CRISPR Therapeutics: From Casgevy to In Vivo
Cas9, base editors, prime editors — and the 2023 watershed: a CRISPR drug on the market.
What's covered
- T01Cas9, Cas12a, Cas13: mechanism and guide design
- T02Base editing (CBE, ABE) and prime editing — bypassing DSBs
- T03Casgevy (exa-cel) for SCD/β-thalassemia: BCL11A enhancer disruption + autologous HSC
- T04In-vivo lipid-nanoparticle delivery: NTLA-2001 (TTR for ATTR), VERVE-101/102 (PCSK9)
- T05AAV-delivered editing for ocular and hepatic targets (EDIT-101 EDIT-301)
- T06Off-target, p53 activation, large-deletion, chromosomal-loss safety signals
- T07Manufacturing & access: Casgevy at $2.2M, ex-vivo myeloablation burden
By the end of this module you will be able to
- L01Explain the mechanistic difference between SpCas9, base editors, and prime editors and when each is preferred.
- L02Describe Casgevy's mechanism (BCL11A enhancer → fetal hemoglobin reactivation) and the autologous HSC manufacturing pathway.
- L03Identify the leading in-vivo CRISPR programs and their delivery strategies.
- L04Discuss the documented safety signals: off-target, large deletions, chromothripsis, p53 activation, AAV immunogenicity.
What you should walk away believing
- Casgevy's December 2023 FDA/MHRA approval was the first CRISPR therapy approved anywhere — the field has crossed from promise to product.
- Base editing (especially ABE) avoids double-strand breaks and is the leading strategy for in-vivo single-nucleotide corrections; Verve's heart.1 (VERVE-102) reads out 2024–2026.
- NTLA-2001 (intellia/Regeneron, ATTR amyloidosis) is the first systemic in-vivo CRISPR drug, with durable knockdown of TTR after a single LNP infusion.
- Off-target detection methods (GUIDE-seq, CIRCLE-seq, DISCOVER-seq, ONE-seq) have matured; large structural events (megabase deletions, chromothripsis, aneuploidy) remain the harder safety question.
What this means at your level
CRISPR-Cas systems are programmable molecular scissors that can be directed to specific DNA sequences using a short guide RNA. Newer variants — base editors and prime editors — make changes without cutting both strands of DNA, reducing collateral damage. The first CRISPR therapy was approved in December 2023 for sickle cell disease and β-thalassemia.
Casgevy (exa-cel) is approved but logistically heavy: autologous CD34+ HSC collection, ex-vivo CRISPR editing of the BCL11A erythroid enhancer, busulfan myeloablation, autologous transplant, ~$2.2 M. Indications: severe SCD and transfusion-dependent β-thalassemia. In-vivo systemic editing (NTLA-2001 for ATTR, Verve PCSK9 for FH/CVD) is in pivotal trials and may reach approval 2026–2028.
Beyond classical Cas9: prime editing 3 (PE3, PE5, twinPE) now reaches editing efficiencies suitable for many in-vivo applications; engineered pegRNAs, MS2-recruited reverse transcriptase. Search-and-replace at the kilobase scale via prime-editor + recombinase (PASTE, Anzalone group) and twin-prime strategies. Watch BridgeRNA / IS200/605 systems (Hsu/Patel 2024) and AlphaProteo-designed Cas variants.
CRISPR therapies are inherently safer than traditional gene therapy because they're 'precise.'
Cas9-induced double-strand breaks generate complex local outcomes (large deletions, translocations, chromothripsis) that current short-read sequencing assays underestimate. Base and prime editors reduce but do not eliminate off-target risk. Lipid-nanoparticle and AAV delivery introduce their own immunogenicity and tropism problems.
What the data say
Test yourself
Key terms & abbreviations
- Cas9 / Cas12a / Cas13
- CRISPR effector nucleases targeting dsDNA (Cas9, Cas12a) or RNA (Cas13). Sourced from S. pyogenes, S. aureus, A. cellulolyticus, etc.
- Base editor
- Cas9-nickase fusion with a cytidine (CBE) or adenine (ABE) deaminase that converts a single base without a DSB.
- Prime editor
- Cas9-nickase fused to reverse transcriptase, programmed by a pegRNA carrying both target and template; supports small indels and any base substitution.
- LNPLipid Nanoparticle
- Ionizable-lipid delivery vehicle for mRNA + sgRNA; the only systemically validated non-viral CRISPR delivery to date (hepatic tropism dominant).
- AAVAdeno-Associated Virus
- Viral vector for delivering Cas9 expression cassettes (often split-Cas9 due to packaging limit). Tissue-tropism varies by capsid serotype.
- PASTEProgrammable Addition via Site-specific Targeting Elements
- Anzalone group method coupling prime editing to serine integrase for kilobase-scale insertions.
Anchor references
- Exagamglogene autotemcel for severe sickle cell disease — Frangoul et al., NEJM 2024
- CRISPR-Cas9 in vivo gene editing for transthyretin amyloidosis — Gillmore et al., NEJM 2021 (NTLA-2001)
- Search-and-replace genome editing without double-strand breaks (prime editing) — Anzalone et al., Nature 2019