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© 2015 The Authors. doi: 10.2340/00015555-2145 Journal Compilation © 2015 Acta Dermato-Venereologica. ISSN 0001-5555 H syndrome (OMIM #602783) is an autosomal recessive genodermatosis, first described in 2008 (1). It has multisystemic involvement and is characterized by numerous clinical features, including cutaneous hyperpigmentation and hypertrichosis, hepatosplenomegaly, hearing loss, heart anomalies, hypogonadism, low height (short stature), hyperglycaemia (insulin-dependent diabetes mellitus), and hallux valgus/flexion contractures. Cutaneous hyperpigmentation, commonly accompanied by hypertrichosis and progressive sclerodermatous induration, typically affecting initially the medial thighs with sparing of the knees, is the hallmark of the disorder and is considered pathognomonic (1–3). Causative mutations have been identified in the SLC29A3 gene encoding the human equilibrative nucleoside transporter 3 (hENT3), which serves as a transporter of nucleosides in the late endosome/lysosomes, and across the inner mitochondrial membrane (4). Although numerous mutations have been reported since the initial description (3, 5–8), a solid genotype–phenotype correlation is lacking (3). Increased recognition of the disorder has led to the identification of approximately 100 patients with H syndrome worldwide (9). Novel clinical features have been associated with this condition, such as lymphadenopathy, pancreatic exocrine deficiency, and recurrent febrile episodes (3). Recently, complete agenesis of the inferior vena cava (IVC) was reported in a patient from Turkey who also had striking varicose veins (8). Oligosymptomatic patients with a very mild phenotype have also been described (2, 3, 7). Interestingly, patients with variable degrees of involvement have been described from the same family or geographical region with similar or even identical genetic mutations, reflecting the pleiotropic nature of this unique disorder (3, 6, 7). Histopathologically, H syndrome may simulate Rosai-Dorfman disease, displaying CD68+ S100+ CD1a– histiocytes with emperipolesis (3, 10, 11). Overall, H syndrome may be regarded as an inherited histiocytic disorder based on demonstration of increased CD68+ histiocytes in the skin and lymph nodes, suggesting a possible role of hENT3 in histiocyte turnover. This paper highlights the pleiotropic nature of H syndrome by reporting 2 unrelated Turkish patients from a single institution who share a common mutation in the SLC29A3 gene but demonstrate different phenotypes.

H Syndrome: A Multifaceted Histiocytic Disorder with Hyperpigmentation and Hypertrichosis