Genome editing is a group of technologies used to make targeted changes to the DNA of a cell or organism. Changes can include substitutions, insertions, or deletions at selected genomic locations. Editing may alter a gene, modify a regulatory sequence, or introduce new genetic information. It is used to investigate biological functions, develop agricultural traits, and produce certain medical treatments. Although often associated with CRISPR, genome editing also encompasses earlier protein-based targeting systems and methods based on homologous recombination. (genome.gov)
Biological principles
Many editing systems combine recognition of a particular DNA sequence with an activity that cuts or chemically modifies DNA. In nuclease-based editing, an engineered enzyme creates a break at the target. The final genetic change then depends substantially on the cell’s DNA repair machinery, rather than on DNA cutting alone. Consequently, targeting a specific location does not guarantee one uniform editing outcome. (nature.com)
Following a double-strand break, non-homologous end joining and related repair processes can generate small insertions or deletions. These can disrupt a coding sequence and produce a gene knockout. Alternatively, homology-directed repair can use a supplied DNA template to introduce a specified sequence change. The balance between outcomes depends on the target sequence, repair pathways, and cellular context. Editing therefore creates deliberately induced mutations, but their identity and frequency require experimental verification. (nature.com)
Development and major platforms
Early targeted genome modification relied on homologous recombination, through which introduced DNA exchanges information with a similar genomic sequence. Researchers developed laboratory applications beginning in the late 1970s. Because targeted recombination can be inefficient, subsequent technologies introduced programmable nucleases to increase modification at selected sites. (genome.gov)
Zinc-finger nucleases combine engineered DNA-binding domains with a DNA-cleaving domain. Transcription activator-like effector nucleases, or TALENs, use another modular DNA-recognition system. Both require engineering a targeting protein for the chosen sequence. CRISPR systems instead use a guide RNA to direct a CRISPR-associated nuclease, such as Cas9, toward complementary DNA, making retargeting possible principally by changing the guide sequence. These systems were adapted from microbial defense mechanisms. (genome.gov)
A programmable CRISPR–Cas9 cutting system was demonstrated in 2012, followed by applications in mammalian cells. Emmanuelle Charpentier and Jennifer Doudna received the 2020 Nobel Prize in Chemistry for developing a method for genome editing. CRISPR expanded access to targeted modification, but did not eliminate the need to assess specificity or repair outcomes. (nobelprize.org)
Base editing and prime editing
Base editing changes particular DNA bases without requiring a targeted double-strand break. The cytosine base editor reported in 2016 combined a modified Cas9 with a cytidine deaminase to enable conversion of C–G base pairs to T–A. Adenine base editors subsequently enabled A–T to G–C conversion. Their usefulness depends on the required substitution and the position of editable bases within the enzyme’s activity window; nearby bases can also be changed unintentionally. (nature.com)
Prime editing, reported in 2019, combines a Cas9 nickase with reverse transcriptase and a prime-editing guide RNA. This RNA both identifies the target and encodes the intended alteration. The original system demonstrated all twelve possible base-to-base substitutions, as well as targeted small insertions and deletions, without requiring a double-strand break or a separate donor DNA template. Efficiency and unwanted products vary among targets and cell types; prime editing is not uniformly superior to other methods. (nature.com)
Research, agriculture, and medicine
In molecular biology, editing enables researchers to disrupt genes, introduce variants, and examine their effects in cells and model organisms. Comparisons between edited and control systems help investigate gene function and mechanisms of disease. Agricultural applications include modifying plants for disease resistance, nutritional characteristics, and responses to environmental conditions. Genome editing can alter existing sequences rather than necessarily adding a gene from another species. (genome.gov)
In gene therapy, editing may occur ex vivo, in cells removed from a patient and subsequently returned, or in vivo, through delivery to cells within the body. On December 8, 2023, the U.S. Food and Drug Administration approved Casgevy for patients aged twelve years and older with sickle cell disease and recurrent vaso-occlusive crises. It was the first FDA-approved CRISPR–Cas9 therapy. The treatment edits patients’ blood-forming stem cells, which are reinfused after conditioning treatment and increase fetal hemoglobin production. (iris.who.int)
Technical limitations and governance
Delivery to the appropriate tissue remains a major constraint. Editing components must reach enough target cells while avoiding unacceptable toxicity or immune responses. Safety assessment also considers unintended changes at other genomic locations and unwanted outcomes at the intended target. Experimental studies have documented large deletions and complex rearrangements following Cas9-induced breaks, showing why verification cannot be restricted to detecting the desired small edit. (genome.gov)
Human applications distinguish somatic editing, which targets non-reproductive cells, from germline editing involving reproductive cells or embryos. Germline modifications can become heritable if edited cells contribute to reproduction. These categories raise different questions about safety, informed consent, and effects on future generations. In July 2021, the World Health Organization published recommendations and a governance framework addressing institutional oversight, research registration, international coordination, and public engagement. The recommendations constitute international guidance rather than a single binding global regulatory system. (genome.gov)