Preimplantation Genetic Testing (PGT) is the earliest form of prenatal diagnosis, done on embryos. During the IVF cycle, a single cell or a few cells are removed (biopsied) from the embryo and are checked for specific genetic abnormalities. The embryos that are reported normal are transferred into the mother’s uterus. PGT offers a new alternative to chorionic villus sampling or amniocentesis. For any PGT testing, one needs to have multiple embryos developed by ICSI.
The steps involved are:
1. Medical evaluation of the couple to check the suitability for an IVF/ICSI cycle.
2. Genetic evaluation of the couple
3. The ICSI cycle.
4. Embryo biopsy which is followed by embryo cryopreservation.
5. Genetic testing of embryos (PGT)
6. Transfer of unaffected embryos
7. A Pregnancy test 14 days later.
Utility of PGT:
Improves pregnancy rates and decreases spontaneous abortion rates in:
1. Older age women
2. Previous unsuccessful IUI/IVF attempts
3. History of recurrent spontaneous miscarriages
4. Male factor infertility
5. Past history of repeated terminations because of an abnormality diagnosed during pregnancy
6. Known carriers of a genetic defect (e.g. Beta-Thalassemia, Sickle cell anemia, Cystic Fibrosis, Spinal Muscular Atrophy, Duchenne Muscular Dystrophy, Fragile X Syndrome, Huntington Chorea or a chromosomal translocation
• Prevents chromosomally or genetically abnormal births in couples having a history of failed fertilization after IVF-ICSI, missed abortions or carriers of balanced translocations.
• Increases the chances of implantation.
• Reduces the incidence of spontaneous miscarriages.
• Reduces the incidence of multiple offspring.
• Eliminates the risk of a child being affected with a familial gene disorder.
• A previously affected child can be cured by a Saviour Sibling (HLA matched sibling) who can then donate the stem cells to the affected child. eg. Thalassemia major.
Different techniques offered at our Centre:
- PGT (PGS or PGT-SR) by Fluorescence In Situ Hybridization (FISH) for telomeric/cryptic translocations and inversions
PGT by the FISH technique is now mainly carried out in cases where the husband or wife is a carrier of a balanced translocation or inversion. This is required to select embryos free of related unbalanced rearrangements. Pre-PGT work up of the couple using specific probes for the rearrangements is necessary to check for an additional tiny or cryptic rearrangement which may not have been noticed earlier. The embryo biopsy is done on Day-5. About 4-8 trophectoderm cells are biopsied from each embryo and are fixed on slides and subjected to the FISH procedure using probes specific for each couple, as per the chromosomes involved. The fluorescent signals are observed using fluorescent microscopy technique. A normal cell will show two signals of each colour for each chromosome. If there are 3 signals or 1 signal present, it indicates abnormal embryos which are not transferred to the mother.
Our team has >20 years of experience in such cases and has reported the first livebirths for Robertsonian /Reciprocal translocations and inversions in India.
- PGT for all 24 chromosome aneuploidies (PGS or PGT-A) using the array Comparative Genomic Hybridization (aCGH) technique:
For 24 chromosome PGT, the biopsied cells are “tubed” and the DNA is amplified and tested for chromosomal aneuploidies using the microarray technique. Detection of all 24 chromosomes is possible on Day 5 of embryonic development. The results are available within 24 hours. The technique can also be used for detection of unbalanced translocations except those involving the telomeres (tips) of the chromosomes.
- PGT for all 24 chromosomal aneuploidies (PGS or PGT-A) using Next Generation Sequencing (NGS):
For PGT, the biopsied cells are “tubed”. The embryonic DNA is further subjected to a series of procedures and is finally sequenced using NGS technology to check for chromosomal aneuploidies. Detection of all 24 chromosomes is possible on Day 5 of embryonic development. The results are available within 24 hours.
4. PGT for single gene disorders (PGT-M or PGD):
Whole genome amplification is carried out to increase the quantity of DNA externally in a PCR machine. Then the amplified product is subjected to further testing for the single gene disorder. The parental genetic markers are matched with the embryonic markers in order to make a diagnosis.