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Linear keratinocytic epidermal nevi on trunk skin caused by a somatic FGFR2 p.C382R mutation

Dear Editor, Non-epidermolytic keratinocytic epidermal nevi (KEN) are caused by somatic mutations in genes associated with the RAS/mitogen-activated protein kinase (MAPK) pathway or phosphoinositide-3 kinase/AKT pathway. Several particular mutations of KRAS have been reported to cause sebaceous nevi on the face and scalp but KEN on the trunk. A 3-year-old girl exhibited brown papules on her right lower extremity at birth and later developed pebbly brownish plaques scattered along the lines of Blaschko in two regions: one distributed from the right axilla to the right side of the trunk and the other from the right groin to the right thigh (Fig. 1a,b). Several yellowish smooth papules were evident on the right ring and little fingers (Fig. 1c). No extracutaneous sign was evident. Biopsy of the right axillary lesion revealed marked papillomatous acanthosis with rete ridge elongation but without hyperkeratosis or granular degeneration (Fig. 1d,e). Examination of multiple step sections revealed that the lesions contained several small pseudohorn cysts within the epidermis but lacked immature follicles, sebaceous glands and large keratinous cysts (Fig. 1d,e). We obtained written informed consent from her parents according to the guidelines of the institutional review board of the Keio University School of Medicine, and performed nextgeneration sequencing using custom targeted exome sequencing panels of the HaloPlex Target Enrichment System (Agilent Technologies, Santa Clara, CA, USA) for genodermatoses. The target resequencing library was constructed from genomic DNA of the lesional epidermis separated from dermis via ex vivo dispase treatment of biopsied specimen. We identified a missense mutation in FGFR2 (NM_000141.4: c.1144T>C [p.C382R]) and several common variations in genes associated with KEN (Table S1). Sanger sequencing confirmed that the FGFR2 c.1144T>C mutation was present in the lesional epidermis but not in blood leukocytes (Fig. 1f); the other genetic variations were evident in both tissues (Table S1). Therefore, we diagnosed unilateral KEN caused by a somatic FGFR2 p.C382R mutation. The FGFR2 p.C382R mutation, which is suspected as an activating mutation, has been reported in papillomatous, pedunculated sebaceous nevi on the face and the scalp in two cases. Our case showed a partly resembling clinicopathological appearance, namely a pebbly appearance and a lack of hyperkeratosis, but lacked sebaceous differentiation. Such phenotypic differences associated with identical mutations at different body sites have also been reported in KRAS p.G12C and p.G12D mutations, which cause sebaceous nevi on the head but KEN on the trunk. Our case and previous cases suggest that FGFR2 p.C382R mutation triggers the development of sebaceous nevi on the head but KEN on the trunk, while another FGFR2 mutation of p.Y376C causes KEN both on the head and trunk (Table S2). Congenital FGFR2 p.Y376C