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Case Reports

Abstract

A 12-year-old girl presented with a solitary 3 mm hyperpigmented, mildly raised, firm lesion on the interdental gingiva, present for 5 months, with occasional bleeding, and recent growth.  An excisional biopsy revealed a pigmented compound melanocytic lesion within a fibrous stroma. Superficial nevomelanocytes were enlarged without definitive high-grade nuclear atypia. Deeper cells were smaller and showed maturation. No pagetosis was seen. However, there was marked inflammation and focal scar tissue. Immunohistochemistry showed retained p16, negative PRAME, HMB-45 reactivity in the superficial component, and negative BRAF V600E, supporting a cytologically atypical mucosal melanocytic proliferation. Given the uncertain malignant potential of oral nevi, molecular evaluation was performed. Genomic profiling revealed a KIF5B-RET fusion, RET-UBE2N rearrangement, and TP53 rearrangement. RET fusions occur in 3-4% of Spitz neoplasms, with KIF5B among the most common partners, and are associated with dome-shaped, symmetrical compound melanocytic proliferations with epithelioid, spindled, and/or small nevoid melanocytes in confluent nests. While RET fusions are found in Spitz neoplasms, the significance of RET-UBE2N rearrangement is not well-studied in this context. TP53 mutations are rare in melanocytic neoplasms. In this case, it may represent a germline polymorphism or potentially a pathogenic, tumor-specific somatic mutation. Intraoral Spitz nevus is extremely rare with very limited data on morphology, molecular landscape, or behavior. To our knowledge, no prior reports describe molecular findings of oral Spitz. This case expands the limited molecular data and highlights the potential diagnostic and pathogenetic significance of RET fusions/rearrangements and TP53 rearrangements in these rare lesions.