Background: BRAF mutations are present in 40 % of human skin melanomas. Mutated tumors with an increased\npercentage of BRAF mutant alleles (BRAF-M%) may have a better response to RAF/MEK inhibitors. We evaluated the\nBRAF-M% in melanomas, and the genetic causes of its variation.\nMethods: BRAF-M% was quantified by pyrosequencing, real-time PCR (rtPCR) and/or picoliter-droplet PCR (dPCR).\nBRAF mutant expression was detected by immunohistochemistry. Chromosomal alterations were analyzed with\nfluorescence in situ hybridization (FISH), and single nucleotide polymorphism (SNP) arrays.\nResults: BRAF-M% quantification obtained with pyrosequencing was highly correlated (R = 0.94) with rtPCR, and with\ndPCR. BRAF-M% quantified from DNA and RNA were also highly correlated (R = 0.98). Among 368 samples with >80 %\ntumor cells, 38.6 % had a BRAFV600E mutation. Only 66.2 % cases were heterozygous (BRAF-M% 30 to 60 %). Increased\nBRAF-M% (>60 %) was observed in 19 % of cases. FISH showed a polysomy of chromosome 7 in 13.6 %, 35.3 %\nand 54.5 % of BRAF wild-type, heterozygous and non-heterozygous BRAF-mutated samples, respectively (P < 0.005).\nAmplification (5.6 %) and loss (3.2 %) of BRAF locus were rare. By contrast, chromosome 7 was disomic in 27/27\nBRAF-mutated nevi.\nConclusions: BRAF-M% is heterogeneous and frequently increased in BRAF-mutant melanomas. Aneuploidy of\nchromosome 7 is more frequent in BRAF mutant melanomas, specifically in those with high BRAF-M%.
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