A clinician's guide to X-linked hypophosphatemia

Identification of ENPP1 Haploinsufficiency in Patients With Diffuse Idiopathic Skeletal Hyperostosis and Early-Onset Osteoporosis

Homozygous ENPP1 mutations are associated with autosomal recessive hypophosphatemic rickets type 2 (ARHR2), severe ossification of the spinal ligaments, and generalized arterial calcification of infancy type 1. There are a limited number of reports on phenotypes associated with heterozygous ENPP1 mutations. Here, we report a series of three probands and their families with heterozygous and compound heterozygous ENPP1 mutations. The first case (case 1) was a 47-year-old male, diagnosed with early-onset osteoporosis and low-normal serum phosphate levels, which invoked suspicion for hypophosphatemic rickets. The second and third cases were 77- and 54-year-old females who both presented with severe spinal ligament ossification and the presumptive diagnosis of diffuse idiopathic skeletal hyperostosis (DISH). Upon workup, fibroblast growth factor 23 (FGF23) was noted to be relatively high in case 2 and serum phosphorous was low-normal in case 3, and the diagnoses of X-linked hypophosphatemic rickets (XLH) and ARHR2 were considered. Genetic testing for genes related to congenital hypophosphatemic rickets was therefore performed, revealing heterozygous ENPP1 variants in cases 1 and 2 (case 1, c.536A>G, p.Asn179Ser; case 2, c.1352A>G, p.Tyr451Cys) and compound heterozygous ENPP1 variants in case 3 constituting the same variants present in cases 1 and 2 (c.536A>G, p.Asn179Ser and c.1352A>G, p.Tyr451Cys). Several in silico tools predicted the two variants to be pathogeneic, a finding confirmed by in vitro biochemical analysis demonstrating that the p.Asn179Ser and p.Tyr451Cys ENPP1 variants possessed a catalytic velocity of 45% and 30% compared with that of wild-type ENPP1, respectively. Both variants were therefore categorized as pathogenic loss-of-function mutations. Our findings suggest that ENPP1 mutational status should be evaluated in patients presenting with the diagnosis of idiopathic DISH, ossification of the posterior longitudinal ligament (OPLL), and early-onset osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Interdisciplinary management of FGF23-related phosphate wasting syndromes: a Consensus Statement on the evaluation, diagnosis and care of patients with X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most frequent cause of hypophosphataemia-associated rickets of genetic origin and is associated with high levels of the phosphaturic hormone fibroblast growth factor 23 (FGF23). In addition to rickets and osteomalacia, patients with XLH have a heavy disease burden with enthesopathies, osteoarthritis, pseudofractures and dental complications, all of which contribute to reduced quality of life. This Consensus Statement presents the outcomes of a working group of the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases, and provides robust clinical evidence on management in XLH, with an emphasis on patients' experiences and needs. During growth, conventional treatment with phosphate supplements and active vitamin D metabolites (such as calcitriol) improves growth, ameliorates leg deformities and dental manifestations, and reduces pain. The continuation of conventional treatment in symptom-free adults is still debated. A novel therapeutic approach is the monoclonal anti-FGF23 antibody burosumab. Although promising, further studies are required to clarify its long-term efficacy, particularly in adults. Given the diversity of symptoms and complications, an interdisciplinary approach to management is of paramount importance. The focus of treatment should be not only on the physical manifestations and challenges associated with XLH and other FGF23-mediated hypophosphataemia syndromes, but also on the major psychological and social impact of the disease.

Clinical practice guidelines for paediatric X-linked hypophosphataemia in the era of burosumab

X-linked hypophosphataemia (XLH), the most common inherited form of rickets, is caused by a PHEX gene mutation that leads to excessive serum levels of fibroblast growth factor 23 (FGF23). This leads to clinical manifestations such as rickets, osteomalacia, pain, lower limb deformity and overall diminished quality of life. The overarching aims in the management of children with XLH are to improve quality of life by reducing overall burden of disease, optimise an individual's participation in daily activities and promote normal physical and psychological development. Burosumab, a monoclonal antibody targeting FGF23, has been shown to improve biochemistry, pain, function and radiological features of rickets in children with XLH and has transformed management of XLH around the world. Burosumab has been recently approved for clinical use in children with XLH in Australia. This manuscript outlines a clinical practice guideline for the use of burosumab in children with XLH to assist local clinicians, encourage consistency of management across Australia and suggest future directions for management and research. This guideline also strongly advocates for all patients with XLH to have multidisciplinary team involvement to ensure optimal care outcomes and highlights the need to consider other aspects of care for XLH in the era of burosumab, including transition to adult care and the effective coordination of care between local health-care providers and specialist services.

Burden of disease of X-linked hypophosphatemia in Japanese and Korean patients: a cross-sectional survey

The burden of disease of X-linked hypophosphatemia (XLH) in East Asia is poorly understood. This was a cross-sectional study using an online questionnaire to evaluate health-related quality of life (HRQOL) and disease complications in Japanese and Korean patients with XLH. Adults with XLH and the caregivers of children <18 years of age with XLH in Japan and Korea were surveyed. Respondents disclosed demographic data, family history, diagnostic history, medical history, surgical history, disease-specific clinical symptoms, treatment, medications, and use of ancillary equipment. Patient-reported outcomes (PROs; the Western Ontario and McMaster Universities Osteoarthritis Index, the brief pain inventory, and the 36-item short form health survey version 2) were used to assess pain, disability, and HRQOL in adults. Of those surveyed, all 14 children (100%) and 30/32 adults (93.8%) were receiving treatment for XLH. However, despite oral phosphate and active vitamin D use, short stature, gait abnormalities, dental conditions, and decreased physical function were reported. Stapling of the growth plates was reported in 14.3% of children but no adults. Adult patients reported high rates of bone pain (59.4%) and joint pain (65.6%). Caregivers of children with XLH also reported the occurrence of bone pain (35.7%) and joint pain (35.7%). Many adult patients had a history of impaired renal function (9.5%), nephrocalcinosis (15.6%), hyperparathyroidism (15.6%), and parathyroidectomy (6.3%), all of which are associated with conventional XLH treatments. These data show that patients (both pediatric and adult) continue to have symptoms such as pain, disability, and various complications despite receiving conventional therapies.

Novel Treatment Options in Childhood Bone Diseases

BACKGROUND

Several novel treatment options have recently become available in childhood bone diseases. The purpose of this article is to provide an update on some of the therapeutic agents used in the treatment of pediatric osteoporosis, X-linked hypophosphatemic rickets, and achondroplasia (ACH).

SUMMARY

Vitamin D3 and Ca supplementation remains the basis of childhood osteoporosis treatment. Bisphosphonate (BP) therapy is the main antiresorptive therapeutic option, while denosumab, a human monoclonal IgG2 antibody with high affinity and specificity for a primary regulator of bone resorption - RANKL, represents a possible alternative. Its potent inhibition of bone resorption and turnover process leads to continuous increase of bone mineral density throughout the treatment also in the pediatric population. With a half-life much shorter than BPs, its effects are rapidly reversible upon discontinuation. Safety and dosing concerns in children remain. Novel treatment options have recently become available in two rare bone diseases. Burosumab, a monoclonal antibody against FGF-23, has been approved for the treatment of children with X-linked hypophosphatemic rickets older than 1 year. It presents an effective, more etiology-based treatment for rickets compared to conventional therapy, without the need for multiple daily oral phosphate supplementation. Its long-term efficacy and safety are currently being investigated. After years of anticipation, a novel treatment option for ACH has become available. C-type natriuretic peptide analog vosoritide effectively increases proportional growth and has a reasonable safety profile in children >2 years. Its effect on other features of the disease and the final height is yet to be determined. Several other treatment options for ACH exploring different therapeutic approaches are currently being investigated.

KEY MESSAGES

Denosumab is effective in the treatment of childhood-onset osteoporosis; however, further studies are necessary to determine the optimal treatment protocol. Burosumab is more etiology-based and convenient in comparison to conventional treatment of X-linked hypophospha--temic rickets in children and adults. Vosoritide importantly changes the natural course of achondroplasia, at least in the short term.

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 ± 0.1 (least squares mean ± 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.

Identification of a Novel Missense Mutation of the PHEX Gene in a Large Chinese Family with X-Linked Hypophosphataemia

X-linked hypophosphataemia (XLH) is an X-linked dominant rare disease that refers to the most common hereditary hypophosphatemia (HH) caused by mutations in the phosphate-regulating endopeptidase homolog X-linked gene (PHEX; OMIM: * 300550). However, mutations that have already been reported cannot account for all cases of XLH. Extensive genetic analysis can thus be helpful for arriving at the diagnosis of XLH. Herein, we identified a novel heterozygous mutation of PHEX (NM_000444.5: c.1768G > A) in a large Chinese family with XLH by whole-exome sequencing (WES). In addition, the negative effect of this mutation in PHEX was confirmed by both bioinformatics analysis and in vitro experimentation. The three-dimensional protein-model analysis predicted that this mutation might impair normal zinc binding. Immunofluorescence staining, qPCR, and western blotting analysis confirmed that the mutation we detected attenuated PHEX protein expression. The heterozygous mutation of PHEX (NM_000444.5: c.1768G > A) identified in this study by genetic and functional experiments constitutes a novel genetic cause of XLH, but further study will be required to expand its use in clinical and molecular diagnoses of XLH.

Musculoskeletal Features in Adults With X-linked Hypophosphatemia: An Analysis of Clinical Trial and Survey Data

CONTEXT

Patients with X-linked hypophosphatemia (XLH) experience multiple musculoskeletal manifestations throughout adulthood.

OBJECTIVE

To describe the burden of musculoskeletal features and associated surgeries across the lifespan of adults with XLH.

METHODS

Three groups of adults were analyzed: subjects of a clinical trial, participants in an online survey, and a subgroup of the online survey participants considered comparable to the clinical trial subjects (according to Brief Pain Inventory worst pain scores of ≥ 4). In each group, the adults were categorized by age: 18-29, 30-39, 40-49, 50-59, and ≥ 60 years. Rates of 5 prespecified musculoskeletal features and associated surgeries were investigated across these age bands for the 3 groups.

RESULTS

Data from 336 adults were analyzed. In all 3 groups, 43% to 47% had a history of fracture, with the proportions increasing with age. The overall prevalence of osteoarthritis was > 50% in all 3 groups, with a rate of 23% to 37% in the 18- to 29-year-old group, and increasing with age. Similar patterns were observed for osteophytes and enthesopathy. Hip and knee arthroplasty was reported even in adults in their 30s. Spinal stenosis was present at a low prevalence, increasing with age. The proportion of adults with ≥ 2 musculoskeletal features was 59.1%, 55.0%, and 61.3% in the clinical trial group, survey group, and survey pain subgroup, respectively.

CONCLUSION

This analysis confirmed high rates of multiple musculoskeletal features beginning as early as age 20 years among adults with XLH and gradually accumulating with age.

Safety and Efficacy of Burosumab in Pediatric Patients With X-linked Hypophosphatemia: A Phase 3/4 Open-Label Trial

Objective

Burosumab, an anti-fibroblast growth factor 23 antibody, was recently approved for the treatment of X-linked hypophosphatemia (XLH).

We evaluated the safety and efficacy of burosumab in pediatric XLH patients.

Methods

This open-label, phase 3/4 trial of ≤ 124 weeks’ duration was conducted at 4 Japanese medical centers. Fifteen children aged 1 to 12 years with XLH were included. All had previously been treated with phosphorus or vitamin D. Subcutaneous burosumab was administered every 2 weeks, starting with 0.8 mg/kg, and adjusted based on serum phosphorus levels and any safety concerns (maximum 2 mg/kg). Safety assessments included the frequency of treatment-emergent adverse events (TEAEs). Efficacy of burosumab on biochemical markers, clinical markers of rickets, motor function, and growth was also evaluated.

Results

The average treatment duration was 121.7 weeks. Frequently reported TEAEs were nasopharyngitis (46.7%), dental caries (40.0%), and influenza (33.3%). At baseline, patients had low serum phosphorus concentrations (2.6 ± 0.3 mg/dL) and low-to-normal 1,25-dihydroxyvitamin D concentrations (24.7 ± 12.7 pg/mL), which increased with burosumab treatment and were maintained during the study period. Alkaline phosphatase decreased continuously. At baseline, the mean ± SD total Thacher Rickets Severity Score (RSS) was 1.3 ± 1.2, and 4 patients (26.7%) had an RSS ≥ 2.0. Mean Radiographic Global Impression of Change and RSS tended to improve, particularly in patients with higher baseline RSS. There was a trend toward increased 6-minute walk test distance. No apparent changes in growth rate were observed.

Conclusion

Burosumab has a good safety profile and is effective in pediatric patients with XLH.

Molecular Diagnoses of X-Linked and Other Genetic Hypophosphatemias: Results From a Sponsored Genetic Testing Program

X-linked hypophosphatemia (XLH), a dominant disorder caused by pathogenic variants in the PHEX gene, affects both sexes of all ages and results in elevated serum fibroblast growth factor 23 (FGF23) and below-normal serum phosphate. In XLH, rickets, osteomalacia, short stature, and lower limb deformity may be present with muscle pain and/or weakness/fatigue, bone pain, joint pain/stiffness, hearing difficulty, enthesopathy, osteoarthritis, and dental abscesses. Invitae and Ultragenyx collaborated to provide a no-charge sponsored testing program using a 13-gene next-generation sequencing panel to confirm clinical XLH or aid diagnosis of suspected XLH/other genetic hypophosphatemia. Individuals aged ≥6 months with clinical XLH or suspected genetic hypophosphatemia were eligible. Of 831 unrelated individuals tested between February 2019 and June 2020 in this cross-sectional study, 519 (62.5%) individuals had a pathogenic or likely pathogenic variant in PHEX (PHEX-positive). Among the 312 PHEX-negative individuals, 38 received molecular diagnoses in other genes, including ALPL, CYP27B1, ENPP1, and FGF23; the remaining 274 did not have a molecular diagnosis. Among 319 patients with a provider-reported clinical diagnosis of XLH, 88.7% (n = 283) had a reportable PHEX variant; 81.5% (n = 260) were PHEX-positive. The most common variant among PHEX-positive individuals was an allele with both the gain of exons 13-15 and c.*231A>G (3'UTR variant) (n = 66/519). Importantly, over 80% of copy number variants would have been missed by traditional microarray analysis. A positive molecular diagnosis in 41 probands (4.9%; 29 PHEX positive, 12 non-PHEX positive) resulted in at least one family member receiving family testing. Additional clinical or family member information resulted in variant(s) of uncertain significance (VUS) reclassification to pathogenic/likely pathogenic (P/LP) in 48 individuals, highlighting the importance of segregation and clinical data. In one of the largest XLH genetic studies to date, 65 novel PHEX variants were identified and a high XLH diagnostic yield demonstrated broad insight into the genetic basis of XLH. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Potential influences on optimizing long-term musculoskeletal health in children and adolescents with X-linked hypophosphatemia (XLH)

In recent years, much progress has been made in understanding the mechanisms of bone growth and development over a lifespan, including the crosstalk between muscle and bone, to achieve optimal structure and function. While there have been significant advances in understanding how to help improve and maintain bone health in normal individuals, there is limited knowledge on whether these mechanisms apply or are compromised in pathological states. X-linked hypophosphatemia (XLH) (ORPHA:89936) is a rare, heritable, renal phosphate-wasting disorder. The resultant chronic hypophosphatemia leads to progressive deterioration in musculoskeletal function, including impaired growth, rickets, and limb deformities in children, as well as lifelong osteomalacia with reduced bone quality and impaired muscle structure and function. The clinical manifestations of the disease vary both in presentation and severity in affected individuals, and many of the consequences of childhood defects persist into adulthood, causing significant morbidity that impacts physical function and quality of life. Intervention to restore phosphate levels early in life during the critical stages of skeletal development in children with XLH could optimize growth and may prevent or reduce bone deformities in childhood. A healthier bone structure, together with improved muscle function, can lead to physical activity enhancing musculoskeletal health throughout life. In adults, continued management may help to maintain the positive effects acquired from childhood treatment, thereby slowing or halting disease progression. In this review, we summarize the opinions from members of a working group with expertise in pediatrics, epidemiology, and bone, joint and muscle biology, on potential outcomes for people with XLH, who have been optimally treated from an early age and continue treatment throughout life.

Oral health-related quality of life in patients with X-linked hypophosphatemia: a qualitative exploration

INTRODUCTION

X-linked hypophosphatemia (XLH) is a rare, hereditary, and lifelong phosphate-wasting disorder characterized by rickets in childhood and impaired teeth mineralization. In the oral cavity, spontaneous abscesses can often occur without any clinical signs of alteration of the causal tooth. The objective of our study was to evaluate the oral care pathway and the oral health-related quality of life (OHRQoL) of patients following in an expert oral medicine department located within a Parisian hospital and working in close collaboration with an endocrinology department expert in this pathology.

METHODS

This study employed a qualitative descriptive design including semi-structured interviews using guiding themes.

RESULTS

Twenty-one patients were included in the study. The topics brought up exceeded the initial objectives as the patients mostly addressed the alteration of their oral health-related and general quality of life; a very chaotic oral health care pathway with oral health professionals not aware of their pathology; consequences on their social, professional, and school integration. Patients declared the importance of having a multidisciplinary team around them, including medical and dental professionals.

CONCLUSIONS

The variety of manifestations in patients with XLH necessitates high coordination of multidisciplinary patient care to optimize quality of life and reduce disease burden. Oral health care pathways are very chaotic for patients who have difficulty in finding professionals with sufficient knowledge of the disease. OHRQoL is therefore diminished. This situation improves when patients enter a coordinated care network.

Verminderte Mineralisation des Knochens: Rachitis und Osteomalazie

Die Mineralisation der Wachstumsfuge und des Knochens benötigt suffiziente Mengen an Kalzium und Phosphat, um ein adäquates Körperwachstum und eine adäquate Knochenstärke zu gewährleisten. Bei Mangelzuständen nehmen Härte und Stärke des Knochens ab (Osteomalazie), mit typischen Veränderungen an der Wachstumsfuge (Rachitis). Die vermeidbaren Komplikationen einer erworbenen Rachitis können lebensbedrohlich sein und sind auch deshalb ein global wichtiges Public-Health-Thema. Typische klinische Zeichen, Laborparameter und radiologische Veränderungen sollten zur unverzüglichen Diagnose führen. Erbliche und erworbene Formen der Rachitis wie X‑chromosomale Hypophosphatämie oder Hypophosphatasie werden, wie oftmals bei seltenen Erkrankungen, spät diagnostiziert. Diagnose, Therapie und Management von Rachitiden sollten durch tertiäre kinderosteologische Spezialisten erfolgen, die in internationalen „Rare-diseases“-Netzwerken arbeiten und in enger Kooperation mit Selbsthilfegruppen stehen.

Cellular and Molecular Alterations Underlying Abnormal Bone Growth in X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets, is caused by inactivating mutations of the phosphate-regulating endopeptidase gene (PHEX). XLH is mainly characterized by short stature, bone deformities and rickets, while in hypophosphatemia, normal or low vitamin D levels and low renal phosphate reabsorption are the principal biochemical aspects. The cause of growth impairment in patients with XLH is not completely understood yet, thus making the study of the growth plate (GP) alterations necessary. New treatment strategies targeting FGF23 have shown promising results in normalizing the growth velocity and improving the skeletal effects of XLH patients. However, further studies are necessary to evaluate how this treatment affects the GP as well as its long-term effects and the impact on adult height.

Emerging therapies for the treatment of rare pediatric bone disorders

In recent years, new therapies for the treatment of rare pediatric bone disorders have emerged, guided by an increasing understanding of the genetic and molecular etiology of these diseases. Herein, we review three such disorders, impacted by debilitating deficits in bone mineralization or cartilage ossification, as well as the novel disease-modifying drugs that are now available to treat these conditions. Specifically, we discuss asfotase alfa, burosumab-twza, and vosoritide, for the treatment of hypophosphatasia, X-linked hypophosphatemia and achondroplasia, respectively. For each skeletal disorder, an overview of the clinical phenotype and natural history of disease is provided, along with a discussion of the clinical pharmacology, mechanism of action and FDA indication for the relevant medication. In each case, a brief review of clinical trial data supporting drug development for each medication is provided. Additionally, guidance as to drug dosing and long-term monitoring of adverse events and pediatric efficacy is presented, to aid the clinician seeking to utilize these novel therapies in their practice, or to become familiar with the healthcare expectations for children receiving these medications through specialized multidisciplinary clinics. The availability of these targeted therapies now significantly augments treatment options for conditions in which past therapy has relied upon less specific, symptomatic medical and orthopedic care.

Fibroblast Growth Factor 2 High Molecular Weight Isoforms in Dentoalveolar Mineralization

Transgenic mice overexpressing human high molecular weight fibroblast growth factor 2 (HMWFGF2) isoforms in osteoblast and odontoblast lineages (HMWTg) exhibit decreased dentin and alveolar bone mineralization, enlarged pulp chamber, and increased fibroblast growth factor 23 (FGF23). We examined if the alveolar bone and dentin mineralization defects in HMWTg mice resulted from increased FGF23 expression and whether an FGF23 neutralizing antibody could rescue the hypomineralization phenotype. HMWTg and VectorTg control mice were given subcutaneous injections of FGF23 neutralizing antibody twice/week starting at postnatal day 21 for 6 weeks. Since Calcitriol (1,25D) have direct effects in promoting bone mineralization, we also determined if 1,25D protects against the defective dentin and alveolar bone mineralization. Therefore, HMWTg mice were given subcutaneous injections of 1,25D daily or concomitantly with FGF23 neutralizing antibody for 6 weeks. Our results showed that HMWTg mice displayed thickened predentin, alveolar bone hypomineralization, and enlarged pulp chambers. FGF23 neutralizing antibody and 1,25D monotherapy partially rescued the dentin mineralization defects and the enlarged pulp chamber phenotype in HMWTg mice. 1,25D alone was not sufficient to rescue the alveolar bone hypomineralization. Interestingly, HMWTg mice treated with both FGF23 neutralizing antibody and 1.25D further rescued the enlarged pulp chamber size, and dentin and alveolar bone mineralization defects. We conclude that the dentin and alveolar bone mineralization defects in HMWTg mice might result from increased FGF23 expression. Our results show a novel role of HMWFGF2 on dentoalveolar mineralization.

Sclerostin antibody improves phosphate metabolism hormones, bone formation rates, and bone mass in adult Hyp mice

X-linked hypophosphatemia (XLH) is caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX). Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23), impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily by osteocytes, suppresses bone formation by antagonizing canonical Wnt-signaling and is reported to be elevated in XLH patients. Our previous study reported that a monoclonal antibody to sclerostin (Scl-Ab) decreases FGF23 and increases phosphate and bone mass in growing Hyp mice (XLH murine model). In the current study, we investigated the efficacy of Scl-Ab in treating XLH pathophysiology in adult Hyp mice that are past the period of rapid skeletal growth (12 and 20-weeks old). We hypothesized that Scl-Ab would not only increase bone formation, bone strength and bone mass, but would also normalize phosphate regulating hormones, FGF23, parathyroid hormone (PTH), and vitamin 1,25(OH)2D. Scl-Ab treatment increased cortical area, trabecular bone volume fraction, trabecular bone formation rate, and the bending moment in both sexes of both age groups. Scl-Ab treatment suppressed circulating levels of intact FGF23 and c-term FGF23 in treated male and female wild-type and Hyp mice of both age groups and improved both vitamin 1,25(OH)2D and PTH. Scl-Ab treated Hyp mice also showed evidence of increased renal expression of the sodium-phosphate co-transporter, NPT2a, specifically in the female Hyp mice. Our study suggests that Scl-Ab treatment can improve several skeletal and metabolic pathologies associated with XLH, further establishes the role of sclerostin in the regulation of FGF23 and provides evidence that Scl-Ab can improve phosphate regulation by targeting the bone-renal axis.

An Expert Perspective on Phosphate Dysregulation With a Focus on Chronic Hypophosphatemia

Because of their rarity, diseases characterized by chronic hypophosphatemia can be underrecognized and suboptimally managed, resulting in poor clinical outcomes. Moreover, serum phosphate may not be measured routinely in primary care practice. Authors participated in several working sessions to advance the understanding of phosphate homeostasis and the causes, consequences, and clinical implications of chronic hypophosphatemia. Phosphate levels are regulated from birth to adulthood. Dysregulation of phosphate homeostasis can result in hypophosphatemia, which becomes chronic if phosphate levels cannot be normalized. Chronic hypophosphatemia may be underrecognized as serum phosphate measurement is not always part of routine analysis in the primary care setting and results might be misinterpreted, for instance, due to age-specific differences not being accounted for and circadian variations. Clinical consequences of chronic hypophosphatemia involve disordered endocrine regulation, affect multiple organ systems, and vary depending on patient age and the underlying disorder. Signs and symptoms of chronic hypophosphatemic diseases that manifest during childhood or adolescence persist into adulthood if the disease is inadequately managed, resulting in an accumulation of clinical deficits and a progressive, debilitating impact on quality of life. Early identification and diagnosis of patients with chronic hypophosphatemia is crucial, and clinical management should be started as soon as possible to maximize the likelihood of improving health outcomes. Furthermore, in the absence of a universally accepted description for "chronic hypophosphatemia," a definition is proposed here that aims to raise awareness of these diseases, facilitate diagnosis, and guide optimal phosphate management strategies by improving monitoring and assessment of patient response to treatment. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Rickets guidance: part II-management

Here, we discuss the management of different forms of rickets, including new therapeutic approaches based on recent guidelines. Management includes close monitoring of growth, the degree of leg bowing, bone pain, serum phosphate, calcium, alkaline phosphatase as a surrogate marker of osteoblast activity and thus degree of rickets, parathyroid hormone, 25-hydroxyvitamin D₃, and calciuria. An adequate calcium intake and normal 25-hydroxyvitamin D₃ levels should be assured in all patients. Children with calcipenic rickets require the supplementation or pharmacological treatment with native or active vitamin D depending on the underlying pathophysiology. Treatment of phosphopenic rickets depends on the underlying pathophysiology. Fibroblast-growth factor 23 (FGF23)-associated hypophosphatemic rickets was historically treated with frequent doses of oral phosphate salts in combination with active vitamin D, whereas tumor-induced osteomalacia (TIO) should primarily undergo tumor resection, if possible. Burosumab, a fully humanized FGF23-antibody, was recently approved for treatment of X-linked hypophosphatemia (XLH) and TIO and shown to be superior for treatment of XLH compared to conventional treatment. Forms of hypophosphatemic rickets independent of FGF23 due to genetic defects of renal tubular phosphate reabsorption are treated with oral phosphate only, since they are associated with excessive 1,25-dihydroxyvitamin D production. Finally, forms of hypophosphatemic rickets caused by Fanconi syndrome, such as nephropathic cystinosis and Dent disease require disease-specific treatment in addition to phosphate supplements and active vitamin D. Adjustment of medication should be done with consideration of treatment-associated side effects, including diarrhea, gastrointestinal discomfort, hypercalciuria, secondary hyperparathyroidism, and development of nephrocalcinosis or nephrolithiasis.

Burosumab for X-linked hypophosphatemia in children and adolescents: Opinion based on early experience in seven European countries

Given the relatively recent introduction of burosumab in the management of X-linked hypophosphatemia (XLH), there is limited real-world data to guide its use in clinical practice. As a group of European physicians experienced with burosumab treatment in clinical practice, we convened with the objective of sharing these practice-based insights on the use of burosumab in children and adolescents with XLH. We attended two virtual meetings, then discussed key questions via Within3, a virtual online platform. Points of discussion related to patient selection criteria, burosumab starting dose, dose titration and treatment monitoring. Our discussions revealed that criteria for selecting children with XLH varied across Europe from all children above 1 year to only children with overt rickets despite conventional treatment being eligible. We initiated burosumab dosing according to guidance in the Summary of Product Characteristics, an international consensus statement from 2019 and local country guidelines. Dose titration was primarily guided by serum phosphate levels, with some centers also using the ratio of tubular maximum reabsorption of phosphate to glomerular filtration rate (TmP/GFR). We monitored response to burosumab treatment clinically (growth, deformities, bone pain and physical functioning), radiologically (rickets and deformities) and biochemically (serum phosphate, alkaline phosphatase, 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D, urine calcium-creatinine ratio and TmP/GFR). Key suggestions made by our group were initiation of burosumab treatment in children as early as possible, from the age of 1 year, particularly in those with profound rickets, and a need for clinical studies on continuation of burosumab throughout adolescence and into adulthood.

Dental health of pediatric patients with X-linked hypophosphatemia (XLH) after three years of burosumab therapy

An inactivating PHEX gene mutation with the resultant accumulation of several mineralization-inhibiting proteins (e.g., FGF23) causes skeletal and dental morbidity in X-linked hypophosphatemia (XLH). This prospective case-control study explored the effect of burosumab, an anti-FGF23 antibody, on dental health of children with XLH. Ten children (age 4.3-15 years) with XLH underwent burosumab treatment per protocol. Assessment of their dental status at treatment initiation and after 1 and 3 years of treatment included clinical, laboratory and radiographic evaluation of rickets and dentition. Orthopantomographic examinations of ten healthy sex- and age-matched controls were selected for comparison. Coronal and pulp dimensions of a selected permanent mandibular molar were measured with Planmeca Romexis^® software. One year of treatment led to improvement of height z-score (p=0.019) and healing of the rickets (p<0.001) in the XLH patients, and those achievements were maintained after three years of treatment. Dental morphology of XLH patients, distinguished by increased pulp-coronal ratios compared to controls (p=0.002), remained larger after the first year of treatment (p<0.001) and did not attain the decrease expected with age after three years of treatment. Five patients had a history of recurrent dental abscesses, with three having undergone at least one episode during the year before burosumab initiation. One patient sustained recurrent abscesses throughout three years of treatment. The persistence of the unique dental morphology of XLH patients undergoing burosumab therapy, as evidenced by excessively larger pulp dimensions, supports the role of other PHEX gene-related local mineralization inhibitors, such as osteopontin, in the pathogenesis of dental morbidity.

Persistent Lower Limb Deformities Despite Amelioration of Rickets in X-Linked Hypophosphatemia (XLH) - A Prospective Observational Study

BACKGROUND

Gait deviations, lower limb pain and joint stiffness represent key symptoms in patients with X-linked hypophosphatemia (XLH, OMIM 307800), a rare disorder of mineral homeostasis. While the pathomechanism for rickets is well understood, the direct role of PHEX (Phosphate-regulating neutral endopeptidase) deficiency in non-rachitic features including complex deformities, skull and dental affections remains unclear. FGF23-inhibiting antibody treatment can normalize serum phosphate levels and to improve rickets in XLH patients. However, linear growth remains impaired and effects on lower limb deformity and gait are insufficiently studied.

AIMS

To characterize and evaluate the course of lower limb deformity in a case series of pediatric XLH patients receiving Burosumab therapy.

METHODS

Comparative assessment of planar radiographs, gait analysis, biochemical and clinical features of pediatric patients before and ≥12 months after initiation of FGF23-inhibiting was performed prospectively. Lower limb maltorsion was quantified by torsional MRI and gait analysis. Standardized deformity analysis of lower limb anteroposterior radiographs was conducted.

RESULTS

Seven patients (age 9.0 +/-3.6 years) were eligible for this study. All patients received conventional treatment before onset of antibody treatment. Maltorsion of the femur was observed in 8/14 legs using torsional MRI (mean antetorsion 8.79°). Maltorsion of the tibia was observed in 9/14 legs (mean external torsion 2.8°). Gait analysis confirmed MRI findings with femoral external malrotation prior to and one year after onset of Burosumab therapy. Internal foot progression (intoeing gait) remained pathological in all cases (mean 2.2°). Knee rotation was pathologically internal 10/14 legs. Mean mechanical axis deviation (MAD) of 16.1mm prior to Burosumab changed in average by 3.9mm. Three children underwent guided growth procedures within the observation period. Mild postprocedural rebound of frontal axis deviation was observed under Burosumab treatment in one patient.

CONCLUSIONS

This is the first study to investigate lower limb deformity parameters quantitatively in children with XLH receiving Burosumab. One year of Burosumab therapy was associated with persistent maltorsion and frontal axis deviation (varus/valgus) despite improved rickets in this small, prospective uncontrolled study.

Prevalence of Enthesopathies in Adults With X-linked Hypophosphatemia: Analysis of Risk Factors

CONTEXT

Enthesopathies are the determinant of a poor quality of life in adults with X-linked hypophosphatemia (XLH).

OBJECTIVE

To describe the prevalence of patients with enthesopathies and to identify the risk factors of having enthesopathies.

METHODS

Retrospective study in the French Reference Center for Rare Diseases of the Calcium and Phosphate Metabolism between June 2011 and December 2020. Adult XLH patients with full body X-rays performed using the EOS® low-dose radiation system and clinical data collected from medical records. The main outcome measures were demographics, PHEX mutation, conventional treatment, and dental disease with the presence of enthesopathies.

RESULTS

Of the 114 patients included (68% women, mean age 42.2 ± 14.3 years), PHEX mutation was found in 105 patients (94.6%), 86 (77.5%) had been treated during childhood. Enthesopathies (spine and/or pelvis) were present in 67% of the patients (n = 76). Patients with enthesopathies were significantly older (P = .001) and more frequently reported dental disease collected from medical records (P = .03). There was no correlation between the PHEX mutations and the presence of enthesopathies. Sixty-two patients had a radiographic dental examination in a reference center. Severe dental disease (number of missing teeth, number of teeth endodontically treated, alveolar bone loss, and proportion of patients with 5 abscesses or more) was significantly higher in patients with enthesopathies.

CONCLUSION

Adult XLH patients have a high prevalence of enthesopathies in symptomatic adults patients with XLH seen in a reference center. Age and severe dental disease were significantly associated with the presence of enthesopathies.

Combined treatment by burosumab and a calcimimetic can ameliorate hypophosphatemia due to excessive actions of FGF23 and PTH in adult XLH with tertiary hyperparathyroidism: A case report

INTRODUCTION

X-linked hypophosphatemia (XLH) is the most prevalent type of heritable fibroblast growth factor 23 (FGF23)-related hypophosphatemic rickets. Recently, anti-FGF23 antibody, burosumab, has become clinically available. We herein report a patient with adult XLH and tertiary hyperparathyroidism.

CASE PRESENTATION

The serum phosphate level and tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR) remained low, despite burosumab treatment. While the influence of the relationship between FGF23 and parathyroid hormone (PTH) on the phosphaturic effect is unclear, it was considered that a high level of PTH due to tertiary hyperparathyroidism remains to suppress renal phosphate reabsorption. A calcimimetic, evocalcet, increased the serum phosphate level and TmP/GFR.

DISCUSSION AND CONCLUSION

Therefore, it is important to evaluate the presence of secondary-tertiary hyperparathyroidism in patients whose serum phosphate level does not increase with burosumab treatment.

Mineral tessellation in bone and the stenciling principle for extracellular matrix mineralization

We review here the Stenciling Principle for extracellular matrix mineralization that describes a double-negative process (inhibition of inhibitors) that promotes mineralization in bone and other mineralized tissues, whereas the default condition of inhibition alone prevents mineralization elsewhere in soft connective tissues. The stenciling principle acts across multiple levels from the macroscale (skeleton/dentition vs soft connective tissues), to the microscale (for example, entheses, and the tooth attachment complex where the soft periodontal ligament is situated between mineralized tooth cementum and mineralized alveolar bone), and to the mesoscale (mineral tessellation). It relates to both small-molecule (e.g. pyrophosphate) and protein (e.g. osteopontin) inhibitors of mineralization, and promoters (enzymes, e.g. TNAP, PHEX) that degrade the inhibitors to permit and regulate mineralization. In this process, an organizational motif for bone mineral arises that we call crossfibrillar mineral tessellation where mineral formations - called tesselles - geometrically approximate prolate ellipsoids and traverse multiple collagen fibrils (laterally). Tesselle growth is directed by the structural anisotropy of collagen, being spatially restrained in the shorter transverse tesselle dimensions (averaging 1.6 × 0.8 × 0.8 μm, aspect ratio 2, length range 1.5-2.5 μm). Temporo-spatially, the tesselles abut in 3D (close ellipsoid packing) to fill the volume of lamellar bone extracellular matrix. Poorly mineralized interfacial gaps between adjacent tesselles remain discernable even in mature lamellar bone. Tessellation of a same, small basic unit to form larger structural assemblies results in numerous 3D interfaces, allows dissipation of critical stresses, and enables fail-safe cyclic deformations. Incomplete tessellation in osteomalacia/odontomalacia may explain why soft osteomalacic bones buckle and deform under loading.

Genotype-phenotype analysis, and assessment of the importance of the zinc-binding site in PHEX in Japanese patients with X-linked hypophosphatemic rickets using 3D structure modeling

X-linked hypophosphatemic rickets (XLH) is an inheritable type of rickets caused by inactivating variants in the phosphate regulating endopeptidase homolog X-linked (PHEX) gene, which results in the overproduction of fibroblast growth factor 23 (FGF23). The mechanism by which PHEX impairment leads to FGF23 overproduction is unknown. Because little is known regarding the genotype-phenotype correlation in Japanese XLH, we summarized the available clinical and genetic data and analyzed the genotype-phenotype relationships using 3-dimensional (3D) structure modeling to clarify the XLH pathophysiology. We retrospectively reviewed the clinical features and performed genetic analysis of 39 Japanese patients with XLH from 28 unrelated pedigrees carrying any known or novel PHEX variant. To predict changes in the 3D structure of mutant PHEX, we constructed a putative 3D model of each mutant and evaluated the effect of structural alteration by genotype-phenotype correlation analysis. Genetic analysis revealed 23 PHEX variants, including eight novel variants. They were associated with high i-FGF23 levels, hypophosphatemia, phosphaturia, high alkaline phosphatase levels, and short stature. No gene dosage effect or genotype-phenotype correlation was observed when truncating and non-truncating variants were compared. However, the conservation of the zinc-binding site and cavity in PHEX had an impact on the elevation of i-FGF23 levels. Via genotype-phenotype relationship analysis using 3D modeling, we showed that the zinc-binding site and cavity in PHEX can play a critical role in its function. These findings provide new genetic clues for investigating the function of PHEX and the pathogenesis of XLH.

FGF23 and Hypophosphatemic Rickets/Osteomalacia

PURPOSE OF REVIEW

X-linked hypophosphatemia and tumor-induced osteomalacia are diseases characterized by hypophosphatemia with impaired proximal tubular phosphate reabsorption. Complete resection of responsible tumors is the first-line therapy for patients with tumor-induced osteomalacia. In contrast, phosphate and active vitamin D have been used for patients with X-linked hypophosphatemia and inoperable ones with tumor-induced osteomalacia. The purpose of this review is to summarize the pathogenesis of these diseases and discuss about the new treatment.

RECENT FINDINGS

Excessive FGF23 production has been shown to underline several kinds of hypophosphatemic rickets/osteomalacia including X-linked hypophosphatemia and tumor-induced osteomalacia. Burosumab, an anti-FGF23 monoclonal antibody, was approved for clinical use, while the indications of burosumab are different depending on countries. The inhibition of excessive FGF23 activity has been approved as a new therapy for several kinds of hypophosphatemic diseases. Further studies are necessary to clarify the long-term effects and safety of burosumab.

Conductive Hearing Loss in the Hyp Mouse Model of X-Linked Hypophosphatemia Is Accompanied by Hypomineralization of the Auditory Ossicles

X-linked hypophosphatemia (XLH) is a hereditary musculoskeletal disorder caused by loss-of-function mutations in the PHEX gene. In XLH, increased circulating fibroblast growth factor 23 (FGF23) levels cause renal phosphate wasting and low concentrations of 1,25-dihydroxyvitamin D, leading to an early clinical manifestation of rickets. Importantly, hearing loss is commonly observed in XLH patients. We present here data from two XLH patients with marked conductive hearing loss. To decipher the underlying pathophysiology of hearing loss in XLH, we utilized the Hyp mouse model of XLH and measured auditory brain stem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) to functionally assess hearing. As evidenced by the increased ABR/DPOAE threshold shifts in the mid-frequency range, these measurements indicated a predominantly conductive hearing loss in Hyp mice compared to wild-type (WT) mice. Therefore, we carried out an in-depth histomorphometric and scanning electron microscopic analysis of the auditory ossicles. Quantitative backscattered electron imaging (qBEI) indicated a severe hypomineralization of the ossicles in Hyp mice, evidenced by lower calcium content (CaMean) and higher void volume (ie, porosity) compared to WT mice. Histologically, voids correlated with unmineralized bone (ie, osteoid), and the osteoid volume per bone volume (OV/BV) was markedly higher in Hyp mice than WT mice. The density of osteocyte lacunae was lower in Hyp mice than in WT mice, whereas osteocyte lacunae were enlarged. Taken together, our findings highlight the importance of ossicular mineralization for hearing conduction and point toward the potential benefit of improving mineralization to prevent hearing loss in XLH. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Novel PHEX gene locus-specific database: Comprehensive characterization of vast number of variants associated with X-linked hypophosphatemia (XLH)

X‐linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemia, is caused by disrupting variants in the PHEX gene, located on the X chromosome. XLH is inherited in an X‐linked pattern with complete penetrance observed for both males and females. Patients experience lifelong symptoms resulting from chronic hypophosphatemia, including impaired bone mineralization, skeletal deformities, growth retardation, and diminished quality of life. This chronic condition requires life‐long management with disease‐specific therapies, which can improve patient outcomes especially when initiated early in life. To centralize and disseminate PHEX variant information, we have established a new PHEX gene locus‐specific database, PHEX LSDB. As of April 30, 2021, 870 unique PHEX variants, compiled from an older database of PHEX variants, a comprehensive literature search, a sponsored genetic testing program, and XLH clinical trials, are represented in the PHEX LSDB. This resource is publicly available on an interactive, searchable website (https://www.rarediseasegenes.com/), which includes a table of variants and associated data, graphical/tabular outputs of genotype‐phenotype analyses, and an online submission form for reporting new PHEX variants. The database will be updated regularly with new variants submitted on the website, identified in the published literature, or shared from genetic testing programs.

Changes in Bone Mineral Density Following Conventional Oral Phosphonate Treatment of Hypophosphatemic Osteomalacia: A Non-Randomized Controlled Study

Purpose

There are limited clinical studies aimed at solving the problem of the efficiency of conventional treatment with oral phosphate and calcitriol in adults with hypophosphatemic osteomalacia (HO). In addition, there still had no good non-hazardous markers to evaluate the severity of bone loss of osteomalacia before and after treatment. Therefore, the purpose of this study was to assess the efficacy of conventional treatment with a self-blended phosphate supplementation and calcitriol on patients with HO and whether bone mineral density (BMD) can be helpful for monitoring the efficacy.

Patients and Methods

A total of 21 HO patients and 105 healthy controls were enrolled. All patients were tested for serum biomarkers and BMD of the lumbar spine (L1-L4), femoral neck, and total left hip. After three years of treatment, 11 of 21 HO patients were recalled for BMD measurement. According to the administration of drugs, HO patients with calcium and calcitriol were divided into three phosphate treatment groups: patients in group A (n = 3) received continuous phosphate supplementation, patients in group B (n = 5) received intermittent phosphate supplementation and patients in group C (n = 3) received no phosphate supplementation.

Results

The diagnoses of 21 HO patients were 5 cases of hereditary hypophosphatemic rickets, 4 cases of Fanconi syndrome with the features of renal tubular acidosis and vitamin D deficiency, and 12 cases of hereditary vitamin D abnormality. The average initial serum phosphorus level of the patient group was approximately 50% lower than that of the control group. Lower BMD was significantly observed in the HO group than the control group at the lumbar spine and total hip. Continuous treatment with the phosphate supplement could increase BMD in the lumbar spine and total hip by 33.4-52.3% and in the femoral neck increased by 43.2-79.3% compared with baseline, and the effect appears to be continued once treatment is discontinued.

Conclusion

These findings suggest that conventional therapy can improve bone mineral defects in patients with HO, especially in the femoral neck. Detection of BMD in HO patients is a good tool to assess the extent of bone defects and the therapeutic effect.

Trial Registration

Chinese Clinical Trial Registry, ChiCTR-OOC-16010095. Registered 7 December 2016. Retrospectively registered.

Prevalence and clinical characteristics of X-linked hypophosphatemia in Paraná, southern Brazil

OBJECTIVE

The aim of this cross-sectional study was to estimate the prevalence of XLH in Paraná, a state in southern Brazil, and report the clinical features and complications of the disease.

METHODS

We invited all endocrinologists (n = 205), nephrologists (n = 221), orthopedic surgeons (n = 1020), and pediatricians (n = 1000) in Paraná to fill out an electronic survey with information on patients with X-linked hypophosphatemia (XLH), and searched the records of the state's health department for all calcitriol prescriptions in 2018.

RESULTS

In all, 244 (10%) specialists responded to the email, of whom 18 (7.4%) reported to be taking care of patients with XLH and answered the online survey. A total of 57 patients with XLH were identified (prevalence 5 per million inhabitants). The median age at diagnosis was 22 years, and 42.2% were children and adolescents. Fifteen patients had genetic testing showing a PHEX mutation. Overall, 91.2% had bone deformities, 30.8% had a history of fragility fractures, and 22.4% had renal complications.

CONCLUSION

This study demonstrated a prevalence of XLH of 5 cases per million inhabitants in the state of Paraná, a rate lower than the one reported in other countries. Manifestations of renal calcification and bone fragility were frequent among the patients. This is the first epidemiological study evaluating the prevalence and clinical presentation of XLH in Latin America.

Adult rheumatologic features, treatment and complications of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare genetic phosphate disorder caused mainly by PHEX mutations. Unlike for children, knowledge of the disease's manifestations in adults is limited. Musculoskeletal symptoms are the main feature of the disease in young adults associated with a heavy burden on patients' life. They include fractures and pseudofractures, pain, joint stiffness, osteoarthritis, enthesopathies, and muscle weakness, eventually leading to impaired quality of life. Conventional treatment with phosphate supplements and vitamin D analogs is indicated in symptomatic patients. Appropriate rehabilitation is also a key to the management of the disease to improve physical function and decrease pain, stiffness, and fatigue. Regarding the incidence and consequences of musculoskeletal features in XLH, all patients should be assessed by a bone disease specialist and, if necessary, managed by a multidisciplinary team.

Higher prevalence of non-skeletal comorbidity related to X-linked hypophosphataemia: a UK parallel cohort study using CPRD

OBJECTIVES

X-Linked hypophosphataemic rickets (XLH) is a rare multi-systemic disease of mineral homeostasis that has a prominent skeletal phenotype. The aim of this study was to describe additional comorbidities in XLH patients compared with general population controls.

METHODS

The Clinical Practice Research Datalink (CPRD) GOLD was used to identify a cohort of XLH patients (1995-2016), along with a non-XLH cohort matched (1 : 4) on age, sex and GP practice. Using the CALIBER portal, phenotyping algorithms were used to identify the first diagnosis (and associated age) of 273 comorbid conditions during patient follow-up. Fifteen major disease categories were used and the proportion of patients having ≥1 diagnosis was compared between cohorts for each category and condition. Main analyses were repeated according to the Index of Multiple Deprivation (IMD).

RESULTS

There were 64 and 256 patients in the XLH and non-XLH cohorts, respectively. There was increased prevalence of endocrine [OR 3.46 (95% CI: 1.44, 8.31)] and neurological [OR 3.01 (95% CI: 1.41, 6.44)] disorders among XLH patients. Across all specific comorbidities, four were at least twice as likely to be present in XLH cases, but only depression met the Bonferroni threshold: OR 2.95 (95% CI: 1.47, 5.92). Distribution of IMD among XLH cases indicated greater deprivation than the general population.

CONCLUSION

We describe a higher risk of mental illness in XLH patients compared with matched controls, and greater than expected deprivation. These findings may have implications for clinical practice guidelines and decisions around health and social care provision for these patients.

X-linked hypophosphatemia: The medical expert's challenges and the patient's concerns on their journey with the disease

X-linked hypophosphatemia (XLH) is a rare inheritable disorder of phosphate handling due to loss of function mutations of the PHEX gene, associated with increased production of FGF23 and impaired bone mineralization. In children, the disease's most common manifestations are bowing deformities of the lower limbs, short stature, and spontaneous dental abscesses. In adults, these are osteomalacia, insufficiency fractures, and enthesopathies associated with bone and joint pain. The XLH patient's journey with the disease may be difficult, reflecting concerns and experiences globally common to all patients with rare genetic diseases. Delays in diagnosis often preclude an optimal treatment outcome. Under-treatment is common as treating physicians, particularly those not familiar with the disease, tend to err on the side of caution, often choosing safety over efficacy. Physical abnormalities, pain, diminished function, and impaired mobility tend not only to isolate the XLH patient from his peers but also to have a significant psychological effect, eventually leading to significant impairment in quality of life. Significant advances in understanding the pathophysiology of XLH, the availability of a very comprehensive Evidence-based Guideline for the diagnosis and management of XLH, and the successful development of an effective and safe disease-specific novel therapy for XLH, have paved the way for a significant improvement in the management of this rare disorder of phosphate metabolism, heralding a significant improvement in the disease's outcome measures. Additional data from long-term observational studies and randomized controlled trials are eagerly awaited to consolidate these promising developments in the field of this rare disease.

Burden of disease and clinical targets in adult patients with X-linked hypophosphatemia. A comprehensive review

X-linked hypophosphataemia (XLH) is a lifelong condition. Despite the mounting clinical evidence highlighting the long-term multi-organ sequelae of chronic phosphate wasting and consequent hypophosphatemia over the lifetime and the morbidities associated with adult age, XLH is still perceived as a paediatric disease.

INTRODUCTION

Children who have XLH need to transition from paediatric to adult healthcare as young adults. While there is general agreement that all affected children should be treated (if the administration and tolerability of therapy can be adequately monitored), there is a lack of consensus regarding therapy in adults.

METHODS

To provide guidance in both diagnosis and treatment of adult XLH patients and promote better provision of care for this potentially underserved group of patients, we review the available clinical evidence and discuss the current challenges underlying the transition from childhood to adulthood care to develop appropriate management and follow-up patterns in adult XLH patients.

RESULTS AND CONCLUSIONS

Such a multi-systemic lifelong disease would demand that the multidisciplinary approach, successfully experienced in children, could be transitioned to adulthood care with an integration of specialized sub-disciplines to efficiently control musculoskeletal symptoms while optimizing patients' QoL. Overall, it would be desirable that transition to adulthood care could be a responsibility shared by the paediatric and adult XLH teams. Pharmacological management should require an adequate balance between the benefits derived from the treatment itself with complicated and long-term monitoring and the potential risks, as they may differ across age strata.

Monitoring response to conventional treatment in children with XLH: Value of ALP and Rickets Severity Score (RSS) in a real world setting

INTRODUCTION

X-linked hypophosphataemia (XLH) is conventionally managed with oral phosphate and active vitamin D analogues.

OBJECTIVES

To evaluate long term treatment response by assessing biochemical disease activity [serum alkaline phosphatase (ALP)], radiological rickets severity score (RSS), growth and morbidity in patients with XLH on conventional therapy and assess the correlation between serum ALP and RSS.

METHODS

XLH patients from 3 UK tertiary centres with ≥3 radiographs one year apart were included. Data was collected retrospectively. The RSS was assessed from routine hand and knee radiographs and ALP z scores were calculated using age-specific reference data.

RESULTS

Thirty-eight (male = 12) patients met the inclusion criteria. The mean ± SD knee, wrist and total RSS at baseline (median age 1.2 years) were 2.0 ± 1.2, 1.9 ± 1.2 and 3.6 ± 1.3 respectively; and at the most recent clinic visit (median age 9.0 years, range 3.3-18.9) were 1.6 ± 1.0, 1.0 ± 1.0 and 2.5 ± 1.5 respectively. The mean ± SD serum ALP z scores at baseline and the most recent visit were 4.2 ± 2.3 and 4.0 ± 3.3. Median height SDS at baseline and most recent visit were -1.2 and -2.1 (p = 0.05). Dental abscess, craniosynostosis, limb deformity requiring orthopaedic intervention and nephrocalcinosis were present in 31.5%, 7.9%, 31.6% and 42.1% of the cohort respectively. There was no statistically significant (p > 0.05) correlation between ALP z scores and knee (r = 0.07) or total (r = 0.12) RSS.

CONCLUSIONS

Conventional therapy was not effective in significantly improving biochemical and radiological features of disease. The lack of association between serum ALP and rickets severity on radiographs limits the value of ALP as the sole indicator of rickets activity in patients receiving conventional therapy.

Microarchitectural parameters and bone mineral density in patients with tumour-induced osteomalacia by HR-pQCT and DXA

INTRODUCTION

Tumour-induced osteomalacia (TIO) is a rare paraneoplastic condition characterised by decreased tubular phosphate reabsorption. The purpose of this study is to evaluate bone mineral density (BMD) and microarchitecture in six TIO patients, compared with 18 healthy controls.

METHODS

Volumetric BMD and microarchitecture were evaluated by high-resolution peripheral quantitative computed tomography (HR-pQCT), and areal BMD by dual-energy X-ray absorptiometry (DXA). Differences between groups were significant for p < .05.

RESULTS

All TIO subjects were healthy until the development of diffuse bone pain and multiple skeletal fractures and deformities. At baseline, sPi and TmPi/GFR were low and patients were on vitamin D and phosphate replacement at the study. Compared with controls, TIO patients had lower aBMD at lumbar spine and hip, and lower vBMD at trabecular, cortical and entire bone, at distal radius (R) and distal tibia (T): trabecular vBMD (R = 118.3 × 177.1; T = 72.3 × 161.3 gHA/cm³ ); cortical vBMD (R = 782.3 × 866.5; T = 789.1 × 900.9 gHA/cm³ ); total region vBMD (R = 234.5 × 317; T = 167.1 × 295.8 gHA/cm³ ). Bone microarchitecture was very heterogeneous among patients and significantly different from controls: lower cortical thickness (R = 0.59 × 0.80; T = 0.90 × 1.31 mm), bone volume-to-total volume ratio (R = 0.09 × 0.14; T = 0.06 × 0.13) and Tb.N (R = 1.46 × 2.10; T = 0.93 × 1.96 mm^-1 ) and also higher Tb.Sp (R = 0.70 × 0.41; T = 1.28 × 0.45 mm) and Tb.1/N.SD (R = 0.42 × 0.18; T = 0.87 × 0.20 mm).

CONCLUSION

In this original study of TIO patients, DXA and HR-pQCT evaluation identified lower areal and volumetric BMD and severely impaired microarchitecture at cortical and trabecular bones, which probably contribute to bone fragility and fractures.

Contribution of imaging to the diagnosis and follow up of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is the most common form of inheritable rickets. The disease is caused principally by PHEX mutations leading to increased concentrations of circulating intact FGF23, hence renal phosphate wasting, hypophosphatemia, and decreased circulating levels of 1,25(OH)₂ vitamin D. The chronic hypophosphatemia leads to rickets and osteomalacia through a combination of mechanisms, including a lack of endochondral ossification and impaired mineralization. Imaging has a major role in determining the diagnosis of rickets and its cause, detecting complications as early as possible, and helping in treatment monitoring.

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.

Hereditary hypophosphatemic rickets and craniosynostosis

BACKGROUND

Craniosynostosis is an underdiagnosed complication associated with hypophosphatemic rickets. The study aims to describe the clinical and auxological characteristic of children with hypophosphatemic rickets and craniosynostosis, describe the usual treatment, and compare the characteristics with those of children without craniosynostosis.

METHODS AND PATIENTS

An observational and retrospective cohort study was conducted. Clinical notes and cranial images were reviewed. Out of 96 children, only the 50 patients who had skull images were included.

RESULTS

Out of 50 patients, 26 (15 males) had craniosynostosis (52%). No differences were observed in birth size, age, height, body proportions, alkaline phosphatase, serum phosphate, or percent tubular reabsorption of phosphate at first appointment among children with or without craniosynostosis. Among patients with craniosynostosis, dolichocephaly was prevalent. The sagittal suture was affected in all patients with craniosynostosis, with 19 of 26 children (73%) affected with isolated scaphocephaly. Pan-sutural craniosynostosis was present in 7 children (27%). None of the children had microcephaly, 7 of them presented macrocephaly and, in the remaining subjects, head circumference was normal. Five patients had undergone at least 1 cranial remodeling surgery. One patient with craniosynostosis was diagnosed with a Chiari I malformation. Molecular characterization of PHEX gene was performed in 14 cases.

CONCLUSIONS

Craniosynostosis is an underdiagnosed complication of hypophosphatemic rickets. Many patients with normal head size and growth may go undiagnosed, thus it is important to consider this association for early diagnosis and possible surgical treatment. A multidisciplinary approach is necessary for a correct long-term follow-up.

Clinical and molecular characterization of Chilean patients with X-linked hypophosphatemia

We report the most comprehensive clinical and molecular characterization of XLH patients performed in Chile. We show high prevalence of musculoskeletal burden and pain, associated with significantly impaired physical capacity and quality of life, with many relevant complications presenting more frequently than previously reported in cohorts from developed countries.

INTRODUCTION

Our current understanding of the clinical presentation and natural history of X-linked hypophosphatemia (XLH) comes mainly from cohorts from developed countries, with limited data on the clinical and genetic abnormalities of XLH patients in South America.

OBJECTIVE

To describe the clinical, biochemical, and molecular presentation of patients with XLH in Chile.

METHODS

Patients with XLH referred by endocrinologist throughout Chile were included. Demographic data and clinical presentation were obtained from a clinical interview. Surveys were applied for quality of life (QoL), pain, and functionality. FGF23 was measured by ELISA, and genetic testing was performed. Imaging studies were conducted to assess skeletal and renal involvement.

RESULTS

We included 26 patients, aged 2-64 years, from 17 unrelated Chilean families. All pediatric patients but only 40% of adults were receiving conventional therapy, while 65% of all patients had elevated alkaline phosphatase. All patients had mutations in PHEX, including 5 novel variants. Radiographic skeletal events (RSE) and enthesopathies in adults were frequent (34% and 85%, respectively). The duration of treatment was associated with fewer RSE (p < 0.05). Most adults reported pain and impaired QoL, and 50% had impaired physical capacity. The number of enthesopathies was associated with worse pain and stiffness scores (p < 0.05).

CONCLUSION

Chilean patients with XLH have a high prevalence of musculoskeletal burden associated with pain and impaired physical capacity and QoL, especially in adults who were generally undertreated. These data identify a significant unmet need, inform our understanding of the current status of patients, and can guide care for XLH patients in similarly socioeconomically defined countries.

Autosomal Dominant Hypophosphatemic Rickets: A Case Report and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is an extremely rare form of genetic rickets caused by mutations in the fibroblast growth factor 23 gene. ADHR is characterized by hypophosphatemia secondary to isolated renal phosphate wasting. Only a few cases of ADHR have been reported in the literature to date. We describe the case of a 17-month-old girl who presented with severe failure to thrive (length: −4.08 standard deviation (SD), weight: −2.2 SD) and hypotonia. Hypophosphatemia, decreased tubular phosphate reabsorption (69%), and rachitic lesions were found. Genetic analysis showed the heterozygous variant c.536G>A (NM_020638.3:c.536G>A) in exon 3 of the FGF23 gene, leading to the diagnosis of ADHR. She was treated with phosphate salts and oral alfacalcidol. After 4 years of treatment, at 5 years of age, the patient’s ADHR resolved spontaneously. Considering the lack of knowledge regarding ADHR, we reviewed the literature to describe the features of this rare and poorly understood disease. Eleven ADHR pediatric cases have been described thus far, with cases tending to be more common in females than males. Similar to the general population, two groups of patients with ADHR can be described depending on the mutations present: patients with an R179 and R176 mutation have early-onset of disease and higher frequency of rickets, and a milder and late-onset of disease, respectively. Symptoms and disease severity may fluctuate. Spontaneous remission may occur during the pediatric age.

Incidence of Complications in 25 Adult Patients With X-linked Hypophosphatemia

CONTEXT

Adults with X-linked hypophosphatemia (XLH) present complications other than osteomalacia.

OBJECTIVE

To describe the incidence and severity of comorbidities in adults with XLH.

METHODS

This observational retrospective study included a total of 25 adults with XLH with thorough investigations, including spinal computed tomography scans, x-rays of hip/knee joints and Achilles tendons, abdominal ultrasounds, and audiograms. The index of ossification of the anterior/posterior longitudinal ligament and yellow ligament (OA/OP/OY index) and the sum of OA/OP/OY index (OS index) were utilized to evaluate the severity of spinal ligament ossification. The Kellgren-Lawrence (KL) classification was adopted to evaluate the severity of the hip/knee osteophytes.

RESULTS

The participants consisted of 13 male patients and 12 female patients from 21 families, with a median age of 43 (range, 18-72) years. In all, 20 patients (80%) showed spinal ligament ossification. The median OA/OP/OY/OS indices were 2 (0-22), 0 (0-15), 6 (0-13), and 12 (0-41), respectively. Hip/knee osteophytes were reported in 24 (96%) and 17 cases (68%). The median KL grade was 3 in the hip joint and 2 in the knee joint, and 18 cases (72%) developed enthesopathy in the Achilles tendon. Nephrocalcinosis and hearing impairment were observed in 18 (72%) and 8 (32%) cases.

CONCLUSION

This study revealed a high prevalence and severity of ectopic ossification and disclosed the incidence of nephrocalcinosis and hearing impairment in adults with XLH. In cases with severe spinal ligament ossification or noticeable osteophytes around the hip/knee joints, undiagnosed XLH should be considered as a possible underlying condition.

INZ-701 Prevents Ectopic Tissue Calcification and Restores Bone Architecture and Growth in ENPP1-Deficient Mice

Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is the major enzyme that cleaves extracellular adenosine triphosphate (ATP) to generate pyrophosphate (PPi), an inorganic metabolite with potent anticalcification activity. Loss-of-function mutations cause hypopyrophosphatemia and lead to a state of ENPP1 deficiency, which has an acute infantile phase known as generalized arterial calcification of infancy (GACI) and a pediatric to adult phase known as autosomal-recessive hypophosphatemic rickets type 2 (ARHR2). ENPP1 deficiency manifests as ectopic calcification of multiple tissues, neointimal proliferation, premature mortality, impaired growth, and bone deformities. INZ-701, a human ENPP1-Fc protein, is in clinical development as an enzyme replacement therapy for the treatment of ENPP1 deficiency. The pharmacokinetic and pharmacodynamic profile and therapeutic effect of INZ-701 were investigated in Enpp1^asj/asj mice, a murine model of ENPP1 deficiency. Enpp1^asj/asj mice have undetectable plasma PPi, lower plasma phosphate, and higher FGF23 levels compared with wild-type (WT) mice. Enpp1^asj/asj mice on the acceleration diet, containing high phosphate and low magnesium, quickly develop clinical signs, including dehydration, rough hair coat, pinned ears, stiffed legs, and hunched back. Enpp1^asj/asj mice treated with vehicle had aforementioned clinical signs plus severe ectopic calcification in multiple tissues and bone defects, characteristics of the clinical phenotype observed in GACI and ARHR2 patients. Our results showed a durable PPi response for more than 3 days after a single dose of INZ-701. Treatment of ENPP1-deficient mice every other day with INZ-701 for 8 weeks restored circulating levels of PPi, prevented pathological calcification in all the tested organs, restored growth parameters, corrected bone defects, improved clinical signs, and decreased mortality in Enpp1^asj/asj mice, demonstrating the potential of INZ-701 to treat ENPP1 deficiency. © 2021 American Society for Bone and Mineral Research (ASBMR).

Characterization of Oral Health Status in Chilean Patients with X-Linked Hypophosphatemia

X-Linked Hypophosphatemia (XLH) is the most common cause of inherited hypophosphatemic rickets. Dental involvement, including spontaneous abscesses and/or fistulae, is an important part of the disease and has not been completely defined, especially in cohorts from developing countries. To describe oral health status in a cohort of Chilean patients with XLH and explore its correlation with biochemical presentation and treatment, we conducted a cross-sectional observational study of patients with PHEX mutation-confirmed XLH. All patients had an oral clinical exam, radiographic evaluation; clinical and biochemical data were obtained to determine their association with oral features. Twenty-six patients were included, 77% adults and 23% children. Most adults (89%) had past or current dental pulp pathology (abscesses and/or fistulae). Pulpal chamber enlargement and radiolucent apical lesions were common radiological features (94 and 74%, respectively). In children, abscess and/or fistulae were also common (33%). Caries index, which was determined by dmft/DMFT, was higher than the Chilean national average. Early and long-term therapy with phosphate and activated vitamin D was associated with lower carious index and attachment loss. XLH patients frequently present with high pulpal involvement and carious index. Conventional therapy was associated with lower carious index and attachment loss. These data highlight the importance of early and periodical dental care in order to prevent dental damage and assure a good quality of oral health for XLH patients.

Validation of the Radiographic Global Impression of Change (RGI-C) score to assess healing of rickets in pediatric X-linked hypophosphatemia (XLH)

BACKGROUND

Rickets is a primary manifestation of pediatric X-linked hypophosphatemia (XLH) - a rare progressive hereditary phosphate-wasting disease. Severity is quantified from radiographs using the Rickets Severity Scale (RSS). The Radiographic Global Impression of Change (RGI-C) is a complementary assessment in which a change score is assigned based on differences in the appearance of rickets on pairs of radiographs compared side by side.

OBJECTIVE

The current study evaluated the reliability, validity, and sensitivity to change of the RGI-C specifically in pediatric XLH.

METHODS

The reliability, validity, and sensitivity to change of the RGI-C were evaluated using data from two studies in pediatric XLH (113 children aged 1-12 years) in which burosumab treatment significantly improved rickets severity. Intra-rater and inter-rater reliability were assessed by three pediatric radiologists.

RESULTS

Intra-rater reliability for RGI-C global score was >90% for agreement within 1 point, with weighted kappa values >0.5, indicating moderate to almost perfect agreement. Inter-rater reliability was also >90% (0.47-0.52 for all reader pairs; moderate agreement). The RGI-C global score showed significant relationships with changes from baseline to week 64 in serum phosphorus (r = -0.397), alkaline phosphatase (-0.611), total RSS (-0.672), standing height (0.268), and patient-reported global functioning (0.306) and comfort/pain functioning (0.409). Based on standardized response means, RGI-C global scores were sensitive to change in RSS, differentiating between those considered improved and greatly improved. Results for validity and sensitivity to change were similar for the RGI-C wrist, knee, and standing long leg scores.

CONCLUSION

The RGI-C is a reliable, valid, and sensitive measure in pediatric XLH, and complementary to the RSS.

Rickets in Children: An Update

Rickets refers to a deficient mineralization of the growth plate cartilage, predominantly affecting longer bones. Despite the fact that preventive measures are available, it is still a common disease worldwide; nutritional rickets, due to vitamin D deficiency or dietary calcium inadequate intake, remains the most common form. Medical history, physical examination, radiologic features and biochemical tests are essential for diagnosis. Although recent studies suggest hypophosphatemia as the leading alteration, rickets is classically divided into two categories: calcipenic rickets and phosphopenic rickets. Knowledge of this categorization and of respective clinical and laboratory features is essential for rapid diagnosis and correct management. The aim of this review is to analyze the epidemiological, pathogenetic, clinical, and therapeutic aspects of the different forms of rickets, describing the novelties on this “long-lived” disease.

Switching from conventional therapy to burosumab injection has the potential to prevent nephrocalcinosis in patients with X-linked hypophosphatemic rickets

OBJECTIVES

X-linked hypophosphatemic rickets (XLH) is a congenital fibroblast growth factor (FGF)23-related metabolic bone disease that is treated with active vitamin D and phosphate as conventional therapies. Complications of these therapies include nephrocalcinosis (NC) caused by excessive urine calcium and phosphate concentrations. Recently, an anti-FGF23 antibody, burosumab, was developed and reported to be effective in poorly-controlled or severe XLH patients. This study aimed to reveal the impact of switching treatments in relatively well-controlled XLH children with the Rickets Severity Scale less than 2.0.

METHODS

The effects of the two treatments in eight relatively well-controlled XLH children with a mean age of 10.4 ± 1.9 years were compared retrospectively for the same treatment duration (31 ± 11 months) before and after the baseline.

RESULTS

Actual doses of alfacalcidol and phosphate as conventional therapy were 150.9 ± 43.9 ng/kg and 27.5 ± 6.3 mg/kg per day, respectively. Renal echography revealed spotty NC in 8/8 patients, but no aggravation of NC was detected by switching treatments. Switching treatments increased TmP/GFR (p=0.002) and %TRP (p<0.001), and improved the high urine calcium/creatinine ratio to the normal range (p<0.001) although both treatments controlled disease markers equally. Additionally, low intact parathyroid hormone during conventional therapy was increased within the normal range by switching treatments.

CONCLUSIONS

Our results suggest that a high dose of alfacalcidol was needed to control the disease, but it caused hypercalciuria and NC. We concluded that switching treatments in relatively well-controlled XLH children improved renal phosphate reabsorption and decreased urine calcium extraction, and may have the potential to prevent NC.

The Metabolic Bone Disease X-linked Hypophosphatemia: Case Presentation, Pathophysiology and Pharmacology

The authors present a stereotypical case presentation of X-linked hypophosphatemia (XLH) and provide a review of the pathophysiology and related pharmacology of this condition, primarily focusing on the FDA-approved medication burosumab. XLH is a renal phosphate wasting disorder caused by loss of function mutations in the PHEX gene (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). Typical biochemical findings include elevated serum levels of bioactive/intact fibroblast growth factor 23 (FGF23) which lead to (i) low serum phosphate levels, (ii) increased fractional excretion of phosphate, and (iii) inappropriately low or normal 1,25-dihydroxyvitamin D (1,25-vitD). XLH is the most common form of heritable rickets and short stature in patients with XLH is due to chronic hypophosphatemia. Additionally, patients with XLH experience joint pain and osteoarthritis from skeletal deformities, fractures, enthesopathy, spinal stenosis, and hearing loss. Historically, treatment for XLH was limited to oral phosphate supplementation, active vitamin D supplementation, and surgical intervention for cases of severe bowed legs. In 2018, the United States Food and Drug Administration (FDA) approved burosumab for the treatment of XLH and this medication has demonstrated substantial benefit compared with conventional therapy. Burosumab binds circulating intact FGF23 and blocks its biological effects in target tissues, resulting in increased serum inorganic phosphate (Pi) concentrations and increased conversion of inactive vitamin D to active 1,25-vitD.