Variations of BRAF mutant allele percentage in melanomas

BRAF mutations are present in 40% of human skin melanomas. Mutated tumors with an increased percentage of BRAF mutant alleles (BRAF-M%) may have a better response to RAF/MEK inhibitors. We evaluated the BRAF-M% in melanomas, and the genetic causes of its variation.

BRAF-M% was quantified by pyrosequencing, real-time PCR (rtPCR) and/or picoliter-droplet PCR (dPCR). BRAF mutant expression was detected by immunohistochemistry. Chromosomal alterations were analyzed with fluorescence in situ hybridization (FISH), and single nucleotide polymorphism (SNP) arrays.

BRAF-M% quantification obtained with pyrosequencing was highly correlated (R = 0.94) with rtPCR, and with dPCR. BRAF-M% quantified from DNA and RNA were also highly correlated (R = 0.98). Among 368 samples with >80% tumor cells, 38.6% had a BRAF (V600E) mutation. Only 66.2% cases were heterozygous (BRAF-M% 30 to 60%). Increased BRAF-M% (>60%) was observed in 19% of cases. FISH showed a polysomy of chromosome 7 in 13.6%, 35.3% and 54.5% of BRAF wild-type, heterozygous and non-heterozygous BRAF-mutated samples, respectively (P < 0.005). Amplification (5.6%) and loss (3.2%) of BRAF locus were rare. By contrast, chromosome 7 was disomic in 27/27 BRAF-mutated nevi.

BRAF-M% is heterogeneous and frequently increased in BRAF-mutant melanomas. Aneuploidy of chromosome 7 is more frequent in BRAF mutant melanomas, specifically in those with high BRAF-M%.