A Brief Primer on Genetic Testing

A Brief Primer on Genetic Testing

World Economic Forum

January 24, 2003

Francis S. Collins, M.D., Ph.D.
Director, National Human Genome Research Institute

What are the current indications for genetic tests and for whom are they appropriate?

Genetic tests can be done to confirm a suspected diagnosis, to predict the possibility of future illness, to detect the presence of a carrier state in unaffected individuals (whose children may be at risk), and to predict response to therapy. Genetic tests may be carried out in the prenatal arena, either through pre-implantation genetic diagnosis (where the diagnosis is made of an individual embryo before implantation), chorionic villus sampling (CVS), or amniocentesis. Most newborns in industrialized countries are tested at birth for a few genetic disorders that require immediate treatment. Genetic tests may be carried out on children (to confirm a diagnosis, but generally not to predict adult-onset disorders unless an intervention in childhood is essential). Genetic tests may be carried out on adults for all of these indications.

What kinds of genetic tests are available?

About 900 genetic tests are now offered by diagnostic laboratories (see Genetic Testing Registry at NCBI for a wealth of information on the specifics). Some genetic tests look at whether the number of chromosomes is correct and whether there is any evidence of a chromosome rearrangement or other abnormality. This kind of test, for instance, would detect Down syndrome (an extra chromosome 21). Most genetic problems are more subtle than this, so tests able to detect them must look at the actual DNA sequence of a particular gene. To detect a carrier of Huntington's disease, for instance, the test must discover a particular expanded repeated sequence of a gene on chromosome 4. If this repeat of CAGCAGCAG... is very long, there is a high likelihood of the future onset of illness. For many genes, however, there are multiple different ways that the gene can be misspelled; in that situation, an effective test may need to detect many possible misspellings (usually referred to as mutations). A standard test for cystic fibrosis, for instance, looks for 32 different mutations in the so-called CFTR gene, but will still miss rare ones. Other types of genetic tests do not look at DNA at all, but look at RNA (the messenger that is transcribed from the gene), or at the actual protein product of the gene. Carrier detection for Tay-Sachs disease, for instance, actually measures the enzyme activity of the protein product.

What kinds of tests are available now for predicting disease susceptibility?

The number of tests is growing, but most of these are currently applied only in families where there is a strong history of the disorder. For instance, BRCA1 and BRCA2 testing are only offered to individuals with a strong family history of breast and ovarian cancer. Similar situations exist for diseases such as colon cancer or Huntington's Disease. But in the next few years, it is expected that a much longer list of susceptibility tests will become available, and may be offered to anyone interested in the information, regardless of family history.

What about tests that predict response to therapy?

This is generally referred to as pharmacogenomics. Some such tests are already available, such as a test for estrogen receptors in a breast tumor sample to see whether the drug Herceptin will be effective. A much larger array of tests that predict drug responsiveness for cancer, heart disease, asthma, and other disorders is under development, and some will reach the market soon.

What will happen to genetic testing over the next decade?

It is likely that the major genetic factors involved in susceptibility to common diseases like diabetes, heart disease, Alzheimer's disease, cancer and mental illness will be uncovered in the course of the next 5 to 7 years. For many of these conditions, altering diet, lifestyle, or medical surveillance could be beneficial for high-risk individuals. That will open the door to wider availability of genetic tests to identify individual predispositions to future illness, potentially for virtually anyone. If applied properly, this could usher in a new era of individualized preventive medicine that could have considerable health benefits. It will be important to remember, however, that most of these tests will not be "yes or no" but rather will predict relative risk. For this paradigm to succeed, it will also be essential that predictive genetic information is used to benefit individuals, rather than to injure them by discriminatory misuse.