Mechanisms of PiT2-loop7 Missense Mutations Induced Pi Dyshomeostasis / 神经科学通报·英文版

PiT2 is an inorganic phosphate (Pi) transporter whose mutations are linked to primary familial brain calcification (PFBC). PiT2 mainly consists of two ProDom (PD) domains and a large intracellular loop region (loop7). The PD domains are crucial for the Pi transport, but the role of PiT2-loop7 remains unclear. In PFBC patients, mutations in PiT2-loop7 are mainly nonsense or frameshift mutations that probably cause PFBC due to C-PD1131 deletion. To date, six missense mutations have been identified in PiT2-loop7; however, the mechanisms by which these mutations cause PFBC are poorly understood. Here, we found that the p.T390A and p.S434W mutations in PiT2-loop7 decreased the Pi transport activity and cell surface levels of PiT2. Furthermore, we showed that these two mutations attenuated its membrane localization by affecting adenosine monophosphate-activated protein kinase (AMPK)- or protein kinase B (AKT)-mediated PiT2 phosphorylation. In contrast, the p.S121C and p.S601W mutations in the PD domains did not affect PiT2 phosphorylation but rather impaired its substrate-binding abilities. These results suggested that missense mutations in PiT2-loop7 can cause Pi dyshomeostasis by affecting the phosphorylation-regulated cell-surface localization of PiT2. This study helps understand the pathogenesis of PFBC caused by PiT2-loop7 missense mutations and indicates that increasing the phosphorylation levels of PiT2-loop7 could be a promising strategy for developing PFBC therapies.

The Pathology of Primary Familial Brain Calcification: Implications for Treatment / 神经科学通报·英文版

Primary familial brain calcification (PFBC) is an inherited neurodegenerative disorder mainly characterized by progressive calcium deposition bilaterally in the brain, accompanied by various symptoms, such as dystonia, ataxia, parkinsonism, dementia, depression, headaches, and epilepsy. Currently, the etiology of PFBC is largely unknown, and no specific prevention or treatment is available. During the past 10 years, six causative genes (SLC20A2, PDGFRB, PDGFB, XPR1, MYORG, and JAM2) have been identified in PFBC. In this review, considering mechanistic studies of these genes at the cellular level and in animals, we summarize the pathogenesis and potential preventive and therapeutic strategies for PFBC patients. Our systematic analysis suggests a classification for PFBC genetic etiology based on several characteristics, provides a summary of the known composition of brain calcification, and identifies some potential therapeutic targets for PFBC.

A family study of autosomal recessive primary familial brain calcification caused by JAM2 gene mutation and literature review / 中华神经科杂志

Objective:To improve the clinician′s recognition of the clinical and molecular characteristics of primary familial brain calcification (PFBC).Methods:The detailed clinical information, imaging and molecular characteristics were analyzed in proband and family members of a genetically confirmed autosomal recessive PFBC family. The clinical and imaging features of junctional adhesion molecule 2 (JAM2) gene related PFBC were analyzed in combination with the literature review.Results:The proband was a 32-year-old man, with slurred speech and paroxysmal limb twitch as the first symptoms, accompanied by cognitive dysfunction, and rigidity in the limbs, with epilepsy in the past. Brain CT showed extensive, symmetrical, and bilateral calcification involving the cerebellum, basal ganglia, thalamus, subcortex and cortex. Other family members showed no related clinical symptoms. Brain CT of the parents of the proband showed no calcification. Gene testing of the proband revealed a homozygous c.685C>T(p.R229*) mutation in JAM2 gene, which has been reported as a pathogenic variation abroad, whereas has not been reported in China. The proband′s parents and children were found with heterozygous c.685C>T (p.R229*) mutation.Conclusions:Autosomal recessive inherited PFBC is a rare disease, and JAM2 mutation is a newly discovered pathogenic gene of PFBC in 2020. Patients with intracranial calcification should be alert of JAM2 gene mutation.