Human Equilibrative Nucleoside Transporter-3 (hENT3) Spectrum Disorder Mutations Impair Nucleoside Transport, Protein Localization, and Stability

Abstract

Accumulating evidence reveals that sole mutations in hENT3 cause a spectrum of human genetic disorders. Among these include H syndrome, characterized by scleroderma, hyperpigmentation, hypertrichosis, hepatomegaly, cardiac abnormalities and musculoskeletal deformities, pigmented hypertrichotic dermatosis with insulin-dependent diabetes syndrome, characterized by autoantibody-negative diabetes mellitus and skin deformities, familial Rosai-Dorfman disease, characterized by short stature, familial histiocytosis and sinus histiocytosis with massive lymphadenopathy (SHML), characterized by severe tissue infiltration of immune cells and swollen lymph nodes. hENT3 spectrum disorders share a common mutation and share overlapping clinical manifestations that display many intriguing resemblances to mitochondrial and lysosomal disorders. Although earlier studies identify hENT3 as a mitochondrial and a lysosomal nucleoside transporter, the precise connections between hENT3 and the pathophysiology of these disorders remain unresolved. In this study, we performed functional and biochemical characterization of these mutations in hENT3. We report severe reductions/losses of hENT3 nucleoside transport functions of hENT3 syndrome mutants. In addition to transport alterations, we provide evidence for possible loss of hENT3 functions in all H and pigmented hypertrichotic dermatosis with insulin-dependent diabetes syndromes due to either mistrafficking or altered stability of mutant hENT3 proteins.H syndrome and pigmented hypertrichotic dermatosis with insulin-dependent diabetes (PHID) 3 syndrome are recently described autosomal-recessive genetic disorders in humans (1, 2). H syndrome patients display symptoms such as cutaneous hyperpigmentation, hypertrichosis, heart anomalities, hearing loss, hepatomegaly hypogonadism, and etc. The SLC29A3 gene encodes human equilibrative nucleoside transporter 3 (hENT3), a member of a largely conserved group of solute carrier (SLC) transporters called the ENT or SLC29 family (4, 5). These facilitative transporters mediate salvage of hydrophilic nucleosides as well as nucleoside analogs used in the treatment of cancers and viral diseases (4, 5). In comparison with the other human ENT members, hENT3 is unique in that it functions intracellularly with maximal activity at an acidic pH range of 5.5-6.5 (6, 7). Whereas Baldwin et al. (6) reported that hENT3 is localized partially in the late endosomes and lysosomes, our recent studies indicate that hENT3 is also localized in the mitochondria (7). Low pH transport properties and subcellular localization of hENT3 in lysosomes and in mitochondria suggest that hENT3 possibly transports nucleosides from the inside of the lysosomes to the cytoplasm (6) as well as across the inner mitochondrial membrane (7). Although these data indicate that hENT3 is likely to perform physiological functions relating to lysosomes and mitochondria, direct evidence linking hENT3 to these organelle functions has not yet been established.In a study involving 10 families affecte...

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