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Preimplantation Genetic Testing

Aneuploid embryos and preimplantation genetic testing

Aneuploidy is the term associated with an abnormal number of chromosomes found in a cell within an early human embryo. This term literally means not or without the correct number of chromosomes. A genetically normal embryo is made up of cells that each contain 46 chromosomes, numbered 1-22 (autosomes) plus the sex chromosomes X and Y. Embryos with the correct number of chromosomes are also called euploid embryos. Unfortunately, aneuploid embryos do not physically look any different than euploid embryos so without preimplantation genetic testing for aneuploidy (PGT-A), aneuploid embryos can and will be transferred and these embryos have been shown to fail implantation in about 96% of the cases1. It is therefore important to identify and selectively transfer euploid embryos which have been shown to have a very high chance of successful implantation and pregnancy.

For research use only. Not for use in diagnostic procedures.

Preimplantation genetic testing-aneuploidy

The most significant recent advance to improve IVF success rates has been the introduction of 24 chromosome preimplantation genetic testing-aneuploidy (PGT-A).

The purpose of PGT-A is to identify embryos with the correct number of chromosomes for IVF transfer. PGT-A cannot correct aneuploid embryos; it can only identify those embryos that are not suitable for transfer.

Selecting only euploid embryos to transfer with PGT-A has been demonstrated to:

  • Reduce the time to pregnancy by reducing the number of cycles/transfers needed to become pregnant 
  • Reduce the risk of miscarriage 
  • Allow only the selection of euploid embryos for freezing, avoiding the expense of storing embryos unsuitable for transfer
  • Overcome the adverse effect of maternal age on IVF success by focusing on euploid embryos 
  • Reduce the risk of multiple pregnancies from IVF
Preimplantation genetic testing for monogenic diseases (PGT-M)

Thousands of different single gene mutations, cancer predisposition genes, and now multi-factorial diseases can be screened in the early human embryo before transfer during an IVF cycle1. Having multiple cells to work with removes some of the potential issues that surrounded early attempts at single cell biopsy including reductions in allele drop-out (ADO) and preferential amplification (PA). Allele drop-out arises in the early stages of amplification by polymerase chain reaction (PCR), where one allele of a heterozygote is not primed and products for this allele are not created. 

In a heterozygote cell, this means that one allele will be missed which can lead to misdiagnosis as a homozygous embryo2. Preferential amplification is similar to ADO except that some product is made for one allele while more product is made for the other allele. Again, depending on the down-stream method of analysis chosen, this can lead to errant results and potential misdiagnosis. Dreesen’s and colleagues have published a thorough review of misdiagnosis during PGT-M (PGD), and Warren and colleagues have shown how using the PG-Seq™ kit with TSE lessens or eliminates the issues of PA and ADO.

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