PGD – What is it?
Preimplantation Genetic Diagnosis (PGD) is an advanced technology combining principles of prenatal diagnosis, molecular biology and genetics. PGD enables screening the in vitro fertilized (IVF) embryo for major chromosomal or genetic abnormalities before embryo transfer.
At Southern California Reproductive Center, we use PGD to screen embryos for hereditary single gene disorders (such as cystic fibrosis and Tay-Sachs), or for the major chromosomal abnormalities (aneuploidy) found in miscarriages and live born infants. The first reports of PGD used in conjunction with IVF treatments appeared in the early 1990’s and since then thousands of normal, healthy babies the world over have been born through the use of this advanced reproductive technology.
Who Can Benefit from PGD?
PGD can benefit couples who have a family or personal history of inherited genetic diseases or chromosomal abnormality. PGD is also the most reliable method of gender determination to enable choosing the sex of the infant, or to prevent a sex chromosome-linked disease. PGD is an excellent option for women over 35 years old who wish to reduce their likelihood of a chromosomally abnormal infant. PGD has also been shown to increase pregnancy rates in older women and those who have a history of recurrent miscarriage or repeated IVF failures.
How is PGD Performed?
PGD requires patients to undergo an IVF cycle in order to produce embryos available for testing. Before the embryos are transferred into the uterus, one or two cells (called blastomeres) are removed from the embryos. These blastomeres are evaluated for genetic or chromosomal abnormalities, and embryos that are negative for the tested abnormality can be transferred.
What Can Be Determined About a Pre-Embryo by PGD Testing?
PGD can be used for determining chromosomal number and oftentimes structure (PGD-FISH, or now, PGD-CGH) or the presence or absence of specific genes related to hereditary disease (PGD-PCR). Chromosomal abnormalities occur as a result of errors in cell division, resulting in an incorrect number of chromosomes within that cell. Some chromosomes are more likely than others to become unpaired, which can lead to early embryonic demise (chromosomes 13, 16, 18, and 22) or affect term births, such as Down syndrome (trisomy 21). Numerical mistakes in the sex chromosomes can lead to Turner’s syndrome (XO), or Klinefelter’s syndrome (XXY). Sometimes, chromosomes can break apart and attach to other chromosomes, a condition known as translocation. Screening for normal chromosome pairing is done using a technique called fluorescent in situ hybridization (FISH) or comparative genomic hybridization. PGD can also help screen the embryo for single gene disorders (SGA) such as cystic fibrosis, Tay-Sachs disease, Duchenne muscular dystrophy, sickle cell anemia and X-chromosome-linked disorders. The method of analysis for specific gene disorders involves a technique that greatly multiplies the genetic code contained in a single blastomere, so that the specific gene in question can be detected. This technique is called polymerase chain reaction (PCR).
PGD-PCR (Polymerase Chain Reaction)
Examples of hereditary conditions caused by gene disorders are cystic fibrosis, Tay-Sachs disease, Duchenne muscular dystrophy, sickle cell anemia and X-chromosome-linked disorders. The method of analysis for gene disorder involves a technique that greatly multiplies the genetic code contained in a single blastomere, so that the specific gene or genes being looked for can be detected. This technique is called polymerase chain reaction (PCR)
Can PGD of Pre-Embryos Improve the Chance of Conceiving Through IVF?
Until the development of PGD, assisted reproductive technology labs could rely only on the appearance of pre-embryos to decide which ones were suitable for implantation into the patient’s uterus. PGD has proven that normal appearing pre-embryos frequently can have an abnormal number of chromosomes, rendering them incompatible with the development of a normal pregnancy.
PGD allows embryologists to select pre-embryos that are balanced for the tested chromosomes. PGD testing allows the implantation of only select pre-embryos increasing the chance for pregnancy, and, since the quality of the pre-embryo has been proven, less pre-embryos are needed to establish a successful pregnancy, greatly diminishing the chances of a multiple pregnancy.
What Does the Future Hold for Genetic Testing of Embryos?
In 1999, the genetic code of an entire human chromosome was sequenced, or ‘mapped’. The completion of this revolutionary development in medical science will allow the continuing discovery of the sequence of the code that comprises specific units of genetic information. Genes are responsible for directing the construction of every cell in our body, down to the tiniest molecular detail. Therefore, it is reasonable to expect that the function of all genes, and the detection of abnormal genes, comprising the human genome will be categorized. This knowledge may in turn be used to reduce or eliminate susceptibility to hereditary, infectious and cancerous diseases. Someday, in ART application, only a single pre-embryo will be needed to establish a pregnancy – and it will be a healthy pregnancy.
PGD Can Improve the Chance of Conceiving Through IVF?
Until the development of PGD, assisted reproductive technology laboratories could rely only on microscopic appearance when selecting embryos for transfer. PGD has demonstrated that normal appearing embryos frequently can have an abnormal number of chromosomes, rendering them incompatible with the development of a normal pregnancy. PGD testing allows the transfer and implantation of embryos that are normal for the chromosomes tested, increasing the chance for pregnancy