Examination of mutations in BRAF, NRAS, and PTEN in primary cutaneous melanoma

Abstract

Frequent somatic mutation of v-raf murine sarcoma viral oncogene homolog B (BRAF), a downstream effector of the rat sarcoma oncogene (RAS) signaling pathway, is described in melanoma and other tumors. Our analysis of melanoma cell lines suggests that activating mutations in BRAF can occur simultaneously with inactivation of phosphatase and tensin homolog (PTEN), but neuroblastoma RAS (NRAS) mutations are not coincident. We determined the concurrent prevalence of mutations in BRAF and NRAS, and alteration of PTEN expression in 69 primary cutaneous melanomas. BRAF mutations were seen in 57% of cases. NRAS was mutated in 17% of samples, exclusively in exon 2. Two cases showed concurrent BRAF and NRAS mutations. Using immunohistochemistry, PTEN protein expression was lost or greatly reduced in 19% of tumors. Seven tumors with reduced PTEN yielded DNA amenable to sequencing, and three also showed mutation in BRAF but none in NRAS. In all, 11 (85%) of 13 tumors showing reduced PTEN expression were greater than 3.5 mm thick, and the association of increasing Breslow thickness and loss or reduction of PTEN expression was statistically significant (P<0.0001). Mutations in NRAS were not coincident with reduced PTEN expression, and the concurrent mutation of NRAS and BRAF was rare.