Chloride channel 7 (ClCN7) gene mutations and autosomal dominant osteopetrosis, type II

Effect of Allele-Specific Clcn7G213R siRNA Delivered Via a Novel Nanocarrier on Bone Phenotypes in ADO2 Mice on 129S Background

Autosomal dominant osteopetrosis type 2 (ADO2) is a rare inherited bone disorder characterised by dense but brittle bones. It displays striking phenotypic variability, with the most severe symptoms, including blindness and bone marrow failure. Disease management largely relies on symptomatic treatment since there is no safe and effective treatment. Most ADO2 cases are caused by heterozygous loss-of-function mutations in the CLCN7 gene, which encodes an essential Cl-/H+ antiporter for proper bone resorption by osteoclasts. Thus, siRNA-mediated silencing of the mutant allele is a promising therapeutic approach, but targeting bone for first-in-human translation remains challenging. Here, we demonstrate the utility of silicon-stabilised hybrid lipid nanoparticles (sshLNPs) as a next-generation nucleic acid nanocarrier capable of delivering allele-specific siRNA to bone. Using a Clcn7G213R knock-in mouse model recapitulating one of the most common human ADO2 mutations and based on the 129S genetic background (which produces the most severe disease phenotype amongst current models), we show substantial knockdown of the mutant allele in femur when siRNA targeting the pathogenic variant is delivered by sshLNPs. We observed lower areal bone mineral density in femur and reduced trabecular thickness in femur and tibia, when siRNA-loaded sshLNPs were administered subcutaneously (representing the most relevant administration route for clinical adoption and patient adherence). Importantly, sshLNPs have improved stability over conventional LNPs and enable 'post hoc loading' for point-of-care formulation. The treatment was well tolerated, suggesting that sshLNP-enabled gene therapy might allow successful clinical translation of essential new treatments for ADO2 and potentially other rare genetic bone diseases.

Patient-Reported Outcomes in Autosomal Dominant Osteopetrosis: Findings from the Osteopetrosis Registry Study

CONTEXT

Autosomal dominant osteopetrosis (ADO) is a rare sclerotic bone disease characterized by impaired osteoclast activity, resulting in high bone mineral density and skeletal fragility. The full phenotype and disease burden on patients' daily lives has not been systematically measured.

OBJECTIVE

We developed an online registry to ascertain population-based data on the spectrum and rate of progression of disease and to identify relevant patient centered outcomes that could be used to measure treatment effects and guide the design of future clinical trials.

DESIGN

Cross-sectional data from participants with osteopetrosis were collected using an online REDCap-based database.

PARTICIPANTS

Thirty-four participants with a confirmed diagnosis of ADO, aged 4-84 years.

MAIN OUTCOME MEASURES

Participants aged 18 years and older completed the PROMIS 57, participants aged 8 to 17 years completed the PROMIS Pediatric 49, and parents of participants aged <18 years completed the PROMIS Parent Proxy 49.

RESULTS

Based on the PROMIS 57, relative to the general population, adults with ADO reported low physical function and low ability to participate in social roles and activities, and high levels of anxiety, fatigue, sleep problems, and pain interference. Daily pain medications were reported by 24% of the adult population. In contrast, neither pediatric participants, nor their parent proxy reported a negative impact on health-related quality of life.

CONCLUSIONS

Data from this registry demonstrate the broad spectrum of ADO disease severity and high impact on health-related quality of life in adults with ADO.

Effect of Roflumilast, a Selective PDE4 Inhibitor, on Bone Phenotypes in ADO2 Mice

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption that usually results from heterozygous missense mutations in the chloride channel 7 (CLCN7) gene. We previously created mouse models of ADO2 (p.G213R) with one of the most common mutations (G215R) as found in humans and demonstrated that this mutation in mice phenocopies the human disease of ADO2. Previous studies have shown that roflumilast (RF), a selective phosphodiesterase 4 (PDE4) inhibitor that regulates the cAMP pathway, can increase osteoclast activity. We also observed that RF increased bone resorption in both wild-type and ADO2 heterozygous osteoclasts in vitro, suggesting it might rescue bone phenotypes in ADO2 mice. To test this hypothesis, we administered RF-treated diets (0, 20 and 100 mg/kg) to 8-week-old ADO2 mice for 6 months. We evaluated bone mineral density and bone micro-architecture using longitudinal in-vivo DXA and micro-CT at baseline, and 6-, 12-, 18-, and 24-week post-baseline time points. Additionally, we analyzed serum bone biomarkers (CTX, TRAP, and P1NP) at baseline, 12-, and 24-week post-baseline. Our findings revealed that RF treatment did not improve aBMD (whole body, femur, and spine) and trabecular BV/TV (distal femur) in ADO2 mice compared to the control group treated with a normal diet. Furthermore, we did not observe any significant changes in serum levels of bone biomarkers due to RF treatment in these mice. Overall, our results indicate that RF does not rescue the osteopetrotic bone phenotypes in ADO2 heterozygous mice.

The PDE4 Inhibitors Roflumilast and Rolipram Rescue ADO2 Osteoclast Resorption Dysfunction

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption caused by heterozygous missense mutations in the chloride channel 7 (CLCN7). Adenylate cyclase, which catalyzes the formation of cAMP, is critical for lysosomal acidification in osteoclasts. We found reduced cAMP levels in ADO2 osteoclasts compared to wild-type (WT) osteoclasts, leading us to examine whether regulating cAMP would improve ADO2 osteoclast activity. Although forskolin, a known activator of adenylate cyclase and cAMP levels, negatively affected osteoclast number, it led to an overall increase in ADO2 and WT osteoclast resorption activity in vitro. Next, we examined cAMP hydrolysis by the phosphodiesterase 4 (PDE4) proteins in ADO2 versus WT osteoclasts. QPCR analysis revealed higher expression of the three major PDE4 subtypes (4a, 4b, 4d) in ADO2 osteoclasts compared in WT, consistent with reduced cAMP levels in ADO2 osteoclasts. In addition, we found that the PDE4 antagonists, rolipram and roflumilast, stimulated ADO2 and WT osteoclast formation in a dose-dependent manner. Importantly, roflumilast and rolipram displayed a concentration-dependent increase in osteoclast resorption activity which was greater in ADO2 than WT osteoclasts. Moreover, treatment with roflumilast rescued cAMP levels in ADO2 OCLs. The key findings from our studies demonstrate that osteoclasts from ADO2 mice exhibit reduced cAMP levels and PDE4 inhibition rescues cAMP levels and ADO2 osteoclast activity dysfunction in vitro. The mechanism of action of PDE4 inhibitors and their ability to reduce the high bone mass of ADO2 mice in vivo are currently under investigation. Importantly, these studies advance the understanding of the mechanisms underlying the ADO2 osteoclast dysfunction which is critical for the development of therapeutic approaches to treat clinically affected ADO2 patients.

Bone marrow transplantation as a therapy for autosomal dominant osteopetrosis type 2 in mice

Autosomal dominant osteopetrosis type II (ADO2) is a heritable bone disease of impaired osteoclastic bone resorption caused by missense mutations in the chloride channel 7 (CLCN7) gene. Clinical features of ADO2 include fractures, osteomyelitis of jaw, vision loss, and in severe cases, bone marrow failure. Currently, there is no effective therapy for ADO2, and patients usually receive symptomatic treatments. Theoretically, bone marrow transplantation (BMT), which is commonly used in recessive osteopetrosis, could be used to treat ADO2, although the frequency of complications related to BMT is quite high. We created an ADO2 knock-in (p.G213R mutation) mouse model on the 129 genetic background, and their phenotypes mimic the human disease of ADO2. To test whether BMT could restore osteoclast function and rescue the bone phenotypes in ADO2 mice, we transplanted bone marrow cells from 6-8 weeks old male WT donor mice into recipient female ADO2 mice. Also, to determine whether age at the time of transplant may play a role in transplant success, we performed BMT in young (12-week-old) and old (9-month-old) ADO2 mice. Our data indicate that ADO2 mice transplanted with WT marrow achieved more than 90% engraftment up to 6 months post-transplantation at both young and old ages. The in-vivo DXA data revealed that young ADO2 mice transplanted with WT marrow had significantly lower whole body and spine areal bone mineral density (aBMD) at month 6 post-transplantation compared to the ADO2 control mice. The old ADO2 mice also displayed significantly lower whole body, femur, and spine aBMD at months 4 and 5 post-transplantation compared to the age-matched control mice. The in-vivo micro-CT data showed that ADO2 experimental mice transplanted with WT marrow had significantly lower BV/TV at months 2 and 4 post-transplantation compared to the ADO2 control mice at a young age. In contrast, ADO2 control and experimental mice displayed similar BV/TV values for all post-transplantation time points at old age. In addition, serum CTX was significantly higher at month 2 post-transplantation in both young and old ADO2 experimental mice compared to the ADO2 control mice. Serum P1NP levels in young ADO2 experimental mice were significantly higher at baseline and month 2 post-transplantation compared to the ADO2 control mice. These data suggest that BMT may provide, at least, some beneficial effect at both young and adult ages.

Unusual Cortical Phenotype After Hematopoietic Stem Cell Transplantation in a Patient With Osteopetrosis

The osteopetroses are a group of rare genetic diseases caused by osteoclast dysfunction or absence. The hallmark of osteopetrosis is generalized increased bone mineral density (BMD). However, the bone is fragile and fractures are common. Autosomal recessive osteopetrosis is usually a severe disorder and often life-threatening in childhood. We present male siblings with autosomal recessive osteopetrosis due to biallelic variants in TCIRG1 who survived childhood and underwent hematopoietic stem cell transplant (HSCT) in adulthood. One sibling died of posttransplant complications. After transplant, the other sibling had improvement of multiple clinical parameters, including some decline in BMD Z-scores by dual-energy X-ray absorptiometry (DXA) and cessation of fractures. However, spine quantitative computed tomography 11 years after transplant demonstrated an anvil pattern of sclerosis with BMD Z-score of +18.3. High-resolution peripheral quantitative computed tomography (HR-pQCT) of the tibia demonstrated near complete obliteration of the marrow space combined with an unusual cortical phenotype, suggesting extensive cortical porosity at the distal tibia. This case highlights that despite successful transplantation and subsequent improvement in clinical parameters, this patient continued to have significantly elevated bone density and decreased marrow space. Transplant-associated increased cortical porosity is multifactorial and occurs in two-thirds of non-osteopetrotic patients undergoing HSCT. This finding after transplant in osteopetrosis may suggest particular sensitivity of the cortical bone to resorptive activity of transplanted osteoclasts. The case also suggests HR-pQCT may be a useful modality for imaging and assessing the therapeutic effects on bone in individuals with osteopetrosis. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Role of the Lysosomal Cl−/H+ Antiporter ClC-7 in Osteopetrosis and Neurodegeneration

CLC proteins comprise Cl⁻ channels and anion/H⁺ antiporters involved in several fundamental physiological processes. ClC-7 is a lysosomal Cl⁻/H⁺ antiporter that together with its beta subunit Ostm1 has a critical role in the ionic homeostasis of lysosomes and of the osteoclasts’ resorption lacuna, although the specific underlying mechanism has so far remained elusive. Mutations in ClC-7 cause osteopetrosis, but also a form of lysosomal storage disease and neurodegeneration. Interestingly, both loss-of- and gain-of-function mutations of ClC-7 can be pathogenic, but the mechanistic implications of this finding are still unclear. This review will focus on the recent advances in our understanding of the biophysical properties of ClC-7 and of its role in human diseases with a focus on osteopetrosis and neurodegeneration.

Natural History of Type II Autosomal Dominant Osteopetrosis: A Single Center Retrospective Study

BACKGROUND

Autosomal dominant osteopetrosis II (ADO II, MIM166600) is a sclerosing bone disorder caused by CLCN7 mutation. The main clinical characteristics include minor trauma-related fracture and hip osteoarthritis, whereas cranial nerve palsy and bone marrow failure rarely develop. Although it is generally believed that ADO II has a relatively benign course, the natural course of the disease in Chinese patients remains unclear.

MATERIALS AND METHODS

Thirty-six patients diagnosed with ADO II in Shanghai Jiao Tong University Affiliated Sixth People's Hospital from 2008 to 2021 were studied retrospectively. Among them, 15 patients were followed for an average of 6.3 years (1-14 years).

RESULTS

In this study, minor trauma-related fractures of the limb were the most typical clinical manifestations. Visual loss (1/36) and bone marrow failure (2/36), was rare in this study. The condition of ADO II seems to be stable in most patients. There were no correlations between markedly elevated bone mineral density (BMD) and minor trauma-related fractures. In total, 21 diseases causing mutations were detected. Among them, the mutation c.2299C>T (p.Arg767Trp) was the most common (16.67%), and mutation c.937G>A [p.(Glu313Lys)] was associated with severe fractures, haematological defects and cranial palsy.

CONCLUSIONS

Minor trauma-related fracture is the most typical clinical manifestation of ADO II and always occurs in. The mutation c.2299C>T (p.Arg767Trp) is in general a relatively common variant, while the mutation c.937G>A [p.(Glu313Lys)] seems to be associated with severe phenotype. In our study, ADO II seems to remain stable over time.

Chloroquine increases osteoclast activity in vitro but does not improve the osteopetrotic bone phenotype of ADO2 mice

Autosomal Dominant Osteopetrosis type II (ADO2) is a bone disease of impaired osteoclastic bone resorption that usually results from heterozygous missense mutations in the chloride channel 7 (CLCN7) gene. We created mouse models of ADO2 by introducing a knock-in (p.G213R) mutation in the Clcn7 gene, which is analogous to one of the common mutations (G215R) found in humans. The mutation leads to severe osteopetrosis and lethality in homozygous mice but produces substantial phenotypic variability in heterozygous mice on different genetic backgrounds that phenocopy the human disease of ADO2. ADO2 is an osteoclast-intrinsic disease, and lysosomal enzymes and proteins are critical for osteoclast activity. Chloroquine (CQ) is known to affect lysosomal trafficking, intracellular signaling and the lysosomal and vesicular pH, suggesting it might improve ADO2 osteoclast function. We tested this hypothesis in cell culture studies using osteoclasts derived from wild-type (WT or ADO2+/+) and ADO2 heterozygous (ADO2+/-) mice and found that CQ and its metabolite desethylchloroquine (DCQ), significantly increased ADO2+/- osteoclasts bone resorption activity in vitro, whereas bone resorption of ADO2+/+ osteoclasts was increased only by DCQ. In addition, we exploited our unique animal model of ADO2 on 129 background to identify the effect of CQ for the treatment of ADO2. Female ADO2 mice at 8 weeks of age were treated with 5 doses of CQ (1, 2.5, 5, 7.5 and 10 mg/kg BW/day) via drinking water for 6 months. Bone mineral density and bone micro-architecture were analyzed by longitudinal in vivo DXA and micro-CT at baseline, 3 and 6 months. Serum bone biomarkers (CTX, TRAP and P1NP) were also analyzed at these time points. CQ treatment at the doses tested failed to produce any significant changes of aBMD, BMC (whole body, femur and spine) and trabecular BV/TV (distal femur) in ADO2 mice compared to the control group (water only). Further, levels of bone biomarkers were not significantly changed due to CQ treatment in these mice. Our findings indicate that while CQ increased osteoclast activity in vitro, it did not improve the osteopetrotic bone phenotypes in ADO2 heterozygous mice.

Clinical and molecular characterization of five Chinese patients with autosomal recessive osteopetrosis

Background

Osteopetrosis is characterized by increased bone density and bone marrow cavity stenosis due to a decrease in the number of osteoclasts or the dysfunction of their differentiation and absorption properties usually caused by biallelic variants of the TCIRG1 and CLCN7 genes.

Methods

In this study, we describe five Chinese children who presented with anemia, thrombocytopenia, hepatosplenomegaly, repeated infections, and increased bone density. Whole‐exome sequencing identified five compound heterozygous variants of the CLCN7 and TCIRG1 genes in these patients.

Results

Patient 1 had a novel variant c.1555C>T (p.L519F) and a previously reported pathogenic variant c.2299C>T (p.R767W) in CLCN7. Patient 2 harbored a novel missense variant (c.1025T>C; p.L342P) and a novel splicing variant (c.286‐9G>A) in CLCN7. Patients 3A and 3B from one family displayed the same compound heterozygous TCIRG1 variant, including a novel frameshift variant (c.1370del; p.T457Tfs*71) and a novel splicing variant (c.1554+2T>C). In Patient 4, two novel variants were identified in the TCIRG1 gene: c.676G>T; p.E226* and c.1191del; p.P398Sfs*5. Patient 5 harbored two known pathogenic variants, c.909C>A (p.Y303*) and c.2008C>T (p.R670*), in TCIRG1. Analysis of the products obtained from the reverse transcription‐polymerase chain reaction revealed that the c.286‐9G>A variant in CLCN7 of patient 2 leads to intron 3 retention, resulting in the formation of a premature termination codon (p.E95Vfs*8). These five patients were eventually diagnosed with autosomal recessive osteopetrosis, and the three children with TCIRG1 variants received hematopoietic stem cell transplantation.

Conclusions

Our results expand the spectrum of variation of genes related to osteopetrosis and deepen the understanding of the relationship between the genotype and clinical characteristics of osteopetrosis.

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).

A Novel Variant in CLCN7 Regulates the Coupling of Angiogenesis and Osteogenesis

Autosomal dominant osteopetrosis type II (ADO II), characterized by increased bone mass and density, is caused by mutations in the chloride channel 7 (CLCN7) gene. In this study, a novel missense variant in CLCN7 (c.1678A > G; p.Met560Val) was identified in three symptomatic subjects and one carrier of a Chinese family with ADO II. Notably, bone formation markers, including osteocalcin and total procollagen type N-terminal propeptide, have increased or presented at the upper limit of the normal range in the three patients. Serum factors secreted by osteoclast lineage cells and affecting the CD31^hiEMCN^hi vessel formation, such as tartrate-resistant acid phosphatase 5b, platelet-derived growth factor-BB, vascular endothelial growth factor, and SLIT3, had a higher expression in three ADO II subjects than in 15 healthy age-matched and sex-matched controls. Moreover, the conditioned medium was obtained from preosteoclast induced from the ADO II patients’ peripheral blood mononuclear cells. It was found to promote the CD31^hiEMCN^hi vessel formation of human microvascular endothelial cells and osteogenic differentiation of bone marrow-derived stem cells. Taken together, our finding revealed a novel CLCN7 variant associated with ADO II and suggested that the sclerotic bone was potentially associated with the increase of the CD31^hiEMCN^hi vessel formation and bone formation.

The Genetic Architecture of High Bone Mass

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

Genotyping, generation and proteomic profiling of the first human autosomal dominant osteopetrosis type II-specific induced pluripotent stem cells

Background

Autosomal dominant osteopetrosis type II (ADO2) is a rare human genetic disease that has been broadly studied as an important osteopetrosis model; however, there are no disease-specific induced pluripotent stem cells (ADO2-iPSCs) that may be valuable for understanding the pathogenesis and may be a potential source of cells for autologous cell-based therapies.

Methods

To generate the first human ADO2-iPSCs from a Chinese family with ADO2 and to identify their characteristics, blood samples were collected from the proband and his parents and were used for genotyping by whole-exome sequencing (WES); the urine-derived cells of the proband were reprogrammed with episomal plasmids that contained transcription factors, such as KLF4, OCT4, c-MYC, and SOX2. The proteome-wide protein quantification and lysine 2-hydroxyisobutyrylation detection of the ADO2-iPSCs and normal control iPSCs (NC-iPSCs) were performed by high-resolution LC-MS/MS and bioinformatics analysis.

Results

WES with filtering strategies identified a mutation in CLCN7 (R286W) in the proband and his father, which was absent in the proband’s mother and the healthy controls; this was confirmed by Sanger sequencing. The ADO2-iPSCs were successfully generated, which carried a normal male karyotype (46, XY) and the mutation of CLCN7 (R286W); the ADO2-iPSCs positively expressed alkaline phosphatase and other surface markers; and no vector and transgene were detected. The ADO2-iPSCs could differentiate into all three germ cell layers, both in vitro and in vivo. The proteomic profiling revealed similar expression of pluripotency markers in the two cell lines and identified 7405 proteins and 3664 2-hydroxyisobutyrylated peptides in 1036 proteins in the ADO2-iPSCs.

Conclusions

Our data indicated that the mutation CLCN7 (R286W) may be a cause of the osteopetrosis family. The generated vector-free and transgene-free ADO2-iPSCs with known proteomic characteristics may be valuable for personalized and cell-based regenerative medicine in the future.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1369-8) contains supplementary material, which is available to authorized users.

Interferon Gamma-1b Does Not Increase Markers of Bone Resorption in Autosomal Dominant Osteopetrosis

In autosomal dominant osteopetrosis type 2 (ADO2) CLCN7 mutations cause impaired osteoclast function. Severe consequences include skeletal fragility despite high bone mass, osteomyelitis, osteonecrosis, bone marrow failure, and severe cranial nerve impingement. There is no effective medical treatment for ADO2. We recruited subjects with ADO2 into a 14-week, open-label, pilot clinical trial of interferon gamma-1b. Doses were titrated based on tolerability and if fasting serum C-telopeptide (CTX) was <25% above baseline at week 8, targeting doses of 100 µg/m² three times a week. The primary outcomes were change from baseline in CTX and N-telopeptide/creatinine ratio (NTX/Cr) at week 14. Secondary outcomes included changes in urine calcium/creatinine ratio, bone formation markers and tolerability. Nine adults and three children were recruited. Severe manifestations of ADO2 included histories of fractures (100%), osteomyelitis (16.7%), vision loss (50%), and anemia (58.3%). Baseline CTX and NTX/Cr were generally low-normal. Procollagen type I N-terminal propeptide was elevated or in the upper-normal range in 11 of 12 (91.6%) subjects. Elevations of aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were common. One subject withdrew due to rash. Five subjects achieved doses of 50 µg/m² 3 days a week, while six reached the full dose of 100 µg/m² 3 days a week. Only 3 of 11 (27.3%) completing subjects achieved the primary outcome of increasing CTX ≥25% above baseline at week 14. The mean ± SD change from baseline in CTX at week 14 was +2.2% ± 43.2%, p = 0.86). Likewise, there was no significant change in NTX/Cr (mean change -2.1%, p = 0.81). Interferon gamma-1b was poorly tolerated. Most subjects had adverse events, and the Mental Health and Mental Component Scales of the SF-36v2 health survey declined slightly (p < 0.05). Over 14 weeks, interferon gamma-1b failed to significantly increase bone turnover markers in ADO2 and was poorly tolerated. Consequently, interferon gamma-1b is unlikely to be effective for decreasing bone mass in ADO2. © 2019 American Society for Bone and Mineral Research.

Testing the Cre-mediated genetic switch for the generation of conditional knock-in mice

The Cre-mediated genetic switch combines the ability of Cre recombinase to stably invert or excise a DNA fragment depending upon the orientation of flanking mutant loxP sites. In this work, we have tested this strategy in vivo with the aim to generate two conditional knock-in mice for missense mutations in the Impad1 and Clcn7 genes causing two different skeletal dysplasias. Targeting constructs were generated in which the Impad1 exon 2 and an inverted exon 2* and the Clcn7 exon 7 and an inverted exon 7* containing the point mutations were flanked by mutant loxP sites in a head-to-head orientation. When the Cre recombinase is present, the DNA flanked by the mutant loxP sites is expected to be stably inverted leading to the activation of the mutated exon.

The targeting vectors were used to generate heterozygous floxed mice in which inversion of the wild-type with the mutant exon has not occurred yet. To generate knock-in mice, floxed animals were mated to a global Cre-deleter mouse strain for stable inversion and activation of the mutation. Unexpectedly the phenotype of homozygous Impad1 knock-in animals overlaps with the lethal phenotype described previously in Impad1 knock-out mice. Similarly, the phenotype of homozygous Clcn7 floxed mice overlaps with Clcn7 knock-out mice. Expression studies by qPCR and RT-PCR demonstrated that mutant mRNA underwent abnormal splicing leading to the synthesis of non-functional proteins. Thus, the skeletal phenotypes in both murine strains were not caused by the missense mutations, but by aberrant splicing. Our data demonstrate that the Cre mediated genetic switch strategy should be considered cautiously for the generation of conditional knock-in mice.

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.

CLCN7 and TCIRG1 mutations in a single family: Evidence for digenic inheritance of osteopetrosis

Osteopetrosis is a monogenic condition with various inheritance patterns, including autosomal dominant, autosomal recessive and X‑linked. Several disease‑causing genes have been identified and three distinguished types of osteopetrosis have been reported. In the present study, a family with osteopetrosis was investigated. Two novel mutations in chloride voltage‑gated channel 7 (CLCN7) and T cell immune regulator 1 (TCIRG1) were identified by exome sequencing, Sanger sequencing and microsatellite marker analysis. The CLCN7 mutation occurred in amino acid R286, the same position as previously reported. The TCIRG1 mutation occurred on a splicing site of exon 15, thereby leading to a truncated transcript. These two mutations were undetected in 496 ethnic‑matched controls. To the best of our knowledge, this is the first report of human osteopetrosis involving digenic inheritance in a single family, which has important implications for decisions on clinical therapeutic regimen, prognosis evaluation and antenatal diagnosis.

Cortical bone quality affectations and their strength impact analysis using holographic interferometry

It is now accepted that bone strength is a complex property determined mainly by three factors: quantity, quality and turnover of the bone itself. Most of the patients who experience fractures due to fragility could never develop affectations related to bone mass density (i.e. osteoporosis). In this work, the effect of secondary bone strength affectations are analyzed by simulating the degradation of one or more principal components (organic and inorganic) while they are inspected with a nondestructive optical technique. From the results obtained, a strong correlation among the hydroxyapatite, collagen and water is found that determines the bone strength.

Sclerosing bone dysplasias

The group of sclerosing bone dysplasia's is a clinically and genetically heterogeneous group of rare bone disorders which, according to the latest Nosology and classification of genetic skeletal disorders (2015), can be subdivided in three subgroups; the neonatal osteosclerotic dysplasias, the osteopetroses and related disorders and the other sclerosing bone disorders. Here, we give an overview of the most important radiographic and clinical symptoms, the underlying genetic defect and potential treatment options of the different sclerosing dysplasias included in these subgroups.

Enhanced Activation of Rac1/Cdc42 and MITF Leads to Augmented Osteoclastogenesis in Autosomal Dominant Osteopetrosis Type II

The autosomal dominant osteopetrosis type II (ADOII) caused by the mutation of chloride channel 7 (ClC‐7) gene is the most common form of adult‐onset osteopetrosis. Despite dysfunctional bone resorption, an augmented osteoclast differentiation was reported recently in ADOII patients. DNA sequencing analysis of the ADOII patient's ClC‐7 gene identified a known heterozygous mutation, c.643G>A in exon 7, encoding p.Gly215Arg. In vitro osteoclast differentiation from the ADOII patient's peripheral blood mononuclear cells (PBMCs) increased compared with control despite their dysfunctional bone resorbing capacity. Osteoclasts from the ADOII patient's PBMCs and ClC‐7 knockdown bone marrow monocytes (BMMs) showed an enhanced Ser‐71 phosphorylation of Rac1/Cdc42 and increase of the microphthalmia‐associated transcription factor (MITF) and receptor activator of NF‐κB (RANK) that can be responsible for the enhanced osteoclast differentiation. © 2018 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

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.

Human Genetics of Sclerosing Bone Disorders

PURPOSE OF REVIEW

The group of sclerosing bone disorders encompasses a variety of disorders all marked by increased bone mass. In this review, we give an overview of the genetic causes of this heterogeneous group of disorders and briefly touch upon the value of these findings for the development of novel therapeutic agents.

RECENT FINDINGS

Advances in the next-generation sequencing technologies are accelerating the molecular dissection of the pathogenic mechanisms underlying skeletal dysplasias. Throughout the years, the genetic cause of these disorders has been extensively studied which resulted in the identification of a variety of disease-causing genes and pathways that are involved in bone formation by osteoblasts, bone resorption by osteoclasts, or both processes. Due to this rapidly increasing knowledge, the insights into the regulatory mechanisms of bone metabolism are continuously improving resulting in the identification of novel therapeutic targets for disorders with reduced bone mass and increased bone fragility.

Cervical spine fractures in osteopetrosis: a case report and review of the literature

While management of appendicular fractures has been well described in the setting of osteopetrosis, there is limited information on managing fractures of the axial spine. Here we present an osteopetrotic patient with multiple traumatic multiple, comminuted, unstable cervical spinal fractures managed with non-operative stabilization, and provide a review of the pathophysiology, genetic characteristics, and special considerations that must be explored when determining operative versus non-operative management of spinal injury in osteopetrosis. A PubMed query was performed for English articles in the literature published up to June 2016, and used the following search terms alone and in combination: "osteopetrosis", "spine", "fractures", "osteoclasts", and "operative management". Within four months after initial injury, treatment with halo vest allowed for adequate healing. The patient was asymptomatic with cervical spine dynamic radiographs confirming stability at four months. On four-year follow up examination, the patient remained without neck pain, and CT scan demonstrated partially sclerotic fracture lines with appropriate anatomical alignment. In conclusion, external halo stabilization may be an effective option for treatment of multiple unstable acute traumatic cervical spine fractures in patients with osteopetrosis. Given the challenge of surgical stabilization in osteopetrosis, further research is necessary to elucidate the optimal form of treatment in this select patient population.

Phenotypic severity of autosomal dominant osteopetrosis type II (ADO2) mice on different genetic backgrounds recapitulates the features of human disease

Autosomal dominant osteopetrosis type II (ADO2) is a heritable osteosclerotic bone disorder due to dysfunctional osteoclast activity. ADO2 is caused by missense mutations in the chloride channel 7 (CLCN7) gene characterized by osteosclerosis with multiple fractures. ADO2 can result in osteomyelitis, visual loss and bone marrow failure. Currently, there is no cure for ADO2, and until recently no appropriate animal model of ADO2 existed to understand better the pathogenesis of this disease and to test new therapies. Therefore, we created ADO2 knock-in mouse model with a G213R (human homolog of G215R) missense mutation in the Clcn7 gene on 129S1 background, and demonstrated that this mouse model phenocopies human ADO2. As ADO2 gives rise to incomplete penetrance (66%) in human and marked phenotypic variability is observed among patients with the same mutation, we hypothesized that the severity and penetrance of ADO2 will also vary in mouse models on different genetic backgrounds. To test this, we created ADO2 mouse models in DBA/D2, C57BL/6J/B6 and Balb/c strains, and compared bone phenotypes and performed serum biochemical analysis between strain- and age-matched wild-type (WT) and ADO2 mice. At 3months of age, whole body aBMD was higher (4-7% in male; 1-5% in female) in the ADO2 mice compared to their wild-type littermates. In addition, ADO2 male mice on 129 background displayed highest percent increase of BV/TV (106%), followed by D2 (92%), B6 (46%), and Balb/c (33%) compared to strain-matched wild-type mice. We observed similar differences for BV/TV between ADO2 and wild-type mice on different genetic backgrounds in female: 129 (96%)>D2 (73%)>Balb/c (39%) and B6 (36%). Serum calcium, phosphorus, alkaline phosphatase and P1NP levels were similar in the WT and ADO2 mice on all genetic backgrounds but TRAP was higher (76% to 220% in male; 33-95% in female) and CTX/TRAP ratio was lower (39-65% in male and 3-41% in female) in the ADO2 mice compared to their strain-matched wild-type littermates. We also found that young (3months) ADO2 mice on 129S1 background exhibited 200% higher trabecular BV/TV whereas old (18months) ADO2 mice displayed 400-700% higher BV/TV compared to their age-matched wild-type controls. In summary, phenotypic severity in ADO2 mice varied markedly on different genetic backgrounds (129>D2>Balb/c>B6) and became more pronounced with age, which resembles the wide variations in phenotype observed in ADO2 patients. These mouse models will help us to identify genes/factors that influence severity and penetrance of ADO2, and test innovative therapies to treat this disease.

Enhanced but hypofunctional osteoclastogenesis in an autosomal dominant osteopetrosis type II case carrying a c.1856C>T mutation in CLCN7

Type II autosomal dominant osteopetrosis (ADO2), which is the most common form of osteopetrosis, is caused by heterozygous mutations in the chloride channel 7 (CLCN7) gene. The osteopetrosis of ADO2 has been attributed to hypofunctional osteoclasts. The mechanism underlying the abnormality in osteoclast function remains largely unknown. This study was designed to investigate gene mutations and osteoclast function in a case that was clinically diagnosed as ADO2. Genomic DNA was extracted from blood samples of this patient, and the 25 exons of CLCN7 were amplified. Peripheral blood from the ADO2 subject and a healthy age- and sex-matched control was used to evaluate osteoclastogenesis, osteoclast morphology, and bone resorption. Analysis of DNA from the patient showed a germline heterozygous missense mutation, c.1856C>T (p.P619L), in exon 20 of CLCN7. A similar homozygous mutation at this site was previously reported in a patient with autosomal recessive osteopetrosis. When cultured, the peripheral blood mononuclear cells (PBMCs) from the ADO2 patient spontaneously differentiated into mature osteoclasts in vitro. The ADO2 patient’s PBMCs formed enhanced, but heterogeneous, osteoclasts in both the presence and absence of macrophage-colony stimulating factor, and nuclear factor-ĸB ligand. Bone resorption was reduced in the ADO2 patient’s osteoclasts, which exhibited aberrant morphology and abnormal distribution of integrin a_vβ₃. Gene analysis found increased c-fos expression and reduced RhoA and integrin beta 3 expression in ADO2 cells. In conclusion, our data suggest that enhanced, heterogeneous osteoclast induction may be an intrinsic characteristic of ADO2.

Two novel mutations of CLCN7 gene in Chinese families with autosomal dominant osteopetrosis (type II)

Autosomal dominant osteopetrosis type II (ADO-II) is a heritable bone disorder characterized by osteosclerosis, predominantly involving the spine (vertebral end-plate thickening, or rugger-jersey spine), the pelvis ("bone-within-bone" structures) and the skull base. Chloride channel 7 (CLCN7) has been reported to be the causative gene. In this study, we aimed to identify the pathogenic mutation in four Chinese families with ADO-II. All 25 exons of the CLCN7 gene, including the exon-intron boundaries, were amplified and sequenced directly in four probands from the Chinese families with ADO-II. The mutation site was then identified in other family members and 250 healthy controls. In family 1, a known missense mutation c.296A>G in exon 4 of CLCN7 was identified in the proband, resulting in a tyrosine (UAU) to cysteine (UGU) substitution at p.99 (Y99C); the mutation was also identified in his affected father. In family 2, a novel missense mutation c.865G>C in exon 10 was identified in the proband, resulting in a valine (GUC) to leucine (CUC) substitution at p.289 (V289L); the mutation was also identified in her healthy mother and sister. In family 3, a novel missense mutation c.1625C>T in exon 17 of CLCN7 was identified in the proband, resulting in an alanine (GCG) to valine (GUG) substitution at p.542 (A542V); the mutation was also identified in her father. In family 4, a hot spot, R767W (c.2299C>T, CGG>TGG), in exon 24 was found in the proband which once again proved the susceptibility of the site or the similar genetic background in different races. Moreover, two novel mutations, V289L and A542V, occurred at a highly conserved position, found by a comparison of the protein sequences from eight vertebrates, and were predicted to have a pathogenic effect by PolyPhen-2 software, which showed "probably damaging" with a score of approximately 1. These mutation sites were not identified in 250 healthy controls. Our present findings suggest that the novel missense mutations V289L and A542V in the CLCN7 gene were responsible for ADO-II in the two Chinese families.

Novel CLCN7 mutation identified in a Han Chinese family with autosomal dominant osteopetrosis-2

Osteopetrosis is a heritable bone condition featuring increased bone density due to defective osteoclastic bone resorption. Exome sequencing and Sanger sequencing were conducted in Han Chinese family members, some of whom had typical osteopetrosis, and a novel missense variant c.2350A>T (p.R784W) in the chloride channel 7 gene (CLCN7) was identified. This variant cosegregated with the disorder in the family but was not observed in 800 controls. The data indicate that exome sequencing is a powerful and effective molecular diagnostic tool for detecting mutations in osteopetrosis, which is a genetically and clinically heterogeneous disorder. This discovery broadens the CLCN7 gene mutation spectrum and has important implications for clinical therapeutic regimen decisions, prognosis evaluations, and antenatal diagnoses.

Novel mutations of CLCN7 cause autosomal dominant osteopetrosis type II (ADO-II) and intermediate autosomal recessive osteopetrosis (IARO) in Chinese patients

SUMMARY

Osteopetrosis is a group of genetic bone disorders. Mutations in the chloride channel 7 gene (CLCN7) lead to chloride channel defect, which results in autosomal dominant osteopetrosis type II (ADO-II), autosomal recessive osteopetrosis (ARO), and intermediate autosomal recessive osteopetrosis (IARO). In the present study, we identified seven novel mutations of the CLCN7 gene and reported the first case of IARO with compound heterozygous mutation in Chinese population.

INTRODUCTION

Osteopetrosis is a heritable bone disorder due to the deficiency of or function defect in osteoclasts. Mutations in the CLCN7 lead to chloride channel defects, which result in osteopetrosis with diverse severity ranging from asymptomatic or relatively mild symptoms in ADO-II to the very severe phenotype in ARO. Heterozygous mutations in CLCN7 are associated to ADO-II, while homozygous and compound heterozygous mutations in CLCN7 may result in ARO and IARO. To date, a total of 24 mutations in CLCN7 were identified in ADO-II, and only 3 mutations were identified in IARO. In the present study, we reported seven unrelated ADO-II patients and one IARO patient from Chinese population and elucidated the characteristics of CLCN7 gene mutations in these patients.

METHODS

All 25 CLCN7 exons and exon-intron boundaries from genomic DNA were amplified and sequenced in eight affected individuals suffering from ADO-II/IARO. The clinical, biochemical, and radiographic analysis were evaluated to compare the differences between ADO-II and IARO both in genotype and phenotype.

RESULTS

The results showed that there were seven novel CLCN7 mutations identified in these ADO-II/IARO patients, including six heterozygous missense mutations (p.L224R, p.S290Y, p.R326G, p.G347R, p.S473N, and p.L564P) and a novel splice mutation (p.K691FS).

CONCLUSIONS

The compound heterozygous mutations (p.L224R and p.K691FS) were firstly observed in one IARO patient. The present study would enrich the database of CLCN7 mutations and improve our understanding of this heritable bone disorder.

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.

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.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

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

Friend or foe: high bone mineral density on routine bone density scanning, a review of causes and management

A finding of high BMD on routine DXA scanning is not infrequent and most commonly reflects degenerative disease. However, BMD increases may also arise secondary to a range of underlying disorders affecting the skeleton. Although low BMD increases fracture risk, the converse may not hold for high BMD, since elevated BMD may occur in conditions where fracture risk is increased, unaffected or reduced. Here we outline a classification for the causes of raised BMD, based on identification of focal or generalized BMD changes, and discuss an approach to guide appropriate investigation by clinicians after careful interpretation of DXA scan findings within the context of the clinical history. We will also review the mild skeletal dysplasia associated with the currently unexplained high bone mass phenotype and discuss recent advances in osteoporosis therapies arising from improved understanding of rare inherited high BMD disorders.

The virulence gene and clinical phenotypes of osteopetrosis in the Chinese population: six novel mutations of the CLCN7 gene in twelve osteopetrosis families

Osteopetrosis is a heritable bone disorder resulting from a deficiency of or a functional defect in osteoclasts. We aimed to characterize the molecular defects and clinical manifestations in Chinese patients with osteopetrosis by studying 12 unrelated osteopetrosis families. The entire coding region and adjacent splice sites of the CLCN7, TCIRG1, LRP5 and SOST genes were amplified and directly sequenced. X-rays of hip and lumbar spine, bone mineral density and bone turnover markers were examined simultaneously. Family history and fracture history were collected using a questionnaire. Among 12 unrelated families, 10 families were diagnosed with autosomal dominant osteopetrosis type II (ADOII) with 10 probands and 3 affected subjects. Two individuals in the other two families were diagnosed with uncategorized osteopetrosis because no mutations were detected in any of the four studied genes. Eight mutations, including two reported mutations (R767W and E798FS) and six novel mutations (E313K, A316G, R743W, G741R, W127G and S290F), were detected in the CLCN7 gene from 12 living ADOII patients. Among them, R767W and R743W mutations were two common mutations that were each found in 20% of 10 ADOII probands. In CLCN7-related ADOII patients, long bone fractures and elevated serum CK level were two major clinical phenotypes, especially in patients younger than 18 years. Further functional studies of the above eight mutations in the CLCN7 gene are needed in the future.

ClC-5 mutations associated with Dent's disease: a major role of the dimer interface

Dent's disease is an X-linked recessive disorder affecting the proximal tubules. Mutations in the 2Cl(-)/H(+) exchanger ClC-5 gene CLCN5 are frequently associated with Dent's disease. Functional characterization of mutations of CLCN5 have helped to elucidate the physiopathology of Dent's disease and provided evidence that several different mechanisms underlie the ClC-5 dysfunction in Dent's disease. Modeling studies indicate that many CLCN5 mutations are located at the interface between the monomers of ClC-5, demonstrating that this protein region plays an important role in Dent's disease. On the basis of functional data, CLCN5 mutations can be divided into three different classes. Class 1 mutations impair processing and folding, and as a result, the ClC-5 mutants are retained within the endoplasmic reticulum and targeted for degradation by quality control mechanisms. Class 2 mutations induce a delay in protein processing and reduce the stability of ClC-5. As a consequence, the cell surface expression and currents of the ClC-5 mutants are lower. Class 3 mutations do not alter the trafficking of ClC-5 to the cell surface and early endosomes but induce altered electrical activity. Here, we discuss the functional consequences of the three classes of CLCN5 mutations on ClC-5 structure and function.

Report of two Chinese patients suffering from CLCN7-related osteopetrosis and root dysplasia

Osteopetrosis is a group of genetic bone disorders. There are three types of osteopetrosis: autosomal recessive osteopetrosis (ARO), autosomal dominant osteopetrosis type II (ADO II), and intermediate autosomal recessive osteopetrosis (IARO). The prevalence of ADO II is about 1:100,000, while no more than 20 cases of IARO have been reported worldwide. We present the first Chinese IARO patient with a novel homozygous variant in CLCN7 gene (p. Pro470Leu) and an ADO II patient with a heterozygous variant in CLCN7 gene (p. Arg286Trp). In addition to general osteosclerosis, the striking features of these two patients are unerupted teeth with root dysplasia. We speculate that ClC-7 in different tooth cells may contribute directly to the root development, the defect of ClC-7 may have a dose dependent effect on the phenotype of root dysplasia, and the tooth position may also affect the root phenotype with dysfunctional ClC-7.

Diseases affecting bone quality: beyond osteoporosis

BACKGROUND

Bone quantity, quality, and turnover contribute to whole bone strength. Although bone mineral density, or bone quantity, is associated with increased fracture risk, less is known about bone quality. Various conditions, including disorders of mineral homeostasis, disorders in bone remodeling, collagen disorders, and drugs, affect bone quality.

QUESTIONS/PURPOSES

The objectives of this review are to (1) identify the conditions and diseases that could adversely affect bone quality besides osteoporosis, and (2) evaluate how these conditions influence bone quality.

METHODS

We searched PubMed using the keywords "causes" combined with "secondary osteoporosis" or "fragility fracture." After identifying 20 disorders/conditions, we subsequently searched each condition to evaluate its effect on bone quality.

RESULTS

Many disorders or conditions have an effect on bone metabolism, leading to fragility fractures. These disorders include abnormalities that disrupt mineral homeostasis, lead to an alteration of the mineralization process, and ultimately reduce bone strength. The balance between bone formation and resorption is also essential to prevent microdamage accumulation and maintain proper material and structural integrity of the bone. As a result, diseases that alter the bone turnover process lead to a reduction of bone strength. Because Type I collagen is the most abundant protein found in bone, defects in Type I collagen can result in alterations of material property, ultimately leading to fragility fractures. Additionally, some medications can adversely affect bone.

CONCLUSIONS

Recognizing these conditions and diseases and understanding their etiology and pathogenesis is crucial for patient care and maintaining overall bone health.

ClC-7 is a slowly voltage-gated 2Cl(-)/1H(+)-exchanger and requires Ostm1 for transport activity

Mutations in the ClC-7/Ostm1 ion transporter lead to osteopetrosis and lysosomal storage disease. Its lysosomal localization hitherto precluded detailed functional characterization. Using a mutated ClC-7 that reaches the plasma membrane, we now show that both the aminoterminus and transmembrane span of the Ostm1 β-subunit are required for ClC-7 Cl(-)/H(+)-exchange, whereas the Ostm1 transmembrane domain suffices for its ClC-7-dependent trafficking to lysosomes. ClC-7/Ostm1 currents were strongly outwardly rectifying owing to slow gating of ion exchange, which itself displays an intrinsically almost linear voltage dependence. Reversal potentials of tail currents revealed a 2Cl(-)/1H(+)-exchange stoichiometry. Several disease-causing CLCN7 mutations accelerated gating. Such mutations cluster to the second cytosolic cystathionine-β-synthase domain and potential contact sites at the transmembrane segment. Our work suggests that gating underlies the rectification of all endosomal/lysosomal CLCs and extends the concept of voltage gating beyond channels to ion exchangers.

Elevated serum lactate dehydrogenase isoenzymes and aspartate transaminase distinguish Albers-Schönberg disease (Chloride Channel 7 Deficiency Osteopetrosis) among the sclerosing bone disorders

Osteopetrosis (OPT) refers to the consequences of generalized failure of skeletal resorption during growth. Most cases are explained by loss-of-function mutation within the genes that encode either chloride channel 7 (CLCN7) or a vacuolar proton pump subunit (TCIRG1), each compromising acid secretion by osteoclasts. Patients suffer fractures and sometimes cranial nerve entrapment and insufficient medullary space for hematopoiesis. In 1996, we reported that a high serum level of the brain isoenzyme of creatine kinase (BB-CK), the CK of osteoclasts, characterizes OPT dueamong the sclerosing bone disorders (J Clin Endocrinol Metab. 1996;11:1438). Now, we show that elevation in serum of multiple lactate dehydrogenase (LDH) isoenzymes with aspartate transaminase (AST) distinguishes autosomal dominant OPT due to loss-of-function mutation in CLCN7 [Albers-Schönberg disease (A-SD)] among these conditions. Serum total LDH and AST levels as high as 3× and 2×, respectively, the upper limits of normal for age-appropriate controls, were persistent and essentially concordant in A-SD. Serum LDH was elevated in 7 of 9 children and in the 2 adults studied with A-SD. LDH isoenzyme quantitation showed excesses of LDH-2, -3, and -4. Neither total LDH nor AST increases were found in other forms of OPT, including bisphosphonate-induced OPT, or in 41 children and 6 adults representing 20 additional sclerosing bone disorders. Serum TRACP-5b and BB-CK also were markedly elevated in A-SD. Hence, high serum levels of several enzymes characterize A-SD. Elevated serum LDH isoenzymes and AST indicate a disturbance (of uncertain clinical significance) within multiple extraosseous tissues when there is CLCN7 deficiency.

Structure of a eukaryotic CLC transporter defines an intermediate state in the transport cycle

CLC proteins transport chloride (Cl(-)) ions across cell membranes to control the electrical potential of muscle cells, transfer electrolytes across epithelia, and control the pH and electrolyte composition of intracellular organelles. Some members of this protein family are Cl(-) ion channels, whereas others are secondary active transporters that exchange Cl(-) ions and protons (H(+)) with a 2:1 stoichiometry. We have determined the structure of a eukaryotic CLC transporter at 3.5 angstrom resolution. Cytoplasmic cystathionine beta-synthase (CBS) domains are strategically positioned to regulate the ion-transport pathway, and many disease-causing mutations in human CLCs reside on the CBS-transmembrane interface. Comparison with prokaryotic CLC shows that a gating glutamate residue changes conformation and suggests a basis for 2:1 Cl(-)/H(+) exchange and a simple mechanistic connection between CLC channels and transporters.

The G215R mutation in the Cl-/H+-antiporter ClC-7 found in ADO II osteopetrosis does not abolish function but causes a severe trafficking defect

BACKGROUND

ClC-7 is a ubiquitous transporter which is broadly expressed in mammalian tissues. It is implied in the pathogenesis of lysosomal storage disease and osteopetrosis. Because of its endosomal/lysosomal localization it is still poorly characterized.

METHODOLOGY/PRINCIPAL FINDINGS

An electrophysiological characterization of rat ClC-7 using solid-supported membrane-based electrophysiology is presented. The measured currents show the characteristics of ClC-7 and confirm its function as a Cl(-)/H(+)-antiporter. We have used rat ClC-7 in CHO cells as a model system to investigate the functionality and cellular localization of the wt transporter and its variant G213R ClC-7 which is the analogue of human G215R ClC-7 responsible for autosomal dominant osteopetrosis type II. Our study shows that rat G213R ClC-7 is functional but has a localization defect in CHO cells which prevents it from being correctly targeted to the lysosomal membrane. The electrophysiological assay is tested as a tool for drug discovery. The assay is validated with a number of drug candidates. It is shown that ClC-7 is inhibited by DIDS, NPPB and NS5818 at micromolar concentrations.

CONCLUSIONS/SIGNIFICANCE

It is suggested that the scenario found in the CHO model system also applies to the human transporter and that mislocalization rather than impaired functionality of G215R ClC-7 is the primary cause of the related autosomal dominant osteopetrosis type II. Furthermore, the robust solid-supported membrane-based electrophysiological assay is proposed for rapid screening for potential ClC-7 inhibitors which are discussed for treatment of osteoporosis.

The pathogenesis of bisphosphonate-related osteonecrosis of the jaw: so many hypotheses, so few data

Bisphosphonate-related osteonecrosis of the jaw (BRONJ) has generated great interest in the medical and research communities yet remains an enigma, given its unknown pathogenesis. The goal of this review is to summarize the various proposed hypotheses underlying BRONJ. Although a role of the oral mucosa has been proposed, the bone is likely the primary tissue of interest for BRONJ. The most popular BRONJ hypothesis-manifestation of necrotic bone resulting from bisphosphonate--induced remodeling suppression--is supported mostly by indirect evidence, although recent data have shown that bisphosphonates significantly reduce remodeling in the jaw. Remodeling suppression would be expected, and has been shown, to allow accumulation of nonviable osteocytes, whereas a more direct cytotoxic effect of bisphosphonates on osteocytes has also been proposed. Bisphosphonates have antiangiogenic effects, leading to speculation that this could contribute to the BRONJ pathogenesis. Compromised angiogenesis would most likely be involved in post-intervention healing, although other aspects of the vasculature (eg, blood flow) could contribute to BRONJ. Despite infection being present in many BRONJ patients, there is no clear evidence as to whether infection is a primary or secondary event in the pathophysiology. In addition to these main factors proposed in the pathogenesis, numerous cofactors associated with BRONJ (eg, diabetes, smoking, dental extraction, concurrent medications) could interact with bisphosphonates and affect remodeling, angiogenesis/blood flow, and/or infection. Because our lack of knowledge concerning BRONJ pathogenesis results from a lack of data, it is only through the initiation of hypothesis-driven studies that significant progress will be made to understand this serious and debilitating condition.

Identification of the CLCN7 gene mutations in two Chinese families with autosomal dominant osteopetrosis (type II)

Here we report the identification of two different mutations in chloride channel 7 gene in two unrelated patients with autosomal dominant osteopetrosis type II. We determined that one patient (a 32-year-old woman) carried a heterozygous gene for a R767W mutation in exon 24, and another patient (a 17-year-old boy) carried a heterozygous gene for a novel frameshift mutation (Glu798FS) in exon 25. Recent studies have reported loss-of-function mutations in the chloride channel 7 (CLCN7) gene as a cause of autosomal dominant osteopetrosis type II (ADO-II). The identification of gene mutations in Chinese with ADO has not been reported previously. In this study, we identified mutations of the CLCN7 gene in two unrelated Chinese families with ADO-II. Two probands with ADO-II were diagnosed based on their bone characteristics on X-rays and their laboratory results. All 25 exons of the CLCN7 gene, including the exon-intron boundaries, were sequenced. We found in family 1 that the proband (a 32-year-old woman) was heterozygous for a CLCN7 mutation. The nonsynonymous mutation consisted of a heterozygous C/T transition at codon 2327 in exon 24, which resulted in an arginine (CGG)-to tryptophan (TGG) substitution at position 767 (R767W). The same heterozygous mutation (C/T) was determined in her father and son, who were asymptomatic with normal skeleton radiography. In family 2, we found that the proband (a 17-year-old boy) carried a novel frameshift mutation (Glu798FS) resulting from a G insertion between codon 60 and codon 61 in exon 25. The heterozygous -/G insertion is predicted to elongate the peptide of CLCN7 by 120 amino acids after position 797 amino acids. Similarly, some individuals of this family carried the same heterozygous mutation, but they are all asymptomatic. Furthermore, the R767W and Glu798FS mutations were not found in 100 unrelated controls. Our present findings suggest that the novel Glu798FS mutation in exon 25 and R767W in exon 24 in the CLCN7 gene were responsible for ADO-II in these Chinese patients.

Characterization of the bone phenotype in ClC-7-deficient mice

Mice deficient in the chloride channel ClC-7, which is likely involved in acidification of the resorption lacuna, display severe osteopetrosis. To fully characterize the osteopetrotic phenotype, the phenotypes of osteoclasts and osteoblasts were evaluated. ClC-7(-/-) mice and their corresponding wild-type littermates were killed at 4-5 weeks of age. Biochemical markers of bone resorption (CTX-I), osteoclast number (TRAP5b), and osteoblast activity (ALP) were evaluated in serum. Splenocytes were differentiated into osteoclasts using M-CSF and RANKL. Mature osteoclasts were seeded on calcified or decalcified bone slices, and CTX-I, Ca(2+), and TRAP were measured. Acidification rates in membrane vesicles from bone cells were measured using acridine orange. Osteoblastogenesis and nodule formation in vitro were investigated using calvarial osteoblasts. ClC-7(-/-) osteoclasts were unable to resorb calcified bone in vitro. However, osteoclasts were able to degrade decalcified bone. Acid influx in bone membrane vesicles was reduced by 70% in ClC-7(-/-) mice. Serum ALP was increased by 30% and TRAP5b was increased by 250% in ClC-7(-/-) mice, whereas the CTX/TRAP5b ratio was reduced to 50% of the wild-type level. Finally, evaluation of calvarial ClC-7(-/-) osteoblasts showed normal osteoblastogenesis. In summary, we present evidence supporting a pivotal role for ClC-7 in acidification of the resorption lacuna and evidence indicating that bone formation and bone resorption are no longer balanced in ClC-7(-/-) mice.

CLCN7 polymorphisms and bone mineral density in healthy premenopausal white women and in white men

INTRODUCTION

Mutations in the chloride channel 7 gene (CLCN7) cause osteopetrosis, and polymorphisms of CLCN7 in the non-disease allele are associated with penetrance of the autosomal dominant osteopetrosis (ADO) phenotype. Studies have also shown an association between CLCN7 polymorphisms and bone mineral density (BMD) in women. However, there is no study to date that has examined whether CLCN7 polymorphisms underlie normal variation of peak BMD in healthy premenopausal white women and in white men.

METHODS

Six single nucleotide polymorphisms (SNPs) and one variable number tandem repeat (VNTR) polymorphism in the CLCN7 gene were genotyped. Association was tested between CLCN7 gene polymorphisms and both lumbar spine and femoral neck BMD. Healthy premenopausal white sisters (age 33.1+/-7.2, n=1692) and healthy white brothers (age 33.6+/-10.9, n=715) were studied.

RESULTS

No significant association between CLCN7 gene polymorphisms and BMD at the lumbar spine or femoral neck was found in white women or white men.

CONCLUSIONS

Genetic variation in the CLCN7 gene is not a major contributor to the variability in peak BMD at the femoral neck and lumber spine in healthy premenopausal white women and in white men.