Characterization of STING SAVI gain of function mutations in mice

Lupus affects nine times more women than men. One explanation for this SAVI (STING-associated vasculopathy with onset in early infancy) is a disease caused by a mutation in the gene that makes a protein called STING. In patients with SAVI, the STING protein is locked in its “on” position, causing immune cells to be constantly active. These active immune cells create inflammation that damages tissues throughout the body in a manner that is similar in many respects to lupus. While lupus is caused by more complex genetic and environmental factors than SAVI, the STING pathway may also play a role in lupus. Dr. Katherine Fitzgerald is exploring how the mutant STING protein goes rogue to trigger the lupus-like symptoms in mice.

What this study means to people with lupus

By focusing on the simpler model system of a single-gene disease like SAVI, Dr. Fitzgerald will gain new information about the STING pathway that can be applied to develop new treatment approaches for SAVI and lupus patients.

Detection of pathogen-derived nucleic acids by surface as well as cytosolic receptors is critical for the initiation of innate immune defense against bacterial, viral and eukaryotic pathogens. However, inappropriate detection of self nucleic acids can result in debilitating human diseases. Genetic studies of humans and mice implicate the cytosolic cGAS-STING pathway in certain autoimmune and inflammatory diseases. A link between this pathway and autoimmune diseases has been established in Aicardi-Gourtiere syndrome, a monogenic inflammatory disorder caused by mutations in TREX1 a nuclease that normally degrades immune stimulatory DNA, typically affecting the brain (encephalopathy) and the skin. Dysregulation of this pathway is also associated with systemic lupus erythematosus where mutations in TREX1 and other Dnases have been identified in a subset of SLE patients. Most recently, direct evidence that activation of STING causes a lupus like human disease has been provided by the identification of gain-of-function mutations in STING resulting in early-onset systemic inflammation, cutaneous vasculopathy and pulmonary fibrosis. The STING mutations in SAVI patients result in spontaneous dimerization and activation of STING resulting in a robust type I interferon gene signature. SAVI patients also present with this interferon signature and additional “lupus-like” symptoms such as production of anti-nuclear autoantibodies (ANAs), malar rash and alveolar/ interstitial inflammation. Surprisingly, certain tissues are particularly inflamed in SAVI patients which is surprising since STING is widely expressed in numerous cell types such as endothelial, epithelial as well as hematopoietic cell types. It remains to be understood how, where and when STING activation contributes to disease in this system. Further understanding of monogenic interferonopathies such as SAVI will be crucial not only to identify new drugs to treat this disease, but also for more complex diseases such as Systemic Lupus Erythematosis where multiple pathways including the STING pathway play a role. To gain mechanistic insight into the pathogenesis of autoimmune disorders, we have generated mice where e introduced the SAVI mutations (N153S and V154M) in individual mouse models using CRISPR/CAS 9. SAVI knock-in mice exhibit features of systemic autoimmune consistent with findings in humans. The heterozygous mutant mice (SAVI HETs) show a robust interferon signature in the sera as well as in the bone- marrow. Similar to what has been observed in humans peliminary characterization of these mice reveal inflammation in the lung, liver and kidney. We proposed to define the impact of STING activity in this model system by characterizing immune activation in cells and tissues. We have also generated stop-flox alleles of the SAVI mutants so that individual cell types can be examined to define their influence on disease pathogenesis. We also propose to test STING antagonists (which block STING activity) in order to block STING pathway activation.