Effects of growth hormone treatment on adult height in severely short children with X-linked hypophosphatemic rickets

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.

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.

X-linked hypophosphatemic rickets: from diagnosis to management

X-linked hypophosphatemia (XLH), the most common cause of hypophosphatemic rickets, affects one in every 20,000 people. Although conventional therapy for XLH was introduced approximately 4 decades ago, the temporary replacement of oral phosphate salts and activated vitamin D cannot completely control chronic hypophosphatemia, leaving patients with incomplete healing and residual skeletal deformity as well as at risk of endocrine abnormalities and adverse drug reactions. However, understanding the pathophysiology has led to the development of a targeted therapy, burosumab, a fibroblast growth factor-23 inhibitor that was recently approved in Korea for the treatment of XLH. This review provides insight into the diagnosis, evaluation, treatment, and recommended follow-up for a typical case of XLH and reviews its pathophysiology.

Linear growth of children with X-linked hypophosphatemia treated with burosumab: a real-life observational study

To assess the long-term efficacy of burosumab for pediatric patients with X-linked hypophosphatemia, focusing on linear growth. This multi-center retrospective study included 35 pediatric patients who began treatment with burosumab between January 2018 and January 2021. We collected clinical data, anthropometric measurements, laboratory results, and Rickets Severity Score (RSS), from 2 years prior to treatment initiation and up to 4 years after. Burosumab was initiated at a mean age of 7.5 ± 4.4 years (range 0.6-15.9), with a mean initial dose of 0.8 ± 0.3 mg/kg, which was subsequently increased to 1.1 ± 0.4 mg/kg. The patients were followed for 2.9 ± 1.4 years (range 1-4) after initiating burosumab. Serum phosphorus levels increased from 2.7 ± 0.8 mg/dl at burosumab initiation to 3.4 ± 0.6 mg/dl after 3 months and remained stable (p < 0.001). Total reabsorption of phosphorus increased from 82.0 ± 6.8 to 90.1 ± 5.3% after 12 months of treatment (p = 0.041). The RSS improved from 1.7 ± 1.0 at burosumab initiation to 0.5 ± 0.6 and 0.3 ± 0.6 after 12 and 24 months, respectively (p < 0.001). Both height z-score and weight z-score improved from burosumab initiation to the end of the study: from - 2.07 ± 1.05 to - 1.72 ± 1.04 (p < 0.001) and from - 0.51 ± 1.12 to - 0.11 ± 1.29 (p < 0.001), respectively. Eight children received growth hormone combined with burosumab treatment. Height z-score improved among those who received growth hormone (from - 2.33 ± 1.12 to - 1.94 ± 1.24, p = 0.042) and among those who did not (from - 2.01 ± 1.01 to - 1.66 ± 1.01, p = 0.001).

CONCLUSION

 Burosumab treatment in a real-life setting improved phosphate homeostasis and rickets severity and enhanced linear growth.

WHAT IS KNOWN

• Compared to conventional therapy, burosumab treatment has been shown to increase serum phosphate levels and reduce the severity of rickets. • The effect of burosumab on growth is still being study.

WHAT IS NEW

• Height z-score improved between the start of burosumab treatment and the end of the study (-2.07 ± 1.05 vs. -1.72 ± 1.04, p < 0.001). • Eight children received burosumab combined with growth hormone treatment without side effects during the concomitant treatments.

Adult height improved over decades in patients with X-linked hypophosphatemia: a cohort study

OBJECTIVES

The aim of this study is to analyze height after cessation of growth (final height [FH]) and its evolution over the last decades in X-linked hypophosphatemia (XLH) patients in France, as the data on natural history of FH in XLH are lacking.

DESIGN

We performed a retrospective observational study in a large cohort of French XLH patients with available data on FH measurements.

MATERIALS AND METHODS

We divided patients into 3 groups according to their birth year: group 1 born between 1950 and 1974, group 2 born between 1975 and 2000, and group 3 born between 2001 and 2006, respectively, and compared their FHs.

RESULTS

A total of 398 patients were included. Mean FHs were the following: for group 1, -2.31 ± 1.11 standard deviation score (SDS) (n = 127), 156.3 ± 9.7 cm in men and 148.6 ± 6.5 cm in women; for group 2, -1.63 ± 1.13 SDS (n = 193), 161.6 ± 8.5 cm in men and 153.1 ± 7.2 cm in women; and for group 3, -1.34 ± 0.87 SDS (n = 78), 165.1 ± 5.5 cm in men and 154.7 ± 6 cm in women. We report a significant increase in mean FH SDS over 3 generations of patients, for both men and women (P < .001). Final height SDS in male (-2.08 ± 1.18) was lower than in female (-1.70 ± 1.12) (P = .002).

CONCLUSION

The FH of XLH patients in France increased significantly over the last decades. Even though men's FHs improved more than women's, men with XLH remain shorter reflecting a more severe disease phenotype. While the results are promising, most patients with XLH remain short leaving room for improvement.

Growth hormone treatment improves final height in children with X-linked hypophosphatemia

BACKGROUND/AIM

Despite optimal conventional treatment (oral phosphate supplements and active vitamin D analogs), about 40-50% of children with well-controlled X-linked hypophosphatemia (XLH) show linear growth failure, making them less likely to achieve an acceptable final height. Here, we studied the hypothesis that rhGH treatment improves final height in children with XLH and growth failure.

METHODS

Two cohorts of children with XLH were included in this retrospective longitudinal analysis: (1) a cohort treated with rhGH for short stature (n = 34) and (2) a cohort not treated with rhGH (n = 29). The mean duration of rhGH treatment was 4.4 ± 2.9 years. We collected the auxological parameters at various time points during follow-up until final height.

RESULTS

In rhGH-treated children, 2 years of rhGH therapy was associated with a significant increase in height from - 2.4 ± 0.9 to - 1.5 ± 0.7 SDS (p < 0.001). Their mean height at rhGH discontinuation was - 1.2 ± 0.9 SDS and at final height was - 1.3 ± 0.9 SDS corresponding to 165.5 ± 6.4 cm in boys and 155.5 ± 6.3 cm in girls. Notably, the two groups had similar final heights; i.e., the final height in children not treated with rhGH being - 1.2 ± 1.1 SDS (165.4 ± 6.8 cm in boys and 153.7 ± 7.8 cm in girls), p = 0.7.

CONCLUSION

Treatment with rhGH permits to improve final height in children with XLH and growth failure, despite optimal conventional treatment. We propose therefore that rhGH therapy could be considered as an option for short stature in the context of XLH.

Growth pattern in children with X-linked hypophosphatemia treated with burosumab and growth hormone

BACKGROUND

X-linked hypophosphatemia (XLH) is characterized by increased serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphatemia and insufficient endogenous synthesis of calcitriol. Beside rickets, odonto- and osteomalacia, disproportionate short stature is seen in most affected individuals. Vitamin D analogs and phosphate supplements, i.e., conventional therapy, can improve growth especially when started early in life. Recombinant human growth hormone (rhGH) therapy in XLH children with short stature has positive effects, although few reports are available. Newly available treatment (burosumab) targeting increased FGF23 signaling leads to minimal improvement of growth in XLH children. So far, we lack data on the growth of XLH children treated with concomitant rhGH and burosumab therapies.

RESULTS

Thirty-six patients received burosumab for at least 1 year after switching from conventional therapy. Of these, 23 received burosumab alone, while the others continued rhGH therapy after switching to burosumab. Children treated with burosumab alone showed a minimal change in height SDS after 1 year (mean ± SD 0.0 ± 0.3 prepubertal vs. 0.1 ± 0.3 pubertal participants). In contrast, rhGH clearly improved height during the first year of treatment before initiating burosumab (mean ± SD of height gain 1.0 ± 0.4); patients continued to gain height during the year of combined burosumab and rhGH therapies (mean ± SD height gain 0.2 ± 0.1). As expected, phosphate serum levels normalized upon burosumab therapy. No change in serum calcium levels, urinary calcium excretion, or 25-OHD levels was seen, though 1,25-(OH)₂D increased dramatically under burosumab therapy.

CONCLUSION

To our knowledge, this is the first study on growth under concomitant rhGH and burosumab treatments. We did not observe any safety issue in this cohort of patients which is one of the largest in Europe. Our data suggest that continuing treatment with rhGH after switching from conventional therapy to burosumab, if the height prognosis is compromised, might be beneficial for the final height.

Rickets guidance: part II-management

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

Recombinant growth hormone therapy for X-linked hypophosphatemia in children

BACKGROUND

Conventional treatment of X-linked hypophosphatemia with oral phosphate and calcitriol can heal rickets, but it does not always raise serum phosphate concentrations significantly, nor does it always normalize linear growth. Some clinical trials suggest that combining recombinant human growth hormone therapy with conventional treatment improves growth velocity, phosphate retention, and bone mineral density, but some clinical trials suggest that it appears to aggravate the pre-existent disproportionate stature of such children. This is an updated version of a previously published review.

OBJECTIVES

To determine whether recombinant human growth hormone therapy for children with X-linked hypophosphatemia is associated with changes in longitudinal growth, mineral metabolism, endocrine function, renal function, bone mineral density, body proportions, and also with any adverse effects.

SEARCH METHODS

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register which comprises references identified from comprehensive electronic database searches and handsearches of relevant journals and abstract books of conference proceedings. In addition, we searched the Cochrane Central Register of Controlled Trials, Ovid MEDLINE and the reference lists of identified trials and other reviews. We also undertook some additional handsearching of relevant journals and conference proceedings. Date of the most recent search: 12 January 2021 SELECTION CRITERIA: All randomized controlled studies or quasi-randomized controlled studies comparing growth hormone (alone or combined with conventional treatment) with either placebo or conventional treatment alone in children with X-linked hypophosphatemia.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed studies for risk of bias and extracted data from eligible studies. GRADE criteria were used to assess the certainty of the evidence for each outcome.

MAIN RESULTS

We included two studies (20 participants) in the review. In one cross-over study, results showed that recombinant human growth hormone therapy may improve the height standard deviation (SDS) score (z score), but we are unsure whether the intervention was the reason behind a transient increase in serum phosphate and tubular maximum for phosphate reabsorption. In the second, parallel study, treatment may also have improved the height SDS from baseline in the rhGH group compared to the control group, although no significant difference was seen between groups after three years, MD 0.50 SDS (95 % CI -0.54 to 1.54) (low-certainty evidence). The treatment was possibly well-tolerated during both studies with only transient adverse effects seen in three participants (low-certainty evidence). We are uncertain whether growth hormone improves serum phosphate levels or change in TmP/GFR (very low-certainty evidence). The treatment may make little or no difference to alkaline phosphatase levels (low-certainty evidence).

AUTHORS' CONCLUSIONS

We do not have enough high-certainty evidence to recommend the use of recombinant human growth hormone therapy in children with X-linked hypophosphatemia.

Rare PHEX variant with insidious presentation leads to a delayed diagnosis of X-linked hypophosphatemia

A 7-year-old girl without a significant previous medical history was diagnosed with X-linked hypophosphatemic rickets (XLHR) due to a rare, most likely pathogenic, PHEX gene variant after a 4-year delayed diagnosis due to mild clinical presentation. At 2 years of age, her intoeing and femoral bowing were attributed to physiologic bowing and borderline vitamin D sufficiency, despite phosphorus not being measured. Hypophosphatemia was eventually detected after incomplete improvement of bowing and leg length discrepancy with suboptimal linear growth. This rare PHEX variant (c.1949T>C, p.Leu650Pro) further supported the clinical diagnosis of XLHR. Treatment with burosumab (an anti-FGF23 monoclonal antibody) normalised phosphorus and alkaline phosphatase levels and improved her bowing. The diverse phenotypic presentation of this variant can result in delayed diagnosis and highlights the importance of prompt assessment of phosphorus levels in patients with skeletal deformities to ensure timely recognition and treatment.

Outcome of primary tubular tubulopathies diagnosed in pediatric age

BACKGROUND AND OBJECTIVE

Primary tubulopathies are rare and usually present at pediatric age. Recent advances in genetic diagnosis and treatment have changed its natural history. This study provides the clinical spectrum of a series of primary tubulopathies diagnosed in a Pediatric Nephrology Unit and to offer long-term follow-up data regarding growth, estimated glomerular filtration (eGFR) and intercurrent complications.

PATIENTS AND METHODS

Observational study in 53 patients with primary tubulopathies and identified genetic defect: Gitelman syndrome (36%), distal renal tubular acidosis (15%), cystinuria (11%), X-linked hypophosphatemic rickets (7%), Dent-syndrome Lowe (7%), cystinosis (6%), and 1-2 cases of other tubulopathies. Demographic, analytical and clinical data were collected at diagnosis, during evolution and at the time of the study.

RESULTS

The age (median and interquartile range) at diagnosis was 5.08 years (1.33-8.50). The most frequent presentation manifestations were metabolic decompensations associated with intercurrent processes (40%) and short stature (38%). Height (mean ± SD) was -1.39 ± 1.49 at diagnosis and 1.07 ± 1.54 after a follow-up of 18.92 (6.25-24.33) years. Sixteen (32%) developed an eGFR <90 ml/min/1.73 m². Three patients required replacement renal replacement. Eleven patients had metabolic decompensations that required hospitalization, 9 renal colic and/or kidney stones and 10 mental problems. Six of 8 patients with distal renal tubular acidosis developed sensorineural deafness.

CONCLUSIONS

Primary tubulopathies are a heterogeneous group of diseases that cause growth impairment, largely reversible with treatment, risk of eGFR reduction and significant extrarenal complications derived or associated.

Upstream Regulators of Fibroblast Growth Factor 23

Fibroblast growth factor 23 (FGF23) has been described as an important regulator of mineral homeostasis, but has lately also been linked to iron deficiency, inflammation, and erythropoiesis. FGF23 is essential for the maintenance of phosphate homeostasis in the body and activating mutations in the gene itself or inactivating mutations in its upstream regulators can result in severe chronic hypophosphatemia, where an unbalanced mineral homeostasis often leads to rickets in children and osteomalacia in adults. FGF23 can be regulated by changes in transcriptional activity or by changes at the post-translational level. The balance between O-glycosylation and phosphorylation is an important determinant of how much active intact or inactive cleaved FGF23 will be released in the circulation. In the past years, it has become evident that iron deficiency and inflammation regulate FGF23 in a way that is not associated with its classical role in mineral metabolism. These conditions will not only result in an upregulation of FGF23 transcription, but also in increased cleavage, leaving the levels of active intact FGF23 unchanged. The exact mechanisms behind and function of this process are still unclear. However, a deeper understanding of FGF23 regulation in both the classical and non-classical way is important to develop better treatment options for diseases associated with disturbed FGF23 biology. In this review, we describe how the currently known upstream regulators of FGF23 change FGF23 transcription and affect its post-translational modifications at the molecular level.

Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium

X-linked hypophosphatemia (XLH) is the most common genetic form of hypophosphatemic rickets and osteomalacia. In this disease, mutations in the PHEX gene lead to elevated levels of the hormone fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting and impaired skeletal and dental mineralization. Recently, international guidelines for the diagnosis and treatment of this condition have been published. However, more specific recommendations are needed to provide guidance at the national level, considering resource availability and health economic aspects. A national multidisciplinary group of Belgian experts convened to discuss translation of international best available evidence into locally feasible consensus recommendations. Patients with XLH may present to a wide array of primary, secondary and tertiary care physicians, among whom awareness of the disease should be raised. XLH has a very broad differential-diagnosis for which clinical features, biochemical and genetic testing in centers of expertise are recommended. Optimal care requires a multidisciplinary approach, guided by an expert in metabolic bone diseases and involving (according to the individual patient's needs) pediatric and adult medical specialties and paramedical caregivers, including but not limited to general practitioners, dentists, radiologists and orthopedic surgeons. In children with severe or refractory symptoms, FGF23 inhibition using burosumab may provide superior outcomes compared to conventional medical therapy with phosphate supplements and active vitamin D analogues. Burosumab has also demonstrated promising results in adults on certain clinical outcomes such as pseudofractures. In summary, this work outlines recommendations for clinicians and policymakers, with a vision for improving the diagnostic and therapeutic landscape for XLH patients in Belgium.

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

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

Mineralized tissues in hypophosphatemic rickets

Hypophosphatemic rickets is caused by renal phosphate wasting that is most commonly due to X-linked dominant mutations in PHEX. PHEX mutations cause hypophosphatemia indirectly, through the increased expression of fibroblast growth factor 23 (FGF23) by osteocytes. FGF23 decreases renal phosphate reabsorption and thereby increases phosphate excretion. The lack of phosphate leads to a mineralization defect at the level of growth plates (rickets), bone tissue (osteomalacia), and teeth, where the defect facilitates the formation of abscesses. The bone tissue immediately adjacent to osteocytes often remains unmineralized ("periosteocytic lesions"), highlighting the osteocyte defect in this disorder. Common clinical features of XLH include deformities of the lower extremities, short stature, enthesopathies, dental abscesses, as well as skull abnormalities such as craniosynostosis and Chiari I malformation. For the past four decades, XLH has been treated by oral phosphate supplementation and calcitriol, which improves rickets and osteomalacia and the dental manifestations, but often does not resolve all aspects of the mineralization defects. A newer treatment approach using inactivating FGF23 antibodies leads to more stable control of serum inorganic phosphorus levels and seems to heal rickets more reliably. However, the long-term benefits of FGF23 antibody treatment remain to be elucidated.

Größenwachstum und Knochengesundheit bei Erkrankungen der Wachstumsfuge und des Knochens: Möglichkeiten und Grenzen einer GH-Therapie

Hintergrund

Genetische, parakrine und endokrine Faktoren beeinflussen das Größenwachstum und die Knochenmineralisation.

Fragestellung

Wie sind spontanes Größenwachstum und Knochengesundheit von Patienten mit seltenen Knochenerkrankungen? Kann man Wachstum und Knochengesundheit bei diesen Erkrankungen mit Wachstumshormon („growth hormone“ [GH]) verbessern?

Material und Methoden

Ergebnisse eines Expertentreffens mit Literaturrecherche zur Knochengesundheit von mit Kleinwuchs assoziierten ossären Erkrankungen und deren Therapieoptionen.

Ergebnisse

Viele Patienten mit einer Osteogenesis imperfecta sind kleinwüchsig. Eine zusätzliche Gabe von GH hat keinen Einfluss auf die Erwachsenengröße und wird derzeit nicht angewendet. Patienten mit unzureichendem Größenwachstum bei Pseudohypoparathyreoidismus (PHP) können bei Nachweis eines GH-Mangels mit GH behandelt werden und von der Therapie profitieren. Kinder mit X‑chromosomal vererbter hypophosphatämischer Rachitis haben unter der bisherigen Therapie mit Phosphat und Calcitriol einen disproportionierten Kleinwuchs. Randomisierte Therapiestudien mit GH führten zwar zu einer vorübergehenden Verbesserung der Körperhöhe, aber zu keiner signifikanten Verbesserung der Erwachsenenkörpergröße. Bei SHOX-Defizienz ist eine GH-Therapie zugelassen und hinsichtlich Köpergrößenzunahme vergleichbar effektiv wie bei Mädchen mit Ullrich-Turner-Syndrom (UTS). Zusätzlich legen Beobachtungsstudien nahe, dass eine GH-Therapie das bei UTS erhöhte Frakturrisiko reduzieren kann. Entzündung, verminderte körperliche Aktivität und Malnutrition führen bei Patienten mit juveniler idiopathischer Arthritis (JIA) zu einem Kleinwuchs mit Verlust an Muskel- und Knochenmasse. Studien konnten einen positiven Effekt von GH auf Längenwachstum, Dichte, Geometrie und Metabolismus des Knochens sowie auf die Muskelmasse zeigen. Die Therapie mit GH ist bei den Patienten mit JIA nicht zugelassen.

Schlussfolgerungen

Bei den genannten Knochenerkrankungen muss jede Wachstumsstörung individuell betrachtet werden. Neben dem Größenwachstum kann sich die GH-Therapie je nach Indikation positiv auf Stoffwechsel, Mineralsalzgehalt und Knochendichte auswirken. Zugelassene Indiktionen für eine GH-Therapie liegen bei PHP nur bei einem GH-Mangel und bei Patienten mit intrauterinem Kleinwuchs (SGA) und UTS/SHOX-Mangel vor. Neben einer Zunahme des Wachstums sollte eine Verbesserung der Knochengesundheit im Kindes- und Jugendalter als Zielparameter einer GH-Therapie diskutiert werden.

Description of a novel SLC34A3.c.671delT mutation causing hereditary hypophosphatemic rickets with hypercalciuria in two adolescent boys and response to recombinant human growth hormone

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is an autosomal recessive disorder characterized by hypophosphatemia, rickets, hyperphosphaturia, elevated 1,25(OH)₂D, and hypercalciuria. Mutations in SLC34A3, the gene encoding the sodium-dependent cotransporter NPT2c, have previously been described as a cause of HHRH. Here, we describe two male siblings with rickets and hypercalciuric nephrolithiasis born to unrelated parents, and their response to oral phosphate supplementation and growth hormone therapy. Whole exome sequencing of the oldest brother, and polymerase chain reaction and Sanger sequence analysis of the identified SLC34A3 mutations, was performed for confirmation and to evaluate his siblings and parents. Serum and urine biochemical parameters of mineral homeostasis before and after therapy were evaluated. Whole exome sequencing analysis identified a previously reported heterozygous deletion SLC34A3.g.2259-2359del101bp on the maternal allele, and a novel heterozygous single nucleotide deletion SLC34A3.c.671delT on the paternal allele of the two affected brothers. The parents and the unaffected brother are heterozygous carriers. Recombinant human growth hormone (rHGH) plus oral phosphate in one affected brother improved the renal phosphate leak and resulted in accelerated linear growth superior to that seen with oral phosphate supplementation alone in the other affected brother. Our case study is the first to demonstrate that rHGH can be considered in addition to oral supplementation with phosphorus to improve linear growth in patients with this disorder, and suggests that renal phosphate reabsorption in response to rHGH is NPT2c-independent.

Nationwide Turkish Cohort Study of Hypophosphatemic Rickets

OBJECTIVE

Hypophosphatemic rickets (HR) is a rare renal phosphate-wasting disorder, which is usually X-linked and is commonly caused by PHEX mutations. The treatment and follow-up of HR is challenging due to imperfect treatment options.

METHODS

Here we present nationwide initial and follow-up data on HR.

RESULTS

From 24 centers, 166 patients were included in the study. Genetic analysis (n=75) showed PHEX mutation in 80% of patients. The mean follow-up period was 6.7±2.4 years. During the first 3-years of treatment (n=91), mild increase in phosphate, decrease in alkaline phosphatase and elevation in parathyroid hormone (PTH) levels were detected. The height standard deviation scores were -2.38, -2.77, -2.72, -2.47 at initial, 1^st, 2^nd and 3^rd year of treatment, respectively (p>0.05). On follow-up 36% of the patients showed complete or significant improvement in leg deformities and these patients had similar phosphate levels at presentation with better levels in 1^st and 2^nd years of treatment; even the treatment doses of phosphate were similar. Furthermore, 27 patients developed nephrocalcinosis (NC), the patients showed no difference in biochemical differences at presentation and follow-up, but 3^rd year PTH was higher. However, higher treatment doses of phosphate and calcitriol were found in the NC group.

CONCLUSION

HR treatment and follow-up is challenging and our results showed higher treatment doses were associated with NC without any change in serum phosphate levels, suggesting that giving higher doses led to increased phosphaturia, probably through stimulation of fibroblast growth factor 23. However, higher calcitriol doses could improve bone deformities. Safer and more efficacious therapies are needed.

X-Linked Hypophosphataemic Rickets and Growth

X-linked hypophosphataemia (XLH) is the most prevalent form of hereditary rickets characterized by an alteration of phosphate metabolism which frequently leads to the appearance of fractures, bone deformities and growth delay. Although the mechanism of growth impairment in patients with XLH still needs to be clarified, it is known that this alteration is not due to genetic or endocrine factors. A potential explanation for the impairment of growth in this disease is the alteration of the growth plate, a structure responsible for longitudinal growth of bones. Some of the findings in the growth plate of patients with XLH include atypical organization of chondrocytes due to low rates of proliferation and apoptosis and disturbance of chondrocyte hypertrophy, overactivation of the mitogen-activated protein kinase (MAPK) signalling pathway and upregulation of phosphorylated extracellular signal-regulated kinase (pERK). Conventional treatment of XLH (consisting of oral phosphate supplements and active vitamin D analogues) is often insufficient for the longitudinal growth of bone, but other strategies based on recombinant growth hormone or therapies targeting fibroblast growth factor 23 (FGF23) or its receptor, such as burosumab, have shown promising results. This article briefly describes the relationship between XLH and growth retardation, and how to address this alteration in patients with XLH.

Diagnosis, treatment-monitoring and follow-up of children and adolescents with X-linked hypophosphatemia (XLH)

Early diagnosis, optimal therapeutic management and regular follow up of children with X-linked hypophosphatemia (XLH) determine their long term outcomes and future quality of life. Biochemical screening of potentially affected newborns in familial cases and improving physician's knowledge on clinical signs, symptoms and biochemical characteristics of XLH for de novo cases should lead to earlier diagnosis and treatment initiation. The follow-up of children with XLH includes clinical, biochemical and radiological monitoring of treatment (efficacy and complications) and screening for XLH-related dental, neurosurgical, rheumatological, cardiovascular, renal and ENT complications. In 2018, the European Union approved the use of burosumab, a humanized monoclonal anti-FGF23 antibody, as an alternative therapy to conventional therapy (active vitamin D analogues and phosphate supplements) in growing children with XLH and insufficiently controlled disease. Diagnostic criteria of XLH and the principles of disease management with conventional treatment or with burosumab are reviewed in this paper.

New Developments in the Treatment of X-Linked Hypophosphataemia: Implications for Clinical Management

X-linked hypophosphataemia (XLH) is due to mutations in phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and represents the most common heritable form of rickets. In this condition, the hormone fibroblast growth factor 23 (FGF23) is produced in excessive amounts for still unknown reasons, and causes renal phosphate wasting and suppression of 1,25-dihydroxyvitamin D, leading to low serum phosphate concentrations. Prolonged hypophosphataemia decreases apoptosis of hypertrophic chondrocytes in growth plates (causing rickets) and decreases mineralisation of existing bone (causing osteomalacia). In contrast to historical conventional treatment with oral phosphate supplements and active vitamin D for the last 50 years, the new anti-FGF23 antibody treatment (burosumab) targets the primary pathology by blocking FGF23, thereby restoring phosphate homeostasis. In this review, we describe the changes in treatment monitoring, treatment targets and long-term treatment goals, including future opportunities and challenges in the treatment of XLH in children.

Positive Response to One-Year Treatment With Burosumab in Pediatric Patients With X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) causes significant burden in pediatric patients in spite of maintained treatment with phosphate supplements and vitamin D derivatives. Administration of burosumab has shown promising results in clinical trial but studies assessing its effect in the everyday practice are missing. With this aim, we analyzed the response to one-year treatment with burosumab, injected subcutaneously at 0.8 mg/kg every 2 weeks, in five children (three females) aged from 6 to 16 years, with genetically confirmed XLH. Patients were being treated with phosphate and vitamin D analogs until the beginning of burosumab treatment. In all children, burosumab administration led to normalization of serum phosphate in association with marked increase of tubular reabsorption of phosphate and reduction of elevated serum alkaline phosphatase levels. Baseline height of patients, from -3.56 to -0.46 SD, increased in the three prepubertal children (+0.84, +0.89, and +0.16 SD) during burosumab treatment. Growth improvement was associated with reduction in body mass index (-1.75, -1.47, and -0.17 SD, respectively), suggesting a salutary effect of burosumab on physical activity and body composition. Burosumab was well-tolerated, mild local pain at the injection site and transient and mild headache following the initial doses of burosumab being the only reported undesirable side effects. No patient exhibited hyperphosphatemia, progression of nephrocalcinosis, worsening of metabolic control or developed hyperparathyroidism. Mild elevation of serum PTH present at the beginning of treatment in one patient 4 was not modified by burosumab administration. These results indicate that in the clinical setting, beyond the strict conditions and follow-up of clinical trials, burosumab treatment for 1 year exerts positive effects in pediatric patients with XLH without major adverse events.

Clinical Evidence for the Benefits of Burosumab Therapy for X-Linked Hypophosphatemia (XLH) and Other Conditions in Adults and Children

Burosumab (KRN23) is an FGF23 neutralizing antibody that has been the subject of several recent clinical trials principally focused on the treatment of hypophosphatemic rickets in patients with X-linked hypophosphatemia (XLH). Since the first publications in 2014, these trials have demonstrated efficacy with minimal safety concerns in both adult and pediatric cohorts. These studies have used dose-escalation to establish a dosing regimen that is well-tolerated in clinical use. This review summarizes the clinical trial data with respect to burosumab treatment in adults and children as well as noting several clinical trials currently underway. While burosumab appears transformative for the treatment of XLH, long term follow-up studies would be required to allay concerns over the potential for nephrocalcinosis and cardiac calcification. While these do not appear to be problematic in current trials, the effects of chronic or lifelong treatment have yet to be established.

FGF23 and Associated Disorders of Phosphate Wasting

Fibroblast growth factor 23 (FGF23), one of the endocrine fibroblast growth factors, is a principal regulator in the maintenance of serum phosphorus concentration. Binding to its cofactor αKlotho and a fibroblast growth factor receptor is essential for its activity. Its regulation and interaction with other factors in the bone-parathyroid-kidney axis is complex. FGF23 reduces serum phosphorus concentration through decreased reabsorption of phosphorus in the kidney and by decreasing 1,25 dihydroxyvitamin D (1,25(OH)2D) concentrations. Various FGF23-mediated disorders of renal phosphate wasting share similar clinical and biochemical features. The most common of these is X-linked hypophosphatemia (XLH). Additional disorders of FGF23 excess include autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Treatment is challenging, requiring careful monitoring and titration of dosages to optimize effectiveness and to balance side effects. Conventional therapy for XLH and other disorders of FGF23-mediated hypophosphatemia involves multiple daily doses of oral phosphate salts and active vitamin D analogs, such as calcitriol or alfacalcidol. Additional treatments may be used to help address side effects of conventional therapy such as thiazides to address hypercalciuria or nephrocalcinosis, and calcimimetics to manage hyperparathyroidism. The recent development and approval of an anti-FGF23 antibody, burosumab, for use in XLH provides a novel treatment option.

FGF23, Hypophosphatemia, and Emerging Treatments

FGF23 is an important hormonal regulator of phosphate homeostasis. Together with its co-receptor Klotho, it modulates phosphate reabsorption and both 1α-hydroxylation and 24-hydroxylation in the renal proximal tubules. The most common FGF23-mediated hypophosphatemia is X-linked hypophosphatemia (XLH), caused by mutations in the PHEX gene. FGF23-mediated forms of hypophosphatemia are characterized by phosphaturia and low or low-normal calcitriol concentrations, and unlike nutritional rickets, these cannot be cured with nutritional vitamin D supplementation. Autosomal dominant and autosomal recessive forms of FGF23-mediated hypophosphatemias show a similar pathophysiology, despite a variety of different underlying genetic causes. An excess of FGF23 activity has also been associated with a number of other conditions causing hypophosphatemia, including tumor-induced osteomalacia, fibrous dysplasia of the bone, and cutaneous skeletal hypophosphatemia syndrome. Historically phosphate supplementation and therapy using analogs of highly active vitamin D (eg, calcitriol, alfacalcidol, paricalcitol, eldecalcitol) have been used to manage conditions involving hypophosphatemia; however, recently a neutralizing antibody for FGF23 (burosumab) has emerged as a promising treatment agent for FGF23-mediated disorders. This review discusses the progression of clinical trials for burosumab for the treatment of XLH and its recent availability for clinical use. Burosumab may have potential for treating other conditions associated with FGF23 overactivity, but these are not yet supported by trial data. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

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

X-linked hypophosphataemia (XLH) is the most common cause of inherited phosphate wasting and is associated with severe complications such as rickets, lower limb deformities, pain, poor mineralization of the teeth and disproportionate short stature in children as well as hyperparathyroidism, osteomalacia, enthesopathies, osteoarthritis and pseudofractures in adults. The characteristics and severity of XLH vary between patients. Because of its rarity, the diagnosis and specific treatment of XLH are frequently delayed, which has a detrimental effect on patient outcomes. In this Evidence-Based Guideline, we recommend that the diagnosis of XLH is based on signs of rickets and/or osteomalacia in association with hypophosphataemia and renal phosphate wasting in the absence of vitamin D or calcium deficiency. Whenever possible, the diagnosis should be confirmed by molecular genetic analysis or measurement of levels of fibroblast growth factor 23 (FGF23) before treatment. Owing to the multisystemic nature of the disease, patients should be seen regularly by multidisciplinary teams organized by a metabolic bone disease expert. In this article, we summarize the current evidence and provide recommendations on features of the disease, including new treatment modalities, to improve knowledge and provide guidance for diagnosis and multidisciplinary care.

Phosphate wasting disorders in adults

A cause of hypophosphatemia is phosphate wasting disorders. Knowledge concerning mechanisms involved in phosphate wasting disorders has greatly increased in the last decade by the identification of phosphatonins, among them FGF-23. FGF-23 is a primarily bone derived factor decreasing renal tubular reabsorption of phosphate and the synthesis of calcitriol. Currently, pharmacological treatment of these disorders offers limited efficacy and is potentially associated to gastrointestinal, renal, and parathyroid complications; therefore, efforts have been directed toward newer pharmacological strategies that target the FGF-23 pathway. This review focuses on phosphate metabolism, its main regulators, and phosphate wasting disorders in adults, highlighting the main issues related to diagnosis and current and new potential treatments.