Sickle cell disease (SCD) is a perfect candidate for gene editing. It is perhaps the best understood single-gene condition, due to a substitution of a single DNA base in the gene that encodes the beta subunit of hemoglobin, the protein that carries oxygen in the blood.
Red blood cells, which bend into the sickle shape in the disease, obstructing circulation and causing excruciating pain, descend from hematopoietic (“blood-forming”) stem cells (HSCs) in the bone marrow. So theoretically, a patient’s stem cells can be removed, the mutation replaced in them, and then the fixed cells infused back into the patient, circumventing an immune response.
The trick is getting enough changed cells into a person to vanquish the symptoms and for the effect to last. A report in Science Translational Medicine published yesterday takes a solid step in that direction by replacing the sickle cell mutation in stem cells from patients and increasing output of functional hemoglobin, decreasing the load of sickle hemoglobin, and at the same time producing a bit of fetal hemoglobin — in the cells and in mice. Not in people. Yet.
A REACHABLE TARGET
In the upcoming new edition of Human Genetics: The Basics (shameless book plug), I tell some little-known vignettes from the sickle cell story: the professor who stole credit for the discovery of sickled hemoglobin from his student; and the deadly consequence of telling young parents that their young kids had sickle cell “trait” without explanation – leading to at least one suicide.
DNA Science covered the protection against malaria that being a SCD carrier confers in the context of other examples of balanced polymorphism. Another bit of sickle cell trivia: the name was changed to sickle cell disease decades ago, to reflect symptoms other than anemia. But I see from the news reports that anemia persists.
The only permanent treatment for SCD is a stem cell or bone marrow transplant, which even today has some risk. Hydroxyurea is a drug that coaxes production of fetal hemoglobin, which can ease and possibly prevent painful sickle cell crises, but must be taken daily. And sickle cell disease has long been a target for gene therapy, as have other conditions with access through the bone marrow.
THREE ROUTES TO MOLECULAR HEALING
Correcting the mutation behind SCD is an obvious strategy. How to do so is where creativity comes in.
The classic tools of gene therapy – viral vectors and liposomes – add functioning genes. Gene editing tools (the older TALENs and zinc fingers and the easier CRISPR-Cas9) instead replace the mutation, harnessing DNA repair systems that naturally exist in cells.
Three ways to deploy gene editing to treat sickle cell disease recently published differ in the targeted cell types and genes. In chronological order:
1. Researchers at Johns Hopkins edited one copy of the mutant beta globin gene in iPS cells from patients, leading some red blood cells to make normal hemoglobin, published in May 2015 and reviewed here at DNA Science.
Induced pluripotent stem (iPS) (aka reprogrammed) cells grow in culture from sampled specialized cells, typically skin fibroblasts, and are then marinated in a cocktail of growth factors and transcription factors to steer development towards a desired cell type. Here that was red blood cells, which are so mature that they jettison their nuclei and so are easy to spot. They resemble squished jelly doughnuts when filled with normal hemoglobin, but the telltale sickles when that one DNA base is altered.
2. Trimming an “enhancer” gene (BCL11A) dialed down the molecular switch from fetal to adult hemoglobin, enabling some of the functioning fetal version to persist, and compensate for sickled adult beta globin. The team, from Dana-Farber/Boston Children’s Cancer and Blood Disorders Center, the Broad Institute of MIT and Harvard, used HSCs and published in November 2015.
It was an indirect approach. “Our goal was to break the enhancer, rather than fix the hemoglobin mutation, but to do so in very precise ways that are only possible since gene editing technologies like CRISPR became available,” said co-author Daniel Bauer, MD, PhD, a pediatric hematologist/oncologist at Dana-Farber/Boston Children’s. An update is here.
3. In yesterday’s Science Translational Medicine, Mark DeWitt, of the University of California, Berkeley and co-workers describe experiments that in a way combine the two above: they edited the beta globin gene in HSCs from SCD patients. Instead of delivering the correct DNA sequence aboard viruses or liposomes, they used a single guide RNA along with single-stranded pieces of DNA to target the beta globin gene.
Normal hemoglobin levels rose in cells, but when the altered HSCs were placed into mice, the initially elevated level fell over the four months of the study – yet was still higher than in previous work, and is approaching the extent of correction that could dampen or prevent symptoms. The procedure would still entail the risk of wiping out some of the patients bone marrow cells to make room for the fixed ones to establish a population. A more philosophical barrier, the researchers point out, is that the difficulty of the intervention, not to mention the cost, would not be feasible for the part of the world with by far the most people with SCD – Africa.
I can see, perhaps a decade from now, a marriage between stem cell transplantation and gene editing in ways that provide lasting treatments for many conditions.