Throughout your pre-implantation testing journey, you might hear terms like PGT-A, PGT-M, PGT-SR, PGT-HLA, NGS, PCR, etc. All these letters can get pretty confusing if you aren’t sure what they mean, so let’s break it down.
There are 4 types of preimplantation genetic testing (PGT). The first and most popular is preimplantation genetic testing for aneuploidy (PGT-A). PGT-A tests for chromosomal abnormalities to ensure that an embryo has the correct number of chromosomes and there are no large deletions or duplications in genetic material. Transferring a euploid, or chromosomally normal, embryo increases the chance of a successful pregnancy while avoiding chromosomal abnormalities such as Down Syndrome and Trisomy 18.
The second type is preimplantation genetic testing for monogenic disorders (PGT-M). This testing is beneficial for patients who have a genetic disorder that runs in their family or are found to be carriers of a disease through carrier screening. PGT-M is used to identify embryos that are affected with the inherited disorder. This allows patients to only use unaffected or carrier embryos to significantly decrease the chance of having a child with the genetic disorder.
Another type of test is preimplantation genetic testing for structural rearrangements (PGT-SR). PGT-SR tests embryos for a specific translocation, inversion or other chromosomal rearrangement that was previously identified in the patient or partner. This testing allows for the identification of embryos with unbalanced translocations. A balanced translocation is when genetic material is exchanged evenly and usually results in a fully functional gene. Patient’s with a balanced translocation are usually not affected by it, however their children may inherit an unbalanced translocation. Unbalanced translocations occur when there is an uneven exchange in genetic material which results in extra or missing genes. Embryos with unbalanced translocations likely will not result in a successful pregnancy.
Last but not least is PGT-HLA for Human Leukocyte Antigen matching. This test identifies embryos that are HLA matches to a living child that is in need of a cord blood or bone marrow transplant due to a genetic disorder. PGT-HLA is often performed alongside PGT-M to determine embryos that are both unaffected with the inherited disorder and an HLA match.
Now that we have briefly gone over the main types of testing, let’s move on to the methods of analysis. Next-Generation Sequencing (NGS) and Polymerase Chain Reaction (PCR) are 2 different methods of DNA analysis and each are used for a particular type of testing.
NGS is a method of sequencing where DNA from the embryo is compared to a known normal control using tens of thousands of genetic markers within the human genome. Computers generate a chromosome map for each embryo which is interpreted by our lab and translated into a final report. NGS allows for the screening of multiple chromosomes in the genome at the same time and is therefore the most appropriate method of analysis for PGT-A testing.
On the other hand, PCR is used in combination with gel electrophoresis to analyse small regions of DNA to identify a specific mutation. PCR is used to amplify the embryo’s DNA to create millions of copies so there is enough material for our lab to analyse. After PCR amplification, gel electrophoresis is performed to determine which embryos have the mutation. Gel electrophoresis is a technique that separates DNA fragments based on size. Different sized fragments move through the gel at different speeds when an electric current is applied. This helps to identify a normal gene from a mutant gene. PCR and gel electrophoresis are used for PGT-M testing to identify embryos with an inherited genetic mutation.
PGT-SR can be completed by NGS or PCR depending on the size of the structural rearrangement. NGS cannot detect structural rearrangements less than 5-10 Mb in size. Therefore, smaller structural rearrangements may need to be analyzed by PCR instead. Our lab director reviews each PGT-SR case to determine the most appropriate analysis method.
While seemingly confusing, the alphabet of PGT can be arranged in a way that is comprehensible. If the letters get away from you, don’t hesitate to reach out to your healthcare provider or ask a friendly clinical team member at your PGT lab.
By Carissa Huber, Case Coordinator