Predictive Genetic Testing: Do You Really Want To Know Your Future?

PRODUCERS: Kathy McAnally

New & Noteworthy, 2007
by Jennifer Jongsma

In 1998, when The DNA Files first visited the science and social implications of predictive testing in Predictive Genetic Tests: Do You Really Want to Know Your Future?, host John Hockenberry said that "A little knowledge is a dangerous thing." Perhaps the most important idea to take away from this program is that predictive genetic testing represents just a tiny bit of knowledge about an extremely complex subject: our genetic makeup. Scientists had hoped that the complete human genome would quickly lead us into an age of individualized medicine: Doctors could perform tests to assess genetic risks for common (and not-so-common) diseases; determine whether a patient was vulnerable to certain environmental factors, including cigarette smoke and air pollution; and determine what drugs would work best for a patient with the fewest harmful side effects. Although we haven't yet reached the age of individualized medicine, a growing number of predictive genetic tests can save lives and influence lifestyle decisions; however, the value of these tests varies because most have uncertain predictive value.

Many genetic tests are now available to assess genetic risks for single-gene disorders and multiple-gene disorders. Huntington's disease is a rare, fatal, single-gene disorder. Patients who inherit the gene for Huntington's disease will develop it, although the age of onset, degree and type of clinical symptoms, and the rate of progression are uncertain. Genetic testing for single-gene disorders is standard in newborns in many states. For example, children in all 50 states are tested for phenylketonuria shortly after birth. These children require a phenylalanine-poor diet in order to prevent mental retardation.

Most newborn screens test for conditions for which treatment is available. On the other hand, prenatal genetic tests often identify conditions without therapy or cure. When the results reveal a disorder or deformity, parents often choose to abort. Many people view this technology as dangerously close to eugenics, thus scientists, ethicists, religious leaders, and parents are debating where to draw the line. Is it ethical for parents to abort a child because the child will have Tay-Sachs disease or Down syndrome? Is abortion okay if the child is not going to be the "right" sex?

Multiple-gene disorders, where several mutated genes trigger the disease, are much more common. Examples include breast cancer, diabetes, and asthma. Testing for multiple-gene disorders is more problematic and in most cases can only provide information on a person's risk of developing a disease, not a definitive diagnosis. Some risks identified by genetic testing are high; for example, women who carry certain BRCA1 and BRCA2 mutations are thought to have a 60 percent to 80 percent chance of developing breast cancer in their lifetimes. However, most breast cancer is not hereditary or genetic. In the case of BRCA mutations and most other conditions, genes do not equal destiny. A combination of factors ultimately determines who will develop cancer, and so far, it remains unclear which ones are important.

Pharmacogenetic tests are a new form of predictive genetic tests that may see increased clinical application. These tests predict a person's response to a drug or class of drugs by testing for gene variants such as those that code for drug-metabolizing enzymes. These tests offer a means to reduce adverse reactions and increase the efficacy of drug treatment. For example, a variant in the enzyme thiopurinemethyltransferase (TPMT) gene increases the risk of life-threatening reactions to mercaptopurine, a drug used to treat acute childhood leukemia. When clinicians know the gene is present, they can adjust the dose.

Rapid advances in research have led to an ever-increasing assortment of genetic tests. They have also led to a growing public debate. Many opponents are concerned how the information gained from genetic tests will be used. They point to cases of discrimination by insurance companies, employers, and society as a result of genetic testing. They also express concern about providing information that offers no ability to act - as in a test for a disease for which there is no cure. Ultimately, individuals must decide whether they want to pursue genetic testing, and if they do, what to do with the information gained.

Original Program Description, 1998

If you were destined to live with a disabling disease, would you want to know now? Would you want to know, for example, that because your family carries a particular gene you are likely to contract Huntington's disease, an incurable condition that eventually devastates both mind and body? It's the kind of question more and more of us are having to face. This program offers an overview of predictive genetic testing. You'll learn which diseases it currently targets, the testing techniques, the reliability of those tests, and, most important of all, what is achievable once the results are obtained.

In only a very small percentage of diseases is the gene the ruler - the unequivocal dictator that determines the onset of the condition. Nonetheless, predictive genetic tests are becoming increasingly available. One company offers a test for the gene that identifies a possible predisposition for breast cancer. And researchers are coming up with ways to look for genes related to cardiovascular disease, asthma, osteoporosis, obesity, depression, stroke, and pulmonary disease. If scientists identify these genes, and companies come up with tests to identify them in you, what will you do with that information? Maybe you'll find it helpful in preparing for the likely debilitation of yourself or a family member. If you're lucky, some treatment will be available, or a lifestyle change might make a difference in your prognosis.

Often, however, there is absolutely nothing that can be done to treat the disease, and having the genetic information creates a tremendous psychological burden for entire families.

This program introduces us to people whose family history has compelled them to decide whether to undergo genetic testing. You'll hear, first-hand, how families have dealt with Huntington's disease and Alzheimer's. And you'll find out how tests for colon cancer and breast cancer are viewed by health care providers, by scientists, and by representatives of the companies that are busy developing genetic tests.

The issues are difficult - and pressing, as the pace of genetic research advances rapidly. In the near future, a predisposition for many more diseases will be identifiable through genetic testing. Will treatment and cure keep up with prediction? Do you really want to know your destiny?