In nature, microorganisms use CRISPR (clustered regularly interspaced palindromic repeats) and CRISPR-associated (Cas) proteins for antiviral immunity through recognition and destruction of specific DNA sequences. Over the past decade, CRISPR genome editing has been developed to create transformative technologies to treat, cure, and prevent human disease.
CRISPR genome editing allows scientists to change DNA sequences in cells at virtually any desired position, enabling both fundamental research and therapeutic applications (Figure). CRISPR-Cas9, the most widely used genome editor, is an RNA-guided DNA-cutting enzyme that makes double-stranded DNA breaks at preselected (on-target) positions in the DNA of living cells.1 Repair of the break site results in either small insertions and deletions (indels) introduced by error-prone repair or insertion of a new DNA donor sequence chosen by the investigator (homology-directed repair). Indels are useful for interrupting gene function, whereas sequence insertion can replace a defective sequence to restore gene function.2 In either case, controlling the exact editing outcome for a particular indication in a specific cell type or organ is challenging, and unintended (off-target) DNA changes can be harmful.