Expression of Melan-A (MART-1) in Various Pigmented Melanocytic Nevi and Close Mimickers

Introduction: Melanocytic nevi are neoplasms resulting from the proliferation of melanocytes. Diagnosis of melanocytic tumours can be tricky, considering two factors,to diagnose origin and determination of its benign or malignant nature. Visualisation of melanin or other pigments are non specific with Hematoxylin and Eosin (H&E) staining and specific with Melan-A Immunohistochemistry (IHC). Intensity and pattern of these reactions shows marked variability in different melanocytic lesions. Aim: To study the intensity and pattern of Melan-A expression in various pigmented melanocytic nevi and to understand its utility to differentiate close mimickers. Materials and Methods: A cross-sectional study of 50 lesions (45 cases and 5 controls) was conducted in the Department of Pathology of Dr. B.R. Ambedkar Medical College (tertiary hospital), Bengaluru, Karnataka, India, between September 2020 to October 2021. The skin biopsies received were fixed in formalin and paraffin embedded. Sections were stained with H&E and Melan-A IHC marker, using A103 antibody and a high temperature antigen retrieval was performed. Pattern and intensity of Melan-A expression were studied and its evaluation was done with normal skin (control). Results: Melan-A showed varied intensity (+ weak; ++ moderate; +++ strong), pattern (patchy, diffuse), distribution and on various pigmented melanocytic lesions and mimickers were analysed. Dermal nevi (18) showed +++ intensity, diffuse pattern in dermis and two cases showed ++ intensity and patchy staining. Five cases of Compound nevi showed ++ intensity, diffuse pattern in Dermoepidermal Junction (DEJ) and dermis, Deep penetrating nevi (in one case) +++ intensity and diffuse pattern in DEJ, Dermis. Two cases of pigmented Basal Cell Carcinoma (BCC) showed + intensity and no definitive pattern in dermis. Four cases in fibrohistiocytic lesions and Malignant Peripheral Nerve Sheath Tumour (MPNST) shows negative Melan-A. In control melanocytes show dendritic pattern of staining and melanophages were negative. Conclusion: Melan-A is specific melanocytic marker as it stains only melanocytic lineage and no other cell types in background thus it is a marker of histogenesis instead of malignancy indicator. It highlights important architectural features and confirms the origin of lesions thus aiding the pathologist towards accurate diagnosis and differentiates from close mimickers.