5. Example - Gene therapy for X-linked severe combined immunodeficiency
Gene therapy has been used successfully to treat X-linked severe combined immunodeficiency (SCID-X1), an inherited disease in which the body produces very few immune cells. Therefore, patients are prone to multiple infections, which can be fatal. It is caused by a mutation in a gene located on the X chromosome, IL2RG, which is why it is called an X-linked disease.
Some of the first successful human gene therapy trials were conducted in patients with SCID-X1, which allowed participants to live relatively normal lives. In one trial, individuals were given gene therapy to introduce a functional version of the IL2RG gene into their bone marrow cells. Bone marrow cells were harvested from patients, then modified outside of the body and infused back into the patients. After 10 years, seven out of nine patients had functional immune systems. During the study, one patient died and four developed acute leukaemia, although all four recovered. An increased risk of leukaemia has been observed in other similar trials and as previously discussed, has been attributed to insertional mutagenesis caused by the use of viral gene therapy vectors.