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.
