Pre-implantation genetic diagnosis or PGD is a technology that allows genetic testing of an embryo prior to implantation and before pregnancy occurs. The technology is used in conjunction with in vitro fertilization (IVF), and allows only embryos diagnosed as unaffected of a specific genetic disease to be transferred into a woman for pregnancy. Similarly, the testing is performed by removing a single cell from a 3-day or 5-day old embryo and having the cell genetically screened for a specific disease.
Types of Pre-implantation Genetic Diagnosis(PGD):
- PGD for Single Gene Disorders (PGT-M for Monogenic Disorders) is a sophisticated reproductive technology used to test early embryos for specific or single gene disorders. This treatment is important for couples that have a history of genetic disease which can be passed down to the child e.g., sickle cell anaemia, cystic fibrosis, beta-thalassemia etc. The test is done using Polymerase chain reaction (PCR) & genetic sequencing techniques.
A genetic work up is required. It involves blood samples from each partner and may need additional samples from other family members. This work up process ultimately enables scientists to identify all the affected and non-affected DNA patterns in the patient’s embryos.
- PGD for 24 chromosomes Aneuploidy (PGT-A for 24-chrom Aneuploidy) formerly known as PGS, is now used to describe screening embryos for sporadic chromosome abnormalities. The process is used to test embryos by counting the 24 chromosomes in order to detect an additional or missing chromosome(s) (“aneuploidy”). The screening helps to reduce the chance of having a child with extra or missing chromosome. The test employs Next Generation Sequencing(NGS) technique. It is recommended for couples with recurrent pregnancy loss, a history of recurrent failed IVF/implantation and for couples whose female partners are 35 years and above.
- PGD for 5 chromosomes Aneuploidy (PGT-A for 5-chrom Aneuploidy) is used to test 5 chromosomes(13, 18, 21, X &Y) which are involved in the most common syndromes such as: Down syndrome, Turners syndrome, Patau’s syndrome and Edward’s syndrome. The test employs Next Generation Sequencing(NGS) technique or Fluorescent In Situ Hybridization(FISH) technique using probe set, specific for those 5 chromosomes. It is recommended for couples with recurrent pregnancy loss as well as a history of recurrent failed IVF/implantation, and for couples whose female partners are 35 years and above.
- PGD for Chromosomal Translocation (PGT-SR for Structural Rearrangements) formerly known as PGD for reciprocal and robertsonian translocations (balanced or unbalanced) or inversions, is used to test embryos for chromosomal abnormalities, abnormal chromosomal positions and rearrangements. It is recommended for couples with balanced translocation/inversion. The test employs Next Generation Sequencing (NGS) or Fluorescent In Situ Hybridization (FISH) technique by using specific fluorescent probes involving the translocation/inversion locus.
- PGD for HLA Typing PGD-HLA involves couples with a history of haematological genetic disorders such as beta-thalassaemia, sickle cell anaemia, Fanconi anaemia, Wiscott-Aldrich syndrome and X-linked adrenoleukodystrophy. In like manner, individuals affected with leukaemia or lymphoma, may benefit from stem cell transplantation, using an HLA-matched, related donor, most often a sibling child. This PGD technique is helpful in two situations:
(a) One child with a haematological inherited disorder and his parents need PGD in order to avoid another affected child. At the same time, HLA typing may bring the hope of saving the already affected sibling.
(b) One child with a non-inherited disease such as leukaemia, and his parents want to apply PGD with HLA typing alone in order to allow the newborn to be a donor to the sick child. The test is employed using Polymerase chain reaction (PCR) and genetic sequencing techniques.
