Hereditary hypophosphatemia in Norway: a retrospective population-based study of genotypes, phenotypes, and treatment complications

Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is a rare metabolic bone disorder caused by pathogenic variants in the PHEX gene, which is predominantly expressed in osteoblasts, osteocytes and odontoblasts. XLH is characterized by increased synthesis of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23), which results in renal phosphate wasting with consecutive hypophosphataemia, rickets, osteomalacia, disproportionate short stature, oral manifestations, pseudofractures, craniosynostosis, enthesopathies and osteoarthritis. Patients with XLH should be provided with multidisciplinary care organized by a metabolic bone expert. Historically, these patients were treated with frequent doses of oral phosphate supplements and active vitamin D, which was of limited efficiency and associated with adverse effects. However, the management of XLH has evolved in the past few years owing to the availability of burosumab, a fully humanized monoclonal antibody that neutralizes circulating FGF23. Here, we provide updated clinical practice recommendations for the diagnosis and management of XLH to improve outcomes and quality of life in these patients.

A literature review of the healthcare resource use and productivity burden of X-linked hypophosphataemia

Introduction

X-linked hypophosphataemia (XLH) is a rare, genetic, renal phosphate wasting disorder that causes a lifelong rapid progression of morbidities, which are associated with substantial humanistic and economic burden. A structured literature review was carried out to identify publications reporting healthcare resource use and productivity impact of XLH to provide a comprehensive description of the burden.

Methods

Literature searches of the Embase®, Medline®, and EconLit electronic databases were carried out in August 2022 using free-text and subject heading search terms regarding XLH-related clinical morbidities and associated healthcare resource use, limited to English language records from 1992 onwards.

Results

After screening by pre-specified inclusion/exclusion criteria, 22 publications were selected for inclusion in the review. Use of conventional pharmacological therapy with oral phosphate and/or active vitamin D was reported in 15 publications, in up to 100% of paediatric patients and 75% of adults. Findings indicated that a high proportion of patients with XLH undergo orthopaedic procedures/surgeries, including a history of osteotomy in up to 25% of paediatric patients and 61% of adults, and a history of growth plate stapling in up to 63% of paediatric patients and 20% of adults. A high prevalence of fractures (in up to 61% of adults) and use of assistive mobility devices was also reported. The findings highlighted a substantial prevalence of morbidities, either due to persistently low phosphate levels or complications of conventional therapy, that had directly associated healthcare resource use, including dental problems, hearing problems, hyperparathyroidism, and nephrocalcinosis. Healthcare resource use and associated clinical events were generally found to be higher in adults compared with paediatric patients, which is consistent with the natural history of XLH as a progressive lifelong condition. Studies also highlighted the negative impact of XLH on school attendance and the ability to work.

Discussion

The results of this structured literature review emphasise the lifelong impact of XLH, showing that it is associated with a substantial economic burden, across many healthcare resource use categories including pharmacological therapy, management of pain and mobility, orthopaedic procedures, morbidities due to XLH or conventional therapy, and work/school productivity.

Advancing patient evidence in XLH (APEX): rationale and design of a real-world XLH global data unification program

X-linked hypophosphatemia (XLH) is a rare, genetic, progressive, lifelong disorder caused by pathogenic variants in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene, resulting in excess fibroblast growth factor 23 (FGF23) and consequent renal phosphate wasting. Chronic hypophosphatemia leads to deficits of the musculoskeletal system affecting bone, muscle, joint, and dental health. XLH treatments include oral phosphate and active vitamin D-which are associated with a burdensome dosing regimen, gastrointestinal disturbances, hyperparathyroidism, and nephrocalcinosis-or burosumab, a fully human anti-FGF23 antibody. Randomized clinical trials (RCTs) demonstrated burosumab to be well tolerated and efficacious in improving serum phosphate, rickets, bone turnover, and patient-reported outcomes. However, there are limited data on the natural history of XLH or real-world comparisons of the safety, effectiveness, and long-term outcomes of XLH treatments. Advancing Patient Evidence in XLH (APEX) is a global data unification project aiming to describe the burden and lifelong progression of XLH, collect real-world data on treatment effectiveness and safety, and investigate regional differences in treatment outcomes. Participants from three observational, noninterventional, retrospective and prospective, multicenter, longitudinal (10-year) studies of patients with XLH will be included: XLH Disease Monitoring Program (NCT03651505), International XLH Registry (NCT03193476), and SUNFLOWER (NCT03745521). Data collected in the Americas, Europe, Israel, Japan, and South Korea will be processed to unify identical and similar data elements. Data unification will be an iterative process with a clinical and programming review, ensuring validity and accuracy. In this observational study, unified data involving approximately 2000 pediatric and adult participants with XLH will be analyzed to address research questions in an exploratory manner. Long-term observational studies and patient registries provide opportunities to generate real-world data and address knowledge gaps in rare diseases. APEX aims to improve clinical decision-making and practice by bridging evidence gaps that cannot be addressed by RCTs or regional registries.

Factors associated with impaired physical functionality in X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare inherited disorder characterized by elevated levels of FGF23, chronic hypophosphatemia, impaired bone mineralization, and chronic long-term manifestations. Treatment for XLH has been mainly focused on normalizing its biochemical abnormalities. Despite treatment, patients with XLH often present impaired physical function and decreased quality of life (QoL). We hypothesize that physical functionality and QoL are more strongly associated with chronic pain and decreased muscle mass than persistent biochemical abnormalities or exposure to conventional treatment. We conducted an observational, cross-sectional study with patients with XLH. Clinical records and biochemical parameters were assessed. QoL surveys SF36v.2 and WOMAC were applied. Functional status was measured by a physiatrist and an occupational therapist. Appendicular lean mass (ALM) was measured and compared with age and sex-matched healthy controls. Enthesopathies and osteoarthritis were evaluated. Pain was assessed using the Brief Pain Inventory, the Visual Analog Scale, and the Doleur Neuropathique-4 scales. Muscle strength was evaluated by the quadriceps muscle isometric strength (QMS) and physical performance with the 6-Minute Walk Test (6MWT) and the Functional Independence Measure (FIM) scale. A total of 30 patients were included: 21 females; median age of 32 yr. All participants had significant functional deficits, chronic pain, and reduced QoL. Limitations in daily activities were significantly associated with higher severity of pain, decreased ALM, lower QMS, and less distance in 6MWT (p < .05). Neither FIM scale, phosphate levels, FGF23, nor the lifetime exposure to conventional treatment was associated with these functional variables. In conclusion, impaired physical functionality in patients with XLH was associated with lower muscle mass, lower muscle strength, and severe chronic pain. These findings highlight the importance of, in addition to optimizing the biochemical control of the disease, expanding patient care including pain prevention and management as well as comprehensive physical therapy and rehabilitation.

Intragenic duplication of PHEX in a girl with X-linked hypophosphatemia: a case report with review of literature

Over 70 intragenic copy-number variations (CNVs) of PHEX have been identified in patients with X-linked hypophosphatemia (XLH). However, the underlying mechanism of these CNVs has been poorly investigated. Furthermore, although PHEX undergoes X chromosome inactivation (XCI), the association between XLH in women with heterozygous PHEX variants and skewed XCI remains unknown. In this study, we determined the precise genomic structure and the XCI status of a girl with XLH who showed short stature and bowing of the legs at 2 years old. Laboratory tests revealed low levels of serum phosphate and elevated levels of alkaline phosphatase and fibroblast growth factor 23. Multiplex ligation-dependent probe amplification and targeted long-read sequencing revealed that she carried a 24.6-kb intragenic duplication of PHEX. The duplication was tandemly aligned in a head-to-tail orientation. The duplication breakpoints shared a 2-bp microhomology, indicating that this CNV resulted from a replication-based error. Trio sequencing results showed that the duplication was a de novo CNV that occurred on the paternally-derived allele. DNA methylation analysis demonstrated random XCI. A literature review of 12 previously reported cases of intragenic CNVs of PHEX revealed that the deletions/duplications can be ascribed to replication-based errors. Our findings and those of previous studies indicate that XLH-causative CNVs in PHEX predominantly arise from replication-based errors. Thus, the genomic region surrounding PHEX may be vulnerable to replication-based errors during gametogenesis or early embryogenesis. Our study provides supporting evidence that heterozygous PHEX variants can lead to XLH in women with random XCI.

Prevalence of Comorbid Hyperparathyroidism and Its Association with Renal Dysfunction in Asian Patients with X-Linked Hypophosphatemic Rickets/Osteomalacia

In patients with X-linked hypophosphatemic rickets/osteomalacia (XLH) in Asia, the current prevalence of hyperparathyroidism and its association with renal dysfunction have not been determined. We used patient data retrospectively collected up to the time of informed consent in the SUNFLOWER study, a long-term observational study, to investigate the current treatment status and prevalence of comorbid hyperparathyroidism and its association with renal dysfunction in patients with XLH in Japan and South Korea. Of 69 patients who met the eligibility criteria, 32 (46.4%) did not have hyperparathyroidism (hereinafter referred to as non-hyperparathyroidism), 33 (47.8%) had secondary hyperparathyroidism, and four (5.8%) had tertiary hyperparathyroidism. Men were more prone to develop secondary and tertiary hyperparathyroidism, use oral phosphate at higher frequencies, and have a higher incidence of Stage ≥ 3 chronic kidney disease and Grade ≥ 3 renal calcification than women. Ongoing treatments for patients with XLH and non-hyperparathyroidism, secondary hyperparathyroidism, and tertiary hyperparathyroidism mainly consisted of active vitamin D (30 [93.8%], 25 [75.8%], and 3 [75.0%], respectively) and oral phosphate (21 [65.6%], 23 [69.7%], and 4 [100.0%], respectively). At informed consent, patients with tertiary hyperparathyroidism had the lowest estimated glomerular filtration rate values. Our study highlights the prevalence of comorbid hyperparathyroidism and its association with renal dysfunction in patients with XLH through a large-scale observational study in Asia.Trial registration: NCT03745521; UMIN000031605.

Disease burden and health-related quality of life among children with X-linked hypophosphataemia in China: a national cross-sectional survey

BACKGROUND

X-linked hypophosphataemia (XLH) is a rare inherited disorder often misdiagnosed and lacking sufficient aetiological treatment. Previous studies have shown that XLH is associated with worse health-related quality of life (HRQoL) and greater economic burden for children and their families compared with the general population, but evidence from China is scarce. This study aimed to comprehensively explore the burden and HRQoL of XLH children in China.

METHODS

An online retrospective survey of paediatric patients with XLH and their caregivers was conducted nationwide during March to June 2021. A self-administered questionnaire was used to collect socio-demographic, clinical and economic data. The EQ-5D-Y-3L instrument was employed to assess HRQoL, and the health utility score was calculated. Direct medical, non-medical and indirect costs were determined. Multivariate regression analysis was performed to explore potential associations between HRQoL and identified influencing factors.

RESULTS

The study included 221 subjects with a mean age of 7.25 years, of whom 119 (53.8%) were girls. Most XLH children (63.8%) lived in rural areas, and 39.4% reported a family history. Over 70% experienced misdiagnosis. The average total annual cost per patient was found to be 34 657.85 CNY. Of direct medical costs, patients' out-of-pocket expenses were substantial. The incidence of catastrophic health expenditure was 19.9%. The means (SD) of EQ-5D-Y-3L health utility and EQ-5D VAS scores were 0.83 (0.14) and 56.07 (10.95). Both univariate and multivariate analyses found older age and lower economic affordability were associated with poorer HRQoL.

CONCLUSIONS

The study underscores the significant burden of XLH on paediatric patients and their families in China, both in terms of HRQoL and economic costs. The findings emphasise the importance of early detection, accurate diagnosis, cost-effective targeted interventions and long-term multidisciplinary management strategies to improve the lives of XLH children and their families.

X-linked hypophosphataemia

X-linked hypophosphataemia is a rare genetic disorder that causes renal phosphate wasting, impaired mineralisation of teeth and bones, skeletal deformities and limited mobility, all of which significantly impact on health and quality of life. This clinical review examines the recommendations for diagnosing, treating and monitoring adults with the condition.

Clinical, Radiographic, and Molecular Analysis of Patients with X-Linked Hypophosphatemic Rickets: Looking for Phenotype-Genotype Correlation

BACKGROUND/OBJECTIVES

X-linked hypophosphataemic rickets (XLH) represents the most frequent type of rickets from genetic origin, it is caused by mutations on the PHEX gene. The main clinical manifestations are short stature and bone deformities. Phenotype variation is observed at the intrafamily and interfamily level. The bases for this variation are not fully understood. The aim of this study was to investigate if there is a phenotype-genotype correlation in a cohort of patients with confirmed diagnosis of XLH.

METHODS

We recruited a total of 130 patients of Mexican Mestizo origin with confirmed molecular diagnosis of XLH; this is one of the largest cohorts reported.

RESULTS

Radiographies for calculating the rickets severity score (RSS) were available from 50 patients. A total of 56 different pathogenic variants were found among the study population; from them, 31 variants have not been previously reported. We compared the RSS values between individuals considering clinical and biochemical characteristics such as age, height, sex, phosphorus, and alkaline phosphatase in serum; no significant differences were observed. Then, we compared the RSS considering if the variant was intronic or exonic and considering the presence of a truncated protein or not. None of the two comparisons showed significant differences.

CONCLUSIONS

We did not find a genotype-phenotype correlation in the study population. Despite the knowledge regarding the genetic cause of XLH, the mechanisms driving the intrafamily and interfamily variability remain elusive. More analyses looking for the genotype-phenotype correlation are necessary in other populations, especially considering the discovery of new mutations in patients from different origin.

X-linked hypophosphataemia

X-linked hypophosphataemia is a genetic disease caused by defects in the phosphate regulating endopeptidase homolog X-linked (PHEX) gene and is characterised by X-linked dominant inheritance. The main consequence of PHEX deficiency is increased production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) in osteoblasts and osteocytes. Chronic exposure to circulating FGF23 is responsible for renal phosphate wasting and decreased synthesis of calcitriol, which decreases intestinal phosphate absorption. These mechanisms result in lifelong hypophosphataemia, impaired growth plate and bone matrix mineralisation, and diverse manifestations in affected children and adults, including some debilitating morbidities and possibly increased mortality. Important progress has been made in disease knowledge and management over the past decade; in particular, targeting FGF23 is a therapeutic approach that has substantially improved outcomes. However, patients affected by this complex disease need lifelong care and innovative treatment strategies, such as gene repair of PHEX, are necessary to further limit the disease burden.

Meta-analysis and systematic review: burosumab as a promising treatment for children with X-linked hypophosphatemia

Objective

The aim of this study was to evaluate the effectiveness of burosumab therapy in children with X-Linked Hypophosphatemia (XLH).

Materials and methods

We systematically reviewed literature from PubMed, Web of Science, The Cochrane Library, and Embase up until January 2024, using EndNote Web for study organization. The Newcastle-Ottawa scale guided quality assessment, while Revman software was used for data analysis and visualization. Study selection, quality evaluation, and data aggregation were independently performed by three researchers.

Results

The meta-analysis encompassed ten studies, including eight cohort studies that examined burosumab's impact pre- and post-administration, and two randomized controlled trials comparing burosumab to standard therapy. The evidence from this review suggests burosumab's superiority in managing XLH in pediatric populations, particularly in improving key biochemical markers including 1,25-dihydroxyvitamin D (1,25-(OH)2D), phosphorus, and alkaline phosphatase (ALP), alongside improvements in the renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR), and significant skeletal improvements as indicated by the rickets severity score (RSS) and the 6-minute walk test (6MWT). However, the long-term safety and effects, including height and quality of life (QOL) data, remains to be elucidated.

Conclusions

Burosumab has shown significant therapeutic effectiveness in treating children with XLH, highlighting its potential as a key treatment option.

Characterization of Novel PHEX Variants in X-linked Hypophosphatemic Rickets and Genotype-PHEX Activity Correlation

BACKGROUND

X-linked hypophosphatemia (XLHR) is the most common genetic form of hypophosphatemic rickets (HR), which is caused by phosphate regulating endopeptidase homolog X-linked (PHEX) gene mutation. At present, the genotype-phenotype relationship of XLHR and the pathogenic role of PHEX are not fully understood.

METHODS

In this study, we summarized clinical features in a new cohort of 49 HR patients and detected 16 novel PHEX and 5 novel non-PHEX variants. Subsequently, we studied the pathogenesis of new variants by protein expression, glycosylation analysis, subcellular localization, and endopeptidase activity.

RESULTS

The results showed that missense variants (Q189H and X750R) slightly reduced protein expression without obviously altering protein length and localization, whereas truncating variants significantly impaired the synthesis of PHEX and produced a shorter immature protein in cells. Interestingly, no evident correlation was observed between mutation types and clinical phenotypes. However, when we analyzed the relationship between PHEX activity and serum phosphorus level, we found that patients with low PHEX activity tended to have severe hypophosphatemia and high rickets severity score. Following this observation, we established 2 new knock-in XLHR mouse models with 2 novel Phex variants (c.T1349C and c.C426G, respectively) using CRISPR/Cas9 technology. Both mouse models demonstrated clinical manifestations of XLHR seen in patients, and PhexC426G mice showed more severe phenotype than PhexT1349C mice, which further confirmed the rationality of genotype-PHEX enzymatic activity correlation analysis.

CONCLUSION

Therefore, our findings demonstrated that novel PHEX variants could disrupt protein function via affecting protein synthesis, post-translational modification, cellular trafficking, and catalytic activity. Our study facilitates a better understanding of XLHR pathogenic mechanism and PHEX activity-phenotype correlation, which is of crucial importance for future diagnosis and treatment of XLHR.

Epidemiological analysis to identify predictors of X-linked hypophosphatemia (XLH) diagnosis in an Italian pediatric population: the EPIX project

PURPOSE

X-linked hypophosphatemia (XLH) is a rare multi-systemic disease characterized by low plasma phosphate levels. The aim of this study was to investigate the annual XLH prevalence and internally evaluate predictive algorithms' application performance for the early diagnosis of XLH.

METHODS

The PediaNet database, containing data on more than 400,000 children aged up to 14 years, was used to identify a cohort of XLH patients, which were matched with up to 10 controls by date of birth and gender. The annual prevalence of XLH cases per 100,000 patients registered in PediaNet database was estimated. To identify possible predictors associated with XLH diagnosis, a logistic regression model and two machine learning algorithms were applied. Predictive analyses were separately carried out including patients with at least 1 or 2 years of database history in PediaNet.

RESULTS

Among 431,021 patients registered in the PediaNet database between 2007-2020, a total of 12 cases were identified with a mean annual prevalence of 1.78 cases per 100,000 patients registered in PediaNet database. Overall, 8 cases and 60 matched controls were included in the analysis. The random forest algorithm achieved the highest area under the receiver operating characteristic curve (AUC) value both in the one-year prior ID (AUC = 0.99, 95% CI = 0.99-1.00) and the two-year prior ID (AUC = 1.00, 95% CI = 1.00-1.00) analysis. Overall, the XLH predictors selected by the three predictive methods were: the number of vitamin D prescriptions, the number of recorded diagnoses of acute respiratory infections, the number of prescriptions of antihistamine for systemic use, the number of prescriptions of X-ray of the lower limbs and pelvis and the number of allergology visits.

CONCLUSION

Findings showed that data-driven machine learning models may play a prominent role for the prediction of the diagnosis of rare diseases such as XLH.

Safety and efficacy of burosumab in improving phosphate metabolism, bone health, and quality of life in adolescents with X-linked hypophosphatemic rickets

BACKGROUND AND OBJECTIVE

X-linked hypophosphatemic rickets (XLH) is due to loss-of-function mutations in the phosphate-regulating endopeptidase homologue on the X chromosome (PHEX) that lead to increased fibroblast growth factor 23 (FGF23) production. FGF23 excess causes renal phosphate wasting and insufficient 1,25-dihydroxyvitamin D (1,25(OH)2D) synthesis with reduced intestinal phosphate absorption, ultimately resulting in chronic hypophosphatemia. Children with XLH show typical skeletal lesions of rickets, deformities of the lower limbs, stunted growth with disproportionate short stature, bone pain, and physical dysfunctions. Burosumab, a fully human IgG1 monoclonal antibody that binds to FGF23 to inhibit its activity, is more effective to improve the biochemical and clinical signs of XLH than conventional treatment with phosphate supplements and vitamin D active metabolites. Data on adolescents with XLH during the transition period to young adulthood are few. In this prospective case series, we aimed to assess safety and efficacy of burosumab in adolescents with XLH who discontinued long-term conventional therapy.

METHODS

Five Caucasian adolescents (4 males, 1 female; mean age 15.4 ± 1.5 years) with XLH were recruited and switched from conventional treatment to burosumab (0.8-1.2 mg/kg, s. c. QW2). Burosumab was continued for 12-48 months and, once discontinued, patients were followed-up for 6-12 months. In all patients, serum calcium, phosphate, alkaline phosphatase (ALP), parathyroid hormone (PTH), and 1,25(OH)2D levels, and renal tubular reabsorption of phosphate (TmP/GFR) values were assessed at entry and during burosumab. Intact FGF23 plasma levels were measured at entry. Patient-reported outcomes (PROs) were assessed at entry and every 3-6 months to evaluate the impact of low extremity pain, stiffness, and difficulties performing daily activities.

RESULTS

At entry, all patients showed hypophosphatemia, increased intact FGF23 levels, reduced TmP/GFR, insufficient 1,25(OH)2D levels, and in four out of five increased ALP levels. Two patients had radiological signs of rickets. During burosumab, all patients showed a significant increase in serum phosphate and 1,25(OH)2D levels, and in TmP/GFR values (P < 0.05 - P < 0.0001). Serum ALP levels significantly declined (P < 0.05) to normal values. No changes of serum calcium and PTH levels (PNS) were found during burosumab. PROs significantly improved (P < 0.02 - P < 0.0001) in all patients. Four patients discontinued burosumab when they turned 18 or 19, whereas one continued the treatment since he was still younger than 18 during the study period. Four patients who suspended burosumab showed a rapid decline in serum phosphate and 1,25(OH)2D levels and in TmP/GFR values; serum ALP levels increased, and PROs progressively worsened with a significant reduction in quality of life. These consequences were not observed in the patient who continued burosumab treatment.

DISCUSSION

Our data showed that conventional treatment improved only in part the signs and symptoms of XLH. Burosumab was well tolerated and was effective in improving phosphate metabolism, bone health, and PROs. All the benefits of burosumab were lost after its discontinuation. These results suggested that continuing burosumab is required to achieve and maintain the clinical benefits of the treatment during the transition to young adulthood in patients with XLH.

The Diagnostic Odyssey in Children and Adolescents With X-linked Hypophosphatemia: Population-Based, Case-Control Study

CONTEXT

X-linked hypophosphatemia (XLH) is a rare genetic disorder causing renal phosphate wasting, which predicates musculoskeletal manifestations such as rickets. Diagnosis is often delayed.

OBJECTIVE

To explore the recording of clinical features, and the diagnostic odyssey of children and adolescents with XLH in primary care electronic healthcare records (EHRs) in the United Kingdom.

METHODS

Using the Optimum Patient Care Research Database, individuals aged 20 years or younger after January 1, 2000, at date of recorded XLH diagnosis were identified using Systematized Nomenclature of Medicine Clinical Terms (SNOMED)/Read codes and age-matched to 100 controls. Recording of XLH-related clinical features was summarized then compared between cases and controls using chi-squared or Fisher's exact test.

RESULTS

In total, 261 XLH cases were identified; 99 met the inclusion criteria. Of these, 84/99 had at least 1 XLH-related clinical feature recorded in their primary care EHR. Clinical codes for rickets, genu varum, and low phosphate were recorded prior to XLH diagnosis in under 20% of cases (median of 1, 1, and 3 years prior, respectively). Rickets, genu varum, low phosphate, nephrocalcinosis, and growth delay were significantly more likely to be recorded in cases.

CONCLUSION

This characterization of the EHR phenotypes of children and adolescents with XLH may inform future case-finding approaches to expedite diagnosis in primary care.

Lessons learned from the real-world diagnosis and management of hereditary hypophosphatemic rickets

Hypophosphatemic rickets, which is often hereditary, is still under- or misdiagnosed in both children and adults, denying these individuals access to optimal management and genetic counseling. There have been recent calls to compile real-world data and share best practice on these rare conditions to guide clinical decision-making. Here we present eight clinical vignettes of patients with hypophosphatemic rickets encountered in our tertiary pediatric endocrinology practice. We describe the clinical features, genetics, and management of four cases of X-linked hypophosphatemia (PHEX mutations), one each of autosomal recessive hypophosphatemic rickets (DMP1 mutation) and autosomal recessive vitamin D-dependent rickets type 1A (CYP27B1 mutation), and two cases of distal renal tubular acidosis with FOXI1 mutation-associated hypophosphatemic rickets. Our cases prompt consideration of the (i) frequent misdiagnosis of hypophosphatemic rickets in clinical practice and the importance of comprehensive genetic testing; (ii) variable expressivity of the causative mutations; and (iii) a lack of responsiveness and/or compliance to conventional therapy and the value of burosumab in modern management, provided access is equitable. These cases highlight common real-world themes and challenges to managing patients presenting with these diverse conditions, especially the burden of disease hidden by misdiagnosis. In sharing these cases, we hope to raise awareness of these conditions, promote best practice in genetic diagnosis and management, and further advocate for reimbursement equity for the best available therapies.

Genetic causes of hypophosphatemia

Phosphate is a key component of mineralized tissues and is also part of many organic compounds. Phosphorus homeostasis depends especially upon intestinal absorption, and renal excretion, which are regulated by various hormones, such as PTH, 1,25-dihydroxyvitamin D, and fibroblast growth factor 23. In this review we provide an update of several genetic disorders that affect phosphate transporters through cell membranes or the phosphate-regulating hormones, and, consequently, result in hypophosphatemia.

Three exonic variants in the PHEX gene cause aberrant splicing in a minigene assay

Background: X-linked hypophosphatemia (XLH, OMIM 307800) is a rare phosphorus metabolism disorder caused by PHEX gene variants. Many variants simply classified as missense or nonsense variants were only analyzed at the DNA level. However, growing evidence indicates that some of these variants may alter pre-mRNA splicing, causing diseases. Therefore, this study aimed to use bioinformatics tools and a minigene assay to ascertain the effects of PHEX variations on pre-mRNA splicing. Methods: We analyzed 174 variants in the PHEX gene described as missense or nonsense variants. Finally, we selected eight candidate variants using bioinformatics tools to evaluate their effects on pre-mRNA splicing using a minigene assay system. The complementary DNA (cDNA) sequence for the PHEX gene (RefSeq NM_000444.6) serves as the basis for DNA variant numbering. Results: Of the eight candidate variants, three were found to cause abnormal splicing. Variants c.617T>G p.(Leu206Trp) and c.621T>A p.(Tyr207*) in exon 5 altered the splicing of pre-mRNA, owing to the activation of a cryptic splice site in exon 5, which produced an aberrant transcript lacking a part of exon 5, whereas variant c.1700G>C p.(Arg567Pro) in exon 16 led to the activation of a cryptic splice site in intron 16, resulting in a partial inclusion of intron 16. Conclusion: Our study employed a minigene system, which has a great degree of flexibility to assess abnormal splicing patterns under the circumstances of patient mRNA samples that are not available, to explore the impact of the exonic variants on pre-mRNA splicing. Based on the aforementioned experimental findings, we demonstrated the importance of analyzing exonic variants at the mRNA level.

Real-world non-interventional post-authorization safety study of long-term use of burosumab in children and adolescents with X-linked hypophosphatemia: first interim analysis

Background

X-linked hypophosphatemia (XLH) is a rare, progressive disorder characterized by excess fibroblast growth factor 23 (FGF23), causing renal phosphate-wasting and impaired active vitamin D synthesis. Burosumab is a recombinant human monoclonal antibody that inhibits FGF23, restoring patient serum phosphate levels. Safety data on long-term burosumab treatment are currently limited.

Objectives

This post-authorization safety study (PASS) aims to monitor long-term safety outcomes in children and adolescents (1-17 years) treated with burosumab for XLH. This first interim analysis reports the initial PASS safety outcomes.

Design

A 10-year retrospective and prospective cohort study.

Methods

This PASS utilizes International XLH Registry (NCT03193476) data, which includes standard diagnostic and monitoring practice data at participating European centers.

Results

At data cut-off (13 May 2021), 647 participants were included in the International XLH Registry; 367 were receiving burosumab, of which 67 provided consent to be included in the PASS. Mean (SD) follow-up time was 2.2 (1.0) years. Mean (SD) age was 7.3 (4.3) years (range 1.0-17.5 years). Mean duration of burosumab exposure was 29.7 (25.0) months. Overall, 25/67 participants (37.3%) experienced ⩾1 adverse event (AE) during follow-up; 83 AEs were reported. There were no deaths, no AEs leading to treatment withdrawal, nor serious AEs related to treatment. The most frequently reported AEs were classified as 'musculoskeletal and connective tissue disorders', with 'pain in extremity' most frequently reported, followed by 'infections and infestations', with 'tooth abscess' the most frequently reported.

Conclusion

In this first interim analysis of the PASS, covering the initial 2 years of data collection, the safety profile of burosumab is consistent with previously reported safety data. The PASS will provide long-term safety data over its 10-year duration for healthcare providers and participants with XLH that contribute to improvements in the knowledge of burosumab safety.

Trial registration

European Union electronic Register of Post-Authorisation Studies: EUPAS32190.

Diagnosis, treatment, and management of rickets: a position statement from the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology

Rickets results from impaired mineralization of growing bone due to alterations in calcium and phosphate homeostasis. Clinical signs of rickets are related to the age of the patient, the duration of the disease, and the underlying disorder. The most common signs of rickets are swelling of the wrists, knees or ankles, bowing of the legs (knock-knees, outward bowing, or both) and inability to walk. However, clinical features alone cannot differentiate between the various forms of rickets. Rickets includes a heterogeneous group of acquired and inherited diseases. Nutritional rickets is due to a deficiency of vitamin D, dietary calcium or phosphate. Mutations in genes responsible for vitamin D metabolism or function, the production or breakdown of fibroblast growth factor 23, renal phosphate regulation, or bone mineralization can lead to the hereditary form of rickets. This position paper reviews the relevant literature and presents the expertise of the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology (SIEDP). The aim of this document is to provide practical guidance to specialists and healthcare professionals on the main criteria for diagnosis, treatment, and management of patients with rickets. The various forms of rickets are discussed, and detailed references for the discussion of each form are provided. Algorithms to guide the diagnostic approach and recommendations to manage patients with rare forms of hereditary rickets are proposed.

Intact FGF23 concentration in healthy infants, children, and adolescents, and diagnostic usefulness in patients with X-linked hypophosphatemic rickets

OBJECTIVE

FGF23 measurement may have a diagnostic role to investigate patients with phosphate disorders. However, normal values for infants, children, and adolescents have not been defined.

METHODS

In a total of 282 (males 145, females 137) healthy infants (n = 30), prepubertal (n = 147), pubertal (n = 59), and postpubertal (n = 46), and in twenty patients with X-linked hypophosphatemic rickets (XLH, age 10.2 ± 5.6 years) serum phosphate (automated analyzer), and plasma intact FGF23 (immunochemiluminescent sandwich assay, DiaSorin) concentrations were measured.

RESULTS

Intact FGF23 concentrations were higher in healthy infants than in prepubertal (P < 0.01) and postpubertal subjects (P < 0.05); pubertal subjects showed higher values (P < 0.05) than postpubertal subjects. Serum phosphate concentrations were higher (P < 0.001) in healthy infants than in prepubertal, pubertal, and postpubertal subjects. Pubertal subjects had higher (P < 0.001) serum phosphate concentrations than postpubertal subjects. Intact FGF23 and serum phosphate concentrations did not differ (P = NS) by sex, age of menarche, and time after menarche. In healthy subjects, there was no correlation between intact FGF23 and serum phosphate concentrations. Intact FGF23 concentrations were higher (P < 0.0001) in patients with XLH than in healthy subjects according to chronological age and pubertal development. In all patients, intact FGF23 concentrations were above 40 pg/mL; intact FGF23 concentrations were inversely correlated with serum phosphate concentrations (r = -0.65; P < 0.01).

CONCLUSION

In healthy subjects, chronological age and puberty were main determinants of intact FGF23 concentrations. Intact FGF23 concentrations may be a useful marker for the early diagnosis of XLH in pediatric patients.

X-linked hypophosphatemia: The value of feedback focus groups to assess patient and caregiver needs

X-linked hypophosphatemia (XLH) is a rare, multi-systemic, invalidating disease requiring a multi-disciplinary approach. No specific action in XLH, neither for the patients' specific needs nor for the methodology for the evaluation of these were found. Thus, to identify the needs of XLH patients and their caregivers, we organised focus groups in our reference centre with a view to build educational sessions. Focus groups including either XLH children, XLH adults, or caregivers ran in parallel. Each group was led by a person trained in therapeutic education (nurse, paediatric nephrologist) with another healthcare provider specialised in XLH (rheumatologist, nephrologist). One additional person with knowledge of XLH (clinical research associate, paediatric resident) took minutes. The duration of each session was 1.5h; XLH patients/caregivers were asked to answer age-adapted "open questions" on their daily life and quality of life. At the end, a global restitution was made. The needs identified were later grouped and analysed, which allowed us to build the educational sessions. The XLH children group included 5 children, the XLH adults group included 10 adults, and the caregivers group included 6 parents or partners. Major needs were identified: knowledge of XLH, treatment, dental care and adapted physical activity, with additional questions on socio-professional adaptations and financial support in adults. Partner patients were also identified to co-build the support programme. The study allowed us to identify the needs of XLH patients and their caregivers using the focus group method and then, using these needs, to build educational sessions and a therapeutic education programme for XLH patients.

The impact of metabolic disorders on management of periodontal health in children

Periodontitis is a chronic inflammatory disease caused by plaque biofilm which shares risk factors with systemic chronic diseases such as diabetes, cardiovascular disease, and osteoporosis. Many studies have found increased prevalence and rate of progression of periodontal disease in children with common metabolic disorders. Although the causal relationship and specific mechanism between them has not been determined yet. The aim of this paper is to progress on the impact of metabolic disorders on periodontal health in children and the underlying mechanisms, which provides new evidences for the prevention and intervention of metabolic disorders and periodontitis in children.

Disease Manifestations and Complications in Dutch X-Linked Hypophosphatemia Patients

X-linked hypophosphatemia (XLH) is the most common monogenetic cause of chronic hypophosphatemia, characterized by rickets and osteomalacia. Disease manifestations and treatment of XLH patients in the Netherlands are currently unknown. Characteristics of XLH patients participating in the Dutch observational registry for genetic hypophosphatemia and acquired renal phosphate wasting were analyzed. Eighty XLH patients, including 29 children, were included. Genetic testing, performed in 78.8% of patients, showed a PHEX mutation in 96.8%. Median (range) Z-score for height was - 2.5 (- 5.5; 1.0) in adults and - 1.4 (- 3.7; 1.0) in children. Many patients were overweight or obese: 64.3% of adults and 37.0% of children. All children received XLH-related medication e.g., active vitamin D, phosphate supplementation or burosumab, while 8 adults used no medication. Lower age at start of XLH-related treatment was associated with higher height at inclusion. Hearing loss was reported in 6.9% of children and 31.4% of adults. Knee deformities were observed in 75.0% of all patients and osteoarthritis in 51.0% of adult patients. Nephrocalcinosis was observed in 62.1% of children and 33.3% of adults. Earlier start of XLH-related treatment was associated with higher risk of nephrocalcinosis and detection at younger age. Hyperparathyroidism longer than six months was reported in 37.9% of children and 35.3% of adults. This nationwide study confirms the high prevalence of adiposity, hearing loss, bone deformities, osteoarthritis, nephrocalcinosis and hyperparathyroidism in Dutch XLH patients. Early start of XLH-related treatment appears to be beneficial for longitudinal growth but may increase development of nephrocalcinosis.

Spectrum of PHEX Mutations and FGF23 Profiles in a Taiwanese Cohort With X-Linked Hypophosphatemia Including 102 Patients

BACKGROUND/AIM

X-linked hypophosphatemia (XLH), the most common form of hereditary rickets, results from loss-of-function mutations in the phosphate-regulating PHEX gene. Elevated fibroblast growth factor 23 (FGF23) contributes to hypophosphatemia in XLH. This study aimed to characterize PHEX variants and serum FGF23 profiles in Taiwanese patients with XLH.

PATIENTS AND METHODS

We retrospectively reviewed the records of 102 patients clinically suspected of having hypophosphatemic rickets from 2006 to 2022. Serum intact Fibroblast growth factor-23 (iFGF23) levels were measured on clinic visit days. PHEX mutations were identified using Sanger sequencing, and negative cases were analyzed using whole-exome sequencing.

RESULTS

The majority (92.1%) of patients exhibited elevated FGF23 compared with normal individuals. Among 102 patients, 44 distinct PHEX mutations were identified. Several mutations recurred in multiple unrelated Taiwanese families. We discovered a high frequency of novel PHEX mutations and identified variants associated with extreme FGF23 elevation and tumorigenesis.

CONCLUSION

Our findings revealed the PHEX genotypic variants and FGF23 levels in Taiwanese patients with XLH. These results are crucial given the recent approval of burosumab, a monoclonal FGF23 antibody, for XLH therapy. This study provides key insights into the clinical management of XLH in Taiwan.

X-Linked Hypophosphatemia: Does Targeted Therapy Modify Dental Impairment?

X-linked hypophosphatemia is a rare, hereditary disorder that significant influences teeth and alveolar bone. The first clinical sign leading to the diagnosis of X-linked hypophosphatemia is often dental impairment with dental abscesses and dentin mineralization defects. Genetic analysis helped find the responsible gene and therefore opened up new ways of therapeutically managing X-linked hypophosphatemia. The human monoclonal antibody Burosumab represents a milestone in the targeted therapy of this hereditary disease by directly addressing its pathophysiology. Targeted therapy has been shown to improve skeletal impairment, pain, and phosphate metabolism. However, the influence of this new therapy on dental impairment has only been addressed in a few recent studies with varying results. Therefore, in this review, we aim to summarize the dental phenotype and analyze the different treatment modalities with a focus on dental impairment.

The contribution of a novel PHEX gene mutation to X-linked hypophosphatemic rickets: a case report and an analysis of the gene mutation dosage effect in a rat model

A Chinese family was identified to have two patients with rickets, an adult female and a male child (proband), both exhibiting signs related to X-linked hypophosphatemic rickets (XLH). Gene sequencing analysis revealed a deletion of adenine at position 1985 (c.1985delA) in the PHEX-encoding gene. To investigate the relationship between this mutation and the pathogenicity of XLH, as well as analyze the effects of different dosages of PHEX gene mutations on clinical phenotypes, we developed a rat model carrying the PHEX deletion mutation. The CRISPR/Cas9 gene editing technology was employed to construct the rat model with the PHEX gene mutation (c.1985delA). Through reproductive procedures, five genotypes of rats were obtained: female wild type (X/X), female heterozygous (-/X), female homozygous wild type (-/-), male wild type (X/Y), and male hemizygous (-/Y). The rats with different genotypes underwent analysis of growth, serum biochemical parameters, and bone microstructure. The results demonstrated the successful generation of a stable rat model inheriting the PHEX gene mutation. Compared to the wild-type rats, the mutant rats displayed delayed growth, shorter femurs, and significantly reduced bone mass. Among the female rats, the homozygous individuals exhibited the smallest body size, decreased bone mass, shortest femur length, and severe deformities. Moreover, the mutant rats showed significantly lower blood phosphorus concentration, elevated levels of FGF23 and alkaline phosphatase, and increased expression of phosphorus regulators. In conclusion, the XLH rat model with the PHEX gene mutation dosage demonstrated its impact on growth and development, serum biochemical parameters, and femoral morphology.

Development of Spinal Enthesopathies in Adults With X-linked Hypophosphatemia

CONTEXT

Musculoskeletal complications are the main manifestations in adults with X-linked hypophosphatemia (XLH). Enthesopathy significantly impairs quality of life.

OBJECTIVE

To identify the risk factors associated with the development and progression of spinal enthesopathies in adults with XLH.

DESIGN AND SETTING

We conducted a retrospective study in the French Reference Center for Rare Diseases of the Calcium and Phosphate Metabolism.

PATIENTS

Adults XLH patients with 2 EOS® imaging performed at least 2 years apart at the same center between June 2011 and March 2022. The progression of enthesopathies was defined as a new enthesopathy at least 1 intervertebral level in patients with or without presence of enthesopathy at baseline.

MAIN OUTCOME MEASURES

Demographic, treatment, PHEX mutation with the progression of enthesopathies.

RESULTS

Fifty-one patients (66.7% of women, mean age 42.1 ± 13.4 years) underwent 2 EOS imaging with an average interval of 5.7 (± 2.31) years.Progression of spinal enthesopathies was observed in 27 (52.9%) patients. In univariate analysis, patients with a progression of spinal enthesopathies were significantly older (P < .0005), were significantly older at treatment initiation (P = .02), presented with dental complications (P = .03), received less frequently treatment during childhood with phosphate and/or vitamin D analogs (P = .06), and presented more frequently with hip osteoarthritis (P = .002) at baseline. In multivariate analysis, none of these factors was associated with a progression of spinal enthesopathies.

CONCLUSION

This study confirms the high proportion of patients with a progression of spinal enthesopathies. Age seems to be the main factor associated with progression.

Burosumab vs Phosphate/Active Vitamin D in Pediatric X-Linked Hypophosphatemia: A Subgroup Analysis by Dose Level

CONTEXT

In an open-label, randomized, controlled, phase 3 trial in 61 children aged 1 to 12 years with X-linked hypophosphatemia (XLH), burosumab improved rickets vs continuing conventional therapy with active vitamin D and phosphate.

OBJECTIVE

We conducted an analysis to determine whether skeletal responses differed when switching to burosumab vs continuing higher or lower doses of conventional therapy.

METHODS

Conventional therapy dose groups were defined as higher-dose phosphate [greater than 40 mg/kg] (HPi), lower-dose phosphate [40 mg/kg or less] (LPi), higher-dose alfacalcidol [greater than 60 ng/kg] or calcitriol [greater than 30 ng/kg] (HD), and lower-dose alfacalcidol [60 ng/kg or less] or calcitriol [30 ng/kg or less] (LD).

RESULTS

At week 64, the Radiographic Global Impression of Change (RGI-C) for rickets was higher (better) in children randomly assigned to burosumab vs conventional therapy for all prebaseline dose groups: HPi (+1.72 vs +0.67), LPi (+2.14 vs +1.08), HD (+1.90 vs +0.94), LD (+2.11 vs +1.06). At week 64, the RGI-C for rickets was also higher in children randomly assigned to burosumab (+2.06) vs conventional therapy for all on-study dose groups: HPi (+1.03), LPi (+1.05), HD (+1.45), LD (+0.72). Serum alkaline phosphatase (ALP) also decreased in the burosumab-treated patients more than in the conventional therapy group, regardless of on-study phosphate and active vitamin D doses.

CONCLUSION

Prior phosphate or active vitamin D doses did not influence treatment response after switching to burosumab among children with XLH and active radiographic rickets. Switching from conventional therapy to burosumab improved rickets and serum ALP more than continuing either higher or lower doses of phosphate or active vitamin D.

Sclerostin antibody improves alveolar bone quality in the Hyp mouse model of X-linked hypophosphatemia (XLH)

X-linked hypophosphatemia (XLH) is a rare disease of elevated fibroblast growth factor 23 (FGF23) production that leads to hypophosphatemia and impaired mineralization of bone and teeth. The clinical manifestations of XLH include a high prevalence of dental abscesses and periodontal disease, likely driven by poorly formed structures of the dentoalveolar complex, including the alveolar bone, cementum, dentin, and periodontal ligament. Our previous studies have demonstrated that sclerostin antibody (Scl-Ab) treatment improves phosphate homeostasis, and increases long bone mass, strength, and mineralization in the Hyp mouse model of XLH. In the current study, we investigated whether Scl-Ab impacts the dentoalveolar structures of Hyp mice. Male and female wild-type and Hyp littermates were injected with 25 mg·kg-1 of vehicle or Scl-Ab twice weekly beginning at 12 weeks of age and euthanized at 20 weeks of age. Scl-Ab increased alveolar bone mass in both male and female mice and alveolar tissue mineral density in the male mice. The positive effects of Scl-Ab were consistent with an increase in the fraction of active (nonphosphorylated) β-catenin, dentin matrix protein 1 (DMP1) and osteopontin stained alveolar osteocytes. Scl-Ab had no effect on the mass and mineralization of dentin, enamel, acellular or cellular cementum. There was a nonsignificant trend toward increased periodontal ligament (PDL) attachment fraction within the Hyp mice. Additional PDL fiber structural parameters were not affected by Scl-Ab. The current study demonstrates that Scl-Ab can improve alveolar bone in adult Hyp mice.

X-Linked Familial Hypophosphatemia: A Case Report of 27-Year Old Male and Review of Literature

X-linked hypophosphatemia (XLH) associated with short stature during childhood are mostly referred to the hospital and diagnosed as vitamin D deficiency rickets and received vitamin D before adulthood. A case is presented with clinical features of hypophosphatemia from childhood who did not seek medical care for diagnosis and treatment, nor did his mother or two brothers, who have short statures, bone pain, and fractures. The patient was assessed for sociodemographic, hematological, and biochemical parameters together with a genetic assessment. A DEXA scan and X-ray were done to determine the abnormalities and deformities of joints and bones despite clinical examination by an expert physician. All imaging, laboratory parameters, and the genetic study confirmed the diagnosis of XLH. A detailed follow-up of his condition was performed after the use of phosphate tablets and other treatments. X-linked hypophosphatemia needs a good assessment, care, and follow up through a complementary medical team including several specialties. Phosphate tablets in adulthood significantly affects clinical and physical improvement and prevention of further skeletal abnormality and burden on daily activity. The patients should be maintained with an adequate dose of phosphate for better patient compliance. More awareness is needed in society and for health professionals when conducting medical checkups during the presence of stress fractures, frequent dental and gum problems, rickets, short stature, or abnormality in the skeleton or walking to think of secondary causes such as hypophosphatemia. Further investigations including a visit to a specialist is imperative to check for the primary cause of these disturbances.

The International X-Linked Hypophosphatemia (XLH) Registry: first interim analysis of baseline demographic, genetic and clinical data

BACKGROUND

X-linked hypophosphatemia (XLH) is a rare, hereditary, progressive, renal phosphate-wasting disorder characterized by a pathological increase in FGF23 concentration and activity. Due to its rarity, diagnosis may be delayed, which can adversely affect outcomes. As a chronic disease resulting in progressive accumulation of musculoskeletal manifestations, it is important to understand the natural history of XLH over the patient's lifetime and the impact of drug treatments and other interventions. This multicentre, international patient registry (International XLH Registry) was established to address the paucity of these data. Here we present the findings of the first interim analysis of the registry.

RESULTS

The International XLH Registry was initiated in August 2017 and includes participants of all ages diagnosed with XLH, regardless of their treatment and management. At the database lock for this first interim analysis (29 March 2021), 579 participants had entered the registry before 30 November 2020 and are included in the analysis (360 children [62.2%], 217 adults [37.5%] and 2 whose ages were not recorded [0.3%]; 64.2% were female). Family history data were available for 319/345 (92.5%) children and 145/187 (77.5%) adults; 62.1% had biological parents affected by XLH. Genetic testing data were available for 341 (94.7%) children and 203 (93.5%) adults; 370/546 (67.8%) had genetic test results; 331/370 (89.5%) had a confirmed PHEX mutation. A notably longer time to diagnosis was observed in adults ≥ 50 years of age (mean [median] duration 9.4 [2.0] years) versus all adults (3.7 [0.1] years) and children (1.0 [0.2] years). Participants presented with normal weight, shorter length or height and elevated body mass index (approximately - 2 and + 2 Z-scores, respectively) versus the general population. Clinical histories were collected for 349 participants (239 children and 110 adults). General data trends for prevalence of bone, dental, renal and joint conditions in all participants were aligned with expectations for a typical population of people with XLH.

CONCLUSION

The data collected within the International XLH Registry, the largest XLH registry to date, provide substantial information to address the paucity of natural history data, starting with demographic, family history, genetic testing, diagnosis, auxology and baseline data on clinical presentation.

A genetic study of a Brazilian cohort of patients with X-linked hypophosphatemia reveals no correlation between genotype and phenotype

Aim

X-linked hypophosphatemia (XLH) is the most common inherited form of rickets, and it is caused by pathogenic inactivating variants of the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene. The main purpose of this study is to identify the presence of a genotype-phenotype correlation in a cohort of XLH patients.

Methods

This is a retrospective study including patients diagnosed with hypophosphatemic rickets, confirmed by clinical, radiological, and laboratory findings. Medical records were reviewed for phenotypic analyses. Genomic DNA was extracted from the peripheral blood lymphocytes, and PHEX sequencing was performed by exomic NGS sequencing. The Wilcoxon rank-sum test and the two-tailed Fisher's exact test were employed for the statistical analyses of this study.

Results

A total of 41 patients were included in this study, and 63.41% (26/41) of the patients were female. The mutation analyses identified 29.27% missense variants and 29.72% nonsense variants, most of them were considered deleterious (66.41%). Six novel deleterious variants in the PHEX gene were detected in seven patients. The median concentrations of pretreatment serum calcium, phosphorus, and parathyroid hormone (PTH) were not significantly different among patients with different genotypes. An orthopedic surgery due to bone deformity was required in 57.69%.

Conclusions

Our analysis did not identify any specific genotype as a predictor. No significant genotype-phenotype correlation was found, suggesting that the recognition of subjacent pathogenic mutation in the PHEX gene may have limited prognostic value. Despite this finding, genetic testing may be useful for identifying affected individuals early and providing appropriate treatment.

Elevated morbidity and mortality in patients with chronic idiopathic hypophosphatemia: a nationwide cohort study

Background

Chronic idiopathic hypophosphatemia (CIH) induced by X-linked hypophosphatemic rickets or tumor-induced osteomalacia is a rare inherited or acquired disorder. However, due to its rarity, little is known about the epidemiology and natural course of CIH. Therefore, we aimed to identify the prevalence and long-term health outcomes of CIH patients.

Methods

Using the Korean Health Insurance Review and Assessment claims database, we evaluated the incidence of hypophosphatemia initially diagnosed from 2003 to 2018. After excluding secondary conditions that could change serum phosphorus levels, we identified 154 patients (76 men and 78 women) with non-secondary and non-renal hypophosphatemia. These hypophosphatemic patients were compared at a ratio of 1:10 with age-, sex-, and index-year-matched controls (n = 1,540).

Results

In the distribution of age at diagnosis, a large peak was observed in patients aged 1-4 years and small peaks were observed in ages from 40-70 years. The age-standardized incidence rate showed non-statistically significant trend from 0.24 per 1,000,000 persons in 2003 to 0.30 in 2018. Hypophosphatemic patients had a higher risk of any complication (adjusted hazard ratio [aHR], 2.17; 95% confidence interval [CI], 1.67-2.69) including cardiovascular outcomes, chronic kidney disease, hyperparathyroidism, osteoporotic fractures, periodontitis, and depression. Hypophosphatemic patients also had higher risks of mortality and hospitalization than the controls (aHR, 3.26; 95% CI, 1.83-5.81; and aHR, 2.49; 95% CI, 1.97-3.16, respectively).

Conclusion

This first nationwide study of CIH in South Korea found a bimodal age distribution and no sex differences among patients. Hypophosphatemic patients had higher risks of complications, mortality, and hospitalization compared to age- and sex-matched controls.

Efficacy of Burosumab Every 2 Weeks in an Adult with X-Linked Hypophosphatemia: Should We Learn from Children?

X-linked hypophosphatemia is a genetic condition that leads to fibroblast-growth-factor 23 (FGF23) increase, causing phosphate renal wasting. Since 2018, burosumab, an anti-FGF23 antibody, has been used for this disease with different dosage in children and adults. We report the case of burosumab administration every 2 weeks, as usually done in children. We retrospectively evaluated parathormone (PTH), alkaline phosphatase, serum phosphate, tubular reabsorption of phosphate (TRP), and 25OH vitamin D every 2 weeks in a 29-year-old man with nephrocalcinosis and tertiary hyperparathyroidism who did not respond to standard treatment with burosumab nor to maximum dosage and was treated with burosumab 90 mg every 2 weeks. His serum phosphate and TRP increased with this regimen compared with 4 weeks frequency (respectively 1.74 ± 0.26 mg/dL vs. 2.3 ± 0.19 mg/dL [p 0.0004] and 71.3% ± 4.8% vs. 83.9% ± 7.9% [p 0.01]) with decrease in PTH (183 ± 24.7 pg/mL vs. 109 ± 12.2 pg/mL [p 0.04]). Burosumab may be a good choice in adult patients with X-linked hypophosphatemia; new data are needed regarding the increase in dosage and/or frequency of administration as usually done in children, to achieve disease control.

Sclerostin antibody improves alveolar bone quality in the Hyp mouse model of X-Linked Hypophosphatemia (XLH)

X-linked hypophosphatemia (XLH) is a rare disease of elevated fibroblast growth factor 23 (FGF23) production that leads to hypophosphatemia and poor mineralization of bone and teeth. The clinical manifestations of XLH include a high prevalence of dental abscesses, likely driven by poorly formed structures of the dentoalveolar complex, including the alveolar bone, cementum, dentin, and periodontal ligament. Our previous studies have demonstrated that sclerostin antibody (Scl-Ab) treatment improves phosphate homeostasis, and increases bone mass, strength and mineralization in the Hyp mouse model of XLH. In the current study, we investigated whether Scl-Ab impacts the dentoalveolar structures of Hyp mice. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or Scl-Ab twice weekly beginning at 12 weeks of age and euthanized at 20 weeks of age. Scl-Ab increased alveolar bone mass in both male and female mice and alveolar tissue mineral density in the male mice. The positive effects of Scl-Ab were consistent with an increase in the fraction of active (non-phosphorylated) β-catenin stained alveolar osteocytes. Scl-Ab had no effect on mineralized tissues of the tooth - dentin, enamel, acellular and cellular cementum. There was a non-significant trend toward increased periodontal ligament (PDL) attachment fraction within the Hyp mice. Additional PDL fibral structural parameters were not affected by Scl-Ab. The current study demonstrates that Scl-Ab can improve alveolar bone in the Hyp mouse model of XLH.

Osteomalacia in Adults: A Practical Insight for Clinicians

The term osteomalacia (OM) refers to a series of processes characterized by altered mineralization of the skeleton, which can be caused by various disorders of mineral metabolism. OM can be genetically determined or occur due to acquired disorders, among which the nutritional origin is particularly relevant, due to its wide epidemiological extension and its nature as a preventable disease. Among the hereditary diseases associated with OM, the most relevant is X-linked hypophosphatemia (XLH), which manifests in childhood, although its consequences persist into adulthood where it can acquire specific clinical characteristics, and, although rare, there are XLH cases that reach the third or fourth decade of life without a diagnosis. Some forms of OM present very subtle initial manifestations which cause both considerable diagnosis and treatment delay. On occasions, the presence of osteopenia and fragility fractures leads to an erroneous diagnosis of osteoporosis, which may imply the prescription of antiresorptive drugs (i.e., bisphosphonates or denosumab) with catastrophic consequences for OM bone. On the other hand, some radiological features of OM can be confused with those of axial spondyloarthritis and lead to erroneous diagnoses. The current prevalence of OM is not known and is very likely that its incidence is much higher than previously thought. Moreover, OM explains part of the therapeutic failures that occur in patients diagnosed with other bone diseases. Therefore, it is essential that clinicians who treat adult skeletal diseases take into account the considerations provided in this practical review when focusing on the diagnosis and treatment of their patients with bone diseases.

Self-Administration of Burosumab in Children and Adults with X-Linked Hypophosphataemia in Two Open-Label, Single-Arm Clinical Studies

INTRODUCTION

X-linked hypophosphataemia (XLH) is a rare, genetic renal phosphate-wasting disease, resulting from excess fibroblast growth factor 23 (FGF23) activity, which has a progressive and profound impact on patients throughout life. The monoclonal anti-FGF23 antibody, burosumab, is a subcutaneous injection indicated for the treatment of XLH in children and adults. Originally, burosumab was approved to be administered by a healthcare professional (HCP), but the option of self-administration would enable patient independence and easier access to treatment. Two open-label, single-arm clinical trials, conducted in Japan and Korea, have assessed the safety and efficacy of self-administration of burosumab in both children and adults with XLH.

METHODS

In KRN23-003 (n = 15 children aged 1-12 years) and KRN23-004 (n = 5 children aged 3-13 years, n = 4 adults aged 21-65 years), children initially received 0.8 mg/kg of burosumab every 2 weeks and adults initially received 1.0 mg/kg of burosumab every 4 weeks. Self-administration was permitted from Week 4, and patients or carers were provided with training to inject correctly.

RESULTS

In both trials, burosumab had an acceptable safety profile with mainly mild-to-moderate adverse events. Following self-administration, no patients reported serious treatment-emergent adverse events ≥ grade 3, injection-site reactions or hypersensitivity reactions related to burosumab. Serum phosphate and active vitamin D levels increased from baseline in children and adults.

CONCLUSIONS

These results indicated that the efficacy and safety of burosumab when administered either by a carer or patient are similar to that when administered by an HCP and show that self-administration is a viable option for patients with XLH.

TRIAL REGISTRATION NUMBERS

NCT03233126 and NCT04308096.

Wnt pathway inhibitors are upregulated in XLH dental pulp cells in response to odontogenic differentiation

X-linked hypophosphatemia (XLH) represents the most common form of familial hypophosphatemia. Although significant advances have been made in the treatment of bone pathology, patients undergoing therapy continue to experience significantly decreased oral health-related quality of life. The following study addresses this persistent oral disease by further investigating the effect of DMP1 expression on the differentiation of XLH dental pulp cells. Dental pulp cells were isolated from the third molars of XLH and healthy controls and stable transduction of full-length human DMP1 were achieved. RNA sequencing was performed to evaluate the genetic changes following the induction of odontogenic differentiation. RNAseq data shows the upregulation of inhibitors of the canonical Wnt pathway in XLH cells, while constitutive expression of full-length DMP1 in XLH cells reversed this effect during odontogenic differentiation. These results imply that inhibition of the canonical Wnt pathway may contribute to the pathophysiology of XLH and suggest a new therapeutic strategy for the management of oral disease.

Presentation and Diagnosis of Pediatric X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare type of hereditary hypophosphatemic rickets. Patients with XLH have various symptoms that lower their QOL as defined by HAQ, RAPID3, SF36-PCS, and SF36-MCS in adult patients and SF-10 and PDCOI in pediatric patients. Early diagnosis and treatment are needed to reduce the burden, but the condition is often diagnosed late in childhood. The present review aims to summarize the symptoms, radiological and biological characteristics, and long-term prognosis of pediatric XLH. Typical symptoms of XLH are lower leg deformities (age six months or later), growth impairment (first year of life or later), and delayed gross motor development with progressive lower limb deformities (second year of life or later). Other symptoms include dental abscess, bone pain, hearing impairment, and Chiari type 1 malformation. Critical, radiological findings of rickets are metaphyseal widening, cupping, and fraying, which tend to occur in the load-bearing bones. The Rickets Severity Score, validated for XLH, is useful for assessing the severity of rickets. The biochemical features of XLH include elevated FGF23, hypophosphatemia, low 1,25(OH)2D, and elevated urine phosphate. Renal phosphate wasting can be assessed using the tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR), which yields low values in patients with XLH. XLH should be diagnosed early because the multisystem symptoms often worsen over time. The present review aims to help physicians diagnose XLH at an early stage.

Case Report of Femoral Neck Stress Fracture in X-linked Hypophosphatemic Rachitis Patient Treated with Mechanical Axis Correction and a Literature Review

Introduction

X-linked hypophosphatemic rachitis (XLHR) is the most common cause of hereditary rickets that can lead to long bone deformities requiring multiple surgical correction procedures. In addition, high rates of fractures are reported in adult XLHR patients. This study aimed to report a case of femoral neck stress fracture in XLHR patient treated with mechanical axis correction. No previous studies demonstrating a combined valgus correction and cephalomedullary nail fixation were identified in the literature.

Case Report

A 47-year-old male patient with XLHR attended the outpatient clinic with severe left hip pain. X-rays revealed a left proximal femoral varus deformity and a femoral neck stress fracture. After 1 month without improvement of pain, and no radiographic sign of healing, correction of the proximal femoral varus deformity and fixation of the cervical neck fracture was achieved by a cephalomedullary nail. At 8 months follow-up, hip pain relief was achieved with radiographic healing of the femoral neck stress fracture and the proximal femoral osteotomy.

Conclusion

A review of the literature was performed to identify any case report of femoral neck fractures fixation due to coxa vara in an adult. Both coxa vara and XLHR can cause femoral neck stress fracture. This study presented the surgical technique for treating a rare case of femoral neck stress fracture in a XLHR patient with coxa vara. Pain relief and bone healing were achieved by combined deformity correction and fracture fixation with a femoral cephalomedullary nail. The technique for deformity correction and cephalomedullary nail insertion in the patient with coxa vara is shown.

Dental health of patients with X-linked hypophosphatemia: A controlled study

Objective

The present study compared the dental health of patients with X-linked hypophosphatemia (XLH) with healthy age- and gender-matched controls to increase our knowledge of the impact of XLH on oral health.

Materials and methods

Twenty-two adult patients with XLH in the Stockholm region of Sweden were referred to the Department of Orofacial Medicine at Karolinska Institutet for an extended clinical and radiological examination. Pre-existing radiologic examinations of 44 healthy age- and gender-matched controls were retrieved from the Department of Oral Radiology, at Karolinska Institutet.

Results

The 22 patients with XLH (15 females, median age 38 years, range 20-71; 7 males, median age 49 years, range 24-67) had a significantly higher number of root-filled teeth compared to healthy controls (p = .001). In the XLH group, females had significantly better oral health than males, especially concerning endodontic and cariological status (p's = .01 and .02, respectively). Periodontal status differed non-significantly between the XLH and control groups.

Conclusion

Patients with XLH had a significantly lower oral health status compared to a healthy population especially concerning endodontic conditions. Male patients with XLH had a higher risk of poor oral health compared to female patients with XLH.

A Novel Synonymous Variant of PHEX in a Patient with X-Linked Hypophosphatemia

X-linked dominant hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets/osteomalacia, is caused by loss-of-function phosphate-regulating endopeptidase homolog X-linked gene (PHEX) variants. However, synonymous PHEX variants are rare in XLH. We report a 7-year-old boy with hypophosphatemia, short stature, and lower limb deformity. Whole-exome sequencing, reverse transcription-polymerase chain reaction, and Sanger sequencing were performed to identify the pathogenicity of the variant. A novel synonymous PHEX variant (NM_000444.4:c.1530 C&gt;T, p.Arg510Arg) was detected in the proband. Further analysis revealed a 58-bp deletion at the 5' site of exon 14 during splicing. This study extends the genetic spectrum of XLH and confirms the rarity and significance of synonymous PHEX variants.

Bone Deformities and Kidney Failure: Coincidence of PHEX-Related Hypophosphatemic Rickets and m.3243A&gt;G Mitochondrial Disease

X-linked hypophosphatemic rickets (XLH) and m.3243A&gt;G mitochondrial disease share several clinical findings, including short stature, hearing impairment (HI), nephropathy, and hypertension. Here, we report on a case with the rare coincidence of these two genetic conditions. In early childhood, the patient presented with hypophosphatemia and bone deformities and was clinically diagnosed with XLH. This was genetically verified in adulthood with the identification of a de novo pathogenic deletion in phosphate-regulating endopeptidase homolog X-linked (PHEX). In addition, the patient developed HI and hypertension and when his mother was diagnosed with m.3243A&gt;G, subsequent genetic testing confirmed the patient to carry the same variant. Over the next two decades, the patient developed progressive renal impairment however without nephrocalcinosis known to associate with XLH which could indicate an m.3243A&gt;G-related kidney disease. Parallel with the progression of renal impairment, the patient developed hyperphosphatemia and secondary hyperparathyroidism. In conclusion, this case represents a complex clinical phenotype with the reversal of hypo- to hyperphosphatemia in XLH potentially mediated by the development of an m.3243A&gt;G-associated nephropathy.

Hereditary Metabolic Bone Diseases: A Review of Pathogenesis, Diagnosis and Management

Hereditary metabolic bone diseases are characterized by genetic abnormalities in skeletal homeostasis and encompass one of the most diverse groups among rare diseases. In this review, we examine 25 selected hereditary metabolic bone diseases and recognized genetic variations of 78 genes that represent each of the three groups, including sclerosing bone disorders, disorders of defective bone mineralization and disorder of bone matrix and cartilage formation. We also review pathophysiology, manifestation and treatment for each disease. Advances in molecular genetics and basic sciences has led to accurate genetic diagnosis and novel effective therapeutic strategies for some diseases. For other diseases, the genetic basis and pathophysiology remain unclear. Further researches are therefore crucial to innovate ways to overcome diagnostic challenges and develop effective treatment options for these orphan diseases.

Post-authorisation safety study of burosumab use in paediatric, adolescent and adult patients with X-linked hypophosphataemia: rationale and description

Background:

X-linked hypophosphataemia (XLH) is a rare, inherited, phosphate-wasting disorder that elevates fibroblast growth factor 23 (FGF23), causing renal phosphate-wasting and impaired active vitamin D (1,25(OH)₂D) synthesis. Disease characteristics include rickets, osteomalacia, odontomalacia, and short stature. Historically, treatment has been oral phosphate and 1,25(OH)₂D supplements. However, these treatments do not correct the primary pathogenic mechanism or treat all symptoms and can be associated with adverse effects. Burosumab is a recombinant human immunoglobulin G1 monoclonal antibody against FGF23, approved for treating XLH in several geographical regions, including Europe and Israel. Burosumab restores normal serum phosphate levels, minimising the clinical consequences of XLH. Safety data on long-term treatment with burosumab are lacking owing to the rarity of XLH. This post-authorisation safety study (PASS) aims to evaluate the safety outcomes in patients aged >1 year.

Methods:

The PASS is a 10-year retrospective and prospective cohort study utilising data from the International XLH Registry (NCT03193476), which includes standard diagnostic and monitoring practice data at participating centres. The PASS aims to evaluate frequency and severity of safety outcomes, frequency and outcomes of pregnancies in female patients, and safety outcomes in patients with mild to moderate kidney disease at baseline, in children, adolescents and adults treated with burosumab for XLH. It is expected that there will be at least 400 patients who will be administered burosumab.

Results:

Data collection started on 24 April 2019. The expected date of the final study report is 31 December 2028, with two interim reports.

Conclusion:

This PASS will provide data on the long-term safety of burosumab treatment for XLH patients and describe safety outcomes for patients receiving burosumab contrasted with those patients receiving other XLH treatments, to help inform the future management of XLH patients. The PASS will be the largest real-world safety study of burosumab.

Registry identification:

The International XLH Registry is registered with clinicaltrials.gov as NCT03193476 (https://clinicaltrials.gov/ct2/show/NCT03193476), and the PASS is registered with the European Union electronic Register of Post-Authorisation Studies as EUPAS32190 (http://www.encepp.eu/encepp/viewResource.htm?id=32191).

Complications and Treatments in Adult X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare inherited disorder involving elevated levels of fibroblast growth factor (FGF) 23, and is caused by loss-of-function mutations in the PHEX gene. FGF23 induces renal phosphate wasting and suppresses the activation of vitamin D, resulting in defective bone mineralization and rachitic changes in the growth plate and osteomalacia. Conventional treatment with combinations of oral inorganic phosphate and active vitamin D analogs enhances bone calcification, but the efficacy of conventional treatment is insufficient for adult XLH patients to achieve an acceptable quality of life. Burosumab, a fully human monoclonal anti-FGF23 antibody, binds and inhibits FGF23, correcting hypophosphatemia and hypovitaminosis D. This review describes a typical adult with XLH and summarizes the results of clinical trials of burosumab in adults with XLH.

A Case of X-Linked Hypophosphatemic Rickets with Dentin Dysplasia in Mandibular Third Molars

X-linked hypophosphatemic rickets (XLH) is a disease characterized by impaired bone mineralization, and its dental features include gingival abscesses and large pulp spaces due to dentin dysplasia. A 20-year-old woman with XLH was referred to oral surgery for extraction of mandibular third molars. She was diagnosed with XLH at approximately 1 year of age and was treated thereafter. There was no history of gingival abscesses, and panoramic radiographic and computed tomographic examinations revealed no evidence of dentin dysplasia. However, histopathological examination of the extracted teeth showed dentin dysplasia, including interglobular dentin. In this XLH patient, dentin dysplasia was revealed histologically even though no obvious abnormality was found on visual and radiographic examinations. These findings suggest that in patients with XLH, oral management must take dentin dysplasia of the permanent teeth into consideration even if the patient’s general condition is well controlled with conventional therapy.

Treatment of X-Linked Hypophosphatemia in Children

The conventional treatment for X-linked hypophosphatemia (XLH), consisting of phosphorus supplementation and a biologically active form of vitamin D (alfacalcidol or calcitriol), is used to treat rickets and leg deformities and promote growth. However, patients’ adult height often remains less than −2 SD. Moreover, adverse events, such as renal calcification and hyperparathyroidism, may occur. The main pathology in XLH is caused by excessive production of fibroblast growth factor 23 (FGF23). Several studies have demonstrated that treatment with burosumab, a blocking neutralizing antibody against FGF23, is better than conventional therapy for severe XLH and has no serious, short-term side effects. Thus, treatment with burosumab may be an option for severe XLH. The present article reviews the conventional and burosumab therapies. In addition to the fact that the long-term efficacy of antibody-based treatment has not been demonstrated, there are other, unresolved issues concerning the burosumab treatment of XLH.