Frequently Asked Questions

1. What are genes?

Genes are the biological units of heredity. They determine obvious traits, such as hair and eye color, as well as more subtle characteristics, such as the oxygen-carrying ability of the blood. Complex traits, such as IQ and physical strength, may be shaped by the interaction of a number of different genes along with environmental influences.

It is estimated that humans have 100,000 genes. A flaw in virtually any one of them can result in disease. Each gene acts as a blueprint for making a specific enzyme or other protein. However, only certain genes in a cell are active at any given moment and, as cells mature, many of their genes become permanently inactive. It is the pattern of active and inactive genes in a cell and its resulting protein composition that determines what kind of cell it is and what it can and cannot do.

2. What is DNA, and how is it related to genes?

In chemical terms, genes are composed of segments of deoxyribonucleic acid, or DNA. DNA is a very long molecule, composed of individual units called nucleotides. Each nucleotide contains phosphate, the sugar deoxyribose, and one of four nucleic acid bases: adenine, thymine, guanine, or cytosine. It is these bases that carry the information content, or "code," of the DNA molecule.

There are about 3 billion pairs of nucleotides in the DNA of a typical human cell. Each individual's genetic material has a unique nucleotide sequence (except in the case of identical twins), although a large percentage of the genome (total set of genes) of every human is identical to that of every other.

DNA is organized in two chains that form a double helix. Wherever adenine appears on one chain, it is matched (and physically linked) with thymine on the other chain. Similarly, guanine is matched with cytosine. This consistent pairing of complementary bases allows DNA to duplicate itself accurately when cells divide.

3. What is RNA?

Ribonucleic acid, or RNA, is a molecule that is chemically similar to DNA and carries the same code, using different bases. When DNA is biochemically "read," or transcribed, the transcription product is RNA. This RNA is read in turn, and converted into a corresponding protein.

In certain viruses, RNA, rather than DNA, serves as the genetic material.

4. How are genes related to chromosomes and cells?

The genes are arranged on chromosomes -- rod-like structures composed of DNA and protein. In humans, each cell -- the basic unit of living organisms -- contains 46 chromosomes (23 pairs), located within a central structure known as the nucleus.

5. Why are the viruses used in gene therapy referred to as "vectors"?

One meaning of the word "vector" is "carrier." In the field of infectious diseases, the term has been used to describe an agent, such as an insect, that carries an infectious organism from one individual to another. By analogy, the genetically disabled viruses used in gene therapy are referred to as vectors because they carry genes to cells. Most often, these vectors are derived from mouse retroviruses.

6. How could gene therapy be used to treat cancer?

Scientists are working on ways to genetically alter immune cells that are naturally or deliberately targeted to cancers. They are interested in arming such cells with cancer-fighting genes and returning them to the body, where they could more forcefully attack the cancer. Clinical trials along these lines are in progress for the treatment of melanoma.

Alternatively, cancer cells can be taken from the body and altered genetically so that they elicit a strong immune response. These cells can then be returned to the body in the hope that they will act as a cancer vaccine. A variety of clinical trials using this approach are now under way.

It is also possible to inject a tumor with a gene that renders the tumor cells vulnerable to an antibiotic or other drug. Subsequent treatment with the drug should kill only the cells that contain the foreign gene. Since other cells would be spared, the treatment should have few side effects. Two trials using this approach are in progress for treatment of brain tumors.

7. How could gene therapy be used to treat AIDS?

Gene therapy could be used to make immune cells resistant to HIV (the AIDS virus). It could also be used to help patients destroy HIV and HIV-infected cells by increasing the body's immune response to these elements.

Results from the ADA trial support the idea that genetically altered lymphocytes or stem cells might help prevent immune system failure in AIDS patients. T-lymphocytes enhanced with genes that block the spread of HIV could be tested in humans soon.

8. AIDS and other diseases are caused by retroviruses. What are these viruses, and how can they be safely used in gene therapy?

Retroviruses are a class of viruses whose genetic material is RNA rather than DNA, and which produce a unique enzyme known as reverse transcriptase. Because retroviruses make this enzyme, they can transform their RNA into DNA, which can be permanently integrated into the DNA of the host cells.

Most gene therapy experiments rely on disabled mouse retroviruses to deliver the healthy gene. These viruses normally carry their genetic information into cells and integrate it into the cells' own genetic material. Thus, they serve as efficient tools for transferring genes.

Scientists use a number of techniques to modify retroviruses for safe use in gene therapy. Generally, they remove crucial retroviral genes so that the virus cannot reproduce after it delivers its genetic cargo. They may also give the retrovirus a new gene that makes the cells it infects susceptible to an antibiotic, so that these cells could easily be destroyed if they became cancerous or the virus delivered genes to the wrong cells.

9. How are specific genes delivered to particular cells?

In a typical scenario, scientists select a retrovirus that normally infects cells of the desired type. The gene chosen for transfer is snipped out of a source cell's DNA using so-called "restriction enzymes" that cut DNA at specific ("restricted") locations. This gene and a marker, or selector, gene (e.g., a gene that confers resistance to the antibiotic neomycin) are inserted together into the retrovirus vector with the help of other enzymes. The altered retrovirus is then permitted to infect the target cells, integrating the new genes with the cells' own genes (transduction). By exploiting properties of the selector gene (e.g., exposing the target cells to neomycin), it is possible to select out those cells that have successfully integrated the new genes.

10. What risks are associated with current gene therapy trials in humans?

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, when DNA is directly injected into a tumor, or when a liposome delivery system is used, there is a slight chance that foreign genes could unintentionally be introduced into germ cells -- sperm or eggs -- producing heritable changes, although this has not occurred in animal tests.

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.

However, scientists use animal testing and other precautions to assess and avoid these risks. These safety measures have been successful to the extent that these potential problems have not occurred in any of the human gene therapy trials performed to date.

11. What is the process by which gene therapy experiments receive approval?

A proposed experiment, or protocol, must pass through at least two review boards at the scientists' institution and must be approved by that institution. The protocol must then be approved by the Recombinant DNA Advisory Committee (RAC) of the National Institutes of Health (NIH) and be signed by the NIH director. All protocols must also receive the approval of the U.S. Food and Drug Administration.

12. Why are there so many steps in this process?

Any experimentation in humans must be approached with great care. Gene therapy in particular is a very powerful technique, and it is relatively new. These factors make it necessary for scientists to take special precautions with gene therapy until they have gained more experience with this new technology.

13. What scientific developments led up to gene therapy?

One crucial step in the evolution of gene therapy occurred in the early 1950s, when James Watson and Francis Crick elucidated the structure of DNA. Other major steps included the cracking of the genetic code in the 1960s and the discovery in the 1970s of restriction enzymes, which enabled researchers to isolate specific genes from DNA and to begin to develop recombinant DNA (gene-splicing) technology. During the 1980s, gene transfer systems using retroviruses and lymphocytes were first developed. Today, improved gene delivery systems are taking shape -- a development expected to move the field far ahead.

14. What major problems must scientists overcome before gene therapy becomes a common technique for treating disease?

As mentioned earlier, scientists need to learn how to isolate and insert curative genes into stem, or progenitor, blood cells, so they can treat immunologic and blood disorders.

Scientists also need to find easier and better ways of delivering genes to the body. To treat cancer, AIDS, and other diseases effectively with gene therapy, they need to develop vectors that can be injected directly into the patient. These vectors must then home in on appropriate target cells (e.g., cancer cells) throughout the body and successfully integrate the desired gene into the DNA of these cells.

New vectors are currently being tested. These include adenoviruses, which, unlike retroviruses, can transfer genetic material to nondividing cells, such as those found in the lungs; and liposomes, or fat droplets, which can adhere to some cells, including tumor cells, and insert genes into these cells.

Two other advances are needed: one or more ways to deliver genes consistently to a precise location in the patient's genetic material (thus diminishing the risk of inducing cancer during gene transfer), and the ability to ensure that transplanted genes are precisely regulated by the body's normal physiologic signals. Insulin is just one example of a protein that must be produced in the right amounts at the right times if it is to help rather than harm the patient.

Although scientists are working hard on these problems, it is impossible to predict when the various obstacles will be overcome.

15. What is the difference between germ cells and somatic cells, and is germ-line gene therapy an option in the foreseeable future?

Somatic cells are the nonreproductive cells of the body, such as those of the skin and brain. Germ cells are the reproductive cells of the body -- the sperm and egg -- which pass genes along to future generations. Although scientists do not currently have the technology to safely alter human reproductive cells genetically, such therapy could become technically feasible in the future.

16. What would be the advantage of germ-line gene therapy?

In the case of hereditary illness, successful germ-line gene therapy would have the potential to eliminate a genetic defect, such as hemophilia, from an entire family line with a single procedure.

17.How long will it be until gene therapy is widely available?

18.If I had gene therapy someday,would it affect my future children?

19.Does gene therapu go "too far"?Does it interfere with nature?

Gene therapy is similar to many other medical interventions to treat disease, from vaccines to surgery, and is an exciting, advanced way to treat a disease at its cause. It does not involve making changes in the crucial sperm or egg cells that affect a patient's offspring.

20.Can gene therapy help with other,non-genetic diseases?

Yes. Many common threats to our health, including hypertension, heart disease, Alzheimer's disease and juvenile diabetes, actually have genetic components. People often have a genetic tendency toward a disease (such as when several family members have cancer). Someday, people in affected families may be able to use gene therapy to reduce their risks of developing a disease.

Gene therapy may be the best weapon medicine has ever brought to the fight against birth defects and many other health problems. As research moves forward, the March of Dimes foresees a day in which gene therapy could prevent or ease hundreds of devastating conditions, including types of mental retardation, congenital heart disorders, blood diseases and many others. It may help people in your own family to overcome problems that limit their lives today.