A SUMMARY OF CYTOGENETIC TESTS
OFFERED BY CMCS
Among the available cytogenetic assays are the following
four types of studies that are particularly applicable when it
is important to determine whether there has been exposure to a
potentially carcinogenic agent. Test descriptions have been written
with the non-biologist in mind. For more information, of a general
or more technical nature, please contact us.
1. Micronucleus test.
This test is based on the idea that pieces of broken chromosomes,
or in some cases whole chromosomes, will become disassociated from the
rest of the nuclear DNA and form a tiny nucleus on their own. While this
technique provides an excellent opportunity for rapid screening of a
population, it is not possible to determine the amount or specific
identity of the DNA contained in the micronucleus or the events that led
to its formation. Moreover, the presence of a micronucleus offers no clue
to the nature of other possible abnormalities contained within the
nucleus of the cell. Thus, the test is most useful when used in
conjunction with G-banded studies, or as an indicator of what direction a
possible study should take.
2. Sister Chromatid Exchange (SCE).
Just prior to cell division DNA replicates. When this process is
complete each chromosome will have made an exact copy of itself. The
two copies are held together at a location called the centromere. In
this configuration the chromosome is said to be composed of two
chromatids. DNA breaks, sometimes spontaneously but more often because of
influences from outside the cell. As soon as DNA breaks it begins its own
repair process. Sometimes in the repair process equal portions of the two
opposing chromatids (sister chromatids) of a chromosome will exchange
places. Special staining techniques allow us to stain one chromatid dark
and the other light making any exchange that has occurred quite
detectable. Thus SCE is a sensitive indicator of a cell's exposure to
hazardous substances. However, with this technique it is not possible to
detect damage that might have occurred during DNA replication prior to
the time the cells were placed in culture.
3. Chromosome breakage (sometimes referred to as aberrations).
This is test whose results whose results are typically reported as
breaks, gaps, and deletions. While this a test also makes it possible to
screen large populations rapidly, it has a good number of disadvantages.
First, the distinction among the terms, "break",
"gap", and "deletion" is a subjective one relating to
whether or not the reader feels the deletion is larger or smaller than
the width of a chromatid and whether the chromatids remain aligned or
not. It is not at all unusual for equally experienced readers to report
different results on the same metaphases. Second, slide preparation,
among other things, can have an influence on the manifestation of the
lesion. Third, because all the chromosomes stain alike it is not possible
to identify one chromosome from another with any degree of certainty and
therefore it is not possible to identify specific chromosomes involved in
rearrangements or to even detect rearrangements where the two chromosome
fragments involved are the same size. Finally, non-banded chromosomes are
often not what they appear to be. For example, the literature abounds
with reports of translocations that clearly cannot be positively
identified and with control populations with two or three dicentrics
(chromosomes with two centromeres) per one hundred cells, a finding that
is simply not consistent with reality.
By using special techniques the cytogeneticist is able to stain
chromosomes in such a way that they are easily and quickly identifiable
one from the other. A particular chromosome will stain in the same
pattern not only from one cell to another within an individual but from
one individual to another within a species. Moreover, the
"banding" pattern achieved by this staining procedure is so
intricate that any minor changes in the structure of the chromosome can
be detected by a technologist properly trained in the technique. Although
this test is the most demanding in terms of time and skill levels, it is
the one we prefer to do, particularly in cases where exposure was in the
past and not going on currently or where exposure has occurred at low
levels over long periods of time.
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