The First CRISPR Cure: How Gene Editing Finally Kept Its Promise

In July 2019, a woman named Victoria Gray drove to Nashville with a disease that had defined every day of her life. She had sickle cell disease — a genetic condition that causes red blood cells to crumple into rigid crescents, clogging vessels and starving tissue of oxygen. Pain crises could last for days. Hospitalizations came in waves. Organ damage accumulates over a lifetime.

In Nashville, she received an infusion of her own stem cells — stem cells that had been removed, reprogrammed at the genetic level, and returned to her body. The tool used to reprogram them was CRISPR-Cas9, the molecular scissors that won a Nobel Prize. Within months, her pain crises stopped. Years later, they still hadn’t returned.

In December 2023, that experimental procedure became medicine. Regulators in the UK, US, and Europe approved Casgevy (exagamglogene autotemcel) — the world’s first CRISPR-based therapeutic — for sickle cell disease. A few weeks later, it was approved for a second blood disorder, beta-thalassemia.

A tool discovered in bacterial immune systems in 2012 had, in eleven years, become a cure.

The Problem Written in One Letter

Sickle cell disease begins with a single nucleotide change — one wrong letter in the six billion that make up a human genome. That change alters the beta-globin protein, which is part of hemoglobin, the molecule that carries oxygen in red blood cells. The defective hemoglobin — called HbS — is functionally normal when oxygen is plentiful, but when it releases oxygen in tissues, it polymerizes. It crystallizes. And as it does, it distorts the red blood cell from a flexible disc into a stiff, sickled crescent.

Those sickled cells can’t navigate capillaries the way normal cells can. They clog. They obstruct. They trigger inflammation. Blood can’t reach bone, muscle, or organ. The pain is severe enough that patients describe it as burning, stabbing, crushing — occurring unpredictably, lasting for days, and recurring throughout their lives. Over time, the crises damage the kidneys, the spleen, the lungs, the brain. The average life expectancy in high-income countries is around 54 years. In low-income settings, most children with the disease don’t survive to adulthood.

Around 300,000 people are born with sickle cell disease every year worldwide. Roughly 100,000 live with it in the United States, the majority of them Black.

Beta-thalassemia has a similar origin: mutations that reduce or eliminate functional beta-globin production. Without enough normal hemoglobin, red blood cells die rapidly, causing anemia severe enough to require blood transfusions every three to five weeks to survive. People with transfusion-dependent beta-thalassemia (TDT) may receive hundreds of transfusions over a lifetime — which causes its own complications, including iron overload that damages the heart and liver.

For decades, the only potential cure was a bone marrow transplant from a matched sibling donor. Fewer than 20% of patients have a suitable donor. And bone marrow transplants carry serious risks of graft-versus-host disease.

A better answer required editing the genome itself.

A Bacterial Immune System, Repurposed

CRISPR — Clustered Regularly Interspaced Short Palindromic Repeats — was first noticed by microbiologists studying strange repetitive sequences in bacterial DNA. Over the following two decades, researchers discovered that these sequences were part of an adaptive immune system: bacteria were storing fragments of past viral invaders as a kind of genetic memory, then using guide RNA molecules and a protein called Cas9 to search for and destroy matching sequences if the virus appeared again.

In 2012, a team led by Jennifer Doudna at UC Berkeley and Emmanuelle Charpentier at Umeå University showed that this system could be reprogrammed. By changing the guide RNA, you could direct Cas9 to cut any DNA sequence you chose — with precision, speed, and extraordinary simplicity compared to prior gene-editing tools. The paper, published in Science in August 2012, described it as “a programmable dual-RNA-guided DNA endonuclease.”

That is an understatement that deserves a place in the history of understatements.

The demonstration that Cas9 could be programmed like a molecular search-and-cut tool catalyzed a worldwide field nearly overnight. Laboratories adapted it for mammalian cells, for human cells, for stem cells. The CRISPR-Cas9 system turned out to be cheap, fast, and remarkably versatile. Within two years, it was being used across biology. Within five, clinical trials had begun. Within eight, it had won Doudna and Charpentier the Nobel Prize in Chemistry (2020).

The Target: Waking Up the Fetal Gene

Here is where the biology gets beautiful.

Adults carry two main hemoglobin genes: the adult beta-globin gene (which, in sickle cell patients, is mutated) and the gamma-globin gene — a fetal version of hemoglobin that fetuses use before birth. Fetal hemoglobin (HbF) carries oxygen just as well as adult hemoglobin. Crucially, it does not sickle. It also actively inhibits the polymerization of HbS. A red blood cell with enough HbF is essentially protected from sickling.

In most people, the gamma-globin gene is switched off a few months after birth — turned down by a master transcription factor called BCL11A. The adult blood system doesn’t need fetal hemoglobin anymore, so BCL11A shuts it off.

The insight behind Casgevy is that you don’t need to fix the broken gene. You can work around it — by putting BCL11A to sleep and waking up the fetal hemoglobin gene that’s been silent since infancy.

The CRISPR approach targets the erythroid-specific enhancer of BCL11A — a regulatory region specifically active in red blood cell precursors. Disabling this enhancer in those cells reduces BCL11A activity, allowing gamma-globin expression to reactivate and fetal hemoglobin levels to rise. Critically, editing this enhancer (rather than BCL11A itself) leaves BCL11A function intact in other cell types where it matters, avoiding unintended effects elsewhere in the body.

The procedure works like this: stem cells are drawn from a patient’s blood. In the laboratory, CRISPR-Cas9 edits the BCL11A enhancer in those cells — in about 80% of alleles, based on early results. The patient then undergoes myeloablative conditioning (high-dose chemotherapy to eliminate the existing bone marrow). The edited stem cells are infused back. They engraft, multiply, and the patient’s new blood supply begins producing red cells rich in fetal hemoglobin.

What the Clinical Trials Found

The first two patients — one with sickle cell disease, one with beta-thalassemia — were reported in The New England Journal of Medicine in January 2021. More than a year after treatment, both had high levels of allelic editing in their bone marrow and blood, dramatically elevated fetal hemoglobin levels, and a complete transformation in their disease trajectory. The sickle cell patient had no vaso-occlusive crises. The thalassemia patient needed no transfusions.

The phase 3 trials confirmed these results at scale.

The CLIMB SCD-121 trial enrolled 44 patients with severe sickle cell disease — people who had experienced at least two serious vaso-occlusive crises in each of the two years before enrollment. After treatment, with median follow-up of 19.3 months (range: 0.8 to 48.1 months), researchers evaluated the 30 patients with sufficient time to assess the primary endpoint: 29 of 30 (97%; 95% CI, 83–100) were free from severe vaso-occlusive crises for at least 12 consecutive months. All 30 of 30 (100%; 95% CI, 88–100) were free from hospitalization for vaso-occlusive crises for at least 12 consecutive months. Both results exceeded the prespecified null hypothesis response rate of 50% with p<0.001.

The CLIMB THAL-111 trial enrolled 52 patients with transfusion-dependent beta-thalassemia. Of 35 with sufficient follow-up for evaluation, 32 (91%; 95% CI, 77–98) achieved transfusion independence lasting at least 12 consecutive months. During periods of transfusion independence, the mean total hemoglobin level was 13.1 g/dL — within the normal range. Fetal hemoglobin accounted for 11.9 g/dL of that total, and it was pancellularly distributed: present in at least 94% of red cells. The HbF was doing the job that HbA was supposed to do.

The safety profile in both trials reflected the myeloablative conditioning regimen rather than the gene editing itself — consistent with what’s observed in standard bone marrow transplantation. No concerning off-target editing events were reported.

Three Approvals in One Month

In November 2023, the UK’s Medicines and Healthcare Products Regulatory Agency (MHRA) became the first regulator in the world to approve Casgevy — for both transfusion-dependent beta-thalassemia and sickle cell disease in patients aged 12 and older with recurrent vaso-occlusive crises.

On December 8, 2023, the U.S. Food and Drug Administration approved Casgevy for sickle cell disease, alongside Lyfgenia, a different gene therapy for the same disease using a viral vector approach. On December 15, the European Medicines Agency approved Casgevy for both indications. The FDA followed with TDT approval in January 2024.

A decade after Doudna and Charpentier published their programmable endonuclease, CRISPR was medicine.

What Remains

None of this means the problem is solved.

Casgevy is priced at approximately $2.2 million in the United States. The procedure requires myeloablation — intensive chemotherapy that carries its own risks and requires a specialized treatment center. The disease predominantly affects Black and Brown populations, precisely the communities least likely to have equitable access to a $2 million treatment. Globally, the countries with the highest burden of sickle cell disease — in sub-Saharan Africa — are nowhere near the distribution network that could deliver this therapy.

These are not small obstacles. They are the next, critical problem to solve.

Researchers are already exploring next-generation approaches that might reduce these barriers: base editing and prime editing, which make targeted changes without creating double-strand DNA breaks, potentially increasing precision and reducing off-target risk. Delivery mechanisms that might reduce the need for myeloablation. Manufacturing improvements that might bring costs down. Simpler in vivo approaches that could work outside specialized bone marrow transplant centers.

The science has cleared a threshold that was not certain it could be cleared. The challenge now is ensuring that what can be done reaches the people who need it.

Victoria Gray continues to thrive. She’s talked publicly about what the treatment meant: a life without constant fear, without watching the calendar for the next crisis, without the weight of a disease that had shaped every decision. She now runs half-marathons.

For the researchers who spent decades working on CRISPR, on BCL11A, on hemoglobin biology — who saw the promise in bacterial immune sequences and in fetal globin genes and in the molecular scissors that could be programmed to find any sequence in three billion base pairs — that life is the point.

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Sources: Jinek et al., Science 2012 (DOI: 10.1126/science.1225829); Frangoul et al., NEJM 2021 (PMID 33283989, DOI: 10.1056/NEJMoa2031054); Frangoul et al., NEJM 2024 (PMID 38661449) — CLIMB SCD-121; Locatelli et al., NEJM 2024 (PMID 38657265) — CLIMB THAL-111; Parums, Medical Science Monitor 2024 (PMID 38425279).