Chapter 3 - Genetic Testing
You’re watching Genetic Testing, chapter three of the genetics and fertility video series from the American Society for Reproductive Medicine.
Now that we’ve gone over what inherited mutations are and how they can be passed from parent to child, let’s talk about how to test for them. The first choice is who or what to test.
- the people who want to have a baby,
- the embryos before they are transferred, or
- the fetus after a woman gets pregnant.
Each test has its own advantages and place in reproductive planning. (Let’s explore the kinds of testing available, and where they may be suitable.)
Testing the potential parents can help a couple tell whether additional genetic testing will be needed.
If a member of the couple has a family history of genetic disease or is a known carrier of a genetic disease, genetic testing may be indicated for the other parent.
If neither parent carries a particular gene mutation, the chances of having an affected child are very low. The couple can choose to conceive naturally and forgo testing of the embryo.
An advantage of testing the parents is that the results tell you the chances of having an affected child, however, this testing does not tell you whether the child is actually affected.
If both potential parents have an autosomal recessive mutation, they may choose to undergo in vitro fertilization or IVF, then biopsy the resulting embryos, and freeze and test each biopsy. Later, only embryos that have tested normal will be transferred generally in a frozen embryo cycle.
If one parent has has an autosomal recessive mutation and the other doesn't, the medical team can help evaluate the risk of having an affected child and the couple can make an informed decision. However, testing of the embryo in this circumstance is extremely unlikely to be helpful.
If parents have known gene mutations for a particular genetic disease, preimplantation genetic testing, PGT, could be considered. This involves testing the individual embryos by removing cells with a biopsy in the IVF laboratory, which are then sent for genetic analysis. This gives information about the specific mutations of that particular embryo before there is a pregnancy. This allows the parents to choose to transfer only embryos that don't have a mutation or combination of mutations that cause the particular genetic disease.
The disadvantage here is that the embryo testing requires IVF even if there is no infertility, which can be expensive and invasive. In some cases the result of the test may not be conclusive for every embryo.
Testing the fetus at risk for a genetic disease during pregnancy gives more accurate information than testing the embryo before a pregnancy begins. The cells that are tested come from the tissues and fluid around the fetus that contains the fetus’s DNA. Testing the fetus at risk for a genetic disease during pregnancy gives more accurate information than testing the embryo before a pregnancy begins, because there are more cells available for analysis. This strategy also can allow couples to avoid IVF and get pregnant on their own.
The disadvantage here is that if a pregnancy is ongoing, a couple may have to choose between having an affected child or terminating the pregnancy, which can be emotionally difficult.
So, how do you decide whether to test the fetus, your embryos, or you?
Several organizations of experts have made recommendations about who should be screened for what. The guidance is based on your family history, ethnicity, or what is a common genetic mutation that occurs in the region where you live. For instance, in the US, everyone should be offered testing for cystic fibrosis and spinal muscular atrophy because these are two of the most common autosomal recessive conditions seen in US children.
Ashkenazi Jews are at much higher risk for Tay Sachs or Canavan disease, so people of Ashkenazi descent should be offered screening for those diseases and several others. As with other risk groups there is a common set of mutations in the Ashkenazi Jewish population. Manufacturerers of genetic tests often bundle a group of tests together into a single panel rather than offering individual tests for each mutation. The individual mutations that each panel tests for varies from manufacturer to manufacturer.
People of African, Asian and Mediterranean heritage should be offered screening for certain blood disorders, like sickle cell disease or thalassemia. You can visit the websites of the CDC, ACOG, ACMG and specialty organizations for the latest recommendations or discuss with your healthcare provider.
Expanded Carrier Screening
Since many of us are of mixed ethnic background, it can be difficult to know what to test for. Expanded carrier screening is one of the options to help with this—just one test allows a person to be screened for up to several hundred disorders.
A disadvantage here is that the results can be hard to interpret. A positive test result tells you whether you carry the trait, but severity and treatment can vary from disease to disease. Also, no one test covers all mutations, so you could carry a condition that wasn’t tested for. Expanded carrier screening is new technology and new tests are being developed all the time. The accuracy of these tests hasn’t been verified independently other than by the manufacturer. These tests can also be expensive and often aren’t covered by insurance.
If you’re considering expanded carrier screening, make sure you have an expert, like a geneticist or genetic counselor, to help you understand your results and check with your insurance carrier about your coverage.
Now, let’s talk about preimplantation genetic testing of embryos
A biopsy is performed in the IVF laboratory, where cells are taken from the embryo and tested. This requires IVF so that the embryos are available for testing, and allows for decisions to be made on the results of the test.
There are two basic types of testing: PGT-A, aneuploidy screening, and testing for specific inherited gene or chromosome changes called PGT-M and PGT-SR
Preimplantation genetic testing that looks for specific specific inherited genetic changes include PGT-M and PGT-SR. PGT-M checks for single gene mutations and PGT-SR is used to detect unbalanced chromosome rearrangements. Remember that a normal or euploid human embryo has 46 chromosomes—23 pairs, one copy of each pair comes from each parent, including the numbered chromosomes one to twenty two and the sex chromosomes X and Y.
PGT-A, which checks for extra or missing chromosomes called aneuploidy can find conditions like Down syndrome—an extra chromosome number 21. However aneuploidy can occur with any of the chromosome pairs. Most cases of aneuploidy result in failed embryo transfers or miscarriages. Therefore, the biggest advantage to PGT-A is to identify embryos with a higher likelihood of resulting in a successful and healthy pregnancy.
Although aneuploidy is a potential testing result it isn't an inherited genetic trait, but is a instead related to the physical condition and quality of the biological mother’s eggs. Aneuploidy is linked to age. By the time a woman is 35 years old, about half her embryos are expected to be aneuploid. By age 40, this increases to about 80%.
What About A Karyotype?
A karyotype is what most people think of when genetic testing is mentioned. It’s a picture of the actual chromosomes arranged in pairs. It can show structural abnormalities on the chromosomes and counts the number of copies of chromosomes.
Because a karyotype is a big-picture view of how the chromosomes are organized, it can’t tell us anything about gene mutations that might be there. It also can’t detect tiny structural errors on individual chromosomes. These could be microdeletions—tiny missing segments of DNA—or microduplications—tiny extra copies of DNA that can lead to disease/health problems.
PGT-A is different from a karyotype in that PGT-A doesn’t provide the image of the chromosomes because there aren’t enough cells to test. Instead, PGT-A analyzes pieces of a chromosome and infers the number and presence of the chromosomes from the pieces that are there.
We hope you found this information helpful. For the next chapter in this series please click here.
For more information about genetic testing in the setting of infertility treatment please visit reproductivefacts.org