Base Editing vs. Prime Editing vs. CRISPR: What’s the Difference?

Base editing, prime editing, and CRISPR-Cas9 are three genome editing technologies that differ in how they alter DNA. Base editing chemically converts one DNA nucleotide to another without cutting the strand; prime editing uses a search-and-replace genome editing mechanism to install any of 12 possible base changes without double-strand breaks; and CRISPR-Cas9 cuts both DNA strands and relies on cellular repair pathways. Researchers choose between these tools based on the mutation type they need to correct and the precision each application requires.

What Is Base Editing and How Does It Work?

Base editing achieves precise gene editing without double-strand breaks by fusing a modified, catalytically inactive Cas9 to a deaminase enzyme. The complex binds the target DNA site and chemically converts one base to another – without cutting the backbone. According to research in the International Journal of Molecular Sciences (Kantor et al., 2020), over 25% of pathogenic single-nucleotide variants can be corrected with the four transitions base editing supports.

Two classes of base editors handle four transition mutations:

• Cytosine Base Editors (CBE): Convert C•G base pairs to T•A. First described in 2016 by David Liu’s lab at the Broad Institute.
• Adenine Base Editors (ABE): Convert A•T base pairs to G•C. Introduced in 2017 by the same group.

Base editing is highly efficient in both dividing and non-dividing cells – a key advantage for gene therapy for sickle cell disease and beta-thalassemia, where non-dividing red blood cell precursors are the target. Beam Therapeutics is currently running active clinical trials using base editing for hemoglobinopathies. The main limitation is that base editing handles only four transition mutations and cannot perform transversions, insertions, or deletions. Bystander editing – unintended changes to nearby bases within the editing window – is also a known challenge.

What Is Prime Editing and How Does It Improve on Base Editing?

Prime editing is a search-and-replace genome editing technique first published in Nature by Anzalone et al. (2019). It uses a fusion of Cas9 nickase and reverse transcriptase, guided by a prime editing guide RNA (pegRNA). The pegRNA both locates the target DNA site and carries the template for the desired edit – enabling any of the 12 possible base-to-base conversions that base editing cannot fully cover.

Prime editing works through these four steps:

1. The Cas9 nickase, guided by the pegRNA, binds to the target site and creates a single-strand nick in the DNA.
2. The nicked strand hybridizes to the primer-binding site on the pegRNA.
3. The reverse transcriptase synthesizes new DNA using the pegRNA template, encoding the desired change.
4. The cell’s own repair machinery completes the edit by incorporating the new sequence.

According to the original Nature publication, as cited in Addgene’s prime editing resource, prime editing can in principle correct up to 89% of pathogenic variants in the NCBI ClinVar database – which tracks over 75,000 disease-associated human variants. It supports insertions of up to 44 base pairs and deletions of up to 80 base pairs, well beyond base editing’s range.

On base editing vs prime editing efficiency: base editors are generally faster and more consistent in most cell types. Prime editing delivers greater versatility and is free from off-target effects in gene editing caused by bystander editing, but requires more optimization per target site. Prime Medicine and other companies are advancing early-stage clinical programs using prime editing.

How CRISPR-Cas9 Works: The Original Gene Editor

CRISPR-Cas9 edits the genome by creating a CRISPR double-strand break at a targeted location, guided by a short RNA sequence. The cell then repairs the break through one of two pathways. For a detailed walkthrough, see how CRISPR-Cas9 works.

The two repair pathways determine the outcome:

• Non-Homologous End Joining (NHEJ): Fast but error-prone – introduces insertions or deletions, disrupting the gene. Best for knockouts.
• Homology-Directed Repair (HDR): Precise but requires a donor DNA template and works mainly in dividing cells. Best for exact sequence replacement.

CRISPR-Cas9 excels at large gene knockouts, insertions, and CAR-T cell engineering. Two CRISPR therapies – Casgevy and Lyfgenia – received FDA approval in December 2023 for sickle cell disease, making CRISPR-Cas9 the most clinically advanced of the three tools. See CRISPR in modern medicine for a full overview of approved and pipeline therapies.

The double-strand break carries risks that base editing and prime editing avoid: chromosomal rearrangements, indel byproducts, and higher off-target rates – particularly when multiple edits are made simultaneously. High-fidelity Cas9 variants reduce these risks significantly.

Base Editing vs. Prime Editing vs. CRISPR: Side-by-Side Comparison

These three next-generation CRISPR tools differ across six key dimensions relevant to research and therapeutic applications:

Real-World Applications: Which Tool Is Used When?

All three tools have roles in modern gene therapy. Their applications reflect their distinct strengths:

• Base Editing: Point mutation correction for sickle cell disease and beta-thalassemia. Beam Therapeutics is running active clinical trials using a cytosine base editor for hemoglobinopathies.
• Prime Editing: Correction of variants that require transversions or indels – including cystic fibrosis (F508del deletion), Huntington’s disease repeat expansions, and other mutations that single nucleotide variants in genetic disease cannot address with base editing alone.
• CRISPR-Cas9: Gene knockouts in research, large deletions, and CAR-T cell engineering. Two CRISPR therapies – Casgevy and Lyfgenia – received FDA approval in December 2023.

The mutation type determines the right tool. For simple C-to-T or A-to-G corrections, a base editor delivers high efficiency with a proven track record. For complex edits involving transversions or indels, prime editing is the correct choice. For gene knockouts or large genomic rearrangements, CRISPR-Cas9 remains the standard.

Frequently Asked Questions About Base Editing, Prime Editing, and CRISPR