Preimplantation Genetic Testing for Monogenic/Single Gene Defects (PGT-M)

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 test carried out on embryos during in vitro fertilization, where one or both the biological parents have a monogenic or single-gene disorder like Tay-Sachs disease, cystic fibrosis or many other conditions.

This test helps reduce the risk of having a child with the same 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 preimplantation genetic diagnosis (PGT)

FertilitySmarts explains Preimplantation Genetic Testing for Monogenic/Single Gene Defects (PGT-M)

PGT-M examines the genetic material inside of embryos. The test involves taking multiple biopsied cells from the outer layer of the embryos (which will make up the placenta) ideally, on day 5 of development. The embryos that are shown to have a higher chance of developing into a healthy baby are selected for transfer to a uterus whereas embryos identified to have a genetic disease component are deselected.

Fertility specialists prefer testing the day-5 embryos due to three main reasons:

  1. 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 appreciated by day 5. This allows to biopsy only the placental and not fetal cells, preventing any damage to the latter. It also greatly optimizes the implantation rates.
  2. The error rates are high with a day-3 biopsy as the embryo layers are not fully developed.
  3. The day-3 biopsy enables examination of only a "single" cell (instead of multiple cells on a day-5 biopsy) and thus may skip 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 donors, as well as from a family member with a known genetic disease; thus, reducing the chances of any errors.

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