Preimplantation genetic diagnosis (PGT) is a complex process that can help couples conceive a child without passing on a specific genetic disorder. As couples begin this process, they may have some PGT misconceptions. It’s easy to feel like you don’t quite understand the whole process and the steps involved. But knowledge is power, so let’s look at three of the most common misconceptions about the uses for PGT that Erin Armenti, MS, LCGC, a genetic counselor at CooperGenomics, a lab that performs PGT, dispels for her patients.
Misconception #1: We Know Our Mutation, So We’re Ready to Start
Armenti believes that this is probably one of the most common PGT misconceptions that her patients have: “Many patients come to me and say, ‘We already know what my mutation is, so why do we have to do additional steps to look for the mutation?'”
The reason, Armenti explains, is that testing an embryo is vastly different from testing an adult because of the amount of DNA involved. When you get tested for a genetic mutation, they draw several tubes of blood. “There are a lot of cells in those tubes, lots of DNA to work with,” she says. With an embryo, there are only a handful of cells instead of hundreds or thousands, so there’s a much smaller amount of DNA to use in testing. This is one of the main factors that leads to certain limitations and challenges with PGT.
“When we’re working with such a tiny amount of DNA, we never want to examine just the mutation itself,” Armenti says. “When we process DNA for testing, one of the steps is to copy the DNA to make more of it for the test. When we do that with a very small sample, not every letter will copy. If the letter that is involved in the mutation doesn’t copy correctly, the embryo could be misdiagnosed,” she says. The letters Armenti refers to are the individual nucleotide bases — A, C, G, and T — that make up a DNA strand. Changes in the order of these bases define the mutations that cause genetic disorders.
To get around this limitation, PGT labs build a test customized to the couple. They look for normal population variants in sections of DNA that are right next to the gene mutation for the disorder — so-called single nucleotide polymorphisms, or SNPs for short. By looking at another family member — usually a child or the couple’s parents — they can identify the patterns of inheritance and see which SNPs are moving with the disease mutation, and which move with the normal copy of the disease gene. The more locations around the gene they examine, the more opportunity they have to confirm the test results.
“This is why, when we do [PGT], we often involve family members, like another child or the couple’s parents,” explains Armenti. “This helps us tease out the pattern.”
Misconception #2: PGT Will Screen for All Genetic Disorders
When couples undergo PGT, the test is built specifically for them, but it’s also built for the disease mutation they want to screen for. While there are many uses for PGT, the process is particular to each disease. “We can only look for what we’ve designed the test to look for,” says Armenti. “A lot of times patients will think that they’re having a test that looks for everything,” she adds.
For example, if the couple has a family history of hemophilia, the test will be built to look for that specific disease. If the couple also has BRCA mutations in their family, that won’t come up on the test at all.
“I often get asked about autism,” says Armenti, “and I explain that we can’t look for that in an embryo, as it’s a combination of poorly-understood genetic mutations and environment.”
Armenti explains to patients that the PGT test is “like a zoom lens. We’re zoomed in on this one gene mutation.”
Misconception #3: PGT Will Only Take a Short Time
When a patient has genetic testing performed, the results typically come back in a couple of weeks. But for PGT, the process can take a longer time, Armenti says. “First we have to build the test, which can take about a month. Then, after the IVF cycle itself is complete, it can take a couple more weeks to test the embryo samples.”
It’s helpful if couples have an open mind about the timeline and steps involved when they begin the PGT process, Armenti notes. It’s important to be aware that you may need to test other family members to build the right PGT test for you. She also recommends that you work with a clinic that will refer you to the PGT lab to answer any questions you may have about the process.
“Most important is for patients to understand that [PGT] is a process,” says Armenti. “It’s something that will always be unique to your family, so it will take a bit of time to develop.”