Lack of distinct molecular profile of Primary Dermal Melanoma

Primary dermal melanoma (PDM) is a rare variant of melanoma which simulates metastatic melanoma to the skin. Diagnosis of PDM cannot be established on histologic grounds alone but requires absence of evidence of melanoma elsewhere by clinical history and/or imaging studies. Despite this entity being clinically well documented, limited data on molecular characterization are available. We performed comprehensive mutation and copy number variation analysis in a series of PDMs in search for distinctive molecular features.Studies were approved by respective institutional review boards. Six cases fulfilling strict histologic criteria of PDM were identified in patients with absent history of melanoma elsewhere, negative sentinel lymph node biopsies and imaging studies. DNA was extracted from formalin-fixed, paraffin-embedded (FFPE) tissue, and five cases passed quality control measures and were subjected to targeted exon sequencing using the Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets 410 panel. Two of five PDM carried NRAS hotspot mutations characteristic of cutaneous melanoma (CM) genomic subtypes. One case showed a probable low-activating NRAS mutation in combination with additional aberrations in other mitogen-activated protein kinase (MAPK) pathway effectors. Hotspot mutations were also identified in the TERT promoter region, and one tumor showed a mutation in the SWI/SNF remodeling complex. No oncogenic mutations were identified in one case. Furthermore, none of the tumors analyzed showed activating mutations in Gα subunits, including GNAQ and GNA11. Copy number alterations included deletions of Chr 9p, characteristic of CM.Despite PDM showing mutational heterogeneity, our findings suggest predominance of MAPK pathway aberrations in agreement with the mutational profile of CMs in general. Given the absence of genetic overlap with other distinct primary dermal melanocytic proliferations, mutational profiling will unlikely aid in the difficult differential diagnosis of PDM versus melanoma metastasis. Thorough metastatic workup remains crucial in establishing this rare diagnosis.