Clonality of basal cell carcinoma arising in an epidermal nevus. New insights provided by molecular analysis.

Non-organoid non-epidermolytic (keratinocytic) epidermal nevi are benign congenital skin lesions following the lines of Blaschko. The occurrence of epithelial tumors in epidermal nevi of this type is very rare, and it remains elusive whether these lesions are clonally related or represent a simple collision of independent tumors. We report a case of a patient who developed 2 basal cell carcinomas in an extensive epidermal nevus on the trunk. Molecular analysis of a biopsy containing both basal cell carcinoma and epidermal nevus revealed a PIK3CA mutation. Further analysis suggested that this mutation is present in the basal cell carcinoma rather than in the epidermal nevus. The results favour an independent origin of the basal cell carcinoma, although a possible clonal relationship between both lesions cannot be completely excluded on the basis of the molecular genetic findings.