Molecular basis of autoimmune disease protection by MDA5 variants

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• Melanoma differentiation-associated gene 5 (MDA5) is crucial for recognizing viral double-stranded RNA (dsRNA).
• MDA5's function relies on cooperative filament formation and ATP-dependent proofreading for sensitive and specific dsRNA detection.
• Genetic variants in MDA5 are linked to autoimmune disease protection but increased infection risk, yet their precise impact on RNA sensing is unknown.

Purpose of the Study:

• To investigate how autoimmune-protective MDA5 variants influence its molecular structure and RNA sensing activities.
• To elucidate the mechanisms by which specific MDA5 variants affect immune signaling pathways.

Main Methods:

• Utilized cryoelectron microscopy (cryo-EM) to determine the structures of MDA5 variants bound to dsRNA.
• Assessed the RNA binding capabilities and ATPase activity of MDA5 variants.
• Measured the interferon response to picornavirus infection and cytokine induction in the presence of MDA5 variants.

Main Results:

• Rare variants E627* and I923V were found to reduce the interferon response to picornavirus infection.
• E627* variant exhibited no RNA binding, while I923V showed hyperactive ATPase, leading to premature dsRNA dissociation.
• Cryo-EM structures revealed that I923V has smaller RNA binding interfaces and excessive proofreading activity.
• Genetically linked variants R843H and T946A did not impact cytokine induction, suggesting indirect disease mechanisms.

Conclusions:

• Autoimmune-protective MDA5 variants can dampen MDA5-dependent signaling through diverse mechanisms, including impaired RNA binding and altered ATPase activity.
• These findings clarify how specific MDA5 genetic variations affect innate immune responses and susceptibility to infections and inflammation.