For the last 20 years, hematologist Linzhao Cheng has been trying to solve a problem that has vexed researchers for even longer: how to produce an ample supply of red blood cells for use in transfusions. For patients suffering from blood disorders such as sickle cell disease, thalassemia, and myelodysplastic syndromes, life-prolonging blood transfusions are often part of a treatment regimen. But donor blood is always in short supply, and mismatched blood types can cause serious problems. Repeated transfusions also come with potential risks—from an unhealthy buildup of iron to allergic reactions to a rejection of the new blood by the patient's immune system, leading to kidney damage and other complications. "No matter how carefully a doctor tries to match a patient's blood, you will eventually have immune rejection because it's another person's blood," says Cheng, a professor of medicine and oncology and chair of the Department of Hematology at the School of Medicine. "The efficacy of transfusion gets worse and worse, and you need more frequent transfusions, leading to a vicious cycle."
In his lab, Cheng, who is also a member of the Johns Hopkins Institute for Cell Engineering, had already created red blood cells from embryonic stem cells, but that didn't solve the issue. Embryonic stem cells would be too challenging to source in sufficient numbers and because the cells are derived from embryos, not the patient, they no more match the patient than cells from a transfusion.
Around 10 years ago, Cheng and his colleagues concentrated on using an emerging technique involving so-called induced pluripotent stem (iPS) cells. These cells are derived from a person's blood or other tissue and are genetically reprogrammed in the lab to return them to an embryonic state. Just like embryonic stem cells, iPS cells can be manipulated to form any cell type—including blood cells—and they also proliferate indefinitely in the laboratory. "Essentially, we were able to reverse the biological clock in the test tube," says Cheng of producing the nascent iPS cells. The promise was that by using a sample of a patient's blood, researchers would be able to grow an unlimited supply of matched, transfusion-ready blood cells. The team published its results in 2008, following pioneering studies by Japanese researcher Shinya Yamanaka, who won a Nobel Prize in 2012.
But everything did not work as planned. While the team successfully produced red corpuscles from iPS cells, when researchers tried to coax the iPS cells into producing blood-forming stem cells—like those found in bone marrow and used in transfusions—they were less successful. In animal trials, the laboratory-grown blood stem cells would not settle and stay in the marrow, where they needed to be to generate mature blood cells.
Cheng decided to change course. Instead of trying to create blood-forming stem cells, he concentrated on "fixing" the genetic defects found in the mature red blood cells. "I thought, 'Do I really have to create the blood-forming stem cell? Can we just go to the final product—that is, reprogram the [defective] red blood cell?'"
With a 2011 grant from the Maryland Stem Cell Research Fund, Cheng and his group were able to test the theory in his laboratory. Using blood samples from people with sickle cell disease, the team reprogrammed mature red blood cells into iPS cells and then employed a relatively new gene-editing technology to prune the gene variant and replace it with a healthy version before growing the stem cells back into mature cells. In theory, since the iPS cells are developed directly from the patient—using his or her own blood cells—there shouldn't be any immune system complications when transfused.
Cheng admits that clinical trials are years away. First, researchers must see if they can produce sufficient numbers of the designer blood cells and, most importantly, make sure they are safe and function correctly before transfusing them into a patient.
If the technique works, it could also have an impact on treating other diseases such as malaria, in which parasites infect and damage red blood cells. The concept would involve creating custom cells resistant to the infection and growth of malaria-causing parasites. "The idea would be to create a genetic modification that is just good enough to block the door of entry to the parasites," he says.
Cheng has applied for grants from the Maryland Stem Cell Research Fund as well as the National Institutes of Health to conduct additional experiments. "The fact that we haven't been able to solve this problem so far is frustrating," he says, "but it's also the reality."