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.

BACKGROUND: 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.
METHODS: Cross-sectional data from participants with osteopetrosis were collected using an online REDCap-based database.
METHODS: Thirty-four participants with a confirmed diagnosis of ADO, aged 4-84 years.
METHODS: 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.

BACKGROUND: Autosomal dominant osteopetrosis (ADO) is a rare genetic disorder resulting from impaired osteoclastic bone resorption. Clinical manifestations frequently include fractures, osteonecrosis (particularly of the jaw or maxilla), osteomyelitis, blindness, and/or bone marrow failure. ADO usually results from heterozygous missense variants in the Chloride Channel 7 gene (CLCN7) that cause disease by a dominant negative mechanism. Variants in the T-cell immune regulator 1 gene (TCIRG1) are commonly identified in autosomal recessive osteopetrosis but have only been reported in 1 patient with ADO.
METHODS: Here, we report 3 family members with a single heterozygous missense variant (p.Gly579Arg) in TCIRG1 who have a phenotype consistent with ADO. Three of 5 protein prediction programs suggest this variant likely inhibits the function of TCIRG1.
CONCLUSIONS: This is the first description of adult presentation of ADO caused by a TCIRG1 variant. Similar to families with ADO from CLCN7 mutations, this variant in TCIRG1 results in marked phenotype variability, with 2 subjects having severe disease and the third having very mild disease. This family report implicates TCIRG1 missense mutations as a cause of ADO and demonstrates that the marked phenotypic variability in ADO may extend to disease caused by TCIRG1 missense mutations.

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.

Autosomal dominant osteopetrosis (ADOP) is an extremely rare congenital bone disease. We report a case of a hip disorder in combination with ADOP, which was treated by total hip arthroplasty, resulting in a good recovery and a significant improvement in the patient\'s ability to perform daily activities.

UNASSIGNED: Osteopetrosis (OP) is a rare, heritable skeletal disorder that is caused by osteoclast dysfunction, leading to failure of bone resorption and increased bone density. The fragility of such dense bone may result in an increased incidence of fractures. Furthermore, surgery in patients with OP is associated with increased technical difficulty and a higher risk of complications.
UNASSIGNED: We report a case of a 20-year-old woman with autosomal dominant OP who developed a subtrochanteric femoral fracture. The fracture was treated by open reduction and internal fixation using a dynamic hip screw. Although technical difficulties were experienced and an intraoperative femoral neck fracture occurred, the surgical outcome was satisfactory. Union of the fractures was visible on radiographs obtained 12 months postoperatively. At 2 years postoperatively, the patient was completely free of any complications resulting from her injury and treatment.
UNASSIGNED: Open reduction and internal fixation may be an effective option for fractures in patients with OP. Orthopedic surgeons should be aware that the increased density and stiffness of osteopetrotic bone increases the risks of intraoperative technical difficulties, iatrogenic fractures, and postoperative complications.

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.

Autosomal dominant osteopetrosis type II (ADO2) is a heritable osteosclerotic disorder dependent on osteoclast impairment. In most patients it results from heterozygous missense mutations in the chloride channel 7 (CLCN7) gene, encoding for a 2Cl(-)/1H(+) antiporter. By a knock-in strategy inserting a missense mutation in the Clcn7 gene, our two research groups independently generated mouse models of ADO2 on different genetic backgrounds carrying the homolog of the most frequent heterozygous mutation (p.G213R) in the Clcn7 gene found in humans. Our results demonstrate that the heterozygous model holds true presenting with higher bone mass, increased numbers of poorly resorbing osteoclasts and a lethal phenotype in the homozygous state. Considerable variability is observed in the heterozygous mice according with the mouse background, suggesting that modifier genes could influence the penetrance of the disease gene.