Preimplantation genetic testing (PGT) allows physicians to analyze an embryo formed outside the mother’s uterus for genetic defects that would prevent that embryo from implanting, produce a miscarriage or cause an inherited disease upon birth.
How does PGT work?
PGT requires patients to use In Vitro Fertilization (IVF) to produce multiple eggs and embryos that will undergo biopsy and genetic testing. The biopsy of the embryos can be performed at different stages. The earliest way of testing the embryos is by studying the polar bodies (small cells consisting of a tiny bit of cytoplasm and a nucleus, formed during maturation of the egg). The polar bodies can be removed and studied to determine the maternal genetic contribution to the developing embryo. Another way of testing occurs three days after the egg has been fertilized in the lab. In this method, one cell from the eight-cell embryo is removed and its genetic material is tested to identify possible defects.
Regardless of the type of cell studied, there are two types of testing available. One technique, the polymerase chain reaction (PCR), produces thousands of copies of a specific DNA fragment. The size of the fragment distinguishes the normal from the abnormal gene. Another technique, fluorescent in situ hybridization (FISH), uses fluorescent dyes to visualize specific chromosomes. A cell with an abnormal chromosomal set will have a characteristic fluorescent pattern.
The results from the PCR and/or FISH tests are usually available within 24 hours. Once identified, a limited number of normal embryos are returned to the mother’s uterus to allow implantation and pregnancy to be established.
What are the indications for PGT?
PGT is most commonly performed in 4 different situations, depending on the couple’s or individual’s needs.
- Genetic disorders – for couples aware of genetic disorders through family history or based on carrier testing, PGT testing can be performed to help avoid a pregnancy or child born with the specific genetic condition.
- Advanced maternal age – chromosomal abnormalities due to advancing maternal age are more likely to occur in women over the age of 35, leading to problems like Down Syndrome or early miscarriage. PGT testing can determine the number and character of the chromosomes and determine which embryos are the most likely to result in a healthy, ongoing pregnancy.
- Recurrent miscarriages – balanced translocation or rearrangement of chromosomes can lead to recurrent miscarriages. PGT testing can determine which eggs and/or embryos are the most likely to result in a healthy pregnancy, screening out the chromosomes that have joined up incorrectly.
- Male factor infertility – in some cases of severe male factor infertility, a high rate of chromosomal abnormality has been seen in embryos that result from men with azoospermia.
Does PGT cause genetic damage or other damage to the embryos?
No. The polar bodies naturally dissolve at implantation and have no significance after fertilization. With embryo biopsy, the three-day old embryo is unaffected by the removal of one or two cells. Thus far, there has been no increase in the rate of congenital abnormalities or pregnancy complications in patients when embryo biopsy has been done.
Why is PGT important?
PGT is important because the technology holds the promise of reducing the incidence of genetic disease. To date, over 40 life-threatening and debilitating genetic diseases may be screened before embryonic implantation. PGT also changes reproductive options for families at risk. Parents who have previously given birth to a child with Cystic Fibrosis or Tay-Sachs, for example, now have the option to genetically screen their embryos before pregnancy. Only those embryos that do not possess the disease-causing genes will be implanted.
Most chromosomally abnormal embryos either do not implant or spontaneously abort shortly after implantation. The use of PGT allows that only normal embryos are transferred back into the uterus, providing a much better chance of achieving a full-term pregnancy with chromosomally healthy embryos.
Who should consider PGT testing?
There are several groups of patients that could benefit from PGT:
- Genetic disorders: PGT testing can be performed for couples aware of genetic disorders through family history or based on carrier testing to help them conceive a healthy child. In genetic disorders where the genetic mutation is known, such as Cystic Fibrosis or Tay-Sachs disease, the actual genes of the embryo are examined for presence of the condition.
- Advanced maternal age: Chromosomal abnormalities due to advancing maternal age are more likely to occur in women over the age of 35. PGT testing can determine the number of the chromosomes and help select the embryos most likely to result in a healthy pregnancy.
- Recurrent miscarriages and repeated IVF failure: These two groups of couples have been found to produce a higher proportion of abnormal embryos. PGT testing can determine which embryos are the most likely to result in a healthy pregnancy.
What is the accuracy of PGT?
Overall PGT is able to diagnose genetic defects with approximately 98% accuracy. Even if a patient undergoes PGT, prenatal testing such as amniocentesis and chorionic villus sampling (CVS) is usually recommended to confirm the PGT diagnosis. Revealing genetic defects before pregnancy can significantly reduce the risk of a fetus being affected.
What ethical concerns does PGT technology present?
At present, nearly all-pregnant women are offered some form of prenatal screening and diagnosis. Since PGT allows for screening embryos before they are transferred, many believe that it spares some couples from the emotionally wrenching ordeal of having to decide about a second trimester pregnancy termination if their baby has a genetic disorder. PGT has been generally applied to identify genetically abnormal embryos known to cause significant health issues in the newborn. The Ethics Committee of the American Society of Reproductive Medicine (ASRM) has established ethical guidelines regarding sex selection and PGD. According to the Committee’s recommendations, PGT used for sex selection for non-medical reasons should not be encouraged. Moreover, the initiation of IVF with PGT solely for sex selection was strongly discouraged.
What is the future of PGT? What is yet to come?
PGT is rapidly advancing with improvements in accuracy and availability. More and more genetic disorders will be able to be diagnosed. New techniques are being developed that will allow all the chromosomes from the embryos to be checked, improving the success rates of IVF.
PGT is an exciting technology that offers couples at risk an alternative to pregnancy termination of an affected fetus. PGT-A of aneuploidy has the potential to decrease miscarriage rates and possibly increase implantation rates. Further studies are required to demonstrate whether or not patients with repeat IVF failures, recurrent pregnancy loss, and advanced maternal age could benefit from routine application of PGT. Finally, PGT offers the promise to become an important tool in clinical IVF to select for transfer a single embryo with the greatest potential for implantation, thereby maximizing pregnancy outcomes and at the same time reducing the risk of multiple births.