A case of indeterminate cell histiocytosis with <i>ETV3</i>‐<i>NCOA2</i> translocation

Indeterminate cell histiocytosis (ICH) is a rare histiocytic disorder characterized by a proliferation of CD1a⁺ and CD207/langerin^- cells. Recent molecular analyses have identified ETV3-NCOA2 translocation as a possible aetiopathogenesis of ICH. Herein, we describe the first Japanese case of ICH with ETV3-NCOA2 translocation. A 79-year-old Japanese man presented with a 1-year history of pruritic erythematous papules and nodules on his trunk and extremities. Histological examination revealed a dense and diffuse sheets-like infiltration of medium-sized histiocyte-like cells from the epidermis to the deep dermis. Immunohistochemically, the atypical cells were positive for CD1a but negative for CD207/langerin. Fluorescence in situ hybridization using NCOA2 break-apart probes confirmed a chromosomal break occurring on NCOA2 monoallele in the tumor cells. Furthermore, ETV3 exon 4-NCOA2 exon 14 translocation was identified in formalin-fixed paraffin-embedded skin samples using reverse transcription polymerase chain reaction and subsequent direct DNA sequencing. He also presented with interspersed eczematous plaques on his trunk and reactive dermatopathic lymphoadenopathy without any infiltration of ICH. He was treated with topical corticosteroids and narrowband UVB phototherapy. Four months later, his ICH skin eruptions, eczematous plaques, and lymphoadenopathy gradually regressed. Our case supports the notion that the detection of ETV3-NCOA2 translocation can be useful for diagnosis of ICH.