Clinical and cellular manifestations of OSTM1-related infantile osteopetrosis

Hematopoietic stem cell transplantation, a curative approach in infantile osteopetrosis

Most patients with osteopetrosis (OPT) can be causally and curatively treated with allogeneic hematopoietic stem cell transplantation (HSCT) because osteoclasts are derived from the HSC. However, HSCT is contraindicated in some forms of OPT, namely OPT with neurodegeneration (in all patients with OSTM1 and about half of patients with CLCN7 mutations) and OPT caused by an osteoblast defect (patients with RANKL mutations). HSCT for OPT risks serious side effects, such as transplant failure, venous occlusive disease, pulmonary hypertension, and hypercalcemic crises. Nevertheless, the success rate of HSCT has improved significantly in recent decades. This applies, in particular, to HSCT from non-HLA compatible (haploidentical) donors. Therefore, nowadays an HSCT can be discussed for intermediate OPT forms. After a successful HSCT, most patients have very good quality of life, but about two-thirds are visually impaired, and in rarer cases show motor and neurological disabilities. Early diagnosis, further improvements in transplantation procedures, and advances to improve quality-of-life after transplantation are challenges for the future.

The clinical features of OSTM1-associated malignant infantile osteopetrosis: A retrospective, single-center experience over one decade

Mutation in OSTM1 give rise to the rarest and most lethal subtype of malignant infantile osteopetrosis (MIOP), and an improved understanding of OSTM1-associated MIOP would help with informed decision-making regarding symptom management and early palliative care referral. This retrospective study describes the clinical and laboratory features of patients with a genetic diagnosis of OSTM1 MIOP made between January 2011 and December 2021 in the Department of Pediatrics, Al-Adan Hospital, Kuwait. Twenty-two children had confirmed homozygous deletion in OSTM1 (13 females, nine males). Consanguinity was reported in almost all parents. 72.7% were diagnosed before the age of two months, most commonly incidentally with a high clinical suspicion. All 22 patients developed upper respiratory symptoms, hepatosplenomegaly, poor feeding, and had severe developmental delay. 80% of patients developed pain and/or irritability, and 40.9% were diagnosed with primary seizures. Bone fractures developed in 27% of patients, most likely iatrogenic, and some patients had hernia and gum abnormalities. The mean survival was 10.9 months. The clinical presentation, symptomatology, and mortality of our cohort were compared with other cases of OSTM1 MIOP identified through a comperhensive search of the PubMed database. The findings conclude that OSTM1 MIOP is a multi-systemic disease with distinct clinical features, of which neurological complications are the most severe and include nociplastic pain and irritability. Although orthopedic complications influence the trajectory of most patients with other forms of osteopetrosis, OSTM1 MIOP is driven by its neurological complications. Hence, OSTM1 should be regarded as a neurodegenerative disease with osteopetrosis as a comorbidity that warrants early palliative care referral.

OSTM1 pleiotropic roles from osteopetrosis to neurodegeneration

Autosomal recessive osteopetroses (ARO) are rare genetic skeletal disorders of high clinical and molecular heterogeneity with an estimated frequency of 1:250,000 worldwide. The manifestations are diverse and although individually rare, the various forms contribute to the prevalence of a significant number of affected individuals with considerable morbidity and mortality. Among the ARO classification, the most severe form is the autosomal recessive-5 (OPTB5) osteopetrosis (OMIM 259720) that results from homozygous mutation in the OSTM1 gene (607649). OSTM1 mutations account for approximately 5 % of instances of autosomal recessive osteopetrosis and lead to a highly debilitating form of the disease in infancy and death within the first few years of life (Sobacchi et al., 2013) [1].

A Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis is a critical mediator of Ostm1 regulatory network in T lymphopoiesis

Ostm1 mutations cause the severe form of osteopetrosis with bone marrow deficiency in humans and mice, yet a role in T cell ontogeny remains to be determined. Herein, we show that thymi of the Ostm1-null mice (gl/gl) from P8-to-P15 become markedly hypocellular with disturbed architecture. Analysis of gl/gl early T cell program determined a major decrease of 3-fold in bone marrow common lymphoid precursors (CLP), 35-fold in early thymic precursors (ETPs) and 100-fold in T cell double positive subpopulations. Ostm1 ablation in T cell double negative (DN) also appears to induce fast-paced differentiation kinetics with a transitory intermediate CD44⁺CD25^int subpopulation. Transgenic targeting Ostm1 expression from the gl/gl DN1 population partially rescued T cell subpopulations from ETP onwards and normalized the accelerated DN differentiation, indicating a cell-autonomous role for Ostm1. Transcriptome of early DN1 population identified an Ostm1 crosstalk with a Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis. Our findings establish that Ostm1 is an essential regulator of T cell ontogeny.

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Loss of Ostm1 causes severe thymus hypocellularity

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Ostm1 is a modulator of the T cell differentiation program from the CLPs onwards

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Targeted CD2-Ostm1 in Ostm1 null mice leads to partial rescue of DN differentiation

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Ostm1 null DN1 transcriptome identifies a Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis

Efficient generation of osteoclasts from human induced pluripotent stem cells and functional investigations of lethal CLCN7-related osteopetrosis

Human induced pluripotent stem cells (hiPSCs) hold great potential for modeling human diseases and the development of innovative therapeutic approaches. Here, we report on a novel, simplified differentiation method for forming functional osteoclasts from hiPSCs. The three-step protocol starts with embryoid body formation, followed by hematopoietic specification, and finally osteoclast differentiation. We observed continuous production of monocyte-like cells over a period of up to 9 weeks, generating sufficient material for several osteoclast differentiations. The analysis of stage-specific gene and surface marker expression proved mesodermal priming, the presence of monocyte-like cells, and of terminally differentiated multinucleated osteoclasts, able to form resorption pits and trenches on bone and dentine in vitro. In comparison to peripheral blood mononuclear cell (PBMC)-derived osteoclasts hiPSC-derived osteoclasts were larger and contained a higher number of nuclei. Detailed functional studies on the resorption behavior of hiPSC-osteoclasts indicated a trend towards forming more trenches than pits and an increase in pseudoresorption. We used hiPSCs from an autosomal recessive osteopetrosis (ARO) patient (BIHi002-A, ARO hiPSCs) with compound heterozygous missense mutations p.(G292E) and p.(R403Q) in CLCN7, coding for the Cl^- /H⁺ -exchanger ClC-7, for functional investigations. The patient's leading clinical feature was a brain malformation due to defective neuronal migration. Mutant ClC-7 displayed residual expression and retained lysosomal co-localization with OSTM1, the gene coding for the osteopetrosis-associated transmembrane protein 1, but only ClC-7 harboring the mutation p.(R403Q) gave strongly reduced ion currents. An increased autophagic flux in spite of unchanged lysosomal pH was evident in undifferentiated ARO hiPSCs. ARO hiPSC-derived osteoclasts showed an increased size compared to hiPSCs of healthy donors. They were not able to resorb bone, underlining a loss-of-function effect of the mutations. In summary, we developed a highly reproducible, straightforward hiPSC-osteoclast differentiation protocol. We demonstrated that osteoclasts differentiated from ARO hiPSCs can be used as a disease model for ARO and potentially also other osteoclast-related diseases. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Managing challenging pain and irritability in OSTM1 mutation-related infantile malignant osteopetrosis

Osteopetrosis describes a heterogeneous group of diseases characterised by increased bone density due to impaired osteoclast. The malignant infantile autosomal recessive (MIOP) form caused by mutations in OSTM1 is the most severe form of osteopetrosis. Children with this phenotype exhibit multisystemic complications, of which the neuropathic manifestations are the most severe. Infants with MIOP may present with pain and irritability that are likely to become continuous and debilitating as the disease progresses. There is limited understanding of the aetiology and management of pain in MIOP. Here, we describe a 2 month-old infant with OSTM1 mutation-related MIOP presenting with severe irritability and pain. This case provides the opportunity to discuss the cause and management of these distressing symptoms. We also review similar cases and the possible underlying mechanisms of pain and irritability to help provide a conceptual framework for the management of these symptoms in infants with OSTM1 MIOP.

Expression pattern of the V5-Ostm1 protein in bacterial artificial chromosome transgenic mice

Mutations in the osteopetrotic transmembrane protein 1 (Ostm1) gene are responsible for the most severe form of autosomal recessive osteopetrosis both in humans and in the gray lethal (gl/gl) mouse. This defect leads to increased bone mass with bone marrow occlusion and hematopoietic defects. To establish the expression profile of the mouse Ostm1 protein in vivo, homologous recombination in bacteria was designed to generate a V5-Ostm1 bacterial artificial chromosome (BAC) that was subsequently integrated in the mouse genome. Tissue expression of the transgene V5-Ostm1 RNA and protein in transgenic mice follow the endogenous expression profile. Immunohistochemistry analysis demonstrated expression in neuronal populations from central and peripheral nervous system and defined a unique cellular expression pattern. Importantly, together with appropriate protein post-translational modification, in vivo rescue of the osteopetrotic bone gl/gl phenotype in BAC V5-Ostm1 gl/gl mice is consistent with the expression of a fully functional and active protein. These mice represent a unique tool to unravel novel Ostm1 functions in individual tissue and neuronal cell populations and the V5-Ostm1 transgene represents an easy visual marker to monitor the expression of Ostm1 in vitro and in vivo.

Ostm1 from Mouse to Human: Insights into Osteoclast Maturation

The maintenance of bone mass is a dynamic process that requires a strict balance between bone formation and resorption. Bone formation is controlled by osteoblasts, while osteoclasts are responsible for resorption of the bone matrix. The opposite functions of these cell types have to be tightly regulated not only during normal bone development, but also during adult life, to maintain serum calcium homeostasis and sustain bone integrity to prevent bone fractures. Disruption of the control of bone synthesis or resorption can lead to an over accumulation of bone tissue in osteopetrosis or conversely to a net depletion of the bone mass in osteoporosis. Moreover, high levels of bone resorption with focal bone formation can cause Paget’s disease. Here, we summarize the steps toward isolation and characterization of the osteopetrosis associated trans-membrane protein 1 (Ostm1) gene and protein, essential for proper osteoclast maturation, and responsible when mutated for the most severe form of osteopetrosis in mice and humans.

Malignant Infantile Osteopetrosis: A Case Report

Osteopetrosis is a rare genetic disease of bone resorption. It includes a variety of hereditary skeletal disorders that have the main radiographic feature of increased bone density and thickness due to differentiation or functional defects in osteoclast. The clinical presentation varies widely based on the type of osteopetrosis and ranges in severity from asymptomatic to a fatal course. Our case is of the infantile malignant osteopetrosis (IMOP) form. It is inherited as an autosomal recessive pattern that generally starts in intrauterine life and manifests at birth or early childhood. It is the most severe form and has an incidence of 1 in 250,000 births. The patient presented at the age of two months with a history of recurrent fever, recurrent pneumonia, developmental delay, and infantile spasms. Upon examination, she was found to have hepatosplenomegaly, axial hypotonia, limb spasticity, and visual impairment. Genetic testing revealed a homozygous variant of OSTM1 gene, which is a known Saudi mutation of autosomal recessive osteopetrosis (ARO). IMOP should be considered as a rare differential of hepatosplenomegaly. Early diagnosis by clinical picture, imaging, and genetic testing is important to direct the appropriate management in order to prevent disease progression before the irreversible neurological sequelae occur. Patients should be managed by a comprehensive approach, and currently, hematopoietic stem cell transplantation (HSCT) provides a better outcome for IMOP patients.

Molecular signaling in bone cells: Regulation of cell differentiation and survival

The achievement of proper bone mass and architecture, and their maintenance throughout life requires the concerted actions of osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells. The differentiation and activity of osteoblasts and osteoclasts are regulated by molecules produced by matrix-embedded osteocytes, as well as by cross talk between osteoblasts and osteoclasts through secreted factors. In addition, it is likely that direct contact between osteoblast and osteoclast precursors, and the contact of these cells with osteocytes and cells in the bone marrow, also modulates bone cell differentiation and function. With the advancement of molecular and genetic tools, our comprehension of the intracellular signals activated in bone cells has evolved significantly, from early suggestions that osteoblasts and osteoclasts have common precursors and that osteocytes are inert cells in the bone matrix, to the very sophisticated understanding of a network of receptors, ligands, intracellular kinases/phosphatases, transcription factors, and cell-specific genes that are known today. These advances have allowed the design and FDA-approval of new therapies to preserve and increase bone mass and strength in a wide variety of pathological conditions, improving bone health from early childhood to the elderly. We have summarized here the current knowledge on selected intracellular signal pathways activated in osteoblasts, osteocytes, and osteoclasts.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Ostm1 Bifunctional Roles in Osteoclast Maturation: Insights From a Mouse Model Mimicking a Human OSTM1 Mutation

Ostm1 mutations are responsible for the most severe form of osteopetrosis in human and mice. To gain insight into Ostm1 cellular functions, we engineered a conditional in-frame deletion of the Ostm1 transmembrane domain and generated the first Ostm1 mouse model with a human mutation. Systemic targeting of Ostm1 loss of transmembrane domain produced osteopetrosis, as in the null Ostm1 gl/gl mouse. Significantly, conditional osteoclast targeting of Ostm1 resulted in similar osteopetrosis, thereby demonstrating that the intrinsic Ostm1 osteoclast deficiency is solely responsible for the mouse phenotype. Our analysis showed oversized osteoclasts with enhanced multinucleation associated with stimulation of intracellular calcium levels, of Nfatc1 nuclear re-localization, and of specific downstream Nfatc1 target genes, providing compelling evidence that Ostm1 is a negative regulator of preosteoclast fusion. Moreover, mature OCs with Ostm1 loss of transmembrane domain show appropriate levels of intracellular acidification but an altered distribution pattern, highlighting misregulation of endolysosome localization and dispersion. Consistently, the hydrolases tartrate-resistant acid phosphatase (TRAP) and cathepsin K (Ctsk) normally produced are sequestered within the osteoclasts and are not extracellularly secreted. These studies defined bifunctional roles for Ostm1 as a major regulator of preosteoclast cytoskeletal rearrangements toward cell multinucleation and of mature osteoclast intracellular lysosomal trafficking and exocytosis mechanism, both of which are essential for bone resorption. Importantly, these Ostm1 molecular and regulatory functions could serve as preclinical targets in this mouse model toward osteoclastogenic pathologies as osteoporosis and inflammation-induced bone loss. © 2018 American Society for Bone and Mineral Research.

Anesthesia Management of a Child with Osteopetrosis

Osteopetrosis is a rare genetic disorder of osteoclast dysfunction leading to anatomical and physiological disorders. We present the anesthesia management for the femur fracture of a 4-year-old girl with malignant infantile type of osteopetrosis. She had a ventriculoperitoneal shunt, impaired motion, visual disturbance, growth failure, facial deformity, heart murmur of moderate tricuspid regurgitation, and left ventricular heart failure, with splenomegaly and severe anemia.

Hematopoietic cell transplantation for a child with OSTM1 osteopetrosis

HCT prior to onset of neurologic symptoms in children with OSTM1 osteopetrosis does not halt neurologic progression.

Regulatory-auxiliary subunits of CLC chloride channel-transport proteins

The CLC family of chloride channels and transporters is composed by nine members, but only three of them, ClC-Ka/b, ClC-7 and ClC-2, have been found so far associated with auxiliary subunits. These CLC regulatory subunits are small proteins that present few common characteristics among them, both structurally and functionally, and their effects on the corresponding CLC protein are different. Barttin, a protein with two transmembrane domains, is essential for the membrane localization of ClC-K proteins and their activity in the kidney and inner ear. Ostm1 is a protein with a single transmembrane domain and a highly glycosylated N-terminus. Unlike the other two CLC auxiliary subunits, Ostm1 shows a reciprocal relationship with ClC-7 for their stability. The subcellular localization of Ostm1 depends on ClC-7 and not the other way around. ClC-2 is active on its own, but GlialCAM, a transmembrane cell adhesion molecule with two extracellular immunoglobulin (Ig)-like domains, regulates its subcellular localization and activity in glial cells. The common theme for these three proteins is their requirement for a proper homeostasis, since their malfunction leads to distinct diseases. We will review here their properties and their role in normal chloride physiology and the pathological consequences of their improper function.

Role of Ostm1 Cytosolic Complex with Kinesin 5B in Intracellular Dispersion and Trafficking

In humans and in mice, mutations in the Ostm1 gene cause the most severe form of osteopetrosis, a major bone disease, and neuronal degeneration, both of which are associated with early death. To gain insight into Ostm1 function, we first investigated by sequence and biochemical analysis an immature 34-kDa type I transmembrane Ostm1 protein with a unique cytosolic tail. Mature Ostm1 is posttranslationally processed and highly N-glycosylated and has an apparent mass of ∼60 kDa. Analysis the subcellular localization of Ostm1 showed that it is within the endoplasmic reticulum, trans-Golgi network, and endosomes/lysosomes. By a wide protein screen under physiologic conditions, several novel cytosolic Ostm1 partners were identified and validated, for which a direct interaction with the kinesin 5B heavy chains was demonstrated. These results determined that Ostm1 is part of a cytosolic scaffolding multiprotein complex, imparting an adaptor function to Ostm1. Moreover, we uncovered a role for the Ostm1/KIF5B complex in intracellular trafficking and dispersion of cargos from the endoplasmic reticulum to late endosomal/lysosomal subcellular compartments. These Ostm1 molecular and cellular functions could elucidate all of the pathophysiologic mechanisms underlying the wide phenotypic spectrum of Ostm1-deficient mice.

A case of infantile osteopetrosis: The radioclinical features with literature update

BACKGROUND

Osteopetrosis is a rare hereditary metabolic bone disorder characterized by generalized skeletal sclerosis caused by a defect in bone resorption and remodelling. Infantile autosomal recessive osteopetrosis is one of three subtypes of osteopetrosis and the most severe form. The correct and early diagnosis of infantile osteopetrosis is important for management of complications and for future genetic counselling. Diagnosis is largely based on clinical and radiographic evaluation, confirmed by gene testing where applicable.

METHODS

Therefore, in this case study the classical clinical and radiological signs of a boy with infantile osteopetrosis will be presented with a comprehensive literature update. The differentiating signs from other causes of hereditary osteosclerosing dysplasias are discussed.

RESULTS

This case study and review of available literature show that there tends to be a highly unique clinical and skeletal radiographic pattern of affection in infantile osteopetrosis.

CONCLUSION

Although tremendous advances have been made in the elucidation of the genetic defect of osteopetrosis over the past years, the role of accurate clinical and radiological assessment remains an important contributor to the diagnosis of infantile osteopetrosis.

Secretion of a truncated osteopetrosis-associated transmembrane protein 1 (OSTM1) mutant inhibits osteoclastogenesis through down-regulation of the B lymphocyte-induced maturation protein 1 (BLIMP1)-nuclear factor of activated T cells c1 (NFATc1) axis

Genetic mutations in osteoclastogenic genes are closely associated with osteopetrotic bone diseases. Genetic defects in OSTM1 (osteopetrosis-associated transmembrane protein 1) cause autosomal recessive osteopetrosis in humans. In particular, OSTM1 mutations that exclude the transmembrane domain might lead to the production of a secreted form of truncated OSTM1. However, the precise role of the secreted form of truncated OSTM1 remains unknown. In this study, we analyzed the functional role of truncated OSTM1 in osteoclastogenesis. Here, we showed that a secreted form of truncated OSTM1 binds to the cell surface of osteoclast (OC) precursors and inhibits the formation of multinucleated OCs through the reduction of cell fusion and survival. Truncated OSTM1 significantly inhibited the expression of OC marker genes through the down-regulation of the BLIMP1 (B lymphocyte-induced maturation protein 1)-NFATc1 (nuclear factor of activated T cells c1) axis. Finally, we demonstrated that truncated OSTM1 reduces lipopolysaccharide-induced bone destruction in vivo. Thus, these findings suggest that autosomal recessive osteopetrosis patients with an OSTM1 gene mutation lacking the transmembrane domain produce a secreted form of truncated OSTM1 that inhibits osteoclastogenesis.

Severe neurodegeneration with impaired autophagy mechanism triggered by ostm1 deficiency

Loss of Ostm1 leads to the most severe form of osteopetrosis in mice and humans. Because functional rescue of the osteopetrotic defect in these mice extended their lifespan from ∼3 weeks to 6 weeks, this unraveled a second essential role of Ostm1. We discovered that Ostm1 is highly expressed in the mouse brain in neurons, microglia, and astrocytes. At 3-4 weeks of age, mice with Ostm1 loss showed 3-10-fold stimulation of reactive gliosis, with an increased astrocyte cell population and microglia activation. This inflammatory response was associated with marked retinal photoreceptor degeneration and massive neuronal loss in the brain. Intracellular characterization of neurons revealed abnormal storage of carbohydrates, lipids, and ubiquitinated proteins, combined with marked accumulation of autophagosomes that causes frequent axonal swelling. Stimulation of autophagy was provided by specific markers and by significant down-regulation of the mammalian target of rapamycin signaling, identifying a cellular pathologic mechanism. A series of transgenic mouse lines specifically targeted to distinct central nervous system cell subpopulations determined that Ostm1 has a primary and autonomous role in neuronal homeostasis. Complete functional complementation demonstrated that the development of severe and rapid neurodegeneration in these mice is independent of the hematopoietic lineage and has clinical implications for treatment of osteopetrosis. Importantly, this study establishes a novel neurodegenerative mouse model critical for understanding the multistep pathogenic cascade of cellular autophagy disorders toward therapeutic strategy design.

Malignant infantile osteopetrosis: case report with review of literature

Malignant Infantile Osteopetrosis (MIOP) is a rare genetic disorder due to osteoclast abnormal activity. We report a thirteen month-old male patient, diagnosed as MIOP while investigating the cause of hepatosplenomegaly associated with hydrocephalus. His medical history revealed non consanguineous parents and one brother's death at the same age of unknown etiology (similar symptoms). Systemic examination showed hepatosplenomegaly, growth failure, developmental milestones delay, and rickets features. Ophthalmic exam yielded bilateral optic atrophy. Skeleton radiographs detected generalized dense bone and rickets. Cerebral CT scan revealed hydrocephalus. Histological examination showed hypoplastic bone marrow and extra-medullary hematopoeisis. Diagnosis was confirmed by genetic testing that showed two heterozygote mutations within the TCIRG1 gene. The patient received supportive treatment. He died from an acute respiratory distress. MIOP should be kept in mind as a rare cause of hepatosplenomegaly. Early diagnosis and timely Hematopoietic stem cell transplantation are the only curative approach for an otherwise fatal disease.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

Osteopetrosis: genetics, treatment and new insights into osteoclast function

Osteopetrosis is a genetic condition of increased bone mass, which is caused by defects in osteoclast formation and function. Both autosomal recessive and autosomal dominant forms exist, but this Review focuses on autosomal recessive osteopetrosis (ARO), also known as malignant infantile osteopetrosis. The genetic basis of this disease is now largely uncovered: mutations in TCIRG1, CLCN7, OSTM1, SNX10 and PLEKHM1 lead to osteoclast-rich ARO (in which osteoclasts are abundant but have severely impaired resorptive function), whereas mutations in TNFSF11 and TNFRSF11A lead to osteoclast-poor ARO. In osteoclast-rich ARO, impaired endosomal and lysosomal vesicle trafficking results in defective osteoclast ruffled-border formation and, hence, the inability to resorb bone and mineralized cartilage. ARO presents soon after birth and can be fatal if left untreated. However, the disease is heterogeneous in clinical presentation and often misdiagnosed. This article describes the genetics of ARO and discusses the diagnostic role of next-generation sequencing methods. The management of affected patients, including guidelines for the indication of haematopoietic stem cell transplantation (which can provide a cure for many types of ARO), are outlined. Finally, novel treatments, including preclinical data on in utero stem cell treatment, RANKL replacement therapy and denosumab therapy for hypercalcaemia are also discussed.

Regulation of lysosome biogenesis and functions in osteoclasts

In order to resorb the mineralized bone extracellular matrix, the osteoclast relies on the generation of a resorption lacuna characterized by the presence of specific proteases and a low pH. Hence, bone resorption by osteoclasts is highly dependent on lysosomes, the organelles specialized in intra- and extracellular material degradation. This is best illustrated by the fact that multiple forms of human osteopetrosis are caused by mutations in genes encoding for lysosomal proteins. Yet, until recently, the molecular mechanisms regulating lysosomal biogenesis and function in osteoclasts were poorly understood. Here we review the latest developments in the study of lysosomal biogenesis and function in osteoclasts with an emphasis on the transcriptional control of these processes.

Severe neuronopathic autosomal recessive osteopetrosis due to homozygous deletions affecting OSTM1

Autosomal recessive osteopetrosis (ARO, MIM 259700) is a genetically heterogeneous rare skeletal disorder characterized by failure of osteoclast resorption leading to pathologically increased bone density, bone marrow failure, and fractures. In the neuronopathic form neurological complications are especially severe and progressive. An early identification of the underlying genetic defect is imperative for assessment of prognosis and treatment by hematopoietic stem cell transplantation. Here we describe for the first time homozygous microdeletions of different sizes affecting the OSTM1 gene in two unrelated consanguineous families with children suffering from neuronopathic infantile malignant osteopetrosis. Patients showed an exceptionally severe phenotype with variable CNS malformations, seizures, blindness, and deafness. Multi-organ failure due to sepsis led to early death between six weeks and five months of age in spite of intensive care treatment. Analysis of the breakpoints revealed different mechanisms underlying both rearrangements. Microdeletions seem to represent a considerable portion of OSTM1 mutations and should therefore be included in a sufficient diagnostic screening.

Association of severe autosomal recessive osteopetrosis and structural brain abnormalities: a case report and review of the literature

We describe a fetus with severe osteopetrosis diagnosed on post-mortem radiographs following termination of pregnancy at 29 weeks for major brain malformations detected on ultrasound. SNP microarray confirmed loss of heterozygosity in 5% of the genome, consistent with parental consanguinity. Sequencing of the genes known to cause severe recessive osteopetrosis, TCIRG1, CLCN7, OSTM1 and SNX10, was negative. Brain malformations are not typically considered part of the phenotypic spectrum of osteopetrosis. We review the literature, and propose that this may represent a novel autosomal recessive variant of osteopetrosis.

Impaired vibration of auditory ossicles in osteopetrotic mice

In the middle ear, a chain of three tiny bones (ie, malleus, incus, and stapes) vibrates to transmit sound from the tympanic membrane to the inner ear. Little is known about whether and how bone-resorbing osteoclasts play a role in the vibration of auditory ossicles. We analyzed hearing function and morphological features of auditory ossicles in osteopetrotic mice, which lack osteoclasts because of the deficiency of either cytokine RANKL or transcription factor c-Fos. The auditory brainstem response showed that mice of both genotypes experienced hearing loss, and laser Doppler vibrometry revealed that the malleus behind the tympanic membrane failed to vibrate. Histological analysis and X-ray tomographic microscopy using synchrotron radiation showed that auditory ossicles in osteopetrotic mice were thicker and more cartilaginous than those in control mice. Most interestingly, the malleal processus brevis touched the medial wall of the tympanic cavity in osteopetrotic mice, which was also the case for c-Src kinase-deficient mice (with normal numbers of nonresorbing osteoclasts). Osteopetrotic mice showed a smaller volume of the tympanic cavity but had larger auditory ossicles compared with controls. These data suggest that osteoclastic bone resorption is required for thinning of auditory ossicles and enlargement of the tympanic cavity so that auditory ossicles vibrate freely.

Osteoclast activity and subtypes as a function of physiology and pathology--implications for future treatments of osteoporosis

Osteoclasts have traditionally been associated exclusively with catabolic functions that are a prerequisite for bone resorption. However, emerging data suggest that osteoclasts also carry out functions that are important for optimal bone formation and bone quality. Moreover, recent findings indicate that osteoclasts have different subtypes depending on their location, genotype, and possibly in response to drug intervention. The aim of the current review is to describe the subtypes of osteoclasts in four different settings: 1) physiological, in relation to turnover of different bone types; 2) pathological, as exemplified by monogenomic disorders; 3) pathological, as identified by different disorders; and 4) in drug-induced situations. The profiles of these subtypes strongly suggest that these osteoclasts belong to a heterogeneous cell population, namely, a diverse macrophage-associated cell type with bone catabolic and anabolic functions that are dependent on both local and systemic parameters. Further insight into these osteoclast subtypes may be important for understanding cell-cell communication in the bone microenvironment, treatment effects, and ultimately bone quality.

Distinct neuropathologic phenotypes after disrupting the chloride transport proteins ClC-6 or ClC-7/Ostm1

The proteins ClC-6 and ClC-7 are expressed in the endosomal-lysosomal system. Because Clcn6-deficient mice display some features of neuronal ceroid lipofuscinosis (NCL), CLCN6 may be a candidate gene for novel forms of NCL. Using landmarks of disease progression from NCL mouse models as a guide, we examined neuropathologic alterations in the central nervous system of Clcn6(-/-), Clcn7(-/-), andgl mice. gl mice bear a mutation in Ostm1, the β-subunit critical for Clcn7 function. Severely affected Clcn7(-/-) and gl mice have remarkably similar neuropathologic phenotypes, with pronounced reactive changes and neuron loss in the thalamocortical system, similar to findings in early-onset forms of NCL. In contrast, Clcn6(-/-) mice display slowly progressive, milder neuropathologic features with very little thalamic involvement or microglial activation. These findings detail for the first time the markedly different neuropathologic consequences of mutations in these two CLC genes. Clcn7(-/-) and gl mice bear a close resemblance to the progressive neuropathologic phenotypes of early onset forms of NCL, whereas the distinct phenotype of Clcn6-deficient mice suggests that this gene could be a candidate for a later-onset form of mild neurologic dysfunction with some NCL-like features.

A novel CLCN7 mutation resulting in a most severe form of autosomal recessive osteopetrosis

Osteopetrosis is a bone disease characterized by osteoclast failure and impaired bone resorption. Genetically, it is classified in three forms with autosomal recessive (ARO), autosomal dominant, and intermediate autosomal recessive inheritance, respectively. Some ARO forms are also associated with primary neurodegeneration, retinal atrophy, and lysosomal storage, which are caused by CLCN7 and OSTM1 gene mutations. Herein, we present a unique consanguineous family with a 26-month-old child with osteopetrosis, neurodegeneration, retinal atrophy, and tubulopathy. Direct sequencing of the CLCN7 gene showed a novel homozygous R561Q variant in the patient. Both healthy parents were heterozygous for this amino acid substitution indicating autosomal recessive inheritance. The same homozygous nucleotide transition was found prenatally in a second child and the pregnancy was terminated at 17th week of gestation. A full autopsy was performed to the fetus, which confirmed the presence of osteopetrosis, thereby indicating that the variant observed indeed represents the disease-causing mutation. This is the first report of ARO associated with a novel recessive R561Q variant in CLCN7 gene, in which prenatal diagnosis was made.

A single-center experience in 20 patients with infantile malignant osteopetrosis

Infantile malignant osteopetrosis (IMO) includes various genetic disorders that affect osteoclast development and/or function. Genotype-phenotype correlation studies in IMO have been hampered by the rarity and heterogeneity of the disease and by the severity of the clinical course, which often leads to death early in life. We report on the clinical and molecular findings and treatment in 20 consecutive patients (11 males, nine females) with IMO, diagnosed at a single center in the period 1991-2008. Mean age at diagnosis was 3.9 months, and mean follow-up was 66.75 months. Mutations in TCIRG1, OSTM1, ClCN7, and TNFRSF11A genes were detected in nine, three, one, and one patients, respectively. Six patients remain genetically undefined. OSTM1 and ClCN7 mutations were associated with poor neurologic outcome. Among nine patients with TCIRG1 defects, six presented with hypogammaglobulinemia, and one showed primary pulmonary hypertension. Fourteen patients received hematopoietic cell transplantation; of these, nine are alive and eight of them have evidence of osteoclast function. These data may provide a basis for informed decisions regarding the care of patients with IMO.

Osteopetrosis

Osteopetrosis ("marble bone disease") is a descriptive term that refers to a group of rare, heritable disorders of the skeleton characterized by increased bone density on radiographs. The overall incidence of these conditions is difficult to estimate but autosomal recessive osteopetrosis (ARO) has an incidence of 1 in 250,000 births, and autosomal dominant osteopetrosis (ADO) has an incidence of 1 in 20,000 births. Osteopetrotic conditions vary greatly in their presentation and severity, ranging from neonatal onset with life-threatening complications such as bone marrow failure (e.g. classic or "malignant" ARO), to the incidental finding of osteopetrosis on radiographs (e.g. osteopoikilosis). Classic ARO is characterised by fractures, short stature, compressive neuropathies, hypocalcaemia with attendant tetanic seizures, and life-threatening pancytopaenia. The presence of primary neurodegeneration, mental retardation, skin and immune system involvement, or renal tubular acidosis may point to rarer osteopetrosis variants, whereas onset of primarily skeletal manifestations such as fractures and osteomyelitis in late childhood or adolescence is typical of ADO. Osteopetrosis is caused by failure of osteoclast development or function and mutations in at least 10 genes have been identified as causative in humans, accounting for 70% of all cases. These conditions can be inherited as autosomal recessive, dominant or X-linked traits with the most severe forms being autosomal recessive. Diagnosis is largely based on clinical and radiographic evaluation, confirmed by gene testing where applicable, and paves the way to understanding natural history, specific treatment where available, counselling regarding recurrence risks, and prenatal diagnosis in severe forms. Treatment of osteopetrotic conditions is largely symptomatic, although haematopoietic stem cell transplantation is employed for the most severe forms associated with bone marrow failure and currently offers the best chance of longer-term survival in this group. The severe infantile forms of osteopetrosis are associated with diminished life expectancy, with most untreated children dying in the first decade as a complication of bone marrow suppression. Life expectancy in the adult onset forms is normal. It is anticipated that further understanding of the molecular pathogenesis of these conditions will reveal new targets for pharmacotherapy.

Infantile malignant, autosomal recessive osteopetrosis: the rich and the poor

Human recessive osteopetrosis (ARO) represents a group of diseases in which, due to a defect in osteoclasts, bone resorption is prevented. The deficit could arise either from failure in osteoclast differentiation or from inability to perform resorption by mature, multinucleated, but nonfunctional cells. Historically, osteopetrosis due to both these mechanisms was found in spontaneous and artificially created mouse mutants, but the first five genes identified in human ARO (CA-II, TCIRG1, ClCN7, OSTM1, and PLEKHM1) were all involved in the effector function of mature osteoclasts, being linked to acidification of the cell/bone interface or to intracellular processing of the resorbed material. Differentiation defects in human ARO have only recently been described, following the identification of mutations in both RANKL and RANK, which define a new form of osteoclast-poor ARO, as expected from biochemical, cellular, and animal studies. The molecular dissection of ARO has prognostic and therapeutic implications. RANKL-dependent patients, in particular, represent an interesting subset which could benefit from mesenchymal cell transplant and/or administration of soluble RANKL cytokine.

OSTM1 bone defect reveals an intercellular hematopoietic crosstalk

The most severe form of bone autosomal recessive osteopetrosis both in humans and in the gray-lethal (gl/gl) mouse is caused by mutations in the Ostm1 gene. Although osteopetrosis is usually associated with a defect in the hematopoietic-derived osteoclast cells, this study determined that Ostm1 is expressed in many hematopoietic cells of the myeloid and lymphoid B- and T-lineages. Hematopoiesis in gl/gl mice is characterized by a marked expansion of the osteoclast lineage but also by deregulation of the lymphoid lineages with a decrease in B-lymphoid cell populations and altered distribution in T-lymphoid double and single CD4 CD8-positive cells. In committed gl/gl osteoclasts, specific Ostm1 transgene targeting showed a requirement of additional factors and/or cells for normal osteoclast function, and importantly, defined the gl osteopetrotic defect as non-cell autonomous. By contrast, gl/gl osteoclast, B- and T-lymphoid lineage phenotypes were rescued when Ostm1 is expressed under PU.1 regulation from a bacterial artificial chromosome transgene, which established an essential role for Ostm1 in hematopoietic cells in addition to osteoclasts. Together these experiments are the first to demonstrate the existence of hematopoietic crosstalk for the production of functional and active osteoclasts.

Prognostic potential of precise molecular diagnosis of Autosomal Recessive Osteopetrosis with respect to the outcome of bone marrow transplantation

Hematopoietic stem cell transplantation (HSCT) is often the only practical approach to fatal genetic defects. One of the first pathologies which HSCT was applied to was Autosomal Recessive Osteopetrosis (ARO), a rare genetic bone disease in which a deficit in bone resorption by osteoclasts leads to increased bone density and secondary defects. The disease is often lethal early in life unless treated with HSCT. In utero transplantation (IUT) of the oc/oc mouse, reproducing the clinical features of a subset of ARO, has demonstrated that the quality of life and the survival of transplanted animals are greatly improved, suggesting that a similar protocol could be applied to humans. However, recently the dissection of the molecular bases of the disease has shown that ARO is genetically heterogeneous and has revealed the presence of subsets of patients which do not benefit from HSCT. This observation highlights the importance of molecular diagnosing ARO to identify and establish the proper therapies for a better prognosis. In particular, on the basis of experimental results in murine models, efforts should be undertaken to develop approaches such as IUT and new pharmacological strategies.

CLC chloride channels and transporters: from genes to protein structure, pathology and physiology

CLC genes are expressed in species from bacteria to human and encode Cl(-)-channels or Cl(-)/H(+)-exchangers. CLC proteins assemble to dimers, with each monomer containing an ion translocation pathway. Some mammalian isoforms need essential beta -subunits (barttin and Ostm1). Crystal structures of bacterial CLC Cl(-)/H(+)-exchangers, combined with transport analysis of mammalian and bacterial CLCs, yielded surprising insights into their structure and function. The large cytosolic carboxy-termini of eukaryotic CLCs contain CBS domains, which may modulate transport activity. Some of these have been crystallized. Mammals express nine CLC isoforms that differ in tissue distribution and subcellular localization. Some of these are plasma membrane Cl(-) channels, which play important roles in transepithelial transport and in dampening muscle excitability. Other CLC proteins localize mainly to the endosomal-lysosomal system where they may facilitate luminal acidification or regulate luminal chloride concentration. All vesicular CLCs may be Cl(-)/H(+)-exchangers, as shown for the endosomal ClC-4 and -5 proteins. Human diseases and knockout mouse models have yielded important insights into their physiology and pathology. Phenotypes and diseases include myotonia, renal salt wasting, kidney stones, deafness, blindness, male infertility, leukodystrophy, osteopetrosis, lysosomal storage disease and defective endocytosis, demonstrating the broad physiological role of CLC-mediated anion transport.