Positive results from several trials with ex vivo gene editing suggest that reactivating fetal hemoglobin in sickle cell disease and β-thalassemia is now a reliable therapeutic strategy after years of uncertainty.
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In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. In the quarter century since the publication of the first human genome sequence, the development of safe, effective gene therapies has been one of medicine’s ‘north stars’. But the journey to the finish line has been far from smooth.
Patient deaths, inconsistent results, off-target effects and manufacturing issues have all proved The field is now building optimism around ex vivo CRISPR therapies that edit a patient’s blood stem cells outside of the body before they are reinfused for the treatment of severe inherited blood disorders such as sickle-cell disease and transfusion‑dependent β‑thalassemia. More than half of children with sickle-cell disease in sub-Saharan Africa and India die by the age of 5 years, noted geneticist and physician Stuart Orkin of the Dana-Farber Cancer Institute in a.
Orkin did foundational work identifying the biological mechanism by which these gene therapies would function, and he was honored with a US $3 million Breakthrough Prize in Life Sciences in April 2026.confirmed that several CRISPR-based editing strategies for reactivating fetal hemoglobin production resulted in remarkably similar rates of gene activity . Andfrom Vertex Therapeutics, the makers of the world’s first approved CRISPR-based therapy, Casgevy, show that the company’s gene-editing tool provides safe and durable protection from transfusions and severe pain episodes in children as young as 5 years of age.
“I think we have enough science on what we need to do,” Orkin says. “The real issue is going to be how to streamline these to get them to as many patients as possible. That's the limiting factor. ” The results mark a sea change in the questions surrounding ex vivo gene therapy, marking a transition from questions around safety and durability to producing benchmarks of routine reliability.
It is a major win for patients, says Alexis Thompson, a hematologist at the Children’s Hospital of Philadelphia.
“Many of us are very excited that it would seem the current options for ex vivo gene therapy, for the most part, appear to be durable,” Thompson says. “The expectations today are that you only would need to go through this once, and that following the current approaches that the vast majority of patients will have a successful outcome. ”: children with sickle-cell disease did not begin showing symptoms until around 6 months of age.
It was, she noted, right around the time babies stop producing fetal hemoglobin and start producing the adult form of the protein. By the 1970s and 1980s, geneticists around the world had noted that people carrying the sickle-cell variant who also maintained high levels of fetal hemoglobin into adulthood had far milder forms of sickle-cell disease.
They did not develop anemia; nor did they suffer from organ damage or, in which sickle-shaped red blood cells get stuck in small blood vessels, obstructing flow and causing excruciating pain. If scientists could find some way to help an adult with sickle-cell disease start producing fetal hemoglobin again, they might be able to create a lasting treatment for the condition, says Orkin. Doing so required a deep dive into the expression of the fetal hemoglobin protein.
In 2008, Orkin and other scientists working in this areaby switching off the gene’s enhancer would block BCL11A production only in red blood cells, triggering the adult production of fetal hemoglobin. Other scientists found that making small genetic edits to the promoter of the fetal hemoglobin gene could create the same effect. This basic research lay the groundwork for hemoglobinopathy gene therapies such as Casgevy and other treatments currently in development.
“Sickle-cell disease is always getting the short end of every stick,” says Julie Kanter, co-director of the Lifespan Comprehensive Sickle Cell Center at the University of Alabama, Birmingham. “If we can enhance our sickle-cell centers to provide better care for more people that will certainly help everyone. ”enhancer, makes a double-stranded DNA break that results in 45% production of fetal hemoglobin — enough to eliminate the need for transfusion and nearly all vaso-occlusive crises.
But like all ex vivo gene therapies, it comes with major drawbacks. Chief among them is the need for intensive chemotherapy with the drug busulfan, to allow the gene-edited blood stem cells to engraft in the bone marrow. This treatment requires lengthy hospitalization, and complications can be life-threatening and cause permanent infertility. Even so, growing evidence that Casgevy is safe and long‑lasting has supported trials in younger children.
The success of Casgevy in the initial and recent phase 3 trials provided a proof of concept that editing of theenhancer could work, says Bill Hobbs, vice president of clinical development in hematology at Vertex. But the drug’s ongoing success shows that its effects are far from a one-off, he says. If a new technology can replace busulfan conditioning with something less toxic, that would be an even bigger win, Hobbs says.
“What's been really exciting to see about these programs is really the consistency and durability of the effect,” Hobbs says. “We're targeting the common biology that's shared by all patients, which is that fetal hemoglobin is protective, and that if you can switch it back on, it's a mechanism that’s independent of age.
”How far the field has come, however, can be seen not just in the different therapies that have been tested in trials but also in how consistent their results are, both for fetal hemoglobin production and for clinical outcomes. The recent papers from the, Editas tested its CRISPR therapy reni‑cel in 28 adults with severe sickle-cell disease.
Rather than editing BCL11A itself, this treatment uses an engineered Cas12a enzyme to disrupt sites in promoter regions of the fetal hemoglobin genes, which prevents BCL11A from binding. Treatment with reni-cel increased fetal hemoglobin levels from an average of 2.5% to 48%, and total hemoglobin levels rose from 9.8 g dLwith the same drug showed similar outcomes in transfusion-dependent β-thalessemia .
All nine patients became transfusion independent after treatment, and the six patients that were followed for over 12 months remained free from transfusion. As in the RUBY trial, the mean total hemoglobin levels topped 12 g dL“It shows really consistent safety patterns,” says Rabi Hanna, a pediatric hematologist-oncologist at the Cleveland Clinic.promoter.
But instead of cutting both strands of DNA, it uses a form of base editing that changes a single DNA letter, which prevents BCL11A from attaching and allows fetal hemoglobin to be produced. Risto-cel also requires busulfan conditioning, but Matthew Heeney, a pediatric hematologist-oncologist at Boston Children’s Hospital and lead investigator of the BEACON trial into risto-cel, says that the base editing resulted in faster engrafting of the stem cells into the bone marrow than other gene-editing therapies.treated 31 patients with sickle-cell disease, 12–35 years of age, all of whom had mean fetal hemoglobin levels exceeding 60% and no severe vaso-occlusive crises after treatment.
It was, says Beam president Giuseppe Ciaramella, the first major clinical demonstration that base editing can be as effective as the nuclease editing used by Casgevy and reni-cel.
“The data is showing that the risto-cel is capable of a very high upregulation of the fetal form of hemoglobin, the greatest extent that has been seen and been achieved so far,” Ciaramella says. Access the most recent journalism from Nature's award-winning team
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