Definition - What does Preimplantation Genetic Testing for Monogenic/Single Gene Defects (PGT-M) mean?
Preimplantation genetic testing for monogenic/single gene defects (PGT-M), is a genetic screening carried out on embryos in coordination with an in vitro fertilization (IVF) cycle, where one or both the biological parents have an inherited monogenic or single-gene disorder like Tay-Sachs disease, cystic fibrosis or many other conditions.
This screening of the embryos helps identify healthy embryos and reduce the risk of having a child with the same inherited genetic disorder. Monogenic disorders are genetic disorders caused by the mutation (alteration) of a single gene; hence, they are also known as single-gene defects.
PGT-M was previously known as preimplantation genetic diagnosis (PGD)
FertilitySmarts explains Preimplantation Genetic Testing for Monogenic/Single Gene Defects (PGT-M)
PGT-M is a test to reduce the chance of having a child that has an inherited condition. examines the genetic material inside of embryos. The test involves:
- Developing a probe or test using the couple's existing DNA.
- An IVF cycle is initiated and the process of making embryos occur in an IVF laboratory.
- If some embryos develop to an advanced stage, multiple biopsied cells from the outer layer of the embryos (which will make up the placenta) ideally, on day 5 of development are removed from the embryos and the cells and the embryo are frozen as the testing process takes longer than the embryo can remain viable outside the body in laboratory conditions.
- The cells are tested at a genetics laboratory against the previously developed prove and are thought to be representative of the corresponding frozen embryo.
Normal embryos are shown to have a higher chance of developing into a healthy baby if they survive the thaw and selected for transfer to a uterus whereas embryos identified to have a genetic disease component are deselected.
Combining PGT-M with aneuploidy testing PGT-A is often combined at additional expense to maximize the amount of information that is known about the embryo and can be tested without additional cell biopsy or risks to the embryo.
Embryo vs. Blastocyst Biopsy
Fertility specialists prefer testing a blastocyst embryo for three main reasons:
- The two layers of these embryos, namely the outer cell layer (that will develop into the placenta) and the inner cell mass (that will mature into the baby) are easily evaluated at the blastocyst stage. This allows the biopsy of only the placental with a lower risk of not disrupting the fetal cells. It also greatly optimizes the implantation rates.
- The error rates of long term embryo damage to development are higher with a day-3 biopsy as the embryo layers are not fully developed.
- The day-3 biopsy enables examination of only a "single" cell (instead of multiple cells on a blastocyst biopsy) and thus may fail to identify the abnormal embryos.
Innovative versions of PGT-M like karyomapping are now being used because the amount of DNA available is very small, which can make the diagnosis difficult. Karyomapping involves studying the DNA from the sperm and egg contributors, as well as from a family member with a known genetic disease; thus, increasing the probability of correctly identifying affected embryos
- Preimplantation Genetic Screening: A Practical Guide. (2013).
- https://www.reproductivefacts.org/news-and-publications/patient-fact-sheets-and-booklets/documents/fact-sheets-and-info-booklets/preimplantation-genetic-testing/. (2014).