Reversing sickle cell disease with CRISPR-edited stem cells

red blood cells
Editing a small portion of stem cells with CRISPR-Cas9 is sufficient for long-term reactivation of fetal hemoglobin to treat blood disorders, scientists at Fred Hutchinson Cancer Research Center have found. (Pixabay)

Currently available gene therapies usually deliver a functional gene to replace a defective one. In a new approach developed at Fred Hutchinson Cancer Research Center, researchers instead introduced a mutation using the gene-editing system CRISPR-Cas9, and they’ve reported promising results from monkey studies aimed at boosting the production of fetal hemoglobin to treat blood disorders.

Sickle cell disease and beta-thalassemia are caused by mutations in the beta-globin gene that cause misshapen red blood cells. Bluebird Bio’s Zynteglo, also known as LentiGlobin, adds functional copies of the gene to the patient’s own blood-producing hematopoietic stem cells through a lentiviral vector.

The Fred Hutch-led team instead used CRISPR-Cas9 to reactivate another type of hemoglobin that normally only operates during fetal development. In monkey studies, the approach led to long-lasting expression of functional fetal hemoglobin, offsetting the defect in adult hemoglobin. They published the results in the journal Science Translational Medicine.

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Some people have a benign condition known as hereditary persistence of fetal hemoglobin (HPHF). These patients continue to express gamma-globin, which normally works with alpha-globin to form hemoglobin in the womb. Previous studies have shown that reactivating this fetal hemoglobin could reverse symptoms in sickle cell disease and beta-thalassemia.

Hans-Peter Kiem, the study’s senior author, and colleagues used CRISPR-Cas9 to remove a piece of genetic code to replicate what’s naturally seen in those with HPHF. This allows red blood cells to continuously produce elevated levels of fetal hemoglobin.

The approach is based on hematopoietic stem cells that “engraft,” or settle, in the patient’s bone marrow to produce red blood cells with functional hemoglobin. Rather than traditional blood stem cells, which have the protein marker CD34, the stem cells Kiem’s team worked with have the CD90 antigen. These CD90 cells can regrow the entire blood and immune system, according to the researchers.

RELATED: Bypassing the anti-cancer p53 gene for better CRISPR editing in blood disorders

The edits were taken up by 78% of target stem cells in the lab dish before they were infused. After transplantation, the cells engrafted, with edits still appearing in 30% of blood cells after one year, and that resulted in 18% of red blood cells expressing fetal hemoglobin, a level that Kiem said “would be close to a level sufficient to reverse symptoms of sickle cell disease.” Because the stem cells can provide long-term production of genetically modified blood cells, they might offer a lifetime cure for the diseases, the scientists figured.

One major obstacle for the adoption of gene therapies is their high costs. Bluebird, for example, recently set the price for Zynteglo at €1.575 million ($1.75 million) in Europe. The approach developed by the Fred Hutch team could address that issue because the researchers cut the number of cells required for transplantation by 10-fold. That means targeting them require fewer editing reagents, and therefore could be less costly, the researchers said.

“Not only were we able to edit the cells efficiently, we also showed that they engraft efficiently at high levels, and this gives us great hope that we can translate this into an effective therapy for people,” Kiem said in a statement.

Boosting the production of fetal hemoglobin in stem cells via ex-vivo gene modification is a strategy being actively pursued by several companies to treat blood disorders. CRISPR Therapeutics and partner Vertex’s phase 1/2 candidate CTX001 recently secured FDA “fast track” designation in sickle cell disease and beta-thalassemia after a temporary clinical hold was lifted. Aruvant Sciences is working on RVT-1801 and recently reported positive preliminary data from an ongoing phase 1/2 trial. Both therapies target CD34 stem cells.

RELATED: Ramaswamy's latest 'vant' to focus on gene therapies for blood disorders

Meanwhile, scientists are working on better ways to improve CRISPR in blood disorders. A Fred Hutch team recently developed gold nanoparticles that they found could serve as an alternative delivery vehicle. Scientists led by the San Raffaele Telethon Institute for Gene Therapy made highly-specific Cas9 “scissors” to limit the activation of the p53 gene, a well-known DNA stabilizer that could prevent edited hematopoietic stem cells from proliferating.

Kiem and colleagues found no harmful off-target mutations in edited cells in their study, and they are currently running long-term follow-up studies to confirm the safety profile. In the future, they hope their technology could help many other patients with blood diseases.

“By demonstrating how this select group of cells can be efficiently edited for one type of disease, we hope to use the same approach for conditions such as HIV and some cancers,” Kiem said in the statement.

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