Iron deficiency drives an autosomal dominant hypophosphatemic rickets (ADHR) phenotype in fibroblast growth factor-23 (Fgf23) knock-in mice

Abstract

Autosomal dominant hypophosphatemic rickets (ADHR) is unique among the disorders involving Fibroblast growth factor 23 (FGF23) because individuals with R176Q/W and R179Q/W mutations in the FGF23 176 RXXR 179 /S 180 proteolytic cleavage motif can cycle from unaffected status to delayed onset of disease. This onset may occur in physiological states associated with iron deficiency, including puberty and pregnancy. To test the role of iron status in development of the ADHR phenotype, WT and R176Q-Fgf23 knock-in (ADHR) mice were placed on control or low-iron diets. Both the WT and ADHR mice receiving low-iron diet had significantly elevated bone Fgf23 mRNA. WT mice on a low-iron diet maintained normal serum intact Fgf23 and phosphate metabolism, with elevated serum C-terminal Fgf23 fragments. In contrast, the ADHR mice on the low-iron diet had elevated intact and C-terminal Fgf23 with hypophosphatemic osteomalacia. We used in vitro iron chelation to isolate the effects of iron deficiency on Fgf23 expression. We found that iron chelation in vitro resulted in a significant increase in Fgf23 mRNA that was dependent upon Mapk. Thus, unlike other syndromes of elevated FGF23, our findings support the concept that late-onset ADHR is the product of gene-environment interactions whereby the combined presence of an Fgf23-stabilizing mutation and iron deficiency can lead to ADHR.Online Mendelian Inheritance in Man no. 193100) is characterized by low serum phosphate concentrations due to isolated renal phosphate wasting, inappropriately normal or low serum 1,25(OH) 2 vitamin D (1,25D) concentrations, and rickets/osteomalacia and fracture (1). Heterozygous missense mutations in the fibroblast growth factor-23 (FGF23) gene cause ADHR (2). These mutations replace the arginine (R) residues at positions 176 or 179 with glutamine (Q) or tryptophan (W) within a 176 RXXR 179 / S 180 subtilisin-like proprotein convertase (SPC) site that separates the conserved FGF-like N-terminal domain from the variable Cterminal tail (2-4). Acting through the coreceptor α-Klotho (5) and a fibroblast growth factor receptor (FGFR) (5, 6), FGF23 reduces renal phosphate reabsorption through down-regulation of the sodium phosphate cotransporters NPT2a and NPT2c and suppresses kidney 1,25(OH) 2 vitamin D production by inhibiting and increasing vitamin D 1α-hydroxylase (Cyp27b1) and 24-hydroxylase expression (Cyp24), respectively (7). Compared with WT Fgf23 protein, ADHR-mutant FGF23 shows increased but not complete resistance to SPC proteolytic cleavage (3, 4). When expressed in mammalian cells, the R176Q-, R179Q-, and R179W-FGF23 proteins are secreted primarily as the full-length (32-kDa) polypeptide, in contrast to the full-length and cleavage products (20 and 12 kDa) typically observed for WT FGF23 (3). This proteolytic event inactivates the mature FGF23 polypeptide, as full-length FGF23, but not N-terminal fragments (residues 25-179) or C-terminal fragments (residues 180-251), reduces serum phosphate concentrations when injected into rodents (4).The ADHR...

Mechanism of FGF23 processing in fibrous dysplasia

“…In an iron-deficient state, humans with ADHR and an ADHR model mouse respond by increasing FGF23 blood levels. (34) Interestingly, in response to iron deficiency, the WT control of the ADHR mice showed a dramatic increase in cFGF23 in the blood but maintained normal blood iFGF23 and phosphorus levels. This finding is paralleled in normal humans, wherein there is a significant inverse correlation between cFGF23 levels and serum iron levels but no such relationship with iFGF23 levels.…”

Fibroblast Growth Factor 23 and Inflammation in CKD

“…Alternatively, functional iron deficiency induced by inflammation-mediated secretion of hepcidin, which inhibits intestinal absorption of iron and prevents its release from macrophages and hepatic stores for erythropoiesis (40), could drive increased FGF23 levels. Indeed, iron deficiency induces transcription of FGF23, which is cleaved intracellularly, leading to elevated circulating levels of C-terminal FGF23 but normal intact levels (unless patients carry the activating FGF23 mutation of autosomal-dominant hypophosphatemic rickets) (41,42). Of note, no studies have examined the regulation of FGF23 secretion in osteocytes by disordered iron metabolism in CKD.…”

Dynamic regulation of FGF23 by Fam20C phosphorylation, GalNAc-T3 glycosylation, and furin proteolysis

“…The mechanisms by which Fam20C is functionally or expressionally regulated are unknown. Recent studies examining the production of intact and/ or C-terminal fragments of FGF23 in fibrous dysplasia and during physiological situations of low iron, such as in ADHR (36,37), support that GalNAc-T3 and furin are likely controlled by multiple factors including 3′,5′-cyclic adenosine monophosphate (38), iron or iron deficiency (39), and inorganic phosphate (40).…”