Background: Type 1 long interspersed elements, or LINE-1, are the only active retroelements in human cells. The retrotransposition process of LINE-1 can trigger the activation of the innate immune system and has been proposed to play a role in the development of several autoimmune diseases, including Aicardi-Goutières syndrome (AGS). In contrast, all known AGS-associated proteins, except MDA5, have been reported to affect LINE-1 activity. Thus, MDA5 is likely to also function as a LINE-1 suppressor.
Results: MDA5 was found to potently suppress LINE-1 activity in a reporter-based LINE-1 retrotransposition assay. Although MDA5 is an endogenous RNA sensor able to activate the innate immune system, increased interferon (IFN) expression only weakly contributed to MDA5-mediated LINE-1 suppression. Instead, MDA5 effectively reduced the levels of LINE-1 ORF1p and ORF2p, as a result of the MDA5-mediated downregulation of the promoter activity of LINE-1 5’-UTR, and the subsequent generation of LINE-1 RNA. Interestingly, despite MDA5 being a multi-domain protein, the N-terminal 2CARD domain alone is sufficient to inhibit LINE-1 activity.
Conclusion: Our data reveal that MDA5 functions as a promoter regulator and suppresses the promoter activity of LINE-1 5’-UTR. Consequently, MDA5 reduces LINE-1 RNA and protein levels, and ultimately inhibits LINE-1 retrotransposition. In contrast, MDA5-induced IFN expression only plays a mild role in MDA5-mediated LINE-1 suppression. In addition, the N-terminal 2CARD domain was found to be a functional region for MDA5 upon inhibition of LINE-1 replication. Thus, our data suggest that besides being an initiator of the innate immune system, MDA5 is also an effector against LINE-1 activity, potentially forming a feedback loop by suppressing LINE-1-induced innate immune activation.