Inducible CRISPR genome-editing tool: classifications and future trends

The discovery of CRISPR-Cas9/dCas9 system has reinforced our ability and revolutionized our history in genome engineering. While Cas9 and dCas9 are programed to modulate gene expression by introducing DNA breaks, blocking transcription factor recruitment or dragging functional groups towards the targeted sites, sgRNAs determine the genomic loci where the modulation occurs. The off-target problem, due to limited sgRNA specificity and genome complexity of many species, has posed concerns for the wide application of this revolutionary technique. To solve this problem and, more importantly, gain power over gene functionality and cell fate control, inducible strategies have been continuously evolved to offer tailored solutions to address specific biological questions. By reviewing recent advances in inducible CRISPR system design and critical elements potentially adding values to such systems, we classify current approaches in this domain into four mechanically distinct categories, namely, "split system", "allosteric system", "combinatorial system", and "transient delivery system", discuss the pros and cons of each system, and point out the under-explored areas and future directions, with the aim of enriching our toolbox of delicate life engineering.