Characterization of XPR1/SLC53A1 variants located outside of the SPX domain in patients with primary familial brain calcification

Uriel López-Sánchez,Jawida Touhami,Gaël Nicolas,Mahmoud Charif,David Maltête,Cyril Goizet,Xavier Ayrignac,Jean-Luc Battini,Gilles Labesse,Marc Sitbon,Anne-Claire Richard

doi:10.1038/s41598-019-43255-x

Abstract

Primary familial brain calcification (PFBC) is a rare neurological disease characterized by deposits of calcium phosphate in the basal ganglia and other regions of the brain. Pathogenic variants in the XPR1/SLC53A1 gene, which encodes the only known inorganic phosphate exporter, cause an autosomal dominant form of PFBC. These variants are typically located in the SPX N-terminal domain of the protein. Here, we characterize three XPR1 variants outside of SPX in three PFBC patients with an apparently sporadic presentation: c.1375C > T p.(R459C), c.1855A > G p.(N619D) and c.1886T > G p.(I629S), with the latter identified as the first XPR1/SLC53A1 de novo mutation to occur in a PFBC proband. When tested in an in vitro physiological complementation assay, the three XPR1 variants were impaired in phosphate export function, although they were normally expressed at the cell surface and could serve as functional receptors for retrovirus entry. Moreover, peripheral blood cells from the p.N619D patient could be assayed ex vivo and displayed significantly impaired phosphate export. Our results establish for the first time the clinical and molecular characteristics of XPR1 variants located outside the SPX domain and assert a direct link between these variants, deficient phosphate export, and PFBC. Moreover, we unveiled new structural features in XPR1 C-terminal domain that play a role in phosphate export and disease.

Highlights

Primary familial brain calcification (PFBC) is a rare neurological disease characterized by the presence of calcium phosphate deposits in the microvessels of the basal ganglia and other brain regions
The p.R459C variant maps to exon 11 (NG_050964, NM_004736.3), corresponding to the predicted largest extracellular loop of XPR1/SLC53A1, while variants p.N619D and p.I629S, encoded by exon 14, are both located in the XPR1/SLC53A1 cytoplasmic C-terminal domain (Fig. 1c,d)
All XPR1 variants demonstrated so far to be causative of PFBC have been mapped within the N-terminal cytoplasmic domain, mostly in the highly conserved SPX domain

Summary

Results

XPR1/SLC53A1 variants outside of the region encoding the N-terminal cytoplasmic domain. The patient was 42 years of age when she presented with fluctuant paresthesia of the left upper limb Her medical history was marked by a diagnosis of celiac disease. Upon introduction of WT XPR1, siRNA-mediated decreased phosphate efflux was fully complemented, while complementation by the p.(L612_696Tdel) construct, which was not expressed at the cell surface, was similar to that of an empty control vector (Fig. 4a, left panel). The three PFBC variants had different levels of complementation for the phosphate exporter function, with the p.N619D variant showing the most marked defect (Fig. 4a, left panel) This defect was observed only for phosphate efflux, as phosphate uptake remained unchanged for all constructions, confirming the specificity of XPR1 for phosphate export (Fig. 4a right panel). When assayed ex vivo for phosphate efflux, PBMC from the p.N619D patient were significantly impaired in phosphate efflux (Fig. 4b left panel), while phosphate uptake remained unaffected in the patient PBMC (Fig. 4b right panel)

Discussion

Author Contributions

Additional Information