Subclinical Fields of BRAF V600E -Mutant Melanocytes Populate Human Skin and Are Enriched Around Melanoma and Naevi

Abstract Background The origins of cutaneous melanoma are often traced to visible precursor lesions (e.g., melanocytic naevi), but ∼2/3 melanomas arise from clinically normal skin with no precursor lesion. BRAF V600E drives both melanoma and naevus formation, but eruptive naevi syndromes and the disproportionate role early-life UV exposure plays in melanoma risk suggest the presence of a hidden field of oncogene-mutant melanocytes within clinically normal skin, which may represent an earlier step in tumourigenesis. Objectives We aimed to determine whether BRAF V600E-mutant melanocytes were commonly present in non-lesional skin from adults at high risk of melanoma. Methods We examined BRAF V600E mutation in 97 histologically and clinically normal, non-lesional skin samples from a high-risk Australian melanoma cohort. The skin selected was adjacent to a naevus or a prior melanoma site, photodamaged skin proximal (∼5cm) from melanoma excision, and photoprotected skin distant from lesions, as well as a low-risk neonatal foreskin-derived melanoblasts. We used immunohistochemistry to locate BRAF V600E-mutant cells in histopathology sections, droplet digital PCR to determine the fractional abundance of BRAF V600E in whole skin, and single-cell RNA sequencing to confirm cells as melanocytes and detect their transcriptional programs. Results We identified BRAF V600E-mutant melanocytes in skin surrounding naevi and primary melanomas, even years after tumour excision. Fields of BRAF V600E-mutant melanocytes are commonly observed in the skin of patients at high risk for melanoma and are up to 20-fold more dense and 50% more frequent in tumour-adjacent skin. Field cells exhibit a gene expression profile characteristic of BRAF V600E-induced growth arrest, consistent with dormant yet genetically primed cells. Conclusions There is a reservoir of oncogene-harbouring melanocytes in normal skin. Our observations challenge the prevailing notion that melanocyte expression of BRAF V600E is inherently tumourigenic. The presence of such fields suggests that the scope of melanoma detection and prevention efforts might be extended beyond visible lesions, to encompass potentially precancerous fields of driver-mutant cells.