Researchers at Dana-Farber/Boston Children's Cancer and Blood Disorders Center and the University of Massachusetts Medical School have developed a strategy to treat two of the most common inherited blood diseases, sickle cell disease and beta thalassemiam applying CRISPR-Cas9 gene editing to patients' own blood stem cells.
The two studies show that the gene-edited cells generate genetically corrected red blood cells producing functional hemoglobin.
Together, sickle cell disease and beta-thalassemia affect 332,000 conceptions or births worldwide each year, according to the World Health Organization.
Both diseases involve mutations in gene for beta globin protein.
In beta-thalassemia, the mutations prevent red blood cells from producing enough of the oxygen-carrying hemoglobin molecule, leading to anemia.
In sickle cell disease, the mutation causes hemoglobin to change shape, distorting red blood cells into stiff "sickle" shapes that block up blood vessels.
The Nature Medicine study used CRISPR-Cas9 technology, in particular a Cas9 protein modified by a team led by Scot Wolfe, PhD at UMass Medical School, to optimize gene editing.
In previous attempts to edit the genomes of human blood stem and progenitor cells, the efficiency, specificity and long-term stability of the edits once the cells engraft in the bone marrow have varied.
The new technique improves the targeting and durability of the edits.
Bauer's team used the strategy to make a highly targeted edit.
Previous work at Boston Children's had showed that inactivating a gene called BCL11A allows red blood cells to keep producing a fetal form of hemoglobin even after birth.
Fetal hemoglobin doesn't sickle and can stand in for defective "adult" hemoglobin.
More recently, Bauer found a safer target: a genetic enhancer of BCL11A that is active only in red blood cells.
The strategy enabled mice carrying blood stem cells from patients with sickle cell disease to produce red blood cells with enough fetal hemoglobin to prevent cell sickling.
The team showed that the gene-edited cells, infused back into the bloodstream, engrafted in the bone marrow and produced genetically corrected red blood cells.
Later, when blood stem cells were isolated from these mice and transplanted into other mice, the cells engrafted again, still carrying the therapeutic gene changes.
Applied to blood stem cells from patients with beta-thalassemia, the same strategy restored the normal balance of the globin chains that make up hemoglobin.
The other study, published in Blood, used a similar gene editing protocol to target forms of beta-thalassemia that involve splicing mutations, errors in bits of DNA near the beta-globin gene that change how the gene is read out to assemble beta-globin protein.
In this study, nine patients with beta thalassemia donated their cells, which were manipulated in a dish.
For some patients, the UMass team produced a different enzyme, Cas12a, to more effectively target their mutations.
The CRISPR system efficiently made edits and restored normal splicing of the beta-globin protein in blood cells from each of the patients. ■