Clinical, Morphologic, and Genomic Findings in Spitz Tumors With RET Fusion: A Series of 31 Cases

RET-fused Spitz neoplasms represent a rare and poorly characterized category of Spitz tumors. Here we describe the clinical, histologic, and molecular findings of 31 Spitz neoplasms with RET fusion diagnosed as Spitz nevus (n = 16), atypical Spitz tumors (n = 13), and Spitz melanoma (n = 2). The lesions mainly occurred in children and young adults of both sexes with a predilection for the extremities. Microscopically, they were mainly symmetrical compound melanocytic neoplasms with a dome-shaped/slightly raised silhouette predominantly composed of epithelioid, spindled, and/or smaller nevoid melanocytes arranged in confluent nests. Dyscohesive melanocytes within the nests in the upper part of the lesions, prominent Kamino bodies, giant multinucleated melanocytes, variable pigmentation, and increased vascularity with vascular ectasia were frequent features. RNA sequencing detected 9 different 5’ (N-terminus) fusion partners, including KIF5B (n = 8), LMNA (n = 7), CCDC6 (n = 6), OPTN (n = 3), MYO5A (n = 2), and NCOA4, ERC1, MYH9, AGAP3 (n = 1). Of these, OPTN::RET and AGAP3::RET represent novel fusions, and 3 further 5’ fusion partners, namely NCOA4, ERC1, and MYH9, have never been reported in Spitz tumors. Although as a whole group, the tumors showed a heterogeneous histopathologic presentation, correlation of the morphologic features and the 5' fusion partners demonstrated certain associations. Nevoid melanocytes were exclusively encountered in cases with KIF5B fusion partner. Neuroid-like appearances with intersecting fascicles of spindled cells typified both MYO5A-fused cases. Epithelioid melanocyte population dominated cases with LMNA and CCDC6 fusion partners. Transepidermal elimination/floating intraepidermal nests of pigmented spindled and epithelioid melanocytes were observed in the OPTN subgroup. The remaining cases with less frequent 5’ fusion partners manifested in general more atypical histopathologic features, including nuclear pleomorphism, high mitotic count, atypical mitoses, and sheet-like growth pattern. Melanoma fluorescence in situ hybridization probe kit targeting RREB1, MYC, CDKN2A, and CCND1, was negative for copy number variation in 4 cases tested, including 2 cases with complete p16 nuclear loss on immunohistochemistry. Array comparative genomic hybridization was performed in 3 lesions and detected numerous segmental chromosomal imbalances in 2 of them that were diagnosed as Spitz melanoma. DNA and RNA sequencing detected several further genomic alterations, including POU2F3 overexpression in 3 highly pigmented lesions. Further studies are needed to confirm possible correlations between the microscopic features and a particular fusion partner (or additional genetic events) in RET-fused Spitz neoplasms.