What is array comparative genomic hybridization?
Array CGH is an advance in technology and is a new screening test that detects a loss or gain of chromosomes. It is more sensitive than the standard test (FISH) which has the limitation of only testing 4 or 5 chromosomes at a time. Array CGH can analyze all 46 chromosomes in a single test and may discover an abnormality that would not have been detected by FISH.
To understand how array CGH works, you have to understand a little about DNA, genes and chromosomes
What is a chromosome?
A chromosome is a structure that contains tightly packaged DNA and genes. Humans have 46 chromosomes made up of 23 pairs. A mother passes 23 chromosomes to her child through her egg and a father passes 23 chromosomes through his sperm. There is one pair of sex chromosomes (two X chromosomes for females and one X and one Y chromosome for males) and 22 pairs of autosomes (non-sex chromosomes). Any alteration in the number of chromosomes can be attributed to changes that can occur during the formation of reproductive cells (eggs and sperm), in early foetal development or in any cell after birth. Aneuploidy is a chromosome loss or gain and the term euploid refers to a complete set of 46 chromosomes.
How does array CGH work?
Array CGH compares the egg or embryo’s DNA with a control (normal) DNA sample and identifies differences between the 2 sets of DNA. In this way, a loss or a gain of chromosomes in the egg or embryo DNA can be identified. The embryo DNA and control DNA are combined with green and red dye respectively and placed onto a slide containing thousands of spots of human genes corresponding to specific regions of chromosomes for analysis. The slide is scanned and the red to green ratio will determine if the DNA is normal or if there is a loss or gain of chromosomes. Euploid (normal) embryos are then selected and transferred improving the chances of a successful pregnancy.
Who should have array CGH testing?
Array CGH is suitable for all patients. Array CGH is helpful in younger women to diagnose the cause of repeated miscarriage, failed IVF or unexplained infertility. In most cases, it is the egg, rather than the sperm that determines the chromosomal integrity of the embryo. In older women approaching the age of 40, the number of eggs produced with abnormal chromosomes increases. These eggs will produce abnormal embryos. However if the normal embryos are identified, there is a higher chance that can result in a successful pregnancy. Also, the risk of miscarriage increases with advanced maternal age.
Array CGH is suitable for men who have been shown to have sperm at risk of carrying abnormal chromosomes, couples who have had a previous pregnancy with abnormal chromosomes or a child with birth defects as well as those with an inherited genetic disorder.
What are the advantages and limitations of array CGH?
Array CGH testing provides the most comprehensive analysis of chromosomes to date. All 46 chromosomes can be analyzed in a single test. Euploid (normal) embryos can be selected improving the chance of a successful pregnancy for couples.
The limitations of array CGH is that this test does not detect balanced chromosome translocations. This is because balanced chromosome translocations do not result in any loss or gain of chromosomal material. Array CGH cannot detect tiny changes such as a single base pair deletion.
Biopsies of the embryos are performed on day 3 or day 5, results will be discussed with the patient on the morning of transfer.
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