Dual NRASQ61R and BRAFV600E mutation-specific immunohistochemistry completes molecular screening in melanoma samples in a routine practice

NRAS and BRAF mutational status has become mandatory to treat patients with metastatic melanomas. Mutation-specific immunohistochemistry (IHC) can help analyze challenging tumor samples. We report our experience integrating NRASQ61R (SP174) and BRAFV600E (VE1) IHC in routine practice in a cancer molecular genetic platform. All samples screened for BRAF and NRAS mutations during the year 2014 were analyzed by IHC and pyrosequencing, with an independent analysis of the 2 methods. Cases with first-line discordant results benefited from a complementary second-round IHC and next-generation sequencing (NGS) with a final interpretation taking into account the results of pyrosequencing, IHC, NGS, and quantification of the tumor cells. We analyzed 111 consecutive formalin-fixed and paraffin-embedded melanoma samples from 101 patients. Twenty-two and 11 samples were concordant for BRAFV600E and NRASQ61R mutations, respectively. Second-round analyses of 9 discordant and 1 molecularly inconclusive samples allowed conclusion in 4 further mutated samples (2 BRAFV600E and 2 NRASQ61R). A sample remained NRASQ61R IHC negative but NRASQ61R mutated with molecular methods. Overall, BRAFV600 and NRASQ61 mutation frequencies were 31.7% and 30.7%, respectively. When compared to molecular results, the sensitivity and specificity of IHC were 100% for BRAFV600E IHC and 92.3% and 98.9% for NRASQ61R IHC, respectively. IHC interpretation required a more stringent cutoff for BRAFV600E IHC than NRASQ61R to minimize false results. We conclude that NRASQ61R and BRAFV600E IHC coupled with NGS allow detection of mutations in melanoma challenging samples.