Mutations exclusively in SLC29A3, which encodes the lysosomal transporter termed equilibrative
nucleoside transporter-3 (ENT3), cause an expanding spectrum of human genetic disorders, such as H
syndrome, PHID syndrome, RDD syndrome, SHML syndrome, dysosteosclerosis etc. They share common
mutations and overlapping clinical manifestations with anemia, erythroid hypoplasia and hepatosplenomegaly
as characteristic signs. Our group has recently described an indispensable role of ENT3 in the maintenance of
hematopoietic stem cell (HSC) homeostasis via the genetic deletion of ENT3 in mice. Intriguingly, ENT3-
deleted mice manifest clinical signs closely resembling human SLC29A3 disorders. The objective of this
application is to evaluate the molecular disease pathogeneses and treatments of SLC29A3 genetic disorders.
Specifically, it is our hypothesis that interference with the lysosome-to-ER mobilization of bile acid (BA)
chemical chaperones and ER stress signaling in HSCs underlies the pathogeneses of SLC29A3 dysfunctions
and that endogenous or synthetic chaperones will serve as suitable treatment agents to overcome these
disorders. This hypothesis is based on preliminary data showing that BA chemical chaperones are novel
cargos of ENT3, showing ENT3 conferring the BA-dependent amelioration of ER stress signaling in HSCs, and
showing improved function of ENT3-deleted HSCs after treatment with salubrinal, a chemical ER stress
reducer. The rationale for this project is that the identification of the mechanisms of ER stress regulation by
ENT3 in HSCs will determine the molecular disease pathogenesis of SLC29A3 disorders, which would provide
therapeutic opportunities to treat the SLC29A3 genetic disorders. This central hypothesis will be tested by
pursuing two specific aims. Specific Aim 1 will evaluate ENT3 transport and the subcellular disposition of BA
chemical chaperones in HSCs. The working hypothesis is that ENT3 promotes the lysosome-to-ER
mobilization of BA and that the loss of ENT3 will reduce the ER accumulation of BAs with increased
sequestration in the lysosome. We will use new investigational models, ENT3 structure-function analysis, novel
BA molecular probes, time lapse imaging, subcellular pharmacokinetics, and mass spectrometry studies to
address this aim. Specific Aim 2 will evaluate aberrant ER stress signaling as the basis of HSC dysfunction in
SLC29A3 disorders. Key questions connecting ENT3-regulated ER stress signaling and SLC29A3 disease
pathologies will be evaluated in steady-state and stressed HSCs using HSC-specific, inducible and conditional
ENT3 KO mice. Furthermore, endogenous and synthetic chaperones and pharmacological modifiers of ENT3
misfolding, trafficking, and degradation will be evaluated to identify suitable treatment agent(s) for overcoming
SLC29A3 disorders. Completion of this project will illuminate the pathogenetic basis and treatments for
SLC29A3 genetic disorders and will provide unique insight into the ENT3-regulated BA chaperone defense
mechanisms in the physiological ER stress signaling in HSCs.