title: High BRAF mutation frequency does not characterize all melanocytic tumor types.
abstract: Cutaneous melanoma (CM) is the most lethal form of skin cancer. Along with some benign melanocytic tumors, the majority shows BRAF or NRAS mutation, but it is not known whether these are essential to all forms of melanocytic neoplasia. We screened 79 melanocytic tumors of different types for BRAF and NRAS mutations and looked at MAPK pathway activity using immunohistochemistry in a subset. Significant differences in BRAF exon 15 mutation frequency were found: 14/16 (87.5%) in common acquired naevi (CANs), 9/12 (75%) in CMs, 0/26 in Spitz naevi and 3/25 (12%) in blue naevi (p < 0.01). We looked at whether Spitz and blue naevi showed a compensatory increase in BRAF exon 11 and/or NRAS exons 1 and 2 mutations to account for the low BRAF exon 15 mutation frequency. NRAS mutations were found in only 1/16 (6.3%) Spitz naevi and 0/15 blue naevi. In addition, NRAS mutations were found in 2/11 (18.2%) CANs and 3/12 (25%) CMs. None of the tumors showed BRAF exon 11 mutations. Despite their low combined BRAF and NRAS mutation frequency, Spitz naevi showed strong MAPK pathway activation as measured by cytoplasmic expression of dually phosphorylated ERK1/2, while blue naevi had weak pathway activation. We conclude that BRAF and NRAS mutations are not necessary for melanocytic tumor development and that some types of tumor must arise by alternative mechanisms.
authors.affiliation: Department of Cancer Studies and Molecular Medicine, University of Leicester, Leicester, United Kingdom. gss4@le.ac.uk
authors.forename: Gerald
authors.initials: G
authors.lastname: Saldanha
authors.forename: David
authors.initials: D
authors.lastname: Purnell
authors.forename: Alan
authors.initials: A
authors.lastname: Fletcher
authors.forename: Linda
authors.initials: L
authors.lastname: Potter
authors.forename: Angela
authors.initials: A
authors.lastname: Gillies
authors.forename: J Howard
authors.initials: JH
authors.lastname: Pringle
mesh_terms: D002471:Cell Transformation, Neoplastic; D004252:DNA Mutational Analysis; D004273:DNA, Neoplasm / Q000235:genetics*; D005091:Exons; D011905:Genes, ras; D006801:Humans; D007150:Immunohistochemistry; D008545:Melanoma / Q000235:genetics* / Q000473:pathology; D009154:Mutation; D009506:Nevus / Q000235:genetics* / Q000473:pathology; D016133:Polymerase Chain Reaction; D012150:Polymorphism, Restriction Fragment Length; D018807:Polymorphism, Single-Stranded Conformational; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics*; D012878:Skin Neoplasms / Q000235:genetics* / Q000473:pathology
chemical_list: D004273:DNA, Neoplasm; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf