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.

Malignant melanoma: genetics and therapeutics in the genomic era

“…Similarly, BRAF V600E mutation alone in TERT-immortalized RB-p53 mutant human melanocytes was found to produce only junctional nevi in the human/mouse skin graft, in contrast to activated NRAS or PI3K p110a mutants, which generated invasive melanoma lesions (Chudnovsky et al 2005). While these biological outcomes indicate distinct roles for NRAS and BRAF activation in melanoma genesis, some functional overlap is suggested by the predominantly nonconcurrent occurrence of activated NRAS and B-RAF alleles in melanoma and other tumor types (Davies et al 2002;Rajagopalan et al 2002;Goel et al 2006). The single and compound mutant mouse models will prove useful in dissecting the common and distinct roles of activated NRAS and BRAF signaling in vivo in the melanocytic lineage.…”

“…PTEN regulates PIP 3 levels, and its inactivation results in accumulation of PIP 3 , AKT hyperphosphorylation, and enhanced cell survival/ proliferation. In melanoma, allelic loss or altered expression of PTEN comprises 20% and 40% of melanoma tumors, respectively (Pollock et al 2002;Mikhail et al 2005;Slipicevic et al 2005;Goel et al 2006), although somatic point mutations and homozygous deletions are rarely observed. Functionally, ectopic expression of PTEN in PTEN-deficient melanoma cells can abolish phospho-AKT activity, induce apoptosis, and suppress growth, tumorigenicity, and metastasis (Robertson et al 1998;Stewart et al 2002;Stahl et al 2003; for review, see Robertson 2005).…”

Melanoma Epidemiology and Prevention

“…BRAF-mutant melanomas are associated with young age at diagnosis, intermittently sun-exposed sites such as the trunk, superficial spreading subtype, absence of solar elastosis, and presence of mitoses [17,60,63,64,93,144,155,228]. NRAS-mutant melanomas are associated with older age at diagnosis, but less associated with specific anatomic location, are more likely to be nodular subtype, and show increased Breslow thickness and presence of mitoses [60,63,64,86,139,228,231,234]. Interestingly, RAC1-mutant melanomas are more common in older men on the head and neck location [128], while TERT promoter mutations in melanomas are associated with older age, increased Breslow thickness, nodular subtype, and tumor ulceration [101].…”

Targeted Therapy for Cutaneous Melanoma: Beyond BRAF...

“…2,3 These mutations are essentially mutually exclusive with BRAF V600 mutations in treatmentnaïve melanomas. 15 In our study of ~700 melanoma patients who underwent mutation testing, BRAF V600 mutations and NRAS mutations were present concurrently in <1% of the tumors tested. 2 Of note, mutations of NRAS are a known mechanism of resistance to BRAF inhibitors, and they are detected in 20-25% of BRAF V600 mutation-(+) melanomas after disease progression on selective BRAF inhibitors.…”