Towards accessible gene therapy: in vivo base editing to cure Sickle Cell Disease

Sickle cell disease (SCD) is a genetic condition that affects more than 8 million people worldwide by altering the shape of oxygen-carrying red blood cells. Normally, red blood cells are shaped like bowl-shaped disc which allows them to flow smoothly through blood vessels and deliver oxygen to all parts of the body. This bowl shape is maintained by the healthy, blob-shaped hemoglobin protein. However, in SCD, mutations in hemoglobin change the protein’s shape. Instead of discrete blobs, their hemoglobin adopts the shape of long fibers. The long, fibrous hemoglobin changes the healthy bowl shape of the red blood cells into the unhealthy sickle shape. When red blood cells are sickle shaped, they cannot carry oxygen as well and clog up small blood vessels, delaying oxygen delivery throughout the body. This can result in severe pain and anemia for people living with the disease.

SCD is caused by a mutation in a single nucleotide of the hemoglobin gene. So far, the only cure for SCD is a bone marrow transplant, and these procedures are rarely done because of the high cost, significant risks and specialized facilities required. Because of these hurdles, bone marrow transplants are largely inaccessible in regions of the world that are most affected by SCD. To address this problem, researchers at the University of Washington Department of Laboratory Medicine and Pathology, in collaboration with the Kiem Lab at Fred Hutch, developed a method to safely correct the SCD mutation in a patient’s body using gene therapy. Their approach requires one viral vector, a couple injections, and minimal selection for hematopoietic stem cells (HSCs) that repopulate the blood system. “This technically simple approach holds the potential for scalable applications in resource-limited regions where sickle cell disease is highly prevalent,” says Dr. Chang Li, lead author of the study. Their findings were recently published in Molecular Therapy.

The team’s approach starts with a viral vector, a modified virus that delivers genetic material to a cell. They chose to use base editing vectors. Instead of generating a break in the DNA as with other forms of gene editing, base editors swap out a single nucleotide without breaking the DNA. To find the most efficient vector design with the fewest off-target effect, the team spent nearly a year on production that normally takes two months. Throughout that time, they tested five different vectors with different combinations of regulatory genes to find the safest, most effective vector. Once they found the best vector design, the group demonstrated that their new vector efficiently corrected the SCD in mouse blood cells in a dish.

After demonstrating that the viruses could work on cells outside of the body, the group wanted to know if they could efficiently edit blood cells in vivo. They used chemotherapy drugs to mobilize mouse HSCs from the bone marrow into the blood. The group then injected the SCD-correcting viral vector and used another chemotherapy drug to cause the corrected cells to divide. After monitoring the mice for several weeks, they found that almost 30% of the mouse blood cells contained the disease-correcting edit. The corrected mice also demonstrated improvements in their SCD symptoms. The mice had higher hemoglobin levels, fewer sickle-shaped red blood cells, and their spleens returned to a normal size. The group found minimal off-target gene editing, further demonstrating the safety and efficacy of this technique.

Previous groups working to develop in vivo gene therapy for SCD succeeded using mouse models, but their viral vectors were unable to edit human HSCs in a dish, indicating that the therapy likely would not translate into patients. Li and his team therefore wanted to show that their new base-editing vectors would successfully correct SCD in human cells. They collected HSCs from patients with SCD and treated the cells with the viral vectors. They found that they could successfully correct SCD in patient samples, and that the correction resulted in more red blood cell colonies in a dish. This indicates that the treatment could potentially be used in humans to cure SCD. Because the vectors successfully edit blood cells in vivo, they open the door for potential in vivo gene therapy in humans, too. Despite the promising results, there are still technical hurdles to overcome. Li highlights the need for improved ways to mobilize HSCs in vivo and selection strategies for edited cells that do not involve chemotherapy drugs. Although the approach still needs development, this work still brings researchers and patients closer to accessible therapy for SCD.

This work was supported by grants from the National Institutes of Health, and the Bill and Melinda Gates Foundation.

Fred Hutch/University of Washington/Seattle Children's Cancer Consortium Members Drs. André Lieber and Hans-Peter Kiem contributed to this work.  

Li C, Georgakopoulou A, Paschoudi K, Anderson AK, Huang L, Gil S, Giannaki M, Vlachaki E, Newby GA, Liu DR, Yannaki E, Kiem HP, Lieber A. 2024. Introducing a hemoglobin G-Makassar variant in HSCs by in vivo base editing treats sickle cell disease in mice. Mol Ther. (32)12:4353-4371.