PGD, or preimplantation genetic diagnosis, is essentially a form of genetic testing that is recommended as a part of IVF. PGD testing is recommended to IVF couples when there is a likelihood of a genetic disorder being passed on to the embryo. PGD is conducted in the form of extensive embryo testing, where embryo samples prepared for implantation into the uterus of the IVF female are tested. As a result, embryos indicating the possibility of genetic defects are removed and healthy embryos are used to proceed with the IVF treatment. Despite the use of many innovative technologies in PGD, this testing methodology presents some serious limitations.
Limitations Due to Need for Embryos
Preimplantation genetic diagnosis needs a certain number of embryos for executing the various tests. However, this is not favorable for all IVF couples. Among many couples, embryos prepared through in vitro conditions are already very limited. This is because the vulnerability of the oocytes or sperm is sustained despite artificial fertilization. This also means that unless the results of preimplantation genetic diagnosis come through, the embryos need to be further cryopreserved.
Damage to cryopreserved embryos is already a major issue in IVF settings, and PGD demands add to the problem. PGD testing is time consuming and the concluding reports can take nearly 72 hours to come through. The higher loss of frozen embryos means the need to repeatedly extract eggs for creating more embryos, which is both demanding and financially straining for the IVF couple. A commonly unacknowledged part of this problem is that PGD testing uses biopsy of the embryos. If this procedure is not done with extreme precision, cell division within the embryo can be delayed, which can further lengthen the cryopreservation requirement since embryos are implanted only after they enter a particular state of cellular development.
Possibility of Errors in Detecting Single Gene Defects
Examples of single gene defects include cystic fibrosis, B-thalassemia, Tay-Sachs syndrome and muscular dystrophy. Single gene defects or Mendelian disorders are among the more difficult of genetic conditions to decode during PGD testing. Though PGD testing has proven somewhat successful in testing for certain types of single gene abnormalities, the results aren't always conclusive. PGD methods like FISH (Fluorescent In-Situ Hybridization) are able to decode the underlying single gene problems but across a limited number of chromosomes. Even the most advanced of PGD testing methods like PCR (Polymerase Chain Reaction) are prone to occasionally failing to detect single gene defects.
Limitations in Detecting Chromosomal Translocations
Translocation refers to abnormal linking between the chromosomes. Chromosomal translocations can cause serious genetic abnormalities like leukemia and infertility being passed on to the offspring. Though PGD testing can decode the likelihood of translocations, it is error prone. The limited supply of embryos for PGD testing and the practice of testing random embryos means that some chromosomal translocations are often undetected.
Limitations in Detecting Mosaic Down Syndrome
Preimplantation genetic diagnosis is commonly used for testing only 11 chromosomal pairs out of 23 present in the embryo. This means that about half of the chromosomal pairs in each embryo are usually untested. While certain genetic defects can be diagnosed by testing only a few of the chromosomal pairs, some like Mosaic Down Syndrome, need comprehensive testing. Mosaic Down Syndrome refers to the trisomy of the 21st chromosome and it is commonly acknowledged that PGD is usually unable to identify this genetic abnormality.