Editing Anemia
Sickle cell anemia (SCA) is the most common genetic disease detected annually in 300,000 newborns worldwide. SKA is based on two mutated copies of the hemoglobin gene, which change the shape of red blood cells from a circular disc-shaped to a sickle-shaped one. This triggers a cascade of changes leading to organ damage, chronic pain, and early mortality.
Researchers from the Broad Institute of the Massachusetts Institute of Technology, Harvard University and St. Jude Children's Research Hospital used nitrogenous base editing technology that allows directly converting one letter in the DNA chain into another, turning a pathological gene into a healthy one, which led to the synthesis of normal hemoglobin. The study was conducted in vitro on human hematopoietic cells and in vivo on a mouse model of SKA.
Currently, the only method of treating SCA is bone marrow transplantation, but it is difficult to find a suitable donor for the patient, and patients undergoing transplantation have a high risk of serious side effects. Some laboratories are developing methods of gene modification of the patient's own bone marrow cells in order to avoid transplantation of donor tissue, but all of them are based on the introduction of new DNA or are accompanied by the breaking of DNA chains in cells, which can also cause side effects.
The only DNA mutation underlying SKA is the replacement of adenine, which should have been in a healthy hemoglobin gene, with thymine. The base editor is not able to reverse this change, but can convert thymine to cytosine. Such editing turns pathological hemoglobin into a non-pathogenic variant of Makassar. (This variant of hemoglobin was discovered in a man in the city of Makassar on the Indonesian island of Sulawesi in 1969.)
In this work, the research team used a base editor that can convert an adenine-thymine (A-T) base pair to guanine-cytosine (G-C), changing the gene at the level of only one pair of nucleotides. Editing tools include an improved and modified version of the Cas9 protein, which directs the editor to the mutated hemoglobin gene in the genome, as well as an enzyme that converts the target adenine to guanine. Next, the complementary DNA strand is restored, completing the conversion of the target base pair A-T to G-C.
The group introduced a base editor into blood stem cells taken from patients with SCA, up to 80% of pathogenic hemoglobin variants were successfully changed into the Makassar variant; undesirable hemoglobin changes were rarely caused by editing.
The researchers injected these blood stem cells into mice to observe how they function in live animals. After 16 weeks, the edited cells were still producing healthy red blood cells. Almost 90% of the cells contained at least one edited copy of the gene.
In a separate series of experiments, the researchers took blood stem cells from mouse models of SKA, edited them and transplanted them to another group of recipient mice. The control group of mice that received unedacted cells showed typical symptoms of SCA: sickle-shaped erythrocytes with a short lifespan and enlarged spleen. On the contrary, mice that received edited cells gave better results compared to the control for each tested indicator of the disease, and all measured blood parameters were observed at levels almost indistinguishable from those in healthy animals.
Finally, to confirm the persistence of the effect of editing blood stem cells, the researchers performed a secondary transplant, taking bone marrow from mice that received edited cells 16 weeks ago and transplanting them to a new group of mice. In this cohort of animals, the edited cells continued to act similarly to healthy blood stem cells, confirming that the effects of base editing were long-lasting.
The scientists concluded that editing at least 20% of the pathogenic hemoglobin genes is sufficient to maintain blood counts in mice at a healthy level.
The researchers are working to test the safety and effectiveness of this concept in additional preclinical studies in order to transfer them to clinical trials as soon as possible.
The article by G.A.Newby et al. Base editing of haematopoietic stem cells rescues sickle cell disease in mice is published in the journal Nature.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on the materials of the Broad Institute: Genetic base editing treats sickle cell disease in mice.