4. Mode of administration
The vector containing the therapeutic gene can be delivered into the body using a number of methods such as injection into a vein (intravenous), injection to the target site or inhalation. Viral vectors will insert their DNA cargo into cells and non-viral vectors will release DNA into cells by penetrating cell membranes.
An alternative way of using gene therapy vectors is to design them to be used in cells that have been taken from the patient. This approach offers the advantage of being able to test that the gene therapy works by looking for expression of the normal protein, before implanting the cells with the corrected gene into the patient.
Gene therapy that targets cells outside of the body is often used to modify blood-derived stem cells (haematopoietic stem cells) because these are easily extracted from the body and transplanted back into the patient. The advantage of modifying a stem cell population is that the corrected cells, once in the body, will integrate into their niche (e.g. blood stem cells will integrate into the bone marrow) where they will continue to multiply and repopulate the body with corrected, functioning cells. This approach has been used in clinical trials aiming to treat X chromosome-linked severe combined immune deficiency and Fanconi anaemia (blood abnormalities and bone marrow failure).