NRASQ61R‐driven atypical melanocytic tumor with blue nevus‐like morphology: A case report

Abstract

AbstractNRAS Q61 mutations are driver genetic alterations associated with common melanocytic nevi. Herein, we describe a case of NRAS‐mutant melanocytic tumor with a blue nevus‐like morphology. A 71‐year‐old Japanese man presented with a 4.6‐mm nodule on his back. Histopathological examination revealed a dense distribution of spindle‐shaped melanocytes in the upper dermis and a sparse distribution of dendritic melanocytes in the mid‐dermis. The vertical periadnexal extension reached the deep dermis at the center of the tumor. A small junctional component, hyperpigmentation, sclerotic stroma, mild nuclear atypia, and a few mitotic figures were observed. Immunohistochemical examination revealed no PRAME expression and preserved p16 expression. Diffuse RASQ61R immunoreactivity was observed in these tumor cells. Nuclear β‐catenin expression was not observed. Targeted RNA sequencing revealed two mutations, NRAS c.182A>G (Q61R) and FGFR2 c.‐157A>G, but no other pathogenic alterations such as BRAF, GNAQ, GNA11, CTNNB1, PRKAR1A, or IDH1 mutations or kinase gene fusions. The histopathology fits that of compound‐type blue nevus, which is called “Kamino nevus”; however, this tumor was genetically considered to be on the spectrum of conventional acquired melanocytic nevi but not on that of blue nevi. Morphologically, NRAS‐driven melanocytic nevi resemble blue nevi without IDH1R132C coexistence.