CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) repurpose a nuclease-dead Cas9 (dCas9), which retains guide-RNA-directed DNA binding but cannot cleave, as a programmable epigenome-editing and transcription-modulation tool: fusing dCas9 to repressor domains (e.g., KRAB, MeCP2) blocks or silences transcription (CRISPRi), while fusing it to transactivation domains (e.g., VP64, or synergistic multi-domain systems like VPR and SAM, which recruit VP64, p65, and HSF1 via an MS2 RNA-aptamer-modified guide scaffold) activates endogenous gene transcription (CRISPRa). Because activity is directed at the endogenous promoter rather than through cDNA overexpression, effect strength depends on guide RNA position relative to the transcription start site (TSS), with CRISPRi most effective downstream of the TSS and CRISPRa most effective upstream, and regulation is reversible, tunable, and acts on all splice isoforms transcribed from that promoter. This belongs to the domain of functional genomics and epigenetics within molecular biology, extending genome-editing tools (originally CRISPR-Cas9 nucleases) into pooled-screen platforms for probing gene function, genetic interactions, and transcriptional networks, distinct from and complementary to direct genome editing.
CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) repurpose a nuclease-dead Cas9 (dCas9), which retains guide-RNA-directed DNA binding but cannot cleave, as a programmable epigenome-editi…