PREIMPLANTATION GENETIC DIAGNOSIS (PGD) OR PREIMPLANTATION GENETIC SCREENING (PGS)
Each cell in our body should have 46 chromosomes; 44+XX in the female and 44+XY in the male. Egg containes 46 chromosomes prior to ovulation and the sperm in the ejaculate has 23 chromosomes (22+X or 22+Y). In order to accomplish a healthy embryo which should have 46 chromosomes, the egg has to release half of its chromosomes to accommodate the other half from the sperm. In the process of ovulation and expulsion of the egg from the ovary, the egg releases half of its chromosomes (22+X) outside the egg which is called the first polar body. First polar body stays inside the shell of the egg called the zona pellucida and should be the mirror image of the egg chromosomally. Polar body can be removed adjacent to the egg and tested to get information about the chromosome make up of the egg. In cases of egg freezing, Dr. Bayrak recommends testing of the polar body to identify chromosomally normal eggs at the time of fertility preservation.
If the egg divides unevenly during ovulation, the final unfertilized egg is chromosomally abnormal (may have any number of chromosomes other than 23). A chromosomally abnormal egg never results in a healthy embryo, but may get fertilized and even implant, but most of the time results in a miscarriage. Rarely, nature will make a mistake and an abnormal egg may get fertilized, embryo may implant and result in a live birth such as Down’s syndrome (47 chromosomes).
Preimplantation genetic diagnosis (PGD) refers to the group of genetic tests that provide genetic and chromosomal information about eggs or embryos prior to embryo transfer with In vitro fertilization (IVF) treatment. PGD was recently renamed preimplantation genetic screening (PGS) due to its imperfect nature in identifying genetic and chromosomal abnormalities.
Historically, there were only two types of PGD tests available with IVF treatment. These included identification of chromosome pairs with a limited methodology called fluorescent in situ hybridization (FISH) and a more advanced method for specific gene testing called polymerase chain reaction (PCR). FISH is used to assess numerical abnormalities of chromosomes, whereas PCR allows for testing of specific gene disorders within a chromosome.
FISH test involves identification of a small part of the chromosome with a florescent dye and two separate signals connote the presence of the chromosome pair being tested. The limitation of test is the fact that it only evaluates the presence of a small piece of the chromosome and not the entire chromosome or the DNA. Additionally, conventional FISH test is not capable of testing all 46 chromosomes, although more recent ones (24-probe FISH/PGD test) allow a full karyotype, but the reliability is being questioned. It’s helpful in gender selection and for limited screening of chromosomes.
The newer generation of FISH test called the 24-probe FISH allows testing for all chromosomes, but the same limitations exist with the older version and its reliability is not well documented. Its use is possibly limited due to high false positive and false negative results as well as testing for a limited piece of the entire chromosome. It’s application to IVF practice is currently limited due to poor predictability and the multi-step complex technique utilized.
PCR is the method that amplifies a very small piece of the DNA within the chromosome and the DNA sequence is compared to a standard. It’s used for testing of specific genetic disorders such as sickle cell disease, cystic fibrosis, etc. Its use is limited for screening the embryos for chromosomal abnormalities.
The most recently developed PGD tests include comparative genomic hybridization (CGH) and single nucleotide polymorphism (SNP). CGH is a test that amplifies the entire DNA and compares to a standard normal DNA. SNP is a test that assesses for changes in the DNA sequence variations between chromosomes.
Based on the medical literature and experience, CGH appears to provide reliable results when testing the eggs and embryos. When CGH and conventional PGD (FISH) are compared, CGH appears to be superior because of the detailed analysis of the entire DNA. CGH may detect 30-40% more genetic abnormalities than PGD (FISH), which would have been undiagnosed. There are also more cases of CGH done for chromosomal screening of eggs and embryos in patients undergoing IVF treatment compared to other recently developed tests.
Although all PGD options provide information before implantation and pregnancy, they are based on testing from a single cell and not 100% reliable or accurate. Prenatal diagnostic tests such as chorionic villus sampling (CVS) and amniocentesis should always be considered in high risk patients