A trial of growth hormone therapy in well-controlled hypophosphataemic rickets

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.

Microindentation: A New Technique for Bone Quality Assessment

Fractures are the result of the application of a greater force on bone than its strength. Therefore, to understand fracture physiopathology, it is essential to know bone strength determinants. These include bone mineral density (BMD), bone spatial structure (bone geometry and microarchitecture) and bone mechanical and tissue properties. While BMD and bone spatial structure can be easily evaluated through imaging technology, assessment of bone tissue and mechanical properties is complex and typically requires invasive techniques that are not suitable in clinical practice. Microindentation is a relatively recently developed technique that directly measures bone tissue and mechanical properties in patients in a fast, safe, feasible and minimally invasive way. It appears to be particularly informative in diseases associated with an increased risk of fracture not explained by BMD values as occurs in X-linked hypophosphataemia (XLH). The aim of this article is to provide an overview on bone microindentation and its potential utility in the evaluation of patients with XLH.

Phosphate homeostasis disorders

Our understanding of the regulation of phosphate balance has benefited tremendously from the molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the glycosyltransferase GALNT3, the endopeptidase PHEX, and the matrix protein DMP1, and molecules that function as downstream effectors of FGF23 such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted treatment of FGF23-dependent hypophosphatemic conditions, but also provide clinically relevant observations related to the dysregulation of mineral ion homeostasis in health and disease.

X-linked hypophosphatemia and growth

X-Linked hypophosphatemia (XLH) is the most common form of hereditary rickets caused by loss-of function mutations in the PHEX gene. XLH is characterized by hypophosphatemia secondary to renal phosphate wasting, inappropriately low concentrations of 1,25 dihydroxyvitamin D and high circulating levels of fibroblast growth factor 23 (FGF23). Short stature and rachitic osseous lesions are characteristic phenotypic findings of XLH although the severity of these manifestations is highly variable among patients. The degree of growth impairment is not dependent on the magnitude of hypophosphatemia or the extent of legs´ bowing and height is not normalized by chronic administration of phosphate supplements and 1α hydroxyvitamin D derivatives. Treatment with growth hormone accelerates longitudinal growth rate but there is still controversy regarding the potential risk of increasing bone deformities and body disproportion. Treatments aimed at blocking FGF23 action are promising, but information is lacking on the consequences of counteracting FGF23 during the growing period. This review summarizes current knowledge on phosphorus metabolism in XLH, presents updated information on XLH and growth, including the effects of FGF23 on epiphyseal growth plate of the Hyp mouse, an animal model of the disease, and discusses growth hormone and novel FGF23 related therapies.

Hypophosphatemia and growth

Over the last decade the discovery of fibroblast growth factor 23 (FGF23) and the progressive and ongoing clarification of its role in phosphate and mineral metabolism have led to expansion of the diagnostic spectrum of primary hypophosphatemic syndromes. This article focuses on the impairment of growth in these syndromes. Growth retardation is a common, but not constant, feature and it presents with large variability. As a result of the very low prevalence of other forms of primary hypophosphatemic syndromes, the description of longitudinal growth and the pathogenesis of its impairment have been mostly studied in X-linked hypophosphatemia (XLH) patients and in Hyp mice, the animal model of this disease. In general, children with XLH have short stature with greater shortness of lower limbs than trunk. Treatment with phosphate supplements and 1α vitamin D derivatives heals active lesions of rickets, but does not normalize growth of XLH patients. Patients might benefit from recombinant human growth hormone (rhGH) therapy, which may accelerate the growth rate without increasing body disproportion or correcting hypophosphatemia. These clinical data as well as research findings obtained in Hyp mice suggest that the pathogenesis of defective growth in XLH and other hypophosphatemic syndromes is not entirely dependent on the mineralization disorder and point to other effects of hypophosphatemia itself or FGF23 on the metabolism of bone and growth plate.

A clinician's guide to X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is the prototypic disorder of renal phosphate wasting, and the most common form of heritable rickets. Physicians, patients, and support groups have all expressed concerns about the dearth of information about this disease and the lack of treatment guidelines, which frequently lead to missed diagnoses or mismanagement. This perspective addresses the recommendation by conferees for the dissemination of concise and accessible treatment guidelines for clinicians arising from the Advances in Rare Bone Diseases Scientific Conference held at the NIH in October 2008. We briefly review the clinical and pathophysiologic features of the disorder and offer this guide in response to the conference recommendation, based on our collective accumulated experience in the management of this complex disorder.

Hypophosphatemic rickets and osteomalacia

The hypophosphatemic conditions that interfere in bone mineralization comprise many hereditary or acquired diseases, all of them sharing the same pathophysiologic mechanism: reduction in the phosphate reabsorption by the renal tubuli. This process leads to chronic hyperphosphaturia and hypophosphatemia, associated with inappropriately normal or low levels of calcitriol, causing osteomalacia or rickets in children and osteomalacia in adults. X-linked hypophosphatemic rickets, autosomal-dominant hypophosphatemic rickets, and tumor-induced osteomalacia are the main syndromes involved in the hypophosphatemic rickets. Although these conditions exhibit different etiologies, there is a common link among them: increased activity of a phosphaturic factor, being the fibroblast growth factor 23 (FGF-23) the most studied one and to which is attributed a central role in the pathophysiology of the hyperphosphaturic disturbances. Activating mutations of FGF-23 and inactivating mutations in the PHEX gene (a gene on the X chromosome that codes for a Zn-metaloendopeptidase proteolytic enzyme which regulates the phosphate) involved in the regulation of FGF-23 have been identified and have been implicated in the pathogenesis of these disturbances. Genetic studies tend to show that the phosphorus homeostasis depends on a complex osteo-renal metabolic axis, whose mechanisms of interaction have been poorly understood so far. This paper reviews the current knowledge status concerning the pathophysiology of phosphate metabolism regulation and the pathophysiologic basis of hypophosphatemic rickets. It also analyzes the clinical picture and the therapeutic aspects of these conditions as well.

Growth in X-linked hypophosphatemic rickets

Growth failure appears frequently in children with X-linked hypophosphatemic rickets (XLHR) due to hypophosphatemia, disease severity, body disproportion, and primary bone abnormality. Recombinant human growth hormone (rhGH) increases phosphate tubular reabsorption and phosphate level in blood and, thus, constitutes an attractive but controversial therapy in short children with XLHR, those efficacy was demonstrated in small uncontrolled series. Our aim was to report our experience regarding growth in XLHR. Twenty-seven children with XLHR--20 girls, seven boys--diagnosed at a median (md) of 1.46 years of age, (range 0.39-8.5 years), were studied at 10.12 years of age (1.58-18.56), md (range). All received oral treatment with phosphate and calcitriol. At the first visit, grouped Z-height was -1; (-4.58; 0.54) md (range). After 5 years' follow-up (0.92-15.6), Z-height was -0.91 (- 4.56; 0.17), not different from that at baseline (P = 0.465). In 16 children entirely controlled in our program upon presentation, a "catch up" phenomenon after the rickets had healed (P = 0.823) or throughout the long-term was not observed (P = 0.995). Eight patients had a Z-height </= -2SD at the last visit, and impaired linear growth was associated with age >2 years at diagnosis, male gender and non-adherence to treatment. Four children, all boys, received rhGH, and in two cases with sufficient follow up stature normalized. No rhGH side effects were observed, and phosphate and calcitriol doses remained stable. Linear growth failure appeared in a third of XLHR children. Efforts need to be made to reduce the age of diagnosis and to improve adherence to treatment. Treatment with rhGH should be considered early, after the rickets has been controlled, in those patients with impaired growth or delayed diagnosis.

[Genetic hypophosphatemia: recent advances in physiopathogenic concept]

Renal proximal tubular reabsorption of phosphate and intestinal absorption both regulate phosphate homeostasis. Brush-border membrane Npt2a cotransporter is the key element in proximal tubular P (i) reabsorption. Inactivating mutations of Npt2a cause bone demineralisation and urolithiasis. An excess of a phosphaturic factor, called "Phosphatonin", could modulate phosphate reabsorption by inhibition on Npt2a. Inactivating mutation of PHEX, an endopeptidase-membrane coding gene, is responsible for X-linked Hypophosphatemia (XLH), because of an impaired degradation of phosphatonine by PHEX product. Autosomic Dominant Hypophosphatemic Rickets (ADHR) is explained by a mutation preventing FGF23 (one of the best identified phosphatonines) from cleavage. According recent data, FGF23, MEPE (Matrix Extracellular Phosphoglycoprotein) et FRP4 (frizzled related protein-4) are 3 putative "phosphatonines".

Effects of growth hormone treatment on body proportions and final height among small children with X-linked hypophosphatemic rickets

BACKGROUND

X-linked hypophosphatemic rickets (XLH) is characterized by rickets, disproportionate short stature, and impaired renal phosphate reabsorption and vitamin D metabolism. Despite oral phosphate and vitamin D treatment, most children with XLH demonstrate reduced adult height.

OBJECTIVE

To determine the beneficial effects of recombinant human growth hormone (rhGH) therapy on body proportions and adult height among patients with XLH.

METHODS

Three initially prepubertal short children (age, 9.4-12.9 years) with XLH were treated with rhGH for 3.1 to 6.3 years until adult height was attained.

RESULTS

rhGH treatment led to sustained increases in standardized height for all children. The median adult height was 0.9 SD (range: 0.5-1.3 SD) greater than that at the initiation of rhGH treatment and exceeded the predicted adult height by 6.2 cm (range: 5.3-9.8 cm). However, longitudinal growth of the trunk was stimulated more than leg growth. During rhGH treatment, the standardized sitting height increased by 1.6 SD (range: 1.1-2.7 SD), compared with baseline values. In contrast, the median subischial leg length did not change consistently (median change: 0.3 SD; range: -0.1 to 0.6 SD).

CONCLUSION

The increase in final height after rhGH treatment is of potential benefit for children with XLH. However, the exaggeration of disproportionate truncal growth observed for our prepubertal patients is a potential negative effect of treatment and should be confirmed with additional studies.

The effect of growth hormone treatment on biochemical indices in hypophosphatemic rickets

BACKGROUND

Growth hormone (GH) in combination with conventional therapy in hypophosphatemic rickets (HR) has been shown to promote renal phosphate (P) conservation and to result in a better metabolic control. This study aimed at investigating the acute biochemical effects of GH in 7 patients (5 female, 2 male) with HR aged between 2.16 and 16 years.

METHODS

Each patient received the following in a sequential design: oral P plus 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] therapy to determine the optimum doses for baseline requirement followed by GH-only therapy and GH +1,25-(OH)(2)D(3) therapy and GH + P +1,25-(OH)(2)D(3) therapy each for 2 weeks with 1 washout week off treatment in between. GH was given at a dose of 0.03 mg/kg/day s.c. on a daily basis. The dose of oral P used ranged between 500 and 2,000 mg/day, and the dose of 1,25-(OH)(2)D(3) ranged between 0.25 and 0.5 microg/day and was kept constant for each child throughout the study.

RESULTS

Laboratory investigations repeated at the end of each treatment, and the first washout period showed that the serum P level was highest (2.9 ng/ml) during the GH + P + 1,25-(OH)(2)D(3) period with higher serum 1,25-(OH)(2)D(3) levels: 50.9 +/- (SD) 23.4 ng/l. Parathyroid hormone and alkaline phosphatase levels did not show a significant difference between the periods. The tubular P reabsorption rate showed an insignificant increase during GH therapy periods.

CONCLUSION

Considering the fixed dose of P and calcitriol, it may be concluded that GH added to conventional treatment in HR resulted in a slight improvement in the biochemical parameters without any side effects at the short term.

Disorders of phosphate metabolism

Correct identification of the disorders of hypophosphatemia and hyperphosphatemia is important for determining therapy. Further research will provide insights into normal phosphate homeostasis, a complex and fascinating process.