PREIMPLANTATION GENETIC DIAGNOSIS – WHAT YOU NEED TO KNOW

Preimplantation genetic diagnosis (PGD) is a method for detecting genetic defects in fertilized embryos obtained through in vitro fertilization (IVF).  PGD is performed when either one or both parents of the embryo are known to carry a genetic disorder.

On the other hand, preimplantation genetic screening (PGS) refers to the testing of embryos before implantation to check for chromosomal defects, when both parents are assumed to have no genetic abnormalities. Preimplantation genetic tests prevent the transfer of genetically compromised embryos to the intended mother, and therefore prenatal diagnostic tests (such as, amniocentesis) may be avoided.

PGD was first successfully practiced in London in the 1980s as a means of testing for X-linked chromosomal disorders. Today, PGD follows well-established procedures for identifying most common genetic disorders. Unlike PGD, preimplantation genetic screening (PGS) is a newer technology that remains somewhat controversial.

Recommendations: The American Society of Reproductive Medicine supports the use of PGD for detecting serious adult-onset conditions that have inadequate available treatment measures. Couples with any of the following conditions may use PGD before embryo transfer:

• a medical history of X-linked disorders
• chromosomal translocations
• autosomal recessive diseases
• autosomal dominant diseases

More recently, PGD has been used for finding an HLA-matched sibling for a child with severe recessive disorders. The hope is that the cord blood cells of the HLA-matched healthy sibling will cure the affected older child.

The factors that may necessitate a preimplantation genetic screening (PGS) include:

• repeated failure with IVF
• recurrent pregnancy loss
• advanced maternal age
• male partner with severe infertility

Process: Whereas PGD looks for specific known genetic diseases in the embryo, PGS tests for overall chromosomal abnormalities, such as aneuploidy.  During these tests, an individual cell from a pre-embryo (8-cell stage, or blastocyst stage) is biopsied and analyzed. The evaluation is completed within 1 – 2 days, and only the healthy embryos are inserted into a woman’s uterus to help start a pregnancy.

Advantages: Many genetic diseases have no available cures in the market. Medical interventions are there which can alleviate the symptoms in certain genetic diseases, even leading to the prolongation of life. However, the side effects from treatment can lead to additional complications. In this regard, PGD provides a means of detecting in the pre-embryo stage the presence of known genetic disorders, such as cystic fibrosis, X-linked dystrophies, Tay-Sachs disease, Huntington disease. Only the healthy embryos are used to initiate a pregnancy.

One way of detecting genetic disorders in the fetus is through prenatal testing, such as amniocentesis or chorionic villus testing, done between 10 – 16 weeks of the pregnancy. However, if a genetic disorder is confirmed at this time, parents have to face the stressful decision of terminating or continuing  the pregnancy.

Previously, couples with known genetic diseases could not think of having their own children in the fears of passing on the heritable disease. PGD provides these couples a way to test the pre-embryos and ensure that the future child does not carry the same defective genes. A few hundreds of children have been born through both PGD and PGS, and so far the live births have not presented any health concerns from these early diagnostic procedures.