Why Is There No PiT1/SLC20A1 Pathogenic Variants Yet Linked to Primary Familial Brain Calcification?

We read with interested the article by Couasnay and colleagues reporting a new role for PiT1 protein, independent of its inorganic phosphate (Pi) transport function, in the regulation and homeostasis of the endoplasmic reticulum (ER) in chondrocytes. The authors showed that PiT1 is necessary for the maintenance, differentiation, and survival of chondrocytes in the growth plate during endochondral ossification. PiT1 protein is encoded by SLC20A1 gene, which, together with PiT2, encoded by SLC20A2, comprises the type III family of sodium-dependent inorganic phosphate transporters, a branch of the solute carrier (SLC) superfamily. According to the PubMed database, in recent years, we have seen an increase in publications involving these two transporters. This increase, which began in 2012, coincides with the publication of the first related gene to Primary Familial Brain Calcification (PFBC), the SLC20A2. Variants in SLC20A2 gene are among the leading causes of PFBC, a rare neuropsychiatric disease, characterized by the presence of abnormal calcium deposits in the brain with bilateral pattern in areas such as the basal ganglia and thalamus. Beyond SLC20A2, so far, variants in four other genes have been identified as causing PFBC: PDGFRβ, PDGFβ, XPR1, and MYORG. PiT1 and PiT2 have a similar distribution in the brains of humans and mice, are localized in neurons, glial cells, and endothelial vascular cells, share the function of importing Pi into the cell, and have high amino acid homology. In view of this, it is intriguing to note that has not yet been reported any variants of SLC20A1 in patients with PFBC. Until now, no research group reported families with PFBC were linked to SLC20A1 variants, including ours, despite active screening in patients excluded for the five previous genes (unpublished data). Although not yet detected in the patients with PFBC, recent studies have identified that variants in SLC20A1 represent a potential risk for other quite distinct diseases, such as vertebral fractures (independent of low bone density) and combined pituitary hormone deficiency. In all of these cases, no brain calcification has been reported. In general, studies show that Pit1 is critical for the maintenance of adhesion and cell proliferation. The inviability of the complete knockout mouse to SLC20A1 shows the importance of this gene for cell maintenance. In addition to that already discussed here, PiT1 is involved in several other functions such as erythropoiesis, increased insulin signaling, decreased hepatic lipogenesis, and regulation of apoptosis induced by TNFα, as is approached by Couasnay and colleagues. Couasnay and colleagues show that in chondrocytes, PiT1 expression is regulated by ATF4, a transcription factor related to the cytoprotection response performed by the ER. In this same work, the authors further suggest that the regulation of PiT1 by ATF4 may reveal roles in other physiological mechanisms. Previous studies have shown that ER stress contributes to the pathogenesis of vascular calcification and ATF4 plays an important role in this process. in vitro studies using VSMC (vascular smooth muscle cells) showed that the knockdown of ATF4 was responsible for the decrease of mRNA levels of PiT1 and PiT2. The relevance of ATF4 and PiT1 in the brain is remarkable, as has already been pointed out, but it is not yet clear what role they play together, especially in relation to cerebral vascular calcifications. It is known that in osteoblastic cellular models (MC3T3-E1), the inactivation of PiT1 or PiT2 does not impair the uptake of Pi by the cell; there is a mechanism of compensation by the transporter that remained activated. The uptake of Pi is only impaired when there is the inactivation of the two at the same time. Would this pattern of compensation also happen in the brain? Evidence shows that PiT1 and PIT2 have differentiated functions in the brain, which still need to be better explored. As discussed by Couasnay and colleagues, PiT1 has shown to be a protein with functions beyond the Na-Pi co-transporter. Although it is difficult to exclude a gene as causing a disease, this seems to be a natural way for SLC20A1 in relation to PFBC. Based on our patient screening and in the successive findings on the different functions of these similar proteins, it seems clear that variations in SLC20A1 will not be associated with PFBC. However, more specific experiments involving these two proteins in the brain environment are needed to elucidate many of the issues this theme raises.