Identification of the first deletion in the LRP5 gene in a patient with autosomal dominant osteopetrosis type I

Molecular Mechanisms of Craniofacial and Dental Abnormalities in Osteopetrosis

Osteopetrosis is a group of genetic bone disorders characterized by increased bone density and defective bone resorption. Osteopetrosis presents a series of clinical manifestations, including craniofacial deformities and dental problems. However, few previous reports have focused on the features of craniofacial and dental problems in osteopetrosis. In this review, we go through the clinical features, types, and related pathogenic genes of osteopetrosis. Then we summarize and describe the characteristics of craniofacial and dental abnormalities in osteopetrosis that have been published in PubMed from 1965 to the present. We found that all 13 types of osteopetrosis have craniomaxillofacial and dental phenotypes. The main pathogenic genes, such as chloride channel 7 gene (CLCN7), T cell immune regulator 1 (TCIRG1), osteopetrosis-associated transmembrane protein 1 (OSTM1), pleckstrin homology domain-containing protein family member 1 (PLEKHM1), and carbonic anhydrase II (CA2), and their molecular mechanisms involved in craniofacial and dental phenotypes, are discussed. We conclude that the telltale craniofacial and dental abnormalities are important for dentists and other clinicians in the diagnosis of osteopetrosis and other genetic bone diseases.

Novel mutation in LRP5 gene cause rare osteosclerosis: cases studies and literature review

To study the effects of low-density lipoprotein receptor-related protein 5 (LRP5) gene mutations on bone, and to open up our view of LRP5 and Wnt pathways on bone mass regulation. Three patients with increased bone mineral density or thickened bone cortex were included, who were 30-year-old, 22-year-old and 50-year-old men, respectively. The latter two patients were son and father of a same family. The characteristics of bone X-rays were evaluated in detail. Bone turnover markers were detected, such as procollagen type 1 amino-terminal peptide (P1NP), alkaline phosphatase (ALP), and type 1 collagen carboxyl terminal peptide (β-CTX). Dual energy X-ray absorptiometry (DXA) was used to measure the bone mineral density (BMD) at lumbar spine and proximal femur of the patients. The targeted next-generation sequencing (NGS) technology was used to detect pathogenic gene mutations, which were further verified by Sanger sequencing. Moreover, the gene mutation spectrum and phenotypic characteristics of reported patients with LRP5 gain-of-function mutations were summarized by reviewing the literature. The main characteristics of the first patient were headache, facial paralysis, high BMD (lumbar vertebrae 1–4: 1.877 g/cm2, Z-score: 5.8; total hip: 1.705 g/cm2, Z-score: 5.7), slightly increased P1NP (87.0 ng/mL) and β-CTX (0.761 ng/mL) level, and with thickened bone cortex, especially the cranial vault. The latter two patients showed enlargement of the mandible and enlarged osseous prominence of the tours palatinus. X-rays showed that the bone cortex of skull and long bones were thickened. The bone turnover markers and BMD were normal. All three cases carried novel missense mutations in LRP5 gene, which were mutation in exon 3 (c.586 T > G, p.Trp196Gly) of the first patient, and mutation in exon 20 (c.4240C > A, p.Arg1414Ser) of the latter two patients. Combined with the reported literature, a total of 19 gain-of-function mutations in LRP5 were detected in 113 patients from 33 families. Hotspot mutations included c.724G > A, c.512G > T and c.758C > T. Furthermore, mutations in the exon 3 of LRP5 may cause severe phenotypes. LRP5 gain-of-function mutations can lead to rare autosomal dominant osteosclerosis type Ι (ADO Ι), which was characterized by increased bone mass and thickened bone cortex. In-depth research on the Wnt pathway will be benefit for discovering important mechanisms of bone mass regulation.

Identification of Compound Heterozygous Variants in LRP4 Demonstrates That a Pathogenic Variant outside the Third β-Propeller Domain Can Cause Sclerosteosis

Sclerosteosis is a high bone mass disorder, caused by pathogenic variants in the genes encoding sclerostin or LRP4. Both proteins form a complex that strongly inhibits canonical WNT signaling activity, a pathway of major importance in bone formation. So far, all reported disease-causing variants are located in the third β-propeller domain of LRP4, which is essential for the interaction with sclerostin. Here, we report the identification of two compound heterozygous variants, a known p.Arg1170Gln and a novel p.Arg632His variant, in a patient with a sclerosteosis phenotype. Interestingly, the novel variant is located in the first β-propeller domain, which is known to be indispensable for the interaction with agrin. However, using luciferase reporter assays, we demonstrated that both the p.Arg1170Gln and the p.Arg632His variant in LRP4 reduced the inhibitory capacity of sclerostin on canonical WNT signaling activity. In conclusion, this study is the first to demonstrate that a pathogenic variant in the first β-propeller domain of LRP4 can contribute to the development of sclerosteosis, which broadens the mutational spectrum of the disorder.

An Extremely Rare, Atypical and Genetically-undetermined Form of Osteopetrosis

INTRODUCTION

Osteopetrosis is an uncommon skeletal disorder characterized by generalized sclerosis of bones due to defective osteoclast function. A wide variation in clinical severity of the disease has been observed. Radiographic features and genetic testing are commonly used to diagnose the condition.

CASE PRESENTATION

In the present study, we present a case of an extremely rare, atypical and genetically- undetermined form of Osteopetrosis.

CONCLUSION

This patient had some clinical and radiological features of craniometaphyseal dysplasia along with atypical radiological signs of osteopetrosis.

The Genetic Architecture of High Bone Mass

The phenotypic trait of high bone mass (HBM) is an excellent example of the nexus between common and rare disease genetics. HBM may arise from carriage of many 'high bone mineral density [BMD]'-associated alleles, and certainly the genetic architecture of individuals with HBM is enriched with high BMD variants identified through genome-wide association studies of BMD. HBM may also arise as a monogenic skeletal disorder, due to abnormalities in bone formation, bone resorption, and/or bone turnover. Individuals with monogenic disorders of HBM usually, though not invariably, have other skeletal abnormalities (such as mandible enlargement) and thus are best regarded as having a skeletal dysplasia rather than just isolated high BMD. A binary etiological division of HBM into polygenic vs. monogenic, however, would be excessively simplistic: the phenotype of individuals carrying rare variants of large effect can still be modified by their common variant polygenic background, and by the environment. HBM disorders-whether predominantly polygenic or monogenic in origin-are not only interesting clinically and genetically: they provide insights into bone processes that can be exploited therapeutically, with benefits both for individuals with these rare bone disorders and importantly for the many people affected by the commonest bone disease worldwide-i.e., osteoporosis. In this review we detail the genetic architecture of HBM; we provide a conceptual framework for considering HBM in the clinical context; and we discuss monogenic and polygenic causes of HBM with particular emphasis on anabolic causes of HBM.

Osteopetroses, emphasizing potential approaches to treatment

Osteopetroses are a heterogeneous group of rare genetic bone diseases sharing the common hallmarks of reduced osteoclast activity, increased bone mass and high bone fragility. Osteoclasts are bone resorbing cells that contribute to bone growth and renewal through the erosion of the mineralized matrix. Alongside the bone forming activity by osteoblasts, osteoclasts allow the skeleton to grow harmonically and maintain a healthy balance between bone resorption and formation. Osteoclast impairment in osteopetroses prevents bone renewal and deteriorates bone quality, causing atraumatic fractures. Osteopetroses vary in severity and are caused by mutations in a variety of genes involved in bone resorption or in osteoclastogenesis. Frequent signs and symptoms include osteosclerosis, deformity, dwarfism and narrowing of the bony canals, including the nerve foramina, leading to hematological and neural failures. The disease is autosomal, with only one extremely rare form associated so far to the X-chromosome, and can have either recessive or dominant inheritance. Recessive ostepetroses are generally lethal in infancy or childhood, with a few milder forms clinically denominated intermediate osteopetroses. Dominant osteopetrosis is so far associated only with mutations in the CLCN7 gene and, although described as a benign form, it can be severely debilitating, although not at the same level as recessive forms, and can rarely result in reduced life expectancy. Severe osteopetroses due to osteoclast autonomous defects can be treated by Hematopoietic Stem Cell Transplant (HSCT), but those due to deficiency of the pro-osteoclastogenic cytokine, RANKL, are not suitable for this procedure. Likewise, it is unclear as to whether HSCT, which has high intrinsic risks, results in clinical improvement in autosomal dominant osteopetrosis. Therefore, there is an unmet medical need to identify new therapies and studies are currently in progress to test gene and cell therapies, small interfering RNA approach and novel pharmacologic treatments.

Idiopathic Acquired Osteosclerosis in a Middle-Aged Woman With Systemic Lupus Erythematosus

Widely distributed osteosclerosis is an unusual radiographic finding with multiple causes. A 42-year-old premenopausal Spanish woman gradually acquired dense bone diffusely affecting her axial skeleton and focally affecting her proximal long bones. Systemic lupus erythematosus (SLE) diagnosed in adolescence had been well controlled. She had not fractured or received antiresorptive therapy, and she was hepatitis C virus antibody negative. Family members had low bone mass. Lumbar spine bone mineral density (BMD) measured by dual-photon absorptiometry (DPA) at age 17 years, while receiving glucocorticoids, was 79% the average value of age-matched controls. From ages 30 to 37 years, dual-energy X-ray absorptiometry (DXA) BMD Z-scores steadily increased in her lumbar spine from +3.8 to +7.9, and in her femoral neck from -1.4 to -0.7. Serum calcium and phosphorus levels were consistently normal, 25-hydroxyvitamin D (25OHD) <20 ng/mL, and parathyroid hormone (PTH) sometimes slightly increased. Her reduced estimated glomerular filtration rate (eGFR) was 38 to 55 mL/min. Hypocalciuria likely reflected positive mineral balance. During increasing BMD, turnover markers (serum bone-specific alkaline phosphatase [ALP], procollagen type 1 N propeptide [P1NP], osteocalcin [OCN], and carboxy-terminal cross-linking telopeptide of type 1 collagen [CTx], and urinary amino-terminal cross-linking telopeptide of type 1 collagen [NTx and CTx]) were 1.6- to 2.8-fold above the reference limits. Those of bone formation seemed increased more than those of resorption. FGF-23 was slightly elevated, perhaps from kidney disease. Serum osteoprotegerin (OPG) and TGFβ1 levels were normal, but sclerostin (SOST) and receptor activator of nuclear factor kappa-B ligand (RANKL) were elevated. Serum multiplex biomarker profiling confirmed a high level of SOST and RANKL, whereas Dickkopf-1 (DKK-1) seemed low. Matrix metalloproteinases-3 (MMP-3) and -7 (MMP-7) were elevated. Iliac crest biopsy revealed tetracycline labels, no distinction between thick trabeculae and cortical bone, absence of peritrabecular fibrosis, few osteoclasts, and no mastocytosis. Then, for the past 3 years, BMD Z-scores steadily decreased. Skeletal fluorosis, mastocytosis, myelofibrosis, hepatitis C-associated osteosclerosis, multiple myeloma, and aberrant phosphate homeostasis did not explain her osteosclerosis. Mutation analysis of the LRP5, LRP4, SOST, and osteopetrosis genes was negative. Microarray showed no notable copy number variation. Perhaps her osteosclerosis reflected an interval of autoimmune-mediated resistance to SOST and/or RANKL. © 2016 American Society for Bone and Mineral Research.

A Novel Domain-Specific Mutation in a Sclerosteosis Patient Suggests a Role of LRP4 as an Anchor for Sclerostin in Human Bone

Mutations in the LRP4 gene, coding for a Wnt signaling coreceptor, have been found to cause several allelic conditions. Among these, two are characterized by a strong skeletal involvement, namely sclerosteosis and Cenani-Lenz syndrome. In this work, we evaluated the role of LRP4 in the pathophysiology of these diseases. First, we report a novel LRP4 mutation, leading to the substitution of arginine at position 1170 in glutamine, identified in a patient with sclerosteosis. This mutation is located in the central cavity of the third β-propeller domain, which is in line with two other sclerosteosis mutations we previously described. Reporter assays demonstrate that this mutation leads to impaired sclerostin inhibition of Wnt signaling. Moreover, we compared the effect of this novel variant to mutations causing Cenani-Lenz syndrome and show that impaired membrane trafficking of the LRP4 protein is the likely mechanism underlying Cenani-Lenz syndrome. This is in contrast to sclerosteosis mutations, previously shown to impair the binding between LRP4 and sclerostin. In addition, to better understand the biology of LRP4, we investigated the circulating sclerostin levels in the serum of a patient suffering from sclerosteosis owing to a LRP4 mutation. We demonstrate that impaired sclerostin binding to the mutated LRP4 protein leads to dramatic increase in circulating sclerostin in this patient. With this study, we provide the first evidence suggesting that LRP4 is responsible for the retention of sclerostin in the bone environment in humans. These findings raise potential concerns about the utility of determining circulating sclerostin levels as a marker for other bone-related parameters. Although more studies are needed to fully understand the mechanism whereby LRP4 facilitates sclerostin action, it is clear that this protein represents a potent target for future osteoporosis therapies and an interesting alternative for the antisclerostin treatment currently under study.

Sclerostin, an emerging therapeutic target for treating osteoporosis and osteoporotic fracture: A general review

Osteoporosis and its associated fracture risk has become one of the major health burdens in our aging population. Currently, bisphosphonate, one of the most popular antiresorptive drugs, is used widely to treat osteoporosis but so far still no consensus has been reached for its application in treatment of osteoporotic fractures. However, in old patients, boosting new bone formation and its remodelling is essential for bone healing in age-related osteoporosis and osteoporotic fractures. Sclerostin, an inhibitor of the Wnt/β-catenin signalling pathway that regulates bone growth, has become an attractive therapeutic target for treating osteoporosis. In this review, we summarize the recent findings of sclerostin and its potential as an effective drug target for treating both osteoporosis and osteoporotic fractures.

Altered Expression of Wnt Signaling Pathway Components in Osteogenesis of Mesenchymal Stem Cells in Osteoarthritis Patients

Introduction

Osteoarthritis (OA) is characterized by altered homeostasis of joint cartilage and bone, whose functional properties rely on chondrocytes and osteoblasts, belonging to mesenchymal stem cells (MSCs). WNT signaling acts as a hub integrating and crosstalking with other signaling pathways leading to the regulation of MSC functions. The aim of this study was to evaluate the existence of a differential signaling between Healthy and OA-MSCs during osteogenesis.

Methods

MSCs of seven OA patients and six healthy controls were isolated, characterised and expanded. During in vitro osteogenesis, cells were recovered at days 1, 10 and 21. RNA and protein content was obtained. Expression of WNT pathway genes was evaluated using RT-qPCR. Functional studies were also performed to study the MSC osteogenic commitment and functional and post-traslational status of β-catenin and several receptor tyrosine kinases.

Results

Several genes were downregulated in OA-MSCs during osteogenesis in vitro. These included soluble Wnts, inhibitors, receptors, co-receptors, several kinases and transcription factors. Basal levels of β-catenin were higher in OA-MSCs, but calcium deposition and expression of osteogenic genes was similar between Healthy and OA-MSCs. Interestingly an increased phosphorylation of p44/42 MAPK (ERK1/2) signaling node was present in OA-MSCs.

Conclusion

Our results point to the existence in OA-MSCs of alterations in expression of Wnt pathway components during in vitro osteogenesis that are partially compensated by post-translational mechanisms modulating the function of other pathways. We also point the relevance of other signaling pathways in OA pathophysiology suggesting their role in the maintenance of joint homeostasis through modulation of MSC osteogenic potential.

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.

Rare diseases in clinical endocrinology: a taxonomic classification system

PURPOSE

Rare endocrine-metabolic diseases (REMD) represent an important area in the field of medicine and pharmacology. The rare diseases of interest to endocrinologists involve all fields of endocrinology, including rare diseases of the pituitary, thyroid and adrenal glands, paraganglia, ovary and testis, disorders of bone and mineral metabolism, energy and lipid metabolism, water metabolism, and syndromes with possible involvement of multiple endocrine glands, and neuroendocrine tumors. Taking advantage of the constitution of a study group on REMD within the Italian Society of Endocrinology, consisting of basic and clinical scientists, a document on the taxonomy of REMD has been produced.

METHODS AND RESULTS

This document has been designed to include mainly REMD manifesting or persisting into adulthood. The taxonomy of REMD of the adult comprises a total of 166 main disorders, 338 including all variants and subtypes, described into 11 tables.

CONCLUSIONS

This report provides a complete taxonomy to classify REMD of the adult. In the future, the creation of registries of rare endocrine diseases to collect data on cohorts of patients and the development of common and standardized diagnostic and therapeutic pathways for each rare endocrine disease is advisable. This will help planning and performing intervention studies in larger groups of patients to prove the efficacy, effectiveness, and safety of a specific treatment.

[Autosomal dominant osteopetrosis: a presentation of 3 cases and a new gene mutation]

Osteopetrosis (OP) is a congenital bone disease which is caused by a functional disorder in osteoclasts with inability for normal bone resorption, leading to increased bone mineral density and bone sclerosis. It can be classified into different groups according to their clinical and their genetic characteristics: autosomal recessive with several subtypes (OPTB) or autosomal dominant type 1 or 2 (OPTA1-2). There is a wide clinical variability of the disease, from asymptomatic to lethal in the first months of life, with variable expressivity in the family members. Diagnosis is mainly clinical with genetic confirmation of the OP, and treatment is symptomatic. Three cases of OP are presented, with the discovery of a new gene mutation in LRP5 which caused OPTA1 in one of them.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

No mutations in the serotonin related TPH1 and HTR1B genes in patients with monogenic sclerosing bone disorders

Since the identification of LRP5 as the causative gene for the osteoporosis pseudoglioma syndrome (OPPG) as well as the high bone mass (HBM) phenotype, LRP5 and the Wnt/β-catenin signaling have been extensively studied for their role in the differentiation and proliferation of osteoblasts, in the apoptosis of osteoblasts and osteocytes and in the response of bone to mechanical loading. However, more recently the direct effect of LRP5 on osteoblasts and bone formation has been questioned. Gene expression studies showed that mice lacking lrp5 have increased expression of tph1, the rate limiting enzyme for the production of serotonin in the gut. Furthermore mice lacking either tph1 or htr1B, the receptor for serotonin on the osteoblasts, were reported to have an increased bone mass due to increased bone formation. This led to the still controversial hypothesis that LRP5 influences bone formation indirectly by regulating the expression of thp1 and as a consequence influencing the production of serotonin in the gut. Based on these data we decided to evaluate the role of TPH1 and HTR1B in the development of craniotubular hyperostoses, a group of monogenic sclerosing bone dysplasias. We screened the coding regions of both genes in 53 patients lacking a mutation in the known causative genes LRP5, LRP4 and SOST. We could not find disease-causing coding variants in neither of the tested genes and therefore, we cannot provide support for an important function of TPH1 and HTR1B in the pathogenesis of sclerosing bone dysplasias in our tested patient cohort.

WNT signaling in bone homeostasis and disease: from human mutations to treatments

Low bone mass and strength lead to fragility fractures, for example, in elderly individuals affected by osteoporosis or children with osteogenesis imperfecta. A decade ago, rare human mutations affecting bone negatively (osteoporosis-pseudoglioma syndrome) or positively (high-bone mass phenotype, sclerosteosis and Van Buchem disease) have been identified and found to all reside in components of the canonical WNT signaling machinery. Mouse genetics confirmed the importance of canonical Wnt signaling in the regulation of bone homeostasis, with activation of the pathway leading to increased, and inhibition leading to decreased, bone mass and strength. The importance of WNT signaling for bone has also been highlighted since then in the general population in numerous genome-wide association studies. The pathway is now the target for therapeutic intervention to restore bone strength in millions of patients at risk for fracture. This paper reviews our current understanding of the mechanisms by which WNT signalng regulates bone homeostasis.

Mutations in sFRP1 or sFRP4 are not a common cause of craniotubular hyperostosis

Sclerosing bone dysplasias are a heterogeneous group of rare diseases marked by increased BMD caused by either increased bone formation or by decreased bone resorption. In this study we have focused on craniotubular hyperostoses mainly affecting the long bones and the skull. Currently, there are three causative genes identified namely LRP5, SOST and LRP4. All three genes are involved in the canonical Wnt signalling pathway. These findings support the role of this pathway in regulating bone formation. The secreted Frizzled related proteins (sFRPs) can modulate the Wnt signalling pathway by binding to Wnt ligands or Frizzled receptors. Studies using mice showed that two members of this family, sFRP1 and sFRP4, have an important effect on bone formation. Sfrp1-/- mice have increased BMD values especially after peak BMD was reached. On the contrary, sfrp4 overexpression mice exhibit reduced BMD. Therefore, we selected sFRP1 and sFRP4, two members of the secreted Frizzled related protein (sFRP) family, as candidate genes for mutation analysis in patients with craniotubular hyperostosis. Using Sanger sequencing we screened the exons and intron/exon boundaries of sFRP1 and sFRP4 in 53 patients. In all patients mutations in LRP5, SOST and LRP4 were excluded. We identified two unknown heterozygous variants both in sFRP1. The first variant in sFRP1 is an intronic variant which, according to prediction programs, does not affect the splicing of the gene. The second variant (p.Trp131Arg/-) was identified in a young boy whose healthy mother does not carry the variant. In conclusion, our studies indicate that mutations neither in sFRP1 nor in sFRP4 are a common cause of craniotubular hyperostoses. As a consequence, further research will be necessary to identify the disease causing gene(s) in this group of patients.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

LRP5 and bone mass regulation: Where are we now?

The discovery of causal mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene underlying conditions of altered bone mass ushered in a new era in bone research. Since those original publications, the role of Lrp5 and the Wnt/β-catenin signaling pathway controlled by Lrp5 and its homologs, Lrp6 and Lrp4, in bone mass regulation has been an intense area of investigation. Studies to date have implicated this pathway in skeletal development, osteoblast differentiation and proliferation, osteoblast/osteocyte apoptosis, regulation of the balance between osteogenesis-chondrogenesis-adipogenesis, regulation of osteoclastogenesis and the response of bone to mechanical loading. Interestingly, the data from knockout and transgenic mice involving Lrp4/5/6 and/or their regulators, as well as β-catenin signaling pathway components, and in vitro studies have sometimes yielded conflicting results. Adding to the complexity of the system are the studies that suggested Lrp5 regulated bone mass through a gut-bone endocrine signaling system involving Lrp5 mediated control of gut serotonin synthesis. However, recent studies have called this into question and so this provocative concept remains an open question. Clearly, the manipulation of Lrp5/Wnt/β-catenin pathway presents as a major target for drug development to treat diseases of low bone mass such as osteoporosis and these new therapies are in full progress. At present, although it is clear that Lrp5 has a role in bone mass regulation, much of the details remain to be elucidated and this is a major and exciting challenge for future studies.