>A). As a consequence, valine is replaced by glutamic acid at amino acid 600 (p.Val600Glu or “V600E”). This substitution results in constitutive activation of BRAF and downstream MAPK/ERK signaling, which promotes tumorigenesis. Less common mutations involving different substitutions at the V600 “hotspot” similarly lead to constitutive BRAF activation. Activating BRAF mutations are present in >16% of common human tumors. Although molecular testing for BRAF is most commonly performed on surgical resection tissue, cytology specimens also represent a rich source of neoplastic cells, further expediting diagnosis, prognostication, and treatment with targeted therapy. Recent technological advances and the use of rapid on-site assessments have allowed for the molecular analysis of cytology samples, improved sampling of unresectable lesions, and sampling of lesions in critically ill patients. Molecular testing now can be performed routinely on a variety of cytologic preparations, including cell blocks, aspirate smears, liquid-based preparations, and needle rinses. Some laboratories have demonstrated the successful use of a BRAF V600E mutation–specific antibody (VE1) for immunohistochemistry of a variety of tumors, but the majority of experts recommend thorough validation of this approach with confirmatory BRAF molecular testing. Given the importance of BRAF alterations in cancer, many laboratories offer BRAF molecular testing, either as an inexpensive rapid polymerase chain reaction–based assay or as part of a more comprehensive next-generation sequencing panel.