Clustered, regularly interspaced, short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems have been employed as a powerful versatile technology for programmable gene editing, transcriptional modulation, epigenetic modulation, and genome labeling, etc. Yet better control of their activity is important to accomplish greater precision and to reduce undesired outcomes such as off-target events. The use of small molecules to control CRISPR/Cas activity represents a promising direction. Here, we provide an updated review on multiple drug inducible CRISPR/Cas systems and discuss their distinct properties. We arbitrarily divided the emerging drug inducible CRISPR/Cas systems into two categories based on whether at transcription or protein level does chemical control occurs. The first category includes Tet-On/Off system and Cre-dependent system. The second category includes chemically induced proximity systems, intein splicing system, 4-Hydroxytamoxifen-Estrogen Receptor based nuclear localization systems, allosterically regulated Cas9 system, and destabilizing domain mediated protein degradation systems. Finally, the advantages and limitations of each system were summarized.
Keywords: 4-OHT, 4-Hydroxytamoxifen; ABA, abscisic acid; ADs, activation domains; CIP, chemically induced proximity; CRISPR, clustered, regularly interspaced, short palindromic repeats; Cas, CRISPR-associated protein; CrRNA, CRISPR RNA; DD, destabilizing domain; DHFR, dihydrofolate reductase; ER, Estrogen Receptor; FKBP, FK506-binding protein; FRB, FKBP-rapamycin-binding domain; GA, gibberellin; HIT, Hybrid drug Inducible CRISPR/Cas9 Technologies; Hsp90, heat shock protein 90; LBD, ligand binding domain; LSL, loxP-stop-loxP; MST, multiplex single transcript; NES, nuclear export sequence; NLS, nuclear localization sequence; Ptet, tetO-containing promoter; Sa, Staphylococcus areus; Sp, Streptococcus pyogenes; TMP, trimethoprim; TRE, tetracycline response element; TRE3G, Tet-On 3G protein; TetO, tet operator; TetR, Tet repressor protein; VPR, VP64-P65-Rta; arC9, allosterically regulated Cas9; dCas9, dead Cas9; dCpf1, dead Cpf1; dLbCpf1, Lachnospiraceae bacterium dCpf1; dox, doxycycline; iPSCs, induced pluripotent stem cells; rtTA, reverse-tTA; sgRNA, single guide RNA.