PGD/S (Pre-implantation Genetic Diagnosis & Screening)
PGD/S is an advanced laboratory technique which involves biopsy of individual embryos and genetic analysis of a few cells in order to identify embryos that carry either an abnormal number of chromosomes (aneuploidy) or specific defective gene(s) passed on by the genetic parents (Genetics). PGD is an invaluable, if invasive, approach to determining which embryos possess the best developmental potential or to minimizing transmission of serious genetic illnesses.
Biopsy of an 8-cell embryo
Biopsy of a blastocyst
Aneuploidy (abnormality in the number of chromosomes) is the most common cause of early miscarriages (Pregnancy Loss, Aneuploidy & PGS). Chromosome number abnormalities increase markedly with maternal age and contribute to the decreasing live birth rate in older women both with and without IVF (What Is Age Factor?). Cystic fibrosis (CF) is a good example of a genetic disease which can be diagnosed by PGD. It is the most common autosomal recessive condition in the general Caucasian population. Only people who have 2 defective genes for CF, one from each genetic parent, are affected and have the actual disease. The likelihood that a Caucasian man or woman carries a CF mutation is about 1 in 30. If two carriers have children, the chance that one of the children will have CF is 1 in 4 (25%) while half of the offspring would be carriers for one CF mutation. In order to detect whether a given embryo has CF mutations from both genetic parents, it is necessary to know the two exact mutations present in the family. To date over 900 deleterious mutations for CF have been described. The standard CF panel includes only the 25-90 of the most common mutations. Therefore, extensive additional testing is required on the partner if one genetic parent is known to be a CF carrier. PGD may allow a couple to decide which embryos are safe to transfer and which to discard because of presence of both mutations for CF. While PGD is quite accurate (~95%), it is not as reliable as prenatal diagnosis which tests a much larger number of cells. Therefore, prenatal genetic diagnosis is still considered in pregnancies achieved after PGD in at risk couples. The other relatively common autosomal recessive conditions which can be detected through PGD include: sickle cell anemia, Tay-Sachs, Thalassemia, Gaucher’s disease and several others. PGD can also be used to detect embryos affected by dominant diseases such as Huntington’s chorea and X-linked diseases such as hemophilia (Genetics). Overall, however, the genetic diseases with classic Mendelian inheritance patterns are rather uncommon. The much more common diseases with a genetic component, such as diabetes and heart disease, involve inheritance of multiple genes and complex interaction with the environment and thus cannot be currently diagnosed by PGD.