Stem Cell Gene Therapy for Sickle Cell Disease Moves Toward Clinical Trials


UCLA stem cell researchers and “gene jockeys” have successfully proven the efficacy of using genetically engineered hematopoietic (blood cell-making) stem cells from a patient’s own bone marrow to treat sickle-cell disease (SCD).

In a study led by Donald Kohn, professor of pediatrics and microbiology at the UCLA Eli & Edythe Broad Center of Regenerative Medicine & Stem Cell Research, an “anti-sickling” gene was introduced into the hematopoietic stem cells (HSCs) from patients with SCD. Because the HSCs divide continuously throughout the life of the individual, all the blood cells they make will possess the anti-sickling gene and will therefore no sickle. This breakthrough gene therapy technique is scheduled to begin clinical trials by early 2014.

SCD results from a specific mutation in the beta-globin gene. Beta-globin is one of the two proteins that makes the multisubunit protein hemoglobin. Hemoglobin is a four-subunit protein that ferries oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. It is tightly packed into each red blood cell.

The structure of hemoglobin, each subunit is in a different color.
The structure of hemoglobin, each subunit is in a different color.

Hemoglobin has a very high affinity for oxygen when oxygen concentrations are high, but a low affinity for oxygen when oxygen concentrations are low. Therefore, hemoglobin does a very good job of binding oxygen when it is in the lungs, where oxygen is plentiful, and a very good job of releasing oxygen in the tissues, where oxygen is not nearly as plentiful. This adaptive ability displayed by hemoglobin is the result of cooperativity between the four polypeptide chains that compose hemoglobin. Two of these polypeptide chains are alpha-globin proteins and the other two a beta globin proteins. Hemoglobin acts as though it is an alpha-beta dimer, or as though it is composed of two copies of an alpha-globin.beta-globin pair. The interactions between these polypeptide chains and the movement of the hemoglobin subunits relative to each other creates the biochemical properties of hemoglobin that are so remarkable.

A mutation in the beta-globin gene that substitutes a valine residue where there should be a glutamic acid residue (position number 6), creates a surface on the outside of the beta-globin subunit that does not like water, and when oxygen concentrations drops, the mutant hemoglobin molecule changes shape and this new water-hating surface becomes a site for protein polymerization.

Sickle cell hemoglobin

The mutant hemoglobin molecules for long, stiff chains that deform the red blood cells into a quarter moon-shaped structure that clogs capillaries.

Sickle cell RBCs

 

This new therapy seeks to correct the genetic mutation by inserting into the genome of the HSC that makes the abnormal red blood cells a gene for beta-globin that encodes a normal version of beta globin rather than a version of it that causes sickle-cell disease.  By introducing those engineered HSCs back into the bone marrow of the SCD patient, the engineered HSCs will make normal red blood cells that do not undergo sickling under conditions of low oxygen concentration.

Dr. Kohn noted that the results from his research group “demonstrate that our technique of lentiviral transduction is capable of efficient transfer and consistent expression of an effective anti-sickling beta-globnin gene in human SCD bone marrow progenitor cells, which improved the physiologic parameters of the resulting red blood cells.” Dr. Kohn’s statement may lead the reader to believe that this was done in a human patient, but that is not the case.  All this work was done in culture and in laboratory animals.

Kohn and his co-workers showed that in laboratory experiments, genetically engineered HSCs from SCD patients produced new non-sickled blood cells at a rate that would effectively allow SCD patients to show significant clinical improvement.  These new red blood cells also survived longer than those made by the nonengineered SCD HSCs.  The in vitro success of this technique has convinced the US Food and Drug Administration to grant Kohn the right to conduct clinical trials in SCD patients by early next year.

SCD affects more than 90,000 patients in the US, but it most affects people of sub-saharan African descent.  As stated before, the mutation that causes SCD produces red blood cells that are stiff, long, and get stuck in the tiny blood vessels known as capillaries that feed organs.  SCD causes multi-organ dysfunction and failure and can lead to death.

Sickle_cell_01

Treatment of SCD include bone marrow transplants, but immunological rejection of such transplants remains a perennial problem.   The success rate of bone marrow transplants is low and it is typically restricted to those patients with very severe disease who are on the verge of dying.

If Kohn’s clinical trials are successful, this stem cell-based treatment will hopefully become the gold standard for treatment of patients with SCD.  One potential problem with this technique is the use of lentiviral vectors to introduce a new gene into the HSCs.  Because lentiviruses are retroviruses, they insert their DNA into the genome of the host cells.  Such insertions can produce mutations, and it will be incumbent on Kohn and his colleagues to carefully screen each transformed HSC line to ensure that the insertion is not problematic and that the transformed cells are not sick or potentially tumorous.  However, such a vector is necessary in order to ensure permanent residence of the newly-introduced gene.

Even with these caveats, Kohn’s SCD treatment should go forward, and we wish all the best to Dr. Kohn, his team, and to the patients treated in this trial.