XB-ART-55715Urolithiasis December 1, 2019; 47 (6): 511-519.
Evaluating pathogenicity of SLC34A3-Ser192Leu, a frequent European missense variant in disorders of renal phosphate wasting.
Loss-of-function mutations of SLC34A3 represent an established cause of a distinct renal phosphate wasting disorder termed hereditary hypophosphatemic rickets with hypercalciuria (HHRH). SLC34A3 encodes the renal phosphate transporter NaPi2c expressed at the apical brush border of proximal renal tubules. Substitution of p.Ser192Leu is one of the most frequent genetic changes among HHRH patients in Europe, but has never been systematically evaluated, clinically or on a cellular level. Identification of a 32-year-old female with a homozgyous c.575C>T, p.Ser192Leu substitution enabled a more comprehensive assessment of the impact of this missense variant. Clinically, the patient showed renal phosphate wasting and nephrocalcinosis without any bone abnormalities. Heterozygous carriers of deleterious SLC34A3 variants were previously described to harbor an increased risk of kidney stone formation and renal calcification. We hence examined the frequency of p.Ser192Leu variants in our adult kidney stone cohort and compared the results to clinical findings of previously published cases of both mono- and biallelic p.Ser192Leu changes. On a cellular level, p.Ser192Leu-mutated transporters localize to the plasma membrane in different cellular systems, but lead to significantly reduced transport activity of inorganic phosphate upon overexpression in Xenopus oocytes. Despite the reduced function in ectopic cellular systems, the clinical consequences of p.Ser192Leu may appear relatively mild, at least in our index patient, and can potentially be missed in clinical practice.
PubMed ID: 30798342
PMC ID: PMC6825645
Article link: Urolithiasis
Species referenced: Xenopus
Genes referenced: slc34a3
Article Images: [+] show captions
|Fig. 1. Clinical characteristic of the index family. a Renal ultrasound of the index patient (II1), her parents (I1—father, I2—mother), and her brother (II2). Bilateral corticomedullary renal calcifications indicating nephrocalcinosis is displayed in the index patient. Mild renal calcifications can be seen in both parents and the brother’s right kidney (II2). Upper panel—right kidney, lower panel—left kidney. b Bone scan of the index patient (II1) shows normal technetium uptake as sign of undisturbed bone metabolism. c, d Dual X-ray absorptiometry (DXA) of the index patient (II1) shows normal bone mineral density at both sites, spinal and femoral|
|Fig. 2. Mutation analysis of the index family. a Pedigree of the index family. The index patient is denoted by a red arrow. b Chromatogram of the index patient (II1) showing the homozgyous c.575C > T (SLC34A3) variant (NM_080877.2) above the heterozygous change, as present in the other family members (I1, I2, II2). c Evolutionary conservation of SLC34A3/NaPi2c at amino acid position Ser192 (in red) and neighboring residues (p. 184–200) according to NM_001177316|
|Fig. 3. Functional evaluation of NaPi2c-Ser192Leu in comparison to wild type. a Plasma membrane localization of GFP-tagged mutant (Ser192Leu) and wild-type (WT)-NaPi2c proteins upon overexpression in HEK293 cells. b Fluorescence intensity, indicating plasma membrane localization, of negative control, positive control, NaPi2c mutant (Ser192Leu), and NaPi2c wild type (WT) upon Xenopus oocyte injection shows no significant difference. c32Pi-uptake upon oocyte injection of NaPi2c mutant (Ser192Leu) is significantly disturbed in comparison to WT in both, GFP-tagged and non-GFP tagged conditions|
|Fig. 4. Localization and predicted topology of SLC34A3/NaPi2c wild type and mutant. a Cartoon of NaPi2c with its renal localization at the brush border of promixal tubules. The hypothetical structure based on sequence similarity with NaPi2b consists of six transmembrane domains (TM), where Ser192 (red) is part of the third transmembrane helix (light gray), adjacent to the substrate binding site. b Model of flNaPi2b (PM0080462)  showing NaPi2c Ser192 (red) (corresponding to NaPi2b Ala192) localization within the third transmembrane (TM) helix (light gray)|
References [+] :
Bacconi, Renouncing electroneutrality is not free of charge: switching on electrogenicity in a Na+-coupled phosphate cotransporter. 2005, Pubmed, Xenbase