Gene therapy carries the promise of cures for many diseases and for types of medical treatment most of us would not have thought 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.

About Genes

Our genes are part of what makes us unique. Inherited from our parents, they go far in determining 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:

1. 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.
2. 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 gene therapy experts. Kids with an altered gene may have mild or severe effects and the severity often can't be determined in infants. So just because some kids appear to have a genetic problem doesn't mean they'll be substantially 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 hopes are high for its role in curing and preventing childhood diseases. One day it may be possible to treat an unborn child for a genetic disease even before symptoms appear.

Scientists hope that the human genome mapping will help lead to cures for many diseases and that successful clinical trials will create new opportunities. For now, however, it's a wait-and-see situation, calling for cautious optimism.

Reviewed by: Louis E. Bartoshesky, MD, MPH
Date reviewed: June 2010
Originally reviewed by: Linda Nicholson, MS, MC