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.