The concept of gene therapy seems straightforward, but this is clearly an oversimplification, and numerous problems and risks exist that prevent gene therapy using viral vectors. Viruses can usually infect more than one type of cell. Thus, when viral vectors are used to carry genes into the body, they might infect healthy cells as well as cancer cells.

Another danger is that the new gene might be inserted in the wrong location in the DNA, possibly causing harmful mutations to the DNA or even cancer. This has occurred in clinical trials for X-linked severe combined immunodeficiency (X-SCID) patients, in which hematopoietic stem cells were transduced with a corrective transgene using a retrovirus, and this led to the development of T cell leukemia in 4 of 20 patients. See reports for first patient, second patient and third patient.

In addition, when viruses are used to deliver DNA to cells inside the patient’s body, there is a slight chance that this DNA could unintentionally be introduced into the patient’s reproductive cells. If this happens, it could produce changes that may be passed on if a patient has children after treatment.

Other concerns include the possibility that transferred genes could be overexpressed, producing so much of the missing protein as to be harmful; that the viral vector could cause an immune reaction; and that the virus could be transmitted from the patient to other individuals or into the environment.

However, this basic mode of gene introduction currently shows much promise and doctors and scientists are working hard to fix any potential problems that could exist. They use animal testing and other precautions to identify and avoid these risks before any clinical trials are conducted in humans.