PGT – Preimplantation Genetic Testing
PGT, PGT-A,PGT-SR,PGT-M,PGS etc… You might have heard these terms many a times and you might have wondered what do they mean! In simple words PGT-Pre Implantation Genetic testing means genetic testing of an embryo before implantation.
What are the different types of testing?
PGT-A, PGT-SR and PGT-M are different types of pre implantation testings.
PGT-A means testing for Embryonic Anuploidies. Previously it was also called as PGS(pre implantation genetic screening). It will simply tell if the given embryo has required number of chromosomes. Ideally an embryo should have 23 pairs of autosomes along with one pair of sex chromosome. If it has any extra or less number then it will be picked up in this test.
Such anomalies are labelled as Embryonic anuploidies. Down’s syndrome(trisomy 21) is one such example.
PGT-SR denotes pre implantation genetic testing for structural rearrangement of chromosomes. Sometimes one part of a chromosome gets detached from its original place and gets attached to a different chromosome. This is called as structural rearrangement or chromosomal translocation. Such anomalies will be picked up by PGT-SR.
PGT-M stands for pre implantation genetic testing for monogenic or single gene disorder. Every chromosome has thousands of genes. Each gene is responsible for many functions. Loss or malfunction of a gene may cause gene specific disorder, e.g Thalassemia. Diagnosis of single gene disorder is possible through PGT-M.
Whenever a patient has previous history of a child with Down’s syndrome , PGT-A is indicated. Possibility of embryonic anuploidies increases with advanced maternal age(mothers age more than 35 years). Hence PGT-A is recommended whenever mother’s age is in above range. Some couples have history of recurrent implantation failure. In such cases as well PGT A may be helpful to select the best embryo for transfer.
PGT -SR is indicated in known cases of chromosomal translocations. Similarly PGT M is indicated when there is a family history of single gene disorder like thalassemia.
How is the testing done?
For PGT -A usually there is no need of any pre PGT work up , but for PGT -M and PGT-SR pre PGT work up is needed.
For this blood samples from the couple, as well as affected child or affected family members may be needed.
The embryos are made by the process of IVF/ICSI and they are cultured till day 5 or 6 /blastocyst stage. At that stage by a delicate procedure few cells from the trophoectoderm(outer layer of the embryo) are removed and sent for testing.
After that the embryos are frozen.
Frozen Embryo transfer of genetically normal embryos is planned after the report.
In some cases the biopsy procedure is done on day 3 of embryonic life where a cell is removed from the embryo for testing and the embryos are cultured further till we get the report.
The technique of PGT is time tested, safe and has been bringing joy to the life of many affected couples worldwide.