Johns Hopkins researchers have used a patient’s own stem cells to correct the genetic defect the genetic lesion that causes sickle-cell disease (SCD). SCD is a painful, disabling inherited blood disorder, which predominantly affects African-Americans. Stem cells from SCD patients were subjected to genetic techniques that corrected the mutation in their hemoglobin genes that causes SCD. This procedure was done in the laboratory, but the genetically manipulated stem cells were not used in the clinic because they are not yet approved for use in patients.
This research, published in the August 31st edition of the journal Blood, brings researchers one step closer to developing a feasible cure or long-term treatment option for patients with SCD. SCD is caused by a single base change in the DNA sequence of the gene that encodes the protein hemoglobin; the principal protein in red blood cells that carries oxygen from the lungs to the tissues. People who have inherited two copies of this mutation (one mutant copy from each parent) produce red blood cells that deform to a sickle-shaped structure under low oxygen concentrations. Since red blood cells are normally round and slightly dished in the center, they can move through small blood vessels rather readily. The sickle-shaped red blood cells clog blood vessels, leading to pain, fatigue, infections, organ damage and premature death.
Various drugs and painkillers can control SCD symptoms, but the lonely known cure is a bone marrow transplant. However, the vast majority of SCD patients are African-American and few African-Americans have registered in the bone marrow registry, which makes it extremely difficult to find bone marrow donors whose tissue types properly match those of the many SCD patients. Linzhao Cheng, professor of medicine and associate director for basic research in the Division of Hematology put it this way: “We’re now one step closer to developing a combination cell and gene therapy method that will allow us to use patients’ own cells to treat them.”
Researchers initiated this work by isolating bone marrow cells from one adult patient at The Johns Hopkins Hospital. Then they used these bone marrow cells to generate induced pluripotent stem (iPS) cells (adult cells that have been reprogrammed to behave like embryonic stem cells) from the bone marrow cells. Into these iPS cells, they introduced one normal copy of the hemoglobin gene that substituted for the defective. Specialized genetic engineering techniques allowed them to not only introduce the normal copy of the hemoglobin gene, but to also swap the normal copy for the mutant copy. After sequencing genomic DNA from 300 different iPS samples, they identified those iPS cell lines that did not have a normal copy of the hemoglobin gene and those that did. At least four cell lines had normal hemoglobin genes, but three of those four iPS cell lines did not pass subsequent tests. Cheng added: “The beauty of iPS cells is that we can grow a lot of them and then coax them into becoming cells of any kind, including red blood cells.” Cheng’s team converted iPS cells that possessed a corrected copy of the hemoglobin gene into immature red blood cells by treat them with various growth factors. Further work showed that the newly introduced normal hemoglobin gene was turned on properly in these cells, although at less than half of normal levels. Cheng explained, “We think these immature red blood cells still behave like embryonic cells and as a result are unable to turn on high enough levels of the adult hemoglobin gene. We next have to learn how to properly convert these cells into mature red blood cells.”
Only one drug treatment has been approved by the FDA for treatment of SCD, hydroxyurea, whose use was pioneered by George Dover, M.D., the chief of pediatrics at the Johns Hopkins Children’s Center. Outside of bone marrow transplants, frequent blood transfusions and narcotics can control acute anemic episodes that can be life-threatening in some cases.