A single approach that could treat nearly everyone with either disease.
For millions living with sickle cell disease and beta thalassemia, the body already carries a quiet solution — a fetal form of hemoglobin, silenced after birth, that functions where the adult version fails. Scientists at St. Jude Children's Research Hospital and the Broad Institute have now demonstrated that base editing, a precise molecular rewriting tool, can reawaken that dormant gene far more reliably than CRISPR/Cas9, achieving two to four times greater fetal hemoglobin levels with a consistency that earlier gene-editing methods could not match. Published in Nature Genetics in July 2023, the findings suggest that a single universal therapy — one that sidesteps the hundreds of distinct mutations driving these disorders — may be within reach. Clinical trials are underway, and with them, the slow, careful work of turning laboratory promise into human healing.
- Sickle cell disease and beta thalassemia together afflict millions worldwide, distorting or depleting the hemoglobin that keeps people alive, with few curative options available.
- CRISPR/Cas9, long heralded as a genetic revolution, has shown a critical weakness here — its DNA-cutting mechanism produces chaotic, inconsistent repairs that leave cells producing wildly uneven amounts of therapeutic hemoglobin.
- Base editing sidesteps the chaos entirely, chemically converting one DNA letter to another without breaking the strand, and in doing so creates a precise molecular switch that reliably reawakens fetal hemoglobin production.
- The edited cells held their changes when tested in mice, engrafting stably and sustaining elevated fetal hemoglobin levels — a crucial proof that the therapy could persist in a living system.
- Because the approach targets fetal hemoglobin reactivation rather than any specific mutation, it offers a single strategy capable of treating nearly all patients with either disease, regardless of their individual genetic variant.
- Clinical trials are now in motion, but researchers are clear-eyed: rigorous safety testing for off-target edits must precede any broad clinical use, placing the therapy at a hopeful but still cautious threshold.
Inside a laboratory at St. Jude Children's Research Hospital, scientists have been pursuing a deceptively elegant idea: rather than repairing the broken genes that cause sickle cell disease and beta thalassemia, what if you could simply wake up the backup?
Both disorders stem from mutations in the gene governing adult hemoglobin. In sickle cell disease, the resulting protein warps red blood cells into crescent shapes that snag in blood vessels; in beta thalassemia, too little functional hemoglobin is made at all. Both are life-threatening, and both have resisted easy treatment. But the human body carries a relic from fetal development — a different hemoglobin gene, gamma-globin, that works perfectly well and is simply switched off after birth. Researchers at St. Jude and the Broad Institute of MIT and Harvard asked whether that switch could be flipped back on.
Their answer, published in Nature Genetics in July 2023, came through base editing — a technology that rewrites individual DNA letters without cutting the strand. Unlike CRISPR/Cas9, which snips DNA and relies on the cell's own imprecise repair machinery, base editing works like an eraser and pen, converting one nucleotide directly into another. The team used it to create a new binding site for TAL1, a protein that acts as a master activator of gamma-globin. The result was striking: two to four times more fetal hemoglobin than CRISPR could produce, and crucially, consistent levels across every edited cell — something CRISPR's variable repair outcomes could not deliver.
In mouse models, the edited blood stem cells engrafted and continued producing elevated fetal hemoglobin stably over time. The safety profile also favored base editing, with fewer genotoxic stress events and large DNA deletions than CRISPR caused, though researchers acknowledged that off-target edits remain a concern requiring further study.
Perhaps most consequentially, the approach is universal. Because it targets fetal hemoglobin reactivation rather than any one mutation, it could theoretically treat nearly all patients with either disease — a single therapy for hundreds of genetic variants. Clinical trials are already underway, though researchers are careful to note that the path from laboratory to widespread use still requires rigorous safety validation. What was once a distant possibility is beginning, carefully, to take shape.
In a laboratory at St. Jude Children's Research Hospital, scientists have been working toward something that seemed distant just a few years ago: a way to cure two of the world's most common blood disorders by rewriting the genetic instructions that govern how our blood cells make hemoglobin.
Sickle cell disease and beta thalassemia affect millions of people globally. Both are caused by mutations in the gene that produces adult hemoglobin, the protein that carries oxygen through the body. The mutations create hemoglobin molecules that misbehave—in sickle cell disease, they distort red blood cells into a crescent shape that gets stuck in blood vessels; in beta thalassemia, they produce too little functional hemoglobin at all. Both conditions are life-threatening. Both have been difficult to treat.
But humans carry a genetic backup plan. During fetal development, we produce a different version of hemoglobin, made from a gene called gamma-globin. This fetal hemoglobin works perfectly well—it's just that after birth, the body normally switches it off and turns on the adult version instead. What if you could flip that switch back on? What if you could make the body produce fetal hemoglobin again, even after birth, to compensate for the broken adult version?
Researchers at St. Jude and the Broad Institute of MIT and Harvard set out to test this idea using a relatively new gene-editing technology called base editing. Their results, published in Nature Genetics in July 2023, showed something striking: base editing outperformed the more famous CRISPR/Cas9 technology at this task—and by a significant margin. When they used base editing to reactivate the gamma-globin gene in patient cells, they achieved two to four times higher levels of fetal hemoglobin than CRISPR could produce. More importantly, the results were consistent. Every cell edited with base editing produced roughly the same amount of fetal hemoglobin. With CRISPR, the results varied wildly from cell to cell.
The difference comes down to how the two technologies work. CRISPR cuts DNA like scissors, creating breaks that the cell then tries to repair. Those repairs are messy—the cell might insert extra DNA, delete some, or make other changes. You end up with a mixture of different genetic outcomes, some better than others at reactivating fetal hemoglobin. Base editing, by contrast, works like a pencil eraser and rewriter. It converts one DNA letter directly into another without cutting the DNA strand. This precision allowed the researchers to create a new binding site for a protein called TAL1, which acts as a master switch to turn gamma-globin back on. That specific edit proved remarkably potent.
When the team tested these edited cells in mice, the genetic changes persisted. The blood stem cells engrafted and continued producing elevated levels of fetal hemoglobin. The safety profile also favored base editing. CRISPR editing triggered more genotoxic events—cellular stress responses and large deletions in the DNA. Base editing caused fewer of these problems. The researchers did note that base editing has its own potential risks, including the possibility of making unwanted changes at off-target sites in the genome, but they found these changes to be relatively small and unlikely to cause harm.
What makes this work particularly significant is that it offers a universal solution. Sickle cell disease and beta thalassemia are caused by hundreds of different mutations. Trying to fix each one individually would require hundreds of different therapies. But reactivating fetal hemoglobin works regardless of which mutation a patient carries. It's a single approach that could theoretically treat nearly everyone with either disease. Clinical trials are already underway, though researchers emphasize that more safety testing will be needed before base editing moves from the laboratory into widespread clinical use. The work represents a step toward something that seemed impossible not long ago: a genuine cure for diseases that have plagued humanity for centuries.
Notable Quotes
Base editors may be able to create more potent and precise edits than other technologies, but we must do more safety testing and optimization.— Jonathan Yen, Ph.D., St. Jude Therapeutic Genome Engineering group director
These findings bring us a step closer to our goal of broadly available cures.— John Tisdale, M.D., National Heart, Lung, and Blood Institute
The Hearth Conversation Another angle on the story
Why does reactivating fetal hemoglobin matter more than just fixing the broken adult hemoglobin gene?
Because there are hundreds of different mutations that cause these diseases. Fixing each one would mean creating hundreds of different therapies. But fetal hemoglobin works for everyone, regardless of which mutation they carry. It's a one-size-fits-all solution.
And base editing is better at reactivating it than CRISPR because?
CRISPR cuts the DNA and lets the cell repair it however it wants. You get a messy mixture of different outcomes. Base editing is more like changing a single letter in a sentence—precise, controlled, the same result every time.
The consistency seems to be the real advantage here.
Exactly. In a therapy, you want every patient's cells to behave the same way. With CRISPR, some cells produce a lot of fetal hemoglobin, others produce very little. With base editing, they're all producing roughly the same amount. That's what makes it clinically viable.
What's the catch? There must be one.
Base editing can make unwanted changes at off-target sites in the genome. The researchers found these changes are small and probably harmless, but they're not certain yet. That's why more safety testing is needed before this moves into patients.
How close are we to actual treatment?
Clinical trials are already running. But we're probably still years away from this being available to patients. The science is moving fast, but medicine moves carefully.