The Principles of Clinical Cytogenetics - Extra Materials - Springer

Automation in the Cytogenetics Laboratory 127
Fig 12. Computer-controlled automated filter wheel. (Courtesy of Applied Imaging.)
chromosomal rearrangements, CGH will give insight in losses or gains of DNA within a chromosome
(see Chapter 17). In CGH, the probes are generated from two different sources: one from genetically
normal cells and the other from the patient sample. The two different probe sets are labeled
with different fluors. These two pools of probes are then hybridized to a slide with normal metaphases.
As the name indicates, the two probe sets will compete for hybridization to the corresponding loci.
The ratio of the of patient DNA to normal DNA will indicate whether the patient DNA is normal (the
ratio is 1 : 1) or whether there is an addition or deletion of DNA in any given region. When there is an
addition, the ratio will increase; when there is a deletion, the ratio will decrease.
Until recent, this technique was able to pick up additions and deletions in the order of 10 megabase
pairs (Mbp). However, current work has increased the resolution to the order of 3 Mbp (9).
Comparative genomic hybridization requires the use of a high-quality and quantitative FISH imaging
system with a dedicated CGH suite. This software suite will perform the following:
• Accurately measure and average the ratio of the two fluors over multiple metaphases. This requires
sophisticated algorithms.
• Correct the measurements for unequal chromosome length.
• Plot the ratios along the chromosome length for ease of interpretation, highlighting the areas of
statistically significant differences (see Chapter 17, Fig. 15).
With microarray CGH, specific DNA targets are “printed” onto a microscope slide and CGH is
performed in situ on the slide (see Chapter 17). A scanner reads the slide and sends the data to a
computer for analysis (see Fig. 14).