- Casgevy: The First CRISPR Medicine
- Blood Diseases: The Richest Pipeline
- Liver Diseases: The LNP Revolution
- Cancer: CRISPR-Edited Immune Cells
- HIV: Excising the Viral Reservoir
- Eye Diseases: The Privileged Site
- Muscle & Neurological Diseases
- How CRISPR Clinical Trials Are Designed
- The Cost Problem & Access Challenge
- The Story: The Cartographer of the Human Body
Section 1 — Casgevy: How the First CRISPR Medicine Works
On 8 December 2023, the US Food and Drug Administration approved Casgevy (exagamglogene autotemcel, exa-cel) for the treatment of sickle cell disease in patients 12 years and older. Two weeks later, it was also approved for transfusion-dependent beta-thalassaemia. The treatment was developed by Vertex Pharmaceuticals and CRISPR Therapeutics. The day the approval was announced, Jennifer Doudna — who received the call while teaching a class — stepped outside to cry.
Casgevy does not correct the sickle cell mutation directly. Instead, it uses a beautifully indirect strategy rooted in developmental biology: it reactivates fetal haemoglobin (HbF), a form of haemoglobin that is normally switched off after birth and replaced by adult haemoglobin (HbA). In sickle cell patients, the adult haemoglobin (HbS) is defective — but if enough fetal haemoglobin can be produced instead, it dilutes the sickle haemoglobin and prevents the sickling that causes pain crises, anaemia, and organ damage.
The switch from fetal to adult haemoglobin is controlled by a transcription factor called BCL11A. BCL11A binds to an enhancer region in the HBG1/HBG2 genes (which encode fetal haemoglobin chains) and represses their expression in adult red blood cells. Casgevy uses CRISPR to disrupt this BCL11A enhancer in haematopoietic stem cells — not destroying BCL11A entirely (which has other important functions in other cell types) but specifically disrupting the erythroid-specific enhancer element that controls its activity in blood cells.
The clinical results: In the pivotal CLIMB-SCD-121 trial, 29 out of 29 patients with sickle cell disease who were followed for at least 12 months had complete freedom from severe vaso-occlusive crises for the entire follow-up period. In the CLIMB-THAL-111 trial for beta-thalassaemia, 42 out of 42 patients achieved transfusion independence. These are extraordinary outcomes for diseases that have had no cure for decades.
Section 2 — Blood Diseases: The Richest CRISPR Therapeutic Pipeline
Blood diseases dominate the CRISPR clinical pipeline for a straightforward reason: haematopoietic stem cells (HSCs) can be harvested from the blood, edited outside the body (ex vivo), quality-controlled, and reinfused — the same approach used for bone marrow transplantation, which has been performed safely for 60 years. The infrastructure, the clinical expertise, and the regulatory framework all exist. CRISPR adds the ability to edit precisely rather than replacing the entire immune system.
Casgevy (exa-cel, Vertex/CRISPR Therapeutics): BCL11A enhancer disruption to reactivate HbF. 29/29 SCD patients free of severe crises; 42/42 thalassaemia patients transfusion-independent at 12 months. Lyfgenia (lovotibeglogene autotemcel, bluebird bio): lentiviral HBB gene addition (not CRISPR) also FDA-approved December 2023 for SCD.
Multiple programmes targeting liver factor VIII (FVIII) or factor IX (FIX) production. CRISPR Therapeutics’ CTX310 targets ANGPTL3 for reduction of cardiovascular risk in haemophilia patients. In vivo LNP approaches delivering CRISPR to liver hepatocytes are in Phase 1/2 with promising initial results showing sustained factor level increases.
Rarer inherited anaemias where CRISPR correction of HSCs may be curative. These diseases lack the patient populations to drive large trials, but CRISPR’s flexibility to address diverse genetic targets makes it uniquely suited for rare disease applications. Regulatory orphan drug designations significantly reduce trial requirements.
Section 3 — Liver Diseases: The LNP-Delivered In Vivo Revolution
The liver is currently the most accessible organ for in vivo CRISPR editing. Hepatocytes (liver cells) avidly take up LNP-delivered mRNA + guide RNA, and a single intravenous infusion can achieve durable editing in a majority of liver cells. The first clinical results from liver-targeted in vivo CRISPR are among the most impressive in the field.
ATTR Amyloidosis: Intellia’s NTLA-2001
Transthyretin (TTR) amyloidosis is a progressive, fatal disease caused by misfolding of the TTR protein produced by liver hepatocytes. Misfolded TTR aggregates damage the heart, nerves, and kidneys over years to decades. Intellia Therapeutics’ NTLA-2001 uses LNPs to deliver SpCas9 mRNA and a guide RNA targeting the TTR gene to liver cells, disrupting TTR production entirely.
The Phase 1 results, published in NEJM in 2021, were remarkable: a single intravenous dose reduced serum TTR levels by 87% at the highest dose, with the reduction maintained at 12-month follow-up. No serious adverse events related to the treatment were observed. Phase 3 trials are ongoing. This is the first demonstration that a single-dose in vivo CRISPR treatment can produce durable, near-complete knockdown of a disease-causing protein in humans.
PCSK9: A Potential One-Time Cholesterol Treatment
PCSK9 is a protein that degrades LDL receptors in the liver, reducing the liver’s ability to clear LDL cholesterol from the blood. People with loss-of-function mutations in PCSK9 have dramatically lower LDL levels and are protected from cardiovascular disease, with no apparent adverse effects. CRISPR editing of PCSK9 in the liver could permanently replicate this natural protection.
Verve Therapeutics is developing an adenine base editor (ABE) delivered by LNPs to edit PCSK9 in the liver — converting a single A to G that mimics the natural loss-of-function variant. Phase 1 results (HEART-1 trial) showed LDL reductions of up to 55% after a single dose. Verve is targeting patients with familial hypercholesterolaemia who have failed other therapies. If a single infusion can permanently lower LDL by 50%, it could replace a lifetime of daily statin medication.
Other Liver Targets
CRISPR Therapeutics CTX310: LNP delivery targeting ANGPTL3 to reduce HAE attacks. Also being explored for cardiovascular risk reduction. Phase 1 initiated 2023.
Liver produces misfolded A1AT that aggregates and causes cirrhosis. CRISPR correction of the Z allele in hepatocytes. Intellia preclinical programme, IND-enabling studies ongoing.
Autosomal recessive copper metabolism disorder caused by ATP7B mutations. Multiple CRISPR/base editing programmes in preclinical development targeting hepatocyte correction.
Liver overproduces oxalate, causing kidney failure. Intellia’s NTLA-2001 platform being extended to target AGXT gene correction. Phase 1 enrolling.
Section 4 — Cancer: CRISPR-Engineered Immune Cells
CRISPR’s entry into oncology is primarily through the engineering of immune cells — T cells and natural killer (NK) cells — to improve their ability to recognise and destroy tumours. This builds on the established field of CAR-T cell therapy, where patient T cells are extracted, engineered to express a chimeric antigen receptor (CAR) targeting a tumour antigen, and reinfused. CRISPR multiplexes additional edits that dramatically improve CAR-T cell function and enable the creation of “off-the-shelf” universal therapies.
The Four Edits That Transform a T Cell
CRISPR-mediated HDR inserts the chimeric antigen receptor gene at the TRAC locus (which encodes the T cell receptor alpha chain), simultaneously knocking in the CAR and knocking out the endogenous TCR. This is cleaner than random lentiviral insertion and allows more consistent CAR expression levels.
Standard autologous CAR-T requires each patient’s own T cells — a 4–6 week manufacturing process costing $400,000+. Disrupting the beta-2-microglobulin (B2M) gene eliminates HLA class I expression, preventing the patient’s immune system from rejecting donor T cells as foreign. This enables “off-the-shelf” allogeneic CAR-T from healthy donors.
Tumours suppress T cell activity by expressing ligands for checkpoint receptors (PD-L1, Galectin-9). CRISPR disruption of PD-1 (PDCD1) or other checkpoint genes produces T cells that remain active in the tumour microenvironment rather than becoming exhausted. This is the cellular equivalent of removing the tumour’s ability to put the immune system to sleep.
CRISPR screens have identified genes whose disruption enhances T cell persistence, proliferation, and memory formation. TET2 disruption has been reported to dramatically improve CAR-T persistence in both mouse models and a single compassionate-use patient case. These insights, discovered through CRISPR screens, are now being incorporated into next-generation cell therapies.
Current Cancer CRISPR Programmes
Section 5 — HIV: Excising the Viral Reservoir
HIV presents one of the most ambitious targets for CRISPR therapeutics. Current antiretroviral therapy (ART) can suppress HIV viral load to undetectable levels, but it cannot eliminate the virus: HIV integrates its genome into long-lived CD4+ T cells, creating a latent reservoir that persists for decades and reignites infection if ART is stopped. CRISPR offers the possibility of directly excising integrated HIV DNA from cells — potentially achieving a sterilising cure.
The strategy pioneered by the Khalili lab (Temple University) uses a dual-guide RNA approach: two guide RNAs targeting sequences at the 5’ and 3’ ends of the integrated HIV genome simultaneously, causing Cas9 to cut both ends and excise the entire viral sequence. The excised viral DNA is degraded, and the two ends of the host chromosome are rejoined. In cell culture and in humanised mouse models, this approach achieves significant reduction of HIV reservoir cells.
The extraordinary challenge is delivery to every HIV-infected cell in the body — including cells in lymph nodes, gut-associated lymphoid tissue, brain, and other reservoirs that are difficult to reach. In 2023, the Khalili group reported the first use of this approach in a primate model (SIV-infected macaques), with significant but incomplete reduction of viral reservoir. Clinical trials for HIV CRISPR therapy are anticipated but have not yet begun at scale as of 2025.
Section 6 — Eye Diseases: The Immunologically Privileged Frontier
The eye has two features that make it ideal for in vivo CRISPR delivery: it is immunologically privileged (the blood-retinal barrier reduces immune surveillance, lowering the risk of inflammatory reactions to viral vectors), and it requires extremely small doses (the entire retinal volume is microlitres, so only tiny amounts of editing reagent are needed, dramatically reducing systemic exposure and toxicity risk).
Leber Congenital Amaurosis Type 10 (LCA10): EDIT-101
Editas Medicine’s EDIT-101 is the first in vivo CRISPR gene editing programme in the eye to enter clinical trials. It targets LCA10, a form of congenital blindness caused by a specific intronic mutation in the CEP290 gene that creates an aberrant splice site, disrupting photoreceptor function. The mutation is an intronic insertion that is too large to correct by HDR, but CRISPR can disrupt the aberrant splice site to restore normal splicing.
EDIT-101 delivers SaCas9 (the smaller Staphylococcal Cas9 that fits in AAV) via subretinal injection of AAV5 directly to the photoreceptors. Phase 1/2 results (BRILLIANCE trial) showed that the procedure is safe and well-tolerated. Some patients showed measurable improvements in visual acuity and light sensitivity tests, though results were variable across patients. Dose escalation continued through 2024 to determine the optimal therapeutic dose.
Other Eye CRISPR Programmes
Multiple groups are pursuing CRISPR approaches for wet age-related macular degeneration (AMD, targeting VEGF or its receptor), Stargardt disease (ABCA4 mutation), and Leber hereditary optic neuropathy (LHON, a mitochondrial DNA disorder requiring unique delivery strategies). The eye is likely to become the most productive organ for in vivo CRISPR due to the combination of privileged immunity, small target volume, and high unmet medical need.
Section 7 — Muscle & Neurological Diseases: The Hard Targets
Duchenne Muscular Dystrophy (DMD)
DMD is caused by mutations in the DMD gene encoding dystrophin, the largest gene in the human genome (~2.4 Mb). Most pathogenic mutations create frameshifts that abolish dystrophin production. CRISPR exon-skipping strategies use a pair of guide RNAs to delete the exon containing or flanking the mutation, restoring the reading frame and enabling production of a shorter-but-functional “mini-dystrophin.”
The delivery challenge for DMD is formidable: skeletal muscle constitutes about 40% of total body mass, and every muscle fibre must be edited to achieve meaningful therapeutic benefit. AAV9 delivered systemically can reach muscle across the body, but achieving therapeutic editing levels in all muscle groups at safe doses remains challenging. Solid Rock Therapeutics (acquired by Sarepta) has shown mini-dystrophin restoration in the heart and limb muscles of DMD dogs, with human trials beginning in 2024. This represents one of the most ambitious in vivo CRISPR programmes attempted.
Huntington’s Disease: Silencing a Dominant Mutation
Huntington’s disease is caused by an autosomal dominant CAG repeat expansion in the HTT gene. Because the mutation is dominant (one mutant copy is enough to cause disease), correction requires either repairing the expanded repeat or silencing the mutant allele selectively. CRISPR approaches include: (1) CRISPRi to silence the mutant HTT allele (using dCas9-KRAB targeted to an allele-specific SNP near the repeat), (2) base editing to disrupt a splice site in mutant HTT, and (3) direct excision of the CAG repeat expansion. All require CNS delivery — the major bottleneck for all neurological CRISPR therapies.
The Blood-Brain Barrier: The Defining Challenge of Neurological CRISPR
Virtually every neurological CRISPR programme faces the same obstacle: the blood-brain barrier (BBB) prevents systemic delivery of LNPs, AAV (most serotypes), and other vectors from reaching brain parenchyma. Strategies under active development include: intrathecal or intracerebroventricular injection (direct CSF delivery, invasive but effective), engineered AAV variants that cross the BBB (AAV-PHP.eB in mice, though primate data is less encouraging), and receptor-targeted LNPs decorated with transferrin receptor antibodies that exploit transcytosis across the BBB. No neurological in vivo CRISPR therapy has yet reached Phase 3, though several are in Phase 1/2.
Section 8 — How CRISPR Clinical Trials Are Designed
Understanding how clinical trials for gene therapies work is essential context for evaluating claims about CRISPR medicines. Gene therapy trials follow the standard Phase 1–3 framework but with important differences driven by the permanent nature of the intervention and the small patient populations in many rare diseases.
Section 9 — The Cost Problem & Access Challenge
Casgevy’s list price in the United States was set at $2.2 million per patient. Lyfgenia (the competing lentiviral therapy for SCD) was priced at $3.1 million. These are the highest prices ever set for a medicine in the US at the time of approval. The pricing provoked immediate controversy, particularly because sickle cell disease disproportionately affects Black Americans and beta-thalassaemia predominantly affects people from South and Southeast Asia and the Mediterranean — populations that often have limited access to the most expensive medical interventions.
The high price reflects the genuine costs of the manufacturing process: each batch of Casgevy is custom-manufactured for a single patient from their own stem cells, requiring sophisticated GMP manufacturing, extensive quality testing, and complex logistics. The ex vivo approach that makes Casgevy safe also makes it expensive. In vivo CRISPR therapies delivered by LNPs or AAV are potentially manufactured at far lower per-patient cost and may eventually be priced more accessibly.
The global access picture is starker: the sickle cell disease burden is highest in sub-Saharan Africa, where Casgevy is not approved, not manufactured, and not affordable for the health systems that bear most of the disease burden. This is not a problem unique to CRISPR — it reflects broader inequities in how medicines are developed and distributed globally — but it is particularly acute for a disease that disproportionately affects low-income countries.
The Cartographer of the Human Body
Imagine you are a cartographer, and you have been given the most ambitious commission in history: map every type of broken machinery in the human body, city by city, building by building, room by room, and then dispatch precision repair teams to each location. The city is the human body. The buildings are organs. The rooms are cells. The broken machinery is disease. And the repair teams are CRISPR therapies.
The first city the cartographers mapped successfully was Blood City. It had a well-known neighbourhood: the bone marrow district, where haematopoietic stem cells produce every blood cell in the body. The broken machinery here was specific — a single wrong letter in a single gene causing every red blood cell to emerge misshapen and dangerous. The repair was elegant: not correcting the broken letter directly, but switching on a different production line (fetal haemoglobin) that bypassed the problem entirely. The stem cells were brought out of Blood City, repaired in the laboratory, and sent back. The neighbourhood now runs correctly. This was Casgevy.
The second city to yield its map was Liver City. The liver is a generous host: its streets are wide and fenestrated, its loading docks open to specific delivery vehicles (LNPs wearing ApoE flags). The cartographers found that a single delivery — a single infusion of LNP-packaged instructions — could reach most of Liver City simultaneously. In one neighbourhood, a rogue protein factory (the TTR gene) was producing misfolded proteins that damaged the heart and nerves of neighbouring cities. The repair team shut down the factory. Eighty-seven percent reduction in the toxic protein. One dose. Permanent effect. This was NTLA-2001.
Cancer District is different from all the others. Here, the broken machinery is not a single wrong letter in a single gene — it is a neighbourhood that has gone rogue, its cells multiplying without control and hiding from the city’s immune patrol. The repair strategy is not to fix the rogue cells directly, but to upgrade the patrol officers (T cells) so they can find and eliminate the rogue cells themselves. CRISPR doesn’t target the tumour directly — it engineers better soldiers. Remove the officers’ badges that the cancer recognises (TCR knockout), give them a new weapon specifically designed to recognise the tumour (CAR insertion), and remove the sedative that the tumour has been slipping into their water supply (PD-1 knockout). The upgraded patrol goes back in and does its job.
Some cities are still unmapped or only partially accessible. Brain City is surrounded by a high wall (the blood-brain barrier) that blocks most delivery vehicles. The few gates through the wall are narrow and heavily guarded. The repair teams are working on new vehicles — engineered carrier pigeons (AAV-PHP.eB), surface-disguised vans (receptor-targeted LNPs) — but access remains the primary obstacle. Muscle City has the opposite problem: it is enormous, spanning the entire territory, and every room must be repaired, not just a neighbourhood. Even the most ambitious delivery approach leaves some rooms unreached.
The cartographers have also discovered that even in cities they can reach, there are equity problems. Casgevy costs $2.2 million per patient in the United States. The city with the most broken machinery — sub-Saharan Africa, where sickle cell disease is most prevalent — cannot access the repair team at all. The most important work of the next decade is not just discovering new repair techniques. It is redesigning the delivery logistics so that the repair reaches every city, not just the wealthiest ones.
The map of the human body is being drawn, room by room. Each clinical trial adds a new district. Each approval confirms a new territory has been secured. The cartographers of 1987 — Ishino, noticing strange repeating sequences in a bacterial footnote — had no idea they were drawing the first line of the most consequential map in the history of medicine.
References & Further Reading
- Frangoul et al. (2021) — CRISPR-Cas9 Gene Editing for Sickle Cell Disease and Beta-Thalassemia. NEJM 384:252. — The pivotal Casgevy Phase 3 trial data.
- Gillmore et al. (2021) — CRISPR-Cas9 In Vivo Gene Editing for Transthyretin Amyloidosis. NEJM 385:493. — The landmark NTLA-2001 Phase 1 results showing 87% TTR reduction.
- Stadtmauer et al. (2020) — CRISPR-engineered T cells in patients with refractory cancer. Science 367:eaba7365. — First-in-human multiplexed CRISPR T cell editing (University of Pennsylvania).
- Maeder et al. (2019) — Development of a gene-editing approach to restore vision loss in Leber congenital amaurosis type 10. Nature Medicine 25:229. — The preclinical foundation for EDIT-101.
- Xu et al. (2019) — Efficient strategies for CRISPR-Cas9 gene disruption in human pluripotent stem cells. — HDR efficiency benchmark data relevant to therapeutic applications.
- ClinicalTrials.gov — clinicaltrials.gov — Search “CRISPR” for the complete updated list of all registered CRISPR clinical trials globally.
- FDA Casgevy Approval — fda.gov/casgevy — Official FDA approval documents, prescribing information, and REMS requirements.
- Casgevy is approved and works. 29/29 SCD patients free of severe crises; 42/42 thalassaemia patients transfusion-independent. BCL11A enhancer disruption reactivates HbF. Ex vivo + electroporation + RNP. $2.2M list price.
- Blood diseases dominate because ex vivo delivery is established. Haematopoietic stem cells can be extracted, edited, and reinfused. The infrastructure from bone marrow transplantation is directly applicable.
- Liver is the most successful in vivo target. NTLA-2001 showed 87% TTR reduction with a single intravenous LNP dose. Verve’s PCSK9 base editor showed 55% LDL reduction. The liver-LNP-ApoE pathway is a validated delivery route.
- Cancer applications use multiple simultaneous edits. TCR KO + B2M KO + CAR insertion + checkpoint KO in a single electroporation session. Off-the-shelf allogeneic cell therapies are the goal. Multiple Phase 1/2 programmes in haematological malignancies.
- Brain and muscle remain the hardest targets. BBB blocks systemic CNS delivery. Muscle requires body-wide editing. No neurological or major muscle disease CRISPR therapy has reached Phase 3 yet.
- Gene therapy trials require 15-year follow-up. Safety monitoring for late-emerging effects (insertional oncogenesis, off-target effects) runs for 15 years post-treatment by FDA requirement. Casgevy patients will be monitored until 2038.
- Access is the critical unsolved problem. $2.2M price point excludes most of the world. Sub-Saharan Africa carries most of the sickle cell burden but has no access to Casgevy. Lower-cost in vivo approaches and manufacturing innovation are essential for global equity.