When is a genetic test appropriate? The first step is to gather a three-generation pedigree to assess risk. A detailed pedigree includes as much data as are available about the current and past medical conditions, stressors, and environmental exposures for each member appearing on the pedigree. If a genetic test is deemed appropriate, selecting the right type of genetic test is the next decision. There are several different types of genetic testing options: predictive/presymptomatic, diagnostic, preimplantation, prenatal, carrier, newborn screening, personalized medicine and forensic testing, each of which are discussed separately below and include resources that may serve as a genomic primer and be helpful when choosing genomic testing in the clinical setting.
The most common reasons for ordering a genetic test include a strong family history (e.g., cancer, enzyme deficiency, hemochromatosis), signs and symptoms of a known genetic condition (i.e., diagnostic testing), and pharmacogenomics testing to guide the selection of a specific drug or therapy (personalized medicine). The GeneEd Web site provides topics in genetic testing, animations, articles, games, and teacher resources (GeneEd, 2017). A good and basic resource PDF regarding genetic testing for patients and family can be found at the http://Genome.gov Web site (National Institute of Health, 2007).
More than 50 hereditary cancer syndromes have been identified to date, most of which are highly penetrant and inherited in a dominant fashion (National Cancer Institute, 2013). Some cancer syndromes that already have available genetic tests include breast cancer, ovarian cancer, Cowden syndrome, familial adenomatous polyposis, Lynch syndrome, and retinoblastoma. Risk assessment is more accurate, and targeted screening and interventions are possible in individuals who have been tested and found to have inherited a deleterious variant. The National Cancer Institute has a Web site with step-by-step approach to genetic testing (National Cancer Institute, 2013).
In vitro fertilization and preimplantation genetic diagnosis are often considered by families who want to avoid passing deleterious mutations along to unborn children. Preimplantation genetic diagnosis is also used by couples who have experienced failed fertility treatments, recurrent pregnancy losses, or are carriers of single or sex-linked genetic disorders (American Pregnancy Association, 2017). Once a pregnancy has been confirmed, additional genetic testing options are available. Prenatal testing can be done via amniocentesis prior to birth, and newborn screening is done on nearly every newborn in the United States immediately after birth. If a fetal anomaly is suspected on ultrasound, genetic testing can be done (again, via amniocentesis) to confirm the diagnosis. The American Congress of Obstetricians and Gynecologists Web site provides helpful information for the provider considering prenatal genetic screening (American Congress of Obstetricians and Gynecologists, 2017).
Carrier testing is usually ordered when a parent knows that he or she has a family history of or is a member of an ethnic group at an increased risk for an autosomal recessive condition (e.g., cystic fibrosis, sickle cell, Tay-Sachs). The BabyCenter and the Johns Hopkins fertility center Web site offers more information about genetic carrier screening (BabyCenter, 2017; Johns Hopkins Fertility Center, 2016).
Health care designed specifically for one individual's genetic makeup, or "personalized" or "precision" medicine is still in its infancy with the exception of pharmacogenomics. The ability to select a specific drug and deliver it at a specific dose that optimizes therapy for an individual is now possible for many drugs/individuals. The Food and Drug Administration has included pharmacogenomic information in the drug labeling for nearly 250 drugs, which includes specific actions that prescribes should take based on biomarker information. A table of medications with pharmacogenomic biomarker testing to determine indication, usage, precaution, and dosing, and administration can be found at the U.S. Food and Drug Web site (U.S. Food and Drug Administration, 2014). Based on an individual's genetic blueprint, providers have access to important information about how a drug will likely behave in that individual's body. Health care organizations are very interested in this line of research and are considering making pharmacogenomics information available to customers in an effort to spare unnecessary emergency room visits, improve outcomes and avoid adverse effects (selecting chemotherapy doses), and reduce cost. Personalized medicine will be covered in more detail in the future column titled, "Demand for Pharmacogenomics." The "Genetic Testing" page authored by the Centers for Disease Control provides resources and evidence-based guidelines for genetic testing recommended or required for particular medications (Centers for Disease Control and Prevention, 2017). This resource is easy to navigate but will also be highlighted in more detail in an upcoming column issue, "Genomic Testing Guidelines." A simple scroll through the posted tables at this Web site will illustrate medications you are currently prescribing but you may not have considered the pharmacogenomics component.
Forensic testing is a very different type of test because it is not done to assess or predict health risks; it is done to identify the victim in a crime or catastrophe or establish biological relationships between people. DNA forensic analysis resources are available; for starters, consider "A Simplified Guide to DNA Evidence" and the "DNA Forensics" section of the GeneEd Web site (GeneEd, 2018; National Forensic Science Tehnology Center, 2013).
The reasoning and approach to genetic testing can be difficult for the clinician who has not been exposed to ordering and interpreting genomic tests. A genetic testing clinical reference for clinicians can be found through the American College of Preventive Medicine Web site (American College of Preventive Medicine, 2009). This clinical reference goes into depth in the types of genetic tests, personalized medicine, challenges, barriers, pros and cons, clinical resources, direct to consumer testing, and other issues that we will plan to cover in more depth in future column issues.
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