A Fluorescence In Situ Hybridization (FISH) test is a genetic diagnostic tool used to identify specific changes within a person’s chromosomes or genes. A “positive” FISH test result indicates a genetic anomaly or alteration has been detected. This article clarifies what such a result signifies, detailing the types of genetic changes it can reveal, examples of conditions it helps identify, and the steps that typically follow a positive diagnosis.
Understanding the Detection of Genetic Changes
A positive FISH test signifies the presence of a genetic alteration, meaning there is an abnormal number, structure, or location of chromosomes or genes within a cell. FISH works by using fluorescently labeled DNA pieces, called probes, that bind to specific DNA sequences on chromosomes. These probes act like highlighted markers, allowing scientists to visualize specific areas of the genetic material under a specialized microscope.
These changes include aneuploidy, an abnormal number of chromosomes (e.g., an extra chromosome). FISH can also detect deletions, where a piece of a chromosome is missing, and duplications, which involve extra copies of a chromosome piece. Additionally, translocations are identified when chromosome pieces break off and attach to a different chromosome.
Specific Conditions Identified by FISH
A positive FISH test can identify various specific genetic conditions. For instance, Down syndrome (Trisomy 21) is often detected by FISH. This condition results from an extra copy of chromosome 21. FISH can identify this extra chromosome, providing an early diagnosis.
DiGeorge syndrome (22q11.2 deletion syndrome) is another condition identified by a positive FISH result. This syndrome is characterized by a deletion on chromosome 22, specifically in the 22q11.2 region. FISH is useful for detecting these microdeletions, which can be too small for conventional chromosome analysis. Symptoms can include heart defects, developmental delays, and immune deficiencies.
In cancer, a positive FISH test can identify specific genetic changes that guide treatment. The Philadelphia chromosome, for example, is a translocation between chromosome 9 and chromosome 22 (BCR-ABL translocation). This change is associated with certain leukemias, such as Chronic Myeloid Leukemia (CML). FISH identifies such translocations, influencing the choice of targeted therapies. Additionally, FISH can detect gene amplifications, like HER2 amplification in breast cancer, indicating extra copies of the HER2 gene in cancer cells, making them responsive to specific HER2-targeted drugs.
Navigating a Positive FISH Diagnosis
Receiving a positive FISH test result often initiates a series of next steps to understand and manage the diagnosis. Initially, confirmatory and further testing may be recommended, especially if FISH was a preliminary or targeted test. While FISH is accurate for its targets, standard karyotyping or chromosomal microarray analysis may be performed to provide a broader view of the chromosomes and detect other potential genetic changes that FISH might not cover.
Consulting with a genetic counselor is an important next step. These professionals explain the implications of the specific genetic change, discuss inheritance patterns, and provide information about the condition’s prognosis. They also help families understand the potential impact on the individual and other family members.
The diagnosis then guides medical management and treatment plans. For conditions like certain cancers, a positive FISH result directly informs targeted therapies, leading to more effective interventions. For genetic syndromes, the diagnosis helps healthcare providers anticipate potential health challenges and implement supportive care tailored to individual needs. Given the emotional impact of a genetic diagnosis, support resources, including patient advocacy groups and counseling services, are available to help individuals and families.