Gene Therapy to Treat Disease - Biotechnology and Your Health: Pharmaceutical Applications - page 87

inserting their genetic information, normally instructions to make
more virus particles, into the cells that they infect. The gene therapy
viral vectors retain the ability to insert their own genetic informa-
tion into the cell's genetic information, but the instructions to make
more viruses have been removed and replaced with the therapeutic
gene, called the payload.
Most gene therapy trials have used RNA viruses as vectors.
RNA viruses, also called retroviruses , use RNA rather than DNA
as their genetic material. The RNA viral genome must be copied
into a DNA sequence before it is inserted into the cell's genetic
material. Although well suited for introducing genes into divid-
ing cells, such as blood-forming cells from the bone marrow,
the retroviral vectors are too fragile to be injected directly into
the body. Retroviral vectors are mixed with blood-forming cells
removed from the patient's bone marrow or blood. After the
vector payload has been delivered to the cells, the cells are re-
introduced into the patient's body through a vein in the same way
that a blood transfusion is given. Retroviral vectors are prepared
to receive the payload by removal of the genes the viruses use
to instruct an infected cell to produce more virus particles. Retro-
viral vectors are efficient at getting genes into the blood-forming
cells and result in the stable insertion of the payload genetic
material into the cells' genetic material. Every time the blood-
forming cell divides, the inserted gene is copied, and the gene
directs the formation of the desired protein. However, the inser-
tion of the new genetic material may change the cell's genetic
information in harmful ways (Figure 6.1).
Two types of DNA viruses have also been used in human gene
therapy trials. Adenoviruses , double-stranded DNA viruses, can
efficiently carry a larger gene into cells even if they are not dividing,
but the genetic information does not become inserted into the
cell's genetic material and the information may rapidly be lost.
Adeno-associated virus vectors, made from small single-stranded