Equine Melanocytic Tumors: A Narrative Review

SIMPLE SUMMARY: Melanocytic tumors are among the most common cutaneous neoplasms in horses. Contrary to popular opinion, the slow rate of growth typical of these tumors does not warrant benign classification. Furthermore, the wide spectrum of treatment modalities can generate some frustration for equine practitioners. This narrative review aims to give an overview of the genetic basis, linked with melanoma development, of alterations in STX17, ASIP and MITF genes, a description of the clinical and pathological differences between naevus, dermal melanoma, dermal melanomatosis and anaplastic malignant melanoma, as well as a description of the role of the different diagnostic tools, mainly fine needle aspiration, histopathology and immunohistochemistry. The most common treatment modalities, such as surgery, chemotherapy and electroporation are reviewed, and a description of less common options, such as immunotherapy and radiotherapy is presented. The article also elucidates future perspectives and research fields about equine melanocytic tumors, mainly in the search for new therapeutical targets, such as CD47, PD-1 and COX-2. ABSTRACT: Adult grey horses have a high incidence of melanocytic tumors. This article narratively reviews the role of some genetic features related to melanoma formation in horses, such as STX17 mutation, ASIP or MITF alterations, and the link between the graying process and the development of these tumors. A clear system of clinical and pathological classification of melanocytic tumors in naevus, dermal melanoma, dermal melanomatosis and anaplastic malignant melanoma is provided. Clinical and laboratorial methods of diagnosing are listed, with fine needle aspiration and histopathology being the most relevant. Relevance is given to immunohistochemistry, describing potentially important diagnostic biomarkers such as RACK1 and PNL2. Different therapeutical options available for equine practitioners are mentioned, with surgery, chemotherapy and electroporation being the most common. This article also elucidatesnew fields of research, perspectives, and new therapeutic targets, such as CD47, PD-1 and COX-2 biomarkers.