Bone Abstracts

Objectives: ADO2 is a genetic bone disease induced by dominant negative mutations of the proton/chloride antiporter ClC7 encoded by the CLCN7 gene. In osteoclasts, ClC7 is crucial for lysosome function and resorption lacuna acidification. However, Clcn7 is expressed in several cell types in various organs, including brain, lungs, kidneys and spleen. Therefore, we asked whether Clcn7 mutations could affect other tissues beyond the bone.

Methods: A mouse model of ADO2, carrying the heterozygous Clcn7^G213R mutation, was subjected to in-depth phenotyping.

Results: ADO2 mice exhibited 1.4 fold increased anxiety (P<0.05) and depression (P<0.01) and their related enzymes Glo1 and Gad1 were more expressed in ADO2 brains (+1.77 and +1.23-fold respectively; P<0.05). Increased β-amyloid accumulation was found in hippocampus (+3.64-fold), thalamus (+4.6-fold) and amygdala (+2.28-fold; P<0.05). Cryosections of ADO2 hippocampus, as well as cultured ADO2 neurons, showed enlarged γ-adaptin-positive areas (+2.5-fold; P<0.02), suggesting alterations of Golgi-related clathrin-coated vesicular trafficking. Immunohistochemistry showed ClC7 expression in kidney tubular cells, lung bronchiolar epithelium and alveolar and spleen macrophages. Masson’s trichrome staining revealed perivascular fibrosis in ADO2 kidneys (+4.4-fold; P<0.0001). Moderate perivascular fibrosis was also observed in lungs (+1.5-fold; P<0.001), which in homozygous Clcn7^G213R mice was more pronounced and associated with severe atelectasis and airway closure. Interestingly, perivascular fibrosis was confirmed in muscle, a tissue that does not express ClC7 but that it is often damaged in ADO2 patients. ADO2 spleens did not show fibrosis but had elevated number of megakaryocytes (+1.4-fold; P=0.03), sign of an enhanced ectopic hematopoiesis.

Conclusion: Our study demonstrates that ADO2 is not only a bone disease, but it affects several organs at multiple cellular levels. It exemplifies the complexity of this pathology and the need to develop a targeted therapy with a systemic effect. In fact, the perivascular fibrosis was rescued, along with the bone phenotype, by systemic administration of an effective Clcn7^G213R-specific siRNA.

Disclosure: The authors declared no competing interests.