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One of the most amazing genetic applications in
medicine is gene therapy. Also known as somatic
gene therapy and therapeutic gene therapy, this
procedure involves inserting (or sometimes deleting)
portions of the genes in diseased patients so that
they can be cured and live healthier lives.
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Thermal cyclers (amplify DNA via PCR)
Preparing amplified DNA for purification
A fluorescent automated DNA sequencer
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Two methods exist for inserting
genetic material into human chromosomes. The
first, called the ex vivo technique, involves
surgically removing cells from the affected
tissue area, injecting or splicing the new DNA
(the DNA that will correct the disease) into
the cells and letting them divide in cultures.
The new tissues are placed back into the affected
area of the patient. Often, doctors need only
culture the patient’s bone marrow because it
produces the blood that will eventually travel
throughout the body. This type of surgery, however,
is especially painful, and patients usually
have to undergo it twice--once to extract the
marrow and then again to replace it--because
the culturing time takes many hours to complete.
The second method is called the in vivo technique
and requires no surgery or even anesthesia.
In this process, the therapeutic DNA is injected
directly into the body cells, usually via one
of two types of viruses. The most frequently
used type is the very simple retrovirus. Dr.
Richard Mulligan of MIT has synthetically created
the perfect retrovirus: it has no reproduction
sequence and exists solely to deliver therapeutic
DNA during gene therapy. It has no viral DNA
(DNA that would make the cell--and you-- sick)
whatsoever and only carries the new DNA that
has been spliced into it. After injecting the
diseased cell with the new therapeutic DNA,
it then dies. Using retroviruses is very safe
and provides long-lasting effects. Unfortunately,
the new DNA it injects will only help the new
daughter cells and not those that already exist.
The second type of virus used for the in vivo
technique is called an adenovirus, the equivalent
of the common cold virus. Although this virus
will also die after injecting its spliced therapeutic
DNA, it will be attacked by the immune system
and the patient will suffer from a temporary
sore throat and runny nose. The adenovirus works
the same way the retrovirus does, but its effects
are much more immediate--within 48 hours. Unlike
the retrovirus, though, the new DNA’s effects
wear off within weeks. Scientists like the fact
that only a few millimeters of altered adenovirus
solution is needed to cure the patient, whereas
several liters of retrovirus are needed to obtain
a much slower result. |
There are other gene therapy techniques, although
they aren’t as frequently used. One method involves
inserting therapeutic DNA into cultured endothelium
tissue (endothelium is the membrane that lines all
of the blood vessels) and then grafting it into
the patient. Another technique requires the patient
to receive an electric shock while submerged in
a bath of a therapeutic DNA solution. The shock
opens the skin pores, allowing the DNA to enter.
Still other options include skin grafts, connective
tissue grafts, and injecting the liver with the
therapeutic DNA.
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