Telomere FISH tests of metaphase chromosome preparations are currently in use in clinical laboratories. The test is also referred to as a "telomere panel" or "telomere testing," as it is the telomeric or distal end of the chromosome that is assessed. The test utilizes a complete set of chromosome arm-specific probes for loci that are within several hundred kilobases of their respective telomeres.33 The short arms of the five acrocentric chromosomes are not included since they contain only repetitive sequences, and imbalance of these regions has no clinical consequence. Regions of homology exist for the terminal ends of Xp and Yp, and also for Xq and Yq; therefore, one set of probes hybridizes to both sex chromosomes. This leaves a minimum number of 41 probes needed for complete analysis. The test uses standard FISH methodology in which two or more probes are grouped together into one reaction mixture by using different probe colors, and multiple hybridizations are carried out with each group of probes for one complete test (Figure 6-5).
Research surveys have used PCR analysis of genetic markers, such as VNTRs or STRs, on DNA samples. A disadvantage to this method is that it requires parental DNA samples, which are not always available. STR analysis is more amenable to automation than the FISH assay, and could ultimately be less costly; however, it is more likely to yield false-positive results because of the presence of sequence polymorphisms.32 The FISH method is more labor-intensive and requires actively dividing cells, but can also determine the position of each signal. This has particular benefit in detecting reciprocal translocations, because a de novo balanced translocation could generate an abnormal phenotype through gene disruption.
A recently introduced testing method uses comparative genomic hybridization (CGH) with subtelomeric FISH probes in a microarray format. The microarray contains bound subtelomeric probes that are hybridized with patient ip signal missing
Figure 6-5. Subtelomeric FISH detection of abnormal chromosome 1,which is missing material from the distal short arm (1p) and contains additional material from distal 9q. (a) Hybridization mixture with probes for subtelomeric regions of 1p, 1q, Xp/Yp, and X centromere (control probe; aqua signals). Note absence of green signal on one 1p ter-
DNA labeled in one color, and a control DNA labeled in a second color. Significant deviation from a 1:1 hybridization ratio indicates an area of imbalance in the patient's DNA.All probes can be assessed on a single chip from one DNA specimen, without the need for cultured cells or parental samples, and with the potential for automation.
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