5.6) Comparative Genomic Hybridization Array
Array CGH is a significant advance in technology that allows detection of chromosome abbreviation that are too small to be detected by looking down the microscope. Karyotyping is with any of the resolution of the microscope is not able to detect subtle chromosome changes. These smaller alterations, called submicroscopic alterations because they cannot be seen down the microscope, can still disrupt growth and development. These very small changes are often called micro deletions and micro duplications.
Array CGH is also called CGH array, aCGH or simply a microarray, compares DNA with a control DNA sample and identifies differences between the two sets of DNA. In this way, deletions or duplications (imbalances) in DNA can be identified.
In microarray tens of thousands of short sequences of DNA (“probes”), labelled in a precise grid on a glass slide called a chip. DNA from the patient is “digested” (chopped up into short lengths or fragments), then these fragments are labelled with a colored fluorescent dye.
Reference DNA, from a person, or pool of people, with no genetic abnormalities, is labelled with a different colored fluorescent dye. The fluorescent dyes commonly used are red and green. Reference and patient samples are mixed together and applied to the chip and hybridization takes place, the fragments of DNA hybridize with their matching probes on the array. The chip is then scanned in a machine called a microarray scanner which measures the amount of red and green fluorescence on each probe.
The computerized microarray scanner with analytical software calculates the ratio of fluorescent dyes to determine whether, for the piece of DNA represented by each probe, the patient sample has the correct amount of DNA, too much DNA (a duplication) which would be shown by too much red, or too little DNA (a deletion) shown by too much green
5.5.1) The benefit of array CGH
Array CGH has uplift results of cytogenetic study from the microscope to the computer, by combining CGH with high-throughput microarrays hundreds or thousands of discrete regions of the genome can be simultaneously analyzed and identification of unbalanced karyotypes can be done.
5.5.2 )Advantages of array CGH Technology
The primary advantage of array CGH is the ability to simultaneously detect aneuploidies, deletions, duplications, and/or amplifications of any locus represented on an array; in fact it is thousands time more promising then FISH experiments, it is efficient in labor intensive and expense features. Additionally array CGH has proven to be a powerful tool for the detection of submicroscopic chromosomal abnormalities in individuals with idiopathic mental retardation and various birth defects.