Genomic Detection of Disease Risk
Clinical research in genomic medicine is done in what are called “genomewide association studies.” These study hundreds of thousands of gene-related components [called single-nucleotide polymorphisms (SNPs)] that are associated with a disease in hundreds or thousands of persons who have the disease. As discussed by Manolio,  these studies have revolutionized the search for genetic influences on complex traits. These complex conditions, unlike single-gene disorders, are caused by many genetic and environmental factors working together, each having a relatively small effect. And few if any are absolutely required for the disease to occur. See the reference for further details.
The following overview is taken from “Frequently Asked Questions about Genetic Testing.” 1
What is genetic testing?
Genetic testing uses laboratory methods to look at your genes, which are the DNA instructions you inherit from your mother and father. Genetic tests may be used to identify increased risks of health problems, to choose treatments, or to assess responses to treatments.
What can I learn from testing?
There are many different types of genetic tests. Genetic tests can help to
- Diagnose disease
- Identify gene changes that are responsible for an already diagnosed disease
- Determine the severity of a disease
- Guide doctors in deciding on the best medicine or treatment to use for certain individuals
- Identify gene changes that may increase the risk of developing a disease
- Identify gene changes that could be passed on to children
- Screen newborn babies for certain treatable conditions
Genetic test results can be hard to understand. However, specialists such as geneticists and genetic counselors can help explain what results might mean to you and your family. Such specialists can be located at several online sites.2 Because genetic testing tells you information about your DNA, which is shared with other family members, sometimes a genetic test result may have implications for blood relatives of the person who had testing.
What are the different types of genetic tests?
Diagnostic testing is used to precisely identify the disease that is making a person ill. The results of a diagnostic test may help you make choices about how to treat or manage your health.
Predictive and pre-symptomatic genetic tests are used to find gene changes that increase a person’s likelihood of developing diseases. The results of these tests provide you with information about your risk of developing a specific disease. Such information may be useful in decisions about your lifestyle and healthcare. For example, if you have a family history of diabetes, it may influence your diet and involvement in physical exercise.
Carrier testing is used to find people who “carry” a change in a gene that is linked to disease. Carriers may show no signs of the disease; however, they have the ability to pass on the gene change to their children, who may develop the disease or become carriers themselves. Some diseases require a gene change to be inherited from both parents for the disease to occur. This type of testing usually is offered to people who have a family history of a specific inherited disease or who belong to certain ethnic groups that have a higher risk of specific inherited diseases. For example, sickle cell trait (also known as being a carrier) occurs when a person has one gene for sickle hemoglobin and one gene for normal hemoglobin. Approximately one in ten African Americans carries sickle cell trait. People who are carriers generally do not have any medical problems and lead normal lives.
Prenatal testing is offered during pregnancy to help identify fetuses that have certain diseases.
Newborn screening is used to test babies one or two days after birth to find out if they have certain diseases known to cause problems with health and development.
Pharmacogenomic testing gives information about how certain medicines are processed by an individual’s body. This type of testing can help your healthcare provider choose the medicines that work best with your genetic makeup.
Research genetic testing is used to learn more about the contributions of genes to health and to disease. Sometimes the results may not be directly helpful to participants, but they may benefit others by helping researchers expand their understanding of the human body, health, and disease.
The following overview is taken from “Frequently Asked Questions about Pharmacogenomics.” 3
What is pharmacogenomics?
Pharmacogenomics uses information about a person’s genetic makeup, or genome, to choose the drugs and drug doses that are likely to work best for that particular person. This new field combines the science of how drugs work, called pharmacology, with the science of the human genome, called genomics.
What might pharmacogenomics mean for you?
Until recently, drugs have been developed with the idea that each drug works pretty much the same in everybody. But genomic research has changed that “one size fits all” approach and opened the door to more personalized approaches to using and developing drugs.
Depending on your genetic makeup, some drugs may work more or less effectively for you than they do in other people. Likewise, some drugs may produce more or fewer side effects in you than in someone else. In the near future, doctors will be able to routinely use information about your genetic makeup to choose those drugs and drug doses that offer the greatest chance of helping you.
Pharmacogenomics may also help to save you time and money. By using information about your genetic makeup, doctors soon may be able to avoid the trial-and-error approach of giving you various drugs that are not likely to work for you until they find the right one. Using pharmacogenomics, the “best-fit” drug to help you can be chosen from the beginning.
How is pharmacogenomic information being used today?
Although doctors are starting to use pharmacogenomic information to prescribe drugs, such tests are routine for only a few health problems. However, given the field’s rapid growth, pharmacogenomics is soon expected to lead to better ways of using drugs to manage heart disease, cancer, asthma, depression, and many other common diseases and conditions.
One current use of pharmacogenomics is with the human immunodeficiency virus (HIV). Before prescribing the antiviral drug abacavir (Ziagen), doctors now routinely test HIV-infected patients for a genetic variant that makes them more likely to have a bad reaction to the drug.
Another example is the breast cancer drug trastuzumab (Herceptin). This therapy works only for women whose tumors have a particular genetic profile that leads to overproduction of a protein called HER2.
The U.S. Food and Drug Administration (FDA) also recommends genetic testing before giving the chemotherapy drug mercaptopurine (Purinethol) to patients with acute lymphoblastic leukemia. Some people have a genetic variant that interferes with their ability to process the drug. This processing problem can cause severe side effects and increase the risk of infection unless the standard dose is adjusted according to the patient’s genetic makeup.
The FDA also advises doctors to test colon cancer patients for certain genetic variants before administering irinotecan (Camptosar), which is part of a combination chemotherapy regimen. The reasoning is that patients with one particular variant may not be able to clear the drug from their bodies as quickly as other patients, resulting in severe diarrhea and increased infection risk. Such patients may need to receive lower doses of the drug.
What other uses of pharmacogenomics are being studied?
Much research is under way to understand how genomic information can be used to develop more personalized and cost-effective strategies for using drugs to improve human health.
In 2007, the FDA revised the label on the common blood-thinning drug warfarin (Coumadin) to explain that a person’s genetic makeup might influence response to the drug. Some doctors have since begun using genetic information to adjust warfarin dosage. Still, more research is needed to conclusively determine whether warfarin dosing that includes genetic information is better than the current trial-and-error approach.
The FDA also is considering genetic testing for another blood thinner, clopidogrel bisulfate (Plavix), used to prevent dangerous blood clots. Researchers have found that Plavix may not work well in people with a certain genetic variant.
Cancer is another very active area of pharmacogenomic research. Studies have found that the chemotherapy drugs gefitinib (Iressa) and erlotinib (Tarceva) work much better in lung cancer patients whose tumors have a certain genetic variant . On the other hand, research has shown that the chemotherapy drugs cetuximab (Erbitux) and panitumumab (Vecitibix) do not work very well in the 40 percent of colon cancer patients whose tumors have a particular genetic variant. .
Pharmacogenomics may also help to quickly identify the best drugs to treat people with certain mental health disorders. For example, while some patients with depression respond to the first drug they are given, many do not, and doctors have to try another drug. Because each drug takes weeks to take its full effect, patients’ depression may grow worse during the time spent searching for a drug that helps.
Recently, researchers identified genetic variations that influence the response of depressed people to citalopram (Celexa), which belongs to a widely used class of antidepressant drugs called selective serotonin reuptake inhibitors (SSRIs). Clinical trials are now under way to learn whether genetic tests that predict SSRI response can improve patients’ outcomes.
Can pharmacogenomics be used to develop new drugs?
Yes. Besides improving the ways in which existing drugs are used, genome research is aimed at developing better drugs. The goal is to produce new drugs that are highly effective and do not cause serious side effects.
Until recently, drug developers usually used an approach that involved screening for chemicals with broad action against a disease. Researchers are now using genomic information to find or design drugs targeting subgroups of patients with specific genetic profiles. In addition, researchers are using pharmacogenomic tools to search for drugs aimed at specific molecular and cellular pathways involved in disease.
Pharmacogenomics may also breathe new life into some drugs that were abandoned during the drug’s development process. For example, development of the beta-blocker drug bucindolol (Gencaro) was stopped after two other beta-blocker drugs won FDA approval to treat heart failure. But interest in Gencaro revived after tests showed that the drug worked well in patients with two genetic variants that regulate heart function. If Gencaro is approved by the FDA, it could become the first new heart drug to require a genetic test before prescription.
1 National Human Genome Research Institute (www.genome.gov/19516567)
2 (1) National Society of Genetics Counselors (www.nsgc.org/); (2) American College of Medical Genetics (https://www.acmg.net/ACMG/Find_Genetic_Services/ACMG/ISGweb/FindaGeneticService.aspx?hkey=720856ab-a827-42fb-a788-b618b15079f9)
3 ‘National Human Genome Research Institute (www.genome.gov/27530645)