Genetic Testing Can Guide Treatment

Genetic testing provides information that helps diagnose diseases and disorders which are directly inherited from one or both biological parents. Results of genetic tests also inform people if they have a condition that they might pass down to future children. Genetic tests can also help to guide treatment decisions for certain medical conditions

What are genes?

Genes are tiny segments made of DNA that determine the physical traits we inherit from our parents. Physical traits include what we look like and how our bodies develop and function.

Each one of our body cells contains 23 pairs of gene-carrying chromosomes, also made of DNA. One chromosome in each pair comes from our biological father and the other chromosome comes from our biological mother. However, only one of the two genes will be the dominant one. For example, if the gene contributed by the mother codes for brown eyes, and the gene contributed by the father codes for blue eyes, the child will probably have brown eyes.

Genetic diseases occur if the dominant gene, or both genes serving the same function, have a harmful mutation(change) or variant. Having a harmful variant doesn’t always mean you will develop the related disease, or if you do, that you will develop a severe form of the disease. Many factors can affect how a genetic variant affects a person and their offspring.

Some genetic diseases are a result of missing or extra chromosomes or pieces of chromosomes.

What are the different kinds of genetic tests?

Predictive testing is done if your blood relatives have a genetic condition, and you want to know if you too have inherited that condition. Symptoms of some genetic conditions can be prevented, delayed, or reduced if treated early. For example, you might be able to prevent genetically linked high cholesterol with medication and lifestyle changes.

Some genetic variants increase the risk of developing various cancers. For example, a harmful variant of the BRCA1 gene (link to article on ovarian cancer) gives a person a high risk — but not a definite answer — of developing ovarian cancer.

A person who knows that they have the gene might decide to remove her ovaries before cancer develops, take medication to lower her risk, or have frequent exams. A person who would not do any of those options, but who has relatives with ovarian cancer, might choose not to be tested for a BRCA mutation.

Diagnostic testing confirms that you do or don’t have a suspected genetic condition. It can also help you find out if you have certain genetic variants of the condition and help doctors plan treatment tailored to that variant. Diagnostic testing can be done in utero and at any age. If you find out that you have a genetic condition, your family members might choose to be tested for that condition.

Newborn screening is done on all babies in the U.S. shortly after birth for conditions such as phenylketonuria, which can cause intellectual disability and/or seizures if untreated.

Pharmacogenomic tests can be done before you begin to take a medication. These tests look at the genes that determine how well your body or your condition will respond to specific medications. It can also guide dosing decisions and predict if you might have serious side effects from a medication. Pharmacogenomic tests are available for hundreds of commonly prescribed medications, including cardiovascular, anti-cancer, and psychiatric medications.

Clinical exome testing looks at all genes that are related to medical conditions. It is usually limited to cases that are unusually complex or rare, or that involve multiple genes.

Panel tests look at several genes that could be related to a particular symptom or condition. For example, to identify (and treat) an unknown condition causing a child’s delayed growth, doctors would test several seemingly unrelated genes which if mutated, would cause delayed growth. If the mutation is found on a gene involved in producing growth hormone, the doctors might prescribe growth hormone injections.

Another form of panel testing looks for mutations that are found at higher rates among specific ethnic groups. For example, a panel that might be recommended to people of African descent would include tests for sickle cell disease and alpha-thalassemia; and a panel designed for Ashkenazi Jews would include tests for Tay-Sachs and Canavan diseases.

Carrier testing can tell if you have a genetic mutation that is linked to a disease. If Person A has a recessive gene mutation inherited from one parent, Person A would not show symptoms but could pass that gene on to a child. If Person A’s partner, Person B, has inherited the same mutation from one parent, Person B can also pass on that gene. The couple’s children each have a 25% chance of developing the genetic condition, a 50% chance of being a carrier, and 25% chance of not inheriting the mutation at all.

Carrier testing is helpful if you have a family history of a genetic condition or if you are a member of an ethnic group at higher risk of inheriting genes for certain conditions.

How can this information help with family planning?

If one partner has a genetic condition, or both partners are carriers, their biological children are at risk for the condition. Knowing this, the couple might choose to:

• Use donor eggs or sperm in a process called in vitro fertilization (IVF)
• Have IVF with preimplantation genetic diagnosis (PGD). In PGD, a genetic test is developed specifically for the couple’s known mutation, a process that can take months.
  • The test must be developed before IVF is started so that the embryos can be tested prior to implantation.
  • Only embryos without the mutation are implanted.
• Have prenatal testing during the pregnancy to find out if the baby will be affected.
  • If the baby is affected some parents decide to terminate the pregnancy.
  • Other parents use the information to begin any available treatment and to plan for the baby’s medical needs after birth, or to plan for hospice care.
• Adopt a baby or child.

Aren’t there prenatal genetic conditions that aren’t inherited?

Each sperm and egg should contain 23 chromosomes, the thread-like structures that carry our genes. In a random occurrence, an egg or sperm might have a different number of chromosomes, or an extra or missing part. Chromosomal abnormalities can cause many conditions, ranging from mild to life-threatening.

Tests for some relatively common chromosomal abnormalities can be offered to expectant parents. The parents don’t need to have a known mutation to have these tests. Prenatal screening assesses a fetus’s risk of some relatively common chromosomal abnormalities including Down syndrome.

• Prenatal screening can’t confirm that there is an abnormality, but it can pick up increased risk. For a definitive diagnosis, parents can choose to follow up with prenatal diagnostic testing.
• Prenatal diagnostic testing can be done if a screening test showed a high risk for a chromosomal abnormality, or if the parents have a known genetic condition.
• Both tests, or neither test. Whether or not to have prenatal testing is up to the parents.

Are there genetic tests for conditions that are not inherited?

Some genetic tests look at gene mutations that a person has acquired during their lifetime. These mutations only affect the genes in a specific body part. For example, sunburns can cause mutations to the genes in sun-exposed skin cells, causing skin cancer. This type of mutation isn’t directly inherited from parents and isn’t directly passed down to offspring. However, inherited gene variants, whether harmful or not, might increase a person’s risk of developing an acquired mutation. For example, light-haired, light-eyed people have a higher risk of developing skin cancer.

Are there genetic tests to guide cancer treatment?

Cancerous tumor cell genes have mutations that cause uncontrolled cell division. There have recently been tremendous advances in the ability to test tumor cell DNA in order to determine the most effective therapeutic agent. In addition, pharmacogenomic tests can help determine how the patients body will react to selected medications.

How do I decide which genetic tests to have?

Choosing the appropriate tests is extremely important. For that reason, genetic testing is usually conducted by a team consisting of a doctor who specializes in genetics, along with a genetic counselor. They will review your medical records and family history with you to determine which tests to do.

They will also explain some of the limitations of genetic testing. For example, a person can have symptoms of a disease but have a negative genetic test result if the genes that are tested are not the genes responsible for the symptoms. Or a person can have a negative test result for a certain disease yet develop that disease later because of an acquired mutation. Also, a positive tests can’t always accurately predict how severe symptoms will be, or when they will start.

What questions should I think about?

The genetic counselor can help you think about how a positive test result might affect you. For example, if the test comes back positive:

• Can I do something to prevent or minimize the symptoms?
  • Would I do what is required to prevent or minimize the symptoms?
  • If I can’t or wouldn’t take steps to prevent or minimize the symptoms, do I still want to know if I will develop the condition
  • Do I have the financial resources needed to pay for treatment?
• How will my mental health be affected?
• Will I feel that I will be better able to plan for my future, or will I feel depressed or anxious about my future? My children’s futures?
• Will being better able to plan for the future help to relieve any depression or anxiety?
• Will I tell my loved ones? How will it affect them, or their own decision whether to be tested for the same gene?
• Will I have protection against genetic discrimination? Will I be able to get health insurance, life insurance, or disability insurance?

Deciding whether to have the test done is a deeply personal decision that only you as the patient can make. That decision is not always obvious. However, by considering the issues that are most important to you, and by talking it through with the genetics team and any loved ones you choose to include, you will make the best decision – the one that feels right to you.

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