Frequent Allelic Imbalance in NRAS Mutant Melanomas.

9578 Background: We recently showed that 19% of BRAF mutant melanomas have an increased mutant allele percentage ( > 60%) (BMC Cancer 2015). This allelic imbalance was mainly related to a polysomy of chromosome 7. We investigated the percentage of NRAS mutant allele in BRAFwild type (WT) melanomas. Methods: We searched for NRAS Q61 mutation by pyrosequencing method in 216 FFPE samples of cutaneous melanomas WT for BRAFV600. FISH was performed with probes specific for NRAS (RP11-245I3) and 1p34.1 (RP11-269F19) on 105 cases NRAS WT (n = 42), or NRAS mutant with equilibrate (E%, n = 43) and high (H%, n = 20) mutant allele percentage. Loss of Heterozygoty (LOH) was evaluated with 8 microsatellite (MS) markers (3 localized near NRAS, 3 at 1p and 2 at 1q) on 17 samples (E% n = 8 or H% n = 9). Pathological data of 73 NRASmutant melanomas, clinical characteristics and survival data of matching patients were analyzed. Results: NRAS mutation was detected in 109 (50.5%) of BRAF WT samples, of which 31 (34%) were H%. Allelic NRAS imbalance was observed in all mutation types: Q61R (n = 22), Q61K (n = 6), Q61L (n = 2) and Q61H (n = 1). Chromosome 1 disomy, disomy with rare polysomic cells and polysomy were observed in 24% (24% WT vs 24% mutant NRAS), in 24% (33% WT vs 18% mutant NRAS) and in 12% (3% WT vs 18% mutant NRAS) of samples, respectively. NRAS amplification and monosomy of chromosome 1 were rare aberrations in both groups (3.4% and 4.6%, respectively). Intra-tumor heterogeneity concerning chromosome 1 was defined as monosomic or polysomic cells (or both) accompanying disomic tumor cells. Heterogeneity was observed in 24% WT and 35% mutant NRAS samples (P < 0.01), and was more frequent in H% than E% mutant NRAS (47% vs 29.7%). MS analysis revealed no LOH in 8 E%, while LOH was present in 9 H% NRAS samples. No differences were observed between E% and H% NRAS mutant melanomas for clinic, histology and survival data. Conclusions: Allelic imbalance is more frequent in NRAS than in BRAF mutant melanomas (34% versus 19% of H%, P < 0.01). Amplification was rare in NRAS, as shown in BRAF mutant melanomas. NRAS mutant melanomas had high intra-tumor heterogeneity. Allelic imbalance of H% NRAS was closely related to LOH.