CRISPR-Cas9 is a molecular tool that lets scientists cut DNA at a precise location and let the cell's own repair process disable, correct or rewrite a gene. The approach moved from a lab technique into an approved medicine in December 2023, when U.S. regulators cleared the first CRISPR-based therapy for sickle cell disease.
CRISPR-Cas9 works by pairing a guide molecule of RNA with the Cas9 enzyme. The guide RNA is built to match a specific stretch of DNA, and once it finds that sequence inside a cell, it leads Cas9 to the spot and the enzyme cuts both strands of the DNA, according to the National Institute of General Medical Sciences (NIGMS), part of the U.S. National Institutes of Health.
The cut itself does not fix anything. What matters is how the cell repairs the break. NIGMS explains that researchers can let the cell patch the cut with random genetic letters, which disables a gene, or they can supply a template so the cell repairs the break with a specific new sequence, which can correct or rewrite the gene. That control over both where the cut happens and how it is repaired is what makes CRISPR a flexible editing tool that researchers can, in principle, direct at many different locations in the genome.
Where the tool came from
CRISPR-Cas9 as a programmable editing tool was developed by Emmanuelle Charpentier and Jennifer Doudna, who published the key experiment in 2012 showing that a bacterial immune mechanism could be reprogrammed to cut DNA at a location of the user's choosing, as described by MIT Technology Review. The two scientists received the 2020 Nobel Prize in Chemistry for the discovery, which the Nobel committee called an "epoch-making" method for genome editing.
From lab tool to approved medicine
The technique's first clinical payoff came on December 8, 2023, when the U.S. Food and Drug Administration approved Casgevy (exagamglogene autotemcel), developed by Vertex Pharmaceuticals and CRISPR Therapeutics, as the first CRISPR/Cas9-based therapy ever cleared for patients, according to the FDA's own announcement. The agency approved Casgevy alongside a second, non-CRISPR gene therapy called Lyfgenia, both for sickle cell disease in patients 12 and older with a history of recurrent painful crises.
The United Kingdom's medicines regulator, the MHRA, had already authorized Casgevy on November 16, 2023, for both sickle cell disease and transfusion-dependent beta-thalassemia in patients 12 and over, making the UK the first country to approve a CRISPR-based medicine, according to Imperial College London, whose NHS trust ran part of the trial.
How the treatment is actually given
Casgevy is not injected directly into the body. Doctors first collect a patient's own blood stem cells, which are shipped to a manufacturing site where CRISPR edits a specific regulatory region near the BCL11A gene, switching it off so the cells start producing fetal hemoglobin, a form of oxygen-carrying protein normally only made before birth, the FDA explained. Before the edited cells are infused back, patients undergo high-dose chemotherapy to clear space in the bone marrow, a conditioning step that carries its own risks, including low blood cell counts and mouth sores, according to both the FDA and a report by Biopharma Dive.
The FDA's own review of the sickle cell trial found that 29 of 31 evaluable patients, or 93.5%, were free of severe pain crises for at least 12 consecutive months after treatment, according to the agency's approval announcement. Imperial College London reported separate figures tied to the UK authorization, which covered both diseases: 28 of 29 sickle cell patients free of severe pain and 39 of 42 beta-thalassemia patients no longer needing transfusions, both for at least 12 months.
Casgevy approval milestones
Date
Regulator
Decision
Nov. 16, 2023
UK MHRA
First worldwide authorization, for sickle cell disease and beta-thalassemia, ages 12+
Dec. 8, 2023
US FDA
First US approval, for sickle cell disease, ages 12+
Cost and access
Vertex set a U.S. list price of $2.2 million for Casgevy, according to Biopharma Dive, which also reported that the company estimates roughly 25,000 people in the U.S. and Europe could qualify for the therapy. STAT News noted that while the edit is designed to last a lifetime, long-term durability still depends on years of follow-up data that are still being collected.
These approvals represent an important medical advance … reflecting the FDA's commitment to facilitating development of safe and effective treatments.
— Dr. Peter Marks, FDA, in the agency's December 2023 approval announcement
Because the edited cells come from the patient's own body and are not inherited by future generations, this type of CRISPR use is classed as somatic gene editing, distinct from germline editing, which would alter DNA passed on to children and remains a separate and far more contested area of the technology.
Questions
What does CRISPR actually do to DNA?
It uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence, where Cas9 cuts both strands; the cell's own repair process is then directed by researchers to disable or rewrite the targeted gene, according to NIGMS/NIH.
Is there an FDA-approved CRISPR treatment?
Yes. The FDA approved Casgevy on December 8, 2023, for sickle cell disease in patients 12 and older, according to the FDA's approval announcement. The UK's MHRA had already authorized Casgevy for both sickle cell disease and beta-thalassemia on November 16, 2023, according to Imperial College London.
Who invented CRISPR gene editing?
Emmanuelle Charpentier and Jennifer Doudna showed in 2012 how CRISPR-Cas9 could be reprogrammed as a precise gene-editing tool, work for which they won the 2020 Nobel Prize in Chemistry, according to MIT Technology Review.
How much does Casgevy cost?
Vertex Pharmaceuticals set a U.S. list price of $2.2 million for Casgevy, according to Biopharma Dive.
Does CRISPR editing get passed on to children?
The approved therapy, Casgevy, edits only a patient's own blood stem cells (somatic editing) and is not inherited by offspring, which is distinct from germline editing of reproductive cells or embryos.
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