# CRISPR in Medicine: Applications, Mechanisms, and Clinical Impact

## Introduction to CRISPR in Medicine

### What is CRISPR?

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is an adaptive immune system originally discovered in bacteria and archaea. These microorganisms capture short fragments of foreign DNA from invading bacteriophages and integrate them into their own genomes at the CRISPR locus. When the same phage attacks again, the bacterium transcribes these spacer sequences into short CRISPR RNAs (crRNAs) that guide Cas (CRISPR-associated) proteins to complementary foreign DNA, resulting in cleavage and destruction of the invader.

The revolutionary adaptation of this system for genome editing came from the work of Jennifer Doudna, Emmanuelle Charpentier, and Feng Zhang, who demonstrated that the Cas9 endonuclease from *Streptococcus pyogenes* (SpCas9) could be reprogrammed to cut any DNA sequence of interest. The engineered system requires two components: a Cas9 protein and a single-guide RNA (sgRNA). The sgRNA is a chimeric molecule that fuses the crRNA (which provides target specificity through Watson-Crick base pairing) with a trans-activating crRNA (tracrRNA, which is required for Cas9 loading and activation). This two-component system is the foundation of most CRISPR-based therapeutic approaches in medicine today.

### Why CRISPR Matters in Medicine

Before CRISPR, targeted genome editing in human cells relied on zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). These protein-based systems required engineering a new DNA-binding protein for every target site—a laborious, expensive, and technically demanding process. CRISPR replaced [protein engineering](/knowledge/molecular-biology/protein-engineering) with simple RNA design: changing the 20-nucleotide spacer sequence of the sgRNA is sufficient to retarget the nuclease. This simplicity democratized genome editing and opened the door to clinical applications.

CRISPR matters in medicine because it offers the possibility of treating the root cause of genetic disease rather than managing symptoms. For monogenic disorders—conditions caused by mutations in a single gene—CRISPR can theoretically correct the pathogenic variant, restore normal protein function, and provide a permanent cure. For complex conditions like cancer, CRISPR can engineer patient immune cells to recognize and attack tumors. The technology also enables the creation of better disease models, high-throughput genetic screens, and rapid diagnostic tools. As of 2024, the first CRISPR-based therapies have received regulatory approval, marking the transition from bench research to clinical medicine.

## Mechanism of CRISPR-Cas9 Gene Editing

### Guide RNA and Target Recognition

The CRISPR-Cas9 system recognizes its DNA target through two distinct molecular interactions. First, the sgRNA contains a 20-nucleotide spacer sequence complementary to the target DNA strand. Second, the Cas9 protein itself recognizes a short, conserved sequence motif adjacent to the target site called the protospacer adjacent motif (PAM). For SpCas9, the PAM is 5'-NGG-3' (where N is any nucleotide). The PAM is essential because it distinguishes self from non-self DNA during bacterial immunity and prevents Cas9 from cleaving the CRISPR locus itself.

Target recognition proceeds through a series of ordered steps. The Cas9-sgRNA ribonucleoprotein (RNP) complex first scans the genome by transiently binding to DNA and unwinding short stretches to sample for PAM sequences. When a PAM is encountered, the adjacent DNA is interrogated for complementarity to the sgRNA spacer. If the first 8–12 nucleotides proximal to the PAM (the "seed region") match perfectly, the RNA-DNA heteroduplex is stabilized and the full 20-nucleotide pairing is completed. This triggers a conformational change in Cas9 that activates its two nuclease domains: the HNH domain cleaves the target strand (complementary to the sgRNA), and the RuvC domain cleaves the non-target strand. The result is a blunt double-strand break (DSB) located 3 nucleotides upstream of the PAM.

### Double-Strand Break Repair: NHEJ vs HDR

The double-strand break created by Cas9 is repaired by the cell's endogenous DNA repair machinery, and the choice of repair pathway determines the editing outcome. There are two major pathways:

**Non-homologous end joining (NHEJ)** is the dominant repair pathway in most cell types, particularly non-dividing cells. NHEJ directly ligates the broken DNA ends without requiring a homologous template. This process is error-prone: the ends are often resected or processed before ligation, resulting in small insertions or deletions (indels) at the break site. When the DSB occurs in the coding sequence of a gene, these indels frequently cause frameshift mutations that introduce premature stop codons, leading to nonsense-mediated decay of the mRNA or production of a truncated, non-functional protein. This is the basis of **CRISPR knockout**—the permanent disruption of a gene's function.

**Homology-directed repair (HDR)** is a high-fidelity pathway that uses a homologous DNA template to repair the break. In the presence of a donor template—either a double-stranded DNA plasmid or a single-stranded oligodeoxynucleotide (ssODN)—the cell can copy the template sequence into the broken locus. By designing the donor template to carry a desired sequence flanked by homology arms matching the target site, researchers can introduce precise base changes, insert reporter genes, or correct pathogenic mutations. However, HDR is inefficient in most cell types and is largely restricted to the S/G2 phases of the cell cycle when sister chromatids are available. In post-mitotic cells such as neurons or muscle fibers, HDR is essentially non-existent, which poses a major challenge for therapeutic gene correction.

The choice between NHEJ and HDR can be influenced experimentally. Inhibiting key NHEJ factors such as DNA ligase IV or KU70/80 shifts the balance toward HDR. Cell cycle synchronization, using thymidine or nocodazole blocks to enrich for S phase, also improves HDR rates. For clinical applications, however, the low efficiency of HDR remains a significant limitation, which has driven the development of alternative technologies like base editing and prime editing (discussed below).

## CRISPR-Based Therapeutic Strategies

### Ex Vivo vs In Vivo Editing

CRISPR therapies can be delivered in two fundamentally different ways, each with distinct advantages and challenges.

**Ex vivo editing** involves removing cells from the patient, editing them in the laboratory, and then infusing the modified cells back into the patient. This approach is feasible only for cell types that can be harvested, cultured, and re-infused—primarily hematopoietic stem cells (HSCs) and T lymphocytes. The workflow typically involves:

1. Mobilization or apheresis to collect the target cells from the patient.
2. Enrichment and activation of the cells in culture.
3. Delivery of CRISPR components (usually as ribonucleoprotein complexes or electroporated mRNA) to induce editing.
4. Expansion of edited cells under selective conditions.
5. Quality control to verify editing efficiency and absence of off-target mutations.
6. Conditioning of the patient (e.g., myeloablative chemotherapy for HSC transplantation) followed by infusion of edited cells.

Ex vivo editing offers several advantages: the editing efficiency can be optimized and verified before administration, the dose of edited cells is controlled, and exposure of the patient to delivery vectors is minimized. The major limitation is that it is restricted to blood and immune cells.

**In vivo editing** delivers CRISPR components directly to the patient, targeting cells within the body. This requires delivery vehicles that can navigate biological barriers, evade the immune system, and achieve cell-type-specific uptake. Viral vectors are the most efficient delivery vehicles. Adeno-associated viruses (AAVs) are commonly used because they are non-pathogenic, infect both dividing and non-dividing cells, and provide long-term expression. However, AAVs have a small packaging capacity (~4.7 kb), which is barely sufficient for the SpCas9 coding sequence (~4.2 kb) plus a guide RNA expression cassette. This constraint has driven the development of smaller Cas9 orthologs such as *Staphylococcus aureus* Cas9 (SaCas9, ~3.2 kb) and Cas12a (~3.9 kb). Lipid nanoparticles (LNPs) are the leading non-viral option; they can deliver mRNA encoding Cas9 and chemically synthesized guide RNAs, providing transient expression that reduces off-target editing and immunogenicity. The FDA-approved CRISPR therapy for transthyretin amyloidosis uses LNP-based delivery.

### Base Editing and Prime Editing

The inefficiency of HDR and the stochastic nature of NHEJ have motivated the development of "next-generation" CRISPR tools that do not require double-strand breaks.

**Base editing** uses a catalytically dead Cas9 (dCas9) or nickase Cas9 (nCas9, where one nuclease domain is inactivated) fused to a deaminase enzyme. Cytosine base editors (CBEs) fuse a cytidine deaminase (e.g., APOBEC1) to nCas9, converting cytosine to uracil within a small editing window (~5 nucleotides) near the PAM. The resulting U:G mismatch is processed by [base excision repair](/knowledge/molecular-biology/base-excision-repair), ultimately yielding a C-to-T (or G-to-A on the opposite strand) transition. Adenine base editors (ABEs) use an engineered adenosine deaminase (TadA) to convert adenine to inosine, which is read as guanine during replication, yielding A-to-G transitions. Base editors can achieve efficiencies of 50–90% in many cell types with minimal indel formation, making them attractive for correcting point mutations that cause disease.

**Prime editing** is a more versatile approach that can introduce all types of base substitutions, small insertions, and small deletions without requiring a DSB or donor template. The prime editor consists of an nCas9 fused to a reverse transcriptase (RT) and a prime editing guide RNA (pegRNA). The pegRNA contains both a spacer sequence that specifies the target site and a 3' extension that carries the desired edit. After nCas9 nicks the target strand, the RT uses the pegRNA extension as a template to synthesize new DNA, which is incorporated into the genome. A second nick on the non-edited strand and cellular DNA repair complete the process. Prime editing is less efficient than base editing but offers far greater flexibility.

## Clinical Applications and Examples of CRISPR in Medicine

### Sickle Cell Disease and Beta-Thalassemia

Sickle cell disease (SCD) and transfusion-dependent beta-thalassemia (TDT) are caused by mutations in the *HBB* gene, which encodes the beta-globin subunit of adult hemoglobin. SCD results from a single nucleotide substitution (GAG to GTG at codon 6) that produces hemoglobin S, which polymerizes under low oxygen conditions, causing red blood cells to assume a sickle shape. TDT results from various mutations that reduce or abolish beta-globin production.

The first CRISPR-based therapy to receive regulatory approval, exagamglogene autotemcel (Casgevy), does not directly correct the *HBB* mutation. Instead, it recreates a naturally occurring condition called hereditary persistence of fetal hemoglobin (HPFH). The approach uses CRISPR-Cas9 to disrupt the *BCL11A* erythroid-specific enhancer in patient-derived CD34+ hematopoietic stem cells. BCL11A is a [transcription factor](/knowledge/molecular-biology/transcription-factor) that represses gamma-globin expression in adult erythroid cells. By knocking out its enhancer, gamma-globin expression is derepressed, leading to production of fetal hemoglobin (HbF), which compensates for the defective adult hemoglobin. Clinical trials demonstrated that 97% of SCD patients and 93% of TDT patients remained free from vaso-occlusive crises or transfusion dependence at 12 months post-treatment. The therapy received FDA approval in December 2023 and European Medicines Agency approval in early 2024.

### CRISPR in Cancer Immunotherapy

CRISPR has been used to engineer chimeric antigen receptor (CAR) T cells for cancer treatment. The standard CAR-T manufacturing process uses viral vectors to introduce a synthetic receptor that recognizes a tumor antigen (e.g., CD19 for B-cell malignancies). CRISPR adds the ability to eliminate genes that impair T-cell function or cause alloreactivity.

In a landmark 2020 clinical trial, researchers used CRISPR-Cas9 to disrupt three genes in donor-derived CAR-T cells: *TRAC* (T-cell receptor alpha constant, to prevent graft-versus-host disease), *B2M* (beta-2-microglobulin, to eliminate HLA class I expression and prevent rejection), and *PDCD1* (programmed cell death protein 1, to enhance anti-tumor activity). The edited cells were infused into patients with refractory cancer. While the trial demonstrated the feasibility of multiplexed CRISPR editing in humans, the clinical efficacy was limited, and some patients experienced rejection of the HLA-negative cells by natural killer cells. Subsequent approaches have focused on knocking out *CD52* and *CD7* to prevent fratricide in CAR-T cells targeting T-cell malignancies, and on using CRISPR to knock in the CAR construct at the *TRAC* locus, which improves T-cell potency by placing CAR expression under endogenous regulatory control.

### Inherited Eye Disorders

The eye is an attractive target for in vivo CRISPR therapy due to its immune-privileged status, anatomical accessibility, and the ability to deliver vectors by intravitreal or subretinal injection. Leber congenital amaurosis type 10 (LCA10) is caused by mutations in *CEP290*, and the most common pathogenic variant is a deep intronic mutation (c.2991+1655A>G) that creates a cryptic splice donor site, leading to aberrant mRNA splicing and truncated CEP290 protein.

EDIT-101 (developed by Editas Medicine) uses CRISPR-Cas9 delivered by AAV5 to excise the pathogenic intronic region. Two guide RNAs flank the mutation, and their simultaneous cleavage deletes the ~800 bp region containing the cryptic splice site, restoring normal splicing. In a Phase 1/2 clinical trial (BRILLIANCE), 14 patients received the therapy, and 11 showed measurable improvement in visual function. The therapy was well-tolerated with no serious adverse events related to the treatment. Although the trial was paused due to business reasons, it demonstrated the feasibility of in vivo CRISPR editing in humans.

## Evidence and Clinical Trials

### Key Clinical Trial Results

The clinical evidence for CRISPR-based therapies has accumulated rapidly. The most robust data come from the ex vivo editing trials for hemoglobinopathies. In the CLIMB-121 trial for SCD, 31 patients received exagamglogene autotemcel; all 31 achieved successful engraftment, and 29 of 30 evaluable patients remained free of vaso-occlusive crises for at least 12 months. In the CLIMB-111 trial for TDT, 42 patients were treated; 39 of 40 evaluable patients achieved transfusion independence. The median time to neutrophil engraftment was 24 days, and the median time to platelet engraftment was 32 days. No cases of acute myeloid leukemia or myelodysplastic syndrome were observed, though long-term follow-up continues.

For in vivo editing, the most notable trial is the Phase 1 study of NTLA-2001 (Intellia Therapeutics) for transthyretin amyloidosis (ATTR). This condition is caused by mutations in *TTR*, leading to accumulation of misfolded transthyretin protein in nerves and the heart. NTLA-2001 consists of an LNP encapsulating Cas9 mRNA and a guide RNA targeting *TTR*. A single intravenous infusion resulted in a mean serum TTR reduction of 87% at 28 days at the highest dose, and the reduction was sustained at 12 months. This trial was the first to demonstrate that in vivo CRISPR editing in the liver is feasible and clinically meaningful in humans.

### Safety and Off-Target Concerns

The primary safety concern for CRISPR therapies is off-target editing—cleavage at genomic sites that are partially complementary to the guide RNA. Off-target effects can disrupt [tumor suppressor genes](/knowledge/molecular-biology/tumor-suppressor-gene), activate oncogenes, or cause chromosomal rearrangements. The frequency of off-target editing depends on the guide RNA sequence, the Cas9 variant, delivery method, and cell type. In the exagamglogene autotemcel trials, comprehensive off-target analysis using CIRCLE-seq and targeted [amplicon sequencing](/blog/guides/amplicon-sequencing) identified no off-target edits above background in the final drug product.

Other safety concerns include on-target but unintended editing outcomes, such as large deletions or chromosomal translocations at the target locus. For example, CRISPR editing of *BCL11A* enhancer can occasionally produce large deletions that extend into neighboring regulatory elements. The clinical significance of these events is unknown but is being monitored in long-term follow-up studies. Additionally, the conditioning regimen required for HSC transplantation (busulfan-based myeloablation) carries its own toxicity, including infertility, organ damage, and increased infection risk.

## Methods Used to Study CRISPR in Medicine

### Guide RNA Design Tools

Computational tools are essential for selecting guide RNAs with high on-target activity and minimal off-target potential. The most widely used algorithms include:

**CRISPOR** integrates multiple scoring models (e.g., MIT specificity score, CFD (cutting frequency determination) score, and Doench 2016 activity score) to rank guide RNAs for a given target. It also identifies all potential off-target sites in the reference genome and provides primer design for on-target validation.

**Benchling** provides a cloud-based platform that combines guide design with cloning workflows, enabling researchers to design sgRNA oligos, assemble plasmids, and track editing results in a single interface.

**CHOPCHOP** is a web-based tool that supports multiple CRISPR systems (Cas9, Cas12a, Cas13) and provides off-target predictions using Bowtie alignment against the reference genome.

**DeepCRISPR** uses deep learning to predict on-target cleavage efficiency based on sequence features, chromatin accessibility, and epigenetic marks.

For clinical applications, guide RNAs must be validated in the actual cell type to be edited, as chromatin state and DNA methylation can affect accessibility and editing efficiency.

### Off-Target Detection Methods

Several experimental approaches are used to identify off-target edits:

**CIRCLE-seq** (circularization for in vitro reporting of cleavage effects by sequencing) is an in vitro method that uses purified Cas9 and genomic DNA to identify cleavage sites genome-wide. The method involves shearing genomic DNA, circularizing fragments, incubating with Cas9-sgRNA complexes, and sequencing the linearized products. CIRCLE-seq is highly sensitive but may identify sites that are not edited in cells due to chromatin inaccessibility.

**GUIDE-seq** (genome-wide unbiased identification of DSBs evaluated by sequencing) detects off-target cleavage in living cells by capturing double-strand breaks through integration of a short double-stranded oligodeoxynucleotide (dsODN) tag. The tagged sites are amplified and sequenced. GUIDE-seq provides a more physiologically relevant picture but requires efficient delivery of the dsODN into the target cells.

**DISCOVER-seq** (discovery of in situ Cas off-targets and verification by sequencing) identifies off-target sites by detecting MRE11 nuclease recruitment to DSBs in cells, followed by chromatin immunoprecipitation and sequencing.

**Targeted amplicon sequencing** of predicted off-target sites is the standard approach for clinical validation. This method amplifies and deeply sequences (10,000× coverage) each candidate off-target locus to detect low-frequency indels. The sensitivity is typically 0.1–0.5% variant allele frequency.

## Ethical and Regulatory Considerations

### Germline vs Somatic Editing

A fundamental ethical distinction exists between somatic and germline genome editing. Somatic editing—the modification of non-reproductive cells—affects only the treated individual and is not inherited by offspring. All approved and ongoing clinical trials use somatic editing, and the ethical framework for this approach is broadly accepted, analogous to other gene therapies.

Germline editing—modifying sperm, eggs, or embryos—produces heritable changes that are passed to future generations. This approach is prohibited in most countries and is not being pursued in regulated clinical trials. The scientific concerns include off-target effects that would be propagated, mosaicism (where not all cells in the embryo are edited), and the unknown long-term consequences of introducing heritable genetic changes. The ethical concerns include issues of consent (future generations cannot consent), equity (access to germline editing could exacerbate social inequalities), and the slippery slope toward non-therapeutic enhancement. The 2018 announcement by He Jiankui of the first germline-edited babies (using CRISPR to disrupt *CCR5* in twins) was widely condemned by the scientific community and led to criminal prosecution in China.

### Regulatory Oversight

CRISPR-based therapies are regulated as gene therapies by national regulatory agencies. In the United States, the FDA's Center for Biologics Evaluation and Research (CBER) oversees these products under the framework for somatic cell therapy products. The regulatory pathway requires extensive preclinical data, including proof-of-concept in animal models, off-target analysis, and toxicology studies, before an Investigational New Drug (IND) application is approved. Clinical development proceeds through Phase 1 (safety), Phase 2 (dose-finding and preliminary efficacy), and Phase 3 (pivotal efficacy) trials before a Biologics License Application (BLA) is submitted.

The European Medicines Agency (EMA) follows a similar framework under the Advanced Therapy Medicinal Products (ATMP) regulation. The International Conference on Harmonisation (ICH) provides guidelines for the quality, safety, and efficacy requirements for gene therapies. A key regulatory challenge is the long-term follow-up requirement: patients treated with CRISPR therapies are typically followed for 15 years to monitor for delayed adverse events, including malignancy.

## Common Pitfalls and Misconceptions

### Misunderstanding Repair Pathways

A common error is assuming that CRISPR-Cas9 itself performs gene correction. In reality, Cas9 only creates the double-strand break; the cell's endogenous repair machinery determines the outcome. Students often confuse NHEJ and HDR:

- **NHEJ** is the default pathway in most cells and produces indels that typically cause gene knockout. It is efficient but imprecise.
- **HDR** requires a donor template and is restricted to dividing cells. It is precise but inefficient.

A related misconception is that providing a donor template guarantees HDR will occur. In practice, NHEJ usually outcompetes HDR, and the majority of edited cells will contain indels rather than the desired correction. Researchers must design experiments to enrich for HDR events, such as using selection markers or optimizing delivery conditions.

### Overestimating Precision

Another common error is assuming that CRISPR is always precise and that off-target effects are rare. In reality, Cas9 can tolerate mismatches in the guide RNA-target DNA duplex, particularly in the PAM-distal region. A guide RNA with a 20-nucleotide spacer may have hundreds of potential off-target sites in the human genome, some of which may be edited at frequencies of 1–10%. The specificity of CRISPR depends on the guide sequence, the Cas9 variant (high-fidelity variants like HiFi Cas9 and eSpCas9 have reduced off-target activity), and the delivery method (RNP delivery results in shorter exposure and fewer off-target edits than plasmid-based delivery).

Students also often overlook the possibility of large deletions, inversions, and translocations at the on-target site. When two guide RNAs are used simultaneously (e.g., for a deletion), the intervening sequence is excised, but the repair can sometimes produce inverted or rearranged fragments. These structural variants are not detected by standard PCR-based genotyping and require [long-read sequencing](/knowledge/bioinformatics/long-read-sequencing-technologies-pacbio-and-oxford-nanopore) or Southern blotting for identification.

### Overlooking Delivery Challenges

A third common pitfall is assuming that delivering CRISPR components to cells is trivial. In reality, delivery is the single greatest barrier to clinical translation. Electroporation works well for ex vivo editing of blood cells but is not applicable in vivo. Viral vectors are efficient but have packaging constraints, immunogenicity, and integration risks. Lipid nanoparticles are safe but primarily target the liver due to natural biodistribution. Achieving cell-type-specific delivery to other tissues (e.g., muscle, brain, lung) remains an unsolved problem. Students should appreciate that the "CRISPR" part of a therapy is often the easiest component; the delivery vehicle determines whether the therapy works at all.

## Future Directions and Summary

### Emerging CRISPR Technologies

Several advances are expanding the scope of CRISPR in medicine:

**CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa)** use catalytically dead Cas9 (dCas9) fused to transcriptional repressors (e.g., KRAB) or activators (e.g., VP64) to modulate gene expression without cutting DNA. These tools are being explored for treating diseases caused by haploinsufficiency (where one functional copy of a gene is insufficient) or for upregulating compensatory genes.

**CRISPR-Cas13** targets RNA rather than DNA, enabling transient knockdown of gene expression without permanent genomic changes. This approach is being developed for treating viral infections (e.g., SARS-CoV-2) and for conditions where permanent gene disruption is undesirable.

**Epigenome editing** uses dCas9 fused to DNA methyltransferases or histone-modifying enzymes to alter gene expression patterns without changing the underlying DNA sequence. This approach could treat diseases caused by aberrant epigenetic silencing, such as some cancers and imprinting disorders.

**Delivery improvements** include engineered AAV capsids with enhanced tissue tropism, virus-like particles (VLPs) that deliver Cas9-ribonucleoprotein complexes, and nanoparticles conjugated with cell-specific ligands. These advances aim to expand in vivo editing beyond the liver.

### Key Takeaways

- CRISPR-Cas9 is an RNA-guided nuclease that creates double-strand breaks at specific genomic loci; the cell's repair machinery (NHEJ or HDR) determines the editing outcome.
- NHEJ produces indels that cause gene knockout; HDR enables precise gene correction but is inefficient and requires a donor template.
- Base editing and prime editing offer alternatives that do not require double-strand breaks and can achieve precise single-base changes.
- Ex vivo editing is used for blood cells and T cells; in vivo editing uses viral vectors or lipid nanoparticles and is currently most feasible for liver-targeted therapies.
- The first approved CRISPR therapy (exagamglogene autotemcel) treats sickle cell disease and beta-thalassemia by reactivating fetal hemoglobin expression.
- Off-target effects, delivery efficiency, and long-term safety remain the primary challenges for clinical translation.
- Somatic editing is ethically and legally accepted; germline editing is prohibited in most jurisdictions due to safety and ethical concerns.

## Frequently Asked Questions

### What is CRISPR in medicine?

CRISPR in medicine refers to the use of CRISPR-based genome editing technologies to treat or prevent human diseases. This includes ex vivo editing of patient cells (e.g., hematopoietic stem cells or T cells) followed by re-infusion, and in vivo editing where CRISPR components are delivered directly to tissues within the body. The goal is to correct disease-causing mutations, disrupt pathogenic genes, or engineer cells with new therapeutic functions.

### What are examples of CRISPR in medicine?

Approved and investigational CRISPR therapies include: exagamglogene autotemcel (Casgevy) for sickle cell disease and beta-thalassemia, which disrupts the *BCL11A* erythroid enhancer to reactivate fetal hemoglobin; NTLA-2001 for transthyretin amyloidosis, which knocks out *TTR* in the liver; EDIT-101 for Leber congenital amaurosis type 10, which excises a pathogenic intronic mutation in *CEP290*; and various CRISPR-engineered CAR-T cell therapies for cancer.

### [How does CRISPR work](/blog/guides/how-does-crispr-work) in medicine?

CRISPR-Cas9 is delivered to target cells as a ribonucleoprotein complex, mRNA, or DNA encoding the Cas9 nuclease and a guide RNA. The guide RNA directs Cas9 to a complementary genomic sequence adjacent to a PAM motif. Cas9 creates a double-strand break, which is repaired by NHEJ (producing indels that disrupt gene function) or HDR (using a donor template to introduce precise sequence changes). Base editing and prime editing achieve precise nucleotide changes without double-strand breaks.

### Is CRISPR used in clinical practice?

Yes. The first CRISPR-based therapy, exagamglogene autotemcel (Casgevy), received FDA approval in December 2023 for sickle cell disease and transfusion-dependent beta-thalassemia. Several other CRISPR therapies are in clinical trials, including NTLA-2001 for transthyretin amyloidosis (Phase 3) and various CRISPR-edited cell therapies for cancer.

### What are the risks of CRISPR in medicine?

The main risks are off-target editing (unintended mutations at sites partially complementary to the guide RNA), on-target but unintended outcomes (large deletions, chromosomal rearrangements), incomplete editing (mosaicism), and delivery-related toxicities (e.g., immune responses to viral vectors or lipid nanoparticles). For ex vivo therapies, the conditioning regimen required for stem cell transplantation carries additional risks.

### What is the difference between ex vivo and in vivo CRISPR?

Ex vivo CRISPR involves removing cells from the patient, editing them in the laboratory, and infusing them back. This is used for blood cells and T cells and allows verification of editing before administration. In vivo CRISPR delivers editing components directly to the patient's body, targeting specific tissues such as the liver or eye. In vivo approaches are less invasive but face greater delivery challenges.

### Can CRISPR cure genetic diseases?

For monogenic disorders, CRISPR has the potential to provide a permanent cure by correcting the underlying mutation or compensating for its effects. This has been demonstrated for sickle cell disease and beta-thalassemia, where the approved therapy provides a functional cure. However, not all genetic diseases are amenable to current CRISPR approaches due to delivery limitations, the need for precise correction in specific cell types, or the complexity of the disease mechanism.

## Further Reading

- Singh A et al. *Revolutionary breakthrough: FDA approves CASGEVY, the first CRISPR/Cas9 [gene therapy for sickle cell disease](/knowledge/molecular-biology/gene-therapy-for-sickle-cell-disease)*. Annals of medicine and surgery (2012). 2024. [PubMed 39118728](https://doi.org/10.1097/MS9.0000000000002146)
- Rahmanian M et al. *CRISPR in Medicine: A Systematic Review of Clinical Trials and Therapeutic Applications*. Human gene therapy. 2026. [PubMed 41810550](https://doi.org/10.1177/10430342251409714)
- Zaib S, Saleem MA, Khan I. *CRISPR-Cas9 Genome Engineering: Trends in Medicine and Health*. Mini reviews in medicinal chemistry. 2022. [PubMed 34517795](https://doi.org/10.2174/1389557521666210913112030)
- Sun JY et al. *CRISPR in medicine: applications and challenges*. Briefings in functional genomics. 2020. [PubMed 32432687](https://doi.org/10.1093/bfgp/elaa011)
- Becú-Villalobos D. *[CRISPR-CAS9 in medicine, the saga continues]*. Medicina. 2019. [PubMed 31829959](https://pubmed.ncbi.nlm.nih.gov/31829959/)
- Liu JL. *Sparks of the CRISPR explosion: Applications in medicine and agriculture*. Journal of genetics and genomics = Yi chuan xue bao. 2017. [PubMed 28992876](https://doi.org/10.1016/j.jgg.2017.09.006)

## Related Topics

- [CRISPR Knockout](/knowledge/molecular-biology/crispr-knockout)
- [CRISPR Knock](/knowledge/molecular-biology/crispr-knock)
- [CRISPR Explained](/knowledge/molecular-biology/crispr-explained)
- [Use CRISPR](/knowledge/molecular-biology/use-crispr)
- [CRISPR Screening](/knowledge/molecular-biology/crispr-screening)

## Related Clinical & Scientific Guides

* [MAPK Pathway: Mechanism, Function, and Clinical Relevance](/knowledge/molecular-biology/mapk-pathway)
* [Mammalian Cell Culture Bioreactors: A Practical Guide](/knowledge/molecular-biology/mammalian-cell-culture-bioreactor)
* [Nucleotide Formation: Biosynthesis and Assembly of DNA/RNA Building Blocks](/knowledge/molecular-biology/nucleotide-formation)