Gene therapy is an experimental technique that aims to treat genetic diseases by altering a disease-causing gene or introducing a healthy copy of a mutated gene to the body. The U.S. Food and Drug Administration approved the first gene therapy for an inherited disease — a genetic form of blindness — in December 2017.

Gene therapy for sickle cell anemia

Sickle cell anemia is caused by a mutation in the HBB gene which provides the instructions to make part of hemoglobin, the protein in red blood cells that carries oxygen.

Researchers are working on two different strategies to treat sickle cell anemia with gene therapy. Both of these strategies involve genetically altering the patient’s own hematopoietic stem cells. These are cells in the bone marrow that divide and specialize to produce different types of blood cells, including the red blood cells.

One strategy is to remove some of the patient’s hematopoietic stem cells, replace the mutated HBB gene in these cells with a healthy copy of the gene, and then transplant those cells back into the patient. The healthy copy of the gene is delivered to the cells using a modified, harmless virus. These genetically corrected cells will then hopefully repopulate the bone marrow and produce healthy, rather than sickled, red blood cells.

The other strategy is to genetically alter another gene in the patient’s hematopoietic stem cells so they boost production of fetal hemoglobin — a form of hemoglobin produced by babies from about seven months before birth to about six months after birth. This type of hemoglobin represses sickling of cells in patients with sickle cell anemia, but most people only produce a tiny amount of it after infancy. Researchers aim to increase production of fetal hemoglobin in stem cells by using a highly specific enzyme to cut the cell’s DNA in the section containing one of the genes that suppress production of fetal hemoglobin. When the cell repairs its DNA, the gene no longer works and more fetal hemoglobin is produced.

Gene therapy offers an advantage over bone marrow transplant, in that complications associated with a bone marrow donation — now the only cure for the disease — such as finding the right match are not a concern.

Gene therapy in clinical trials for sickle cell anemia

Twelve clinical trials studying gene therapy to treat sickle cell anemia are now ongoing. Nine of the 12 are currently recruiting participants.

Four trials (NCT02186418, NCT03282656, NCT02247843, NCT02140554) are testing the efficacy and safety of gene therapy to replace the mutated HBB gene with a healthy HBB gene. These Phase 2 trials are recruiting both children and adults in the United States and Jamaica.

Three trials (NCT02193191, NCT02989701, NCT03226691) are investigating the use of Mozobil (plerixafor) in patients with sickle cell anemia to increase the production of stem cells to be used for gene therapy. This medication is already approved to treat certain types of cancer. All three are recruiting U.S. participants.

One trial (NCT00669305) is recruiting sickle cell anemia patients in Tennessee to donate bone marrow to be used in laboratory research to develop gene therapy techniques.

The final study (NCT00012545) is examining the best way to collect, process and store umbilical cord blood from babies with and without sickle cell anemia. Cord blood contains abundant stem cells that could be used in developing gene therapy for sickle cell anemia. This trial is open to pregnant women in Maryland — both those who risk having an infant with sickle cell anemia, and those who do not.

One clinical trial (NCT02151526) conducted in France is still active but no longer recruiting participants. It is investigating the efficacy of gene therapy in seven patients. For the trial, a gene producing a therapeutic hemoglobin that functions similarly to fetal hemoglobin is introduced into the patient’s stem cells.  A case study from one of the seven was published in March 2017; it showed that the approach was safe and could be an effective treatment option for sickle cell anemia.

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