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Overview

What Is It?Genetic testing is used to confirm the presence of genetic diseases, as well as to measure your risk of developing a disease or of passing along a genetic disorder to a child.Today, there are hundreds of genetic tests, some of them for relatively common disorders, such as cystic fibrosis, and others for very rare diseases. A genetic test is fundamentally different from other kinds of diagnostic tests you might take. Indeed, a whole new field, genetic counseling, has grown up around the need to help incorporate family history and genetic testing into modern health care.

The purposes of genetic tests vary. Some genetic tests are used to confirm a preliminary diagnosis based on symptoms. But others measure your risk of developing a disease, even if you are healthy now (presymptomatic testing), or determine whether you and your partner are at risk of having a child with a genetic disorder (carrier screening).

As the name suggests, a genetic test looks at your genes, which consist of DNA (deoxyribonucleic acid). DNA is a chemical message to produce a protein, which has a specific function in the body. Proteins are essential to lifethey serve as building blocks for cells and tissues; they produce energy and act as messengers to make your body function. In addition to studying genes, genetic testing in a broader sense includes biochemical tests for the presence or absence of key proteins that signal aberrant gene function.

What do Genetic Tests Test For?

Chromosome AbnormalitiesLong strings of DNA condense together, packaging the DNA in the form of a chromosome. Most people have 23 pairs of chromosomes in the nucleus of each cell. One of each chromosome pair is inherited from the mother and the other is inherited from the father. Some tests look at chromosomes for abnormalities such as extra, missing or transposed chromosomal material. The chromosomes hold 20,000 to 25,000 genes, meaning that each chromosome is densely packed with genes. Extra or missing pieces of chromosomes can have a significant impact on the health of an individual. Also, sometimes pieces of chromosomes become switched, or transposed, so that a gene ends up in a location where it is permanently and inappropriately turned on or off. The genes on the chromosomes are responsible for making proteins, which direct our biological development and the activity of about 100 trillion cells in our bodies.

If something goes wrong with an essential protein, the consequences can be severe. For example, a protein called alpha-1 antitrypsin (AAT) clears the lungs of a caustic agent called neutrophil elastase. If the body has an alteration in the gene that makes the protein AAT, the AAT protein may not be made correctly or at all. Then neutrophil elastase will build up in the lungs, and the individual can develop emphysema and other complications.

MutationsMost genetic conditions are the result of mutations in the DNA, which alter the instructions for making a given protein. Some mutations are inherited on genes passed down from parents, while others occur during an individual's lifetime. These mutations can lead to diseases ranging from those we think of as "genetic diseases," such as cystic fibrosis or AAT deficiency, to those we think of as degenerative diseases, such as heart disease. In the case of diseases like heart disease, asthma or diabetes, a combination of factorssome genetic, some related to environmental or lifestylemay work together to trigger the disease.

It's possible to have a mutation, even one for a severe disease, such as cystic fibrosis (CF) and never know it. Almost all humans have two copies of each chromosome and therefore have two copies of each gene, one inherited from the mother and the other from the father. If only one copy of a given gene has a mutation, you are a healthy carrier of the disorder. You "carry" the mutation but do not have the disease. If both copies of a gene have a mutation, you will have the disease. Such disorders are called autosomal recessive. If you are a carrier, the unaltered gene in the pair retains the function. Those who are diagnosed with a recessive disease have inherited two copies of a gene, both carrying a mutation. Therefore, since one of those copies came from the mother and the other from the father, both parents must have at least one copy of the gene with a mutation. If two carriers of the same disease-causing gene have children, each pregnancy has a 25 percent chance of having the disease (because of a 25 percent chance of inheriting both the mother's and the father's mutated copies of the gene), a 50 percent chance of being a carrier and a 25 percent chance of not inheriting the mutation at all.

Some disorders, such as Huntington disease, are autosomal dominant. If a person has one mutated gene, its effects will cause the disease, even if the matching gene is normal. Thus, each child of a parent with Huntington disease has a 50 percent chance of inheriting the gene causing the disease. Osteogenesis imperfecta, which causes brittle bones, is another example of a dominant disorder.

Chromosomes can be one of two types: sex chromosomes or autosomes. Sex chromosomes are X and Y. Most men have an X and a Y, and most women have two Xs. If each parent contributes an X chromosome, the child is a girl; if the father passes on his Y chromosome, the child is a boy. Because girls have two X chromosomes, and therefore two copies of every X-linked gene, they are less likely than boys to have symptoms from X-linked genetic diseases because boys don't have a backup copy if an X-chromosome gene has a mutation. Examples of X-linked diseases include forms of hemophilia and fragile X syndrome (the most common inherited cause of mental impairment). Autosomes are the remaining 22 pairs of chromosomes. Therefore, most diseases are autosomal, or due to genes on the autosomes.

What Genetic Tests Can Find

Unclear Results Although genetic testing can be very useful in diagnosis, prevention and medical decision-making, genetic tests do not always provide clear answers. One such result is a "variant of uncertain significance." All people have differences in their DNA, so if a new DNA alteration is detected, it may be uncertain as to whether it is associated with disease or is part of normal human variation. Another limitation is that not all genetic tests are created as equals. Since genetic testing can be very expensive, some tests only look for the most common disease-causing mutations. Instead of examining the entire gene, these tests only look for specific, common mutations. If you or your family has a mutation in a portion of the gene that wasn't tested, you will have a negative result, even though you do have a disease-associated mutation. Since genetic tests are not perfect, it is always important that genetic test results be interpreted in combination with medical and family history by a genetic counselor or other genetics-credentialed professional.

The Cost of Genetic Testing

The cost of a genetic test varies dramatically, ranging from $100 to more than $3,200. The difference stems largely from the variation in labor intensity of different tests. Some tests look for a limited number of mutations (sometimes only one) known to cause a disease. This type of test may only look at one piece of DNA code, for one specific mutation. Other genetic tests require sequencing of the entire gene, where they examine each piece of DNA code comprising the gene, which can be thousands of pieces of code.

The explosion of genetic research now taking place is expected to bring prices down and dramatically increase the number of tests available. Tests are becoming available to predict your genetic risk of more common disease, such as heart disease and diabetes. This information will help you and your health care professional develop specific strategies for prevention. Preventive efforts can include changing your lifestyle or perhaps taking certain medications, which may be tailored to your specific genetic profile, and early screening to head off the worst complications should you develop the disease.

Facts to Know

A genetic test examines some aspect of a person's genetic makeup, either directly through gene sequencing or indirectly through the measure of marker chemicals. Such a test usually aims to determine whether a person has, is at above-average risk of having or is a carrier of a disease-causing genetic mutation.

Because the nature of genetic testing is so complex, with implications for both the person being tested and his or her family, genetic counseling is desirable before taking any genetic test and essential for proper interpretation of test results.

Genetic counselors are committed to protecting your privacy. They will not contact other family members without your permission, though they may encourage you to share results that might affect your relatives.

A maternal serum screening test indicates whether a fetus is at above-average risk of being born with certain genetic disorders, most notably Down syndrome, trisomy 18 and open neural tube defects. The test is not diagnostic and a positive result is usually followed up with a diagnostic amniocentesis or chorionic villus sampling test. Out of 1,000 serum screening tests, 50 will suggest increased risk for open neural tube defects, but only one or two of the fetuses will have such a defect. Likewise 40 of 1,000 will test positive for increased risk of Down syndrome, but only one or two will fetuses will actually have the disease.

Some genetic disorders are recessive and X-linked, which means they are caused by a mutation in a gene that resides on the X chromosome. Females have two X chromosomes, but males have only one. If a mother has a disease-linked recessive gene mutation in one of her X chromosomes, she is a carrier of the disorder but will have no or minimal symptoms herself. If she has a son, he will have a 50 percent risk of inheriting the disorder; a daughter will have a 50 percent chance of being a carrier.

In addition to disorders that have surfaced in your family, you may want to consider carrier testing for genetic conditions that occur with greater frequency in your particular ethnic group. For example, Caucasians have a higher risk of cystic fibrosis, while those of African descent are at high risk of carrying a mutation that can cause sickle cell disease. A battery of tests exists for those of Ashkenazi (Eastern European) Jewish descent. Remember that the best time for carrier testing is before a pregnancy.

Children should not be screened for carrier status or for diseases that won't trouble them until much later in life because the information is not relevant to their health care. Most geneticists and genetic counselors consider such testing unethical, since children are not in the position to make their own decisions as to whether or not they want the test (known as informed consent).

Within a family, two or more incidences of the same type of cancer or related cancers, or one at under age 50 may indicate a hereditary pattern. A genetic counselor can take a closer look at your family history to determine whether an inherited mutation appears to be responsible for the cancers in your family and can advise you as to whether testing is available.

The best-known cancer predisposition tests look for mutations in the BRCA1 and BRCA2 genes. Women with a BRCA mutation face a lifetime breast cancer risk of up to 88 percent, compared to about 13 percent in the general population, and lifetime ovarian cancer risk of up to 60 percent, compared to a population risk of about 1.4 percent.

If your family has a history of colorectal and related cancers, you may want to consider genetic counseling and risk assessment. Several colorectal cancer syndromes can be responsible for hereditary cancer risk. One such syndrome is Lynch Syndrome. The syndrome increases lifetime risk of colorectal cancer to 80 percent vs. a 5.4 percent population risk, but also boosts risk of endometrial cancer (to 60 percent), ovarian cancer (to 12 percent) and gastric cancer (to 13 percent). Those with Lynch Syndrome also face a higher risk of cancers of the kidney and ureter, brain and small bowel.

Questions to Ask

Review the following Questions to Ask about genetic testing so you're prepared to discuss this important health issue with your health care professional.

General

Could my symptoms be caused by a genetic disorder? Is testing available?

Are you experienced in diagnosing and treating genetic disorders? If not can you make a referral?

How accurate is this test?

What are the risks of the test?

What information will come out of the test?

What will a positive or negative result tell me?

Is an uncertain result possible, and what would that mean?

What are my options for preventing or treating the disease if a mutation is found?

What other family members might be affected?

How do I broach the subject with them?

Could this disorder affect my children before they're grown? Should they be tested?

What measures are in place to protect my privacy?

How often have you performed the test?

How experienced is the lab in performing this test?

How long will it take to get results back?

How could this test affect my health care?

Cancer Predisposition Testing

Does my family history suggest a pattern of inherited cancer?

Is there a test available to determine which family members are most at risk?

What are my chances of developing cancer if I test positive for a mutation?

How does my risk change with age?

What are my options if I test positive?

How frequently should I have screenings?

Are preventive measures such as surgery or pharmaceuticals available?

Carrier Screening And Preconception Counseling

Based on family history and ethnicity, which carrier tests should my partner and I consider?

What criteria are you using to determine which tests are right for us?

Would other centers recommend a different lineup of tests?

What are the options if a result suggests the possibility of having a child with a genetic disorder?

Prenatal Testing

How early or late in my pregnancy can this test be performed?

What are the risks of the test?

Is this a risk screening test or a diagnostic test?

What are the options if the test finds a problem?

Key Q&A

What is genetic testing?

A genetic test looks at a particular aspect of your genetic makeup, either directly through gene sequencing or indirectly through measure of marker chemicals. Testing may be done for a variety of purposes:

Diagnosis, to determine if a person has a genetic disorder (often performed in conjunction with analysis of symptoms)

Risk screening, to determine if a person is at increased risk of having a genetic disorder (with follow-up diagnostics usually called for if a test is positive)

Predisposition testing, to determine if a person is at higher risk of developing a particular disease later in life

Carrier testing, to determine if a person is a carrier of a disease-causing mutation and may be at risk of having a child with the disease

What does it mean if I'm a carrier for a disease?

Genes come in pairs, and a carrier of a recessive disease has one mutated, disease-causing gene and a corresponding normal gene. The normal gene compensates for the mutated copy and the person never develops the disease. If two carriers of the same disease-causing gene have a child, however, that child has a 25 percent chance of having the disease (because of a 25 percent chance of inheriting two mutated copies of the gene), a 50 percent chance of being a carrier and a 25 percent chance of not inheriting the mutation at all.

If my partner and I have carrier testing, will the results tell us whether or not our children will be affected?

In most cases, the test will provide only guidance as to your child's risk for being born with a particular disorder or being a carrier of the disease. Because you contribute only one of the two copies you have of each gene, each child has a 50 percent chance of inheriting any particular mutation from you. Each child likewise has a 50 percent chance of inheriting any particular mutation your partner has. Thus, if you are both carriers of the same autosomal recessive disorder, each child has a 25 percent risk of being born with the disease, a 50 percent risk of being a carrier and a 25 percent chance of not inheriting a mutation at all. A genetic counselor can help you sort through the possible combinations in your situation and describe options for pregnancy planning and prenatal testing.

Why do I need a genetic counselor in addition to my doctor?

Most counselors and geneticists have extensive training and certification specifically related to genetics and genetic testing. Additionally, most physicians do not have time to spend an hour or more providing education, information collection, risk assessment and informed consent. Hence, many physicians make referrals when the issue arises. Genetic counselors usually work with geneticists (MDs or PhDs), particularly for more complex cases.

If I have a test, will I face job or insurance discrimination if the result is positive?

The Genetic Information Nondiscrimination Act of 2008 (GINA), a new federal law that protects Americans from being treated unfairly because of genetic diseases and mutations that may affect their health, was recently passed. This law specifically addresses protections in regard to health insurance and the workplace.

Why are some genetic tests so much more expensive than others?

Some tests look for mutations by actually sequencing the entire gene; these tests, which may cost more than $3,000, look for mutations by determining the exact order of the chemicals that comprise the gene and compare the order to that of a normal gene. Other, less expensive tests look for individual, commonly known disease-causing mutations. It's like going to a grocery store. If you have never been to that store before and you are looking for a bottle of ketchup, you may go through every aisle. This is the equivalent of sequencing; looking through the entire gene for the mutation. If you have been there before and know where the ketchup is, you can go directly to the location in the store, which is like specific point mutation testingyou know exactly where the mutation is located.

A relative has canceram I at risk, too?

Your family history provides the best clues. Two or more relatives with early onset (before age 50 or 60, depending on the cancer) of related cancers or diagnosis of two or more related cancers in the same person suggest the possibility of a genetic link that could put you at risk. Related cancers are not always as obvious as you might think. For example, colon cancer and endometrial cancer can be caused by the same genetic mutation. Talk to a genetic counselor to get a better idea of your risk and find out whether predisposition testing is available.

Isn't my health my own business? Why should my extended family be involved?

By their very nature, genetic diseases are a family affair, with mutations passed on to multiple generations. When a disease is clearly hereditary, testing positive for a disease-causing mutation or being diagnosed with the disease provides knowledge that other family members may be at risk. A genetic counselor can help you identify who may be at risk and should be notified and can help you handle the situation if there is estrangement between relatives.

What's the difference between amniocentesis and chorionic villus sampling? How do I decide which is right for me?

Both procedures provide for diagnosis of specific chromosomal and genetic disorders in the fetus. Amniocentesis is more likely to be offered as a follow-up to an abnormal maternal serum screening test because results of the screen are obtained too late in pregnancy for CVS. However, CVS, which is done at 10 to 12 weeks gestation, or amniocentesis, are offered in the following situations:

You will be 35 or older at delivery.

A genetic disorder has surfaced on either side of the family.

You or your partner has had a previous child with a birth defect.

You and your partner are carriers of the same recessive disorder.

Both chorionic villus sampling (CVS) and amniocentesis can cause cramping, and a small number of women have miscarriages following the procedures (the risk is higher with CVS). It takes one to two weeks to get results from either test.

Amniocentesis is performed more frequently and should be the choice if you're at risk having a child with neural tube defects. The procedure is performed at 15 to 18 weeks of pregnancy.

CVS can be performed earlier, at 10 to 12 weeks, and is popular with parents who would like to know results before the pregnancy starts to show. The procedure is not available everywhere, however.

If I get a negative result from a cancer predisposition test, can I still develop that particular kind cancer?

Yes. Your lifetime risk for breast cancer, even in the absence of a gene mutation, is about 12 percent. At least 90 percent of breast cancer is not due to a single, inherited cancer predisposition gene. A negative BRCA test result simply means you don't face a higher-than-average risk for the disease due to a hereditary cancer syndrome.

Genetic Counseling

What Is Genetic Counseling?

Because the nature of genetic testing is so complex, with implications for both the person being tested and his or her family, genetic counseling is an important part of pre- and post-genetic testing. Unlike most medical appointments, a counseling session may be a family affair, with participation of all concerned relatives.

Read more:
Genetic Testing | HealthyWomen

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