The clinicopathologic spectrum and genomic landscape of de-/trans-differentiated melanoma

De- and trans-differentiation is a rare and only poorly understood phenomenon in cutaneous melanoma. To study this disease more comprehensively we have retrieved 11 primary cutaneous melanomas from our pathology archives showing biphasic features characterized by a conventional melanoma and additional areas of de-/trans-differentiation as defined by a lack of immunohistochemical expression of all conventional melanocytic markers (S-100 protein, SOX10, Melan-A and HMB-45). The clinical, histologic and immunohistochemical findings were recorded and follow-up was obtained. The patients were mostly elderly (median: 81 years; range: 42-86 years) without significant gender predilection, and the sun-exposed skin of the head and neck area was most commonly affected. The tumors were deeply invasive with a mean tumor thickness of 7 mm (range: 4-80 mm). The dedifferentiated component showed atypical fibroxanthoma-like features in the majority (7), while additional rhabdomyosarcomatous and epithelial transdifferentiation was noted histologically and/or immunohistochemically in two tumors each. The background conventional melanoma component was of desmoplastic (4), superficial spreading (3), nodular (2), lentigo maligna (1) or spindle cell (1) types. For the 7 patients with available follow-up data (median follow-up period of 25 months; range: 8-36 months), 2 died from their disease and 3 developed metastases. Next-generation sequencing of the cohort revealed somatic mutation of established melanoma drivers including mainly NF1 mutations in the conventional component (5 cases), which were also detected in the corresponding de-/trans-differentiated components. In summary, the diagnosis of de-/trans-differentiated melanoma is challenging and depends on the morphologic identification of the conventional melanoma component. Molecular analysis is diagnostically helpful as the mutated gene profile is shared between the conventional and de-/trans-differentiated components. Importantly, de-/trans-differentiation does not appear to confer a more aggressive behavior.

• 18 samples
• DAC: EGAC00001000205