Mice Knocked Out for the Primary Brain Calcification-Associated Gene Slc20a2 Show Unimpaired Prenatal Survival but Retarded Growth and Nodules in the Brain that Grow and Calcify Over Time

Brain calcification of especially the basal ganglia characterizes primary familial brain calcification (PFBC). PFBC is a rare neurodegenerative disorder with neuropsychiatric and motor symptoms, and only symptomatic treatment is available. Four PFBC-associated genes are known; approximately 40% of patients carry mutations in the gene SLC20A2, which encodes the type III sodium-dependent inorganic phosphate transporter PiT2. To investigate the role of PiT2 in PFBC development, we studied Slc20a2-knockout (KO) mice using histology, microcomputed tomography, electron microscopy, and energy-dispersive X-ray spectroscopy. Slc20a2-KO mice showed histologically detectable nodules in the brain already at 8 weeks of age, which contained organic material and were weakly calcified. In 15-week-old mice, the nodules were increased in size and number and were markedly more calcified. The major minerals in overt calcifications were Ca and P, but Fe, Zn, and Al were also generally present. Electron microscopy suggested that the calcifications initiate intracellularly, mainly in pericytes and astrocytes. As the calcification grew, they incorporated organic material. Furthermore, endogenous IgG was detected around nodules, suggesting local increased blood-brain barrier permeabilities. Nodules were found in all 8-week-old Slc20a2-KO mice, but no prenatal or marked postnatal lethality was observed. Thus, besides allowing for the study of PFBC development, the Slc20a2-KO mouse is a potential solid preclinical model for evaluation of PFBC treatments.