Activating kinase fusions have recently been described as early oncogenic events that are mutually exclusive with HRAS and BRAF mutations in Spitz tumors. Here, we report a series of 32 Spitz tumors with ALK fusions (6 Spitz nevi, 22 atypical Spitz tumors, and 4 Spitzoid melanomas) in patients ranging from 5 months to 64 years (median=12 y) of age. The tumors typically presented as exophytic papules on the extremities and were occasionally darkly pigmented. In addition to ALK fusions previously described in other tumor types (NPM1-ALK, TPR-ALK) we identified 2 novel ALK fusions (CLIP1-ALK and GTF3C2-ALK) in our cohort of Spitz tumors. Array comparative genomic hybridization of 19 of these tumors demonstrated a high frequency of chromosome 2 aberrations (where ALK resides, 63%) and chromosome 1p loss in 37% of the cases. Spitz tumors with ALK fusions demonstrated unique histopathologic features. Clefts and small vesicle-like spaces were arrayed between plump spindled melanocytes with fibrillar cytoplasm and enlarged nuclei. These melanocytes were typically arrayed in elongated and fusiform nests with radial orientation. The tumors often had extension into the dermis or subcutis with a wedge-shaped or bulbous lower border (45% and 17% respectively). An infiltrative growth pattern was often present at the periphery of the tumor and was highlighted by ALK immunohistochemistry. In conclusion, Spitz tumors with ALK rearrangement show distinct histopathologic features that should aid in improving classification of these diagnostically challenging tumors.

Oncogenic gene fusions have been identified in many cancers and many serve as biomarkers or targets for therapy. Here we identify six different melanocytic tumors with genomic rearrangements of MET fusing the kinase domain of MET in-frame to six different N-terminal partners. These tumors lack activating mutations in other established melanoma oncogenes. We functionally characterize two of the identified fusion proteins (TRIM4-MET and ZKSCAN1-MET) and find that they constitutively activate the mitogen-activated protein kinase (MAPK), phosphoinositol-3 kinase (PI3K), and phospholipase C gamma 1 (PLCγ1) pathways. The MET inhibitors cabozantinib (FDA-approved for progressive medullary thyroid cancer) and PF-04217903 block their activity at nanomolar concentrations. MET fusion kinases thus provide a potential therapeutic target for a rare subset of melanoma for which currently no targeted therapeutic options currently exist.

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A cutaneous melanocytic tumor with morphologic overlap with clear cell sarcoma, but defined by CRTC1-TRIM11 gene fusion, was recently described in a series of five adult patients. Here, we expand the clinicopathologic features of this entity by four additional cases which include pediatric presentation, exophytic growth, and propensity to occur on the head. Patients (2F; 2M) had a median age of 41 years (range 11-59). Sites of involvement included leg, ear, and face. Tumors were circumscribed, unencapsulated, mostly limited to the dermis, and varied from 5 to 35 mm. One case was exophytic. Lesional cells were arranged in nests and fascicles, and were monomorphic and fusiform with moderate pale to clear cytoplasm, occasional nuclear pseudo-inclusions, and small to prominent nucleoli. Mitotic rate was variable (rare to 12/10 HPF, median 3/10 HPF). The pediatric case showed increased nuclear pleomorphism, tumor necrosis, and mitotic figures. All cases showed strong, diffuse nuclear staining for SOX10, but were negative or focal for S100 protein, HMB45 and Melan-A expression. Cases were positive by FISH technique and/or RNA sequencing for a TRIM11 rearrangement/fusion, and negative for EWSR1 rearrangement. This series is presented to aid in further characterization of this novel melanocytic tumor. K E Y W O R D Sclear cell sarcoma, CRTC1-TRIM11, melanocytoma, melanoma, translocation