Targeting, Skipping, and Editing RNA in Mammalian Cells Using CRISPR-Cas13 Effectors: Expanding the CRISPR Toolbox Beyond DNA Editing

RNA modifications are involved in many aspects of biological functions. RNA modification controls cell fate transition in the mammalian embryonic stem cells. Some modifications have been proved to affect normal development, control the turnover and/or translation of transcripts during cell-state transitions, so as to affect the tissue development and homeostasis. The potential for gene therapy has been evident for addressing diseases in the past years, with progresses have been made in the precision and frequency of genome editing. CRISPR has now become synonymous with genome editing, and is most commonly associated with DNA editing. The discovery of Cas13 enzymes has facilitated the creation of a flexible and programmable toolbox capable of targeting RNA. Cas13 cleaves a target RNA via an intrinsic RNase, whose activity is activated by the binding of a CRISPR RNA (crRNA) guide molecule. CRISPR-Cas13 has been used for the highly efficient and specific degradation of multiple non-coding RNAs both in vitro and in vivo. Moreover, dead Cas13 protein (dCas13) can function as a programmable RNA-binding platform, and can be fused with a catalytic domain or an enzyme with the aim of RNA modification within cells.

Although recently discovered, the CRISPR-Cas13 system has been harnessed to develop various diagnostic tools and transcriptome engineering methods. The distinctive biochemical properties of Cas13 make it a versatile enzyme that can be efficiently used for programmable RNA knockdown, editing, splicing, translation, and chemical modifications. Future studies will further clarify the potentials and limitations of CRISPR-Cas13. As with the CRISPR-Cas9 system, the restrictions of CRISPR-Cas13 can be gradually reduced, and efficiency can be increased. Therefore, we would like to discuss their strengths in comparison with the traditional gene-modifying systems and highlight their emerging therapeutic applications. In addition, clinical trials using the CRISPR-Cas13 system in the diagnosis and treatment of the disease may be possible, and perhaps, in the future, Cas13-based methods may be used in clinical practice as next-generation cancer diagnostics and therapeutics.

The current Research Topic aims to develop molecular tools for RNA modifications based on reprogrammed CRISPR-Cas13 technologies. Areas to be covered in this Research Topic may include, but are not limited to:
• CRISPR-Cas13 system for RNA base editing
• Alternative mRNA splicing/polyadenylation
• Programmable RNA methylation/demethylation
• Live-cell RNA imaging
• Mapping RNA-protein interactions
• Blocking RNA-binding proteins in cells