A gene
can be transplanted into people curing them of severe diseases.
The treatment is called human gene therapy.
What
is gene therapy?
Instead
of giving a patient a drug to treat or control the symptoms of a
genetic disorder, gene therapy attempts to eradicate the disease
by healing the gene itself. Gene therapy does this by inserting
a functioning copy of the gene into DNA and augmenting the cell's
production of the lacking protein.
Examples
of some genetic diseases are cystic fibrosis, sickle cell anemia,
and hemophilia. Hemophilia is caused by the malfunctioning of the
gene that makes the factor that causes blood to clot. Inserting
normal genes with correct information into the DNA of the cells
that contain flawed genes, instead of repeatedly treating a patient
with hemophilia with clotting factor, would allow those cells to
make their own clotting factor.
It
seems likely that human gene therapy will also be used to combat
certain diseases that may not be genetic. For cancer patients who
are not helped by surgery or chemotherapy, researchers are now planning
to treat the patients' disease with genetically-altered white blood
cells that are targeted to cancers.
Gene
therapy could also be used as a drug delivery system. To accomplish
this, a gene that produces a useful product would be inserted into
the DNA of the patient's cells. For example, during blood vessel
surgery, a gene that makes an anticlotting factor could be inserted
into the DNA of cells lining blood vessels to help prevent dangerous
blood clots from forming.
| Basic
Genetics |
Human
cell:
Each of the 100 trillion cells in the human body (except
blood cells contains the entire human genome--all the
genetic information necessary to build a human being).
This information is encoded in 6 billion base pairs, subunits
of DNA. (Egg and sperm cells each have half this amount
of DNA.) |
Cell
nucleus:
Inside the cell nucleus, 6 feet of DNA are packaged into
23 pairs of chromosomes (one chromosome in each pair coming
from each parent). |
Chromosome:
Each of the 46 human chromosomes contains the DNA for
thousands of individual genes, the units of heredity. |
Gene:
Each gene is a segment of double-stranded DNA that holds
the recipe for making a specific molecule, usually a protein.
These recipes are spelled out in varying sequences of
the four chemical bases in DNA: adenine (A), thymine (T),
guanine (G), and cytosine (C). The bases form interlocking
pairs that can fit together in only one way: A pairs with
T; G pairs with C. |
Protein:
Proteins, which are made up of amino acids, are the body's
workhorses--essential components of all organs and chemical
activities. Their function depends on their shapes, which
are determined by the 50,000 to 100,000 genes in the cell
nucleus Source: The National Human Genome Research Institute
(NHGRI) |
|
How
does gene therapy work?
The
techniques for isolating human genes and making multiple copies
of them are well established. Now doctors are working on how to
insert those genes into cells and how to make those genes work properly
once inside the cells.
One
method for inserting genes into cells is to link the genes with
a virus that has been rendered harmless. As part of the modification,
such a virus, sometimes called a vector, has been deliberately altered
so that it can carry genes into cells but cannot then escape to
infect other cells.
After
the cells to be treated have been temporarily removed from a patient's
body, the virus or vector is used to carry the desired gene into
them. The final step will be to return the treated cells, which
now contain the correct genetic information, to the patient's body.
For example, bone marrow, liver cells, or white blood cells could
be removed from the body of a patient, treated in the laboratory,
and returned to the patient.
Whether
bone marrow cells or some other type of human cells were used, the
added genes would be inserted only into non-reproductive cells,
such as those of the skin and brain and not into reproductive (sperm
or egg) cells.
Therefore,
newly inserted genes could not be passed to future generations.
The therapy would be called somatic cell gene therapy and would
not attempt to affect the germ line cells, which carry genetic information
to the next generation.
What
are the risks associated with gene therapy?
Viruses
usually can 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. Also, whenever a gene is added to
DNA, there is the danger that the new gene could be inserted in
the wrong place, possibly causing cancer or other damage. Also,
there is a slight chance that foreign genes could unintentionally
be introduced into germ cells -- sperm or eggs -- producing changes
in future generations, although this has not occurred.
Other
worries 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, especially if administered repeatedly; and that
the virus could be transmitted from the patient to other individuals
or into the environment.
If
most diseases can be traced to an alteration in a gene, then gene
therapy could redefine the practice of medicine in the next century.
It should be a powerful tool for treating many of the more than
4,000 known genetic disorders, as well as heart disease, cancer,
arthritis, and other illnesses.
These
new treatments will add to the costs of health care. On the other
hand, it is possible that gene therapy will eventually be used to
prevent or cure diseases that now kill or disable millions of Americans.
If
so, it has the potential to revolutionize health care by enabling
more people to remain productive members of society and by eliminating
or reducing the need for costly medications and other treatments
that ameliorate symptoms but do not cure disease.
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