CASE REPORT A diagnostically-challenging case of melanoma ex blue nevus with comprehensive molecular analysis, including the 23-gene expression signature (myPath melanoma) Stephanie A. Castillo 1 | Anh K. Pham 2 | Dorothea T. Barton 1,2 | Joel A. Lefferts 1,3 | Shaofeng Yan 1,3 | Julia A. Bridge 4 | Konstantinos Linos 1,3 1 Geisel School of Medicine at Dartmouth, Hanover, New Hampshire 2 Section of Dermatology, Department of Surgery, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire 3 Department of Pathology and Laboratory Medicine, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire 4 Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, Nebraska Correspondence Konstantinos Linos, MD, Department of Pathology and Laboratory Medicine, Dartmouth-Hitchcock Medical Center, 1 Medical Center Drive, Lebanon, NH 03756 Email: konstantinos.linos@hitchcock.org Melanoma ex blue nevus (MEBN) is a rare, aggressive, and potentially lethal neoplasm. Distin- guishing MEBN from an atypical cellular blue nevus can be very challenging. We report a diag- nostically difficult case of MEBN with lymph node metastases, in which single nucleotide polymorphism array and fluorescence in situ hybridization were used to arrive at the correct diagnosis. It was also analyzed by the recently-introduced proprietary 23-gene expression signa- ture test. To the best of our knowledge, this is the second reported case of MEBN analyzed by the 23-gene expression signature, which provided a false-negative result. More studies are needed to assess the sensitivity and specificity of this test in various melanocytic proliferations. KEYWORDS malignant blue nevus, melanoma, melanoma FISH, Myriad, SNP array 1 | INTRODUCTION Melanoma ex blue nevus (MEBN) is a rare and highly aggressive neo- plasm, with approximately 150 cases reported in the literature. 1 Dis- tinguishing MEBN from an atypical cellular blue nevus (ACBN) can be very challenging, even amongst experts. 2,3 We describe a diagnosti- cally difficult case of MEBN with lymph node metastases, in which an array of molecular studies was used to arrive at the correct diagnosis. We also present the second reported application of the proprietary 23-gene expression signature test (myPath Melanoma, Myriad Genetics Inc., Salt Lake City, Utah) in analyzing MEBN. 2 | CASE REPORT A 39-year-old male presented to the dermatology clinic with a long- standing, asymptomatic, 7 mm firm, tan-pink papule on the left parie- tal scalp (Figure 1). Dermoscopy showed an asymmetric peripheral pigment network and an atypical amorphous center. On biopsy, there was a subtle biphasic proliferation (Figure 2A). At the periphery were fascicles of bland ovoid amelanotic melanocytes within a fibrotic stroma, consistent with a typical cellular blue nevus (Figure 2B). At the center, there was a hypercellular proliferation of spindled to epithelioid hyperchromatic melanocytes with conspicuous nucleoli (Figure 2C). There were ≤2 mitotic figures per mm 2 and probable perineural invasion, but no necrosis (Figure 2C, inset). Gene sequencing using the Ion AmpliSeq Cancer Hotspot Panel v2 for 50 genes detected a GNAQ exon 5 (c.626A>T, Q209L) muta- tion consistent with a melanocytic proliferation in the “blue nevus” category. Immunohistochemical staining (IHC) showed retention of p16 and BAP1 expression (Figure 2E,F). Because of the small size of the lesion, the entire proliferation was marked for macrodissection and submitted for single-nucleotide polymorphism (SNP) array analysis (OncoScan FFPE Assay Kit, Affymetrix, Santa Clara, California). This identified multiple genomic aberrations including gains in Xq, 1q, 17q and losses in 1p, 3p, 6q, and 18p (Figure 3D). Gain of 8q was equivocal. Fluorescent in situ hybridization (FISH) conducted with a four-color probe set consisting of CCND1 (11q13), RREB1 (6p25), MYB (6q23), Received: 10 August 2018 Revised: 28 October 2018 Accepted: 26 November 2018 DOI: 10.1111/cup.13400 © 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd. J Cutan Pathol. 2018;1–5. wileyonlinelibrary.com/journal/cup 1