The idea behind most gene therapy is to either correct a genetic disease
by adding a new functional gene to
cells with a defective version the gene, or to add a gene that somehow alters
the target cells to provide a treatment or cure for a disease. For example, a
new gene could be introduced to fix a congenital genetic defect, or to label
some diseased cells, like cancer cells, so the immune system will attack them or make them more
susceptible to a subsequent treatment. The most efficient way to introduce a
new gene into a cell's DNA is to use organisms naturally designed for it.
Viruses have evolved to enter cells, splice a copy of their DNA into the
genome, then replicate it and make more viruses. For gene therapy, a patient's
cells are infected with a modified virus that cannot replicate but carries the
functional gene. Cells can be isolated from the patient, infected, the put back
into the patient (called an ex
vivo protocol), or the virus
can directly injected in the patient to infect the target cells (called an in vivo protocol). With either approach, the
idea is that certain affected cells, after infection, continue on as they
normally would but with a working version of the gene.
Vectors to Carry Genes into Cells
In animal models, researchers use several
types of viral vectors, as well as other non-viral approaches to get DNA into
cells. However, many of these are not suitable for human gene therapy due to
safety concerns. For human gene therapy, DNA is introduced into cells mostly
using highly modified DNA from natural viruses, although other chemical
approaches can sometimes be used too. Obviously, vectors made from viruses need
to be extensively altered before they can safely be used for gene therapy. For
example, the components that enable a virus to replicate itself and affect the
function of other genes in cells, need to be removed. Only very well understood
viruses can be modified appropriately, and then, the vectors derived from these
modifications must be extensively tested and evaluated. DNA vectors made from
viruses only contain enough of the viral DNA to enable them to enter a cell and
set up a gene. The parts that enable a virus to control the cell make copies of
itself or coat itself in proteins so it can infect other cells are deleted.
Most human gene therapy studies use DNA vectors derived from one of three virus
types: retrovirus, adenoviruses, and adeno-associated viruses.
Adenovirus Vectors for Temporary Fixes
Vectors made from adenoviruses do not become part of a cell's core
DNA. As with chemical methods to introduce DNA into cells, genes introduced to
cells using adenovirus vectors do not integrate into the chromosomes and become
part of the genome. The DNA from an adenovirus is put in the same compartment
of the cell as the genomic DNA (i.e., the nucleus)
and the cell decodes the gene just like other genes in the genome. However, the
gene stays on a extra piece of adenovirus DNA and is eventually lost after the
cells divide several times. For temporary changes, for example, making specific
cancer cells more noticeable to the immune system so it will respond and kill
them, adenovirus vectors offer a safer option than ones like retroviruses that
insert a gene directly into genomic DNA. For this reason, the adenovirus vector
can be used for protocols where the viral vector is injected directly into the
patient. However, a permanent cure to a genetic disease requires a different
sort of vector and different approach.
True Genome Changes with DNA Integration
Unlike adenovirus vectors, DNA vectors made
from retroviruses insert a new gene right into the
genome. It becomes part of the core cellular DNA in the chromosomes and is
copied and maintained in daughter cells when the cell divides. The cell and its
descendants are permanently altered. For this reason, when these
genome-integrating vectors are employed, an ex
vivo protocol used, which
means cells are extracted from a patient, infected in a cell culture plate in
the lab, then put back in the patient later. Since a retroviral vector places a
gene virtually anywhere in the genome, one safety risk with retroviral vectors
involves disruption of native cellular genes at the insertion site. This sort
of disruption led to the development of leukemia in some patients in early
trials. However, making a permanent genetic change to cure a congenital disease
requires this type of vector. A lot of work has been done to reduce the risk of
retroviral vectors, much of it using vectors made fromlentiviruses, a
specific type of retrovirus.
Vectors made from adeno-associated viruses (AAV) can also be used to permanently
introduce DNA to a cell's genome. AAV almost always inserts at a specific spot
on chromosome 19 so it minimizes the risk of disrupting other genes. AAV
vectors, though, are somewhat difficult to produce and manipulate and they can
only be used to insert small genes. However, AAV vectors have been shown to be
very effective to treat
Leber congenital amaurosis a
congenital blindness caused by a defective retinal pigment gene (RPE65).
SOURCE
http://biotech.about.com/od/technicaltheory/a/Delivering-New-Genes-To-Cure-Disease.htm
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