Chronic endoplasmic reticulum stress in myotonic dystrophy type 2 promotes autoimmunity via mitochondrial DNA release

Abstract Myotonic dystrophy type 2 (DM2) is a tetranucleotide CCTG repeat expansion disease associated with an increased prevalence of autoimmunity. Here, we identified an elevated type I interferon (IFN) signature in peripheral blood mononuclear cells and primary fibroblasts of DM2 patients as a trigger of chronic immune stimulation. Although RNA-repeat accumulation was prevalent in the cytosol of DM2-patient fibroblasts, type-I IFN release did not depend on innate RNA immune sensors but rather the DNA sensor cGAS and the prevalence of mitochondrial DNA (mtDNA) in the cytoplasm. Sublethal mtDNA release was promoted by a chronic activation of the ATF6 branch of the unfolded protein response (UPR) in reaction to RNA-repeat accumulation and non-AUG translated tetrapeptide expansion proteins. ATF6-dependent mtDNA release and resulting cGAS/STING activation could also be recapitulated in human THP-1 monocytes exposed to chronic endoplasmic reticulum (ER) stress. Altogether, our study demonstrates a novel mechanism by which large repeat expansions cause chronic endoplasmic reticulum stress and associated mtDNA leakage. This mtDNA is, in turn, sensed by the cGAS/STING pathway and induces a type-I IFN response predisposing to autoimmunity. Elucidating this pathway reveals new potential therapeutic targets for autoimmune disorders associated with repeat expansion diseases

Structural basis of oligomerization in the stalk region of dynamin like MxA

The interferon-inducible dynamin-like myxovirus resistance protein 1 (MxA; also called MX1) GTPase is a key mediator of cell-autonomous innate immunity against pathogens such as influenza viruses. MxA partially localizes to COPI-positive membranes of the smooth endoplasmic reticulum-Golgi intermediate compartment. At the point of infection, it redistributes to sites of viral replication and promotes missorting of essential viral constituents. It has been proposed that the middle domain and the GTPase effector domain of dynamin-like GTPases constitute a stalk that mediates oligomerization and transmits conformational changes from the G domain to the target structure; however, the molecular architecture of this stalk has remained elusive. Here we report the crystal structure of the stalk of human MxA, which folds into a four-helical bundle. This structure tightly oligomerizes in the crystal in a criss-cross pattern involving three distinct interfaces and one loop. Mutations in each of these interaction sites interfere with native assembly, oligomerization, membrane binding and antiviral activity of MxA. On the basis of these results, we propose a structural model for dynamin oligomerization and stimulated GTP hydrolysis that is consistent with previous structural predictions and has functional implications for all members of the dynamin family