Population Genetics: PGT for Couples With Genetic Predispositions

by | Nov 7, 2018 | Current Technology, Disease-Specific Information | 0 comments

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When considering the risk of having a child with a genetic disease, most people are quick to think about their family history — but that isn’t the only risk factor.

Most intended parents don’t realize that certain ethnic backgrounds or ancestry is associated with an increased genetic predisposition, or increased chance, to have an infant with a specific genetic disease. They also may not be aware that preimplantation genetic testing (PGT) can help them manage that risk.

Risk Assessment

Doctors have long known that certain ethnic groups are more likely to have children with a specific genetic disease due to a mix of biological, historical and geographic reasons.

For example, people of Ashkenazi Jewish, Cajun or French Canadian ancestry have an increased chance of having a child with Tay-Sachs disease. Sickle cell disease — a group of disorders that cause red blood cells to contort into a sickle shape, which causes abnormal blood flow causing pain — occurs more frequently in African-Americans. People of Asian and Mediterranean ancestry have an increased risk of having children with alpha and beta thalassemia, respectively. Cystic fibrosis — a chronic lung disease — more often affects those with Northern European and Hispanic ancestry.

While these diseases occur more frequently in certain ethnic groups, they are not exclusive to those groups. There are also some genetic diseases that are pan-ethnic — they occur in all ethnic groups equally — such as Fragile X syndrome and spinal muscular atrophy.

Because of the increased risks associated with certain ethnicities, the American College of Obstetrics and Gynecology (ACOG) recommends that people who are considering pregnancy or are already pregnant be offered carrier screening.

What Is Carrier Screening?

Carrier screening is a genetic test to determine if someone is at risk of passing a specific disease-causing gene on to their children. Most of the genes tested in carrier screening cause diseases that have autosomal recessive inheritance. With this pattern of inheritance, both parents must have and pass on the same disease-causing gene in order for their child to have the disease.

Traditionally, carrier screenings were only offered to people of certain ethnicities as recommended by the ACOG, but with advanced technology and lowered cost, it is now possible to screen for many disorders regardless of ethnicity — a form of carrier screening called expanded carrier screening. This type of screening may be a good option for people that have a mixed ethnic background, are not sure of their ethnic background or simply want the most comprehensive carrier screening available.

Carrier screening requires a simple blood draw or saliva test, and is ideally performed prior to pregnancy to give couples the most options to reduce their risk of having a child with a genetic condition. If you are found to be a carrier for a specific genetic disease — meaning that you have a “positive” test result — your partner should be offered testing to see if they are also a carrier for that genetic disease. If you are both found to be carriers for the same disease, there are options that you can take to manage your risk.

Managing Your Risk

A good place to start managing your risk and learning more about your options would be talking to a genetic counselor. They can answer your questions and help you decide which options are best for you.

At-risk couples have the option of using either donor sperm or donor eggs — which can be tested to make sure the donor doesn’t carry the same recessive gene — for their pregnancy. They can also consider adoption or test the fetus during pregnancy (prenatal diagnosis) to see if the fetus has inherited the disease-causing genes.

PGT is another option for at-risk couples prior to becoming pregnant. It is genetic testing of an embryo before implantation and involves in vitro fertilization (IVF). IVF is usually done for fertility issues, but, in the case of PGT, it is performed to have the option to test embryos for genetic disease and implant embryos that tested negative.

What Is the PGT Process?

For IVF and PGT, the same steps are used at the beginning of the process — stimulation of the ovaries to produce eggs, retrieval of the eggs, fertilization with sperm and a waiting a period of time for embryo development. At this point in an IVF cycle, the embryos with normal development would be transferred back to the uterus for implantation. With PGT, the process has additional steps.

Once the embryos have developed to the blastocyst stage (typically five to six days), cells are removed from the embryo. These embryonic cells are sent to a genetics lab and their DNA is tested to determine if the embryo has inherited the disease-causing genes. While waiting for the lab results, the embryos are carefully cryopreserved in order to arrest their growth and development. Once the lab results are received, embryos that are disease-free are chosen for transfer so that they can implant in the uterus and possibly result in a pregnancy. While these additional steps lengthen the process, they can provide peace of mind.

No test can guarantee a perfectly healthy baby, but carrier testing can help you determine your genetic predisposition risks, and PGT can help you manage those risks.

Kathleen Fergus, MS LCGC is a freelance medical writer in northern California. She is also a genetic counselor with over twenty years clinical experience with an extensive background in many medical specialties including oncology, prenatal screening, carrier screening, DNA testing methodologies, newborn screening, public health genetics and rare genetic diseases. Her interest in genetics, health communications strategies and health literacy evolved into a freelance medical writing career. She has written for VeryWell, Exploragen, ThinkGenetic and many other publications.

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