Array CGH (comparative genomic hybridisation) is a laboratory technique used to look for alterations in chromosomes which are too small to see down a microscope. This technique still allows for the entire genome or chromosome to be analysed, enabling clinical scientists to detect alterations in a patient’s genome which may cause a genetic condition.
Array CGH bridges the gap between techniques which look for large chromosomal alterations (such as karyotyping) and those looking for changes in the sequence of a particular section of DNA (such as sequencing).
Array CGH compares patient DNA samples to a control sample which has no alterations.
Equal amounts of a patient DNA sample and a control DNA sample (which is known to have no genetic alterations) are labelled with fluorescent dyes and mixed together. The DNA mix is added to a microscope slide which is spotted with thousands of DNA probes. Each DNA probe represents one small region of a chromosome.
Patient and control DNA attach themselves to the DNA probes. Where there is no difference between the patient and control DNA equal amounts of each sample DNA will bind to the probes. Where there is a gain of genetic material in the patient sample, more patient DNA than control DNA will bind to the corresponding probes. Where there is a loss of genetic material in the patient DNA sample, less patient DNA than control DNA will bind to the corresponding probe.
Computer software detects areas of loss or gain of genetic material in the patient DNA sample by analysing the amount of fluorescence colour detected at each probe spot.
Computer software analyses differences in the amount of fluorescent colour at each probe on the microscope slide. Results are shown on a chart and provide the following indication:
Array CGH is often used to look for genetic alterations in children with developmental delay or congenital anomalies, where clinicians suspect the cause may be genetic.