Preimplantation genetic diagnosis (PGD) refers to the procedure that is performed on embryos prior to implantation, sometimes even on oocytes prior to fertilization. This is one of the forms for prenatal diagnosis. PGD is an adjunctive procedure to In Vitro Fertilization (IVF). PGD starts with embryo biopsy which can be performed on day 3, day 4 or day 5 of development. It is used to prevent genetic diseases or to select the most suitable embryos for transfer to establish a pregnancy. Therefore, controlled ovarian stimulation, egg retrieval, fertilization and embryo development occur as usual with IVF. Usually embryos are biopsied by our experienced embryologists using micromanipulation techniques to remove one cell without impacting the further development of the embryo. PGD is a safe procedure, it is used for screening for a specific genetic disease, is usually performed on women who are at a higher risk of an unhealthy birth. The removed cell can be tested by several methods including:
- Polymerase Chain Reaction (PCR) for possible gene disorders
- Fluorescence in Situ Hybridization (FISH)
- Chromosomal Microarray Analysis (CMA)
Let's cover in more details the Fluorescence in Situ Hybridization (FISH) method.
FISH is the most commonly used test to determine the chromosomal constitution of an embryo. This method identifies certain regions on chromosomes using fluorescent DNA probes. FISH tests can find small pieces of chromosomes that are missing or have extra copies. These small changes can be missed by the overall karyotype test (another type of chromosomal test). FISH analysis can, for instance, be used to reveal the missing fragments of DNA on chromosome 22 that are characteristic of velocardiofacial syndrome. In contrast to karyotyping, it can be used on interphase chromosomes, so that it can be used on PBs, blastomeres and TE samples. The cells are fixated on glass microscope slides and hybridised with DNA probes. Each of these probes are specific for part of a chromosome, and are labeled with a fluorochrome. There is a limited number of fluorochromes, therefore the number that can be analyzed concurrently is strictly limited.
When the X-linked chromosome disorder is not present, the X and Y chromosomes are used instead as an internal FISH control for sex determination. More probes can be added to check for aneuploidies, particularly those that could give rise to a viable pregnancy (such as a trisomy 21). The use of probes for chromosomes X, Y, 13, 14, 15, 16, 18, 21 and 22 has the potential of detecting 70% of the aneuploidies found in spontaneous abortions.
Three consecutive rounds of FISH can be transported out on the same chromosomes. In the case of chromosome rearrangements, specific combinations of probes have to be chosen that flank the region of interest. The FISH technique is considered to have a low error rate.
The main problem of the use of FISH to study the chromosomal constitution of embryos is the elevated mosaic rate observed at the human preimplantation stage. More than 800 embryos were tested and the result from the analysis showed that approximately 75% of preimplantation embryos are mosaic, of which approximately 60% are diploid-aneuploid mosaic and approximately 15% aneuploid mosaic. Li and co-workers found that 40% of the embryos diagnosed as aneuploid on day 3 turned out to have a euploid inner cell mass at day 6. Staessen and collaborators found that 17.5% of the embryos diagnosed as abnormal during PGS, and subjected to post-PGD reanalysis, were found to also contain normal cells, and 8.4% were found grossly normal. However, it is questionable as to whether two cells from an embryo can result in proper test result. And the question remains as to whether viable embryos are being wasted due to the constrictions of the techniques used.
Who is the best candidate for preimplantation genetic diagnosis? PGD is indicated for patients with:
- Advanced maternal age
- Implantation failures
- Previous miscarriages
- Family history (including previous child born with chromosomal and genetic disorder)
- Parental aneuploidy
- Balanced translocation
- Some male factor infertility
- Family balance and sex selection
Preimplantation Genetic Diagnosis can be used to help:
- Decrease miscarriages
- Reduce the occurrence of Trisomic or Turner Syndrome offspring and other chromosomal abnormalities
- Reduce multiple pregnancies
- Determine when to use donor eggs in poor prognosis patients (For example: Advanced maternal age)