What is gene therapy? Gene therapy is a new approach to treat, cure, and ultimately
prevent disease by changing the expression of a person's genes.
Gene therapy is in its infancy, and primarily experimental, with
most human clinical trials only in the research stages. It is
being studied for many different types of cancer and some other
diseases.
What are the objectives of gene therapy?
One of the goals of gene therapy is to produce cells with healthy
copies of missing or altered genes. Instead of giving a patient a
drug, doctors attempting to correct the problem by altering the
genetic makeup of some of the patient's cells. Examples of
diseases that could be treated in this way include cystic fibrosis
and haemophilia.
Gene therapy is also being studied as a way to change how a cell
functions; for example, by stimulating immune system cells to
attack cancer cells or by introducing resistance to human
immunodeficiency virus (HIV), the virus that causes AIDS.
How are genes transferred into cells so that gene therapy can take
place?
In general, a gene cannot be directly inserted into a person's
cell. It must be delivered to the cell using a carrier known as a
"vector”. The most common types of vectors used in gene therapy
are viruses. Scientists use viruses because they have a unique
ability to enter a cell's DNA. Viruses used as vectors in gene
therapy are genetically disabled; they are unable to reproduce
themselves.
What are the basic steps involved in gene therapy?
In most gene therapy clinical trials, cells from the patient's
blood or bone marrow are removed and grown in the laboratory. The
cells are exposed to the virus that is carrying the desired gene.
The virus enters the cells, and the desired gene becomes part of
the cells' DNA. The cells grow in the laboratory and are then
returned to the patient by injection into a vein. This type of
gene therapy is called ex vivo, which means "outside the body”.
The gene is transferred into the patient's cells during the time
the cells are outside the patient's body.
In other studies, vectors or liposomes (fat particles) are used to
deliver the desired gene to cells in the patient's body. This form
of gene therapy is called in vivo, because the gene is transferred
to cells inside the patient's body.
What risks are associated with gene therapy?
Viruses can usually infect more than one type of cell. Thus, when
viral vectors are used to carry genes into the body, they might
alter more than the intended cells. Another danger is that the new
gene might be inserted in the wrong location in the DNA, possibly
causing cancer or other damage.
In addition, when DNA is injected directly into a tumour, or when
a liposome delivery system is used, there is a slight chance that
this DNA could unintentionally be introduced into reproductive
cells, producing inheritable changes. 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 inflammation or an immune reaction;
and that the virus could be transmitted from the patient to other
individuals or into the environment.
However, scientists use animal testing and other precautions to
identify and avoid these risks before any clinical trials are
conducted in humans.
What major problems must scientists overcome before gene therapy
becomes a common technique for treating disease?
Scientists need to identify easier and better ways to deliver
genes to the body. To treat cancer, AIDS, and other diseases
effectively with gene therapy, researchers must develop vectors
that can be injected directly into the patient. These vectors must
then focus on appropriate target cells (such as cancer cells)
throughout the body and successfully integrate the desired gene
into the DNA of these cells.
Other advances that are needed include the ability to deliver
genes consistently to a precise location in the patient's DNA
(thus diminishing the risk of causing cancer), and ensure that
transplanted genes are precisely controlled by the body's normal
physiologic signals.
What are some of the social and ethical issues surrounding human
gene therapy?
Gene therapy is currently focused on correcting genetic flaws and
curing life-threatening disease, and regulations are in place for
conducting these types of studies. But in the future, when the
techniques of gene therapy have become simpler and more
accessible, society will need to deal with more complex ethical
questions.
One such question is related to the possibility of genetically
altering human eggs or sperm, the reproductive cells that pass
genes on to future generations. Since reproductive cells are also
called germ cells, this type of gene therapy is referred to as
germ-line therapy. Another question is related to the potential
for enhancing human capabilities—for example, improving memory and
intelligence—by genetic intervention. Although both germ-line gene
therapy and genetic enhancement have the potential to produce
benefits, possible problems with these procedures worry many
scientists.
Germ-line gene therapy would forever change the genetic make-up of
an individual's descendants. Thus, the human gene pool would be
permanently affected. Although these changes would presumably be
for the better, an error in technology or judgment could have
far-reaching consequences. In the case of genetic enhancement,
there is anxiety that such manipulation could become a luxury
available only to the rich and powerful. Some also fear that
widespread use of this technology could lead to new definitions of
"normal" that would exclude individuals who are, for example, of
merely average intelligence. And, justly or not, some people
associate all genetic manipulation with past abuses of the concept
of "eugenics," or the study of methods of improving genetic
qualities through selective breeding.
Trial Begins for HIV Gene Therapy
Clinical development of
gene therapy for colorectal cancer
Viruses can usually
infect more than one type of cell
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