Genetic and functional investigation of a Mendelian form of systemic lupus erythematosus

Abstract We have shown that bi-allelic mutations in ACP5 result in a deficiency of the encoded protein, tartrate-resistant acid phosphatase (TRAP), which causes the immuno-osseous disease spondyloenchondrodysplasia. In addition to having a bone and neurological phenotype, patients with spondyloenchondrodysplasia exhibit a wide spectrum of autoimmune manifestations, including autoantibody production, systemic lupus erythematosus, increased interferon α (IFNα), and an interferon signature. In bone, TRAP produced by osteoclasts regulates cell migration by dephosphorylation of osteopontin. However, TRAP is also produced by immune cells, particularly macrophages and dendritic cells (DCs). In murine plasmacytoid dendritic cells (pDC), phosphorylated osteopontin is integral to IFN-α production. Whereas osteopontin has been shown to be a TRAP substrate in murine bone cells, nothing is known about whether TRAP regulates osteopontin in immune cells. We hypothesise that TRAP dephosphorylates osteopontin in human pDCs and negatively regulates IFN-α production. We studied the interaction between TRAP and osteopontin. By confocal microscopy, we showed that TRAP co-localised with osteopontin in the Golgi in primary human macrophages, DCs, and pDCs. In a TRAP and osteopontin overexpression system in human embryonic kidney 293 cells, we co-immunoprecipitated TRAP and osteopontin, suggesting that they interact with each other. We also showed in an in-vitro assay that recombinant human TRAP could dephosphorylate osteopontin, demonstrating that osteoponin is a substrate for TRAP. These findings suggest that osteopontin is a target for TRAP in immune cells. To test whether TRAP deficiency leads to hyperphosphorylation of osteopontin and increased IFNα, we are generating TRAP knockdowns in a pDC cell line. Understanding of the mechanism by which TRAP regulates IFNα, potentially via osteopontin, may eventually allow for directed therapeutic approaches in spondyloenchondrodysplasia and other autoimmune diseases associated with an interferon signature, particularly systemic lupus erythematosus. Funding Wellcome Trust.