What is Genetic screening of Embryos in IVF or Preimplantation genetic screening for Aneuploidies (PGT- A) ?
Have you wondered what is genetic screening of embryos in IVF ? How is it done? Do you require it? Read on to know more……
Preimplantation genetic screening for Aneuploidies (PGT- A), earlier known as Preimplantation genetic screening involves testing the genetic material of the embryos before putting the genetically normal/ healthy embryos back into the uterus for implantation.
The genetic material is carried on structures known as chromosomes inside the nucleus of human cells. Every cell of the human body has 46 number of chromosomes. If there is a difference in the number of chromosomes for example 45 or 47, it means the cells are aneuploid.
PGT-A (Preimplantation genetic testing for aneuploidy) is based on 3 key factors:
- Developed embryos are often aneuploid, which means that they have a changed number of chromosomes or chromosomal abnormalities, and the percentage of these issues increases with the woman’s age (see the table).
- Transfer of genetically incompetent embryos into the uterus does not result in a successful pregnancy.
- The effect of chromosomal abnormalities on the embryonic morphology or appearance may be insignificant, which means that “nice-looking” embryos may be aneuploid.
|Patient’s age||Genetically normal embryos (euploid);%||Embryos with genetic abnormalities (aneuploid);%|
If embryos are assessed only morphologically, there is a likelihood of selecting aneuploid embryos for the transfer into the woman’s uterus, because visually they look just as good as or even better than other related embryos.
Initially the concept of Preimplantation genetic Diagnosis or PGD came to check for specific genes of chromosomes which may be passed on to children. The first baby was born by PGD in 1990.
Later on the application was expanded to check for most reproductively competent embryos to avoid failed attempts at pregnancy, miscarriages or the need for medical termination of pregnancies
Genetic testing of developed embryos is carried out by taking one or few cells from the embryo and testing it further.
Biopsy that is taking out a cell for testing can be carried out
- From polar body
- from cleavage stage embryo
- from trophoectoderm of blastocyst
The testing was initially carried out using FISH or PCR techniques but now the latest techniques of aCGH method (array Comparative Genomic Hybridization), or NGS method (Next generation sequencing) allow for simultaneous testing of all chromosomes in an embryo before the embryo is transferred into the uterus
As a result, PGS by aCGH or NGS makes it possible to choose the best and most viable genetically competent embryos, thus effectively increasing the rate of implantation, clinical pregnancy and successful birth.
- IVF/ICSI manipulation
- Cultivation of the received embryos up to day 5
- Embryo biopsy – several trophectoderm cell collection without harming the embryo
- Embryo freezing (vitrification) for later use
- PGS – Preimplantation Genetic Screening – Chromosomal analysis
- Selection of genetically competent embryos for embryo transfer
- Embryo transfer
PGT – A is indicated in the following patients who have a high chance of developing aneuploid embryos
- Women over 37 years
- Women over 35 years, if good morphology embryos develop, but pregnancy fails to occur
- Infertility of unexplained cause in young couples
- Two or more unsuccessful IVF cycles (transfer of five or more embryos)
- A history of two or more spontaneous abortions (including the missed abortion)
- Decreased sperm quality of the partner (oligoasthenoteratozoospermia, severe oligozoospermia or azoospermia, poor sperm morphology, high DNA fragmentation)
- Male infertility factor associated with AZF microdeletions (Y-chromosome)
- One or both parents are carriers of balanced chromosomal aberrations (Robertsonian or reciprocal translocations)
- One or both parents are carriers of X-chromosome mutation (risk of development of e.g. Duchenne/Becker muscular dystrophy, fragile X chromosome syndrome, haemophilia and other disorders)
- In case of altered karyotype in parents (one or both) or if there is a high risk of passing a genetic disorder to the child
A recent study comparing 600 patients undergoing IVF with half of them undergoing PGT- A and half of them not found that PGT-A may be detrimental for those <38 years old undergoing their first IVF cycle. PGT-A has the greatest clinical impact when a transfer is achieved in the ≥38 years old population (1). So PGT-A may be offered to patients > 38 years undergoing their first cycle also.
1.Lauren A Murphy, Emily A Seidler, Denis A Vaughan, Nina Resetkova, Alan S Penzias, omas L Toth, Kim L ornton,Denny Sakkas; To test or not to test? A framework for counselling patients on preimplantation genetic testing for aneuploidy(PGT-A), Human Reproduction, Volume 34, Issue 2, 1 February 2019, Pages 268–275, https://doi.org/10.1093/humrep/dey346