Fluorescence In Situ Hybridization (FISH) is a specialized laboratory technique used to visualize specific segments of DNA or entire chromosomes within a cell. This method detects the presence, absence, and location of genetic material, making it a valuable tool in diagnosing genetic conditions and certain cancers. Understanding a FISH report requires translating the visual pattern of colored signals into meaningful genetic information. Interpretation involves analyzing visual components, quantitative counts, and standardized written notation.
The Visual Basics of FISH Results
The foundation of a FISH result lies in the use of “probes,” which are small pieces of purified DNA tagged with a fluorescent dye. These probes are designed to bind only to a specific, complementary sequence of DNA on the patient’s chromosomes, a process called hybridization. Once bound, the fluorescent tag lights up when viewed under a special microscope, marking the target genetic region.
The visual result is a nucleus illuminated by distinct, colored dots or signals. The color and location of these signals correspond to the specific DNA sequence being analyzed, allowing scientists to quickly assess the status of the targeted DNA sequence.
Distinguishing Normal Versus Abnormal Findings
Interpretation begins with a quantitative assessment to determine the normal number of signals for the targeted region. Since humans have two copies of most chromosomes, a normal cell nucleus is expected to display two separate signals for any single gene or chromosomal region being tested. These two signals correspond to the gene’s location on each paired chromosome.
A result is abnormal when the signal count deviates from this expected two-signal pattern. Only one signal suggests a deletion or loss of the targeted DNA segment, known as monosomy if the entire chromosome is missing. Conversely, three or more signals indicate a gain or duplication of genetic material, such as trisomy.
To ensure accuracy, the laboratory must count and analyze a substantial number of cells, often ranging from 50 to 200. The final report specifies the percentage of cells counted that exhibit the abnormal signal pattern, and a finding is only reported as abnormal if this percentage exceeds a statistically determined cutoff.
Decoding Specific Types of Chromosomal Changes
FISH is used to identify specific structural alterations in the chromosomes beyond simple loss or gain. These alterations are categorized into several types.
Aneuploidy
Aneuploidy refers to a change in the total number of chromosomes, such as Trisomy 21 (Down syndrome), where three signals for chromosome 21 are seen instead of two. These numerical changes are detected using probes that target the centromere or specific regions of a whole chromosome.
Deletion or Microdeletion
A deletion is the loss of a small segment of DNA. A deletion is visually identified when a cell shows only one fluorescent signal for the targeted gene or region, while the control probe on the other chromosome remains present. The location of the missing signal reveals the precise genetic region that has been lost.
Translocation or Fusion
A translocation occurs when a piece of one chromosome breaks off and joins another. This is often detected using a dual-color, break-apart probe set. In a normal cell, two separate colors are positioned close together, sometimes appearing as a single fused yellow signal. If a translocation occurs, the two colors separate onto different chromosomes, resulting in distinct, separated red and green signals.
Understanding the FISH Report Notation
The final component of interpreting a FISH result is understanding the formal written notation provided by the laboratory. This notation uses the International System for Human Cytogenetic Nomenclature (ISCN) to summarize the findings in a standardized, concise format.
For interphase FISH analysis, which is performed on non-dividing cells, the report begins with the prefix “nuc ish” (nuclear in situ hybridization). This prefix is followed by the name of the probe or the chromosomal region tested, and a multiplication sign followed by the number of signals observed. For instance, “nuc ish 8cen(D8Z2x2)” indicates a normal result with two signals for the centromere probe on chromosome 8. An abnormal result, like a trisomy for chromosome 8, would be written as “nuc ish 8cen(D8Z2x3),” showing three signals.