Genetics of Osteopetrosis

A comprehensive report of the clinical and mutational profiles of 30 Iranian malignant infantile osteopetrosis patients

Osteopetrosis is a group of genetically and clinically diverse inherited disorders characterized by an increase in bone density. The main known cause is an abnormality in the development or function of osteoclasts. Hence, the process of bone resorption is impaired, resulting in: 1- a reduction in bone marrow volume and, subsequently, a decrement in the hematopoietic capacity of bone marrow, which leads to anemia and compromised immunological function; 2- improper bone development, which leads to pressure on peripheral nerves, causing auditory, visual, and movement impairments; and 3- disturbance in the formation of bone microstructure that leads to susceptibility to bone fracture. This study aimed to evaluate the clinical symptoms and genetic causes of 30 patients (probands) who suffered from malignant infantile osteopetrosis, a subtype of this disorder. The Sanger sequencing technique was used to sequence four common genes (TCIRG1, CLCN7, SNX10, and OSTM1) in osteopetrosis. Subsequently, the selected variants were subjected to segregation analysis between the probands and their parents. Consequently, the sequencing of these four genes in probands revealed 16 pathogenic and likely pathogenic mutations, five of which had never been reported before. The TCIRG1 gene has three novel splice site variations and one frameshift variant. The CLCN7 gene had a novel missense variant. Also, a total of five variants of uncertain significance (VUSs) were identified in the analyzed sequences, of which three haven't been reported to date, and two were observed in osteopetrosis patients. Therefore, by documenting these novel likely pathogenic variants and VUS in known genes associated with this disease in patients, specialists can conduct more accurate genetic analysis and counseling when encountering these variants. Additionally, this documentation will facilitate the reclassification of these variants.

FAM98 Family Proteins Play Distinct Roles in Osteoclastogenesis and Bone Resorption

There are three FAM98 family proteins (FAM98A/B/C) in humans and mice. Their physiological functions remain largely unknown. We have previously reported that Fam98a interacts with Plekhm1 in murine osteoclasts and functions in lysosome trafficking/secretion and bone resorption in osteoclasts in vitro. In this study, we found that all three Fam98 genes were expressed in precursor and mature osteoclasts. While the knockdown of Fam98c by a specific short-hairpin RNA (shRNA) in osteoclast precursors attenuated osteoclastogenesis, depletion of Fam98b by an shRNA specifically disrupted osteoclast lysosome trafficking and bone resorption with phenotypes similar to Fam98a shRNA-knockdown in our previous study. Loss of Fam98a in myeloid osteoclast precursors was dispensable for trabecular and cortical bone mass in mice, as well as osteoclastogenesis/bone resorption in vitro, possibly due to compensation by increased Fam98b expression in Fam98a-null osteoclasts. These findings indicate that the three Fam98 proteins play distinct roles in osteoclastogenesis and osteoclast function and need further investigation in future studies.

Unlocking the potential of histone modification in regulating bone metabolism

Bone metabolism plays a crucial role in maintaining normal bone tissue homeostasis and function. Imbalances between bone formation and resorption can lead to osteoporosis, osteoarthritis, and other bone diseases. The dynamic and complex process of bone remodeling is driven by various factors, including epigenetics. Histone modification, one of the most important and well-studied components of epigenetic regulation, has emerged as a promising area of research in bone metabolism. Different histone proteins and modification sites exert diverse effects on osteogenesis and osteoclastogenesis. In this review, we summarize recent progress in understanding histone modifications in bone metabolism, including specific modification sites and potential regulatory enzymes. Comprehensive knowledge of histone modifications in bone metabolism could reveal new therapeutic targets and treatment strategies for bone diseases.

SNX10 regulates osteoclastogenic cell fusion and osteoclast size in mice

Bone-resorbing osteoclasts (OCLs) are formed by differentiation and fusion of monocyte precursor cells, generating large multinucleated cells. Tightly regulated cell fusion during osteoclastogenesis leads to formation of resorption-competent OCLs, whose sizes fall within a predictable physiological range. The molecular mechanisms that regulate the onset of OCL fusion and its subsequent arrest are, however, largely unknown. We have previously shown that OCLs cultured from mice homozygous for the R51Q mutation in the vesicle trafficking-associated protein sorting nexin 10, a mutation that induces autosomal recessive osteopetrosis in humans and in mice, display deregulated and continuous fusion that generates gigantic, inactive OCLs. Fusion of mature OCLs is therefore arrested by an active, genetically encoded, cell-autonomous, and SNX10-dependent mechanism. To directly examine whether SNX10 performs a similar role in vivo, we generated SNX10-deficient (SKO) mice and demonstrated that they display massive osteopetrosis and that their OCLs fuse uncontrollably in culture, as do homozygous R51Q SNX10 (RQ/RQ) mice. OCLs that lack SNX10 exhibit persistent presence of DC-STAMP protein at their periphery, which may contribute to their uncontrolled fusion. To visualize endogenous SNX10-mutant OCLs in their native bone environment, we genetically labeled the OCLs of WT, SKO, and RQ/RQ mice with enhanced Green Fluorescent Protein (EGFP), and then visualized the 3D organization of resident OCLs and the pericellular bone matrix by 2-photon, confocal, and second harmonics generation microscopy. We show that the volumes, surface areas and, in particular, the numbers of nuclei in the OCLs of both mutant strains were on average 2-6-fold larger than those of OCLs from WT mice, indicating that deregulated, excessive fusion occurs in the mutant mice. We conclude that the fusion of OCLs, and consequently their size, is regulated in vivo by SNX10-dependent arrest of fusion of mature OCLs.

Rankl genetic deficiency and functional blockade undermine skeletal stem and progenitor cell differentiation

BACKGROUND

Skeletal Stem Cells (SSCs) are required for skeletal development, homeostasis, and repair. The perspective of their wide application in regenerative medicine approaches has supported research in this field, even though so far results in the clinic have not reached expectations, possibly due also to partial knowledge of intrinsic, potentially actionable SSC regulatory factors. Among them, the pleiotropic cytokine RANKL, with essential roles also in bone biology, is a candidate deserving deep investigation.

METHODS

To dissect the role of the RANKL cytokine in SSC biology, we performed ex vivo characterization of SSCs and downstream progenitors (SSPCs) in mice lacking Rankl (Rankl-/-) by means of cytofluorimetric sorting and analysis of SSC populations from different skeletal compartments, gene expression analysis, and in vitro osteogenic differentiation. In addition, we assessed the effect of the pharmacological treatment with the anti-RANKL blocking antibody Denosumab (approved for therapy in patients with pathological bone loss) on the osteogenic potential of bone marrow-derived stromal cells from human healthy subjects (hBMSCs).

RESULTS

We found that, regardless of the ossification type of bone, osteochondral SSCs had a higher frequency and impaired differentiation along the osteochondrogenic lineage in Rankl-/- mice as compared to wild-type. Rankl-/- mice also had increased frequency of committed osteochondrogenic and adipogenic progenitor cells deriving from perivascular SSCs. These changes were not due to the peculiar bone phenotype of increased density caused by lack of osteoclast resorption (defined osteopetrosis); indeed, they were not found in another osteopetrotic mouse model, i.e., the oc/oc mouse, and were therefore not due to osteopetrosis per se. In addition, Rankl-/- SSCs and primary osteoblasts showed reduced mineralization capacity. Of note, hBMSCs treated in vitro with Denosumab had reduced osteogenic capacity compared to control cultures.

CONCLUSIONS

We provide for the first time the characterization of SSPCs from mouse models of severe recessive osteopetrosis. We demonstrate that Rankl genetic deficiency in murine SSCs and functional blockade in hBMSCs reduce their osteogenic potential. Therefore, we propose that RANKL is an important regulatory factor of SSC features with translational relevance.

Molecular Heterogeneity of Osteopetrosis in India: Report of 17 Novel Variants

Osteopetrosis is a clinically and genetically heterogeneous group of inherited bone disorders that is caused by defects in osteoclast formation or function. Treatment options vary with the disease severity and an accurate molecular diagnosis helps in prognostication and treatment decisions. We investigated the genetic causes of osteopetrosis in 31 unrelated patients of Indian origin. Screening for the genetic variants was done by Sanger sequencing or next generation sequencing in 48 samples that included 31 samples from index patients, 16 from parents' and 1 chorionic villus sample. A total of 30 variants, including 29 unique variants, were identified in 26 of the 31 patients in the study. TCIRG1 was the most involved gene (n = 14) followed by TNFRSF11A (n = 4) and CLCN7 (n = 3). A total of 17 novel variants were identified. Prenatal diagnosis was done in one family and the foetus showed homozygous c.807 + 2T > G variant in TCIRG1. Molecular diagnosis of osteopetrosis aids in therapeutic decisions including the need for a stem cell transplantation and gives a possible option of performing prenatal diagnosis in affected families. Further studies would help in understanding the genetic etiology in patients where no variants were identified.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12288-023-01732-4.

Osteopetrosis and related osteoclast disorders in adults: A review and knowledge gaps On behalf of the European calcified tissue society and ERN BOND

Osteopetrosis refers to a group of related rare bone diseases characterized by a high bone mass due to impaired bone resorption by osteoclasts. Despite the high bone mass, skeletal strength is compromised and the risk of fracture is high, particularly in the long bones. Osteopetrosis was classically categorized by inheritance pattern into autosomal recessive forms (ARO), which are severe and diagnosed within the first years of life, an intermediate form and an autosomal dominant (ADO) form; the latter with variable clinical severity and typically diagnosed during adolescence or in young adulthood. Subsequently, the AD form was shown to be a result of mutations in the gene CLCN7 encoding for the ClC-7 chloride channel). Traditionally, the diagnosis of osteopetrosis was made on radiograph appearance alone, but recent molecular and genetic advances have enabled a greater fidelity in classification of osteopetrosis subtypes. In the more severe ARO forms (e.g., malignant infantile osteopetrosis MIOP) typical clinical features have severe consequences and often result in death in early childhood. Major complications of ADO are atypical fractures with delay or failure of repair and challenge in orthopedic management. Bone marrow failure, dental abscess, deafness and visual loss are often underestimated and neglected in relation with lack of awareness and expertise. Accordingly, the care of adult patients with osteopetrosis requires a multidisciplinary approach ideally in specialized centers. Apart from hematopoietic stem cell transplantation in certain infantile forms, the treatment of patients with osteopetrosis, has not been standardized and remains supportive. Further clinical studies are needed to improve our knowledge of the natural history, optimum management and impact of osteopetrosis on the lives of patients living with the disorder.

Molecular and functional mapping of Plekhm1-Rab7 interaction in osteoclasts

Mutations in PLEKHM1 cause osteopetrosis in humans and rats. The germline and osteoclast conditional deletions of Plekhm1 gene in mice lead to defective osteoclast bone resorption and increased trabecular bone mass without overt abnormalities in other organs. As an adaptor protein, pleckstrin homology and RUN domain containing M1 (PLEKHM1) interacts with the key lysosome regulator small GTPase RAB7 via its C-terminal RUBICON homologous (RH) domain. In this study, we have conducted a structural-functional study of the PLEKHM1 RH domain and RAB7 interaction in osteoclasts in vitro. The single mutations of the key residues in the Plekhm1 RH predicted from the crystal structure of the RUBICON RH domain and RAB7 interface failed to disrupt the Plekhm1-Rab7 binding, lysosome trafficking, and bone resorption. The compound alanine mutations at Y949-R954 and L1011-I1018 regions decreased Plekhm1 protein stability and Rab7-binding, respectively, thereby attenuated lysosome trafficking and bone resorption in osteoclasts. In contrast, the compound alanine mutations at R1060-Q1068 region were dispensable for Rab7-binding and Plekhm1 function in osteoclasts. These results indicate that the regions spanning Y949-R954 and L1011-I1018 of Plekhm1 RH domain are functionally important for Plekhm1 in osteoclasts and offer the therapeutic targets for blocking bone resorption in treatment of osteoporosis and other metabolic bone diseases.

Osteopetrosis complicated by multilevel spondylolysis

Osteopetrosis is a heterogenous group of inheritable disorders which manifests as increased bone density and brittleness. The most common and mildest variant typically presents in adulthood with bone pain and pathologic fractures, including spondylolysis. We present the case of an otherwise healthy, active 17-year-old male with a history of osteopetrosis and 1 year of chronic back pain, found to have multilevel (L1-L4) spondylolysis in the setting of severe diffuse bony sclerosis consistent with osteopetrosis. While single-level spondylolysis is an uncommon complication of osteopetrosis, multilevel spondylolysis in the pediatric population is extremely rare and the genetics of prior cases studies have not been reported. Spondylolysis should be considered as one of the types of fractures that may occur in patients with osteopetrosis.

Dental abnormalities in rare genetic bone diseases: Literature review

Currently, over 500 rare genetic bone disorders are identified. These diseases are often accompanied by dental abnormalities, which are sometimes the first clue for an early diagnosis. However, not many dentists are sufficiently familiar with phenotypic abnormalities and treatment approaches when they encounter patients with rare diseases. Such patients often need dental treatment but have difficulties in finding a dentist who can treat them appropriately. Herein we focus on major dental phenotypes and summarize their potential causes and mechanisms, if known. We discuss representative diseases, dental treatments, and their effect on the oral health of patients and on oral health-related quality of life. This review can serve as a starting point for dentists to contribute to early diagnosis and further investigate the best treatment options for patients with rare disorders, with the goal of optimizing treatment outcomes.

Osteoclast Methods in Protein Phosphatase Research

Osteoclasts are specialized cells that degrade bone and are essential for bone formation and maintaining bone homeostasis. Excess or deficient activity of these cells can significantly alter bone mass, structure, and physical strength, leading to significant morbidity, as in osteoporosis or osteopetrosis, among many other diseases. Protein phosphorylation in osteoclasts plays critical roles in the signaling pathways that govern the production of osteoclasts and regulate their bone-resorbing activity. In this chapter, we describe the isolation of mouse splenocytes and their differentiation into mature osteoclasts on resorptive (e.g., bone) and non-resorptive (e.g., plastic or glass) surfaces, examining matrix resorption by osteoclasts, immunofluorescence staining of these cells, and knocking out genes by CRISPR in the mouse osteoclastogenic cell line RAW264.7.

Metabolic bone disorders and the promise of marine osteoactive compounds

Metabolic bone disorders and associated fragility fractures are major causes of disability and mortality worldwide and place an important financial burden on the global health systems. These disorders result from an unbalance between bone anabolic and resorptive processes and are characterized by different pathophysiological mechanisms. Drugs are available to treat bone metabolic pathologies, but they are either poorly effective or associated with undesired side effects that limit their use. The molecular mechanism underlying the most common metabolic bone disorders, and the availability, efficacy, and limitations of therapeutic options currently available are discussed here. A source for the unmet need of novel drugs to treat metabolic bone disorders is marine organisms, which produce natural osteoactive compounds of high pharmaceutical potential. In this review, we have inventoried the marine osteoactive compounds (MOCs) currently identified and spotted the groups of marine organisms with potential for MOC production. Finally, we briefly examine the availability of in vivo screening and validation tools for the study of MOCs.

The molecular spectrum of Turkish osteopetrosis and related osteoclast disorders with natural history, including a candidate gene, CCDC120

BACKGROUND

Osteopetrosis and related osteoclastic disorders are a heterogeneous group of inherited diseases characterized by increased bone density. The aim of this study is to investigate the molecular spectrum and natural history of the clinical and radiological features of these disorders.

METHODS

28 patients from 20 families were enrolled in the study; 20 of them were followed for a period of 1-16 years. Targeted gene analysis and whole-exome sequencing (WES) were performed.

RESULTS

Biallelic mutations in CLCN7 and TCIRG1 were detected in three families each, in TNFRSF11A and CA2 in two families each, and in SNX10 in one family in the osteopetrosis group. A heterozygous variant in CLCN7 was also found in one family. In the osteopetrosis and related osteoclast disorders group, three different variants in CTSK were detected in five families with pycnodysostosis and a SLC29A3 variant causing dysosteosclerosis was detected in one family. In autosomal recessive osteopetrosis (ARO), a malignant infantile form, four patients died during follow-up, two of whom had undergone hematopoietic stem cell transplantation. Interestingly, all patients had osteopetrorickets of the long bone metaphyses in infancy, typical skeletal features such as Erlenmeyer flask deformity and bone-in-bone appearance that developed toward the end of early childhood. Two siblings with a biallelic missense mutation in CLCN7 and one patient with the compound heterozygous novel splicing variants in intron 15 and 17 in TCIRG1 corresponded to the intermediate form of ARO (IARO); there was intrafamilial clinical heterogeneity in the family with the CLCN7 variant. One of two patients with IARO and distal tubular acidosis was found to have a large deletion in CA2. In one family, two siblings with a heterozygous mutation in CLCN7 were affected, whereas the father with the same mutation was asymptomatic. In WES analysis of three brothers from a family without mutations in osteopetrosis genes, a hemizygous missense variant in CCDC120, a novel gene, was found to be associated with high bone mass.

CONCLUSION

This study extended the natural history of the different types of osteopetrosis and also introduced a candidate gene, CCDC120, potentially causing osteopetrosis.

Successful complete oral rehabilitation of a patient with osteopetrosis with extensive pre-treatments, bone grafts, dental implants and fixed bridges: a multidisciplinary case report

BACKGROUND

Osteopetrosis comprises a group of inherited disorders that are rare and result in abnormal bone structure. Bone remodeling is extremely inhibited because osteoclasts are nonfunctional or lacking. This condition causes overgrowth of bone with disappearance of the bone marrow, leading to aplastic anemia; obstruction of nerve passages in the skull leads to blindness and often hearing impairment. In most cases, osteopetrosis results in oral complications such as tooth deformation, hypomineralization, and delayed or absent tooth eruption. The only curative treatment is hematopoietic stem cell transplantation (HSCT). The main treatment of the oral complications during childhood and adolescence consists in protecting the erupted teeth against caries disease through prophylactic treatment aimed at optimal oral hygiene through frequent regular dental visits throughout life. Many patients with osteopetrosis require major oral rehabilitation to treat complications of the disease. Improved results of HSCT increase the likelihood that dental professionals will encounter patients with osteopetrosis.

CASE PRESENTATION

In this case report, we show that individuals with osteopetrosis who have severe oral complications can be treated successfully if they are treated for osteopetrosis at an early age. The boy had his dental care in pedodontics, and regular multidisciplinary meetings were held for future treatment planning. At the age of 15, he was then referred for rehabilitation. The initial evaluations revealed no further growth in the alveolar bone. The rehabilitation was done stepwise, with extraction of malformed and malpositioned teeth. Initially, the patient received a removable partial denture followed by reconstruction of the width of the alveolar process, titanium implants, temporary fixed bridges, and finally screw-retained titanium-ceramic bridges with titanium frames for the upper and lower jaws.

CONCLUSIONS

The three-year follow-up after loading indicated a stable marginal bone level and optimal oral hygiene as a result of frequent professional oral hygiene care. The patient showed no signs of symptoms from the temporomandibular joint and has adapted to the new jaw relation without any functional or phonetical issues.

BASP1 down-regulates RANKL-induced osteoclastogenesis

The cytokine RANKL (Receptor Activator of NFκB Ligand) is the key driver of differentiation of monocytes/macrophages to form multi-nucleated, bone-resorbing osteoclasts, a process that is accompanied by significant changes in gene expression. We show that exposure to RANKL rapidly down-regulates expression of Brain Acid Soluble Protein 1 (BASP1) in cultured primary mouse bone marrow macrophages (BMMs), and that this reduced expression is causally linked to the osteoclastogenic process in vitro. Knocking down BASP1 expression in BMMs or eliminating its expression in these cells or in RAW 264.7 cells enhanced RANKL-induced osteoclastogenesis, promoted cell-cell fusion, and generated cultures containing larger osteoclasts with increased mineral degrading abilities relative to controls. Expression of exogenous BASP1 in BMMs undergoing osteoclastogenic differentiation produced the opposite effects. Upon exposure to RANKL, primary mouse BMMs in which BASP1 had been knocked down exhibited increased expression of the key osteoclastogenic transcription factor Nfatc1and of its downstream target genes Dc-stamp, Ctsk, Itgb3, and Mmp9 relative to controls. The knock-down cells also exhibited increased sensitivity to the pro-osteoclastogenic effects of RANKL. We conclude that BASP1 is a negative regulator of RANKL-induced osteoclastogenesis, which down-regulates the pro-osteoclastogenic gene expression pattern induced by this cytokine. Decreased expression of BASP1 upon exposure of BMMs to RANKL removes a negative regulator of osteoclastogenesis and promotes this process.

Defects in Bone and Bone Marrow in Inherited Anemias: the Chicken or the Egg

PURPOSE OF REVIEW

Recently, there has been an increasing number of studies on the crosstalk between the bone and the bone marrow and how it pertains to anemia. Here, we discuss four heritable clinical syndromes contrasting those in which anemia affects bone growth and development, with those in which abnormal bone development results in anemia, highlighting the multifaceted interactions between skeletal development and hematopoiesis.

RECENT FINDINGS

Anemia results from both inherited and acquired disorders caused by either impaired production or premature destruction of red blood cells or blood loss. The downstream effects on bone development and growth in patients with anemia often constitute an important part of their clinical condition. We will discuss the interdependence of abnormal bone development and growth and hematopoietic abnormalities, with a focus on the erythroid lineage. To illustrate those points, we selected four heritable anemias that arise from either defective hematopoiesis impacting the skeletal system (the hemoglobinopathies β-thalassemia and sickle cell disease) versus defective osteogenesis resulting in impaired hematopoiesis (osteopetrosis). Finally, we will discuss recent findings in Diamond Blackfan anemia, an intrinsic disorder of both the erythron and the bone. By focusing on four representative hereditary hematopoietic disorders, this complex relationship between bone and blood should lead to new areas of research in the field.

Clinical, genetic aspects and molecular pathogenesis of osteopetrosis

Osteopetrosis ("marble bone", ICD-10-78.2) includes a group of hereditary bone disorders distinguished by clinical variability and genetic heterogeneity. The name "osteopetrosis" comes from the Greek language: 'osteo' means 'bone' and 'petrosis' means 'stone', which characterizes the main feature of the disease: increased bone density caused by imbalances in bone formation and remodeling, leading to structural changes in bone tissue, predisposition to fractures, skeletal deformities. These defects, in turn, affect other important organs and tissues, especially bone marrow and the nervous system. The disease can be autosomal recessive, autosomal dominant, X-linked or sporadic. Autosomal dominant osteopetrosis has an incidence of 1 in 20,000 newborns and autosomal recessive one has 1 in 250,000. To date, 23 genes have been described, structural changes in which lead to the development of osteopetrosis. Clinical symptoms in osteopetrosis vary greatly in their presentation and severity. The mildest skeletal abnormalities are observed in adulthood and occur in the autosomal dominant form of osteopetrosis. Severe forms, being autosomal recessive and manifesting in early childhood, are characterized by fractures, mental retardation, skin lesions, immune system disorders, renal tubular acidosis. Clinical examination and review of radiographs, bone biopsy and genetic testing provide the bases for clinical diagnosis. The early and accurate detection and treatment of the disease are important to prevent hematologic abnormalities and disease progression to irreversible neurologic consequences. Most patients die within the first decade due to secondary infections, bone marrow suppression and/or bleeding. This article summarizes the current state of the art in this field, including clinical and genetic aspects, and the molecular pathogenesis of the osteopetrosis.

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.

Severe hypophosphataemia can be an early sign of osteopetrorickets: a case report

Osteopetrorickets is a rare complication of autosomal recessive ("malignant") osteopetrosis. Its prompt diagnosis is essential, because early suspicion of infantile osteopetrosis enables treatment with human stem cell transplantation, depending on the gene involved. It is important to identify not only the characteristic radiological changes of rickets, but also the coexistence of increased bone density, so as not to miss this very rare entity. Herein, a brief case report is presented.

Outlining the Clinical Profile of TCIRG1 14 Variants including 5 Novels with Overview of ARO Phenotype and Ethnic Impact in 20 Egyptian Families

TCIRG1 gene mutations underlie osteopetrosis, a rare genetic disorder impacting osteoclast function with consequent brittle bones prone to fracture, in spite of being characterized by increased bone density. The disorder is known to exhibit marked genetic heterogeneity, has no treatment, and is lethal in most instances. There are reports of ethnic variations affecting bone mineral density and variants' expression as diverse phenotypes even within individuals descending from the same pedigree. We herein focus on one of osteopetrosis's three types: the autosomal recessive malignant form (MIM 259700) (ARO) that is almost always associated with severe clinical symptoms. We reviewed the results of about 1800 Egyptian exomes and we did not detect similar variants within our Egyptian dataset and secondary neurological deficit. We studied twenty Egyptian families: sixteen ARO patients, ten carrier parents with at least one ARO affected sib, and two fetuses. They were all subjected to thorough evaluation and TCIRG1 gene sequencing. Our results of twenty-eight individuals descending from twenty Egyptian pedigrees with at least one ARO patient, expand the phenotype as well as genotype spectrum of recessive mutations in the TCIRG1 gene by five novel pathogenic variants. Identifying TCIRG1 gene mutations in Egyptian patients with ARO allowed the provision of proper genetic counseling, carrier detection, and prenatal diagnosis starting with two families included herein. It also could pave the way to modern genomic therapeutic approaches.

Genome sequencing identifies a large non-coding region deletion of SNX10 causing autosomal recessive osteopetrosis

Autosomal recessive osteopetrosis (ARO) is a rare genetic disorder caused by impaired osteoclast activity. In this study, we describe a 4-year-old boy with increased bone density due to osteopetrosis, autosomal recessive 8. Using genome sequencing, we identified a large deletion in the 5'-untranslated region (UTR) of SNX10 (sorting nexin 10), where the regulatory region of this gene is located. This large deletion resulted in the absence of the SNX10 transcript and led to abnormal osteoclast activity. SNX10 is one of the nine genes known to cause ARO, shown to interact with V-ATPase (vacuolar type H( + )-ATPase), as it plays an important role in bone resorption. Our study highlights the importance of regulatory regions in the 5'-UTR of SNX10 for its expression while also demonstrating the importance of genome sequencing for detecting large deletion of the regulatory region of SNX10.

Clinical and genetic diagnosis of autosomal dominant osteopetrosis type II in a Chinese family: A case report

BACKGROUND

Osteopetrosis is a rare genetic disorder characterized by increased bone density due to defective bone resorption of osteoclasts. Approximately, 80% of autosomal dominant osteopetrosis type II (ADO-II) patients were usually affected by heterozygous dominant mutations in the chloride voltage-gated channel 7 (ClCN7) gene and present early-onset osteoarthritis or recurrent fractures. In this study, we report a case of persistent joint pain without bone injury or underlying history.

CASE SUMMARY

We report a 53-year-old female with joint pain who was accidentally diagnosed with ADO-II. The clinical diagnosis was based on increased bone density and typical radiographic features. Two heterozygous mutations in the ClCN7 and T-cell immune regulator 1 (TCIRG1) genes by whole exome sequencing were identified in the patient and her daughter. The missense mutation (c.857G>A) occurred in the CLCN7 gene p. R286Q, which is highly conserved across species. The TCIRG1 gene point mutation (c.714-20G>A) in intron 7 (near the splicing site of exon 7) had no effect on subsequent transcription.

CONCLUSION

This ADO-II case had a pathogenic CLCN7 mutation and late onset without the usual clinical symptoms. For the diagnosis and assessment of the prognosis for osteopetrosis, genetic analysis is advised.

Osteopetrosis associated with PLEKHM1 and SNX10 genes, both involved in osteoclast vesicular trafficking

The clinical and radiological variability seen in different forms of osteopetrosis, all due to impaired osteoclastic bone resorption, reflect many causal genes. Both defective differentiation of osteoclasts from hematopoietic stem cells as well as disturbed functioning of osteoclasts can be the underlying pathogenic mechanism. Pathogenic variants in PLEKHM1 and SNX10 can be classified among the latter as they impair vesicular transport within the osteoclast and therefore result in the absence of a ruffled border. Some of the typical radiological hallmarks of osteopetrosis can be seen, and most cases present as a relatively mild form segregating in an autosomal recessive mode of inheritance.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Hereditary Metabolic Bone Diseases: A Review of Pathogenesis, Diagnosis and Management

Hereditary metabolic bone diseases are characterized by genetic abnormalities in skeletal homeostasis and encompass one of the most diverse groups among rare diseases. In this review, we examine 25 selected hereditary metabolic bone diseases and recognized genetic variations of 78 genes that represent each of the three groups, including sclerosing bone disorders, disorders of defective bone mineralization and disorder of bone matrix and cartilage formation. We also review pathophysiology, manifestation and treatment for each disease. Advances in molecular genetics and basic sciences has led to accurate genetic diagnosis and novel effective therapeutic strategies for some diseases. For other diseases, the genetic basis and pathophysiology remain unclear. Further researches are therefore crucial to innovate ways to overcome diagnostic challenges and develop effective treatment options for these orphan diseases.

Overcoming therapeutic resistance to platinum-based drugs by targeting Epithelial–Mesenchymal transition

Platinum-based drugs (PBDs), including cisplatin, carboplatin, and oxaliplatin, have been widely used in clinical practice as mainstay treatments for various types of cancer. Although there is firm evidence of notable achievements with PBDs in the management of cancers, the acquisition of resistance to these agents is still a major challenge to efforts at cure. The introduction of the epithelial-mesenchymal transition (EMT) concept, a critical process during embryonic morphogenesis and carcinoma progression, has offered a mechanistic explanation for the phenotypic switch of cancer cells upon PBD exposure. Accumulating evidence has suggested that carcinoma cells can enter a resistant state via induction of the EMT. In this review, we discussed the underlying mechanism of PBD-induced EMT and the current understanding of its role in cancer drug resistance, with emphasis on how this novel knowledge can be exploited to overcome PBD resistance via EMT-targeted compounds, especially those under clinical trials.

Case report of mild TCIRG1-associated autosomal recessive osteopetrosis in Vietnam

Autosomal recessive osteopetrosis (ARO) is a group of disease characterized by osteoclast dysfunction inhibiting bone resorption and bone turnover, with TCIRG1-associated ARO being more common leading to autosomal recessive infantile malignant osteopetrosis (OPTB1, MIM entry number # 259700). While most patients with TCIRG1-associated osteopetrosis present a malignant clinical course and shortened lifespan, a few cases of non-malignant TCIRG1-associated osteopetrosis have been reported. 24-year-old female patient came to us with limp gait, hip pain in both sides, and severe stiffness. She had suffered many fractures, bilateral hip osteoarthritis, right leg was 2 cm shorter compared with left leg. Whole Exome Sequencing was conducted, the result and subsequent Sanger's sequencing shown the patient had a compound heterozygous genotype at TCIRG1 (c.1194dup, p.Gly399ArgTer and c.334G>A, p.Gly112Arg), these two variants found were not previously reported. Sanger's sequencing revealed two other siblings whom suffer the same disorder had similar genotype to the proband; the parents were found to be heterozygous. This is the first case of TCIRG1-associated osteopetrosis reported in Vietnam and one of the few cases of nonmalignant TCIRG1-associated osteopetrosis, in which detailed clinical and genetic work-up were performed.

A cross-sectional nationwide survey of osteosclerotic skeletal dysplasias in Japan

BACKGROUND

The osteosclerotic skeletal dysplasias (OSSDs) are a heterogeneous group of disorders characterized by systemic bone sclerosis. Little is known about OSSDs because of their rarity. We conducted a cross-sectional nationwide survey of OSSDs and examined the incidence, epidemiology, and therapeutic interventions on these disorders.

METHODS

This study consisted of a two-step survey. The number of patients with OSSDs who had visited medical institutions between April 2017 and March 2018 was reported from a total of 341 facilities (1364 departments from pediatrics, orthopaedic surgery, neurosurgery, and otolaryngology in each facility) by the first questionnaire. In the secondary survey, their clinical features were assessed by collecting demographic data, diagnostic details, current status, family histories, therapeutic interventions, histories of bone fracture and osteomyelitis, severity assessed by the modified Rankin Scale (mRS) and recent lifestyle conditions of the patient by the EQ-5D.

RESULTS

In the first survey, 51 facilities (56 departments) reported one or more OSSDs patients, including 50 patients with osteopetrosis and 57 patients of other OSSDs. Among 87 patients eligible for inclusion in the analysis in the secondary survey, we investigated detailed information on the 42 patients with osteopetrosis. The number of initial visits of osteopetrosis patients during the surveillance period was five per year, indicating that the estimated incidence of osteopetrosis seemed to be 0.6 per 100,000 live births. Eighty-six bone fractures were reported in 22 patients (52%), and interventions of pseudarthrosis were conducted in five patients. Nine patients (23%) showed significant disabilities with the mRS of grade 3 or higher. Neurological complications and severe anemia were the factors that deteriorate patients' quality of life.

CONCLUSIONS

This is the first study to examine the detailed epidemiology of OSSDs in Japan. We demonstrated that the incidence of OSSDs is extremely rare. Bone fragility and delayed fracture healing seem to be important orthopaedic problems for patients with osteopetrosis.

Critical Roles of NF-κB Signaling Molecules in Bone Metabolism Revealed by Genetic Mutations in Osteopetrosis

The nuclear factor-κB (NF-κB) transcription factor family consists of five related proteins, RelA (p65), c-Rel, RelB, p50/p105 (NF-κB1), and p52/p100 (NF-κB2). These proteins are important not only for inflammation and the immune response but also for bone metabolism. Activation of NF-κB occurs via the classic and alternative pathways. Inflammatory cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β, activate the former, and cytokines involved in lymph node formation, such as receptor activator of NF-κB ligand (RANKL) and CD40L, activate the latter. p50 and p52 double-knockout mice revealed severe osteopetrosis due to the total lack of osteoclasts, which are specialized cells for bone resorption. This finding suggests that the activation of NF-κB is required for osteoclast differentiation. The NF-κB signaling pathway is controlled by various regulators, including NF-κB essential modulator (NEMO), which is encoded by the IKBKG gene. In recent years, mutant forms of the IKBKG gene have been reported as causative genes of osteopetrosis, lymphedema, hypohidrotic ectodermal dysplasia, and immunodeficiency (OL-EDA-ID). In addition, a mutation in the RELA gene, encoding RelA, has been reported for the first time in newborns with high neonatal bone mass. Osteopetrosis is characterized by a diffuse increase in bone mass, ranging from a lethal form observed in newborns to an asymptomatic form that appears in adulthood. This review describes the genetic mutations in NF-κB signaling molecules that have been identified in patients with osteopetrosis.

Transcription Factor KLF7 Promotes Osteoclast Differentiation by Suppressing HO-1

Background: Osteoporosis is a common orthopedic disease with high prevalence in patients older than 50 years. Osteoporosis is often detected only after the fracture and is hard to treat. Therefore, it is of great significance to explore the molecular mechanism of the occurrence of osteoporosis.

Methods: The expression of Heme oxygenase-1 (HO-1) in people with different bone mineral density (BMD) was analyzed based on public databases. GenHacncer and JASPAR databases were adopted to search and verify the upstream transcription factor of HO-1. qRT-PCR, western blot and tartrate-resistant acid phosphatase assays were performed to explore the impact of HO-1 and Kruppel-like factor 7 (KLF7) on osteoclast differentiation. Chromatin immunoprecipitation (ChIP) assay confirmed the binding relationship between KLF7 and HO-1. Finally, Hemin, the agonist of HO-1, was applied in rescue assays, thereby verifying the mechanism of KLF7 modulating osteoclast differentiation by HO-1.

Results: Bioinformatics analysis revealed that HO-1 was highly-expressed while KLF7 lowly-expressed in people with high BMD. Besides, a potential binding site of KLF7 was found on the promoter region of HO-1. ChIP assay further manifested the targeting relationship between HO-1 and KLF7. Western blot and TRAP staining unveiled that osteoclast differentiation was suppressed by HO-1, while facilitated by KLF7. Rescue experiments indicated that over-expressed HO-1 could reverse of the promoting effect of KLF7 on osteoclast differentiation.

Conclusion: The study confirmed that osteoclast differentiation was promoted by KLF7 constraining HO-1, thereby facilitating osteoporosis. The cognation of the pathogenesis of osteoporosis was further enriched. New treatment could be developed on this basis.

The Treatment of Subtrochanteric Fracture with Reversed Contralateral Distal Femoral Locking Compression Plate (DF-LCP) Using a Progressive and Intermittent Drilling Procedure in Three Osteopetrosis Patients

OBJECTIVE

To describe the application of reversed contralateral distal femoral locking compression plate (DF-LCP) inserted through a progressive and intermittent drilling procedure in the treatment of osteopetrotic subtrochanteric fracture (OSF).

METHODS

Three patients (one male and two females with an average age of 45.33 ± 11.09 years) with OSF hospitalized between September 2015 and September 2020, were included in this present study. Lateral approach was applied in all patients who accepted open reduction and internal fixation (ORIF) with a reversed contralateral DF-LCP inserted through a progressive and intermittent drilling procedure. The operation time and intraoperative blood loss were recorded to evaluate the efficiency of this surgical method. Physical examination and imaging examination of the fracture site were used to evaluate the fracture union status, the position and stability of the implant, and the alignment of the injured limb at 1, 3, 6, and 12 months after operation, then a subsequent visit was conducted at least once a year. Harris Hip Score (HHS) was used to evaluate the hip joint function at 6 and 12 months after operation.

RESULTS

The average operation time was 140 ± 21.60 min (110, 160, and 150 min); The average intraoperative blood loss was about 333.33 ± 23.57 ml (300, 350, and 350 ml). The average follow-up time was 22.33 ± 7.41 months (29, 26, and 12 months). All patients achieved bone union with an average time of 6.67 ± 0.94 months (6, 8, and 6 months). At the time of 6 months after operation, case 1 and 3 were almost pain-free and could walk with full weight bearing while case 2 could walk only with partial weight bearing using a crutch. The HHS scores of cases 1, 2, and 3 were 84/100, 74/100, and 92/100, respectively. At the follow-up at 12 months after operation, the HHS score improved to 91/100, 81/100, and 96/100, respectively. The contralateral incomplete old subtrochanteric fracture was deteriorated in case 1 at 26 months after operation. After 3 months of limited weight bearing using a crutch, bone union was verified in radiograph imaging. Fresh contralateral subtrochanteric fracture occurred in case 2 at 26 months after operation, which was treated using a similar surgical approach, and its clinical outcome is under follow-up. Moreover, no perioperative complications including operation-related death, vascular/nerve injury, deep venous thrombosis, pulmonary embolism, and incision infection, and long-term complications involving malunion, nonunion, implant failure, ankylosis, heterotopic ossification, osteonecrosis, and osteomyelitis were identified.

CONCLUSION

The application of reversed contralateral DF-LCP in OSF is practicable and reliable. Progressive and intermittent drilling is a safe and efficient method for implant insertion in this complicated situation.

Genetics and Epigenetics of Bone Remodeling and Metabolic Bone Diseases

Bone metabolism consists of a balance between bone formation and bone resorption, which is mediated by osteoblast and osteoclast activity, respectively. In order to ensure bone plasticity, the bone remodeling process needs to function properly. Mesenchymal stem cells differentiate into the osteoblast lineage by activating different signaling pathways, including transforming growth factor β (TGF-β)/bone morphogenic protein (BMP) and the Wingless/Int-1 (Wnt)/β-catenin pathways. Recent data indicate that bone remodeling processes are also epigenetically regulated by DNA methylation, histone post-translational modifications, and non-coding RNA expressions, such as micro-RNAs, long non-coding RNAs, and circular RNAs. Mutations and dysfunctions in pathways regulating the osteoblast differentiation might influence the bone remodeling process, ultimately leading to a large variety of metabolic bone diseases. In this review, we aim to summarize and describe the genetics and epigenetics of the bone remodeling process. Moreover, the current findings behind the genetics of metabolic bone diseases are also reported.

Genetic analysis in a familial case with high bone mineral density suggests additive effects at two loci

Osteoporosis is the most common bone disease, characterized by a low bone mineral density (BMD) and increased risk of fracture. At the other end of the BMD spectrum, some individuals present strong, fracture‐resistant, bones. Both osteoporosis and high BMD are heritable and their genetic architecture encompasses polygenic inheritance of common variants and some cases of monogenic highly penetrant variants in causal genes. We have investigated the genetics of high BMD in a family segregating this trait in an apparently Mendelian dominant pattern. We searched for rare causal variants by whole‐exome sequencing in three affected and three nonaffected family members. Using this approach, we have identified 38 rare coding variants present in the proband and absent in the three individuals with normal BMD. Although we have found four variants shared by the three affected members of the family, we have not been able to relate any of these to the high‐BMD phenotype. In contrast, we have identified missense variants in two genes, VAV3 and ADGRE5, each shared by two of out of three affected members, whose loss of function fits with the phenotype of the family. In particular, the proband, a woman displaying the highest BMD (sum Z‐score = 7), carries both variants, whereas the other two affected members carry one each. VAV3 encodes a guanine‐nucleotide‐exchange factor with an important role in osteoclast activation and function. Although no previous cases of VAV3 mutations have been reported in humans, Vav3 knockout (KO) mice display dense bones, similarly to the high‐BMD phenotype present in our family. The ADGRE5 gene encodes an adhesion G protein‐coupled receptor expressed in osteoclasts whose KO mouse displays increased trabecular bone volume. Combined, these mouse and human data highlight VAV3 and ADGRE5 as novel putative high‐BMD genes with additive effects, and potential therapeutic targets for osteoporosis. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Clinical presentation and analysis of genotype-phenotype correlations in patients with malignant infantile osteopetrosis

Malignant infantile osteopetrosis (MIOP) is the autosomal recessive, severe form of osteopetrosis. This rare genetic syndrome usually presents soon after birth and is often fatal if left untreated. Early diagnosis is key for proper management but clinical presentation is diverse, and oftentimes diagnosis may be challenging. In this study, we retrospectively collected data of genetic mutations and phenotypic characteristics at the initial presentation of 81 MIOP patients and analyzed genotype-phenotype correlations. The most common genetic mutation was in the TCIRG1 gene (n = 46, 56.8%), followed by SNX10 (n = 20, 25%). Other genetic mutations included RANK (n = 7, 8.7%), CLCN7 (n = 5, 6.2%) and CA2 (n = 3, 3.7%). More than half of the patients presented with growth retardation (n = 46, 56.8%). Twenty-one of the patients were blind (26%) and thirty-seven patients had other neurological deficits (45.7%) at the time of initial presentation. Most patients presented with hematological signs of bone marrow failure including anemia (n = 69, 85.2%) and thrombocytopenia (n = 33, 40.7%). Thrombocytopenia at initial presentation was significantly more prevalent in patients with mutations in the TCIRG1 gene (p = 0.036). Other phenotypic presenting features were not found to be significantly correlated to specific gene mutations. In conclusion, the initial presentation of MIOP is variable, but some features are common such as growth retardation, visual impairment, and cytopenias. High awareness of MIOP presenting signs is essential for prompt diagnosis of this challenging disease.

OUP accepted manuscript

Osteopetrosis is a rare inherited disease characterized by impaired osteoclast activity causing defective bone resorption and bone marrow aplasia. It is fatal in early childhood unless hematopoietic stem cell transplantation is performed. But, the transplant course is complicated with engraftment failure. Recently, osteoclasts have been described as the potential regulators of hematopoietic stem cell (HSC) niche. Here we investigated the alterations in the HSC and mesenchymal stromal cell (MSC) components of osteopetrotic niche and their interactions to mimic the stem cell dynamics/trafficking in the BM niche after HSC transplantation. Induced pluripotent stem cells were generated from peripheral blood mononuclear cells of patients with osteopetrosis carrying TCIRG1 mutation. iPSC lines were differentiated into hematopoietic and myeloid progenitors, then into osteoclasts using a step-wise protocol. We first demonstrated a shift toward monocyte-macrophages lineage regarding hematopoietic differentiation potential of osteopetrotic iPSC-derived hematopoietic progenitors (HPCs) and phenotypically normal and functionally defective osteoclast formation. The expression of the genes involved in HSC homing and maintenance (Sdf-1, Jagged-1, Kit-L, and Opn) in osteopetrotic MSCs recovered significantly after coculture with healthy HPCs. Similarly, the restoration of phenotype, impaired differentiation, and migratory potential of osteopetrotic iHPCs were observed upon interaction with healthy MSCs. Our results establish significant alterations in both MSC and HPC compartments of the osteopetrotic niche, and support the impact of functionally impaired osteoclasts in defective niche formation.

Neonatal hydrocephalus: an atypical presentation of malignant infantile osteopetrosis

PURPOSE

Autosomal recessive osteopetrosis has a variable presentation, most commonly including failure to thrive, hypocalcemia, seizures, hepatosplenomegaly, hydrocephalus, vision or hearing loss, and cytopenias. Multiple symptoms are usually seen at presentation. The variability of presentation often delays diagnosis and subsequent treatment. Here, we present a case of an infant with this condition who initially presented with triventricular hydrocephalus with Chiari I malformation. This alone is not a common presentation of this disease, and we present this case to highlight autosomal recessive osteopetrosis as a potential diagnosis in infants presenting with hydrocephalus and discuss the other associated symptoms, management, and prognosis of this condition.

CASE REPORT

The patient was a full-term infant with a routine newborn period. At 6 months, the infant had macrocephaly and frontal bossing with a bulging fontanelle. She was found to have hydrocephalus with moderate ventriculomegaly involving the third and lateral ventricles with an associated Chiari 1 malformation. The infant was asymptomatic at the time. The infant was promptly referred to neurosurgery and underwent an uncomplicated ventriculoperitoneal shunt placement. Post-operative X-rays showed increased density of the skull with other bone changes suggestive of autosomal recessive osteopetrosis. Subsequent lab work and imaging studies were consistent with this condition. The diagnosis was confirmed by genetic testing, and the patient has undergone treatment with hematopoietic stem cell transplant.

CONCLUSION

Hydrocephalus is a common feature of this condition, typically seen in conjunction with other systemic symptoms and laboratory findings. Our patient had a limited initial presentation of triventricular hydrocephalus with Chiari I malformation and was otherwise clinically asymptomatic. There is limited literature of such a presentation, and we highlight this case to increase awareness, as timely diagnosis of these patients is critical for treatment and future outcomes.

Genetic analysis of osteopetrosis in Pakistani families identifies novel and known sequence variants

Osteopetrosis is a genetically heterogenous, fatal bone disorder characterized by increased bone density. Globally, various genetic causes are reported for osteopetrosis with all forms of inheritance patterns. A precise molecular diagnosis is necessary for prognosis and for prescribing treatment paradigms in osteopetrosis. Here we report on thirteen individuals diagnosed with infantile malignant osteopetrosis coming from ten unrelated Pakistani families; nine of whom are consanguineous. We performed whole exome sequencing and Sanger sequencing in all families and identified homozygous variants in genes previously reported for autosomal recessive inheritance of osteopetrosis. All the identified variants are expected to affect the stability or length of gene products except one nonsynonymous missense variant. TCIRG1 was found as a candidate causal gene in majority of the families. We report six novel variants; four in TCIRG1 and one each in CLCN7 and OSTM1. Our combined findings will be helpful in molecular diagnosis and genetic counselling of patients with osteopetrosis particularly in populations with high consanguinity.

Differential diagnosis of a diffuse sclerosis in an identified male skull (early 20th century Coimbra, Portugal): A multimethodological approach for the identification of osteosclerotic dysplasias in skeletonized individuals

OBJECTIVE

This work aims to discuss the difficulties in diagnosing osteosclerotic changes in skeletonized individuals and to raise awareness of osteosclerotic dysplasias as a group of rare ancient diseases.

MATERIALS

The skull of a 62-year-old male individual from the International Exchange Skull Collection, curated by the University of Coimbra, who died in 1928 presenting albuminous nephritis (Bright disease)/uraemia as the registered cause of death.

METHODS

The skull was macroscopically and radiologically examined and bone elemental analysis was investigated. The genealogy and medical records of the individual were also searched.

RESULTS

The lesions are in accordance with an osteosclerotic process possibly pointing to osteosclerosis, osteosclerotic metaphyseal dysplasia, or dysosteosclerosis, but osteoclasia with hyperphosphatasia, endosteal hyperostosis, sclerosteosis, or osteopathia striata with cranial sclerosis cannot be ruled out.

CONCLUSIONS

Representativeness of the skeleton is a crucial feature in diagnosing rare diseases and, to avoid a misdiagnosis, the final diagnosis should include a group of diseases rather than a definite disease.

SIGNIFICANCE

Difficulties in diagnosing rare diseases are discussed and best approaches in the study osteosclerotic dysplasias in skeletonized individuals are offered in the light of current clinical knowledge.

LIMITATIONS

The absence of the postcranial skeleton and of pathognomonic lesions associated with osteosclerotic dysplasias limits diagnosis. Although rare diseases often have a genetic basis, specific genetic testing for the diagnosis of rare diseases in paleopathological cases are not yet available.

SUGGESTIONS FOR FURTHER RESEARCH

Future genetic studies might help narrow down the diagnosis.

Identification of novel mutation in RANKL by whole-exome sequencing in a Thai family with osteopetrosis; a case report and review of RANKL osteopetrosis

Background

Osteopetrosis is a rare form of skeletal dysplasia characterized by increased bone density that leads to bone marrow failure, compressive neuropathy, and skeletal dysmorphism. Molecular diagnosis is essential as it guides treatment and prognosis. We report Thai siblings with an ultra‐rare form of osteopetrosis.

Methods

The older brother and the younger sister presented with chronic mandibular osteomyelitis in their 20s. Since childhood, they had visual impairment, pathological fracture, and skeletal dysmorphism.

Quadruplet whole‐exome sequencing was performed and confirmed with Sanger sequencing. Novel mutation in TNFSF11 (RANKL) c.842T>G, p.Phe281Cys was identified in a homozygous state in both siblings.

Results

Surgical debridement, antibiotic, and hyperbaric oxygen therapy were used and discontinued over a 6‐month period with normalization of C‐reactive protein. Hematopoietic stem cell transplantation candidacy was excluded by molecular diagnosis.

Conclusion

We report a novel mutation in an ultra‐rare form of osteopetrosis. Our siblings manifested with a milder phenotype in comparison with nine cases previously published.

A novel mutation in TNFRSF11A gene causes pediatric osteopetrosis: case report

BACKGROUND

Osteopetrosis is a rare inherited bone disorder affected individual by osteoclast disfunction and increasing bone density. Surgery was taken for histological examination of the specimen and evidence of malignancy was not found. Finally, X-ray and gene detection lead to the diagnosis.

CASE PRESENTATION

We report a 10-year-old girl with two years history of pus rhinorrhea, nasal obstruction and smelly nose. She was diagnosed and treated as sinusitis. But the symptoms were recurrent. Ten months ago, she was afflicted with persistent swelling and broken skin on the right cheek. All the laboratory findings showed normal. During surgery, we resected the right gingiva, the right nasal mucosa and the right facial tissue for biopsies. Histological examination showed proliferation of granulation tissue in chronic inflammatory mucosa. X-rays showed generalized sclerosis. Genetic analysis strongly supported a novel mutation of TNFRSF11A gene which caused osteoporosis. We found a novel mutation of the c.1196C > G (p.S399X) in exon 9 of TNFRSF11A. The TNFRSF11A gene encodes RANK, which is fundamental for osteoclast formation.

CONCLUSION

Osteopetrosis is a rare genetic bone disease characterized by increased bone density because of bone resorption failure. Diagnosis is based on X-ray and gene analyze. Osteoclasts are bone-related cells derived from hematopoietic cell lines. Since osteoclasts arise from a hematopoietic progenitor cell of the monocytic lineage, the defect can be corrected by hematopoietic stem cell transplantation (HSCT). Better understanding of this pathological situation and pathogenesis is so important to plan appropriate immunotherapy to benefit.

Sorting Nexin 10 as a Key Regulator of Membrane Trafficking in Bone-Resorbing Osteoclasts: Lessons Learned From Osteopetrosis

Bone homeostasis is a complex, multi-step process, which is based primarily on a tightly orchestrated interplay between bone formation and bone resorption that is executed by osteoblasts and osteoclasts (OCLs), respectively. The essential physiological balance between these cells is maintained and controlled at multiple levels, ranging from regulated gene expression to endocrine signals, yet the underlying cellular and molecular mechanisms are still poorly understood. One approach for deciphering the mechanisms that regulate bone homeostasis is the characterization of relevant pathological states in which this balance is disturbed. In this article we describe one such “error of nature,” namely the development of acute recessive osteopetrosis (ARO) in humans that is caused by mutations in sorting nexin 10 (SNX10) that affect OCL functioning. We hypothesize here that, by virtue of its specific roles in vesicular trafficking, SNX10 serves as a key selective regulator of the composition of diverse membrane compartments in OCLs, thereby affecting critical processes in the sequence of events that link the plasma membrane with formation of the ruffled border and with extracellular acidification. As a result, SNX10 determines multiple features of these cells either directly or, as in regulation of cell-cell fusion, indirectly. This hypothesis is further supported by the similarities between the cellular defects observed in OCLs form various models of ARO, induced by mutations in SNX10 and in other genes, which suggest that mutations in the known ARO-associated genes act by disrupting the same plasma membrane-to-ruffled border axis, albeit to different degrees. In this article, we describe the population genetics and spread of the original arginine-to-glutamine mutation at position 51 (R51Q) in SNX10 in the Palestinian community. We further review recent studies, conducted in animal and cellular model systems, that highlight the essential roles of SNX10 in critical membrane functions in OCLs, and discuss possible future research directions that are needed for challenging or substantiating our hypothesis.

An SNX10-dependent mechanism downregulates fusion between mature osteoclasts

Homozygosity for the R51Q mutation in sorting nexin 10 (SNX10) inactivates osteoclasts (OCLs) and induces autosomal recessive osteopetrosis in humans and in mice. We show here that the fusion of wild-type murine monocytes to form OCLs is highly regulated, and that its extent is limited by blocking fusion between mature OCLs. In contrast, monocytes from homozygous R51Q SNX10 mice fuse uncontrollably, forming giant dysfunctional OCLs that can become 10- to 100-fold larger than their wild-type counterparts. Furthermore, mutant OCLs display reduced endocytotic activity, suggesting that their deregulated fusion is due to alterations in membrane homeostasis caused by loss of SNX10 function. This is supported by the finding that the R51Q SNX10 protein is unstable and exhibits altered lipid-binding properties, and is consistent with a key role for SNX10 in vesicular trafficking. We propose that OCL size and functionality are regulated by a cell-autonomous SNX10-dependent mechanism that downregulates fusion between mature OCLs. The R51Q mutation abolishes this regulatory activity, leading to excessive fusion, loss of bone resorption capacity and, consequently, to an osteopetrotic phenotype in vivo.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: Fusion of monocytes to become bone-resorbing osteoclasts is limited by an SNX10-dependent cell-autonomous mechanism. Loss of SNX10 function deregulates fusion and generates giant inactive osteoclasts.

Autosomal recessive osteopetrosis: mechanisms and treatments

Autosomal recessive osteopetrosis (ARO) is a severe inherited bone disease characterized by defective osteoclast resorption or differentiation. Clinical manifestations include dense and brittle bones, anemia and progressive nerve compression, which hamper the quality of patients' lives and cause death in the first 10 years of age. This Review describes the pathogenesis of ARO and highlights the strengths and weaknesses of the current standard of care, namely hematopoietic stem cell transplantation (HSCT). Despite an improvement in the overall survival and outcomes of HSCT, transplant-related morbidity and the pre-existence of neurological symptoms significantly limit the success of HSCT, while the availability of human leukocyte antigen (HLA)-matched donors still remains an open issue. Novel therapeutic approaches are needed for ARO patients, especially for those that cannot benefit from HSCT. Here, we review preclinical and proof-of-concept studies, such as gene therapy, systematic administration of deficient protein, in utero HSCT and gene editing.

Summary: Autosomal recessive osteopetrosis is a heterogeneous and rare bone disease for which effective treatments are still lacking for many patients. Here, we review the literature on clinical, preclinical and proof-of-concept studies.

Role of OSCAR Signaling in Osteoclastogenesis and Bone Disease

Formation of mature bone-resorbing cells through osteoclastogenesis is required for the continuous remodeling and repair of bone tissue. In aging and disease this process may become aberrant, resulting in excessive bone degradation and fragility fractures. Interaction of receptor-activator of nuclear factor-κB (RANK) with its ligand RANKL activates the main signaling pathway for osteoclastogenesis. However, compelling evidence indicates that this pathway may not be sufficient for the production of mature osteoclast cells and that co-stimulatory signals may be required for both the expression of osteoclast-specific genes and the activation of osteoclasts. Osteoclast-associated receptor (OSCAR), a regulator of osteoclast differentiation, provides one such co-stimulatory pathway. This review summarizes our present knowledge of osteoclastogenesis signaling and the role of OSCAR in the normal production of bone-resorbing cells and in bone disease. Understanding the signaling mechanism through this receptor and how it contributes to the production of mature osteoclasts may offer a more specific and targeted approach for pharmacological intervention against pathological bone resorption.

Further understanding on osteopetrotic femoral fractures: a case report and literature review

BACKGROUND

Osteopetrosis is a genetic disease characterized by defects in osteoclast formation and function. There were a few cases of subtrochanteric femur fractures treated with dynamic hip screw (DHS) in patients with osteopetrosis, but unfortunately the healing outcome was rather poor.

CASE PRESENTATION

We present our experience for treating a patient with intermediate autosomal recessive osteopetrosis (IRO) suffering from subtrochanteric femur fracture. In this case, we successfully used dynamic hip screw (DHS) internal fixation through meticulous preoperative planning and postoperative care, as well as application of surgical techniques. The patient displayed stable internal fixation with no limitation of activities during follow-up for 15 months. In addition to this case, a review of previous case reports showed an increasing number of case reports demonstrating that surgical treatment-related complications could be avoided preoperatively, intraoperatively, and postoperatively.

CONCLUSION

DHS for this patient, who suffered from subtrochanteric fractures with osteopetrosis, was successfully implemented. In the light of a comprehensive literature review, preoperative planning, surgical techniques, and postoperative rehabilitation care can significantly reduce the complications.

How we approach malignant infantile osteopetrosis

Malignant infantile osteopetrosis (MIOP) is a rare hereditary disorder characterized by excessive bone overgrowth due to a defect in bone marrow resorption by osteoclasts. In most cases, hematopoietic stem cell transplantation (HSCT) may correct bone metabolism but the rapidly progressing nature of this condition necessitates early diagnosis and prompt treatment to minimize irreversible cranial nerve damage. The management of patients with MIOP presents many unique challenges. In this review, the clinical management of patients with MIOP is discussed, including diagnosis, preparation for HSCT and special transplant considerations, management of unique HSCT complications, and long-term follow-up.

Pathobiologic Mechanisms of Neurodegeneration in Osteopetrosis Derived From Structural and Functional Analysis of 14 ClC-7 Mutants

ClC-7 is a chloride-proton antiporter of the CLC protein family. In complex with its accessory protein Ostm-1, ClC-7 localizes to lysosomes and to the osteoclasts' ruffled border, where it plays a critical role in acidifying the resorption lacuna during bone resorption. Gene inactivation in mice causes severe osteopetrosis, neurodegeneration, and lysosomal storage disease. Mutations in the human CLCN7 gene are associated with diverse forms of osteopetrosis. The functional evaluation of ClC-7 variants might be informative with respect to their pathogenicity, but the cellular localization of the protein hampers this analysis. Here we investigated the functional effects of 13 CLCN7 mutations identified in 13 new patients with severe or mild osteopetrosis and a known ADO2 mutation. We mapped the mutated amino acid residues in the homology model of ClC-7 protein, assessed the lysosomal colocalization of ClC-7 mutants and Ostm1 through confocal microscopy, and performed patch-clamp recordings on plasma-membrane-targeted mutant ClC-7. Finally, we analyzed these results together with the patients' clinical features and suggested a correlation between the lack of ClC-7/Ostm1 in lysosomes and severe neurodegeneration. © 2020 American Society for Bone and Mineral Research (ASBMR).

PTPRJ promotes osteoclast maturation and activity by inhibiting Cbl-mediated ubiquitination of NFATc1 in late osteoclastogenesis

Bone-resorbing osteoclasts (OCLs) are multinucleated phagocytes, whose central roles in regulating bone formation and homeostasis are critical for normal health and development. OCLs are produced from precursor monocytes in a multistage process that includes initial differentiation, cell-cell fusion, and subsequent functional and morphological maturation; the molecular regulation of osteoclastogenesis is not fully understood. Here, we identify the receptor-type protein tyrosine phosphatase PTPRJ as an essential regulator specifically of OCL maturation. Monocytes from PTPRJ-deficient (JKO) mice differentiate and fuse normally, but their maturation into functional OCLs and their ability to degrade bone are severely inhibited. In agreement, mice lacking PTPRJ throughout their bodies or only in OCLs exhibit increased bone mass due to reduced OCL-mediated bone resorption. We further show that PTPRJ promotes OCL maturation by dephosphorylating the M-CSF receptor (M-CSFR) and Cbl, thus reducing the ubiquitination and degradation of the key osteoclastogenic transcription factor NFATc1. Loss of PTPRJ increases ubiquitination of NFATc1 and reduces its amounts at later stages of osteoclastogenesis, thereby inhibiting OCL maturation. PTPRJ thus fulfills an essential and cell-autonomous role in promoting OCL maturation by balancing between the pro- and anti-osteoclastogenic activities of the M-CSFR and maintaining NFATc1 expression during late osteoclastogenesis.

miRNAs in osteoclast biology

MicroRNAs (miRNAs) are a class of short RNA molecules that mediate the regulation of gene activity through interactions with target mRNAs and subsequent silencing of gene expression. It has become increasingly clear the miRNAs regulate many diverse aspects of bone biology, including bone formation and bone resorption processes. The role of miRNAs specifically in osteoclasts has been of recent investigation, due to clinical interest in discovering new paradigms to control excessive bone resorption, as is observed in multiple conditions including aging, estrogen deprivation, cancer metastases or glucocorticoid use. Therefore understanding the role that miRNAs play during osteoclastic differentiation is of critical importance. In this review, we highlight and discuss general aspects of miRNA function in osteoclasts, including exciting data demonstrating that miRNAs encapsulated in extracellular vesicles (EVs) either originating from osteoclasts, or signaling to osteoclast from divergent sites, have important roles in bone homeostasis.