CRISPR Meets Stem Cells: Curing Sickle Cell Diseases

Red blood cells in sickle-cell disease have a characteristic elongated shape. Image: Junior D. Kannah/AFP/Getty Images

One day, Luke walked into a clinic in Kinshasa, Democratic Republic of the Congo, and he witnessed Meena — a woman who was suffering from excruciating pain in one of her limbs. The pain, caused by a blocked blood vessel, is common in people with a blood disorder called sickle-cell disease (SCD). The disease is caused by mutations in the gene encoding the hemoglobin subunit β (HBB). Hemoglobin (Hb) is a protein that has different combinations of globin subunits, each of which is associated with cofactor heme — an inorganic molecule that carries oxygen. A single nucleotide substitution in HBB from adenine (A) to thymine (T) results in the replacement of glutamic acid with valine in the globin chain. When Hb is not bound to oxygen, Hb molecules polymerize, distorting the red blood cells into the characteristic crescent or sickled shape. While disc-shape, normal red blood cells normally move in a single file in capillaries, the sickle-shaped cells hang on one another, blocking the blood vessels and giving rise to painful episodes that Meena had to suffer once in a while.

Though the biology behind SCD has been well-understood since the 1980s, hematologists, frustratingly enough, have not found an effective cure. The disease continues to strike 300,000 to 400,000 newborns globally each year, mostly in Sub-Saharan Africa, and is estimated by the Centers for Disease Control and Prevention to affect 100,000 people in the United States alone.

As opposed to the high number of patients, only 10% of people with SCD are able to get a bone-marrow transplant from a healthy family member with a matching tissue type. This is not to mention the grueling procedure, where patients must undergo chemotherapy to eliminate their own bone marrow before the transplant. After receiving treatments, patients still have to return to the hospitals for frequent transfusions. Given millions of dollars piling up in the economy every year due to treatments and readmission to hospitals for patients with SCD, the call for an effective cure is more and more urgent.

The answer: CRISPR and Stem Cells. BAAAM!

As sickle cell disease is caused by mutation in the HBB region, we can essentially use CRISPR to correct the mutation in the β-globin gene. Another way is to encourage the production of fetal haemoglobin, the form of haemoglobin used by humans in the womb. Made from subunits that does not contain the β-globins, fetal hemoglobins bind to oxygen more strongly than does adult haemoglobin, helping the fetus to take up oxygen from the mother’s blood. The explanation for this is fetal hemoglobins do not interact with 2,3-bisphosphoglycerate (2,3-BPG or 2,3-DPG) — the substance that decreases the binding affinity with oxygen in adult red blood cells. However, the level of fetal hemoglobins decline after six months as adult hemoglobin synthesis is activated while fetal hemoglobin synthesis is activated. On the other hand, it turns out that patients with SCD have genetic variation that allows to produce fetal hemoglobins well into adulthood, which help them lessen the severity of the disease.

If doctors can edit the patient’s haematopoietic stem cells, commonly found in the bone marrow where they self-renew and begin differentiating into all different types of blood cells, either by fixing the β-globin mutation or to restart the reproduction of fetal hemoglobin, and then use them to repopulate the bone marrows, we eliminate the need for a compatible bone-marrow donor, and more patients can be cured.

How Biotech Companies are Working on This Solution

CRISPR Therapeutics has recently collaborated with Vertex Pharmaceuticals to conduct a joint ex vivo program, where they develop CTX001 to treat both Sickle cell disease and β-thalassemia. The company aims to extract a haematopoietic stem cell from the patient’s body, then use CRISPR to disrupt the function of repressors such as BCL11A, allowing edited cells to begin producing fetal haemoglobin. These cells are then reinfused into the patient’s body and are expected to continue producing fetal Hb, thus ameliorating the deficiencies associated with the disease. The company also recently reported that 90% of edited cells dramatically increase fetal Hb in these cells. CTX001 is expected to enter clinical trial in Europe for β-thalassemia and in the United States for Sickle cell disease by mid-2018.

Editas Medicine is working on a similar approach, using CRISPR/Cas9 homology directed repair (HDR) and CRISPR/Cpf1-directed editing in human CD34+ cells. Matthew Porteus, a pediatrician and stem cell biologist at Stanford University, is also developing a clinical trial using this approach.

Despite the promising avenues towards the treatment of sickle cell diseases, uncertainties remain.

First of all, true hematopoietic stem cells are rare, fragile, and difficult to work with. These stem cells are generally in a quiescent state, in which they do not grow or divide. The goal is to maintain this state until they are delivered into the patient’s body — at which point they will start dividing into healthy red blood cells. Researchers also expect corrected blood cells to eventually surpass sickled ones in a patient’s body. Sickle cells live only 10 to 20 days, but normal red blood cells last from 90 to 120 days. At the same time, to verify that researchers have successfully edited the cells and have not caused off-target effects, the cells need to be tested, which means they are growing outside of the body for a longer period of time. Many worry that if the cells are not transplanted into the bone marrow early enough, they will start differentiating.

Another uncertainty is there is limited clinical data on how well the cells will be rooted in the bone and how long they will persist. According to a study at Rice University in mice, the engrafted cells last for a few months. They were also able to repair from 20% to 40% of the cells. But it is unclear while this amount of correction is enough to cure a patient.

The third problem is accessibility. While drugs can be mass manufactured and be given to many people, cell therapies must be made individually for each patient and handled carefully in ultraclean conditions at specialized facilities. It is not certain whether these technology will be developed to parts of the developing world that need it the most, as in Africa. In other words, 90% of people may not have access to the innovative CRISPR-stem cell therapy. On the bright side, as the amount of research and understanding of SCD increases, new discoveries of small molecule drugs that can be more easily distributed to many people are forthcoming.

Regardless of these uncertainties, a future where sickle cell disease are effectively cured is in our sight! There is one thing for certain: the more we do, the more we learn. With the upcoming clinical trials, I am really excited to learn more about the efficacy of CRISPR-mediated stem cells in humans, and look forward to the new insights we can gain. Hopefully one day, when Luke — or any of us — walks into that clinic in Kinshasa, the pain caused by sickle cell diseases will be out of sight.