Inherited fibroblast growth factor 23 excess

Syndromes of inherited fibroblast growth factor 23 (FGF-23) excess encompass a wide spectrum that includes X-linked hypophosphataemia (XLH), autosomal dominant and recessive forms of rickets as well as various syndromic conditions namely fibrous dysplasia/McCune Albright syndrome, osteoglophonic dysplasia, Jansen's chondrodysplasia and cutaneous skeletal hypophosphataemia syndrome. A careful attention to patient symptomatology, family history and clinical features, supported by appropriate laboratory tests will help in making a diagnosis. A genetic screen may be done to confirm the diagnosis. While phosphate supplements and calcitriol continue to be the cornerstone of treatment, in recent times burosumab, the monoclonal antibody against FGF-23 has been approved for the treatment of children and adults with XLH. While health-related outcomes may be improved by ensuring adherence and compliance to prescribed treatment with a smooth transition to adult care, bony deformities may persist in some, and this would warrant surgical correction.

The diagnostic odyssey in children and adolescents with X-linked hypophosphataemia: population-based, case-control study

CONTEXT

X-linked hypophosphataemia (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 (EHR) in the United Kingdom.

METHODS

Using the Optimum Patient Care Research Database, individuals aged 20 years or younger after 1st Jan 2000 at date of recorded XLH diagnosis were identified using SNOMED/Read codes, and age-matched to 100 controls. Recording of XLH-related clinical features was summarised, then compared between cases and controls using Chi-squared or Fisher's exact tests.

RESULTS

261 XLH cases were identified; 99 met 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 characterisation of the EHR phenotypes of children and adolescents with XLH may inform future case-finding approaches to expedite diagnosis in primary care.

Prevalence and characteristics of paediatric X-linked hypophosphataemia in Australia and New Zealand: Results from the Australian and the New Zealand Paediatric Surveillance Units survey

BACKGROUND

X-linked hypophosphataemia (XLH) is the most common heritable form of rickets. Prevalence data varies across the literature between 1 in 20,000 and 1 in 200,000 per population.

METHODS

Australian and New Zealand Paediatric Surveillance Units collected cross-sectional data from paediatricians on existing cases to estimate prevalence and characteristics of paediatric XLH in Australia and New Zealand.

RESULTS

Seventy-five cases in Australia and 18 cases in New Zealand were identified. Estimated minimum prevalence based on these cases was 1.33 (1.04-1.66) per 100,000 and 1.60 per 100,000 (95%CI 0.97-2.58) in Australia and New Zealand respectively, with actual prevalence likely higher due to incomplete ascertainment. Despite a family history in most cases, delayed diagnosis was common, with 49 % diagnosed after 2 years of age. Delayed diagnosis was more common in sporadic versus familial cases. Most common clinical characteristics included leg bowing (89 %), bone and joint pain (68 %), abnormal gait (57 %) and short stature (49 %). There was a significant burden of orthopaedic disease and surgeries and a high rate of complications of nephrocalcinosis and hyperparathyroidism (32 % and 20 % respectively). Additionally, while guidelines stress the importance of multidisciplinary care, many did not have access to recommended health professionals, with only 3 % seeing a psychologist and 68 % seeing a dentist. This is despite the high psychological burden of XLH and a significant proportion (41 %) of this cohort having dental issues (tooth abscess, dental capping, tooth extraction). There were two cases from NZ without data available. Of the 91 cases with data collected, 46 % were on burosumab therapy. Consistent with clinical trials, those on burosumab had a higher serum phosphate levels (p < 0.001) at most recent follow-up. Three cases reported cancellation of orthopaedic surgery due to improvement in lower limb deformity after commencement of burosumab.

CONCLUSION

These data describe the multisystem burden of disease for children with XLH with care impacted by delayed diagnosis and a lack of access to many health professionals, especially psychological support.

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.

X-linked hypophosphatemic osteomalacia (XLH): Study of 5 adult patients

BACKGROUND

X-linked hypophosphatemic osteomalacia (XLH) is an inherited disorder that can cause highly disabling musculoskeletal comorbidities in adulthood.

OBJECTIVE

To analyze the clinical-radiological characteristics, comorbidities and complications associated with the disease and treatment in an adult population with XLH.

METHOD

Retrospective study of patients treated for XLH in a rheumatology department in the last 10 years, evaluating the clinical-radiological findings, comorbidities and associated complications.

RESULTS

Five patients between 39 and 75 years of age were included. All had short stature, osteoarticular symptoms and radiological enthesopathy. Four patients had early degenerative arthropathy of the knees and hips, and dental alterations associated with their disease. All patients older than 50 years required some type of prosthetic replacement. Two patients had femoral stress fractures, one had renal lithiasis and another developed tertiary hyperparathyroidism.

CONCLUSIONS

Musculoskeletal manifestations are frequent and disabling in the adult population with XLH, so proper diagnosis and management from childhood are essential to prevent the development of complications in adulthood associated with this disease.

Patient-reported outcomes measures of X-linked hypophosphataemia participants: findings from a prospective cohort study in the UK

BACKGROUND

X-linked hypophosphataemia (XLH) is a rare genetic condition passed on through the X chromosome which causes multiple symptoms including weakened teeth, bones, and muscles. Due to the rarity of the condition, little is known about the health outcomes as reported by people with the disease. The objectives of this study were threefold: to characterise key patient reported outcome measures (PROMs) in adults with XLH, to identify clusters of symptom-severity groups based on PROMs, and to analyse the longitudinal progression of available PROMs.

METHODS

Data from 48 participants from the Rare and Undiagnosed Diseases cohort Study (RUDY) was used to analyse both cross-sectional and longitudinal patient-reported outcomes. We analysed data for health-related quality of life (HRQL): EuroQol 5 dimensions-5 levels (EQ-5D-5L), Short-form 36 (SF-36) Physical Component Score (PCS), and SF-36 Mental Component Score (MCS), sleep: Pittsburgh sleep quality index (PSQI) and Epworth Sleepiness scale (ESS), fatigue: Fatigue Severity Scale (FSS) and Functional assessment of chronic illness therapy-fatigue (FACIT-F), pain: Short form McGill pain questionnaire version 2 (SF-MPQ-2) and PainDETECT, and mental well-being: Hospital anxiety and depression scale (HADS) anxiety and depression. Summary statistics, tests of mean differences, mixed-effects models, and cluster analysis were used to describe and examine the various health dimensions of individuals with XLH.

RESULTS

Overall mean scores were EQ-5D-5L = 0.65, SF-36-PCS = 32.7, and SF-36-MCS = 48.4 for HRQL, ESS = 5.9 and PSQI = 8.9 for sleep, FSS = 32.8 and FACIT-F = 104.4 for fatigue, SF-MPQ-2 = 1.9 for pain, and HADS-depression = 4.7 and HADS-anxiety = 6.2 for mental well-being. 7% reported neuropathic pain (PainDETECT). Whilst many adults with XLH reported good outcomes, extreme or severe problems were reported across all outcomes. Cluster analysis identified that adults with XLH could be divided into two distinct groups, one reporting worse (35.3%) and the other better outcomes (64.7%) (less pain, fatigue, depression, and higher levels of sleep). Longitudinal analysis showed that FACIT-F and HADS-anxiety scores worsened slightly over two years with statistically significant (p < 0.05) time coefficients (b =  - 2.135 and b = 0.314, respectively).

CONCLUSION

Although about two thirds of adult participants of the RUDY cohort with XLH report good health outcomes, for a considerable third much worse outcomes are reported. More research is needed to examine why some experience good and others poor health outcomes and the characteristics which identify them.

Left Ventricular Hypertrophy in Patients with X-Linked Hypophosphataemia

X-linked hypophosphatemia (XLH) is a rare genetic disorder with X-linked dominant inheritance. Mutations in the PHEX gene increase fibroblast growth factor 23 (FGF23) concentrations, causing loss of phosphorus at the proximal tubule. Most pediatric patients debut in the first two years with short stature and bowed legs. Conventional treatment consists of oral supplements with phosphorus and calcitriol. Since 2018, burosumab has been approved as a novel therapeutic option for XLH, with promising results. The purpose of this study was to share our experience with two cases of XLH treated with burosumab. These patients presented with a broad phenotypical differences. One had the most severe radiological phenotype and developed left ventricular hypertrophy (LVH) and left ventricular dysfunction with preserved ejection fraction. Treatment with burosumab was well-tolerated and was followed by radiological stability and a striking improvement in both blood biochemistry and quality of life. The LVH was stable and left ventricular function normalized in the patient with cardiac involvement. In recent years many studies have been carried out to explain the role of FGF23 in cardiovascular damage, but the exact pathophysiological mechanisms are as yet unclear. The most intensively studied populations are patients with XLH or chronic kidney disease, as both are associated with high levels of FGF23. To date, cardiovascular involvement in XLH has been described in patients treated with conventional treatment, so it would be of interest to investigate if early use of burosumab at the time of diagnosis of XLH would prevent the occurrence of cardiovascular manifestations.

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.

Burden of disease associated with X-linked hypophosphataemia in adults: a systematic literature review

This systematic review collated evidence on the burden of XLH in adults. Data captured highlight the substantial ongoing burden of XLH in adulthood and identified unmet needs. Greater awareness and understanding of the impact of XLH in adulthood are needed to improve care and outcomes in adults with XLH.

INTRODUCTION

X-linked hypophosphataemia (XLH) is a rare metabolic bone disease characterized by renal phosphate wasting and musculoskeletal manifestations. Whilst the disease's impact in children is well documented, information on the effects of this progressive, debilitating condition on adults is lacking. This systematic review aimed to collate existing evidence on the burden of XLH in adulthood to identify unmet needs.

METHODS

MEDLINE, Embase and Cochrane Library databases and recent congress reports were searched on 19 February 2019 for English-language publications describing the medical, humanistic and socio-economic impact of XLH in adults (≥ 18 years old). In addition, a structured Internet search was conducted.

RESULTS

Of the 2351 articles identified, 91 met the selection criteria along with 44 congress abstracts. Data show that adults with XLH experience a range of clinical manifestations, particularly skeletal deformities and (pseudo)fractures, along with pain, dental abnormalities and impaired physical function and mobility. XLH in adulthood impacts on quality of life and places limitations on daily activities. The level of healthcare resource utilization among adults with XLH is indicative of substantial socio-economic burden; further research is needed to quantitate the economic impact on the healthcare system, society and patients. Adults with XLH may not receive appropriate care and treatment; a possible explanation for this is a lack of awareness among healthcare professionals.

CONCLUSION

XLH in adults is associated with considerable disease burden and unmet needs. Forthcoming studies and increased awareness of the impact of XLH in adulthood should help to improve management of XLH in adulthood and patient outcomes.

Oral health-related quality of life in X-linked hypophosphataemia and osteogenesis imperfecta

X‐linked hypophosphataemia (XLH) and osteogenesis imperfecta (OI) are rare congenital disorders characterised by skeletal dysplasia. The two disorders may include dental anomalies potentially affecting individual well‐being. The aims of study were (a) to assess the oral health‐related quality of life (OHRQoL) in Danish adults with XLH or OI, and (b) to compare the results of the groups. A cross‐sectional study including 35 adults with XLH, 56 adults with OI type I and 17 adults with OI types III‐IV was conducted. The OHRQoL was assessed by the 49‐item version of the questionnaire Oral Health Impact Profile (OHIP). Summed domain scores (seven) were compared between XLH and OI groups. Prevalence of severe impact on OHRQoL (scores 3‐4) was compared between groups. The median scores in XLH group exceeded the medians in OI (P < .05) in the domains functional limitation (XLH:6.5; OI:4.0), pain (XLH:9.5; OI:5.0), psychological discomfort (XLH:5.5; OI:2.0), psychological disability (XLH:2.0; OI:0.0), handicap (XLH:2.0; OI:0.0) and total OHIP (XLH:35.0; OI:14.0). Differences in domains physical disability (XLH: 4.0; OI: 1.0) and social disability (XLH: 0.0; OI: 0.0) were not significant. Prevalence of severe impact on OHRQoL in the XLH group significantly exceeded the level in OI group in the domains functional limitation (XLH: 59%; OI: 35%), psychological discomfort (XLH: 38%; OI: 20%) and physical disability (XLH: 32%; OI: 13%). In conclusion, adults with XLH experience a higher negative impact on their OHRQoL than adults with OI. Only to a minor degree, individuals with OI types III‐IV experience a higher impact on OHRQoL than individuals with OI type I.

Pharmacological management of X-linked hypophosphataemia

The most common heritable disorder of renal phosphate wasting, X-linked hypophosphataemia (XLH), was discovered to be caused by inactivating mutations in the phosphate regulating gene with homology to endopeptidases on the X-chromosome (PHEX) gene in 1995. Although the exact molecular mechanisms by which PHEX mutations cause disturbed phosphate handling in XLH remain unknown, focus for novel therapies has more recently been based upon the finding that the bone-produced phosphaturic hormone fibroblast growth factor-23 is elevated in XLH patient plasma. Previous treatment strategies for XLH were based upon phosphate repletion plus active vitamin D analogues, which are difficult to manage, fail to address the primary pathogenesis of the disease, and can have deleterious side effects. A novel therapy for XLH directly targeting fibroblast growth factor-23 via a humanized monoclonal antibody (burosumab-twza/CRYSVITA, henceforth referred to just as burosumab) has emerged as an effective, and recently approved, pharmacological treatment for both children and adults. This review will provide an overview of the clinical manifestations of XLH, the molecular pathophysiology, and summarize its current treatment.

Developments in rare bone diseases and mineral disorders

In the last decade, there have been a number of significant advances made in the field of rare bone diseases. In this review, we discuss the expansion of the classification system for osteogenesis imperfecta (OI) and the resultant increase in therapeutic options available for management of OI. Bisphosphonates remain the most widely used intervention for OI, although the effect on fracture rate reduction is equivocal. We review the other therapies showing promising results, including denosumab, teriparatide, sclerostin, transforming growth factor β inhibition and gene targeted approaches. X-linked hypophosphataemia (XLH) is the most common heritable form of osteomalacia and rickets caused by a mutation in the phosphate regulating endopeptidase gene resulting in elevated serum fibroblast growth factor 23 (FGF23) and decreased renal phosphate reabsorption. The traditional treatment is phosphate replacement. We discuss the development of a human anti-FGF23 antibody (KRN23) as a promising development in the treatment of XLH. The current management of primary hypoparathyroidism is replacement with calcium and active vitamin D. This can be associated with under or over replacement and its inherent complications. We review the use of recombinant parathyroid hormone (1-84), which can significantly reduce the requirements for calcium and vitamin D resulting in greater safety and quality of life for individuals with hypoparathyroidism. The use of receptor activator of nuclear factor κB ligand infusions in the treatment of a particular form of osteopetrosis and enzyme replacement therapy for hypophosphatasia are also discussed.