Elsevier

Bone

Volume 36, Issue 3, March 2005, Pages 542-548
Bone

An intermediate form of juvenile Paget's disease caused by a truncating TNFRSF11B mutation

https://doi.org/10.1016/j.bone.2004.12.004Get rights and content

Abstract

Juvenile Paget's disease (JPD) is a rare condition with an autosomal recessive mode of inheritance. Typically presenting in infancy or early childhood, the disorder is characterized by a generalized widening of the long bones and thickening of the skull combined with sustained elevation of serum alkaline phosphatase levels. The extremely rapid bone turnover results in osteopenia, fractures, and progressive skeletal deformity. In 2002, mutations in TNFRSF11B, the gene encoding osteoprotegerin, were described as underlying JPD. We evaluated a patient with JPD at the clinical, biochemical, radiological, and molecular level. Mutation analysis of TNFRSF11B revealed a homozygous insertion/deletion in exon 5, predicted to result in truncation of the protein at amino acid 325. The residual activity of the mutated protein product was investigated by Western blotting and ELISA upon transient overexpression. Absence of the C-terminal domain abolished homodimerization and was shown to lead to a decreased capacity of the mutant protein to bind its ligand RANKL. We conclude that truncation of the C-terminal part of osteoprotegerin negatively affects functional activity. As a consequence, osteoclast formation and function are up-regulated, causing the increased bone turnover seen in this patient.

Introduction

Juvenile Paget's disease (JPD; MIM 239000) – also referred to as (Chronic Congenital) Idiopathic Hyperphosphatasia and Osteoectasia with Hyperphosphatasia [1], [2], [3], [4] – was initially described by Bakwin and Eiger in 1956 [5]. Since the first publication, more than 50 cases have been reported [6].

Radiographs of JPD patients show that virtually every bone is affected. The long bones show symmetrical lateral or anterior bowing and are broadened. Their cortices may be ill defined and can be either hyperostotic or osteopenic. Inferior quality of the bone leads to non-traumatic fractures. Gross cranial enlargement is typical as hyperostosis can affect the facial bones, calvaria, and skull base. Areas of both increased and decreased bone density give the skull a cotton-wool like appearance. The diploic spaces may be widened. Pelvis, scapular bones, clavicles, ribs, spine, hands, and feet are seen to be affected ([1], [2], [3], [4], [5], [7], [8], [9], [10], [11], [12]; reviewed in Ref. [6]).

The first symptoms of the disorder typically appear in infancy or early childhood, but can even be present from birth [7]. Patients suffer from bone and joint pain. Skeletal deformities include a disproportionately enlarged head, visibly widened and bowed long bones, kyphoscoliosis and chest wall deformity. Bone encroachment on cranial nerves can cause visual and auditory impairment. The combination of skeletal deformities and muscle weakness prevents the patients from running or even walking [2], [4], [8], [9], [10].

The chronic sustained elevation of serum ALP – a marker for bone turnover – is the most striking biochemical abnormality. Other abnormalities which are present in most patients include increased serum levels of acid phosphatase and increased urinary levels of hydroxyproline, both markers of osteoclast activity [1], [2], [3], [4], [8], [9], [10], [11], [12].

The RANK/RANKL/OPG system is indispensable in the osteoblast-mediated regulation of osteoclastogenesis and bone resorption [13], [14], [15], [16]. The importance of this system was confirmed by the identification of activating mutations in TNFRSF11A – the gene coding for RANK – as the cause of Familial Expansile Osteolysis and Expansile Skeletal Hyperphosphatasia, two related bone dysplasias [17], [18]. In addition, inactivating mutations in TNFRSF11B – encoding OPG – were detected in JPD [19], [20], [21]. Evidence for genetic heterogeneity is, however, present [19], [21], implying the involvement of other genes in this disorder.

In this study, a JPD patient was evaluated at a clinical, biochemical, radiological and molecular level.

Section snippets

Mutation analysis

Genomic DNA from the patient was isolated from fresh leukocytes according to standard protocol. Primers were designed from intronic sequences flanking exons 1 through 5 of the TNFRSF11B gene to amplify the exons and intron–exon boundaries. PCR products were sequenced on both forward and reverse strands on an ABI 3100 automated sequencer (Applied Biosystems) using the DYEnamic ET terminator cycle sequencing kit (Amersham Biosciences).

Expression constructs and transfection

A BamHI–EcoRI fragment containing the full-length wild-type

Case report

At two years of age, a diagnosis of juvenile Paget's disease was made in a patient of Croatian origin, currently 27 years of age. He originates from a non-consanguineous marriage; his parents have no history of any bone abnormalities. He suffers from fatigue and experiences bone and joint pain, which seriously limits his walking abilities: he can only walk approximately 500 m and is unable to run. He does not show muscle atrophy or muscle weakness. In adulthood, he reached a height of 130 cm

Discussion

The physical interaction between the osteoclast receptor RANK and its osteoblast-derived ligand RANKL activates (among others) NF-κB- and MAPK-mediated signalling cascades, leading to regulated transcription of genes involved in osteoclast differentiation, function and survival [23], [24]. The system is down-regulated by the expression and secretion by osteoblasts of OPG that acts as a decoy receptor of RANKL, thereby preventing RANK–RANKL interaction (reviewed in Ref. [15]). In 2002, mutations

Acknowledgments

The authors would like to thank Josie Meaney for critical reading of the manuscript. This study is supported by the Fund for Scientific Research Flanders with a research project (G.0404.00) and by an Interuniversity Attraction Poles program P5/19 of the Belgian Federal Science Policy Office grant to WVH. KJ is a postdoctoral researcher of the Fund for Scientific Research Flanders.

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