Loss of Functional Osteoprotegerin: More Than a Skeletal Problem

Rhizomelia and Impaired Linear Growth in a Girl with Juvenile Paget Disease: The Natural History of the Condition

In ultra-rare bone diseases, information on growth during childhood is sparse. Juvenile Paget disease (JPD) is an ultra-rare disease, characterized by loss of function of osteoprotegerin (OPG). OPG inhibits osteoclast activation via the receptor activator of nuclear factor-κB (RANK) pathway. In JPD, overactive osteoclasts result in inflammatory-like bone disease due to grossly elevated bone resorption. Knowledge on the natural history of JPD, including final height and growth, is limited. Most affected children receive long-term antiresorptive treatment, mostly with bisphosphonates, to contain bone resorption, which may affect growth. In this study, we report the follow-up of height, growth velocity, and skeletal maturation in a 16-year-old female patient with JPD. The patient was treated with cyclic doses of pamidronate starting at 2.5 years of age and with 2 doses of denosumab at the age of 8 years, when pamidronate was paused. In the following years, a sustainable decline in a height z-score and a stunted pubertal growth spurt; despite appropriate maturation of the epiphyseal plates of the left hand, the proximal right humerus and both femora were observed. Whether this reflects the growth pattern in JPD or might be associated to the antiresorptive treatments is unclear, since there is very limited information available on the effect of bisphosphonates and denosumab on growth and the growth plate in pediatric patients. Studies are needed to understand the natural history of an ultra-rare bone disease and to assess the effects of antiresorptive treatment on the growing skeleton.

Juvenile Paget's Disease: Report of a successful treatment throughout the complete growth of a patient with a missense TNFRSF11B mutation

INTRODUCTION

Juvenile Paget's Disease (JPD) is an ultra-rare inherited osteopathy featuring markedly accelerated bone turnover. Several clinical characteristics have been reported, including bone deformities developing in childhood and hearing loss.

CASE REPORT

We report the case of a 2 ¾-year-old girl that presented with progressive bowing of both legs since the age of 2, lower limb pain and frequent falls with one consequent femur fracture. Plain radiographs revealed osteoectasia of the long bone's diaphysis, and laboratory tests showed extremely high serum total alkaline phosphatase levels. A missense mutation on the gene TNFRSF11B was identified in homozygosity, and the diagnosis of JPD was made. Treatment with bisphosphonates was initiated early and markedly improved lower limb bowing and pain. The patient reached adulthood with normal height, minor bone deformities, and no functional impairment. Despite the good skeletal symptom's response, bisphosphonates failed to prevent or improve sensorineural hearing loss.

CONCLUSIONS

In this clinical case, early treatment with bisphosphonates was effective for the treatment of JPD skeletal deformities. New therapeutic strategies need to be developed to better control the extraskeletal manifestations of JPD.

Local Production of Osteoprotegerin by Osteoblasts Suppresses Bone Resorption

Osteoprotegerin (OPG) inhibits the ability of receptor activator of nuclear factor κB (NF-κB) ligand (RANKL) to stimulate the differentiation, activity, and survival of bone-resorbing osteoclasts. Genetic studies in mice show that osteocytes are an important source of RANKL, but the cellular sources of OPG are unclear. We use conditional deletion of Tnfrsf11b, which encodes OPG, from different cell populations to identify functionally relevant sources of OPG in mice. Deletion from B lymphocytes and osteocytes, two cell types commonly thought to supply OPG, has little or no impact on bone mass. By contrast, deletion of Tnfrsf11b from osteoblasts increases bone resorption and reduces bone mass to an extent similar to germline deletion, demonstrating that osteoblasts are an essential source of OPG. These results suggest that, in addition to producing new bone matrix, osteoblasts also play an active role in terminating the resorption phase of the bone remodeling cycle by suppressing RANKL activity.

Clinical course in two children with Juvenile Paget's disease during long-term treatment with intravenous bisphosphonates

Juvenile Paget disease (JPD) is an ultra-rare disease, characterized by loss of function of osteoprotegerin. Osteoprotegerin inhibits osteoclast activation via the receptor activator of nuclear factor κB (RANK) pathway. Severely affected children suffer from bone deformities and pain and require long term anti-resorptive treatment. Due to the rarity of the disease, few long-term follow-up data on the clinical course in children are available.

In this report, motor development during infancy and early childhood and the activity of the bone disease based on clinical, radiographic and biochemical parameters are reported in 2 children with severe forms of JPD during long term treatment (4 and 14 years) with bisphosphonates. Results of a bone biopsy in patient 1 after 10 years of treatment and video material of the motor development of patient 2 are provided.

Doses per year of pamidronate ranged from 4 to 9 mg/kg bodyweight and were administered in 4–10 courses, yearly. Treatment was adjusted individually according to the presence of bone pain. Motor development was delayed in both children before treatment with bisphosphonates was commenced and improved thereafter. Bone histology revealed a significantly higher heterogeneity of mineralization which was mainly attributed to the increased percentage of low mineralized bone areas. Individualized intravenous treatment with pamidronate resulted in sufficient control of bone pain and suppression of bone turnover with few side effects over the observation period.

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Long-term individualized treatment with intravenous bisphosphonates in children with JPD is safe and effective to control bone turnover and pain

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Histomorphometric analyses reveal signs of high bone turnover despite long-term anti resorptive treatment.

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Without curative treatment options, severe forms of JPD are a debilitating disease with high morbidity and increased mortality

Meta-analysis of sample-level dbGaP data reveals novel shared genetic link between body height and Crohn's disease

To further explore genetic links between complex traits, we developed a comprehensive framework to harmonize and integrate extensive genotype and phenotype data from the four well-characterized cohorts with the focus on cardiometabolic diseases deposited to the database of Genotypes and Phenotypes (dbGaP). We generated a series of polygenic risk scores (PRS) to investigate pleiotropic effects of loci that confer genetic risk for 19 common diseases and traits on body height, type 2 diabetes (T2D), and myocardial infarction (MI). In a meta-analysis of 20,021 subjects, we identified shared genetic determinants of Crohn's Disease (CD), a type of inflammatory bowel disease, and body height (p = 5.5 × 10^-5). The association of PRS-CD with height was replicated in UK Biobank (p = 1.1 × 10^-5) and an independent cohort of 510 CD cases and controls (1.57 cm shorter height per PRS-CD interquartile increase, p = 5.0 × 10^-3 and a 28% reduction in CD risk per interquartile increase in PRS-height, p = 1.1 × 10^-3, with the effect independent of CD diagnosis). A pathway analysis of the variants overlapping between PRS-height and PRS-CD detected significant enrichment of genes from the inflammatory, immune-mediated and growth factor regulation pathways. This finding supports the clinical observation of growth failure in patients with childhood-onset CD and demonstrates the value of using individual-level data from dbGaP in searching for shared genetic determinants. This information can help provide a refined insight into disease pathogenesis and may have major implications for novel therapies and drug repurposing.

Mechanisms of Bone Fragility: From Osteogenesis Imperfecta to Secondary Osteoporosis

Bone material strength is determined by several factors, such as bone mass, matrix composition, mineralization, architecture and shape. From a clinical perspective, bone fragility is classified as primary (i.e., genetic and rare) or secondary (i.e., acquired and common) osteoporosis. Understanding the mechanism of rare genetic bone fragility disorders not only advances medical knowledge on rare diseases, it may open doors for drug development for more common disorders (i.e., postmenopausal osteoporosis). In this review, we highlight the main disease mechanisms underlying the development of human bone fragility associated with low bone mass known to date. The pathways we focus on are type I collagen processing, WNT-signaling, TGF-ß signaling, the RANKL-RANK system and the osteocyte mechanosensing pathway. We demonstrate how the discovery of most of these pathways has led to targeted, pathway-specific treatments.

Genetic disorders associated with the RANKL/OPG/RANK pathway

The RANKL/OPG/RANK signalling pathway is a major regulatory system for osteoclast formation and activity. Mutations in TNFSF11, TNFRSF11B and TNFRSF11A cause defects in bone metabolism and development, thereby leading to skeletal disorders with changes in bone density and/or morphology. To date, nine kinds of monogenic skeletal diseases have been found to be causally associated with TNFSF11, TNFRSF11B and TNFRSF11A mutations. These diseases can be divided into two types according to the mutation effects and the resultant pathogenesis. One is caused by the mutations inducing constitutional RANK activation or OPG deficiency, which increase osteoclastogenesis and accelerate bone turnover, resulting in juvenile Paget's disease 2, Paget disease of bone 2, familial expansile osteolysis, expansile skeletal hyperphosphatasia, panostotic expansile bone disease, and Paget disease of bone 5. The other is caused by the de-activating mutations in TNFRSF11A or TNFSF11, which decrease osteoclastogenesis and elevate bone density, resulting in osteopetrosis, autosomal recessive 2 and 7, and dysosteosclerosis. Here we reviewed the current knowledge about these genetic disorders with paying particular attention to the updating genotype-phenotype association in the TNFRSF11A-caused diseases.

Blockade of the angiotensin II type 1 receptor increases bone mineral density and left ventricular contractility in a mouse model of juvenile Paget disease

Juvenile Paget disease (JPD1), an autosomal-recessive disorder, is characterized by extremely rapid bone turnover due to osteoprotegerin deficiency. Its extra-skeletal manifestations, such as hypertension and heart failure, suggest a pathogenesis with shared skeletal and cardiovascular system components. In spite of this, the effects of anti-hypertensive drugs on bone morphometry remain unknown. We administered an angiotensin II type 1 receptor blocker, olmesartan (5 mg/kg/day) to 8-week-old male mice lacking the osteoprotegerin gene, with and without 1 μg/kg/min of angiotensin II infusion for 14 days. Olmesartan treatment decreased systolic blood pressure, and echocardiography showed increased left ventricular systolic contractility. Three-dimensional micro-computed tomography scans demonstrated that olmesartan treatment increased trabecular bone volume (sham, +176%; angiotensin II infusion, +335%), mineral density (sham, +150%; angiotensin II infusion, +313%), and trabecular number (sham, +407%; angiotensin II infusion, +622%) in the tibia. Olmesartan increased cortical mineral density (sham, +19%; angiotensin II infusion, +24%), decreased the cortical bone section area (sham, -16%; angiotensin II infusion, -18%), decreased thickness (sham, -18%; angiotensin II infusion, -31%), and decreased the lacunar area (sham, -41%; angiotensin II infusion, -27%) in the tibia. Similar trend was observed in the femur. Moreover, olmesartan decreased angiotensin II-induced increases in tartrate-resistant acid phosphatase concentrations in plasma, but it affected neither type I procollagen N-terminal propeptides, nor the receptor activator of nuclear factor kappa-B ligand. Our data suggest that blockade of the angiotensin II type 1 receptor improves bone vulnerability, and helps to maintain the heart's structural integrity in osteoprotegerin-deficient mice.

Rare Inherited forms of Paget's Disease and Related Syndromes

Several rare inherited disorders have been described that show phenotypic overlap with Paget's disease of bone (PDB) and in which PDB is a component of a multisystem disorder affecting muscle and the central nervous system. These conditions are the subject of this review article. Insertion mutations within exon 1 of the TNFRSF11A gene, encoding the receptor activator of nuclear factor kappa B (RANK), cause severe PDB-like disorders including familial expansile osteolysis, early-onset familial PDB and expansile skeletal hyperphosphatasia. The mutations interfere with normal processing of RANK and cause osteoclast activation through activation of nuclear factor kappa B (NFκB) independent of RANK ligand stimulation. Recessive, loss-of-function mutations in the TNFRSF11B gene, which encodes osteoprotegerin, cause juvenile PDB and here the bone disease is due to unopposed activation of RANK by RANKL. Multisystem proteinopathy is a disorder characterised by myopathy and neurodegeneration in which PDB is often an integral component. It may be caused by mutations in several genes including VCP, HNRNPA1, HNRNPA2B1, SQSTM1, MATR3, and TIA1, some of which are involved in classical PDB. The mechanisms of osteoclast activation in these conditions are less clear but may involve NFκB activation through sequestration of IκB. The evidence base for management of these disorders is somewhat limited due to the fact they are extremely rare. Bisphosphonates have been successfully used to gain control of elevated bone remodelling but as yet, no effective treatment exists for the treatment of the muscle and neurological manifestations of MSP syndromes.

Loss of the Hematopoietic Stem Cell Factor GATA2 in the Osteogenic Lineage Impairs Trabecularization and Mechanical Strength of Bone

The transcription factor GATA2 is required for expansion and differentiation of hematopoietic stem cells (HSCs). In mesenchymal stem cells (MSCs), GATA2 blocks adipogenesis, but its biological relevance and underlying genomic events are unknown. We report a dual function of GATA2 in bone homeostasis. GATA2 in MSCs binds near genes involved in skeletal system development and colocalizes with motifs for FOX and HOX transcription factors, known regulators of skeletal development. Ectopic GATA2 blocks osteoblastogenesis by interfering with SMAD1/5/8 activation. MSC-specific deletion of GATA2 in mice increases the numbers and differentiation capacity of bone-derived precursors, resulting in elevated bone formation. Surprisingly, MSC-specific GATA2 deficiency impairs the trabecularization and mechanical strength of bone, involving reduced MSC expression of the osteoclast inhibitor osteoprotegerin and increased osteoclast numbers. Thus, GATA2 affects bone turnover via MSC-autonomous and indirect effects. By regulating bone trabecularization, GATA2 expression in the osteogenic lineage may contribute to the anatomical and cellular microenvironment of the HSC niche required for hematopoiesis.

Juvenile Paget disease

Juvenile Paget disease (JPD) is a rare disorder, mainly caused by mutations in the gene TNFRSF11B that encodes osteoprotegerin (OPG). Loss of OPG action causes generalized, extremely rapid bone turnover. The clinical manifestations are both skeletal - progressive skeletal deformity that develops in childhood - and extra-skeletal, including hearing loss, retinopathy, vascular calcification and internal carotid artery aneurysm formation. The severity of the phenotype seems to be related to the severity of TNFRSF11B gene deactivation. JPD is characterized biochemically by very high alkaline phosphatase activity, as well as other bone turnover markers. Bisphosphonates are commonly used to reduce the greatly accelerated bone turnover and can ameliorate the skeletal phenotype, if started early enough in childhood and continued at least until growth is complete. Limited evidence from patients treated with recombinant OPG or denosumab also provided favorable results. Recombinant OPG would represent a replacement treatment, but it is unavailable for clinical use. It seems that life-long treatment with anti-resorptives is required, since the disease is reactivated after treatment discontinuation. An international collaborating network for the continuous registration and follow-up of JPD patients could be helpful in the future.