Mutations in XPR1 cause primary familial brain calcification associated with altered phosphate export

Abstract

Primary familial brain calcification (PFBC) is a neurological disease characterized by calcium phosphate deposits in the basal ganglia and other brain regions and has thus far been associated with SLC20A2, PDGFB or PDGFRB mutations. We identified in multiple families with PFBC mutations in XPR1, a gene encoding a retroviral receptor with phosphate export function. These mutations alter phosphate export, implicating XPR1 and phosphate homeostasis in PFBC.

Figure 1. Localization of the Variants Identified in the XPR1 Protein and Effect of the p.Leu145Pro on Protein Expression and Function

(a) Schematic representation of the XPR1 protein and the variants identified in this study. The SPX domain is in orange. (b) Efflux of inorganic ³³P in HEK293T cells transfected with siLUC (Lane 1), siXPR1 alone (Lane 2) or in combination with HA-tagged wild type (Lane 3) or p.Leu145Pro XPR1 (Lane 4). Results are means ± s.e.m. in a representative experiment (n=3); **P≤0.01 and ***P≤0.001 as tested against siLUC. (c) Immunoblot of HA-tagged XPR1 expression in cell lysates with an anti-HA antibody (upper panel). β-actin was used as loading control (lower panel). (d) Cell-surface detection of XPR1 on HEK293T cells transfected as in (b). Nonspecific staining with the secondary IgG antibody (filled histograms) and specific binding with X-MLV RBD (solid line histograms) are represented. Numbers indicate specific mean fluorescence intensity of a representative experiment (n=3). (e) Efflux of ³³P in PBMC collected from two control healthy donors (each symbolized by circles or squares), and two affected patients harboring the p.Leu145Pro mutation (each symbolized by triangles pointing up or down). Bars represent means ± s.e.m., ***P ≤ 0.001.