Pre-implantation Genetic Testing (PGT) is a technique used alongside In-vitro Fertilization (IVF) to check if the embryos created in an IVF cycle have abnormalities in their chromosomes or genes, before transferring them back into the womb. Couples who are known to have abnormalities either in the chromosomes or in the genes may wish to consider PGT so that an embryo not affected by the specific genetic disease can be selected and transferred to the womb and hopefully will result in the birth of a healthy baby. PGT can also be used by couples with no genetic disease but who wish to increase the chance of IVF success by selecting an embryo with a correct number of chromosomes.
To understand genetic diseases, we must know what chromosomes and genes are. Chromosomes are the “visible” genetic materials present in the cell nucleus. With the exception of a few cell types, such as red blood cells, sperm and eggs, every human cell carries 23 pairs of chromosomes, including 22 pairs of autosomes and one pair of sex chromosomes. A gene is the basic unit of genetic information and is composed of deoxyribonucleic acid (DNA). There are over 20,000 genes distributed over the 23 pairs of chromosomes. Each gene defines different function, controlling the various activities of the human body.
Genetic diseases are defined as diseases caused by abnormalities in genetic materials either in the chromosomes or in the genes. These diseases can be passed on from parents to the next generation, or new mutations can occur to an individual while his/ her parents have normal chromosomes or genes. Therefore, genetic diseases are not always inherited from parents.
Chromosomal abnormalities: These are caused by an incorrect number or an abnormal structure of the chromosomes. A well-known example of this group is Down syndrome, caused by an extra Chromosome 21, i.e. an incorrect number of chromosomes.
Single gene disorders: These are caused by abnormalities or mutations in individual genes. An example is Thalassemia. These disorders follow specific modes of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive and Y-linked inheritance.
Multi-factorial disorders: These diseases are caused by a combination of multiple genetic and environmental factors. Examples of these diseases include diabetes mellitus and hypertension.
There are three types of PGT classified by the genetic diseases that they are specifically designed to detect:
Pre-implantation Genetic Testing for Aneuploidies (PGT-A), previously called Pre-Implantation Genetic Screening (PGS), aims at detecting whether embryos have problems with the number of their chromosomes, because transferring an embryo with an incorrect number of chromosomes is thought to be the most common reason for IVF treatment failure. An embryo with a correct number of chromosomes can therefore be chosen to be transferred thereby increasing the pregnancy rate for each transfer attempt, reducing miscarriage rate and hopefully increasing the chance of having a healthy baby.
Pre-implantation Genetic Testing for Structural Chromosomal Rearrangements (PGT-SR), previously called Pre-implantation Genetic Diagnosis (PGD), is used when either one of the couple is known to have a structural chromosomal abnormality. The technique aims at detecting whether embryos have inherited the chromosomal abnormality so that an unaffected embryo can be transferred to the womb and hopefully will result in the birth of an unaffected baby.
Pre-implantation Genetic Testing for Monogenic / Single Gene Disorders (PGT-M), previously known as Pre-implantation Genetic Diagnosis (PGD), is used when either one of the couple is or both are known to carry an abnormal gene. The technique aims at detecting whether embryos have inherited the abnormal gene so that an unaffected embryo can be transferred to the womb and hopefully will result in the birth of an unaffected baby.