Cytogenetic analysis

Definition

Cytogenetics is the analysis of blood or bone marrow cells that reveals the organization of chromosomes. Chromosomes are the physical structures that contain the genetic material, DNA.

Purpose

Cytogenetic analyses are essential to the diagnosis and treatment of different forms of cancer, especially leukemia, cancers of the blood cell-forming system. The results of cytogenetic tests can help to confirm the diagnosis of a particular form of leukemia, and permit the best treatment to be selected for each patient.

Precautions

This test is performed on tissue or cells that have been removed during the initial surgery or diagnostic procedure used to determine the precise nature of the leukemia or other cancer. It usually does not require any new surgery or blood draw on the patient and, so, does not entail any additional precautions for the patient.

Description

The development of leukemia and other cancers involves alterations, or mutations, in the cellular genetic material. The types of changes seen differ among various forms of cancer, but include changes in the specific sequence of DNA substituent units (termed nucleotides), as well as more dramatic alterations. Some of these more dramatic alterations include loss or duplication of large stretches of DNA sequence, or chromosomal rearrangements that correspond to the movement of genetic sequences from one chromosome to another. These rearrangements can lead to the production of novel, and often characteristic, molecules that are believed to play essential parts in the development of particular cancers.

Cytogenetic analysis focuses upon chromosomal rearrangements. In essence, this type of testing is a hybrid approach that combines genetics, analysis of mutations, with examination of cells. The cells to be tested, usually obtained from circulating blood or bone marrow, are treated in such a way that the chromosomes are made visible. Cells that are about to divide and which have condensed and organized their chromosomes into pairs are most suitable for this type of analysis. Often cells will be treated in the laboratory to increase the frequency of such cells, in which the chromosome are visible as what are called mitotic figures. Cells containing such mitotic figures are then chemically stained in a way that makes it possible to identify specific chromosomes. When such stained chromosomes are visualized and enumerated, the resulting pattern is termed the karyotype of that cell.

The karyotypes of many cells are usually scrutinized to establish whether some fraction of cells display a reproducible genetic alteration that can be associated with a specific cancer. Historically, the first such abnormality recognized was the Philadelphia chromosome, which is associated with chronic myelogenous leukemia (also called chronic myelocytic leukemia , or CML). In virtually all cases of CML, cytogenetic analyses will reveal a Philadelphia chromosome. The presence of this marker can be used to monitor response to treatment. There are a variety of other genetic abnormalities that are associated with specific forms of cancer. Most of these have been recognized using cytogenetics, and to varying degrees have become useful in diagnosis of leukemia and solid tumors, or in predicting treatment outcome.

Cytogenetics examines microscopically visible chromosomal changes. More recently developed molecular approaches can recognize the same sorts of genetic rearrangements as seen in abnormal karyotypes. In addition, these molecular tests can recognize smaller, more subtle alterations affecting just one or a few of the nucleotide units within a cancer-related gene. These techniques can, in some cases, be more sensitive than cytogenetic approaches, and along with the large body of information derived from the human genome project, hold the promise or providing more accurate tests for diagnosis and treatment of cancer.

Preparation and Aftercare

The only preparation and aftercare would be the preparation and aftercare required for the sample collection—a blood draw or a bone marrow aspiration and biopsy . The cytogenetic analysis itself requires no additional preparation or aftercare on the part of the patient.

Risks

This test is performed on tissue or cells that have been removed during the initial surgery or diagnostic procedure used to determine the precise nature of the leukemia or other cancer. It usually does not require any new surgery or blood draw on the patient and, so, does not entail any additional risk to the patient.

Results

Human body cells, exclusive of reproductive cells, have 23 pairs of chromosomes. Any deviation from this is considered abnormal. Cytogenetic analysis directed toward leukemia or other cancer cells is considered to have an abnormal result when a specific, distinctive genetic alteration is seen in the configuration of these chromosomes. The absence of a cytogenetic alteration is not, by any means, a basis to conclude there is an absence of a particular disease or that the prognosis is better than if a genetic abnormality was observed. For example, in acute lymphocytic leukemia (ALL) there are several different genetic rearrangements that are each associated with a different outcome. The patients who do the best are not those without any cytogenetic abnormality, but rather those with one specific chromosomal rearrangement. Therefore the potential use of cytogenetics, and similar molecular analyses, is to enable doctors to more accurately determine the diagnosis, treatment, and follow-up of patients with leukemia and other forms of cancer.

See Also Chromosome rearrangements

Resources

PERIODICALS

Nowell P.C., J. D. Rowley, A. D. Knudson, Jr. "Cancer genetics, cytogenetics: defining the enemy within." Nature Medicine 4 (October 1998):1107-11.

Warren Maltzman, Ph.D.

KEY TERMS

Chromosome

—The self-replicating genetic the structures in cells that containing the cellular DNA, the hereditary material, that includes the genes in a particular order.

DNA sequence

—The relative order of nucleotides, the chemical subunits of a DNA molecule.

Gene rearrangement

—A change in the structure of a chromosome that changes the order of genes.

Karyotype

—A complete set of chromosomes that are examined to look for genetic abnormalities

QUESTIONS TO ASK THE DOCTOR

• Are the cytogenetic results what you would expect based upon other diagnostic tests and examinations I have undergone?
• Will you want to repeat this test? At what point in my treatment will you want to do so?
• Do these test results tell you anything about my prospects for a disease-free survival?