An additional site mutation in MITA/STING gain-of-function mutants abolishes the autoimmune SAVI phenotypes and directs a therapeutic strategy

STING-associated vasculopathy with onset in infancy (SAVI) is an autoimmune disease caused by gain-of-function mutations (GOFs) of MITA/STING and the most frequent GOFs for SAVI are MITA^N154S and MITA^V155M. However, how MITA GOFs are spontaneously activated remains incompletely understood. Here, we show that the activity of MITA hinge-region GOFs is compromised by an additional mutation at Lys150 and that the SAVI phenotypes of MITA^N153S/WT mice are completely abolished in the MITA^K150N/N153S (MITA^NS/NS) mice. Mechanistically, MITA GOFs constitutively associate with iRhom2 for the spontaneous ER-to-Golgi translocation, which is substantially inhibited by the introduction of a mutation at Lys150. Interestingly, cGAMP binds to MITA^NS, triggers the ER-to-Golgi translocation of MITA^NS as well as the MITA^NS-iRhom2 interaction, and induces the expression of downstream genes in Mita^NS/NS cells similarly as in Mita^+/+ cells. Consistently, structural studies demonstrate an inactive open conformation of apo-MITA^NS characterized by connector region crossover and a curved filament of cGAMP-bound MITA^NS characterized by parallel connector regions, similar to those observed in wild-type MITA. Furthermore, we design a SAVI-inhibitory peptide (SIP) that selectively inhibits the interaction between hMITA^N154S (mMITA^N153S) and iRhom2 and the activity of MITA GOFs and thereby abolishes the SAVI phenotypes of the MITA^N153S/WT→WT chimeric mice. These findings reveal a previously uncharacterized mechanism for the spontaneous activation of MITA GOFs and highlight a potential therapeutic intervention for SAVI.