Preimplantation Genetic Diagnosis: the ultimate treatment for couples at risk?
Preimplantation genetic diagnosis (PGD) is the earliest form of prenatal testing and is fast becoming an established technique with well over 10,000 cycles performed to date. Embryos from the “at risk” couple are created through IVF. One or two cells are biopsied from each embryo at the 6-10 cell stage and tested for the relevant genetic condition. Unaffected embryos are selected for transfer to the patient, obviating the need for testing, and possible termination, during pregnancy.
PGD is commonly performed for high risk couples including carriers of monogenic conditions and chromosomal rearrangements. Chromosomal translocations are detected using fluorescent in situ hybridisation (FISH) with a combination of telomeric and centromeric probes that that will detect all possible segregants. Whenever possible, additional probes are used to detect ploidy of other chromosomes.
Detection of monogenic conditions using single cell PCR almost always involves direct detection of the mutation as well as confirmation using a number of linkage markers providing increased certainty of diagnosis.
Single cell molecular diagnostics have advanced considerably over recent years and, at Melbourne IVF, we obtain an informative result on 97% of all embryos tested using PGD.
The benefits of PGD include a high certainty of a good outcome once a pregnancy is established and avoiding the need for a termination of pregnancy. Costs include that PGD is invasive, expensive and has a relatively low (~20%) chance of pregnancy with each treatment.
We find that most patients who embark on PGD have already had one or more miscarriages or pregnancy terminations after prenatal diagnosis. The majority of patients return for subsequent PGD cycles if they are not successful after the first attempt.