- The ARC - California Edition -

Gene Therapy and Mental Retardation

The US Human Genome Project officially began October 1, 1991, and is part of an international effort to determine the location and structure of the entire set of human genes. Many disorders associated with mental retardation have a genetic cause, with Down syndrome and fragile X syndrome being the most common.

Specific genes have already been discovered for a number of single gene conditions, including fragile X syndrome, for which a treatment may be a possibility in the future. No one has yet been "cured" of a genetic condition by gene therapy, although it is considered to be a promising tool for treating single gene defects causing mental retardation.

The following materials have been abstracted from a report of The Arc’s Human Genome Education Project (Volume 2, Number 3) and are included to assist our readers in becoming informed about gene therapy ethical issues, in order to express their views and personal experiences to policy-makers.


What Is Gene Therapy?

Gene therapy is an experimental treatment in which normal genes are introduced into the body’s cells in order to correct or modify the function of the cell which is not working. The initial purpose of gene therapy was to treat hereditary diseases by adding good genes to function for missing or defective genes. Researchers soon discovered that genes could also be introduced into cells to help patients fight disease.

There are two kinds of gene therapy. Somatic cell gene therapy is directed at treating the somatic cells (the non-sex cells) which are all the body cells except the reproductive cells. Somatic cell gene therapy is generally viewed as a conventional medical therapy that is not a major departure from established medical practice. It would result in correcting a genetic defect only in the non-sex cells of the person being treated.

In contrast, germ line gene therapy would treat cells of the reproductive tissues of a patient, the germ or sex cells, in such a way that the disorder would also be corrected in any offspring of the individual. (The sex cells are called germ cells and include the egg and sperm.) Because future generations are affected, germ line gene therapy is the focus of concern and discussion regarding whether it is an ethical approach to treating genetic disorders. The federal government does not fund this type of gene therapy at this time.

Status of Gene Therapy

More than 250 research trials are under way using somatic cell gene therapy. Many studies have involved treatment of cancer; others involve inherited disorders like cystic fibrosis. While researchers are learning a lot about how to conduct gene therapy, there has not yet been a cure for any of the conditions being treated.
Most studies have only involved a small number of patients to test the possibilities of the transfer of genes into body cells and to evaluate the effects of this treatment.

Members of a National Institutes of Health panel that investigated the status of research on gene therapy believe gene therapy holds great promise. However, the panel noted the public’s mistaken and widespread perception that gene therapy is further developed and more successful than it actually is.

The panel expressed concern that patients, their families and health providers may make unwise decisions regarding treatment alternatives, believing cures are "just around the corner". Scientists disagree on how many years of development are needed before gene therapy becomes a proven treatment, but most of them in the field believe gene therapy is on track to be the "most powerful curative and diagnostic tool ever".

Basic Gene Therapy Techniques

In gene therapy, healthy copies of genes are given to someone whose own versions are faulty or missing. There are two basic methods for doing this. The ex vivo method removes selected cells from the patient’s body, introduces the normal copy of the defective gene (therapeutic gene) into these cells and then returns the cells to the body. The in vivo method inserts the therapeutic genes directly into the targeted body cells generally using a type of virus as a carrier.

There are many challenges to accomplishing gene therapy. One hurdle is to get the therapeutic genes into cells without causing harm to the body. Researchers also need to control the specific types of cells a therapeutic gene enters. For example, if the condition affects liver function, the therapeutic genes will be targeted to reach the liver cells. Once the gene is inside the cell, scientists would like to be able to control the level of activity needed to correct the problem. This means the new gene must function normally in the cell.

Can Gene Therapy Be Used to Treat or Cure Mental Retardation?

At this time, the most promising use of gene therapy is to treat single gene defects causing significant disability that cannot be treated satisfactorily by other approaches. Gene therapy studies to test the safety and the effects of the treatment have begun in a number of genetic disorders causing mental retardation, including phenylketonuria (PKU), Gaucher Disease, Lesch-Nyhan syndrome, urea cycle disorders and Duchenne muscular dystrophy.

The current approaches to gene therapy cannot be used to treat the most common chromosomal abnormality that causes Down syndrome. Scientists have not yet identified the specific number and location of genes involved. The gene causing fragile X syndrome, another common genetic cause of mental retardation, has been identified. At some time in the future, it may become possible to apply gene therapy to fragile X. However, much more knowledge is needed about the function and control of the fragile X gene before gene therapy experiments are undertaken.

GENETIC ISSUES IN MENTAL RETARDATION is a series of reports which have been published by The Arc’s Human Genome Education project.

Volume 1-1 Introduction to Genetics / Mental Retardation
Volume 1-2 Genetic Discrimination
Volume 1-3 Genetic Testing, Screening & Counseling
Volume 2-1 Protecting Genetic Privacy
Volume 2-2 Fragile X Syndrome
Volume 2-3 Gene Therapy and Mental Retardation

Single copies of these reports are free. Send a self addressed envelop with sufficient postage to:

The Arc - National Headquarters
P.O. Box 1047
Arlington, TX, 76004