Expansile skeletal hyperphosphatasia: a new familial metabolic bone disease

Early identification of a 12-bp tandem duplication in TNFRSF11A encoding receptor activator of nuclear factor-kappa B (RANK): Clinical characterization and response to bisphosphonate therapy

INTRODUCTION

Ultra-rare mendelian osteolytic disorders caused by different length in-frame activating duplications within exon 1 of TNFRSF11A encoding receptor activator of nuclear factor-kappa B (RANK) comprise familial expansile osteolysis (FEO), expansile skeletal hyperphosphatasia (ESH), early-onset familial Paget's disease of bone (PDB2), juvenile Paget's disease 2 (JPD2), and panostotic expansile bone disease (PEBD). FEO typically presents with childhood-onset deafness followed by resorption of permanent dentition, and then appendicular bone pain, fractures, and deformities from progressive focal expansile osteolytic lesions emerging from a background of generalized high bone turnover. An 18-bp duplication in TNFRSF11A has been reported in all kindreds with FEO, whereas a 12-bp duplication was found in the young man with PEBD complicated by a massive jaw tumor. We report the clinical course and successful treatment with bisphosphonates of a girl with the 12-bp duplication yet a skeletal phenotype seemingly milder than PEBD.

CASE PRESENTATION AND DISCUSSION

This 10-year-old girl presented for dental and orthodontic treatment and was found to have progressive external tooth root resorption. Speech delay was identified at age 18 months, and audiological evaluation showed both conductive and sensorineural hearing loss subsequently treated with a cochlear implant at age 3 years. Biochemical studies indicated increased bone turnover with elevated urinary N-telopeptide levels and serum alkaline phosphatase in the upper normal range. Low lumbar spine bone mineral density (BMD) was revealed by dual-energy X-ray absorptiometry, but whole-body Technetium-99 m bone scintigraphy was normal. Genetic testing identified the identical de novo 12-bp duplication within exon 1 of TNFRSF11A harbored by the young man with PEBD and massive jaw tumor. Bisphosphonate treatment, initiated with one dose of intravenous zoledronic acid that caused prolonged hypocalcemia, then comprised weekly oral alendronate that decreased bone turnover markers and normalized her BMD.

CONCLUSION

Constitutive activation of RANK signaling should be considered a possible cause in any young person with rapid bone turnover, particularly in the context of early-onset deafness and/or root resorption of permanent teeth. Early diagnosis and anti-resorptive treatment, given judiciously to avoid sudden and prolonged hypocalcemia, may prevent further skeletal disease.

Familial Paget's disease of bone with ocular manifestations and a novel TNFRSF11A duplication variant (72dup27)

INTRODUCTION

Paget's disease of bone (PDB) is a skeletal disorder characterized by disorganized bone remodeling due to abnormal osteoclasts. Tumor necrosis factor receptor superfamily member 11A (TNFRSF11A) gene encodes the receptor activator of nuclear factor kappa B (RANK), which has a critical role in osteoclast function. There are five types of rare PDB and related osteolytic disorders due to TNFRSF11A tandem duplication variants so far, including familial expansile osteolysis (84dup18), expansile skeletal hyperphosphatasia (84dup15), early-onset familial PDB (77dup27), juvenile PDB (87dup15), and panostotic expansile bone disease (90dup12).

MATERIALS AND METHODS

We reviewed a Japanese family with PDB, and performed whole-genome sequencing to identify a causative variant.

RESULTS

This family had bone symptoms, hyperphosphatasia, hearing loss, tooth loss, and ocular manifestations such as angioid streaks or early-onset glaucoma. We identified a novel duplication variant of TNFRSF11A (72dup27). Angioid streaks were recognized in Juvenile Paget's disease due to loss-of-function variants in the gene TNFRSF11B, and thought to be specific for this disease. However, the novel recognition of angioid streaks in our family raised the possibility of occurrence even in bone disorders due to TNFRSF11A duplication variants and the association of RANKL-RANK signal pathway as the pathogenesis. Glaucoma has conversely not been reported in any case of Paget's disease. It is not certain whether glaucoma is coincidental or specific for PDB with 72dup27.

CONCLUSION

Our new findings might suggest a broad spectrum of phenotypes in bone disorders with TNFRSF11A duplication variants.

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.

Early-onset Paget's disease of bone in a Mexican family caused by a novel tandem duplication (77dup27) in TNFRSF11A that encodes RANK

Four heterozygous in-frame tandem duplications of different lengths in TNFRSF11A, the gene that encodes receptor activator of nuclear factor κB (RANK), constitutively activate RANK and lead to high turnover skeletal disease. Each duplication elongates the signal peptide of RANK. The 18-base pair (bp) duplication at position 84 (84dup18) causes familial expansile osteolysis (FEO), the 15-bp duplication at position 84 (84dup15) causes expansile skeletal hyperphosphatasia (ESH), the 12-bp duplication at position 90 (90dup12) causes panostotic expansile bone disease (PEBD), and the 27-bp duplication causes early-onset Paget's disease of bone (PDB2). The severity of the associated skeletal disease seems inversely related to the duplication's length. Additional 15- and 18-bp duplications of TNFRSF11A fit this pattern. Herein, we delineate the skeletal disease of a middle-aged man of Mexican descent who we found to harbor a novel 27-bp tandem duplication at position 77 (77dup27) of TNFRSF11A. His disorder shares features, particularly hand involvement, with the single Japanese (75dup27) and Chinese (78dup27) kindreds with PDB2 (PDB2_Jpn and PDB2_Chn, respectively). However, his distinct hearing loss developed later in adulthood compared to the other 27-bp families. He reported no morbidities during childhood, but in his late 20s developed unexplained tooth loss, low-trauma fractures, post-operative hypercalcemia, and painless enlargement of his fingers. Biochemical studies showed elevated serum alkaline phosphatase (ALP), bone-specific ALP, C-telopeptide, and osteocalcin consistent with rapid bone remodeling. Radiologic imaging revealed remarkably lucent bones with vertebral compression fractures, calvarial lucencies, and thinned long bone cortices. DXA showed extremely low bone mineral density. His disorder genetically and phenotypically fits best with PDB2 and can be called PDB2_Mex.

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.

The Use of Patient-Specific Induced Pluripotent Stem Cells (iPSCs) to Identify Osteoclast Defects in Rare Genetic Bone Disorders

More than 500 rare genetic bone disorders have been described, but for many of them only limited treatment options are available. Challenges for studying these bone diseases come from a lack of suitable animal models and unavailability of skeletal tissues for studies. Effectors for skeletal abnormalities of bone disorders may be abnormal bone formation directed by osteoblasts or anomalous bone resorption by osteoclasts, or both. Patient-specific induced pluripotent stem cells (iPSCs) can be generated from somatic cells of various tissue sources and in theory can be differentiated into any desired cell type. However, successful differentiation of hiPSCs into functional bone cells is still a challenge. Our group focuses on the use of human iPSCs (hiPSCs) to identify osteoclast defects in craniometaphyseal dysplasia. In this review, we describe the impact of stem cell technology on research for better treatment of such disorders, the generation of hiPSCs from patients with rare genetic bone disorders and current protocols for differentiating hiPSCs into osteoclasts.

Juvenile Paget's disease with heterozygous duplication within TNFRSF11A encoding RANK

Mendelian disorders of RANKL/OPG/RANK signaling feature the extremes of aberrant osteoclastogenesis and cause either osteopetrosis or rapid turnover skeletal disease. The patients with autosomal dominant accelerated bone remodeling have familial expansile osteolysis, early-onset Paget's disease of bone, expansile skeletal hyperphosphatasia, or panostotic expansile bone disease due to heterozygous 18-, 27-, 15-, and 12-bp insertional duplications, respectively, within exon 1 of TNFRSF11A that encodes the signal peptide of RANK. Juvenile Paget's disease (JPD), an autosomal recessive disorder, manifests extremely fast skeletal remodeling, and is usually caused by loss-of-function mutations within TNFRSF11B that encodes OPG. These disorders are ultra-rare. A 13-year-old Bolivian girl was referred at age 3years. One femur was congenitally short and curved. Then, both bowed. Deafness at age 2years involved missing ossicles and eroded cochleas. Teeth often had absorbed roots, broke, and were lost. Radiographs had revealed acquired tubular bone widening, cortical thickening, and coarse trabeculation. Biochemical markers indicated rapid skeletal turnover. Histopathology showed accelerated remodeling with abundant osteoclasts. JPD was diagnosed. Immobilization from a femur fracture caused severe hypercalcemia that responded rapidly to pamidronate treatment followed by bone turnover marker and radiographic improvement. No TNFRSF11B mutation was found. Instead, a unique heterozygous 15-bp insertional tandem duplication (87dup15) within exon 1 of TNFRSF11A predicted the same pentapeptide extension of RANK that causes expansile skeletal hyperphosphatasia (84dup15). Single nucleotide polymorphisms in TNFRSF11A and TNFRSF11B possibly impacted her phenotype. Our findings: i) reveal that JPD can be associated with an activating mutation within TNFRSF11A, ii) expand the range and overlap of phenotypes among the Mendelian disorders of RANK activation, and iii) call for mutation analysis to improve diagnosis, prognostication, recurrence risk assessment, and perhaps treatment selection among the monogenic disorders of RANKL/OPG/RANK activation.

Sclerosing bone dysplasias: leads toward novel osteoporosis treatments

Sclerosing bone dysplasias are a group of rare, monogenic disorders characterized by increased bone density resulting from the disturbance in the fragile equilibrium between bone formation and resorption. Over the last decade, major contributions have been made toward better understanding of the pathogenesis of these conditions. These studies provided us with important insights into the bone biology and yielded the identification of numerous drug targets for the prevention and treatment of osteoporosis. Here, we review this heterogeneous group of disorders focusing on their utility in the development of novel osteoporosis therapies.

Genotype-phenotype correlation in juvenile Paget disease: role of molecular alterations of the TNFRSF11B gene

Juvenile Paget disease (JPD) {MIM 239000} is a rare inherited bone disease that affects children. The patients affected with JPD present an altered bone turnover, therefore, show a phenotype characterized by progressive bone deformities, fractures, and short stature. Deletions or missense mutations of the TNFRSN11B gene are common in these children. This gene encodes a soluble protein, the osteoprotegerin, which leads to uncontrolled osteoclastogenesis when mutated. JPD is characterized by a strong genotype-phenotype correlation, so depending on the alteration of the TNFRSN11B gene, the phenotype is variable. This review describes the different clinical features which are characteristic of JPD and the correspondence with the different molecular alterations of the TNFRSN11B gene.

Pathogenesis of Paget disease of bone

Paget disease of bone (PDB) is a common disease characterized by focal areas of increased and disorganized bone turnover. Some patients are asymptomatic, whereas others develop complications such as pain, osteoarthritis, fracture, deformity, deafness, and nerve compression syndromes. PDB is primarily caused by dysregulation of osteoclast differentiation and function, and there is increasing evidence that this is due, in part, to genetic factors. One of the most important predisposing genes is SQSTM1, which harbors mutations that cause osteoclast activation in 5-20 % of PDB patients. Seven additional susceptibility loci for PDB have been identified by genomewide association studies on chromosomes 1p13, 7q33, 8q22, 10p13, 14q32, 15q24, and 18q21. Although the causal variants remain to be discovered, three of these loci contain CSF1, TNFRSF11A, and TM7SF4, genes that are known to play a critical role in osteoclast differentiation and function. Environmental factors are also important in the pathogenesis of PDB, as reflected by the fact that in many countries the disease has become less common and less severe over recent years. The most widely studied environmental trigger is paramyxovirus infection, but attempts to detect viral transcripts in tissues from patients with PDB have yielded mixed results. Although our understanding of the pathophysiology of PDB has advanced tremendously over the past 10 years, many questions remain unanswered, such as the mechanisms responsible for the focal nature of the disease and the recent changes in prevalence and severity.

Low fatness, reduced fat intake and adequate plasmatic concentrations of LDL-cholesterol are associated with high bone mineral density in women: a cross-sectional study with control group

BACKGROUND

Several parameters are associated with high bone mineral density (BMD), such as overweight, black background, intense physical activity (PA), greater calcium intake and some medications. The objectives are to evaluate the prevalence and the main aspects associated with high BMD in healthy women.

METHODS

After reviewing the database of approximately 21,500 BMD scans performed in the metropolitan area of São Paulo, Brazil, from June 2005 to October 2010, high BMD (over 1400 g/cm² at lumbar spine and/or above 1200 g/cm² at femoral neck) was found in 421 exams. Exclusion criteria were age below 30 or above 60 years, black ethnicity, pregnant or obese women, disease and/or medications known to interfere with bone metabolism. A total of 40 women with high BMD were included and matched with 40 healthy women with normal BMD, paired to weight, age, skin color and menopausal status. Medical history, food intake and PA were assessed through validated questionnaires. Body composition was evaluated through a GE-Lunar DPX MD + bone densitometer. Radiography of the thoracic and lumbar spine was carried out to exclude degenerative alterations or fractures. Biochemical parameters included both lipid and hormonal profiles, along with mineral and bone metabolism. Statistical analysis included parametric and nonparametric tests and linear regression models. P < 0.05 was considered significant.

RESULTS

The mean age was 50.9 (8.3) years. There was no significant difference between groups in relation to PA, smoking, intake of calcium and vitamin D, as well as laboratory tests, except serum C-telopeptide of type I collagen (s-CTX), which was lower in the high BMD group (p = 0.04). In the final model of multivariate regression, a lower fat intake and body fatness as well a better profile of LDL-cholesterol predicted almost 35% of high BMD in women. (adjusted R2 = 0.347; p < 0.001). In addition, greater amounts of lean mass and higher IGF-1 serum concentrations played a protective role, regardless age and weight.

CONCLUSION

Our results demonstrate the potential deleterious effect of lipid metabolism-related components, including fat intake and body fatness and worse lipid profile, on bone mass and metabolism in healthy women.

Signal peptide mutations in RANK prevent downstream activation of NF-κB

Familial expansile osteolysis and related disorders are caused by heterozygous tandem duplication mutations in the signal peptide region of the gene encoding receptor activator of NF-κB (RANK), a receptor critical for osteoclast formation and function. Previous studies have shown that overexpression of these mutant proteins causes constitutive activation of NF-κB signaling in vitro, and it has been assumed that this accounts for the focal osteolytic lesions that are seen in vivo. We show here that constitutive activation of NF-κB occurred in HEK293 cells overexpressing wild-type or mutant RANK but not in stably transfected cell lines expressing low levels of each RANK gene. Importantly, only cells expressing wild-type RANK demonstrated ligand-dependent activation of NF-κB. When overexpressed, mutant RANK did not localize to the plasma membrane but localized to extensive areas of organized smooth endoplasmic reticulum, whereas, as expected, wild-type RANK was detected at the plasma membrane and in the Golgi apparatus. This intracellular accumulation of the mutant proteins is probably the result of lack of signal peptide cleavage because, using two in vitro translation systems, we demonstrate that the mutations in RANK prevent cleavage of the signal peptide. In conclusion, signal peptide mutations lead to accumulation of RANK in the endoplasmic reticulum and prevent direct activation by RANK ligand. These results strongly suggest that the increased osteoclast formation/activity caused by these mutations cannot be explained by studying the homozygous phenotype alone but requires further detailed investigation of the heterozygous expression of the mutant RANK proteins.

Bone resorption control of tooth eruption and root morphogenesis: Involvement of the receptor activator of NF-κB (RANK)

Activation of the receptor activator of NF-κB (RANK) is a crucial step in osteoclastogenesis. Loss- and gain-of-function mutations in the Rank gene cause, respectively, osteopetrosis and several forms of extensive osteolysis. Tooth and alveolar bone alterations are associated with these pathologies but remain to be better characterized. The aim of the present study was to establish the tooth and alveolar bone phenotype of a transgenic mouse model of RANK over-expression in osteoclast precursors. Early tooth eruption and accelerated tooth root elongation were observed subsequent to an increase in osteoclast numbers surrounding the tooth. The final root length appeared not to be affected by RANK over-expression, but a significant reduction in root diameter occurred in both control and root-morphogenesis-defective Msx2 null mutant mice. These results indicate that root length is independent of the surrounding bone resorption activity. In contrast, root diameter is sensitive to the activity of alveolar bone osteoclasts. These data suggest that early eruption and thin root are phenotypic features that could be associated with extensive osteolytic pathologies.

Genetic aspects of the Paget's disease of bone: concerns on the introduction of DNA-based tests in the clinical practice. Advantages and disadvantages of its application

BACKGROUND

A large amount of genetic studies have clearly demonstrated the existence of a genetic susceptibility to Paget's disease of bone (PDB). Although the disease is genetically heterogeneous, the SQSTM1/p62 gene, encoding a protein with a pathophysiological role in both osteoclast differentiation and activity, has been found worldwide to harbour germline mutations in most of the PDB patients from geographically distant populations originating from different areas of Europe, both in sporadic and familial cases.

MATERIALS AND METHODS

Thus, SQSTM1/p62 gene mutations may confer an increased lifetime risk of developing PDB.

RESULTS

Several different genotype-phenotype analyses have shown a high penetrance for such mutations. These results suggest the opportunity to perform genetic testing in affected individuals and then, after the identification of a SQSTM1/p62 gene germline mutation, in their relatives as a real and concrete strategy to increase the diagnostic sensitivity in most of the asymptomatic mutant carriers. However, it is of note to underlie that an incomplete penetrance for SQSTM1/p62 gene mutations has also been reported.

CONCLUSIONS

In light of all these contradictory evidences, a review on whether, when and why apply the DNA test to those subjects, its interpretation and clinical application is necessary. In fact, a growing number of preventive care options are now available to affected patients and families and the process of systematically assessing risk is becoming increasingly important for both patients and physicians.

Osteogenic sarcoma in a child with familial expansile osteolysis syndrome: an accidental association?

We present the first reported case of a child with familial expansile osteolysis syndrome (FEO) who developed osteogenic sarcoma (OS) of the iliac bone. A 17-year-old adolescent presented with pain and a mass on the left pelvis. He was from a family with several members who had been diagnosed with FEO, from which he also suffered. The median life expectancy of affected members of the family was reported as 25 to 30 years, with death ensuing as a result of various respiratory and cardiac complications of severe skeletal deformations, characteristic of increased bone turnover as seen in FEO. Biopsy of the patient's mass revealed chondroblastic OS. He was treated according to the P9754 protocol for patients with newly diagnosed nonmetastatic OS. Chemotherapy consisted of HD-MTX, ifosfomide, doxorubicin, and cisplatin. Complete resection of the tumor was carried out, but the patient subsequently developed metastatic disease and died (histologic response to neoadjuvant chemotherapy-85%). The patient's alkaline phosphatase level that was highly elevated before the start of chemotherapy, dropped significantly during treatment, with repeated elevation soon after definitive surgery, while he was recuperating and not on treatment. We speculate that chemotherapy affected not only the malignant cells of OS but normal osteoblasts as well, with a decreasing level of alkaline phosphatase even in the absence of any clinical and radiographic signs of OS. We also think that increased bone turnover, characteristic of a condition such as FEO, may facilitate de novo development of OS.

RANK/RANKL: regulators of immune responses and bone physiology

Bone-related diseases, such as osteoporosis and rheumatoid arthritis, affect hundreds of millions of people worldwide and pose a tremendous burden to health care. By deepening our understanding of the molecular mechanisms of bone metabolism and bone turnover, it became possible over the past years to devise new and promising strategies for treating such diseases. In particular, three tumor necrosis factor (TNF) family molecules, the receptor activator of NF-kappaB (RANK), its ligand RANKL, and the decoy receptor of RANKL, osteoprotegerin (OPG), have attracted the attention of scientists and pharmaceutical companies alike. Genetic experiments revolving around these molecules established their pivotal role as central regulators of osteoclast development and osteoclast function. RANK-RANKL signaling not only activates a variety of downstream signaling pathways required for osteoclast development, but crosstalk with other signaling pathways also fine-tunes bone homeostasis both in normal physiology and disease. In addition, RANKL and RANK have essential roles in lymph node formation, establishment of the thymic microenvironment, and development of a lactating mammary gland during pregnancy. Consequently, novel drugs specifically targeting RANK, RANKL, and their signaling pathways in osteoclasts are expected to revolutionize the treatment of various ailments associated with bone loss, such as arthritis, periodontal disease, cancer metastases, and osteoporosis.

RANKL/RANK as key factors for osteoclast development and bone loss in arthropathies

Osteoporosis or rheumatoid arthritis are bone diseases affecting hundreds of millions of people worldwide and thus pose a tremendous burden to health care. Ground-breaking discoveries made in basic science over the last decade shed light on the molecular mechanisms of bone metabolism and bone turnover. Thereby, it became possible over the past years to devise new and promising strategies for treating such diseases. In particular, three molecules, the receptor activator of NF-kappaB (RANK), its ligand RANKL and the decoy receptor of RANKL, osteoprotegerin (OPG), have been a major focus of scientists and pharmaceutical companies alike, since experiments using mice in which these genes have been inactivated unanimously established their pivotal role as central regulators ofosteoclast function. RANK(L) signaling not only activates a variety of downstream signaling pathways required for osteoclast development, but crosstalk with other signaling pathways also fine-tunes bone homeostasis both in normal physiology and disease. Consequently, novel drugs specifically targeting RANK-RANKL and their signaling pathways in osteoclasts are expected to revolutionize the treatment ofvarious bone diseases, such as cancer metastases, osteoporosis, or arthropathies.

Pathogenesis of Paget's disease of bone

Paget's disease of bone is a common condition characterised by increased and disorganised bone turnover which can affect one or several bones throughout the skeleton. These abnormalities disrupt normal bone architecture and lead to various complications such as bone pain osteoarthritis, pathological fracture, bone deformity, deafness, and nerve compression syndromes. Genetic factors play an important role in PDB and mutations or polymorphisms have been identified in four genes that cause classical Paget's disease and related syndromes. These include TNFRSF11A, which encodes RANK, TNFRSF11B which encodes osteoprotegerin, VCP which encodes p97, and SQSTM1 which encodes p62. All of these genes play a role in the RANK-NFkappaB signalling pathway and it is likely that the mutations predispose to PDB by disrupting normal signalling, leading to osteoclast activation. Although Paget's has traditionally be considered a disease of the osteoclast there is evidence that stromal cell function and osteoblast function are also abnormal, which might account for the fact that the disease is associated with increased bone formation as well as resorption. Environmental factors also contribute to Paget's disease. Most research has focused on paramyxovirus infection as a possible environmental trigger but evidence in favour of the involvement of viruses in the disease remains conflicting. Other factors which have been implicated as possible disease triggers include mechanical loading, dietary calcium and environmental toxins. Further work will be required to identify additional genetic variants that predispose to Paget's disease and to determine how the causal mutations and predisposing polymorphisms interact with environmental factors to influence bone cell function and cause the focal bone lesions that are characteristic of the disease.

Clinical and biochemical response of TNFRSF11A-mediated early-onset familial Paget disease to bisphosphonate therapy

Early-onset familial Paget disease of bone (EoPDB) is a rare condition caused by a 27-bp insertion mutation affecting the signal peptide of TNFRSF11A, which encodes RANK. EoPDB shows phenotypic overlap to both familial expansile osteolysis and expansile skeletal hyperphosphatasia, which are caused by similar mutations in TNFRSF11A. Although EoPDB is characterized by elevated bone turnover, there is no published information on the response of this condition to antiresorptive therapy. Here, we describe the clinical and biochemical response to bisphosphonate therapy in three patients with EoPDB. In all cases, treatment with the first-generation bisphosphonate etidronate at high doses reduced biochemical markers of bone turnover but the response was incomplete and short-lived. In contrast, treatment with aminobisphosphonates resulted in greater suppression of biochemical markers of bone turnover with an extended duration of response. From a clinical perspective, the results were less impressive and there was no clear benefit from antiresorptive treatment in terms of bone deformity, deafness, and tooth loss, although bone pain improved in one patient. We conclude that intravenous aminobisphosphonate therapy may be the preferred mode of treatment for EoPDB to provide long-term suppression of bone turnover. The long-term clinical effects of treatment on the natural history of the bone disease remain uncertain however, and this will require further study.

Benign fibro-osseous lesions of the craniofacial complex. A review

Benign fibro-osseous lesions of the craniofacial complex are represented by a variety of disease processes that are characterized by pathologic ossifications and calcifications in association with a hypercellular fibroblastic marrow element. The current classification includes neoplasms, developmental dysplastic lesions and inflammatory/reactive processes. The definitive diagnosis can rarely be rendered on the basis of histopathologic features alone; rather, procurement of a final diagnosis is usually dependent upon assessment of microscopic, clinical and imaging features together. Fibrous dysplasia and osteitis deformans constitute two dysplastic lesions in which mutations have been uncovered. Other dysplastic bone diseases of the craniofacial complex include florid osseous dysplasia, focal cemento-osseous dysplasia and periapical cemental dysplasia, all showing a predilection for African descent individuals; although no specific genetic alterations in DNA coding have yet to be uncovered and most studies have been derived from predominant high African descent populations. Ossifying fibromas are neoplastic lesions with four subtypes varying with regard to behavior and propensity for recurrence after surgical excision. The clinicopathologic and molecular features of this unique yet heterogeneous group of diseases are reviewed.

CLINICAL Review #: the role of receptor activator of nuclear factor-kappaB (RANK)/RANK ligand/osteoprotegerin: clinical implications

CONTEXT

Receptor activator of nuclear factor-kappaB ligand (RANKL), receptor activator of nuclear factor-kappaB (RANK), and osteoprotegerin (OPG) play a central role in bone remodeling and disorders of mineral metabolism.

EVIDENCE ACQUISITION

A PubMed search was conducted from January 1992 until 2007 for basic, observational, and clinical studies in subjects with disorders related to imbalances in the RANK/RANKL/OPG system.

EVIDENCE SYNTHESIS

RANK, RANKL, and OPG are members of the TNF receptor superfamily. The pathways involving them in conjunction with various cytokines and calciotropic hormones play a pivotal role in bone remodeling. Several studies involving mutations in the genes encoding RANK and OPG concluded in the discovery of a number of inherited skeletal disorders. In addition, basic and clinical studies established a consistent relationship between the RANK/RANKL/OPG pathway and skeletal lesions related to disorders of mineral metabolism. These studies were a stepping stone in further defining the role of the RANK/RANKL/OPG pathway in osteoporosis, rheumatoid arthritis, bone loss associated with malignancy-related skeletal diseases, and its relationship to vascular calcifications. Subsequently, the further understanding of this pathway led to the development of new therapeutic modalities including the human monoclonal antibody to RANKL and recombinant OPG as a target for treatment of postmenopausal osteoporosis and multiple myeloma.

CONCLUSIONS

The RANK/RANKL/OPG system mediates the effects of calciotropic hormones and, consequently, alterations in their ratio are key in the development of several clinical conditions. New agents with the potential to block effects of RANKL have emerged for treatment of postmenopausal osteoporosis and malignancy-related skeletal disease.

Hyperphosphatasia with massive osteoectasia: a 45-year follow-up

Hyperphosphatasia is a heterogeneous group of disorders characterized by a generalized skeletal disease and increased alkaline phosphatase. Increased bone remodeling secondary to increased osteoclastic activity appears to be the underlying feature of these disorders. These disorders include juvenile Paget's disease, expansile skeletal hyperphosphatasia, hyperostosis generalisata with striations, and Camurati-Engelmann's disease, type II. The genetic mutations for a number of these disorders have been identified. We present a patient with congenital hyperphosphatasia whose clinical and radiographic features were somewhat different from these other well-defined syndromes. The patient was followed for 45 years until his death of at age 49. The patient had massive osteoectasia with dense striations involving the entire shaft of his long bones. His spine, pelvis, short tubular bones, and calvarium were also involved. He suffered hearing loss and optic atrophy, but he kept his teeth throughout his life. He was tall with a marfanoid habitus, and he had hypogonadism and hypothyroidism. There was no evidence of mental retardation, and other laboratory studies where within normal limits. This case, as well as other manifestations of hyperphosphatasia, attests to the complexity of the bone remodeling system.

Sporadic hyperphosphatasia syndrome featuring periostitis and accelerated skeletal turnover without receptor activator of nuclear factor-kappaB, osteoprotegerin, or sequestosome-1 gene defects

CONTEXT

A middle-aged woman with recent-onset painful swollen fingers and widespread periostitis, elevated serum alkaline phosphatase (ALP) activity and erythrocyte sedimentation rate, and accelerated skeletal turnover was found not to have mutations in the gene sequences for exon 1 of receptor activator of nuclear factor-kappaB (RANK), osteoprotegerin (OPG), or sequestosome-1.

INTRODUCTION

Hyperphosphatasia refers to disorders that feature elevated serum ALP activity (hyperphosphatasemia) usually from excesses of the bone isoform of ALP. Such conditions include familial expansile osteolysis, expansile skeletal hyperphosphatasia, and a familial form of early-onset Paget's disease of bone (PDB2), all from constitutive activation of RANK, and juvenile Paget's disease from OPG deficiency.

PATIENT AND METHODS

A 38-yr-old woman developed painful swollen fingers and achy bones after an episode of unexplained pericarditis and restrictive lung disease. Sequence analysis of exon 1 of TNFRSF11A encoding RANK, TNFRSF11B encoding OPG, and SQSTM1 encoding sequestosome-1 searched for mutations responsible for familial expansile osteolysis, expansile skeletal hyperphosphatasia, or PDB2, juvenile Paget's disease, or Paget's disease of bone (PDB), respectively.

RESULTS

Serum ALP and osteocalcin and urinary hydroxyproline were increased. Radiographs showed widespread, symmetric hyperostosis in the limbs where bone scintigraphy demonstrated enhanced radionuclide uptake. Iliac crest histology revealed accelerated skeletal turnover. No mutations were detected in the three genes examined. Three years of therapy with 70 mg alendronate orally once weekly improved symptoms, radiographic abnormalities, and biochemical markers.

CONCLUSIONS

Our patient manifested a unique, sporadic hyperphosphatasia syndrome. Unexplained, transient inflammation seemed to cause her pericarditis, restrictive lung disease, and periostitis with accelerated skeletal turnover that responded well to antiinflammatory drugs and alendronate therapy.

Contribution of genetic factors to the pathogenesis of Paget's disease of bone and related disorders

Paget's disease of bone (PDB) is a common condition with a strong genetic component that is characterized by focal increases in bone turnover, leading to bone deformity, pathological fractures, and various other complications. Several rare disorders have also been described that show phenotypic overlap with PDB. Genome-wide searches have identified several susceptibility loci for PDB and PDB-like disorders, and mutations that cause these disorders have now been identified in four genes, all of which are involved in the RANK-NF-kappaB signaling pathway. Mutations in SQSTM1, which encodes an important scaffold protein in this pathway, have been found to be a common cause of classical PDB. Thus far, all disease-causing mutations in SQSTM1 affect the ubiquitin-associated (UBA) domain of the gene product and cause loss of ubiquitin binding. The rare PDB-like disorders of familial expansile osteolysis, early-onset familial PDB, and expansile skeletal hyperphosphatasia are caused by duplication mutations in exon 1 of the TNFRSF11A gene, which encodes the RANK receptor. This gene does not seem to be involved in the pathogenesis of classical PDB. Inactivating mutations in the TNFRSF11B gene, which encodes osteoprotegerin, cause juvenile PDB, and TNFRSF11B polymorphisms seem to increase the risk of classical PDB. The rare syndrome of hereditary inclusion body myopathy, PDB, and frontotemporal dementia (IBMPFD) is caused by mutations in the VCP gene, which is involved in regulating I-kappaB degradation by the proteasome. The disease-causing mutations in VCP cluster in and around a domain involved in ubiquitin binding. Whereas SQSTM1 has emerged as an important gene for classical PDB, most kindreds with familial PDB do not carry SQSTM1 mutations, indicating that additional genes for PDB remain to be discovered. In light of the molecular defects that have been identified thus far, it seems likely that these genes will also be involved in the RANK-NF-kappaB signaling pathway or its interactions with the ubiquitin-proteasome system.

Familial expansile osteolysis--not exclusively an adult disorder

Familial expansile osteolysis (FEO, MIM174810) is a rare syndrome which was observed world-wide in only three kinships and in two unrelated American individuals. We report a patient with familial expansile osteolysis from the Czech Republic, not related to the previously reported cases. This patient's extraordinary clinical course does not conform to the ordinary. Her radiographic bone involvement was unusually extensive, involving most of the peripheral skeleton and the skull. This case documents that familial expansile osteolysis is not only a disease of adults but does occur in childhood.

Paget's Disease of Bone and Genetic Disorders of RANKL/OPG/RANK/NF- B Signaling

Identification of the RANKL/OPG/RANK/NF-kB (receptor activator of nuclear factor kappa-B ligand / osteoprotegerin) signaling pathway as the major regulatory system for osteoclastogenesis began with discovery of these ligands and receptors in the tumor necrosis factor (TNF) superfamily. Subsequently, genetically altered mice revealed physiologic roles for these proteins in bone biology. However, full appreciation of their significance for the human skeleton came from clinical characterization of several extremely rare, heritable disorders followed by discovery of their genetic bases. Familial expansile osteolysis (FEO) is an autosomal dominant disorder featuring constitutive activation of RANK due to an 18-bp tandem duplication in its gene (TNFRSF11A). A similar, 27-bp duplication causes what has been called a familial form of early-onset Paget's disease of bone (PDB2). Expansile skeletal hyperphosphatasia (ESH) is allelic to FEO and PDB2 and involves a 15-bp tandem duplication in TNFRSF11A. Autosomal recessive inheritance of deactivating mutations of the gene encoding OPG (TNFRSF11B) causes most cases of juvenile Paget disease. These disorders feature high bone turnover, deafness during early childhood, "idiopathic external lysis" of adult teeth, and sometimes focal lesions in appendicular bones that mimic active PDB. Biochemical markers indicate rapid skeletal remodeling. In FEO, osteolysis progresses to fat-filled bone rather than to osteosclerosis. Antiresorptive therapy with bisphosphonates can be effective for each disorder.

Metabolic Bone Disease in Children

Metabolic bone disease in children includes many hereditary and acquired conditions of diverse etiology that lead to disturbed metabolism of the bone tissue. Some of these processes primarily affect bone; others are secondary to nutritional deficiencies, a variety of chronic disorders, and/or treatment with some drugs. Some of these disorders are rare, but some present public health concerns (for instance, rickets) that have been well known for many years but still persist. The most important clinical consequences of bone metabolic diseases in the pediatric population include reduced linear growth, bone deformations, and non-traumatic fractures leading to bone pain, deterioration of motor development and disability. In this article, we analyze primary and secondary osteoporosis, rickets, osteomalacia (nutritional and hereditary vitamin D-dependent, hypophosphatemic and that due to renal tubular abnormalities), renal osteodystrophy, sclerosing bony disorders, and some genetic bone diseases (hypophosphatasia, fibrous dysplasia, skeletal dysplasia, juvenile Paget disease, familial expansile osteolysis, and osteoporosis pseudoglioma syndrome). Early identification and treatment of potential risk factors is essential for skeletal health in adulthood. In most conditions it is necessary to ensure an appropriate diet, with calcium and vitamin D, and an adequate amount of physical activity as a means of prevention. In secondary bone diseases, treatment of the primary disorder is crucial. Most genetic disorders await prospective gene therapies, while bone marrow transplantation has been attempted in other disorders. At present, affected patients are treated symptomatically, frequently by interdisciplinary teams. The role of exercise and pharmacologic therapy with calcium, vitamin D, phosphate, bisphosphonates, calcitonin, sex hormones, growth hormone, and thiazides is discussed. The perspectives on future therapy with insulin-like growth factor-1, new analogs of vitamin D, strontium, osteoprotegerin, and calcimimetics are presented.

Short, local duplications in eukaryotic genomes

Short, local duplications lead to an increase in the local copy number of a 1-100 bp sequence motif. They are usually unstable and evolve rapidly. When they involve a functional sequence such as a transcription factor binding site or a protein-protein interaction domain, they can drive phenotypic diversity. Short, local duplications have been implicated in the dramatic morphological differences among different dog breeds, and in the differences in social structure between two sister species of voles. Several human diseases and disorders are also caused by this class of duplication, which encompasses microsatellites, minisatellites and doublets.

Genetics of Paget's disease of bone

PDB (Paget's disease of bone) is a common condition characterized by focal increases in bone turnover affecting one or more sites throughout the skeleton. Genetic factors are important in the pathogenesis of PDB and many families have been described where PDB is inherited in an autosomal-dominant fashion. Several candidate loci for susceptibility to PDB and related syndromes have been identified by genome-wide scans and recent evidence suggests that mutations in genes that encode components of the RANK [receptor activator of NF-κB (nuclear factor-κB)]/NF-κB signalling pathway play an important role in the pathogenesis of this group of diseases. Insertion mutations in the TNFRSF11A gene encoding RANK have been identified as the cause of familial expansile osteolysis, some cases of early onset PDB and expansile skeletal hyperphosphatasia. Inactivating mutations in the TNFRSF11B gene that encodes OPG (osteoprotegerin) have been found to cause the syndrome of juvenile PDB. Polymorphisms in OPG also appear to increase the risk of developing PDB. The most important causal gene for classical PDB is Sequestosome 1 (SQSTM1), which is a scaffold protein in the NF-κB signalling pathway, and mutations affecting the UBA (ubiquitin-associated) domain of this protein occur in between 20–50% of familial and 10–20% of sporadic PDB cases. The rare syndrome of IBMPFD (inclusion body myopathy, PDB and fronto-temporal dementia) is due to mutations in the VCP gene and these also cluster in the domain of VCP that interacts with ubiquitin, suggesting a common disease mechanism with SQSTM1-mediated PDB.

[Familial expansive osteolysis otological and dental manifestations of genetic origin]

OBJECTIVES

Familial Expansive Osteolysis (FEO) ist a rare autosomal dominant bone dysplasia. The disease can show general and focal skeletal alterations, the latter having a predominantly peripheral distribution. Onset occurs after the second decade of life.

PATIENTS AND METHODS

We present the study, of 30 years, of a family consisting of 49 members covering five generations.

RESULTS

Among the 35 members studied, 18 have familial expansive osteolysis (FEO). The first clinical sign of the condition is transmission deafness at an early age. The features of the teeth has a unique and characteristic appearance. Thinning of the cortical bone leads to severe, painful, disabling deformities. Serum alkaline phosphatase, and urinary hydroxyproline and deoxipyridinoline are elevated. Calcium and parathyroid hormone are normal. Treatment with diphosphonates, calcitonin and vitamin D has been unsuccessful. We present the surgical technology and the results to short and long term of 13 interventions on 8 patients.

CONCLUSION

Progressive osteoclastic reabsorption accompanied by weak osteoblastic activity results in medullary expansion characterized by rarefaction of the bone marrow, which is replaced by fibrous tissue and fat. FEO is histologically similar to Paget disease, but the age of onset, the distribution of the bone lesions, the dental and middle ear alterations, and the clinical progression are different. These features also differentiate FEO from fibrous dysplasia, fibrocystic osteitis and imperfect osteogenesis. The gene responsible for FEO is located in the 18q21-22 chromosome region. Mutations in TNFRSF11A, the gene encoding receptor activator of nuclear factor-kappa-B (RANK), has been recently identified as the cause of FEO. A duplication of 18 base pairs in exon 1 of the TNFRSF11A gene suggests that this corresponds to the site of the anomaly and can be considered a "hot spot" for mutations.

Osteoclast diseases and dental abnormalities

Tooth eruption depends on the presence of osteoclasts to create an eruption pathway through the alveolar bone. In diseases where osteoclast formation, or function is reduced, such as the various types of osteopetrosis, tooth eruption is affected. Diseases in which osteoclast formation or activity is increased, such as familiar expansile osteolysis and Paget's disease, are associated with dental abnormalities such as root resorption and premature tooth loss. Less is known about the origin of the dental problems in these conditions as there are no rodent models of these diseases as yet. In this short review, the genes currently known to be mutated in human osteoclast diseases will be reviewed and, where known, the effect of osteoclast dysfunction on dental development described. It will focus on human conditions and only mention rodent disease where no clear data in the human are available.

Susceptibility to Paget's disease of bone is influenced by a common polymorphic variant of osteoprotegerin

To clarify the role of the TNFRSF11B gene encoding osteoprotegerin (OPG), in Paget's disease of bone (PDB) we studied TNFRSF11B polymorphisms in an association study of 690 UK subjects and in a worldwide familial study of 66 kindreds. We found that the G1181 allele of TNFRSF11B, encoding lysine at codon 3 of the OPG protein, predisposes to both sporadic and familial PDB.

INTRODUCTION

Paget's disease of bone (PDB) is a common disorder characterized by focal abnormalities of bone turnover. Genetic factors are important in the pathogenesis of PDB, and studies have shown that inactivating mutations of the TNFRSF11B gene, encoding osteoprotegerin (OPG), cause the rare syndrome of juvenile Paget's disease. In this study, we sought to determine whether polymorphisms of the TNFRSF11B gene contribute to the pathogenesis of classical PDB.

MATERIALS AND METHODS

We screened for polymorphisms of the TNFRSF11B gene by DNA sequencing of the proximal promoter, coding exons, and intron-exon boundaries in 20 PDB patients and 10 controls. Informative single nucleotide polymorphisms (SNPs), including a G1181C SNP, which predicts a lysine-asparagine substitution at codon 3 of the OPG signal peptide and haplotypes, were related to the presence of PDB in 312 cases compared with 378 controls and to transmission of PDB in 140 affected offspring from 66 kindreds with familial PDB.

RESULTS AND CONCLUSIONS

The G1181 allele was significantly over-represented in PDB patients (chi(2) = 5.7, df = 1, p = 0.017, adjusted alpha = 0.024), equivalent to an odds ratio for PDB of 1.55 (95% CI: 1.11-2.16). The distribution of TNFRSF11B haplotypes significantly differed in sporadic PDB cases and controls (chi(2) = 30.2, df = 9, p < 0.001) because of over-representation of haplotypes containing the G1181 allele in cases. The family study showed that the most common haplotype containing the G1181 allele was transmitted more frequently than expected to 140 individuals with familial PDB (chi(2) = 7.35, df = 1, p < 0.01), and the transmission disequilibrium was even more pronounced in a subgroup of 78 familial PDB patients who did not carry mutations of the SQSTM1 gene (chi(2) = 8.44, df = 1, p < 0.005). We conclude that the G1181 allele of TNFRSF11B, encoding lysine at codon 3 of the OPG protein, predisposes to the development of sporadic PDB and familial PDB that is not caused by SQSTM1 mutations.

Osteoclast diseases

Osteoclasts are the only cells capable of resorbing mineralised bone, dentine and cartilage. Osteoclasts act in close concert with bone forming osteoblasts to model the skeleton during embryogenesis and to remodel it during later life. A number of inherited human conditions are known that are primarily caused by a defect in osteoclasts. Most of these are rare monogenic disorders, but others, such as the more common Paget's disease, are complex diseases, where genetic and environmental factors combine to result in the abnormal osteoclast phenotype. Where the genetic defect gives rise to ineffective osteoclasts, such as in osteopetrosis and pycnodysostosis, the result is the presence of too much bone. However, the phenotype in many osteoclast diseases is a combination of osteosclerosis with osteolytic lesions. In such conditions, the primary defect is hyperactivity of osteoclasts, compensated by a secondary increase in osteoblast activity. Rapid progress has been made in recent years in the identification of the causative genes and in the understanding of the biological role of the proteins encoded. This review discusses the known osteoclast diseases with particular emphasis on the genetic causes and the resulting osteoclast phenotype. These human diseases highlight the critical importance of specific proteins or signalling pathways in osteoclasts.

Phenotypic characterization of early onset Paget's disease of bone caused by a 27-bp duplication in the TNFRSF11A gene

Three different insertion mutations in the TNFRSF11A gene affecting the signal peptide of RANK have been described. An 18-bp duplication at position 84 (84dup18) is associated with the clinical syndrome of familial expansile osteolysis (FEO), whereas a 15-bp duplication at the same site (84dup15) causes the syndrome of expansile skeletal hyperphosphatasia (ESH). Here we report the phenotype of patients harboring a 27-bp duplication at position 75 (75dup27) in RANK. Affected individuals had hearing impairment and tooth loss beginning in the second or third decade. Radiographs of affected bones showed lytic and sclerotic lesions with bony enlargement and deformity. Serum alkaline phosphatase levels were elevated between 2 and 17 times above the normal range. Most patients had pelvic and skull involvement, and all had involvement of the mandible and maxilla. Most patients also had bony enlargement of the small joints of the hands, and one developed hypercalcemia during a period of immobilization. We conclude that the 75dup27 mutation of RANK causes a Paget's disease of bone-like phenotype that is distinct from, but which overlaps with, FEO and ESH. A particularly striking feature was involvement of the mandible and maxilla, but it remains to be seen if this is a specific feature of the 75dup27 mutation until further kindreds with this mutation are reported.

Osteoprotegerin deficiency and juvenile Paget's disease

BACKGROUND

Juvenile Paget's disease, an autosomal recessive osteopathy, is characterized by rapidly remodeling woven bone, osteopenia, fractures, and progressive skeletal deformity. The molecular basis is not known. Osteoprotegerin deficiency could explain juvenile Paget's disease because osteoprotegerin suppresses bone turnover by functioning as a decoy receptor for osteoclast differentiation factor (also called RANK ligand).

METHODS

We evaluated two apparently unrelated Navajo patients with juvenile Paget's disease for defects in the gene encoding osteoprotegerin (TNFRSF11B) using polymerase-chain-reaction (PCR) amplification followed by direct sequencing and Southern blotting of genomic DNA. Genetic markers near TNFRSF11B were evaluated by both a PCR method that involved sequence-tagged site-content mapping of a deletion of TNFRSF11B and PCR spanning the DNA break points.

RESULTS

Both patients had a homozygous deletion of TNFRSF11B, with identical break points, on chromosome 8q24.2. The defect spans approximately 100 kb, but neighboring genes are intact. We found that serum levels of osteoprotegerin and soluble osteoclast differentiation factor were undetectable and markedly increased, respectively.

CONCLUSIONS

Juvenile Paget's disease can result from osteoprotegerin deficiency caused by homozygous deletion of TNFRSF11B.

Expansile skeletal hyperphosphatasia is caused by a 15-base pair tandem duplication in TNFRSF11A encoding RANK and is allelic to familial expansile osteolysis

Expansile skeletal hyperphosphatasia (ESH) is a singular disorder characterized in the year 2000 in a mother and daughter with early-onset deafness, premature loss of teeth, progressive hyperostotic widening of long bones causing painful phalanges in the hands, accelerated bone remodeling, and episodic hypercalcemia likely inherited as a highly penetrant, autosomal dominant trait. Absence of large osteolytic lesions with cortical thinning in major long bones, together with bouts of hypercalcemia, indicated that ESH is not a variant of familial expansile osteolysis (FEO). Here, we investigated the molecular basis of ESH after three families with FEO were reported to have an identical 18-base pair tandem duplication (84dup18) in the signal peptide sequence of the TNFRSF11A gene that encodes receptor activator of nuclear factor-kappaB (RANK). We find that ESH is caused by a remarkably similar 15-base pair tandem duplication (84dup15) in TNFRSF11A. Hence, ESH and FEO are allelic diseases and ESH, like FEO, probably reflects increased activity in the skeleton of the RANK target, nuclear factor-kappaB (NF-kappaB).