Effects of Treatment on Body Proportions and Final Height Among Small Children With X-Linked Hypophosphatemic

Dieter Haffner, MD*; Richard Nissel, MD*; Elke Wu¨hl, MD‡; and Otto Mehls, MD‡

ABSTRACT. Background. X-linked hypophosphatemic in the PHEX gene, encoding a membrane-bound en- rickets (XLH) is characterized by rickets, disproportion- dopeptidase. PHEX is expressed in bones and teeth ate short stature, and impaired renal phosphate reabsorp- but not in kidney, and efforts are underway to elu- tion and D metabolism. Despite oral phosphate cidate how PHEX function relates to the mutant phe- and vitamin D treatment, most children with XLH dem- notype.2 onstrate reduced adult height. Pharmacologic treatment consists of oral phos- Objective. To determine the beneficial effects of re- combinant human growth hormone (rhGH) therapy on phate supplementation and calcitriol administration. body proportions and adult height among patients with Although this therapy usually leads to an improve- XLH. ment of rickets, the effects on longitudinal growth Methods. Three initially prepubertal short children are often disappointing.3 Despite adequate phos- (age, 9.4–12.9 years) with XLH were treated with rhGH phate and calcitriol treatment, most previous studies for 3.1 to 6.3 years until adult height was attained. reported reduced adult height among children with Results. rhGH treatment led to sustained increases in XLH.4–7 In addition, children with XLH present with standardized height for all children. The median adult disproportionate growth, ie, relatively preserved height was 0.9 SD (range: 0.5–1.3 SD) greater than that at trunk growth but severely diminished leg growth.8 the initiation of rhGH treatment and exceeded the pre- Previous studies demonstrated that treatment with dicted adult height by 6.2 cm (range: 5.3–9.8 cm). How- ever, longitudinal growth of the trunk was stimulated recombinant human growth hormone (rhGH) was more than leg growth. During rhGH treatment, the stan- able to improve short- and long-term longitudinal dardized sitting height increased by 1.6 SD (range: 1.1– growth among small children with XLH.9–15 How- 2.7 SD), compared with baseline values. In contrast, the ever, we and others11,12 noted that short-term rhGH median subischial leg length did not change consistently treatment could aggravate disproportionate growth median change: 0.3 SD; range: ؊0.1 to 0.6 SD). among these children. Here we report the effects of) Conclusion. The increase in final height after rhGH long-term rhGH treatment on longitudinal growth treatment is of potential benefit for children with XLH. and body proportions among 3 initially prepubertal, However, the exaggeration of disproportionate truncal short children with XLH who were prospectively growth observed for our prepubertal patients is a poten- monitored after the initiation of rhGH treatment, tial negative effect of treatment and should be confirmed with additional studies. Pediatrics 2004;113:e593–e596. until their final heights were achieved. URL: http://www.pediatrics.org/cgi/content/full/113/ 6/e593; growth hormone treatment, hypophosphatemic PATIENTS AND METHODS rickets, growth failure, disproportionate growth, final height. Three short prepubertal children with XLH, with ages of 12.9 years (patient 1), 9.4 years (patient 2), and 11.1 years (patient 3), began to receive rhGH treatment. Patient 1 was female, and pa- ABBREVIATIONS. XLH, X-linked hypophosphatemic rickets; tients 2 and 3 were male siblings. At the time of diagnosis, all patients fulfilled the clinical, biochemical, and radiologic criteria GH, growth hormone; rhGH, recombinant human growth 1 hormone. for diagnosis of the . In each case, X-linked dominant inheritance was confirmed by analysis of the pedigrees. The chil- dren had been treated with calcitriol and phosphate supplements -linked hypophosphatemic rickets (XLH) is an for at least 5 years. They had prescriptions of fixed individual doses of calcitriol (1–1.5 ␮g/day) and oral phosphate supplemen- inherited disorder of phosphate homeostasis tation (0.8–2.1 g/day) during the year before rhGH treatment. The Xcharacterized by disproportionate short stat- individual prescriptions were not changed during the study pe- ure, rickets and , hypophosphatemia, riod. Growth hormone (GH) deficiency was excluded with GH stimulation tests with l-arginine (peak serum GH concentration of aberrant phosphate reabsorption, and disturbance of Ͼ ␮ Ͼ 1 10 g/L) and normal night serum GH concentration profiles ( 2 vitamin D metabolism. XLH is caused by mutations peaks of Ͼ10 ␮g/L). rhGH (0.33 mg/kg per week) was adminis- tered daily, in the evening, as subcutaneous injections. Anthropometric measurements were performed at 3-month in- From the *Department of Pediatric Nephrology, University Hospital tervals, with standard techniques.16 For calculations of age- and Charite´, Berlin, Germany; and ‡Division of Pediatric Nephrology, Univer- gender-related SD scores, the first Zurich longitudinal study was sity Children’s Hospital, Heidelberg, Germany. used as a reference.17 SD values for sitting height and subischial Received for publication Jun 4, 2003; accepted Dec 29, 2003. leg length were used to evaluate disproportionate growth. Bone Address correspondence to Otto Mehls, MD, Division of Pediatric Nephrol- age was assessed at yearly intervals with the method described by ogy, University Children’s Hospital Heidelberg, Im Neuenheimer Feld 150, Greulich and Pyle.18 Pubertal stage was assessed according to the 69120 Heidelberg, Germany. E-mail: [email protected] method described by Tanner.19 Predicted adult height was calcu- PEDIATRICS (ISSN 0031 4005). Copyright © 2004 by the American Acad- lated from the patient’s height and bone age by the method emy of Pediatrics. described by Bayley and Pinneau.20 Final (adult) height was de-

http://www.pediatrics.org/cgi/content/full/113/6/Downloaded from www.aappublications.org/newse593 by guestPEDIATRICS on October 1,Vol. 2021 113 No. 6 June 2004 e593 fined as epiphysial closure on hand radiographs and/or a height TABLE 1. Anthropometric Characteristics of rhGH-Treated increase of Ͻ1 cm/year in the preceding year. The laboratory Children with XLH methods used were described elsewhere.11 Ultrasonographic im- aging of the kidneys was performed at yearly intervals. The study Patient Patient Patient protocol was approved by the ethics committee of the University 1 2 3 of Heidelberg, and written informed consent was obtained from Gender Female Male Male the parents, with assent by the patients. Age at baseline* (y) 12.9 9.4 11.1 Bone age at baseline* (y) 11.4 8.0 9.0 RESULTS Height at baseline* (SD) Ϫ2.1 Ϫ2.0 Ϫ2.0 For each of the 3 patients, rhGH treatment led to a Subischial leg length at Ϫ2.3 Ϫ2.2 Ϫ2.3 baseline* (SD) sustained increase in standardized height (Fig. 1). Sitting height at baseline* (SD) Ϫ1.5 Ϫ1.6 Ϫ1.4 The median adult height was 0.9 SD (range: 0.5–1.3 Genetic target height (cm) 170.5 166.8 166.8 SD) greater than that at the initiation of rhGH treat- Genetic target height (SD) 1.0 Ϫ1.6 Ϫ1.6 ment, exceeded the predicted adult height by 6.2 cm Predicted adult height at 147.3 156.8 161.6 (range: 5.3–9.8 cm), and was close to the genetic baseline* (cm) Predicted adult height at Ϫ2.9 Ϫ3.1 Ϫ2.4 target height for 2 of the 3 patients (median differ- baseline* (SD) ence: Ϫ3.8 cm; range: Ϫ13.4 to 0.1 cm) (Table 1). At Duration of rhGH treatment (y) 3.1 5.6 6.3 baseline, trunk growth was more preserved than leg Age at adult height (y) 16.0 15.0 17.4 growth. During rhGH treatment, the standardized Adult height (cm) 157.1 163.0 166.9 Adult height (SD) Ϫ1.2 Ϫ0.7 Ϫ1.5 sitting height continuously increased for all patients Subischial leg length at adult Ϫ1.7 Ϫ2.3 Ϫ2.0 (Fig. 1). At the final height, the median sitting height height (SD) was increased by 1.6 SD (range: 1.1–2.7 SD), com- Sitting height at adult 0.1 1.1 Ϫ0.3 pared with baseline values. In contrast, the median height (SD) subischial leg length did not change consistently * Time of initiation of rhGH treatment. (median change: 0.3 SD; range: Ϫ0.1 to 0.6 SD). Dur- ing rhGH treatment, there was no obvious change in the degree of bowing of the legs. The median dis- sented with body disproportion, ie, short legs and tance between the medial condylars of the tibia was relatively preserved trunk growth. For all children, 5.8 cm (range: 4.3–7.6 cm) at baseline and was 6.3 cm rhGH therapy predominantly stimulated trunk (range: 5.2–8.7 cm) at the final height. growth, whereas leg growth was stimulated less, During the first 6 months of rhGH treatment, a thereby increasing the preexisting body dispropor- transient increase in the renal tubular maximal reab- tion. sorption of phosphate, relative to the glomerular fil- Despite early positive reports of short-term stud- tration rate, from 0.53 mmol/L (range: 0.43–0.58 ies, previous long-term studies of children with XLH mmol/L) to 0.67 mmol/L (range: 0.52–0.69 mmol/L) could not demonstrate sustained catch-up growth and a concomitant increase in serum phosphate lev- during calcitriol and phosphate treatment.3,5 In the els from 0.7 mmol/L (range: 0.5–0.8 mmol/L) to 0.9 present study, vitamin D and phosphate administra- mmol/L (range: 0.7–1.1 mmol/L) were noted. In ad- tion was not changed during the observation period. dition, serum intact parathyroid hormone concentra- In addition, metabolic control with respect to serum tions transiently increased above the upper normal levels of phosphate, parathyroid hormone, and alka- limit of 6 pmol/L. During rhGH treatment, there line phosphatase did not change with time. How- were no changes in creatinine clearance, urinary cal- ever, the final height was 5 to 10 cm greater than the cium and phosphate excretion, serum glucose levels, predicted adult height for our patients. It has been or the degree of nephrocalcinosis on renal ultrasono- demonstrated that height prediction for children grams, which was noted for 2 of 3 patients at baseline with XLH is quite accurate and might actually over- (data not shown). estimate adult height for these children.5 Therefore, we assume that rhGH treatment increased the adult DISCUSSION height for our patients. However, crucial method- Our study provides evidence that rhGH treatment ologic factors in this study were the small number of improves longitudinal growth and final height patients and the lack of a control group. among small prepubertal children with XLH. Before Several clinical trials (mostly uncontrolled) on the the initiation of rhGH treatment, the children pre- effect of rhGH treatment on longitudinal growth

Fig. 1. Effects of rhGH treatment on standardized height, sitting height, and subischial leg length for three children with XLH.

e594 GH TREATMENT INDownloaded XLH from www.aappublications.org/news by guest on October 1, 2021 among short children with XLH have been report- that the patients reported in those studies differed ed.9–15 The patients were treated for periods of 0.5 to significantly with respect to the degree of stunting at 9 years, with rhGH doses of 0.15 to 0.56 mg/kg per the initiation of rhGH therapy. The median standard- week. The median increase in standardized height ized height at baseline was Ϫ2.0 SD in the present amounted to 1.2 SD (range: 0.7–1.4). Baroncelli et al15 study and was Ϫ3.4 SD in the Italian study, which recently reported final height data for 6 children with used the old reference values of Tanner,19 thereby XLH who were treated with rhGH for up to 9 years. underestimating the degree of stunting by at least 0.5 Whereas the mean standardized height increased by SD, compared with more recent standards.17 There- 1.0 SD at the final height for the rhGH-treated chil- fore, the conflicting results might be influenced dren, the standardized height did not change signif- partly by differences in stunting, ie, severity of the icantly in the control group. disease. It is unclear whether rhGH therapy affects the Recent studies indicate that in various forms of onset or duration of the pubertal growth spurt short stature, eg, uremic growth failure, longitudinal among children with XLH. Among children with growth of the lower limbs is generally more affected XLH without rhGH treatment, a pubertal growth than trunk growth, leading to body disproportion.25 spurt of normal magnitude has been observed.5 In a It has been speculated that this is attributable to the previous study of rhGH treatment of children with vital need of the body to maintain the function of the XLH, longitudinal growth was stimulated mainly internal organs within the trunk. rhGH therapy during the prepubertal growth period, whereas the among short children with chronic renal failure has standardized height during the pubertal growth pe- been shown to improve body disproportion.25 The riod even decreased.15 In accordance with this find- finding that this is not the case for patients with XLH ing, 2 of our patients demonstrated a slight decrease suggests that, in this particular disease, the underly- in standardized height after 3 years of rhGH treat- ing osteoblast defect might be more evident in the ment during the pubertal growth period. These ob- long bones of the legs than in the vertebrae. servations suggest that the expected increase in final When these findings are taken together, it is tempt- height during the prepubertal growth period might ing to speculate that the growth-stimulating proper- be counterbalanced partly by decreased pubertal ties of rhGH only partly overcome the underlying growth. osteoblast defect in XLH, thereby preferentially stim- Why patients with XLH demonstrate dispropor- ulating the less diseased part of the skeleton. Unfor- tionate growth is not fully understood. Various skel- tunately, data on the effect of rhGH on bone forma- etal abnormalities have been demonstrated in an an- tion in Hyp mice are lacking. 21,22 imal model of XLH, ie, Hyp mice. In addition to It has been speculated that the growth-stimulating rickets and osteomalacia, Hyp mice exhibit cranio- effect of rhGH in XLH might be attributable to in- synostosis because of premature fusion of the coro- creased renal phosphate reabsorption, promoting nal suture and abnormal skull morphology, with bone formation. In accordance with this hypothesis, deficient linear growth of the nasal bone. It has been Baroncelli et al15 reported a sustained increase in the assumed that the abnormal bone formation in Hyp renal phosphate threshold concentration during mice is determined not only by hypophosphatemia rhGH treatment. However, we and others7,12 did not but also by an intrinsic osteoblast defect in the mu- 23 observe consistent effects of rhGH treatment on renal tant strain. In view of an underlying osteoblast phosphate reabsorption. In addition, the finding of defect among patients with XLH, the predominant Roy et al26 that rhGH treatment failed to stimulate involvement of the lower limbs, compared with renal phosphate reabsorption in Hyp mice argues lesser involvement of the spine, might be attributable against a major effect of rhGH treatment on the partly to the greater mechanical stress of the legs, phosphorous threshold in XLH. compared with the spine, and/or to a differential pattern of organ-specific activation of PHEX within the skeleton.24 CONCLUSIONS Phosphate and vitamin D treatment fails to nor- Treatment with rhGH might have potential posi- 8 malize body proportions among children with XLH. tive effects on final height and thus psychosocial In vitro studies showed that bone formation in bone rehabilitation among short patients with XLH. How- cells from Hyp mice was only partly normalized ever, the aggravation of disproportion observed for with high-dose 1,25-dihydroxy-vitamin D3 treat- 23 our patients must be viewed critically. Randomized, ment. In the present study, rhGH treatment pref- clinical trials are needed to clarify the effects of rhGH erentially stimulated trunk growth rather than leg on segmental growth among patients with XLH. growth, thereby aggravating the preexisting body disproportion. This was not explained by increased bowing of the legs.11 Body proportions have been REFERENCES measured in only 3 previous studies of children with 1. Rasmussen H, Tenenhouse HS. Hypophosphatemias. In: Scriver CR, XLH treated with rhGH.11,12,15 In accordance with Beadet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Basis of our study, Reusz et al12 reported an increase in body Inherited Disease. 7th ed. New York, NY: McGraw-Hill; 1995:3117–3145 disproportion among children treated with rhGH for 2. 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