Exon 15 BRAF mutations are uncommon in canine oral malignant melanomas

An activating mutation in codon 599 of BRAF has been identified in approximately 60% of human cutaneous nevi and melanomas, but not melanomas of mucosal origin. The purpose of this study was to determine if BRAF mutations occur in canine oral malignant melanomas. The canine BRAF gene was first cloned from normal canine testicular cDNA, and a novel previously unreported splice variant involving exon 5 was identified during this process. To screen canine melanoma samples for BRAF mutation in codon 599, cDNA and genomic DNA were isolated from canine malignant melanoma cell lines and primary tumor samples respectively, all from cases seen at the Veterinary Medical Teaching Hospital at the University of California, Davis. Polymerase chain reaction (PCR) was performed for exon 15 using primers based at the 5' end of exon 15 and the 5' end of intron 15 and the resultant products were directly sequenced. No mutations in codon 599 or exon 15 were identified in any of the 17 samples evaluated. However, all of the melanoma cell lines expressed BRAF and demonstrated high levels of basal ERK phosphorylation suggesting that dysregulation of this pathway is present. Therefore, similar to the case with human mucosal melanomas, canine oral malignant melanomas do not possess codon 599 BRAF mutations commonly identified in human cutaneous melanomas. This finding supports the notion that melanomas arising from non-sun-exposed sites exhibit distinct mechanisms of molecular transformation.