Key Difference – PGS vs PGD
In vitro fertilization (IVF) is a complex process that is used to treat fertility and genetic problems and help in the conception of a child. When a married couple faces problems associated with the conception of a child, IVF serves as a good solution and is the most effective form of medical treatment. Mature egg cells are retrieved from the female ovary and fertilized from the sperms of the respective male under the laboratory conditions (in vitro) as an initial step. Then the fertilized egg is implanted in the uterus of the female. However, IVF is a complex, expensive and time-consuming process which has a lot of challenges. Hence, several medical tests are performed before its implantation. Preimplantation Genetic Screening (PGS) and Preimplantation Genetic Diagnosis (PGD) are two such experiments that are used to determine whether the embryo is free from genetic disorders and the baby is healthy. The key difference between PGS and PGD is that PGD is performed to detect single gene defects which can lead to specific genetic diseases while PGS is performed to detect chromosomal normalcy.
CONTENTS
1. Overview and Key Difference
2. What is PGS
3. What is PGD
4. Side by Side Comparison – PGS vs PGD
5. Summary
What is PGS?
Preimplantation Genetic Screening (PGS) is a genetic test which is performed to detect the chromosomal normalcy of in vitro fertilized embryo. PGS screens embryo cells for the normal chromosomal number. The human body contains a total of 46 chromosomes in 23 pairs. PGS does not look for specific genetic diseases; instead, it looks for the usual number of chromosomes in cells of the embryo. The embryo is evaluated for missing chromosomes or extra chromosomes which change the total number of chromosomes.
The objective of performing PGS is to identify abnormal chromosome number in the embryos prior to implantation in order to avoid genetic syndromes caused due to numerical aberrations of chromosomes. PGS reveals diseases such as Down syndrome, Klinefelter syndrome, etc. Down syndrome is a common disease caused due to trisomy of chromosome 21. It can be easily detected by PGS. Gender selection is also possible by PGS.
There are several methods used in PGS such as Next Generation Sequencing (NGS), Array Comparative Genomic Hybridization (aCGH), Single nucleotide polymorphism microarrays (SNP), Quantitative real-time polymerase chain reaction (qPCR), etc.
Figure 01: Preimplantation Genetic Screening
What is PGD?
Preimplantation Genetic Diagnosis (PGD) is a technique performed to identify single gene disorders in the in vitro fertilized embryo before implantation inside the uterus of the mother to establish the pregnancy. Single gene disorder is a genetic disease or a Mendelian disorder caused due to a mutation or DNA change in a particular gene. The gene which is mutated is known as the mutant or diseased allele. These disorders are inherited from parent to offspring. Hence, it is important to check whether the embryo is free from single gene disorders or not prior to implantation of the embryo. Single gene disorders are very rare in occurrence. However, if both mother and father are carriers of that particular genetic disease, offspring can inherit the disease. Hence, PGD offers an opportunity for the couples to select healthy and disease-free embryos.
PGD is performed for single cells taken from the embryo. It can reveal the presence of chromosomal translocations or mutants effectively. There are many single gene disorders. The most common diseases which are detected by the PGD are Cystic fibrosis, sickle cell anemia, Tay-Sachs, Fragile X, Myotonic Dystrophy, and Thalassemia. PGD offers a chance to determine the sex of the embryo before transplantation. It is an important offer for couples to balance their families. Hence, PGD serves as an important test to give birth to a healthy and disease-free baby in preferred sex. However, the result of the PGD test relies on a single cell evaluation. Hence, the effectiveness of the PGD is questionable. The invasiveness of the biopsy and chromosomal mosaicism also reduce the effectiveness of the PGD.
Figure 02: Preimplantation Genetic Diagnosis
Various methods such as Fluorescent in situ hybridization (FISH) and Polymerase chain reaction (PCR) are used to perform PGD. PCR is used to detect single gene disorders and FISH is used to find chromosomal abnormalities. Apart from FISH and PCR, single cell genome sequencing is also used as a method in PGD to characterize the genome of the embryo.
What is the difference between PGS and PGD?
PGS vs PGD |
|
| PGS is the genetic test performed to detect whether the in vitro fertilized embryo has the normal number of chromosomes in a cell or not. | PGD is a technique used to determine whether the in vitro fertilized embryo carries a specific gene mutation or DNA change in a gene which results in a genetic disorder. |
| Detection of Single Gene Mutations | |
| PGS does not detect single gene mutations. | PGD detect single gene mutations. |
| Diseases | |
| PGS is an important test to detect Down syndrome, Klinefelter syndrome, etc. | PGD is an important test to detect Cystic fibrosis, sickle cell anemia, Tay Sachs, Fragile X, Myotonic Dystrophy, Thalassemia etc. |
Summary – PGS vs PGD
PGS and PGD are two genetic tests employed to detect whether the in vitro fertilized embryo is suitable to be implanted in the uterus to establish the pregnancy. PGS is performed to detect whether the embryo has the usual number of chromosomes in a cell. PGD is performed to detect whether the embryo has single gene mutations which cause genetic disorders. This is the difference between PGS and PGD. Both tests are very important and should be carried out before implantation to screen the embryo for a healthy embryo implantation.
Reference:
1.Stern, Harvey J. “Preimplantation Genetic Diagnosis: Prenatal Testing for Embryos Finally Achieving Its Potential.” Journal of Clinical Medicine. MDPI, Mar. 2014. Web. 06 May 2017
2.Gleicher, Norbert, Vitaly A. Kushnir, and David H. Barad. “Preimplantation genetic screening (PGS) still in search of a clinical application: a systematic review.” Reproductive Biology and Endocrinology : RB&E. BioMed Central, 2014. Web. 06 May 2017