FISH Tests for Diagnosing Cancers

Fluorescence in situ hybridization (FISH) is one of several techniques used to search your cells’ DNA, looking for the presence or absence of specific genes or portions of genes.

Many different types of cancer are associated with known genetic abnormalities. And by genetic, we are not just talking about heredity. Over a lifetime, cells can make mistakes when they divide and grow. Mutations in the DNA that are associated with cancer may accumulate in these cells.

How It Works

FISH is a technique that uses fluorescent probes to detect specific genes or parts of genes (DNA sequences). Medical center lab personnel and oncologists use FISH to help assess patients who may have cancer, and sometimes to monitor a patient who has already been diagnosed with cancer and treated.

FISH can be done using various types of samples according to the location and the type of cancer suspected: Tumor cells obtained from peripheral blood, from a bone marrow biopsy or from a lymph node biopsy, and formalin fixed paraffin-embedded tissue (this refers to a sample of tissue that is processed in the lab and embedded into a type of wax, making it more rigid, so that it can be sliced into thin sections and mounted for viewing under the microscope).

What the Letters Mean

The “H” in FISH refers to hybridization. In molecular hybridization, a labeled DNA or RNA sequence is used as a probe—visualize a red Lego brick, if you will. The probe is used to find a counterpart Lego brick, or DNA sequence, in a biological sample.

DNA in your specimen is like piles of Lego bricks, and most bricks in these piles won't match our red probe. And all of your bricks are neatly organized into 23 pairs of brick piles—each pile is one of your paired homologous chromosomes, more or less. Unlike Lego bricks, our red Lego probe is like a strong magnet and finds its match without having to sort through the piles.

The “F” refers to fluorescence. Our red probe might get lost in the piles of bricks, so it is labeled with colored fluorescent dye so that it will glow. When it finds its match amongst the 23 paired piles, a fluorescent tag reveals its location. So, you now can see how researchers and clinicians might use FISH to help identify where (which pile, or which chromosome) a particular gene is located for a given individual.

The “I” and the “S” stand for in situ. This refers to the fact that our red Lego brick is looking for its match within the sample you gave.

FISH and Specific Blood Cancers

FISH and other in situ hybridization procedures are used to diagnose a variety of chromosomal abnormalities—changes in the genetic material, changes in chromosomes, including the following:

• Deletion: part of a chromosome is gone
• Translocation: part of one chromosome breaks off and sticks onto another chromosome
• Inversion: part of a chromosome breaks off and reinserts back in, but in reverse order
• Duplication: part of a chromosome is present in too many copies within the cell

Each type of cancer may have its own set of chromosomal changes and relevant probes. FISH not only helps identify the initial genetic changes in a disease process like cancer, but it can also be used to monitor response to therapy and disease remission.

The genetic changes detected by FISH sometimes offer additional information about how an individual’s cancer is likely to behave, based on what’s been observed in the past in people with the same kind of cancer and similar genetic changes. Sometimes FISH is used after the diagnosis has already been made, to glean additional information that might help predict a patient's outcome or best treatment.

FISH can identify chromosomal abnormalities in leukemias, including in chronic lymphocytic leukemia (CLL). For chronic lymphocytic leukemia/small lymphocytic lymphoma, FISH allows patients to find out their prognostic category: good, intermediate, or poor. In acute lymphoblastic leukemia (ALL), genetics of the leukemic cells can tell you about the risk level of cancer and help to guide therapeutic decisions.

FISH panels are also available for lymphoma, multiple myeloma, plasma cell proliferative disorders, and myelodysplastic syndrome. In the case of mantle cell lymphoma, for example, there is a translocation that FISH can detect called GH/CCND1 t (11;14) that is frequently associated with this lymphoma.

Why FISH?

An advantage of FISH is that it doesn't have to be performed on cells that are actively dividing. Cytogenetic testing usually takes about three weeks, because the cancer cells must grow in lab dishes for about 2 weeks before they can be tested. In contrast, FISH results are usually available from the lab within a few days.