A mutation in the ZNF687 gene that is responsible for the severe form of Paget's disease of bone causes severely altered bone remodeling and promotes hepatocellular carcinoma onset in a knock-in mouse model

Paget’s disease(PDB)is a late-onset bone remodeling disorder with a broad spectrum of symptoms and complications.One of the most aggressive forms is caused by the P937R mutation in the ZNF687 gene.Although the genetic involvement of ZNF687 in PDB has been extensively studied,the molecular mechanisms underlying this association remain unclear.Here,we describe the first Zfp687 knock-in mouse model and demonstrate that the mutation recapitulates the PDB phenotype,resulting in severely altered bone remodeling.Through microcomputed tomography analysis,we observed that 8-month-old mutant mice showed a mainly osteolytic phase,with a significant decrease in the trabecular bone volume affecting the femurs and the vertebrae.Conversely,osteoblast activity was deregulated,producing disorganized bone.Notably,this phenotype became pervasive in 16-month-old mice,where osteoblast function overtook bone resorption,as highlighted by the presence of woven bone in histological analyses,consistent with the PDB phenotype.Furthermore,we detected osteophytes and intervertebral disc degeneration,outlining for the first time the link between osteoarthritis and PDB in a PDB mouse model.RNA sequencing of wild-type and Zfp687 knockout RAW264.7 cells identified a set of genes involved in osteoclastogenesis potentially regulated by Zfp687,e.g.,Tspan7,Cpe,Vegfc,and Ggt1,confirming its role in this process.Strikingly,in this mouse model,the mutation was also associated with a high penetrance of hepatocellular carcinomas.Thus,this study established an essential role of Zfp687 in the regulation of bone remodeling,offering the potential to therapeutically treat PDB,and underlines the oncogenic potential of ZNF687.

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

Autosomal dominant osteopetrosis type 2 (ADO2) is a high-density brittle bone disease characterized by bone pain,multiple fractures and skeletal-related events,including nerve compression syndrome and hematological failure.We demonstrated that in mice carrying the heterozygous Clcn7^G213R mutation,whose human mutant homolog CLCN7^G215R affects patients,the clinical impacts of ADO2 extend beyond the skeleton,affecting several other organs.The hallmark of the extra-skeletal alterations is a consistent perivascular fibrosis,associated with high numbers of macrophages and lymphoid infiltrates.Fragmented clinical information in a small cohort of patients confirms extra-skeletal alterations consistent with a systemic disease,in line with the observation that the CLCN7 gene is expressed in many organs.ADO2 mice also show anxiety and depression and their brains exhibit not only perivascular fibrosis but also β-amyloid accumulation and astrogliosis,suggesting the involvement of the nervous system in the pathogenesis of the ADO2 extra-skeletal alterations.Extra-skeletal organs share a similar cellular pathology,confirmed also in vitro in bone marrow mononuclear cells and osteoclasts,characterized by an impairment of the exit pathway of the Clcn7 protein product,ClC7,through the Golgi,with consequent reduced ClC7 expression in late endosomes and lysosomes,associated with high vesicular pH and accumulation of autophagosome markers.Finally,an experimental siRNA therapy,previously proven to counteract the bone phenotype,also improves the extra-skeletal alterations.These results could have important clinical implications,supporting the notion that a systematic evaluation of ADO2 patients for extra-skeletal symptoms could help improve their diagnosis,clinical management,and therapeutic options.