Preimplantation genetic diagnosis (PGD) is a laboratory procedure that helps to improve a couple's chance of having a healthy child. PGD allows a couple's embryos to be analyzed for the copy number of specific chromosomes or single gene disorders before pregnancy. Embryos that have the highest chance of resulting in a healthy pregnancy are then chosen for transfer into the woman's uterus.
Healthy pregnancies become much more likely if the early embryo can be genetically screened for abnormalities. In vitro fertilization offers an ideal opportunity to conduct such embryo screening. Following fertilization, the zygote naturally cleaves into “blastomeres”. These cells are virtually identical to each other. Genetically testing these blastomeres can usually reflect the genetic integrity of the entire embryo. During PGD, one or two blastomeres are carefully removed from an embryo via micro-manipulation procedures. In this manner; genetic testing can be safely and efficiently conducted prior to the implantation procedure.
Chromosomes are the packaging of our genetic material, or DNA. Human cells generally contain 46 chromosomes or 23 matching pairs, half from our mother and half from our father. If an incorrect number of chromosomes are present in an embryo (aneuploidy), there is an increased chance for the pregnancy to miscarry or for the birth of a child with physical and/or mental handicaps. The chance of numerical chromosome abnormalities increases with maternal age, regardless of family history, therefore affecting many women attempting to become pregnant. Trisomies (an extra chromosome) and monosomies (a missing chromosome) have a higher chance of miscarriage or may fail to implant, decreasing pregnancy success.
The most common chromosome abnormalities studied by PGD are trisomy 21 (Down syndrome), trisomy 13 and trisomy 18. Furthermore, PGD can test for unbalanced chromosome rearrangements, when one member of the couple is a known carrier of a balanced rearrangement such as a translocation.
PGD can be attempted for individuals or couples undergoing in-vitro fertilization (IVF) most commonly if there is a maternal age risk (> 35), a history of multiple miscarriages, a family history of a genetic disorder, and possibly other indications.
PGD is performed by blastomere biopsy. An embryo, artificially fertilized for IVF, is cultured in the laboratory for 3 days. By this time, the embryo is comprised of 6 to 8 blastomeres. A biopsy is then performed on the embryo with micro-manipulation techniques. Typically, 1 or 2 blastomeres are removed for analysis. At this early stage of embryonic development, the blastomeres have yet to differentiate, and are typically identical to each other in every meaningful way.
Blastomeres are genetically tested using one of two methods: Fluorescence In Situ Hybridization (FISH), or PCR-based DNA amplification. Results can be available within two days. During testing, embryos can be cultured or cryopreserved for future implantation.
Blastomere biopsies can test for maternal and paternal genetic contributions, as well as X-linked disorders by gender determination. Cryopreservation technology makes PGD even more practical and convenient. Essentially, blastomere testing has become the most popular and accepted method of PGD. Future applications even suggest the culturing of biopsied blastomeres for more exhaustive diagnostics.
A technology called FISH (fluorescent in situ hybridization) is used to examine the copy number of specific chromosomes in the embryo. This process uses different colored, fluorescent-tagged genetic probes, which bind to their designated chromosome. These fluorescent probes can then be visualized under the microscope and counted. Probes for chromosomes 13, 18, 21, X, and Y are routinely analyzed in blastomeres since they represent the most frequent aneuploidies in live births.
Overall detection rates of 92-93% can be expected when testing for aneuploidy. When testing for chromosome translocations, the specifics of the translocation involved can influence the accuracy of the results.
PGD does not rule out problems involving chromosomes not tested for, structural chromosome abnormalities (unless a specific translocation analysis is requested) or mosaicism. Mosaicism means some cells carry chromosome abnormalities and other cells of the same embryo do not. Studies indicate that mosaicism can occur in up to 30-50% of embryos. This phenomenon cannot be completely ruled out since only one cell is analyzed from each embryo. Either false-positive or false-negative results are possible. Additionally, in any pregnancy, there is about a 2-4% chance for birth defects regardless of family history that PGD cannot detect.
Yes, prenatal diagnosis is always recommended due to its greater sensitivity and specificity for chromosomal abnormalities, and broader diagnostic range. Options include the procedures of CVS, early and standard amniocentesis to confirm a normal chromosome constitution. These options can be discussed further with a genetic counselor.
Results for chromosome copy number are routinely reported out within 48 hours. Therefore, embryo transfer is usually performed 5 days after egg retrieval.
In addition to detecting chromosome abnormalities, PGD can diagnose some single gene disorders. One genetic disease currently tested for is cystic fibrosis (CF). Sex selection for X-linked disorders can also be performed.
Years of successful PGD work in animals and numerous human tests have proven the procedure to be both safe and reliable.
Note that potential problems with FISH technology include hybridization errors, other technical difficulties, and an incapacity to detect structural abnormalities frequently associated with birth defects. Problems with PCR technology include possible amplification complications, and other technical concerns. These may result in a small probability of missing an abnormality, or excluding a normal embryo. The estimated error rate, however, is less than 10%.
In some cases, mosaicism may further complicate the diagnosis. Although irrelevant to blastomere biopsy work, a correlation has been observed between mosaicism as identified by FISH, and that analyzed through karyotyping of amniotic cells. As a precaution, though, all patients who have utilized PGD should also routinely undergo prenatal testing.
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