Autosomal recessive hypophosphatemic rickets type 2 due to ENPP1 deficiency (ARHR2).

Autosomal recessive hypophosphatemic rickets type 2 (ARHR2; MIM #613312) is a very rare disorder caused by biallelic loss-of-function mutations in the ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) gene. ENPP1 deficiency encompasses a spectrum of phenotypes that includes, in addition to ARHR2, generalized arterial calcification of infancy (GACI), ossification of the posterior longitudinal ligament (OPLL), and pseudoxanthoma elasticum. ARHR2 can be found in GACI survivors, but it may also be the first manifestation of ENPP1 deficiency. Although the precise mechanisms are not fully elucidated, patients with GACI and ARHR2 have elevated serum FGF23 levels, leading to renal phosphate wasting and hypophosphatemia. As a result, the clinical and radiological phenotype of ARHR2 patients is very similar to that of patients affected with other forms of hypophosphatemic rickets, such as X-linked hypophosphatemia. Patients show signs of rickets (abnormal mineralization of growth plates in children) and osteomalacia (abnormal bone mineralization in children and adults) of varying severity. Clinical manifestations specific to ENPP1 loss-of-function mutations and common to GACI, such as ectopic calcifications (valvular, arterial, or periarticular), deafness, OPLL, and PXE, may also be found. Genetic confirmation of the disease is important so as to ensure that patients receive the appropriate treatment or have the opportunity to participate in clinical trials to evaluate the safety and efficacy of novel and promising recombinant enzyme therapies.

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.

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.

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.

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.

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.

Burosumab Improved Histomorphometric Measures of Osteomalacia in Adults with X-Linked Hypophosphatemia: A Phase 3, Single-Arm, International Trial.

In adults with X-linked hypophosphatemia (XLH), excess FGF23 impairs renal phosphate reabsorption and suppresses production of 1,25-dihydroxyvitamin D, resulting in chronic hypophosphatemia and persistent osteomalacia. Osteomalacia is associated with poor bone quality causing atraumatic fractures, pseudofractures, delayed fracture healing, and bone pain. Burosumab is a fully human monoclonal antibody against FGF23. UX023-CL304 is an ongoing, open-label, single-arm, phase 3 study investigating the efficacy of subcutaneous burosumab, 1.0 mg/kg administered every 4 weeks, in improving osteomalacia in adults with XLH who have not been treated for at least 2 years before enrollment. The primary endpoint was improvement in osteoid volume/bone volume assessed by transiliac bone biopsies obtained at baseline and week 48. Additional assessments included serum phosphorus, markers of bone turnover, fracture/pseudofracture healing, and safety. Fourteen subjects enrolled, 13 completed 48 weeks, and 11 completed paired biopsies. All osteomalacia-related histomorphometric measures improved significantly at week 48 (mean percent change: osteoid volume/bone volume, -54%, osteoid thickness, -32%, osteoid surface/bone surface, -26%, [median] mineralization lag time, -83%). Mean serum phosphorus concentration averaged across the mid-point of the dose cycle between weeks 0 and 24 was 3.3 mg/dL, a 50% increase from 2.2 mg/dL at baseline. Markers of bone formation and resorption increased at week 48 (least squares [LS] mean increase: P1NP, +77%; CTx, +36%; both p < 0.0001). All subjects had one or more treatment-emergent adverse event (AE). Most AEs were mild to moderate in severity. Two subjects experienced serious AEs (migraine; paresthesia) that were unrelated to treatment and resolved. Eleven subjects had 18 biopsy procedure-related AEs: 14 for pain, two for itch, and one each for headache and bandage irritation. No deaths or incidents of hyperphosphatemia occurred. In conclusion, by normalizing phosphate homeostasis, burosumab significantly improved osteomalacia in adults with XLH, which likely explains the improved fracture healing and amelioration of skeletal complications. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

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.

Therapeutic Effects of FGF23 c-tail Fc in a Murine Preclinical Model of X-Linked Hypophosphatemia Via the Selective Modulation of Phosphate Reabsorption.

Fibroblast growth factor 23 (FGF23) is the causative factor of X-linked hypophosphatemia (XLH), a genetic disorder effecting 1:20,000 that is characterized by excessive phosphate excretion, elevated FGF23 levels and a rickets/osteomalacia phenotype. FGF23 inhibits phosphate reabsorption and suppresses 1α,25-dihydroxyvitamin D (1,25D) biosynthesis, analytes that differentially contribute to bone integrity and deleterious soft-tissue mineralization. As inhibition of ligand broadly modulates downstream targets, balancing efficacy and unwanted toxicity is difficult when targeting the FGF23 pathway. We demonstrate that a FGF23 c-tail-Fc fusion molecule selectively modulates the phosphate pathway in vivo by competitive antagonism of FGF23 binding to the FGFR/α klotho receptor complex. Repeated injection of FGF23 c-tail Fc in Hyp mice, a preclinical model of XLH, increases cell surface abundance of kidney NaPi transporters, normalizes phosphate excretion, and significantly improves bone architecture in the absence of soft-tissue mineralization. Repeated injection does not modulate either 1,25D or calcium in a physiologically relevant manner in either a wild-type or disease setting. These data suggest that bone integrity can be improved in models of XLH via the exclusive modulation of phosphate. We posit that the selective modulation of the phosphate pathway will increase the window between efficacy and safety risks, allowing increased efficacy to be achieved in the treatment of this chronic disease. © 2017 American Society for Bone and Mineral Research.

Magnetic Resonance Imaging Features as Surrogate Markers of X-Linked Hypophosphatemic Rickets Activity.

OBJECTIVE: X-linked hypophosphatemic rickets (XLH) is the most common form of inheritable rickets. Rickets treatment is monitored by assessing alkaline phosphatase (ALP) levels, clinical features, and radiographs. Our objectives were to describe the magnetic resonance imaging (MRI) features of XLH and to assess correlations with disease activity. STUDY DESIGN: Twenty-seven XLH patients (median age 9.2 years) were included in this prospective single-center observational study. XLH activity was assessed using height, leg bowing, dental abscess history, and serum ALP levels. We looked for correlations between MRI features and markers of disease activity. RESULTS: On MRI, the median maximum width of the physis was 5.6 mm (range 4.8-7.8; normal <1.5), being >1.5 mm in all of the patients. The appearance of the zone of provisional calcification was abnormal on 21 MRI images (78%), Harris lines were present on 24 (89%), and bone marrow signal abnormalities were present on 16 (59%). ALP levels correlated with the maximum physeal widening and with the transverse extent of the widening. CONCLUSIONS: MRI of the knee provides precise rickets patterns that are correlated with ALP, an established biochemical marker of the disease, avoiding X-ray exposure and providing surrogate quantitative markers of disease activity.

A Mutation in the Dmp1 Gene Alters Phosphate Responsiveness in Mice.

Mutations in the dentin matrix protein 1 (DMP1) gene cause autosomal recessive hypophosphatemic rickets (ARHR). Hypophosphatemia in ARHR results from increased circulating levels of the phosphaturic hormone, fibroblast growth factor 23 (FGF23). Similarly, elevated FGF23, caused by mutations in the PHEX gene, is responsible for the hypophosphatemia in X-linked hypophosphatemic rickets (XLH). Previously, we demonstrated that a Phex mutation in mice creates a lower set point for extracellular phosphate, where an increment in phosphorus further stimulates Fgf23 production to maintain low serum phosphorus levels. To test the presence of the similar set point defect in ARHR, we generated 4- and 12-week-old Dmp1/Galnt3 double knockout mice and controls, including Dmp1 knockout mice (a murine model of ARHR), Galnt3 knockout mice (a murine model of familial tumoral calcinosis), and phenotypically normal double heterozygous mice. Galnt3 knockout mice had increased proteolytic cleavage of Fgf23, leading to low circulating intact Fgf23 levels with consequent hyperphosphatemia. In contrast, Dmp1 knockout mice had little Fgf23 cleavage and increased femoral Fgf23 expression, resulting in hypophosphatemia and low femoral bone mineral density (BMD). However, introduction of the Galnt3 null allele to Dmp1 knockout mice resulted in a significant increase in serum phosphorus and normalization of BMD. This increased serum phosphorus was accompanied by markedly elevated Fgf23 expression and circulating Fgf23 levels, an attempt to reduce serum phosphorus in the face of improving phosphorus levels. These data indicate that a Dmp1 mutation creates a lower set point for extracellular phosphate and maintains it through the regulation of Fgf23 cleavage and expression.

Two Case Reports of FGF23-Induced Hypophosphatemia in Childhood Biliary Atresia.

Cholestatic liver disease has long been associated with childhood rickets, secondary to impaired absorption of fat-soluble vitamin D. Elevated serum levels of fibroblast growth factor 23 (FGF23), secondary to genetic defects or tumor-induced osteomalacia, causes hypophosphatemic rickets in childhood. We present 2 infants with end-stage liver disease due to biliary atresia (BA) who developed hypophosphatemia with renal phosphate wasting. Serum FGF23 levels were elevated more than 8 times the upper limit of normal, and the older infant showed radiographic evidence of rickets. Both infants required large supplements of phosphate in addition to calcitriol. Following liver transplantation, FGF23 normalized in both patients and phosphate and calcitriol supplementation were discontinued. Immunohistochemistry revealed ectopic overexpression of FGF23 by hepatocytes in the BA liver. These observations highlight a unique cause of hypophosphatemic rickets in childhood and suggest the need for further investigation into the relationship between BA and other cholestatic disorders, and bone metabolism.

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.

Prolonged Correction of Serum Phosphorus in Adults With X-Linked Hypophosphatemia Using Monthly Doses of KRN23.

CONTEXT: In X-linked hypophosphatemia (XLH), elevated fibroblast growth factor 23 (FGF23) decreases the renal tubular maximum reabsorption rate of phosphate/glomerular filtration rate (TmP/GFR) and serum inorganic phosphorus (Pi), resulting in rickets and/or osteomalacia. OBJECTIVE: The objective was to test the hypothesis that monthly KRN23 (anti-FGF23 antibody) would safely improve serum Pi in adults with XLH. DESIGN: Two sequential open-label phase 1/2 studies were done. SETTING: Six academic medical centers were used. PARTICIPANTS: Twenty-eight adults with XLH participated in a 4-month dose-escalation study (0.05-0.6 mg/kg); 22 entered a 12-month extension study (0.1-1 mg/kg). INTERVENTION: KRN23 was injected sc every 28 days. MAIN OUTCOME MEASURE: The main outcome measure was the proportion of subjects attaining normal serum Pi and safety. RESULTS: At baseline, mean TmP/GFR, serum Pi, and 1,25-dihydroxyvitamin D [1,25(OH)2D] were 1.6 ± 0.4 mg/dL, 1.9 ± 0.3 mg/dL, and 36.6 ± 14.3 pg/mL, respectively. During dose escalation, TmP/GFR, Pi, and 1,25(OH)2D increased, peaking at 7 days for TmP/GFR and Pi and at 3-7 days for 1,25(OH)2D, remaining above (TmP/GFR, Pi) or near [1,25(OH)2D] pre-dose levels at trough. After each of the four escalating doses, peak Pi was between 2.5 and 4.5 mg/dL in 14.8, 37.0, 74.1, and 88.5% of subjects, respectively. During the 12-month extension, peak Pi was in the normal range for 57.9-85.0% of subjects, and ≥25% maintained trough Pi levels within the normal range. Serum Pi did not exceed 4.5 mg/dL in any subject. Although 1,25(OH)2D levels increased transiently, mean serum and urinary calcium remained normal. KRN23 treatment increased biomarkers of skeletal turnover and had a favorable safety profile. CONCLUSIONS: Monthly KRN23 significantly increased serum Pi, TmP/GFR, and 1,25(OH)2D in all subjects. KRN23 has potential for effectively treating XLH.

PHEX 3'-UTR c.*231A>G near the polyadenylation signal is a relatively common, mild, American mutation that masquerades as sporadic or X-linked recessive hypophosphatemic rickets.

Heritable forms of hypophosphatemic rickets (HR) include X-linked dominant (XLH), autosomal recessive, and autosomal dominant HR (from deactivating mutations in PHEX, DMP1 or ENPP1, and activating mutations in FGF23, respectively). Over 30 years, we have cared for 284 children with HR. For those 72 deemed sporadic XLH, we preliminarily reported mutation analysis for 30 subjects. Eleven had PHEX mutations. However, the remaining 19 lacked readily identifiable defects in PHEX, DMP1, or FGF23. In 2008, a novel single-base change near the polyadenylation (pA) signal in the 3'-UTR of PHEX was identified in XLH by other investigators. This c.*231A > G mutation is 3-bp upstream of the putative pA signal (AATAAA) in PHEX. Accordingly, we investigated whether this 3'-UTR defect accounted for HR in any of these 19 sporadic XLH patients. PCR amplification and sequencing of their 3'-UTR region showed the c.*231A > G mutation in four unrelated boys. Then, among an additional 22 of our 72 "sporadic" XLH patients, one boy and one girl were found to have the 3'-UTR defect, totaling six patients. Among these 52 sporadic XLH patients with PHEX analysis, 36 were girls and 16 were boys; ie, a ∼2:1 gender ratio consistent with XLH. However, finding five boys and only one girl with this 3'-UTR mutation presented an unexplained gender bias (p = 0.02). Haplotyping for the five boys, all reportedly unrelated, showed a common core haplotype suggesting a founder. Five of their six mothers had been studied clinically and biochemically (three radiologically). Remarkably, the seemingly unaffected mothers of four of these boys carried the 3'-UTR mutation. These healthy women had normal height, straight limbs, lacked the radiographic presentation of XLH, and showed normal or slight decreases in fasting serum Pi levels and/or TmP/GFR. Hence, PHEX c.*231A > G can masquerade as sporadic or X-linked recessive HR.

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.

Randomized trial of the anti-FGF23 antibody KRN23 in X-linked hypophosphatemia.

BACKGROUND: X-linked hypophosphatemia (XLH) is the most common heritable form of rickets and osteomalacia. XLH-associated mutations in phosphate-regulating endopeptidase (PHEX) result in elevated serum FGF23, decreased renal phosphate reabsorption, and low serum concentrations of phosphate (inorganic phosphorus, Pi) and 1,25-dihydroxyvitamin D [1,25(OH)2D]. KRN23 is a human anti-FGF23 antibody developed as a potential treatment for XLH. Here, we have assessed the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and immunogenicity of KRN23 following a single i.v. or s.c. dose of KRN23 in adults with XLH. METHODS: Thirty-eight XLH patients were randomized to receive a single dose of KRN23 (0.003-0.3 mg/kg i.v. or 0.1-1 mg/kg s.c.) or placebo. PK, PD, immunogenicity, safety, and tolerability were assessed for up to 50 days. RESULTS: KRN23 significantly increased the maximum renal tubular threshold for phosphate reabsorption (TmP/GFR), serum Pi, and 1,25(OH)2D compared with that of placebo (P<0.01). The maximum serum Pi concentration occurred later following s.c. dosing (8-15 days) compared with that seen with i.v. dosing (0.5-4 days). The effect duration was dose related and persisted longer in patients who received s.c. administration. Changes from baseline in TmP/GFR, serum Pi, and serum 1,25(OH)2D correlated with serum KRN23 concentrations. The mean t1/2 of KRN23 was 8-12 days after i.v. administration and 13-19 days after s.c. administration. Patients did not exhibit increased nephrocalcinosis or develop hypercalciuria, hypercalcemia, anti-KRN23 antibodies, or elevated serum parathyroid hormone (PTH) or creatinine. CONCLUSION: KRN23 increased TmP/GFR, serum Pi, and serum 1,25(OH)2D. The positive effect of KR23 on serum Pi and its favorable safety profile suggest utility for KRN23 in XLH patients. Trial registration. Clinicaltrials.gov NCT00830674. Funding. Kyowa Hakko Kirin Pharma, Inc.

Elevated fibroblast growth factor 23 exerts its effects on placenta and regulates vitamin D metabolism in pregnancy of Hyp mice.

Fibroblast growth factor 23 (FGF23) functions in an endocrine fashion and requires α-Klotho to exert its effects on the target organs. We have recently demonstrated that the human placenta also expresses α-Klotho, which led us to hypothesize that FGF23 may exert effects on the placenta. Immunohistochemical analysis demonstrated the expression of FGF receptor 1 (FGFR1) as well as that of α-Klotho in the feto-maternal interface of both mouse and human normal-term placentas, which suggested that these areas might be receptive to FGF23. Therefore, we next investigated whether FGF23 has some roles in the placenta using Hyp mice with high levels of circulating FGF23. Hyp and wild-type (WT) females were mated with WT males, and the mothers and their male fetuses were analyzed. FGF23 levels in Hyp mothers were elevated. FGF23 levels were about 20-fold higher in Hyp fetuses than in Hyp mothers, whereas WT fetuses from Hyp mothers exhibited low levels of FGF23, as did fetuses from WT mothers. We analyzed the placental gene expression and found that the expression of Cyp24a1 encoding 25OHD-24-hydroxylase, a target gene for FGF23 in the kidney, was increased in the placentas of fetuses from Hyp mothers compared with fetuses from WT mothers. In an organ culture of WT placentas, treatment with plasma from Hyp mothers markedly increased the expression of Cyp24a1, which was abolished by the simultaneous addition of anti-FGF23 neutralizing antibody. The direct injection of recombinant FGF23 into WT placentas induced the expression of Cyp24a1. The increase in the placental expression of Cyp24a1 in fetuses from Hyp mothers resulted in decreased plasma 25-hydroxyvitamin D levels. These results suggest that increased levels of circulating FGF23 in pathological conditions such as Hyp mice exerts direct effects on the placenta and affects fetal vitamin D metabolism via the regulation of Cyp24a1 expression.