Equilibrative nucleotide transporter ENT3 (SLC29A3): A unique transporter for inherited disorders and cancers

Clinicogenetic characterisation of SLC29A3-related syndromes: a case series, tracing ancestral variants and molecular dynamics simulation

BACKGROUND

SLC29A3-related syndromes (SLC29A3-RS) are characterised by severe and multiorgan involvement that has a severe impact on the quality of life of the affected persons and therefore merit further genetic and clinical research. We investigated the clinical and genetic aspects of patients with SLC29A3-RS.

METHODS

Six pathogenic variants of the SLC29A3 gene were identified in eight families in the current study. RNA sequencing was used for evaluating SLC29A3 variant gene expression and protein stability by molecular dynamics (MD) simulations. This study conducted a Preferred Reporting Items for Systematic Reviews and Meta-Analyses-compliant systematic review of cases across five electronic databases.

RESULTS

Genetic analysis revealed six pathogenic variants of the SLC29A3 gene in eight families; one variant was shared among three families, indicating a possible founder effect. The estimated most recent common ancestor for these patients lived approximately 8.5 generations ago. MD studies revealed structural instability in mutant proteins. RNA sequencing also demonstrated that the expression of SLC29A3 was downregulated while the expression of the immune markers CD68 and LYZ was upregulated. A systematic search of 197 patients of different ethnic backgrounds revealed that the following symptoms were frequent findings: hyperpigmentation, hypertrichosis, hearing loss, short stature and hepatomegaly. The age of onset of SLC29A3-RS was 5.53±5.24 years with an IQR of 1.4-8.25 years.

CONCLUSIONS

The characterisation of the founder variants and the genotype-phenotype correlations helps delineate the phenotype spectrum of SLC29A3-RS, which will facilitate the genetic counselling and screening of the high-risk population. Findings on SLC29A3 variants show the way to proceed in the process of developing the diagnostic and therapeutic methods in the management of SLC29A3-RS.

Genetic Factors Related to the Development or Progression of Mesoamerican Endemic Nephropathy

Over the past two decades, Mesoamerican endemic nephropathy (MeN) has become a major public health problem in certain regions of Mexico and Central American countries. The etiology of this disease is multifactorial, and important environmental factors have been described, such as chronic heat stress, recurrent episodes of dehydration, infections, and exposure to toxins of chemical and biological origin. Genetic and epigenetic factors have been proposed to play significant roles in MeN. Recent studies have analyzed the role of these factors in MeN. In some cases, these factors appear to be associated with accelerated deterioration of established kidney disease due to preexisting endothelial dysfunction and tubulopathy. In other cases, they appear to be associated with early kidney damage, even before occupational exposure, suggesting that they may play a relevant role in the genesis of the disease. Other factors appear to act as risk reducers for developing MeN in areas with a high prevalence of the disease. Therefore, this disease has a rather complex multifactorial etiology, with possible polygenic contributions, possible epigenetic phenomena, and multiple environmental factors.

Bile acids inhibit equilibrative adenosine transport to alter adenosine receptor signaling in cholestasis

High plasma bile acid (BA) levels in individuals with cholestasis affect adenosine (Ado) receptor (AdoR) signaling, but the underlying mechanisms are unclear. Here, we investigated BA interference with cellular Ado transport as a putative mechanism for altering extracellular Ado availability for AdoR signaling. Computational modeling and experimental studies revealed that equilibrative nucleoside transporter 2 (ENT2), but not ENT1, is capable of translocating BAs across the mammalian plasma membrane. ENT2-mediated BA transport has low affinity, is pH independent, and is partially sensitive to inhibition by nitrobenzylthioinosine (NBMPR). At cholestatic plasma concentrations of BAs, however, BAs interfere with Na+-independent, NBMPR-sensitive, ENTs without affecting Na+-driven, NBMPR-insensitive, concentrative nucleoside transporters. Interestingly, this BA interference with ENT transport was largely selective for Ado, with minimal to no impact on the transport of other purine or pyrimidine nucleosides. Xenopus oocyte-based studies demonstrated that BA inhibition of Ado transport is in the order ENT3≥ENT2>ENT1, which also corresponds to the intrinsic ability of individual ENTs to transport BAs. In silico analysis revealed that Ado and BA tend to occupy similar spaces within the ENT translocation pores and that the polar and hydrophilic pore-lining residues determine the interaction of ENTs with BAs. Furthermore, in vivo studies indicated that the accumulation of extraneously administered Ado decreases in the livers of cholestatic mice and that interference with Ado transport alters AdoR signaling. Together, these findings reveal novel ENT-dependent BA‒Ado interactions that may have implications for BA dysregulation of AdoR signaling in cholestatic liver diseases.

Equilibrative nucleoside transporter 3 supports microglial functions and protects against the progression of Huntington's disease in the mouse model

Huntington's disease (HD) is a hereditary neurodegenerative disorder characterized by involuntary movements, cognitive deficits, and psychiatric symptoms. Currently, there is no cure, and only limited treatments are available to manage the symptoms and to slow down the disease's progression. The molecular and cellular mechanisms of HD's pathogenesis are complex, involving immune cell activation, altered protein turnover, and disturbance in brain energy homeostasis. Microglia have been known to play a dual role in HD, contributing to neurodegeneration through inflammation but also enacting neuroprotective effects by clearing mHTT aggregates. However, little is known about the contribution of microglial metabolism to HD progression. This study explores the impact of a microglial metabolite transporter, equilibrative nucleoside transporter 3 (ENT3), in HD. Known as a lysosomal membrane transporter protein, ENT3 is highly enriched in microglia, with its expression correlated with HD severity. Using the R6/2 ENT3-/- mouse model, we found that the deletion of ENT3 increases microglia numbers yet worsens HD progression, leading to mHTT accumulation, cell death, and disturbed energy metabolism. These results suggest that the delicate balance between microglial metabolism and function is crucial for maintaining brain homeostasis and that ENT3 has a protective role in ameliorating neurodegenerative processes.