<i>NRAS</i>^Q61R‐driven atypical melanocytic tumor with blue nevus‐like morphology: A case report

NRAS 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 IDH1^R132C coexistence.