Loss of DMP1 causes rickets and osteomalacia and identifies a role for osteocytes in mineral metabolism

Abstract

The osteocyte, a terminally differentiated cell comprising 90%-95% of all bone cells 1,2 , may have multiple functions, including acting as a mechanosensor in bone (re)modeling 3 . Dentin matrix protein 1 (encoded by DMP1) is highly expressed in osteocytes 4 and, when deleted in mice, results in a hypomineralized bone phenotype 5 . We investigated the potential for this gene not only to direct skeletal mineralization but also to regulate phosphate (P i ) homeostasis. Both Dmp1-null mice and individuals with a newly identified disorder, autosomal recessive hypophosphatemic rickets, manifest rickets and osteomalacia with isolated renal phosphate-wasting associated with elevated fibroblast growth factor 23 (FGF23) levels and normocalciuria. Mutational analyses showed that autosomal recessive hypophosphatemic rickets family carried a mutation affecting the DMP1 start codon, and a second family carried a 7-bp deletion disrupting the highly conserved DMP1 C terminus. Mechanistic studies using Dmp1-null mice demonstrated that absence of DMP1 results in defective osteocyte maturation and increased FGF23 expression, leading to pathological changes in bone mineralization. Our findings suggest a bone-renal axis that is central to guiding proper mineral metabolism.Human disorders of phosphate (P i ) handling and skeletal mineralization can result from mutations in PHEX 6 , which cause X-linked hypophosphatemic rickets (XLH). A similar phenotype is also observed in Hyp mice, which have mutant Phex 7 and show increased osteocyte expression of the phosphaturic factor FGF23 (ref. 8 Individuals F1-1 and F1-3 presented with rickets and progressive lower limb deformity in late infancy, whereas sister F1-2 had rachitic changes on a chest X-ray at age 7 months. In contrast, F2-1 presented with a mild genu valgum at 8 years of age. The pre-or off-treatment age-related metabolic profiles for both kindreds were similar, characterized by hypophosphatemia owing to renal phosphate-wasting (serum P i : 0.7-0.9 mmol/l, normal: 1.2-1.8; threshold maximum for renal tubular phosphate reabsorption/glomerular filtration rate (TmP/GFR): 0.61-0.81 mmol/l, lower limit of normal: ≥1.0), high normal to moderately elevated alkaline phosphatase, normal intact parathyroid hormone (PTH) levels (4.6-6.9 pmol/l, normal: 1.6-6.9), normocalcemia (ionized calcium: 1. Resolution of rickets and normalization of alkaline phosphatase were observed during treatment with phosphate supplementation and calcitriol; however, the TmP/GFR remained low. Linear growth trajectories were heterogeneous among the affected individuals: affected individuals in F1 had a mid-parental height of 154.5 cm (5 th -10 th percentiles), with F1-1 and F1-2 measuring 153 cm (5 th percentile) and 136.5 cm (<5 th percentile) at final adult height, respectively. F1-3 had a height of 153.5 cm at 10 months post-menarche, well within the genetic target. The affected individual in F-2 had a final adult height of 172 cm (90 th -95 th percentile), 3 cm above the upper limit of her genetic ta...

Identification of Full-Length Dentin Matrix Protein 1 in Dentin and Bone

“…This purported biological function is supported by observations that MC3T3-E1 cells overexpressing DMP1 demonstrate an earlier onset of mineralization and the formation of a significantly larger size of the induced mineralized nodules compared to nontransfected control cells [4]. Findings from Dmp1 knockout mouse experiments and gene mutation studies on human osteomalacia strengthen the conclusion that DMP1 plays an important role in bone and dentin mineralization [5][6][7]. In addition to its direct role in biomineralization, studies indicated that DMP1 may regulate osteoblast-specific and/or odontoblast-specific genes [8,9].…”

“…In addition to its direct role in biomineralization, studies indicated that DMP1 may regulate osteoblast-specific and/or odontoblast-specific genes [8,9]. More recent studies indicated that DMP1 may also be involved in the regulation of phosphate homeostasis through fibroblast growth factor 23 (FGF23), a newly identified hormone that is released from bone and targeted in the kidneys; deletion of the Dmp1 gene leads to a dramatic increase of FGF23 mRNA in osteocytes [7].…”

“…In addition to its direct role in biomineralization, studies indicated that DMP1 may regulate osteoblast-specific and/or odontoblast-specific genes [8,9]. More recent studies indicated that DMP1 may also be involved in the regulation of phosphate homeostasis through fibroblast growth factor 23 (FGF23), a newly identified hormone that is released from bone and targeted in the kidneys; deletion of the Dmp1 gene leads to a dramatic increase of FGF23 mRNA in osteocytes [7].Full-length DMP1 cDNA from a number of species has been cloned and sequenced [1,2,3,10,11], but the corresponding full-length form of the protein has not been identified. In searching for naturally occurring DMP1, we discovered that the extracellular matrix (ECM) of bone and dentin contains fragments originating from intact DMP1, namely, a 37-kDa fragment from the NH 2 -terminal region and a 57-kDa fragment from the COOH-terminal region of the DMP1 amino acid sequence [12].…”

Mutational analysis of PHEX, FGF23, DMP1, SLC34A3 and CLCN5 in patients with hypophosphatemic rickets

“…Including this novel frameshift mutation, a total of seven mutations to date has been reported in DMP1 (Figure 2). 6,[33][34][35][36] As ENPP1…”

Mechanism of FGF23 processing in fibrous dysplasia

“…Various hyperphosphatemic and hypophosphatemic disorders have been shown to be directly related to altered FGF23 levels in the circulation. (1)(2)(3)(4) FGF23 is known to regulate blood Pi by decreasing the expression of the Pi transporter genes, Npt2A and Npt2C, in renal proximal tubule cells, thereby reducing renal ARHR), (9,10) tumor-induced osteomalacia (TIO), (8) and fibrous dysplasia of bone (FD). (11) Conversely, low levels of iFGF23 and elevated levels of cFGF23 are found in familial tumoral calcinosis (FTC) and hyperphosphatemic hyperostosis syndrome (HHS).…”