PGD, or preimplantation genetic diagnosis, is recommended to IVF couples who have a personal or family history of genetic disorders. This testing is vital because the parental genetic disorder can be transmitted to the embryo. This essentially means an increased likelihood of the IVF couple having a child with genetic abnormalities. However, with timely PGD testing, embryos with genetic defects can be identified for systematic elimination. One of the most common forms of chromosomal abnormality is aneuploidy. This refers to an unusual number of chromosomes in the embryonic cells. Embryos with aneuploidies have a very high tendency to develop birth defects and most of these are chronic, life-threatening conditions.
The abnormality in the number of chromosomes can be of both types: excessive or deficient number of chromosomes. The embryo is formed from the male and female reproductive cells, each with 23 chromosomes. Thus, normal embryonic cells should have 46 chromosomes.
The more common form of aneuploidy is the presence of an extra chromosome. Extra chromosomes 13, 18 and 21 are the most common of aneuploidies. These are also referred to as chromosomal trisomy. Down syndrome or trisomy 21 is perhaps the most acknowledged form of aneuploidy disorders. Trisomy 18 is called Edward Syndrome and is less prevalent than Down syndrome, whereas Trisomy 13 or Patau syndrome is extremely rare.
Fluorescence In Situ Hybridization (FISH)
FISH is among the most recommended of PGD testing measures. It is used for determining sex-linked genetic diseases and chromosomal abnormalities, including aneuploidy. In fact, FISH testing is commonly referred to as aneuploidy screening. This is essentially a color-based testing. Before proceeding with FISH, several samples of DNA are prepared through a DNA amplification method. Multiple DNA samples are created, each similar to the original DNA of the embryo cells. Then each DNA strand is used for testing with a fluorescent dye.
The dye-coated samples are microscopically examined by the technician, and chromosomes are identifiable due to the color they develop upon absorbing the dye. The aneuploid cells are easily distinguished due to presence of extra or fewer colored segments. Fluorescence in situ hybridization is most useful for detecting aneuploidy in chromosomes 1, 16 and the sex chromosomes (X, Y), in addition to chromosomes 13, 18 and 21.
Application of Aneuploidy Screening
Aneuploidy screening is also recommended even if the IVF couple doesn't present a family or personal history of inherited disorders. The screening is recommended as a part of PGS, or preimplantation genetic screening, which is done to gauge the overall health of embryos and not just inherited disorders. In such cases, FISH is more likely to be recommended if the IVF procedure involves a woman of advanced maternal age or if the IVF male is suffering from severe male infertility. These two groups have a dangerously high propensity of developing chromosomal abnormalities.
Similarly, IVF couples with a history of recurrent IVF failure are recommended aneuploidy screening to ensure that aneuploidy-linked loss of pregnancy is averted. However, this testing method cannot identify all the chromosomal with aneuploidies, since PGD testing is aimed at testing a restricted number of chromosomes.