Genome Editing Mechanisms and DNA Repair Outcomes
Repair Biology as the Real Editor
DNA Repair Determines the Edit
Genome editing outcomes are governed as much by host repair machinery as by the targeting platform. For example, a Cas nuclease merely creates a lesion; the cell’s repair systems determine whether the result is a knockout, a precise insertion, a large deletion, or an unexpected rearrangement.
Key Repair Pathways
Sources of Variability
Editing efficiency varies with cell cycle stage, chromatin accessibility, target sequence context, DNA methylation state, repair factor abundance, and local transcriptional activity. In many systems, the same editor can produce different outcomes in stem cells, neurons, or tumor cells because the repair environment differs substantially.
Desired vs Undesired Outcomes
Desired outcomes
- High on-target editing fraction
- Low indel and rearrangement burden
- Predictable genotype-phenotype mapping
Undesired outcomes
- Off-target cleavage or binding
- Large deletions or chromothripsis-like events
- Mosaicism and allele-specific bias
Which repair pathway most commonly generates small indels after a double-strand break in many mammalian cells?
Non-homologous end joining is fast and error-prone, often producing small insertions or deletions.
Correct answer: Non-homologous end joining
Why is chromatin context important in genome editing?
Chromatin state influences whether the target site is accessible to editing proteins and repair factors.
Correct answer: It can affect access of editors and repair machinery to the target site
What is one reason the same CRISPR editor may perform differently in two cell types?
Cell-state differences strongly influence the observed editing outcome distribution.
Correct answer: The two cell types may differ in chromatin accessibility and DNA repair pathway usage.
Name one class of undesirable genomic outcome that can accompany double-strand-break editing beyond small indels.
Double-strand breaks can lead to substantial structural variants, not just local sequence changes.
Correct answer: Large deletions or chromosomal rearrangements.
Interpretation Warning
A high on-target rate does not guarantee a safe or useful edit. In advanced studies, measure the full outcome spectrum: intended edit, byproducts, structural variation, mosaicism, and functional consequence.