CRISPR can treat genetic diseases by cutting out, replacing, or disabling faulty DNA sequences responsible for a condition. One CRISPR-based therapy, Casgevy, has already received FDA approval for sickle cell disease and transfusion-dependent beta-thalassemia, and several more treatments are in late-stage clinical trials for conditions ranging from hereditary blindness to a rare swelling disorder called hereditary angioedema.
How CRISPR Edits Disease-Causing Genes
CRISPR works in three basic steps: recognition, cutting, and repair. Scientists design a short piece of genetic material called a guide RNA that matches the exact DNA sequence they want to change. This guide RNA leads an enzyme called Cas9 to the right spot on the genome, where Cas9 cuts both strands of the DNA double helix.
Once the DNA is cut, the cell’s own repair machinery kicks in. There are two ways the cell can fix the break. The simpler method just glues the cut ends back together, which typically disables the gene at that location. The more precise method uses a template provided by researchers to rewrite the sequence with a corrected version. Which repair method doctors want depends on the disease. Sometimes the goal is to silence a harmful gene. Other times, it’s to fix a single-letter typo that causes the entire problem.
Two Ways to Deliver the Treatment
CRISPR therapies reach patients through two broad approaches: editing cells outside the body (ex vivo) or injecting the editing tools directly into the patient (in vivo). Each has trade-offs, and the choice depends largely on which organ or cell type needs to be corrected.
Ex Vivo: Editing Cells in the Lab
In ex vivo therapy, doctors harvest a patient’s own cells, send them to a specialized lab where the CRISPR editing happens, and then return the modified cells through an infusion. This is the approach behind Casgevy, the first approved CRISPR treatment. Stem cells are collected from the patient’s bone marrow, edited in a facility, and reinfused after the patient undergoes chemotherapy to clear out the old, unedited stem cells. The advantage is precision: scientists can verify that the edits worked correctly before putting anything back. The downside is the process is intensive, requiring hospitalization and a chemotherapy conditioning regimen.
In Vivo: Editing Inside the Body
In vivo therapy skips the cell-harvesting step entirely. Instead, the CRISPR components are packaged into a delivery vehicle, often tiny fat-based particles called lipid nanoparticles, and injected into the patient. The particles are engineered to travel to a specific organ. Liver-targeted nanoparticles, for instance, are designed so that when they enter the bloodstream, they’re preferentially taken up by liver cells. This approach is especially promising for diseases affecting organs where you can’t easily remove and replace cells, like the liver, the eye, or the heart. The challenge is making sure the editing tools reach the right cells without triggering an immune reaction or editing unintended tissues.
Sickle Cell Disease and Beta-Thalassemia
The most advanced success story is Casgevy, approved by the FDA for sickle cell disease and transfusion-dependent beta-thalassemia. Both conditions stem from defective hemoglobin, the protein in red blood cells that carries oxygen. Sickle cell disease causes red blood cells to deform into rigid, crescent shapes that block blood vessels and trigger excruciating pain episodes called vaso-occlusive crises.
Casgevy doesn’t fix the broken hemoglobin gene directly. Instead, it edits a different gene to reactivate fetal hemoglobin, a form of the protein that humans naturally produce before birth but switch off shortly after. With fetal hemoglobin flowing again, the body compensates for the faulty adult version. In clinical trials, 29 out of 31 evaluable patients (93.5%) remained free of severe pain crises for at least 12 consecutive months after treatment. That’s a dramatic shift for people who previously experienced repeated hospitalizations.
Hereditary Blindness
In 2020, a patient became the first person to receive a CRISPR therapy injected directly into the body, in this case into the eye, as a treatment for Leber congenital amaurosis, a rare inherited form of blindness. The treatment, called EDIT-101, was designed to snip out the genetic mutation preventing light-sensing cells in the retina from functioning.
Results from the BRILLIANCE trial, published in the New England Journal of Medicine, showed that 11 of 14 participants experienced measurable improvements in vision and quality of life. About 79% of participants saw some degree of improvement after a single treatment to one eye. No serious safety concerns were reported. Because the eye is a small, enclosed space that’s relatively shielded from the immune system, it turned out to be an ideal early target for in vivo CRISPR editing.
Liver Diseases and Beyond
The liver is the other organ where in vivo CRISPR editing has shown striking early results. In a trial for transthyretin amyloidosis, a progressive disease where the liver produces a misfolded protein that accumulates in the heart and nerves, researchers used lipid nanoparticles to deliver CRISPR directly to liver cells. The goal was to disable the gene responsible for producing the toxic protein.
Published in the New England Journal of Medicine, early results showed that patients who received the higher dose experienced an average 87% reduction in the harmful protein within 28 days. Even the lower dose group saw a 52% reduction. These are single-infusion treatments, not ongoing medications, which represents a fundamentally different model from the lifelong drug regimens many genetic disease patients currently face.
A Phase 3 trial is also now recruiting for hereditary angioedema, a condition that causes sudden, severe swelling episodes. The CRISPR approach targets a gene in the liver that produces an enzyme involved in triggering the swelling, aiming to reduce or eliminate attacks with a one-time treatment.
Off-Target Editing Risks
The primary safety concern with CRISPR is off-target editing, where the Cas9 enzyme cuts DNA at the wrong location. Even a single unintended cut could theoretically disrupt an important gene or, in a worst-case scenario, contribute to cancer by hitting a tumor-suppressor gene. Researchers have developed multiple methods to detect these errors, including whole-genome sequencing that compares a cell’s DNA before and after editing to spot any changes that shouldn’t be there.
So far, clinical trials have not reported serious adverse events from off-target editing. But the tools for detecting these errors continue to improve, and long-term follow-up data is still limited. Patients who receive CRISPR therapies are monitored for years after treatment to watch for any delayed effects. For ex vivo therapies, scientists can screen edited cells before reinfusing them, which adds a layer of quality control that in vivo approaches lack.
Cost and Access Challenges
Casgevy carries a list price of $2.2 million per patient in the United States. That price reflects the complexity of the process: harvesting stem cells, shipping them to a specialized editing facility, the chemotherapy conditioning, and the hospital stay for reinfusion. It’s a one-time cost rather than a recurring expense, and manufacturers argue it compares favorably to the lifetime cost of managing a chronic condition like sickle cell disease. Still, $2.2 million creates an obvious barrier, particularly in lower-income countries where sickle cell disease is most prevalent.
In vivo therapies could eventually bring costs down because they skip the labor-intensive cell harvesting and reinfusion steps. A single injection is simpler to manufacture and administer than a personalized cell therapy. But in vivo approaches are still earlier in development, and their pricing remains to be seen. The infrastructure needed to deliver even the simpler version, including hospitals capable of monitoring patients and managing potential immune reactions, isn’t universally available.