A mosaic mutation of phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) in X-linked hypophosphatemic rickets with mild bone phenotypes.

X-linked hypophosphatemic rickets (XLH) is primarily characterized by renal phosphate wasting with hypophosphatemia, short stature, and bone deformity of the leg. Here we present a male case of XLH with relatively mild bone deformity caused by a mosaic mutation of the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX). Polymerase chain reaction (PCR) direct sequencing revealed a novel in-frame deletion, NM-000444.6:c.671-685del p.Gln224-Ser228del, at exon 6 in PHEX as a mosaic pattern. This mutation was not found in any database and may result in a significant change in higher-order protein structure and function. TA cloning of the PCR product and clone sequencing estimated the mutation allele frequency at 21%. Literature review of the previously reported three cases with novel mosaic mutations in PHEX, together with the present case, suggests that the rates of the mutation allele correlate with phenotype severity to some extent. We initially treated him with nutritional vitamin D supplements and phosphate salts. However, to avoid the development of secondary/tertiary hyperparathyroidism, we had switched nutritional to active vitamin D supplementation with reduced phosphorus salts. The present report contributes to understanding the relationship between the mosaic rate, in addition to the mutation locus, of the PHEX gene, and clinical features of XLH.

Mutation update and long-term outcome after treatment with active vitamin D3 in Chinese patients with pseudovitamin D-deficiency rickets (PDDR).

Pseudovitamin D-deficiency rickets is a rare disease which is caused by CYP27B1. In this study, we identified 9 mutations in 7 PDDR patients. In addition, we observed the response to long-term treatment of calcitriol in 15 Chinese patients with PDDR, which showed that the biochemical abnormalities had been corrected satisfactorily after 1-year treatment. INTRODUCTION: Pseudovitamin D-deficiency rickets is a rare autosomal recessive disorder resulting from a defect in 25-hydroxyvitamin D 1α-hydroxylase, which is encoded by CYP27B1. The purpose of this study was to identify the CYP27B1 mutations and investigate the response to long-term treatment of calcitriol in Chinese patients with PDDR. METHODS: We investigated CYP27B1 mutations in seven individuals from six separate families. To investigate the response to long-term (13 years) treatment with calcitriol in PDDR patients, we additionally collected clinical data of eight families from our previous report and analyzed their biochemical parameter and radiographic changes during the treatment. RESULTS: Nine different mutations were identified: two novel missense mutations (G194R, R259L), three novel and one reported deletion mutations (c1442delA, c1504delA, c311-321del, and c. 48-60del), two novel nonsense mutations (c.85G>T, c.580G>T), and a reported insertion mutation (c1325-1332insCCCACCC). The statistical analysis revealed that parathyroid hormone (PTH) and ALP significantly decreased after 6-month and 1-year treatment with calcitriol respectively. Urine calcium was measured in all the patients without kidney stones being documented. After 6-year treatment, the radiographic abnormalities had also been improved. Two patients who had reached their final height are both with short stature (height Z-score below - 2.0). CONCLUSIONS: We identified seven novel mutations of CYP27B1 gene in seven Chinese PDDR families. Our findings revealed after 1-year treatment of active vitamin D3, PTH and ALP significantly decreased. The correction of the biochemical abnormalities had not improved the final height satisfactorily.

Elevated Bone Remodeling Markers of CTX and P1NP in Addition to Sclerostin in Patients with X-linked Hypophosphatemia: A Cross-Sectional Controlled Study.

Aspects of bone remodeling have only been scarcely studied in X-linked hypophosphatemia (XLH). In this cross-sectional controlled study, we assessed biochemical indices of bone remodeling and sclerostin in 27 adult patients (median age 47 [range 24-79] years, 19 women, 8 men) with XLH matched with 81 healthy control subjects (1:3) with respect to age-, sex-, and menopausal status. Markers of bone resorption (carboxyterminal cross-linked telopeptide of type 1 collagen, CTX) and formation (N-terminal propeptide of type 1 procollagen, P1NP) were higher in XLH patients compared to controls (median [IQR] 810 [500-1340] vs 485 [265-715] ng/l and 90 [57-136] vs 49 [39-65] ug/l, respectively, both p < 0.001) as well as sclerostin (0.81 [0.60-1.18] vs 0.54 [0.45-0.69] ng/ml, p < 0.001). Similar differences were found when comparing currently treated (with phosphate and alfacalcidol) (n = 11) and untreated (n = 16) XLH patients with their respective controls. We found no significant associations with treatment status and indices of bone remodeling or sclerostin although sclerostin tended to be increased in untreated versus treated (p = 0.06). In contrast to previous histomorphometric studies suggesting a low remodeling activity in XLH, these biochemical indices suggest high osteoblast and osteoclast activity. Further studies are needed to ascertain if the higher sclerostin level in XLH is related to osteocyte dysfunction or represents a secondary phenomenon.

The elevation of circulating fibroblast growth factor 23 without kidney disease does not increase cardiovascular disease risk.

High circulating fibroblast growth factor 23 (FGF23) levels are probably a major risk factor for cardiovascular disease in chronic kidney disease. FGF23 interacts with the receptor FGFR4 in cardiomyocytes inducing left ventricular hypertrophy. Moreover, in the liver FGF23 via FGFR4 increases the risk of inflammation which is also found in chronic kidney disease. In contrast, X-linked hypophosphatemia is characterized by high FGF23 circulating levels due to loss of function mutations of the phosphate-regulating gene with homologies to an endopeptidase on the X chromosome (PHEX), but is not characterized by high cardiovascular morbidity. Here we used a novel murine X-linked hypophosphatemia model, the PhexC733RMhda mouse line, bearing an amino acid substitution (p.Cys733Arg) to test whether high circulating FGF23 in the absence of renal injury would trigger cardiovascular disease. As X-linked hypophosphatemia patient mimics, these mice show high FGF23 levels, hypophosphatemia, normocalcemia, and low/normal vitamin D levels. Moreover, these mice show hyperparathyroidism and low circulating soluble αKlotho levels. At the age of 27 weeks we found no left ventricular hypertrophy and no alteration of cardiac function as assessed by echocardiography. These mice also showed no activation of the calcineurin/NFAT pathway in heart and liver and no tissue and systemic signs of inflammation. Importantly, blood pressure, glomerular filtration rate and urea clearance were similar between genotypes. Thus, the presence of high circulating FGF23 levels alone in the absence of renal impairment and normal/high phosphate levels is not sufficient to cause cardiovascular disease.

Active vitamin D deficiency mediated by extracellular calcium and phosphorus results in male infertility in young mice.

We used mice with targeted deletion of 25-hydroxyvitamin D-1 α-hydroxylase [1α(OH)ase(-/-)] to investigate whether 1,25(OH)2D3 deficiency results in male infertility mediated by 1,25(OH)2D3 or extracellular calcium and phosphorus. Male 1α(OH)ase(-/-) and their wild-type littermates fed either a normal diet or a rescue diet from weaning were mated at 6-14 wk of age with female wild-type mice on the same diet. The fertility efficiency of females was analyzed, and the reproductive phenotypes of males were evaluated by histopathological and molecular techniques. Hypocalcemic and hypophosphatemic male 1α(OH)ase(-/-) mice on a normal diet developed infertility characterized by hypergonadotropic hypogonadism, with downregulation of testicular calcium channels, lower intracellular calcium levels, decreased sperm count and motility, and histological abnormalities of the testes. The proliferation of spermatogenic cells was decreased with downregulation of cyclin E and CDK2 and upregulation of p53 and p21 expression, whereas apoptosis of spermatogenic cells was increased with upregulation of Bax and p-caspase 3 expression and downregulation of Bcl-xl expression. When serum calcium and phosphorus were normalized by the rescue diet, the defective reproductive phenotype in the male 1α(OH)ase(-/-) mice, including the hypergonadotropic hypogonadism, decreased sperm count and motility, histological abnormalities of testis, and defective spermatogenesis, was reversed. These results indicate that the infertility seen in male 1,25(OH)2D3-deficient mice is not a direct effect of active vitamin D deficiency on the reproductive system but is an indirect effect mediated by extracellular calcium and phosphorus.

Nationwide survey of fibroblast growth factor 23 (FGF23)-related hypophosphatemic diseases in Japan: prevalence, biochemical data and treatment.

A nationwide epidemiologic survey of fibroblast growth factor 23 (FGF23)-related hypophosphatemic diseases was conducted in 2010 to clarify the prevalence and the clinical presentations of the disorders. A questionnaire inquiring the experience of patients with these diseases was sent to randomly selected hospitals throughout Japan. The estimated annual incidence of the diseases was 117 cases (95% CI 75 - 160), 55 males (95% CI 30 - 81) and 62 females (95% CI 40 - 84). Tumor-induced osteomalacia (TIO) and X-linked hypophosphatemic rickets (XLH) were the most prevalent causes of acquired and genetic FGF23-related hypophosphatemic diseases, respectively. The estimated incidence of XLH was about 1 in 20,000. We have also collected clinical data of the patients by a secondary survey. These patients showed FGF23 levels of above 30 pg/mL by intact assay in the presence of hypophosphatemia. While complete resection of responsible tumors improved biochemical abnormalities in patients with TIO, treatment with phosphate and/or active vitamin D3 did not normalize serum phosphate and tubular maximum transport of phosphate in patients with XLH. Our results suggest that there is no racial difference in the incidence of XLH. While FGF23 measurement is useful for the diagnosis of FGF23-related hypophosphatemic diseases, the better management is necessary especially for patients with genetic hypophosphatemic rickets caused by excessive actions of FGF23.

Coupling fibroblast growth factor 23 production and cleavage: iron deficiency, rickets, and kidney disease.

PURPOSE OF REVIEW: High levels of fibroblast growth factor 23 (FGF23) cause the rare disorders of hypophosphatemic rickets and are a risk factor for cardiovascular disease and death in patients with chronic kidney disease (CKD). Despite major advances in understanding FGF23 biology, fundamental aspects of FGF23 regulation in health and in CKD remain mostly unknown. RECENT FINDINGS: Autosomal dominant hypophosphatemic rickets (ADHR) is caused by gain-of-function mutations in FGF23 that prevent its proteolytic cleavage, but affected individuals experience a waxing and waning course of phosphate wasting. This led to the discovery that iron deficiency is an environmental trigger that stimulates FGF23 expression and hypophosphatemia in ADHR. Unlike osteocytes in ADHR, normal osteocytes couple increased FGF23 production with commensurately increased FGF23 cleavage to ensure that normal phosphate homeostasis is maintained in the event of iron deficiency. Simultaneous measurement of FGF23 by intact and C-terminal assays supported these breakthroughs by providing minimally invasive insight into FGF23 production and cleavage in bone. These findings also suggest a novel mechanism of FGF23 elevation in patients with CKD, who are often iron deficient and demonstrate increased FGF23 production and decreased FGF23 cleavage, consistent with an acquired state that mimics the molecular pathophysiology of ADHR. SUMMARY: Iron deficiency stimulates FGF23 production, but normal osteocytes couple increased FGF23 production with increased cleavage to maintain normal circulating levels of biologically active hormone. These findings uncover a second level of FGF23 regulation within osteocytes, failure of which culminates in elevated levels of biologically active FGF23 in ADHR and perhaps CKD.

Anti-fibroblast growth factor 23 antibody therapy.

PURPOSE OF REVIEW: The review is intended to provide an update on the expanding knowledge regarding diseases caused by the excess actions of fibroblast growth factor 23 (FGF23) and also on the new therapeutic measures for these diseases, with an emphasis on the anti-FGF23 antibody. RECENT FINDINGS: FGF23 decreases serum phosphate and 1,25-dihydroxyvitamin D levels. After the cloning of FGF23, several hypophosphatemic diseases, including tumor-induced osteomalacia and X-linked hypophosphatemic rickets (XLHR), were shown to be caused by excess actions of FGF23. In addition, recent studies indicated that mutations in the family with sequence similarity 20, member C (FAM20C), HRAS and NRAS genes, also caused FGF23-related hypophosphatemic diseases. The inhibition of FGF23 production or activity is, theoretically, an ideal treatment for these hypophosphatemic diseases. The C-terminal fragment of FGF23, inhibitors of FGF receptor and extracellular signal-regulated kinase, and anti-FGF23 antibody were shown to inhibit FGF23 actions both in vitro and in vivo. A phase I clinical trial of anti-FGF23 antibody has shown that this antibody increases serum phosphate in patients with XLHR. SUMMARY: These recent findings confirm that FGF23 has a pivotal role in phosphate metabolism. The inhibition of FGF23 production or activity is promising as a new therapy for FGF23-related hypophosphatemic diseases. Further studies are clearly necessary to establish the clinical utility and long-term safety of these measures.

Nephrocalcinosis and kidney function in children and adults with X-linked hypophosphatemia: baseline results from a large longitudinal study.

BACKGROUND: In patients with X-linked hypophosphatemia (XLH), conventional therapy with oral phosphate salts and active vitamin D has been associated with nephrocalcinosis. However, the nature of the relationships among XLH, its treatment, nephrocalcinosis, and kidney function remain poorly understood. METHODS: Renal ultrasounds were performed and glomerular filtration rates were estimated (eGFR) at baseline in burosumab-naïve patients with XLH who participated in burosumab clinical trials (NCT02181764, NCT02526160, NCT02537431, NCT02163577, NCT02750618, NCT02915705) or enrolled in the XLH Disease Monitoring Program (XLH-DMP; NCT03651505). In this cross-sectional analysis, patient, disease, and treatment characteristics were described among patients with and without nephrocalcinosis. RESULTS: The analysis included 196 children (mean [SD] age 7.6 [4.0] yr) and 318 adults (40.3 [13.1] yr). Mean (SD) height z-score was -1.9 (1.2) for children and -2.3 (1.7) for adults. Nearly all children (97%) and adults (94%) had previously received conventional therapy. Nephrocalcinosis was detected in 22% of children and 38% of adults. In children, reduced eGFR <90 mL/min/1.73 m2 was more prevalent in those with nephrocalcinosis (25%) than in those without (11%), a finding that was not observed in adults. Children with nephrocalcinosis had lower mean values of TmP/GFR (p<.05), serum 1,25(OH)2D (p<.05), and eGFR (p<.001) and higher mean serum calcium concentrations (p<.05) than did those without nephrocalcinosis. Adults with nephrocalcinosis had lower mean serum phosphorus (p<.01) and 1,25(OH)2D (p<.05) concentrations than those without. Exploratory logistic regression analyses revealed no significant associations between the presence of nephrocalcinosis and other described patient or disease characteristics. CONCLUSIONS: Nephrocalcinosis was observed in nearly one-quarter of children and more than one-third of adults with XLH. Further study is needed to better understand the predictors and long-term consequences of nephrocalcinosis, with surveillance for nephrocalcinosis remaining important in the management of XLH.

Conventionally, patients with X-linked hypophosphatemia (XLH) were treated with phosphate and vitamin D taken by mouth. However, this therapy might lead to a buildup of calcium in the kidney, called nephrocalcinosis. Here, we tried to better understand how XLH, conventional therapy, nephrocalcinosis, and kidney function are related. Nephrocalcinosis was detected with kidney ultrasounds. Kidney function, called the estimated glomerular filtration rate (eGFR), was determined using blood levels of creatinine. Patients had been part of burosumab clinical trials or part of the XLH Disease Monitoring Program. Data were collected from patients before they received burosumab. The study included 196 children and 318 adults. Almost all children and adults had received conventional therapy. 22% of children and 38% of adults had nephrocalcinosis. Some lab values were different among patients with vs without nephrocalcinosis. Children with nephrocalcinosis had significantly greater loss of phosphate by the kidneys, lower blood levels of the active form of vitamin D (1,25(OH)2D), lower eGFR, and higher blood levels of calcium than those without nephrocalcinosis. Adults with nephrocalcinosis had significantly lower blood levels of phosphorus and 1,25(OH)2D concentrations than those without. It remains important to monitor patients with XLH for nephrocalcinosis. Further study is needed to better understand nephrocalcinosis.

Fibroblast growth factor 23 levels in cord and peripheral blood during early neonatal period as possible predictors of affected offspring of X-linked hypophosphatemic rickets: report of three female cases from two pedigrees.

OBJECTIVES: The role of serum fibroblast growth factor 23 (FGF23) level in early neonatal period on the diagnosis of X-linked hypophosphatemic rickets (XLH) remains unclear. CASE PRESENTATION: Two female patients from the first pedigree had an affected mother, and the other female from the second pedigree had an affected father. In all three cases, FGF23 levels were high in cord blood and peripheral blood at day 4-5. Additionally, the FGF23 levels considerably increased from birth to day 4-5. We identified a PHEX pathogenic variant and initiated treatment during infancy in each case. CONCLUSIONS: In neonates with a parent diagnosed as PHEX-associated XLH, FGF23 in cord blood and peripheral blood at day 4-5 may be useful markers for predicting the presence of XLH.

FGF23 analysis of a Chinese family with autosomal dominant hypophosphatemic rickets.

Autosomal dominant hypophosphatemic rickets (ADHR; MIM 193100) is a hereditary disorder characterized by isolated renal phosphate wasting, hypophosphatemia, and inappropriately normal 1,25-dihydroxyvitamin D(3) levels. Recent studies have shown that the fibroblast growth factor 23 (FGF23) gene is responsible for this disease. FGF23 protein is a phosphaturic factor that is elevated in several diseases associated with hypophosphatemia and rickets but varies with disease status in ADHR. In the present study we observed a Chinese family of Han ethnic origin diagnosed with ADHR. The proband is a 30-year-old woman with no history of rickets but with multiple tooth abscesses as a young adult. She presented with progressive painful swelling of the left ankle after a blunt trauma at 26 years of age. She developed back pain, generalized weakness, and fatigue, and she could barely walk at age 27. She was found to have severe hypophosphatemia, low ratio of phosphorus tubule maximum (TmP) to glomerular filtration rate (GFR) (TmP/GFR), and elevated alkaline phosphatase at age 28. Her brother, 26 years old, presented with fatigue at 24 years of age and is normophosphatemic. The parents of this family had no history of rickets or hypophosphatemia. Direct sequence analysis of genomic DNA demonstrated a single heterozygous c.527G>A (p.R176Q) mutation in the FGF23 gene in three family members, including the proband, her brother, and their mother. Intact FGF23 assay of seven time points during the oral phosphate loading test showed no significant relationship between intact FGF23 and serum phosphorus levels of the subject with ADHR and a control. It is probably the first report of a Chinese family with ADHR.

Sustained Efficacy and Safety of Burosumab, a Monoclonal Antibody to FGF23, in Children With X-Linked Hypophosphatemia.

PURPOSE: In X-linked hypophosphatemia (XLH), excess fibroblast growth factor-23 causes hypophosphatemia and low calcitriol, leading to musculoskeletal disease with clinical consequences. XLH treatment options include conventional oral phosphate with active vitamin D, or monotherapy with burosumab, a monoclonal antibody approved to treat children and adults with XLH. We have previously reported outcomes up to 64 weeks, and here we report safety and efficacy follow-up results up to 160 weeks from an open-label, multicenter, randomized, dose-finding trial of burosumab for 5- to 12-year-old children with XLH. METHODS: After 1 week of conventional therapy washout, patients were randomized 1:1 to burosumab every 2 weeks (Q2W) or every 4 weeks (Q4W) for 64 weeks, with dosing titrated based on fasting serum phosphorus levels between baseline and week 16. From week 66 to week 160, all patients received Q2W burosumab. RESULTS: Twenty-six children were randomized initially into each Q2W and Q4W group and all completed treatment to week 160. In 41 children with open distal femoral and proximal tibial growth plates (from both treatment groups), total Rickets Severity Score significantly decreased by 0.9 ±â€…0.1 (least squares mean ±â€…SE; P < 0.0001) from baseline to week 160. Fasting serum phosphorus increases were sustained by burosumab therapy throughout the study, with an overall population mean (SD) of 3.35 (0.39) mg/dL, within the pediatric normal range (3.2-6.1 mg/dL) at week 160 (mean change from baseline P < 0.0001). Most adverse events were mild to moderate in severity. MAIN CONCLUSIONS: In children with XLH, burosumab administration for 160 weeks improved phosphate homeostasis and rickets and was well-tolerated. Long-term safety was consistent with the reported safety profile of burosumab. CLINICALTRIALS.GOV: NCT02163577.

Low Levels of Serum Sclerostin in Adult Patients With Tumor-Induced Osteomalacia Compared With X-linked Hypophosphatemia.

CONTEXT: Sclerostin inhibits Wnt-ß-catenin signaling, regulating bone formation. Circulating sclerostin was reported to be elevated in X-linked hypophosphatemia (XLH) patients, and sclerostin antibody (Scl-Ab) increased bone mass and normalized circulating phosphate in Hyp mice. However, circulating sclerostin levels in patients with acquired hypophosphatemia due to tumor-induced osteomalacia (TIO) are rarely reported. OBJECTIVE: This study was designed to evaluate serum sclerostin levels in TIO patients compared with age- and sex-matched healthy controls and XLH patients to analyze correlations with bone mineral density (BMD) and laboratory parameters. METHODS: This cross-sectional study determined serum sclerostin levels in 190 individuals, comprising 83 adult TIO patients, 83 adult healthy controls and 24 adult XLH patients. RESULTS: TIO patients (43 male, 40 female) aged 44.3 ±â€…8.7 (mean ± SD) years had lower levels of circulating sclerostin than controls (94.2 ±â€…45.8 vs 108.4 ±â€…42.3 pg/mL, P = 0.01), adjusted for age, gender, BMI, and diabetes rate. Sclerostin levels were positively associated with age (r = 0.238, P = 0.030). Male patients had higher sclerostin than female patients (104.7 ±â€…47.3 vs 83.0 ±â€…41.8 pg/mL, P = 0.014). Sclerostin levels were positively associated with L1-4 BMD (r = 0.255, P = 0.028), femoral neck BMD (r = 0.242, P = 0.039), and serum calcium (r = 0.231, P = 0.043). Comparison of sclerostin levels in TIO patients (n = 24, age 35.9 ±â€…7.3 years) vs XLH patients vs healthy controls revealed significant differences (respectively, 68.4 ±â€…31.3, 132.0 ±â€…68.8, and 98.6 ±â€…41.1 pg/mL, P < 0.001). CONCLUSION: Circulating sclerostin was decreased in TIO patients but increased in XLH patients, possibly due to histological abnormality and bone mass.

A Reference Range for Plasma Levels of Inorganic Pyrophosphate in Children Using the ATP Sulfurylase Method.

PURPOSE: Generalized arterial calcification of infancy, pseudoxanthoma elasticum, autosomal recessive hypophosphatemic rickets type 2, and hypophosphatasia are rare inherited disorders associated with altered plasma levels of inorganic pyrophosphate (PPi). In this study, we aimed to establish a reference range for plasma PPi in the pediatric population, which would be essential to support its use as a biomarker in children with mineralization disorders. METHODS: Plasma samples were collected from 200 children aged 1 day to 18 years who underwent blood testing for medical conditions not affecting plasma PPi levels. PPi was measured in proband plasma utilizing a validated adenosine triphosphate (ATP) sulfurylase method. RESULTS: The analytical sensitivity of the ATP sulfurylase assay consisted of 0.15 to 10 µM PPi. Inter- and intra-assay coefficients of variability on identical samples were below 10%. The standard range of PPi in the blood plasma of children and adolescents aged 0 to 18 years was calculated as 2.36 to 4.44 µM, with a median of 3.17 µM, with no difference between male and female probands. PPi plasma levels did not differ significantly in different pediatric age groups. MAIN CONCLUSIONS: Our results yielded no noteworthy discrepancy to the reported standard range of plasma PPi in adults (2-5 µM). We propose the described ATP sulfurylase method as a diagnostic tool to measure PPi levels in plasma as a biomarker in the pediatric population.

High iFGF23 level despite hypophosphatemia is one of the clinical indicators to make diagnosis of XLH.

X-linked hypophosphatemic rickets (XLH) is caused by inactivating mutations in the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) gene. Deletion of Phex leads to increased serum fibroblast growth factor23 (FGF23) levels in mouse. The aim is to assure the clinical usefulness of FGF23 determination in the diagnosis of XLH. Participants were 21 patients with XLH having abnormalities in PHEX from 13 kindred (PtPHEX: 1 to 42 years old; 10 males, 11 females) and 55 healthy controls (1 month to 18 years old; 27 males, 28 females). Temporal changes in FGF23 were determined by a single oral phosphate administration in PtPHEX and an ad lib diet in controls. Reference ranges of intact FGF23 (iFGF23) for children were determined. iFGF23 level which distinguish between controls and PtPHEX were validated. Correlations between iFGF23 and the severity of XLH (gender, age of onset, bone deformity, The ratio of maximum rate of renal tubular reabsorption of phosphate to glomerular filtration rate (TmPO(4)/GFR), inorganic phosphate (IP), Alkaline Phosphatase (ALP), therapeutic dose) were investigated. Increasing tendency after phosphate administration and no general tendency after breakfast in iFGF23 were observed. Reference range (5(th) and 95(th) percentiles) of iFGF23 for children (12.9 and 51.2 pg/mL) was similar to that for adults. iFGF23 were above the reference range in 19 of 21 PtPHEX (40 to 4710 pg/mL). iFGF23 did not correlate with any index of severity of XLH. Relatively high iFGF23 despite hypophosphatemia is one of the clinical indicators to diagnose XLH.

X-Linked Hypophosphatemic Rickets: Multisystemic Disorder in Children Requiring Multidisciplinary Management.

X-linked hypophosphatemic rickets (XLH) is the commonest inherited form of rickets. It is caused by an impaired regulation of fibroblast growth factor 23 (FGF23) due to a PHEX gene mutation, which leads to reduced tubular reabsorption of phosphate and renal 1α-hydroxylase activity and increased renal 24-hydroxylase activity. Hypophosphatemia associated with renal phosphate wasting, normal serum levels of calcium, parathyroid hormone, and 25-hydroxyvitamin D represents the main biochemical sign in affected patients. Patients with XLH show rickets and osteomalacia, severe deformities of the lower limbs, bone and muscular pain, stunted growth, and reduced quality of life. However, XLH is a multisystemic disorder requiring multidisciplinary approaches in specialized subdisciplines. Severe complications may occur in patients with XLH including craniosynostosis, hearing loss, progressive bone deformities, dental and periodontal recurrent lesions, and psychosocial distress. Moreover, long-term conventional treatment with active vitamin D metabolites and oral inorganic phosphate salts may cause endocrinological complications such as secondary or tertiary hyperparathyroidism, and adverse events in kidney as hypercalciuria, nephrocalcinosis, and nephrolithiasis. However, conventional treatment does not improve phosphate metabolism and it shows poor and slow effects in improving rickets lesions and linear growth. Recently, some trials of treatment with recombinant human IgG1 monoclonal antibody that targets FGF23 (burosumab) showed significant improvement of serum phosphate concentration and renal tubular reabsorption of phosphate that were associated with a rapid healing of radiologic signs of rickets, reduced muscular and osteoarticular pain, and improved physical function, being more effective for the treatment of patients with XLH in comparison with conventional therapy. Therefore, a global management of patients with XLH is strongly recommended and patients should be seen regularly by a multidisciplinary team of experts.

Genotype and phenotypic spectrum of vitamin D dependent rickets type 1A: our experience and systematic review.

BACKGROUND: Vitamin D dependent rickets type 1 (VDDR1) is a rare disease due to pathogenic variants in 1-α hydroxylase gene. We describe our experience with systematic review of world literature to describe phenotype and genotype. METHODS: Seven patients from six unrelated families with genetically proven VDDR1 from our cohort and 165 probands from systematic review were analyzed retrospectively. The clinical features, biochemistry, genetics, management, and long-term outcome were retrieved. RESULTS: In our cohort, the median age at presentation and diagnosis was 11(4-18) and 40(30-240) months. The delayed diagnoses were due to misdiagnoses as renal tubular acidosis and hypophosphatemic rickets. Four had hypocalcemic seizures in infancy whereas all had rickets by 2 years. All patients had biochemical response to calcitriol, however two patients diagnosed post-puberty had persistent deformity. Genetic analysis revealed two novel (p.Met260Arg, p.Arg453Leu) and a recurring variant (p.Phe443Profs*24). Systematic review showed that seizures as most common presentation in infancy, whereas delayed motor milestones and deformities after infancy. Diagnosis was delayed in 27 patients. Patients with unsatisfactory response despite compliance were >12 years at treatment initiation. Inappropriately normal 1,25(OH)2D may be present, however suppressed ratio of 1,25(OH)2 D/25(OH)D may provide a clue to diagnosis. Various region specific and hot-spot recurrent variants are described. Patients with truncating variants had higher daily calcitriol requirement and greatly suppressed ratio of 1,25(OH)2D/25(OH)D. CONCLUSION: Delayed diagnosis may lead to permanent short stature and deformities. Truncating variants tend to have severe disease as compared to non-truncating variants. Diagnostic accuracy of 1,25(OH)2 D/25(OH)D ratio needs further validation.

Osteocytes but not osteoblasts directly build mineralized bone structures.

Bone-forming osteoblasts have been a cornerstone of bone biology for more than a century. Most research toward bone biology and bone diseases center on osteoblasts. Overlooked are the 90% of bone cells, called osteocytes. This study aims to test the hypothesis that osteocytes but not osteoblasts directly build mineralized bone structures, and that defects in osteocytes lead to the onset of hypophosphatemia rickets. The hypothesis was tested by developing and modifying multiple imaging techniques, including both in vivo and in vitro models plus two types of hypophosphatemia rickets models (Dmp1-null and Hyp, Phex mutation mice), and Dmp1-Cre induced high level of ß-catenin models. Our key findings were that osteocytes (not osteoblasts) build bone similar to the construction of a high-rise building, with a wire mesh frame (i.e., osteocyte dendrites) and cement (mineral matrices secreted from osteocytes), which is a lengthy and slow process whose mineralization direction is from the inside toward the outside. When osteoblasts fail to differentiate into osteocytes but remain highly active in Dmp-1-null or Hyp mice, aberrant and poor bone mineralization occurs, caused by a sharp increase in Wnt-ß-catenin signaling. Further, the constitutive expression of ß-catenin in osteocytes recaptures a similar osteomalacia phenotype as shown in Dmp1 null or Hyp mice. Thus, we conclude that osteocytes directly build bone, and osteoblasts with a short life span serve as a precursor to osteocytes, which challenges the existing dogma.