Primary familial brain calcification with a novel SLC20A2 mutation: Analysis of PiT-2 expression and localization

Primary familial brain calcification (PFBC) is an autosomal dominant rare disorder characterized by bilateral and symmetric brain calcifications, and neuropsychiatric manifestations. Four genes have been linked to PFBC: SLC20A2, PDGFRB, PDGFB, and XPR1. In this study, we report molecular and clinical data of a PFBC patient carrying a novel SLC20A2 mutation and we investigate the impact of the mutation on PiT-2 expression and function. Sanger sequencing of SLC20A2, PDGFRB, PDGFB, XPR1 led to the identification of a novel duplication of twelve nucleotides (c.1876_1887dup/ p.Trp626_Thr629dup) in SLC20A2 gene. SLC20A2 encodes for a cell membrane transporter (PiT-2) involved in maintenance of inorganic phosphate homeostasis. We performed an analysis of expression and functionality of PiT-2 protein in patient primary cultured fibroblasts. In patient fibroblasts, the mutation does not affect PiT-2 expression but alter sub-cellular localization. The Pi-uptake assay revealed a less Pi depletion in patient than in control fibroblasts, suggesting that SLC20A2 duplication may impair Pi internalization. This is the first study reporting sub-cellular expression analysis of mutant PiT-2 in primary cultured fibroblasts from a PFBC patient, showing that p.Trp626_Thr629dup in SLC20A2 alters PiT-2 sub-cellular localization and reduces Pi-uptake, leading to onset of PFBC in our patient.