Ablation of systemic phosphate-regulating gene fibroblast growth factor 23 (Fgf23) compromises the dentoalveolar complex

Fibroblast growth factor-23 (FGF23) is a hormone that modulates circulating phosphate (P(i)) levels by controlling P(i) reabsorption from the kidneys. When FGF23 levels are deficient, as in tumoral calcinosis patients, hyperphosphatemia ensues. We show here in a murine model that Fgf23 ablation disrupted morphology and protein expression within the dentoalveolar complex. Ectopic matrix formation in pulp chambers, odontoblast layer disruption, narrowing of periodontal ligament space, and alteration of cementum structure were observed in histological and electron microscopy sections. Because serum P(i) levels are dramatically elevated in Fgf23(-/-), we assayed for apoptosis and expression of members from the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family, both of which are sensitive to elevated P(i) in vitro. Unlike X-linked hypophosphatemic (Hyp) and wild-type (WT) specimens, numerous apoptotic osteocytes and osteoblasts were detected in Fgf23(-/-) specimens. Further, in comparison to Hyp and WT samples, decreased bone sialoprotein and elevated dentin matrix protein-1 protein levels were observed in cementum of Fgf23(-/-) mice. Additional dentin-associated proteins, such as dentin sialoprotein and dentin phosphoprotein, exhibited altered localization in both Fgf23(-/-) and Hyp samples. Based on these results, we propose that FGF23 and (P(i)) homeostasis play a significant role in maintenance of the dentoalveolar complex.

Aberrant cementum phenotype associated with the hypophosphatemic hyp mouse

BACKGROUND

Cementogenesis is sensitive to altered local phosphate levels; thus, we hypothesized a cementum phenotype, likely of decreased formation, would be present in the teeth of X-linked hypophosphatemic (Hyp) mice. Mutations in the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (Phex) cause X-linked hypophosphatemia, characterized by rickets, osteomalacia, and hypomineralized dentin formation, a phenotype recapitulated in the Hyp mouse homolog. Here, we report a developmental study of tooth root formation in Hyp mouse molars, focusing on dentin and cementum.

METHODS

Light and transmission electron microscopy were used to study molar tissues from wild-type (WT) and Hyp mice. Demineralized and hematoxylin and eosin-stained tissues at developmental stages 23 to 96 days postcoital (dpc) were examined by light microscopy. Immunohistochemistry methods were used to detect bone sialoprotein (BSP) distribution in Hyp and WT mouse molar tissues, and transmission electron microscopy was used to study similar molar tissues in the non-demineralized state.

RESULTS

Dentin in Hyp mice exhibited mineralization defects by 33 dpc, as expected, but this defect was partially corrected by 96 dpc. In support of our hypothesis, a cementum phenotype was detected using a combination of immunohistochemistry and transmission electron microscopy, which included thinner BSP-positive staining within the cementum, discontinuous mineralization, and a globular appearance compared to WT controls.

CONCLUSION

Mutations in the phosphate-regulating Phex gene of the Hyp mouse resulted in defective cementum development.

Amelioration of the premature ageing-like features of Fgf-23 knockout mice by genetically restoring the systemic actions of FGF-23

Genetic ablation of fibroblast growth factor 23 from mice (Fgf-23(-/-)) results in a short lifespan with numerous abnormal biochemical and morphological features. Such features include kyphosis, hypogonadism and associated infertility, osteopenia, pulmonary emphysema, severe vascular and soft tissue calcifications, and generalized atrophy of various tissues. To determine whether these widespread anomalies in Fgf-23(-/-) mice can be ameliorated by genetically restoring the systemic actions of FGF-23, we generated Fgf-23(-/-) mice expressing the human FGF-23 transgene in osteoblasts under the control of the 2.3 kb alpha1(I) collagen promoter (Fgf-23(-/-) /hFGF-23-Tg double mutants). This novel mouse model is completely void of all endogenous Fgf-23 activity, but produces human FGF-23 in bone cells that is subsequently released into the circulation. Our results suggest that lack of Fgf-23 activities results in extensive premature ageing-like features and early mortality of Fgf-23(-/-) mice, while restoring the systemic effects of FGF-23 significantly ameliorates these phenotypes, with the resultant effect being improved growth, restored fertility, and significantly prolonged survival of double mutants. With regard to their serum biochemistry, double mutants reversed the severe hyperphosphataemia, hypercalcaemia, and hypervitaminosis D found in Fgf-23(-/-) littermates; rather, double mutants show hypophosphataemia and normal serum 1,25-dihydroxyvitamin D(3) levels similar to pure FGF-23 Tg mice. These changes were associated with reduced renal expression of NaPi2a and 1 alpha-hydroxylase, compared to Fgf-23(-/-) mice. FGF-23 acts to prevent widespread abnormal features by acting systemically to regulate phosphate homeostasis and vitamin D metabolism. This novel mouse model provides us with an in vivo tool to study the systemic effects of FGF-23 in regulating mineral ion metabolism and preventing multiple abnormal phenotypes without the interference of native Fgf-23.

Cytotoxic therapy of gastroenteropancreatic (GEP) tumors

Neuroendocrine gut and pancreatic tumors have provided a diagnostic and therapeutic challenge over the years. Although they are considered slow growing, when disseminated their prognosis is poor. Currently, the various therapeutic options available for patients with inoperable GEP tumors aim either to control symptoms engendered by bioactive mediators, released by tumor cells, or to achieve control of tumor proliferation. The interpretation of the efficacy of the cytotoxic therapy on GEP tumors is hampered by the small size of the studies performed and the inconsistence in tumor type, origin and degree of differentiation. Nevertheless, it is widely recognized that chemotherapy has some role in the management of Neuroendocrine Pancreatic Tumors (NPTs); while in Metastatic Carcinoid Tumors (MCTs) it has only a minimal effect.

A Bsr BI polymorphism in exon 1C of the human interleukin-1 receptor type I (IL-1RI) gene

We have identified a single nucleotide polymorphism in the 5' region of the human interleukin-1 receptor type I (IL-1RI) gene, a C-->A transversion at position 52 in exon 1C (GenBank accession number AF172151) which creates a Bsr BI restriction endonuclease site. Allele frequencies in a Caucasian population were 0.72 (C allele) and 0.28 (A allele).

Identification of novel single nucleotide polymorphisms in intron 1B and exon 1C of the human interleukin-1 receptor type I (IL-1RI) gene

We have identified two novel single nucleotide polymorphisms in the 5' region of the human IL-1RI gene: (1) A-->G at position 52 in intron 1B (GenBank accession number AF146426), which creates an Mspl restriction endonuclease site. Allele frequencies in a Caucasian population were 0.1 (A allele) and 0.9 (G allele). (2) A-->T at position 140 in exon 1C (GenBank accession number AF146427). Allele frequencies in a Caucasian population were 0.27 (A allele) and 0.73 (T allele).

Chronic bronchial suppuration and antineutrophil cytoplasmic antibody (ANCA) positive systemic vasculitis

Two patients with long-standing chronic bronchial suppuration developed antineutrophil cytoplasmic antibody (ANCA) positive Wegener's granulomatosis and microscopic polyarteritis respectively. There is published evidence of an association between previous suppurative respiratory disease and Wegener's granulomatosis. We believe that our cases provide further evidence that chronic lung infection may play an aetiological role in the development of ANCA-positive systemic necrotizing vasculitis in some individuals.