Gene therapy carries the excitement of a cure-all for many
diseases and the promise of a type of medical treatment most of us
would never imagine possible. With its potential to eliminate and
prevent hereditary diseases such as
cystic fibrosis
and hemophilia and its use as a possible cure for heart disease,
AIDS
, and
cancer
, gene therapy is a potential medical miracle-worker.
But what about gene therapy for children? There's a fair
amount of risk involved in trials of this kind of therapy, and to
date, only kids who are seriously ill or have illnesses incurable
by conventional means have been involved in clinical trials using
gene therapy.
For those with serious illnesses that aren't responsive to
conventional therapies, however, gene therapy may soon offer hope
that didn't exist just a short time ago.
What Are Genes?
Our
genes
are part of what makes us unique. Inherited from our parents, they
determine our physical traits - like the color of our eyes and the
color and texture of our hair. They also determine things like
whether babies will be male or female, the amount of oxygen
blood can carry, and IQ.
Genes are composed of strands of a molecule called DNA and are
located in single file within the chromosomes. The genetic message
is encoded by the building blocks of the DNA, which are called
nucleotides. Approximately 3 billion pairs of nucleotides are in
the chromosomes of a human cell, and each person's genetic
makeup has a unique sequence of nucleotides. This is mainly what
makes us different from one another.
Scientists believe that every human has about 25,000 genes per
cell. A mutation, or change, in any one of these genes can result
in a disease, physical disability, or shortened life span. These
mutations can be passed from one generation to another, inherited
just like a mother's red hair or a father's brown eyes.
Mutations also can occur spontaneously in some cases, without
having been passed on by a parent. With gene therapy, the treatment
or elimination of inherited diseases or physical conditions due to
these mutations could become a reality.
Gene therapy involves the manipulation of genes to fight or
prevent diseases. Put simply, it introduces a "good" gene
into a person who has a disease caused by a "bad"
gene.
Two Types of Gene Therapy
The two forms of gene therapy are:
-
Somatic gene therapy
involves introducing a "good" gene into targeted cells
to treat the patient - but not the patient's future children
because these genes do not get passed along to offspring. In
other words, even though some of the patient's genes may be
altered to treat a disease, it won't change the chance that
the disease will be passed on to the patient's children. This
is the more common form of gene therapy being done.
-
Germline gene therapy
involves modifying the genes in egg or sperm cells, which will
then pass any genetic changes to future generations. In
experimenting with this type of therapy, scientists injected
fragments of DNA into fertilized mouse eggs. The mice grew into
adults and their offspring had the new gene. Scientists found
that certain growth and fertility problems could be corrected
with this therapy, which led them to think that the same could be
true for humans. However, although it has potential for
preventing inherited disease, this type of therapy is
controversial and very little research is being done in this
area, both for technical and ethical reasons.
Possible Effects of Gene Therapy
Gene therapy is done only through clinical trials, which often
take years to complete. After new drugs or procedures are tested in
laboratories, clinical trials are conducted with human patients
under strictly controlled circumstances. Such trials usually last 2
to 4 years and go through several phases of research. In the United
States, the U.S. Food and Drug Administration (FDA) must then
approve the new therapy for the marketplace, which can take another
2 years.
The most active research being done in gene therapy for kids has
been for genetic disorders such as cystic fibrosis. Other gene
therapy trials involve children with severe immunodeficiencies,
such as adenosine deaminase (ADA) deficiency (a rare genetic
disease that makes kids prone to serious infection), and those with
familial hypercholesterolemia (extremely high levels of serum
cholesterol).
Gene therapy does have risks and limitations. The viruses and
other agents used to deliver the "good" genes can affect
more than the cells for which they're intended. If a gene is
added to DNA, it could be put in the wrong place, which could
potentially cause cancer or other damage.
Genes also can be "overexpressed," meaning they can
drive the production of so much of a protein that they can be
harmful. Another risk is that a virus introduced into one person
could be transmitted to others or into the environment.
Although the National Cancer Institute reports that such
problems have not occurred in any of the human gene therapy trials
to date, not everyone is excited about this new kind of
treatment.
Gene therapy trials in children present an ethical dilemma,
according to some experts. They say that children shouldn't be
tested to see if they're carriers of genetic diseases, because
at present, nothing can be done about it. And, just because
some kids have a genetic problem doesn't mean they'll be
affected by it, but they'll have to live with the
knowledge of that problem.
Kids could be tested for disorders if there is a medical
treatment or a lifestyle change that could be beneficial - or if
knowing they don't carry the gene reduces the medical
surveillance needed. For example, finding out a child doesn't
carry the gene for a disorder that runs in the family might mean
that he or she doesn't have to undergo yearly screenings or
other regular exams.
The Future of Gene Therapy
To cure genetic diseases, scientists must first determine which
gene or set of genes causes each disease. The Human Genome Project
and other international efforts have completed the initial work of
sequencing and mapping virtually all of the 25,000 to 35,000 genes
in the human cell. This research will provide new strategies to
diagnose, treat, cure, and possibly prevent human diseases.
Although this information will help scientists determine the
genetic basis of many diseases, it will be a long time before
diseases actually can be treated through gene therapy.
Gene therapy's potential to revolutionize medicine in the
future is exciting, and its expectations for curing and preventing
childhood diseases is encouraging. One day it may be possible to
treat an unborn child for a genetic disease even before symptoms
appear.
Scientists are hoping the mapping of the human genome will lead
the way toward cures for many diseases and that the successes of
current clinical trials will create new opportunities and
challenges. For now, however, it's a wait-and-see situation,
calling for cautious optimism.
Reviewed by:
Linda Nicholson, MS, MC
Date reviewed: October 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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