Cutaneous Melanocytoma With CRTC1-TRIM11 Fusion: Report of 5 Cases Resembling Clear Cell Sarcoma.

We report 5 cases of primary intradermal nodular unpigmented tumors with a melanocytic immunophenotype associated with a novel CRTC1-TRIM11 fusion. Clinically, the cutaneous nodules were slowly growing in 3 women and 2 men (25 to 82 y old, median, 28 y) with no specific topography. Lesion size ranged from 4 to 12 mm (median, 5 mm). The tumors were strictly located in the dermis with a nodular pattern. The cells were arranged in confluent nests and fascicules. Central fibronecrotic areas were present in 2 cases. Cells were medium to large, sometimes multinucleated, and presented a spindled and epithelioid cytology with prominent nucleoli. Cytonuclear atypia was constant, and mitotic activity in hotspot areas ranged from 1 to 5/mm². Immunohistochemistry found a constant positivity with S100, MiTF, and Sox10, and a heterogenous staining by MelanA or HMB45. NTRK1 was strongly positive in 3 cases. In all cases, RNA sequencing found an invariable CRTC1(e1)-TRIM11(e2) fusion, confirmed by fluorescent in situ hybridization techniques with a TRIM11 break-apart probe. In 4/4 cases, nuclear TRIM11 expression was positive by immunohistochemistry. Fluorescent in situ hybridization techniques showed no rearrangement of NTRK1 or EWSR1, and array-comparative genomic hybridization displayed no alteration (1 case) or only a whole chromosome 7 gain (2 cases) when performed. No relapse or metastatic event was observed during follow-up [3 to 72 months (median, 14 mo)]. Cutaneous clear cell sarcoma was the main differential diagnosis. Overlapping morphologic features previously described in primary dermal melanomas and paraganglioma-like melanocytic tumors were present. The CRTC1-TRIM11 fusion appears to be specific of an unpigmented nodular tumor combining a melanocytic phenotype and low-grade tumor behavior.