Preimplantation Genetic Diagnostics (PGD), Pregnancy Chances and Genetic Analysis Techniques
PGD or PIGD refers to procedures that are performed on embryos prior to implantation, sometimes even on oocytes prior to fertilization. This is considered another way form of prenatal diagnosis. When screening for a specific genetic disease, a PGD is a procedure that safe, and often is performed on women who are at a lesser risk of an unhealthy birth. PGD techniques are used, along with assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation.
The term pre-implantation genetic screening (PGS) is used to denote procedures that use PGD techniques to identify embryos at risk. An oocyte or early-stage embryo has no symptoms of disease. They might have certain genetic conditions that can lead to certain birth defects. To “screen” means to test for anatomical, physiological, or genetic condition. Therefore, both PGD and PGS should be referred to as types of embryo screening.
The procedures may also be called preimplantation genetic profiling as they are sometimes used on oocytes or embryos prior to implantation for other reasons than diagnosis or screening.
Oocyte selection or sperm selection procedures is done on prior to fertilization, although the methods and aims partly overlap with PGD.
Preimplantation genetic profiling (PGP) is a method of assisted reproductive technology to perform embryo selection of an embryo that is strong and healthy, thus increasing the chance of live birth. The results of PGP rely on the assessment of a single cell, therefore, the tested cell may not be representative of the embryo because of mosaicism.
Randomized controlled trials were performed and the conclusion is that there is no evidence of a beneficial effect of PGP as measured by live birth rate. It should be known that PGP is less successful with women of advanced maternal age. The inefficacy of PGP is often the result of the invasive techniques used in the biopsy, as well as the chromosomal mosaicism.
PGP is mainly carried out as a screening for detection of chromosomal abnormalities such as aneuploidy, reciprocal and Robertsonian translocations, and few cases for other abnormalities such as chromosomal inversions or deletions in women of advanced maternal age as well as those with repetitive IVF failure. Numerical chromosomal abnormalities explain most cases of pregnancy loss, and a large proportion of the human embryos are aneuploid, the selective replacement of euploid embryos often increases the chances of a successful IVF treatment. Comprehensive chromosome analysis methods include array-comparative genomic hybridization (aCGH), quantitative PCR and SNP arrays. These procedures combined with single blastomere biopsy on day-3 embryos, aCGH often has low error rates.
Techniques are in development that can avail for up to full genome sequencing, from which genetic profiling can score the DNA patterns , along with specific screening tests, by comparing with ones that have previously been found among embryos in successful or unsuccessful pregnancies.
Genetic Analysis Techniques
Fluorescent in situ hybridization (FISH) and Polymerase chain reaction (PCR) are the two commonly used, first-generation technologies in PGD. Other technologies are currently in development (such as whole genome amplification and comparative genomic hybridization). PCR is generally used to diagnose monogenic disorders and FISH is used for the detection of chromosomal abnormalities (for instance, aneuploidy screening or chromosomal translocations). Various advancements in PGD testing have allowed for an improvement in the comprehensiveness and accuracy of results available depending on the technology used. Developments to fix metaphase plates from single blastomeres has been rather successful. This technique in conjunction with FISH, m-FISH can produce more reliable results, since analysis is done on whole metaphase plates.