🧬 PubMed RNA Editing Daily Digest

The ability to engineer synthetic biomolecular condensates in living cells offers new opportunities to control intracellular organization, yet robust and programmable RNA-based systems have remained limited. Here, we introduce genetically encoded, modular platforms that generate RNA-driven condensates using nanostar-derived scaffolds. Systematic comparison of repeat-based and

CRISPR-Cas13 systems, harnessed for RNA-guided transcriptome editing, hold significant promise for clinical and in vivo therapeutic applications. However, understanding their in vivo target specificity and recognition rules remains a challenge. In this study, we employed the uSpyCLIP method, which enhances sensitivity and specificity for identifying RNA-binding protein (RBP) binding sites, to map the transcriptome-wide binding sites of catalytically inactive PspCas13b (dPspCas13b) and RfxCas13d (dRfxCas13d) in HEK293T cells, using a variety of single guide RNAs (gRNAs). Surprisingly, we identified both gRNA-dependent and gRNA-independent off-target binding sites for both dCas13 complexes. These gRNA-independent off-target sites exhibited distinct RNA structural and sequence signatures: dPspCas13b's gRNA-independent binding was associated with specific RNA structural features, while dRfxCas13d's was linked to unique sequence motifs. Analysis of gRNA-dependent off-target sites revealed the crucial role of the DR-distal and middle regions of the gRNA in determining binding specificity. Further analysis demonstrated that some off-target binding events led to changes in gene expression at the messenger RNA and/or protein level. Collectively, our findings provide important insights into the characteristics of gRNA-dependent and gRNA-independent off-target binding for PspCas13b and RfxCas13d, offering valuable guidance for optimizing Cas13 and gRNA design in future applications.

APOBEC3A catalyzes cytosine-to-uracil deamination in single-stranded DNA and RNA. Physiologically, APOBEC3A functions in innate immunity and aberrant deamination is associated with cytosine mutations in enzymatically preferred YTCW substrate motifs in multiple cancers. Much less is known about the potential contribution of APOBEC3A-catalyzed RNA editing to virus and cancer evolution. Here, we present HAMMER (hairpin-based APOBEC3A-mediated mRNA editing reporter), a rapid luminescence-based cellular assay for measuring RNA editing by APOBEC3A. HAMMER reports APOBEC3A activity as a reduction in the ratio of firefly to renilla luciferase activity. Briefly, tandem renilla and firefly luciferase open reading frames are separated by an optimal APOBEC3A hairpin substrate, in which C-to-U editing of a CGA motif yields a UGA stop codon thus preventing translation of the downstream firefly luciferase reporter, without impacting the upstream renilla reporter. HAMMER activation is dose-responsive, catalytic activity-dependent, and specific to human APOBEC3A. A panel of herpesviral ribonucleotide reductase constructs was used to show that direct inhibition of APOBEC3A results in a dose-responsive recovery of firefly luciferase expression. HAMMER is therefore a scalable and easy-to-use method for quantifying cellular APOBEC3A RNA editing activity and characterizing inhibitors.

APOBEC cytidine deaminases can convert cytosines to uracils in DNA as well as in RNA. The knowledge of DNA deamination motifs preferred by individual APOBECs revealed APOBEC3A as a major source of hypermutation in cancer. However, the extent and relative contribution of specific APOBECs into RNA editing remains unclear as their preferred RNA-editing motifs have not been defined. Here, using a parallel DNA and RNA sequencing strategy, coupled with motif-centered statistical analyses, we sought to identify mRNA edits and diagnostic editing motifs in yeast and human cells overexpressing individual APOBEC enzymes. This approach revealed a prevailing global enrichment for the uCg trinucleotide motif with even greater preference to the motif's cytosines located in 3' base of a loop within a hairpin-loop secondary structure when APOBEC3A, but not any other tested APOBEC, was overexpressed. Further analysis revealed the APOBEC3A-like diagnostic motif enrichment in editing calls from human cancers and blood cells. The APOBEC3A-like editing motif also prevailed in the RNA genomes of SARS-CoV-2 pandemic isolates, as well as in infectious persistent rubella viruses, and in polioviruses emerging from live-attenuated vaccine strains. Together, our results indicate that APOBEC3A is the predominant global APOBEC RNA editor with a potential to impact cell physiology and viral evolution.

Advanced CRISPR-based therapies benefit from CRISPR RNA (crRNA) with high nuclease resistance and enhanced drug-like properties, which is primarily achieved through chemical replacement of the RNA ribose moiety. However, for gene editing enzymes like CRISPR-Cas9 a handful of residues cannot be replaced with chemical ribose analogues, limiting the scope of therapeutic strategies. The mechanism underlying this restriction has remained unclear. Here, using nucleic acid chemistry, biochemistry, cryo-EM, and molecular dynamics simulations, we show that the ribose 2'-hydroxyl group at specific crRNA residues is required to achieve a conformational state competent for Cas9 target DNA binding. Based on the mechanistic principles uncovered, we combined site-specific phosphorothioate linkage chemistry with ribose replacement chemistry to restore binding and activity, resulting in high Cas9 editing efficiency and fidelity with a ribose-free crRNA. This study offers novel mechanistic insight and crRNAs with full chemical stabilization, making rational design of guide RNAs with complete nuclease protection for CRISPR-based medicines possible.