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Salk researchers develop safe way to repair sickle cell disease genes

sickle cells

To assure that the skin cells were in fact reprogrammed into stem cells, the researchers coaxed them into becoming muscle cells, indicated by the presence of muscle-specific protiens (red) in this image. The cell nuclei—the central compartments that contain the DNA—are seen in blue.

Sickle cell disease is one of the most prevalent devastating genetic disorders in the world. It is caused by genetic mutations in the HBB gene, whose normal function is to make hemoglobin, the iron-containing protein that allows red blood cells to carry oxygen. The sickle cell mutation in hemoglobin causes red blood cells to become rigid and sticky and deform to a sickle-like shape. The disorders can be cured by stem cell or bone marrow transplantation, but the scarcity of compatible donors and the high risk of immunorejection prevent most patients from benefiting from these therapies.

Researchers in the lab of Juan Carlos Izpisua Belmonte set out to devise a safe method using induced pluripotent stem cells (iPSCs) to correct the HBB gene in patients who have defective copies of it. Traditional iPSC generation and gene therapy techniques have proven to be potentially unsafe, according to the researchers. Many have used viruses that insert themselves into the human genome to convert adult cells to stem cells and to deliver a normal HBB gene to repair hematopoietic stem cells—stem cells that give rise to all blood cells. But when these repaired stem cells are returned to patients, they may include unwanted mutations introduced by the viruses that are inserted in the genome. In the most severe cases, these mutations may lead to cancer formation.

To fix the mutation so that it did not leave any unwanted traces in a patient's genome, the researchers used a two-step approach, first creating iPSCs using a technique that avoids the use of viruses, then using a modified adenovirus (common cold virus) that, unlike viruses used in other methods, neither replicates itself in the body, nor expresses viral genes. The engineered adenovirus contains a normal HBB gene. Once inside the iPSCs, the modified adenovirus performs a precise swap between the normal HBB gene it carries and the broken gene that was in the patient's cells.

The correction of the mutant HBB gene, reported in Cell Research, was highly efficient, and the research team conducted multiple tests to ensure that no errant genes were integrated into the genome. By replacing a relatively large region of DNA, the technique allows the scientists to fix many gene mutations at once, which suggests it might provide a way to treat hundreds of HBB-related diseases.