Gene Testing

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In the Spotlight

March 10, 2000

Gene Testing

By Lee Phillips M.D.
Personal MD.com Advisory Board

There has been a tremendous expansion in the understanding of the genetic basis of disease. Tests for gene mutations make it possible not only to detect diseases already in progress but also determine one's risk of disease in the future. Gene testing holds out the possibility of saving thousands of lives through prevention or early detection.

Gene mutations and disease

Many, if not most, diseases have their roots in our genes. More than 4,000 diseases stem from altered genes inherited from one's mother and/or father. Hereditary mutations are carried in the DNA of the reproductive cells. When reproductive cells containing mutations combine to produce offspring, the mutation will be in all of the offspring's body cells.

Acquired mutations are changes in DNA that develop throughout a person's lifetime. Common disorders such as heart disease and most cancers arise from a complex interplay among multiple genes and between genes and factors in the environment.

Gene tests

Gene testing involves examining a person's DNA - taken from cells in a sample of blood, or occasionally, from other body fluids or tissues - for some anomaly that flags a disease or disorder. Testing individuals to determine the presence of genetic defects falls into four basic categories:

Newborn screening is widely used to detect and possibly treat serious genetic diseases. Phenylketonuria (PKU), for example, a genetic disease for which a test is available, and a special diet can prevent most of the serious effects of the disease.
Carrier testing lets couples learn if they carry and risk passing to their children a gene or a chromosome abnormality. Carrier testing is usually requested by adults who have some indication that they may be carriers of a genetically-linked disorder (e.g., because they are members of an ethnic group known to have a high incidence of the disorder, because a relative has a genetic disease, or because the other spouse knows that he or she is a carrier).
Prenatal testing is aimed at detecting the presence of genetic or chromosomal abnormalities in fetuses. During pregnancy the genetic makeup of the fetus is examined through amniocentesis, chorionic villi sampling, blood sampling from the umbilical cord, and blood samples from the mother.
This is often done when either parent's side of the family has a history of an inherited disease, or if the mother is over age 34.
Predictive gene testing determines the probability that a person will develop a genetically-linked disease at some point in the future.

Genetic tests find mutations, not disease

An accurate gene test will tell you if you do or do not have a disease-related gene mutation. An accurate gene test can tell if a mutation is present, but that does not guarantee that disease will develop. A variety of factors can influence the gene's penetrance and the chances that you will actually develop disease.

Some mutations detected by a positive test may never lead to disease. Furthermore, because existing tests look only for the more common mutations in a gene, some disease-causing mutations may escape detection.

In cystic fibrosis, for example, the gene that controls mucus production can have more than 300 different mutations; some cause severe symptoms; some, mild symptoms; and some, no symptoms at all. And some disorders that "run in families" can be traced to shared environmental exposures rather than any inherited susceptibility.

Personal decision

The decision to undergo testing is a very personal one, and it should be totally voluntary. Persons considering genetic testing should meet with genetic counselors before testing begins, when they receive the test results, and in the weeks and months afterwards.

Before testing, the counselors try to make sure that the person is psychologically prepared to cope with the possibility of a positive test, and that he or she has enough balanced information to be able to formulate a truly informed consent.

If the person decides to proceed with testing, counselors help the individual and the family adjust to the test results, and they help them arrange whatever prevention and screening measures are appropriate. After receiving genetic counseling, a person should agree to the test only if she or he wants the information, not to accommodate relatives, or anyone else.

Potential risks of genetic testing have to do with the way the results of the test might change a person's life. First, there are the emotions aroused by learning that one is likely to develop a serious disease. Someone identified as carrying the gene may feel anger, while one who has escaped may be overwhelmed by guilt for avoiding a disease that afflicts a close relative.

People considering gene tests may want to find out how their relatives would feel about knowing whether or not they have a disease gene or allowing the information to be given to others. Perhaps the most serious limitation of gene testing is that test information is not matched by definitive treatment. To receive positive test results when there is no adequate treatment can be tragic.

Gene testing offers several benefits. A negative result can create a tremendous sense of relief and may eliminate the need for frequent checkups and tests that are routine in families with a high risk of cancer.

Even a positive result can relieve uncertainty and allow a person to make informed decisions about the future. Someone who tests positive for a cancer susceptibility gene may opt for preventive or therapeutic measures.

Because our genes hold a wealth of information about us, and indirectly about our relatives, and future generations, confidentiality is a major concern. The concern is that test results might someday be used against a person. Some people have been denied health insurance, some have lost jobs or promotions, and some have been turned down for adoptions because of their gene status.

Clinical test results are normally included in a person's medical records. Might a genetic flaw constitute a "pre-existing condition" that would be excluded from insurance coverage? Even if gene testing information could be kept out of the medical record, a person's need for more frequent medical checkups, for example, could provide a tip-off to susceptibility.

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