Learning Objectives

• Describe the mechanism of FISH using fluorescent DNA/RNA probes.
• Identify chromosomal anomalies detectable by FISH, including microdeletions and translocations.
• Compare the resolution of FISH to standard Karyotyping.

1. Mechanism of Action

Fluorescence In Situ Hybridization (FISH) is a cytogenetic technique that uses fluorescent probes that bind to only those parts of a nucleic acid sequence with a high degree of sequence complementarity.

• The Probe: A fluorescently labeled DNA or RNA fragment.
• The Target: Specific genes or sites of interest on a patient’s chromosomes.
• Visualization: Under a fluorescence microscope, the “glow” indicates the presence and exact localization of the target sequence.

2. Detection of Chromosomal Anomalies

FISH is far more sensitive than standard karyotyping for detecting submicroscopic changes.

• Microdeletions: Recognized by the absence of fluorescence on one chromosome compared to the presence of fluorescence at the same locus on its homologous pair (e.g., DiGeorge Syndrome).
• Translocations: Fluorescence signals appear on “the wrong chromosome” or show abnormal combinations. This is used to visualize complex rearrangements, such as unbalanced translocations between chromosomes 17 and 19.
• Duplications/Aneuploidy: Extra copies of a chromosome result in increased fluorescence signals, indicating trisomy or tetrasomy (e.g., detecting an extra copy of chromosome 8).

Activity

3. Clinical High-Yields

• Resolution: FISH can detect deletions as small as 100 kb, whereas standard G-banded karyotyping typically requires a loss of 5 Mb to be visible.
• Speed: FISH can be performed on interphase cells, meaning you don’t always have to wait for cells to divide (unlike karyotyping).

Activity