PREIMPLANTATION GENETIC SCREENING
It is a very advanced technique used to correct the genetic abnormalities in embryos formed through the IVF /ICSI prior to implantation.
After IVF/ICSI embryos are chosen on basis of morphology and not genetically tested,
hence can not ensure the chance of euploid embryos. As a result, the implantation rates
and subsequent pregnancy are not high. Embryos can have chromosomal anomalies
structural or numerical and also can be mosaic where embryos contain both normal and
abnormalities materials. Low-level mosaics is unable to implant. Knowledge about the abnormalities whether it is numerical or structural increases the chance of a euploid embryo, hence the implantation rate improves. The embryo is probed for each of the 24 chromosomes. Thus, by PGS the care provider can choose a better
embryo.
PGS ensures that certain genetic diseases due to the numerical aberration of the chromosome like Down’s Syndrome can be avoided.
INDICATION
- Maternal age is 35 (increased age itself has a higher chance of having a baby with a chromosomal problem (such as Down’s syndrome)
- Family history of chromosome problems or genetic disorders.
- History of recurrent miscarriages
- Recurrent implantation failure.
- Partner’s sperm is known to be at high risk of having chromosome problems
METHODS
First, we do control ovarian stimulation followed by ovum pick up and by IVF/ICSI makes the embryo then grow them up to day 3 or day5. Though the biopsy can be performed both on day 3 or day5 now it is routinely performed on day 5 embryo as the chance of loss of embryo is lower in day 5 and also the chance of mosaic embryo is less on day 5. A skilled embryologist then takes out cells (blastomeres) from the outer layer of the embryo.
The DNA of the cell is first amplified and then analyzed using different techniques
Comparative genomic hybridization technique helps to test complete chromosome sets that can be tested without cell fixation. It has also the advantage to detect chromosome abnormalities as well as translocation defects. It is almost 98 -99%accurate.
It can also be done like the recent technique of next generation sequencing.it reduces the introduction of artifacts during the amplification process.
BIOPSY TIMING
Can be done on polar body or day 3 (8 cell embryo) or day 5 embryo that is the blastocyst
stage of embryo.
But now day 5 is routinely practiced,
• Chance of mosaic embryo in day 5 is less.
• Inner cell mass remains intact.
• More cellular material can be obtained for genetic testing(<10 trophectoderm calls can be taken out).
Therefore now undoubtedly day 5 embryos are biopsied.
Prerequisite for PGS
- A highly trained embryologist capable of doing biopsy on a day 5 embryo.
- Perfect laboratory (IVF culture) conditions so that the embryo can grow to day 5, otherwise the embryos will not grow because of poor environment.
- Good freezing facilities as embryos after biopsy need to be freezed for result to come out. Then only the correct normal embryos are kept for future transfer and rest are discarded.
SUCCESS RATE
Chromosomal abnormalities are the most common cause of recurrent early miscarriages and recurrent implantation failure in IVF despite the transfer of best quality embryos morphologically as almost 40% of embryos will be genetically abnormal. In older women, 60-90% of the embryos may be abnormal. PGS does not change the genetic composition of the embryo hence does not improve the live birth rate but it improves the embryo selection efficacy and increases the implantation rate(40%to 60%) and decreases the miscarriage rates (from 33.5% to about 6.9%).
DISADVANTAGES
- Damage to embryo specially when performed on day 3 embryo.
- High chance of cancelation of cycle in trial of making blastocyst embryo.
- After the result comes out all embryo can be abnormally resulting in no transfer and wastage of whole money. If the embryo results co.es as low mosaic after counseling it can be transferred.
CONCLUSION
It can be the only gateway to any couples who are carriers of a genetic disease or facing recurrent
implantation failure. At the same time, it is a highly advanced technique requiring highly trained an
embryologist and high-quality IVF lab and the close collaboration of geneticists and fertility specialists is
required.