Extra-skeletal manifestations in mice affected by Clcn7-dependent autosomal dominant osteopetrosis type 2 clinical and therapeutic implications

Novel hybrid silicon-lipid nanoparticles deliver a siRNA to cure autosomal dominant osteopetrosis in mice. Implications for gene therapy in humans

Rare skeletal diseases are still in need of proper clinically available transfection agents as the major challenge for first-in-human translation relates to intrinsic difficulty in targeting bone without exacerbating any inherent toxicity due to used vector. SiSaf's silicon stabilized hybrid lipid nanoparticles (sshLNPs) constitute next-generation non-viral vectors able to retain the integrity and stability of constructs and to accommodate considerable payloads of biologicals, without requiring cold-chain storage. sshLNP was complexed with a small interfering RNA (siRNA) specifically designed against the human CLCN7G215R mRNA. When tested via single intraperitoneal injection in pre-puberal autosomal dominant osteopetrosis type 2 (ADO2) mice, carrying a heterozygous mutation of the Clcn7 gene (Clcn7G213R), sshLNP, this significantly downregulated the Clcn7G213R related mRNA levels in femurs at 48 h. Confirmatory results were observed at 2 weeks and 4 weeks after treatments (3 intraperitoneal injections/week), with rescue of the bone phenotype and demonstrating safety. The pre-clinical results will enable advanced preclinical development of RNA-based therapy for orphan and genetic skeletal disorders by safely and effectively delivering biologicals of interest to cure human systemic conditions.

CLCN7, a gene shared by autosomal recessive and autosomal dominant osteopetrosis

After the discovery of abundant v-ATPase complexes in the osteoclast ruffled membrane it was obvious that in parallel a negative counter-ion needs to be transported across this membrane to allow for efficient transport of protons into the resorption lacuna. While different candidate proteins were discussed the osteopetrosis phenotype of Clcn7 knockout mice suggested that the chloride/proton-exchanger ClC-7 might be responsible for transporting the negative charge. In the following, individuals with autosomal recessive osteopetrosis (ARO) were found to carry biallelic CLCN7 pathogenic variants. Shortly thereafter, heterozygous pathogenic variants were identified as the exclusive cause of autosomal dominant osteopetrosis type 2 (ADO2). Since in most cell types other than osteoclasts ClC-7 resides in late endosomes and lysosomes, it took some time until the electrophysiological properties of ClC-7 were elucidated. Whereas most missense variants lead to reduced chloride currents, several variants with accelerated kinetics have been identified. Evidence for folding problems is also known for several missense variants. Paradoxically, a heterozygous activating variant in ClC-7 was described to cause lysosomal alteration, pigmentation defects, and intellectual disability without osteopetrosis. The counter-intuitive 2 Cl^-/H⁺ exchange function of ClC-7 was shown to be physiologically important for intravesicular ion homeostasis. The lysosomal function of ClC-7 is also the reason why individuals with CLCN7-ARO can develop a storage disorder and neurodegeneration, a feature that is variable and difficult to predict. Furthermore, the low penetrance of heterozygous pathogenic CLCN7 variants and the clinical variability of ADO2 are incompletely understood. We aim to give an overview not only of the current knowledge about ClC-7 and its related pathologies, but also of the scientists and clinicians that paved the way for these discoveries.

Experimental therapies for osteopetrosis

The medical treatment of osteopetrosis is an ongoing clinical problem. There are no effective and safer therapeutic approaches for all its forms. However, recent discoveries concerning the etiology and the pathogenesis of osteopetrosis, the development of dedicated cellular and animal models, and the advent of new technologies are paving the way for the development of targeted and safer therapies for both lethal and milder osteopetrosis. This review summarizes the huge effort and successes made by researchers to identify and develop new experimental approaches with this objective, such as the use of non-genotoxic myeloablation, gene correction of inducible Pluripotent Stem Cells (iPSCs), lentiviral-based gene therapy, protein replacement, prenatal treatment, osteoclast precursors transplantation and RNA Interference.

Pediatric patient with a bilateral Salter-Harris II fracture and slipped capital femoral epiphysis secondary to autosomal recessive osteopetrosis

Treatment of femoral neck fractures secondary to osteopetrosis is an uncertain and puzzled decision. Experience in the treatment, especially in the pediatric population, is scarcely reported. The duration of conservative treatment is prolonged and poses the risks of non-union and development of coxa vara deformity. The recommended treatment is closed reduction and internal fixation; however, surgery on osteopetrotic bone is challenging due to defective bone marrow function, delayed consolidation and higher risk of intraoperative fractures. Slipped capital femoral epiphysis secondary to osteopetrosis is very rarely reported. This article presents the case of a 5-year-old female patient with rapidly deteriorating physical function due to bilateral proximal femoral Salter-Harris type II fractures with associated slippage of the growth plates secondary to confirmed autosomal recessive osteopetrosis. Operative treatment was performed in a tertiary level orthopedic center with closed reduction and internal fixation with cannulated screws. A loss of fixation with coxa vara deformity was seen on the left side 7 months postoperatively with increasing pain. A revision surgery with reosteosynthesis and a valgus osteotomy was thus performed which showed good subjective and objective results 1 year postoperatively with complete bony union.

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.

Multiomics landscape of the autosomal dominant osteopetrosis type II disease-specific induced pluripotent stem cells

BACKGROUND

Autosomal dominant osteopetrosis type II (ADO2) is a genetically and phenotypically metabolic bone disease, caused by osteoclast abnormalities. The pathways dysregulated in ADO2 could lead to the defects in osteoclast formation and function. However, the mechanism remains elusive.

MATERIALS AND METHODS

To systematically explore the molecular characterization of ADO2, we performed a multi-omics profiling from the autosomal dominant osteopetrosis type II iPSCs (ADO2-iPSCs) and healthy normal control iPSCs (NC-iPSCs) using whole genome re-sequencing, DNA methylation and N6-methyladenosine (m6A) analysis in this study.

RESULTS

Totally, we detected 7,095,817 single nucleotide polymorphisms (SNPs) and 1,179,573 insertion and deletions (InDels), 1,001,943 differentially methylated regions (DMRs) and 2984 differential m6A peaks, and the comprehensive multi-omics profile was generated from the two cells. Interestingly, the ISG15 m6A level in ADO2-iPSCs is higher than NC-iPSCs by IGV software, and the differentially expressed m6A-modified genes (DEMGs) were highly enriched in the osteoclast differentiation and p53 signaling pathway, which associated with the development of osteopetrosis. In addition, combining our previously published transcriptome and proteome datasets, we found that the change in DNA methylation levels correlates inversely with some gene expression levels.

CONCLUSION

Our results indicate that the global multi-omics landscape not only provides a high-quality data resource but also reveals a dynamic pattern of gene expression, and found that the pathogenesis of ADO2 may begin early in life.

A Roadmap to Gene Discoveries and Novel Therapies in Monogenic Low and High Bone Mass Disorders

Genetic disorders of the skeleton encompass a diverse group of bone diseases differing in clinical characteristics, severity, incidence and molecular etiology. Of particular interest are the monogenic rare bone mass disorders, with the underlying genetic defect contributing to either low or high bone mass phenotype. Extensive, deep phenotyping coupled with high-throughput, cost-effective genotyping is crucial in the characterization and diagnosis of affected individuals. Massive parallel sequencing efforts have been instrumental in the discovery of novel causal genes that merit functional validation using in vitro and ex vivo cell-based techniques, and in vivo models, mainly mice and zebrafish. These translational models also serve as an excellent platform for therapeutic discovery, bridging the gap between basic science research and the clinic. Altogether, genetic studies of monogenic rare bone mass disorders have broadened our knowledge on molecular signaling pathways coordinating bone development and metabolism, disease inheritance patterns, development of new and improved bone biomarkers, and identification of novel drug targets. In this comprehensive review we describe approaches to further enhance the innovative processes taking discoveries from clinic to bench, and then back to clinic in rare bone mass disorders. We highlight the importance of cross laboratory collaboration to perform functional validation in multiple model systems after identification of a novel disease gene. We describe the monogenic forms of rare low and high rare bone mass disorders known to date, provide a roadmap to unravel the genetic determinants of monogenic rare bone mass disorders using proper phenotyping and genotyping methods, and describe different genetic validation approaches paving the way for future treatments.

Transcriptomic and bioinformatic analysis of Clcn7-dependent Autosomal Dominant Osteopetrosis type 2. Preclinical and clinical implications

Autosomal Dominant Osteopetrosis type 2 (ADO2) is a rare genetic disease characterized by dense yet fragile bones. To date, the radiological approach remains the gold standard for ADO2 diagnosis. However, recent observations unveiled that ADO2 is a systemic disease affecting various organs beyond bone, including lung, kidney, muscle, and brain. Monitoring disease status and progression would greatly benefit from specific biomarkers shared by the affected organs. In this work, data derived from RNA deep sequencing (RNA dSeq) of bone, lung, kidney, muscle, brain, and osteoclasts isolated from wildtype (WT) and Clcn7^G213R ADO2 mice were subjected to gene ontology and pathway analyses. Results showed the presence of alterations in gene ontology terms and pathways associated with bone metabolism and osteoclast biology, including JAK-STAT, cytokine-cytokine receptor, and hematopoietic cell lineage. Furthermore, in line with the multiorgan alterations caused by ADO2, the analysis of soft organs showed an enrichment of PPAR and neuroactive ligand-receptor interaction pathways known to be involved in the onset of tissue fibrosis and behavioral alterations, respectively. Finally, we observed the modulations of potential ADO2 biomarkers in organs and cells of ADO2 mice and in the peripheral blood mononuclear cells of patients, using conventional methods. Of note, some of these biomarkers could be possibly responsive to an effective experimental therapy based on a mutation-specific siRNA. Overall, the identified gene signature and the soluble forms of the encoded proteins could potentially represent reliable disease biomarkers that could improve the ADO2 diagnosis, the monitoring of both the skeletal and non-skeletal dysfunctions, and the assessment of the response to therapy.

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).

Neurodegeneration Upon Dysfunction of Endosomal/Lysosomal CLC Chloride Transporters

The regulation of luminal ion concentrations is critical for the function of, and transport between intracellular organelles. The importance of the acidic pH in the compartments of the endosomal-lysosomal pathway has been well-known for decades. Besides the V-ATPase, which pumps protons into their lumen, a variety of ion transporters and channels is involved in the regulation of the organelles' complex ion homeostasis. Amongst these are the intracellular members of the CLC family, ClC-3 through ClC-7. They localize to distinct but overlapping compartments of the endosomal-lysosomal pathway, partially with tissue-specific expression. Functioning as 2Cl⁻/H⁺ exchangers, they can support the vesicular acidification and accumulate luminal Cl⁻. Mutations in the encoding genes in patients and mouse models underlie severe phenotypes including kidney stones with CLCN5 and osteopetrosis or hypopigmentation with CLCN7. Dysfunction of those intracellular CLCs that are expressed in neurons lead to neuronal defects. Loss of endosomal ClC-3, which heteromerizes with ClC-4, results in neurodegeneration. Mutations in ClC-4 are associated with epileptic encephalopathy and intellectual disability. Mice lacking the late endosomal ClC-6 develop a lysosomal storage disease with reduced pain sensitivity. Human gene variants have been associated with epilepsy, and a gain-of-function mutation causes early-onset neurodegeneration. Dysfunction of the lysosomal ClC-7 leads to a lysosomal storage disease and neurodegeneration in mice and humans. Reduced luminal chloride, as well as altered calcium regulation, has been associated with lysosomal storage diseases in general. This review discusses the properties of endosomal and lysosomal Cl⁻/H⁺ exchange by CLCs and how various alterations of ion transport by CLCs impact organellar ion homeostasis and function in neurodegenerative disorders.

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.