Melanocytic tumour in a black sheep never exposed to ultraviolet radiation

A slow growing skin tumour was identified on the head of a 6-year-old male Serra da Estrela black sheep. The animal had no previous history of exposure to ultraviolet radiation or illness. The tumour consisted of an irregular mass subdivided into two polypoid regions and there were small alternating pigmented and non-pigmented areas in the surrounding epidermis. Microscopical and immunohistochemical features were consistent with a melanocytic tumour of the melanocytoma type, without signs of vascular or perineural invasion. The tumour cells contained pigment stained by the Masson Fontana reaction, expressed S100 protein and vimentin and displayed a low proliferative rate (Ki67 labelling <1%). No metastases were found at the time of gross necropsy examination. Analyses of the homologous regions of the hot spot mutational exons of BRAF and NRAS (the genes that are most often mutated in human melanocytic tumours) did not reveal alterations, but there were silent polymorphic variations in these genes. No such variation was observed in the GNAQ gene sequence that is mutated in human melanocytomas.