The Pros and Cons of Genetic Testing

By: David Fein, MD

“It’s tough to make predictions, especially about the future”
– Yogi Berra

We all want to find a crystal ball that can foretell our future, especially about our health. With the decoding of the human genome in 2003 expectations ran high that within a few years we would be able to pinpoint the genetic cause of many diseases. And it is true that medical research is increasingly finding associations between a wide range of disease and genetic mutations.

The complexity and cost of performing genetic testing has rapidly come down. Many genetic tests are now widely available at a reasonable cost. This has led to a number of companies offering genetic screening tests to the general public, often without the need for a physician prescription.

It’s an enticing idea to have a painless test that can potentially tell your health future. But before you get a genetic test there are some pitfalls to keep in mind.

Genetic “screening tests” are now are offered “direct to the consumer” by a variety of companies such as 23andMe and Navigenics. Genetic screening tests are not a complete sequencing of your DNA such as was done for the Human Genome Project. That is still far too costly and time consuming. Instead, these tests look for Single Nucleotide Polymorphisms (SNPs) which are pinpoint variations in a gene. These genetic variants can alter how a gene functions. The tests typically involve analyzing hundreds of thousands of SNPs that are believed to be associated with the risk of certain diseases or changes in how you may metabolize specific medications.

23andMe, for example, currently provides information on 162 gene-trait associations, including genes for disease risk, drug response and whether you are a carrier of inheritable diseases such as Tay-Sachs or breast cancer. According to their website, they divide the significance of the associations into “established research” and “preliminary research”. Established research associations are those that have been confirmed in at least two large studies or “have gained widespread scientific acceptance in the scientific community.” Preliminary research associations are those that “still need to be confirmed by the scientific community” and “may not stand the rigors of scientific replication.”

That is an important distinction to keep in mind. Many of those associations between an SNP and a particular disease may not be as clear-cut as we think. Data that “still need to be confirmed by the scientific community” could be a weak link for making major life decisions.

Even putting aside the concerns about the accuracy of the data interpretation, there is still the issue of what to do with your results.

Genetic testing can’t tell you with any certainty whether you will develop a disease or not. Your genes load the gun but it is your environment that pulls the trigger. Simply having a genetic predisposition to a disease, such as Type II Diabetes, does not mean you will actually become diabetic. That gene will interact with your weight, diet, level of physical activity and many other factors. Simply finding the presence of a genetic variant can’t tell us whether the gene ends up being expressed as a disease.

Not having the gene doesn’t mean you are safe, either. Even if you don’t have a gene for Type II Diabetes, it is likely that a rigorous effort at overeating and avoiding physical activity will get you there anyway.

You could argue that knowing you have the gene for Type II Diabetes might make you more careful about your lifestyle in hopes of avoiding the disease. If you are successful in making those changes then perhaps the test is useful.
On the other hand, the test results could lead you to feel that since you are more likely to get Diabetes at some point anyway, there is no point in depriving yourself now. In that case, the test results may actually increase your risk and become a self-fulfilling prophecy.

Using Type II Diabetes as an example has the advantage of there being the potential for you to do something that may change your outcome. Many other conditions included in genetic screening tests have no known effective means of prevention or add little to the advice you would get anyway.

There is likely to be very limited value in a test that tells you that the risk of Alzheimer’s Disease in the general population is 4% but your risk is 30%. That information will probably cause you significant long-term anxiety even though it is still more likely that you will not get the disease. And, the test does not tell you when it might strike. Every little normal moment of distraction or forgetfulness is likely to taken as the first sign of the disease. You may never develop Alzheimer’s Disease or it might not start until you are in your 80’s, but losing your car keys at 50 will have you shopping for a nursing home. Knowing you are at higher risk could push you to make decisions about your life that turn out to be unwarranted.

It is important to make a distinction between a genetic test ordered by your physician to look for a evidence of a specific condition versus the genetic screening tests. There is a growing list of very useful genetic tests for diagnostic and treatment decisions. Physicians are increasingly shifting to looking at genetic markers to diagnose cancers, metabolic disease, clotting disorders and many other conditions. Genetic tests can also help to guide decisions about the choice of medications that you are likely to respond to best and to determine the optimum dosage.

Genetic testing will become increasingly important in the coming years. If properly used it can be a powerful tool. At this point genetic screening tests appear unlikely to offer enough benefit to warrant widespread use without medical counseling as to the appropriate use and the meaning of the results.