Animal-type melanoma: A mini-review concerning with one of the rarest variants of human melanoma

Luca Roncati, Francesco Piscioli


Described for centuries in the equines, especially gray horses, under the terminology of ‘equine melanotic disease’ and, subsequently recognized in non-equine animal models and in humans, particularly on non UV-exposed skin, animal-type melanoma, also known as pigmented epithelioid melanocytoma (PEM), is characterized by nodules and fascicles of epithelioid transformed melanocytes with pleomorphic nuclei and striking pigmentation, dendritic cells, numerous melanophages and, sometimes, lymphocytic infiltrate. Up today, only small series have been reported in humans and, therefore, its biological behavior remains unclear. Recently, some authors have supported that the tumor follows an indolent clinical course, with very low risk of spread beyond regional lymph nodes. Given the complexity of the matter, Elder and Murphy proposed a histological categorization of PEM and PEM-like lesions in: epithelioid blue nevus resembling PEM, strongly associated with the Carney complex; PEM, not associated with the Carney complex; tumoral melanosis mimicking PEM. A proper diagnostic framing is crucial in these controversial cases and a good histology in the hands of an expert dermatopathologist remains the most reliable diagnostic starting point. Moreover, a loss of expression of cAMP-dependent protein kinase type I-alpha regulatory subunit, an enzyme encoded by the tumor-suppressor gene PRKAR1A, has been found in PEM, but not in common melanoma or other melanocytic lesions. Therefore, it appears to have a great diagnostic value in helping to distinguish PEM from PEM-like lesions, which mimic the former histologically.



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