- Genetic testing is an important health-care tool that can tell people a lot about their bodies.
- These tests analyze a sample of a person’s DNA and look for specific changes associated with different conditions.
- Often, test results can help doctors diagnose and predict a person’s risk for developing a disease.
- Other DNA tests can tell people about whether they’re predisposed to certain traits.
- INSIDER spoke with Dr. William G. Feero, M.D., Ph.D., an associate professor of Community and Family Medicine at the Geisel School of Medicine at Dartmouth, to get a better understanding of how different genetic DNA tests work, and what they can teach people about their health.
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Getting a DNA test can give you a window into your health.
Today, there are numerous genetic tests available to people at the clinical and consumer level. In general, they involve a mouth swab, hair sample, or a blood test to allow doctors, scientists, and individuals to look closely at a person’s DNA for variations (mutations) that are linked to particular traits or conditions.
But it’s important to note that clinical and consumer tests are not created equal. In fact, many home DNA tests aren’t regulated to the degree that medical DNA tests are.
INSIDER spoke with Dr. William G. Feero, MD, PhD, associate professor of Community and Family Medicine at the Geisel School of Medicine at Dartmouth, to learn how medical DNA tests work and the different ways they can be useful for understanding our health. Regardless of your motivations for seeking DNA testing, you should always consult a physician about interpreting your results.
Diagnostic tests can confirm whether you have a hereditary disease like Huntington’s disease, sickle cell anemia, Marfan syndrome, and cystic fibrosis
Most of the time, doctors use genetic testing as a tool to help confirm a genetic disease diagnosis in patients who are experiencing specific symptoms, said Feero.
During a medical DNA test, a licensed practitioner such as a nurse or doctor collects a small sample of hair, skin, saliva, or blood from a patient. Pregnant people may also give a sample of amniotic fluid (aka the liquid around an unborn child.)
According to the National Institutes of Health, genetic tests (at the clinical level) can be used to identify 2,000 hereditary diseases and conditions. Some examples of common hereditary diseases that these tests might find include cystic fibrosis, familial hyperlipidemia, and muscular dystrophy.
“Diagnostic genetic tests look for specific changes in the DNA, in specific genes that are relatively well-proven to cause those genes not to work properly, and for the person to develop disease,” Feero said. “They are highly reliable and very accurate.”
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One of the benefits of these types of genetic tests is that they allow doctors to detect hereditary diseases at every stage of life. In some cases, if doctors are able to identify the disease before symptoms progress to a severe level, they can help patients plan for managing the condition.
This is especially useful for conditions like familial hyperlipidemia, which can result in premature heart attacks and stroke. Catching these types of diseases during their early stages (as in, before a first heart attack) means a patient will possibly have more options for treating the disease.
This type of testing is usually only done in a health-care setting.
Presymptomatic genetic tests may tell you whether you’re going to eventually develop a (hereditary) disease you don’t actually have yet
When a healthy person with no symptoms has a documented family history of a disease, such as diabetes, genetic tests can analyze specific markers in their genes that are related to that disease, said Feero. Doctors may then use this information to predict whether that person is at risk of developing the condition.
“One clear cut example is Huntington’s disease. It is a single-gene disorder that causes dementia, and it usually starts when someone is in their 50s or 60s,” he told INSIDER. “If you have the mutation associated with that disease, you are 100% going to get it. (In the case of Huntington’s disease, that mutation is an expansion of the region of DNA in a particular gene.)”
But when it comes to diseases that involve multiple mutations, occurring in one or a few genes, determining a person’s risk is a little more complex, said Feero. Some conditions aren’t just caused by changes in a single gene, but rather, a whole range of changes across, potentially, multiple genes. Feero said genetic testing can be useful by allowing doctors to analyze the different types of gene changes. This can give them an idea of whether someone will get a disease, how bad it will be, and perhaps insights on potential treatments.
A carrier test can tell you if your DNA contains a mutation for a genetic disorder that might affect your unborn child
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Sometimes people who are totally healthy can have mutations in their DNA that are associated with different genetic disorders, explained Feero. This is called being a carrier, and it basically means their DNA inherited a single copy of the gene mutation for that hereditary condition or disease. Generally, the condition doesn’t actually affect that person or their health, because their DNA still has an additional copy of that same gene that doesn’t have the mutation.
For example, if you’re a carrier for a disease mutation linked to cystic fibrosis, it doesn’t mean you have cystic fibrosis. But there’s a 50% chance that you will pass along a copy of that same mutation to your future children, and cause them to also be a carrier for that condition. However, when two future parents turn out to be carriers for the same genetic disorder, their child may develop the disease, explained Feero.
He told INSIDER, “If a woman is a carrier of a mutation for cystic fibrosis, and her partner is also a carrier of a mutation in one of his cystic fibrosis genes, their baby has a one in four chance of inheriting two bad copies of those genes. [As a result,] there’s an increased chance that baby will be born with cystic fibrosis.”
Expanded carrier status screening is a type of genetic DNA test that can estimate reproductive risks in healthy individuals, said Feero.
He told INSIDER, “Expanded carrier status screening can be useful if you are a woman who is contemplating pregnancy, and you want to know what genetic changes you might have in your DNA that could predict risk for multiple relatively rare, serious diseases in your newborn. For example, [certain] changes in your DNA might be associated with cystic fibrosis or Tay-Sachs risk for the baby.”
Today, many fertility clinics and other health-care facilities offer tests that can tell you if you are a carrier for different genetic disorders. There are also several direct-to-consumer DNA tests that provide these types of results, too.
According to the National Human Genome Research Institute, hereditary conditions that might be detected in a carrier test include Huntington’s disease, sickle cell anemia, spinal muscular atrophy, polycystic kidney disease, Tay-Sachs disease, and Down syndrome.
Knowing this information allows parents to make well-informed choices about family planning.
A predictive genetic test may give you an idea of your risk for developing some hereditary cancers, but they aren’t 100% certain
According to the National Cancer Institute, some genetic tests can provide people with information about whether they inherited mutations associated with several types of hereditary cancer. This includes certain types of breast cancer, colon cancer, melanoma, and sarcoma. One well-known cancer-related mutation that a DNA might check for is the BRCA 1/BRCA 2 gene mutation.
“If you have one of those classic BRCA genes mutations, your risk of developing breast cancer is very high,” said Feero.
For some people, receiving genetic test results that come back positive for a mutation like this can be life-changing. Feero noted this can make that person eligible for additional tests like ovarian cancer and breast cancer screenings, starting at an early age. It may also influence them to talk with their family members about potentially getting tested for mutation as well.
And depending on the other risk factors a person has (like their weight, diet, and other health conditions), it may lead them to take further action to help prevent the disease before it begins. In this case, if someone has the BRCA 1 or BRCA 2 gene mutation, along with other risk factors, they may decide to get a mastectomy (breast removal) to minimize their chances of cancer occurring.
However, it’s important to remember that not all cancers are caused by hereditary mutations. In fact, many of the mutations that cause cancers are sporadic, which means they happen randomly and unexpectedly due to environmental factors. This means a genetic test can’t always say for certain whether someone will get cancer, said Feero.
Conversely, if someone does have the BRCA 1 or BRCA 2 gene mutation, it doesn’t mean they’re 100% guaranteed to get breast cancer, either (though their risk is MUCH higher than the general population).
Still, if you have a family history of cancer and you’re concerned about your risk, you should talk with your doctor about this type of testing.
Depending on the type of DNA test you get, you may also be able to learn about whether you have some increased risk for developing conditions that are caused by multiple factors, like diabetes
Using genetics to predict whether someone will develop a disease is a very complex process. Tests that are done in a clinical setting like a doctor’s office may look for different things than a DNA test you can buy at home. A direct-to-consumer test usually involves a swab that is then mailed into a lab.
“Direct-to-consumer DNA tests use what is called SNP-based [pronounced ‘snip’], or single-based nucleotide polymorphism testing. They look for changes in DNA that are associated potentially with mutations that actually cause disease, that often aren’t the mutations themselves,” Feero told INSIDER.
Most of the time, these tests can tell you if your DNA shows variations that make you predisposed to developing a disease or condition. But this type of result is a lot different than saying you’re definitely going to get a disease.
This is especially the case when it comes to diseases where your genes and lifestyle choices play a role in whether you get it. Results from a SNP test are just one piece of the puzzle, while other factors like your diet, fitness level, blood pressure, and environment also contribute to whether you’ll get that disease.
Currently, the FDA says that some DNA tests are approved to share information regarding a person’s genetic health risk for developing 10 medical conditions, including Parkinson’s disease, celiac disease, Late-onset Alzheimer’s (a progressive brain disorder that affects memory), along with several blood-clotting and tissue disorders.
Still, Feero noted much the mainstream medical community remains uncertain about how accurate and useful these polygenic risk scores are in predicting whether someone will get a disease.
“These risk scores for conditions like type 2 diabetes are really probabilistic,” said Feero. “They predict some increased risk, but are definitely not deterministic, which means they don’t tell you for certain that you will develop type 2 diabetes. Often the increased risk predicted only accounts for a small amount of the total risk a person has for developing a disease.”
He added that it’s important to consider these genetic risk scores only account for one portion of your risk.
“People should be careful about evaluating their reasons for seeking DNA testing,” said Feero. “They should try to understand from a knowledgeable healthcare provider what the test can and can’t tell them about the particular health conditions they may be interested in testing for.”
Some home DNA tests may be capable of telling you whether you’re likely to have certain traits like lactose intolerance or a higher body weight
Many consumer DNA tests now provide people with information that isn’t related to whether or not they have a serious genetic disorder.
SNP testing can detect variations in a person’s genome that are associated with different traits that aren’t necessarily diseases as well as medical conditions, said Feero. As is the case for diseases in which multiple genetic variants play a role, the associations for traits are made based on research that has linked genetic variations in certain populations to these traits. By comparing your DNA to the DNA of others with these traits, these tests can infer traits you might have.
For example, some test results might tell a person if they’re predisposed to lactose intolerance or moving around when they sleep.
It’s important to note these trait tests, as well as SNP-based tests for disease risk, do have limitations
Feero pointed out that one issue with these consumer tests is that they only test for a limited number of relatively common genetic variants. This can be problematic if you’re someone from an underrepresented population because your results might not be as accurate as those for someone from a population group that has been extensively studied for associations between variants and traits and diseases.
And even if you are from a population that has been well studied and is highly represented in available research databases, it’s important to remember these SNP-based test results are not considered a medical diagnosis.
But that doesn’t mean they can’t be beneficial to you. Learning that you may be predisposed to a certain trait might influence you to make positive lifestyle changes that can improve your health overall. For instance, if you learn that you’re more likely to have a higher weight than the average person, you might feel inspired to work out more often at the gym or start a new fitness plan. In addition, some results may encourage you to have important conversations with your doctor about your diet or fitness plan.
But as far validity goes, perhaps it’s best to take these types of results with a grain of salt.