Project Summary/Abstract
To prevent or limit infection, upon sensing foreign nucleic acids (e.g., viral RNA) interferon (IFN) is produced and
elicits a potent innate immune response orchestrated by hundreds of IFN-stimulated genes. However, cells face
several challenges when attempting to differentiate between foreign and self nucleic acids. For instance, cells
often produce double-stranded RNAs (dsRNA) that can be mistakenly recognized as foreign and lead to
autoimmunity. This is exemplified by the fact that naturally occurring mutations in an RNA-editing enzyme called
ADAR1 (adenosine deaminase acting on RNA 1) cause an autoinflammatory disorder named Aicardi-Goutières
syndrome characterized by an aberrant IFN response in the absence of infection. In addition to this link to
autoimmunity, recent studies have demonstrated that depleting ADAR1 sensitizes tumor cells to innate or
therapy-induced immune responses and ADAR1 may therefore be a promising target for anti-cancer therapies.
Unfortunately, there is a gap in our basic understanding of how ADAR1 enables cells to differentiate self from
non-self RNAs. With this grant, we aim to address this knowledge gap.
ADAR1 belongs to a family of proteins that modifies adenosines (A) to inosines (I) on dsRNA. These A-
to-I modifications, through an unknown mechanism, prevent the sensing of dsRNAs by receptor proteins
including PKR (protein kinase R), which normally triggers translational inhibition and cell death. It has been
speculated that these modifications alter the secondary structures of dsRNAs and prevent them from activating
PKR. However, ADAR1 frequently targets unpaired adenosine bases, and therefore A-to-I conversion is unlikely
to cause major disruption to dsRNA structures. Here, we propose a novel hypothesis that one or more unknown
RNA-binding proteins (RBPs) are involved in differentiating ADAR1-edited vs. unedited dsRNAs. This RBP may
preferentially bind to inosine-containing RNAs and prevent them from activating PKR.
We will use a combination of genome-wide and high-coverage RBP-targeted CRISPR knockout screens
to identify protein factors, which together with ADAR1, prevent self RNA from triggering immune responses and
cell death. We will evaluate potential hits using a semi-arrayed siRNA library and include additional readouts
such as PKR activation and IFN production to prioritize candidates for further investigation. Lastly, we will initiate
preliminary mechanistic studies to determine their RNA interactions and potential protein interactions with
ADAR1 and PKR. This work will lay the foundation for future detailed mechanistic studies.
In conclusion, our proposal has the potential to open new avenues of research to understand the
biological functions and importance of inosines within RNA. Our study may reveal novel and paradigm-shifting
mechanistic insights into how cells differentiate self vs. non-self RNA. It will also provide critical information for
developing ADAR1-based cancer immunotherapies.