In the most successful demonstration of the potential of gene therapy as a treatment for complex hereditary diseases, the approach has been used to treat a blood disorder called thalassemia that occurs in thousands of people worldwide.
Gene therapy has suffered some well-publicized ups and downs over the past decade, but the past few years have proven it to be a viable–and powerful–option for treating genetic disease. Until recently, however, the most successful attempts have addressed serious disorders that are also relatively rare.
Thalassemia is a genetic disease in which the body doesn’t produce enough hemoglobin, the iron-containing protein in red blood cells responsible for ferrying oxygen throughout the body. In the most serious cases, people require bone marrow transplants (if a match can be found) or frequent blood transfusions just to stay alive. Three years ago, researchers in Paris used gene therapy to treat an 18-year-old patient with a more severe form of the disease. Now, in research published yesterday in the journal Nature, the team reports that this patient has remained relatively healthy and transfusion-free for 21 months.
Gene therapy works on the theory that a genetic disorder can be treated by replacing the gene that causes disease with a “corrected” version. For the current study, scientists removed bone marrow stem cells from a patient with a form of thalassemia called beta-thalassemia, cultured the cells, and used a lentivirus to insert a healthy, working version of the gene. (Lentiviruses, like HIV, are a subtype of retrovirus that work well for gene therapy–they can insert themselves into the genome of both dividing and nondividing cells, correcting as many cultured cells as possible.)
The researchers, led by Philippe Leboulch, a geneticist at Brigham and Women’s Hospital in Boston, then gave the patient a dose of chemotherapy to kill off his original population of bone marrow stem cells and injected the corrected ones back in. The patient’s revised stem cells repopulated his bone marrow, and the revised versions now make up about 10 to 15 percent. Levels of globin (the oxygen-bearing part in hemoglobin) are about two-thirds the normal level–in other words, the patient is mildly anemic–but his condition is not life-threatening.
“We are three years after the treatment, and for 21 months he hasn’t received any transfusions,” Leboulch says.
The concept of gene therapy has excited great expectations over the past few decades, but public enthusiasm has been dampened by a few widely publicized failures. One of the early human trials, in 1999, resulted in a teen’s death and prompted a rapid evaluation of the underlying technology. In another study, four of the 10 children treated with gene therapy for severe combined immunodeficiency disease (SCID, or “bubble boy” syndrome) developed leukemia; as it turned out, the corrected gene was inserted too close to a cancer-causing gene, which it activated.