Concordant BRAF^V600E mutation status in primary melanomas and associated naevi: implications for mutation testing of primary melanomas

There is concern that BRAF mutant naevus cells admixed with melanoma cells could cause false positive mutation tests in BRAF wild-type melanomas. We sought to assess the frequency of BRAF^V600E mutations in primary melanomas arising with/without associated naevi and determine BRAF^V600E concordance between melanomas and associated naevi. Formalin fixed, paraffin embedded (FFPE) tissue from 57 patients with primary melanomas with/without associated naevi was immunohistochemically stained to detect BRAF ^V600E mutation. In a subset of patients (n=29), molecular mutation testing was also carried out using a panel of 238 known genetic variants. Of the primary melanomas with an associated naevus (n=29), 55% were BRAF ^V600E mutant with 100% concordance between the melanoma and associated naevus. In contrast, only 21% of the primary melanomas unassociated with naevi were BRAF^V600E mutant (p=0.009). Our results suggest that melanomas with associated naevi have a higher frequency of BRAF^V600E mutations than melanomas unassociated with naevi. Furthermore, melanomas and their associated naevi were concordant in BRAF^V600E status, which suggests that false positive mutation tests occurring as a consequence of admixed BRAF mutant naevus cells in BRAF wild-type primary melanomas are unlikely to be a problem in clinical practice. The findings have important implications for adjuvant clinical trials of targeted therapies.