Conspectus
RNA editing or “epitranscriptomic
modification” refers
to the processing of RNA that occurs after transcription to alter
the sequence or structure of the nucleic acid. These chemical alterations
can be found on either the ribose sugar or the nucleobase, and although
many are “silent” and do not change the Watson–Crick–Franklin
code of the RNA, others result in recoding events. More than 170 RNA
modifications have been identified so far, each having a specific
biological purpose. Additionally, dysregulated RNA editing has been
linked to several types of diseases and disorders. As new modifications
are discovered and our understanding of their functional impact grows,
so does the need for selective methods of identifying and mapping
editing sites in the transcriptome.
The most common methods
for studying RNA modifications rely on
antibodies as affinity reagents; however, antibodies can be difficult
to generate and often have undesirable off-target binding. More recently,
selective chemical labeling has advanced the field by offering techniques
that can be used for the detection, enrichment, and quantification
of RNA modifications. In our method using acrylamide for inosine labeling,
we demonstrated the versatility with which this approach enables pull-down
or downstream functionalization with other tags or affinity handles.
Although this method did enable the quantitative analysis of A-to-I
editing levels, we found that selectivity posed a significant limitation,
likely because of the similar reactivity profiles of inosine and pseudouridine
or other nucleobases.
Seeking to overcome the inherent limitations
of antibodies and
chemical labeling methods, a more recent approach to studying the
epitranscriptome is through the repurposing of proteins and enzymes
that recognize modified RNA. Our laboratory has used Endonuclease
V, a repair enzyme that cleaves inosine-containing RNAs, and reprogrammed
it to instead bind inosine. We first harnessed EndoV to develop a
preparative technique for RNA sequencing that we termed EndoVIPER-seq.
This method uses EndoV to enrich inosine-edited RNAs, providing better
coverage in RNA sequencing and leading to the discovery of previously
undetected A-to-I editing sites. We also leveraged EndoV to create
a plate-based immunoassay (EndoVLISA) to quantify inosine in cellular
RNA. This approach can detect differential A-to-I editing levels across
tissue types or disease states while being independent of RNA sequencing,
making it cost-effective and high-throughput. By harnessing the molecular
recognition capabilities of this enzyme, we show that EndoV can be
repurposed as an “anti-inosine antibody” to develop
new methods of detecting and enriching inosine from cellular RNA.
Nature has evolved a plethora of proteins and enzymes that selectively
recognize and act on RNA modifications, and exploiting the affinity
of these biomolecules offers a promising new direction for the field
of epitranscriptomics.