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Genetic advances, biochemical and clinical features and critical approach to treatment of patients with X-linked hypophosphatemic rickets.

X-linked hypophosphatemic rickets (XLH) is an hereditary form of rickets due to isolated renal tubular phosphate wasting and impaired production of 1,25-dihydroxyvitamin D [1,25(OH)2D]. XLH is caused by mutations in the PHEX (phosphate regulating gene with homology to endopeptidases) gene, which is located on Xp22.1. The pathogenetic mechanisms by which mutations in the PHEX gene cause XLH are not completely known. Hypophosphatemia associated with disproportionate short stature and bone deformities of the lower limbs are the main findings in XLH patients. Some studies have shown that conventional treatment with vitamin D metabolites, such as 1,25(OH)2D3 or 1 alpha-hydroxyvitamin D3, combined with inorganic phosphate salts is able to improve serum phosphate concentrations and linear growth, as well as healing rickets. However, some patients may have poor beneficial effects by this therapy. On the other hand, some important treatment complications, such as hypervitaminosis D, nephrocalcinosis and secondary/tertiary hyperparathyroidism may occur during the current therapy. Despite conventional treatment, some patients may require surgical correction of bone deformities. In the light of the recent genetic advances the mechanisms that could be involved in the pathogenesis of XLH are discussed. Furthermore, the article reviews the effects of the medical treatment providing current recommendations for the management of XLH patients.

Humans↗

Renal phosphate--wasting disorders in childhood.

The purpose of this review paper is to provide a summary of recent developments in the childhood disorders of renal phosphate-wasting, with particular emphasis on two of the hereditary conditions, X-linked hypophosphatemia (XLH) and autosomal dominant hypophosphatemic rickets (ADHR), as they are distinguished from the paraneoplastic syndrome, oncogenic hypophosphatemic osteomalacia (OHO). An overview of the clinical manifestations, pathogenesis, diagnosis, and treatment of these conditions, with attention to newly-discovered genetic and hormonal signatures, will be discussed. The information may be invaluable in the complex diagnosis and successful treatment of OHO and hereditary hypophosphatemias of childhood.

Child↗

X-linked hypophosphatemia: dental and histologic findings.

The recurrent spontaneous formation of abscesses affecting multiple noncarious primary as well as permanent teeth is the principle clinical dental feature in cases of hypophosphatemia, a condition inherited through the X chromosome. Patients often have high pulp horns, large pulp chambers and dentinal clefts. We report a case of hypophosphatemic vitamin D-resistant rickets in a patient who reported to our department on multiple occasions with spontaneous abscesses in relation to his primary teeth. The aim of this article is to review the features of this disorder and to discuss the risks and benefits of the treatment options suggested in the literature.

Child, Preschool↗

Characterization of the defect in the Na(+)-phosphate transporter in vitamin D-resistant hypophosphatemic mice.

Hypophosphatemic vitamin D-resistant rickets is the most common form of vitamin D-resistant rickets in man. The hypophosphatemic mouse model (Hyp) is phenotypically and biochemically similar to the human disease. Biochemically, hypophosphatemia is the hallmark of this disorder. The cause of the hypophosphatemia is thought to be secondary to a defect in the renal and/or intestinal Na(+)-phosphate transporter. The current studies were designed to investigate and characterize the localization of the defect in the Na(+)-phosphate transporter in this disorder. Phosphate uptake by renal brush border membrane vesicles (BBMV) showed a significant decrease in the slope of the initial rate of phosphate uptake in (Hyp) compared with control mice (0.009 versus 0.013, respectively). The slopes representing initial rates of phosphate uptake by jejunal BBMV were similar in (Hyp) and control mice (0.004 and 0.004, respectively). Kinetics of jejunal Na(+)-dependent phosphate uptake showed a Vmax of 0.63 +/- 0.12 and 0.64 +/- 0.12 nmol/mg protein/15 s in (Hyp) and control mice, respectively, whereas Km values were 0.12 +/- 0.08 and 0.2 +/- 0.11 mM, respectively. Similar kinetic analysis in the kidney showed a Vmax of 0.32 +/- 0.06 and 1.6 +/- 0.1 (p less than 0.01) and Km of 0.07 +/- 0.06 and 0.39 +/- 0.05 (p less than 0.02) in (Hyp) and control mice, respectively. Na(+)-dependent D-glucose uptake by BBMVs of intestine and kidney showed typical overshoot phenomena in (Hyp) and control mice. In order to explore these findings further, Na(+)-phosphate transporter expression from intestine and kidney was accomplished by microinjection of 50 ng of poly(A)+ RNA into Xenopus laevis oocytes. Na(+)-dependent phosphate uptake was expressed 6 days after the microinjection of intestinal and kidney poly(A)+ RNA from control mice. However, expression of the transporter from (Hyp) mice occurred only from the intestine, and not from the kidney. The decrease in the expression of the Na(+)-dependent phosphate transporter was not secondary to accelerated efflux of phosphate or decreased metabolism in oocytes injected with poly(A)+ RNA from (Hyp) mice.(ABSTRACT TRUNCATED AT 400 WORDS)

Alkaline Phosphatase↗

Prevalence of dental abscess in a population of children with vitamin D-resistant rickets.

A population of patients with vitamin D-resistant rickets whose present ages range from 4 to 22 years was examined to determine the prevalence of dental abscess associated with that condition. For those patients affected with abscesses in the primary dentition, four associated factors were examined: 1) the age at onset of the first abscess; 2) the tooth/teeth most commonly involved; 3) whether or not other teeth were involved over time; and 4) the order of occurrence of multiple abscesses. Six of 24 patients (25%) were affected with abscesses of the primary dentition; all had multiple abscesses. Males were more commonly involved than females. The sequence of involvement generally followed the pattern of tooth eruption; however, dental treatment in susceptible individuals shortened the time between eruption and abscess. Medication dose and measurement of radiographs were not reliable predictors of the occurrence of abscesses in this population; however, the results indicate that one abscess is a predictor of future abscesses for that patient.

Abscess↗

The effect of medical therapy on dentin formation in vitamin D-resistant rickets.

Vitamin D-resistant rickets (VDRR) is one of the most common forms of rickets in developed countries today. Widely practiced medical treatment regimens usually include replacing phosphorus and calcium lost through urination. In this study, 20 teeth from five patients with varying grades of dental manifestation of VDRR were examined to relate the institution of medical therapy with any observed improvement of dentin calcification. In all the teeth examined, there was no correlation between the time of appearance of dentin changes and the commencement of medical therapy. Furthermore, in several teeth from the patients with Grade III severity, there were no signs of improvement in dentin calcification even after prolonged medical therapy. This study suggests that current medical treatment of VDRR has no beneficial effect on the dental structures and dentists should not rely on medical treatment to correct dental calcification problems.

Calcitriol↗