Clinical
Cytogenetics is the specialty that studies the structure of chromosomes and
their application to the diagnosis and monitoring of acquired and inherited
abnormalities. Clinical Cytogenetics involves the examination of dividing cells
for the recognition of chromosomal abnormalities either inherited or acquired.
The most widely recognized use of cytogenetics today is in prenatal diagnosis
by amniocentesis and more recently chorionic villus sampling. Cytogenetic
technology is also utilized in the study of acquired
chromosomal abnormalities present in various forms of cancer.
The primary responsibility of the clinical cytogenetic technologist is the
analysis and karyotyping of human chromosome preparations. The clinical
genetics technologist must be able to perform all aspects of the cytogenetic
procedure. Cytogenetics requires a basic understanding of cell culture and the
cell growth cycle, the biochemistry of staining procedures, microscopy,
photomicroscopy, darkroom techniques, and new techniques in molecular biology.
Cytogenetic
analysis is crucial for patient care in the diagnosis, prognosis, treatment and follow-up of chromosomal genetic disorders.
Cytogenetic results may also provide an indication that genetic counseling is
required for family members other than the patient.
Indications:
1. Peripheral Blood:
To rule out or confirm trisomy,
monosomy, duplication, deletion, translocation, inversion and other chromosomal
abnormalities involving the autosomes or sex chromosomes in individuals who
have multiple congenital malformations, unusual physical or facial features,
unexplained developmental delay or mental retardation, ambiguous external
genitalia, delayed sexual development, short stature, or a history of multiple
miscarriages, stillbirths or sterility.
Cytogenetic studies are performed on blood samples for all standard medical
indications including birth defects, developmental disorders, and reproductive
problems.
Indications
for cytogenetic analysis:
·
Multiple congenital
anomalies or dysmorphic features
·
Failure to thrive,
developmental delay
·
Unexplained mental
retardation
·
Family history of a
chromosome abnormality
·
Primary or secondary
amenorrhea
·
Couples experiencing
multiple pregnancy losses or infertility
(analyses for all of the above are generally performed on peripheral blood
samples)
·
Advanced maternal age
(prenatal test)
·
Abnormal MSAFP or Triple
Test results (prenatal test)
·
Unexplained pregnancy
loss (performed on products of conception tissue samples)
·
Anemia, leukemia, lymphoma
(performed on bone marrow aspirates)
Cytogenetics is a highly
complex specialty area of the clinical diagnostic laboratory. Cytogenetic
technologists study the hereditary material at the cellular level by examining
the structure and behavior of chromosomes. Chromosomes are the condensed form
of the genetic material. Inherited or acquired genetic disorders may occur when
there is a change in the number or structural arrangement of the genetic
material composing the chromosomes. Cytogenetic analysis is crucial for patient
care in providing diagnosis, prognosis, treatment and follow-up for chromosomal
genetic disorders.
The CYTOGENETICIST examines many
cells from an individual's specimen to determine the cytogenetic diagnosis. The
patient specimen must contain living cells; the most frequently used tissue
types are blood, skin, bone marrow, amniotic fluid, tumors and chorionic villi.
Chromosomes, the condensed form of the genetic material, are visible only for a
short time period during each cell's lifetime. The technologist performs cell
culture, manipulations of the cell cycle, cell harvesting and chromosome
banding techniques. This allows visualization and identification of the
chromosomes so they can perform analysis using state-of-the-art light
microscopes.
A sophisticated computer imaging system is
used to generate an image of the chromosomes from single cells and
produce karyotypes using POWER GENE KARYOTYPING
STATION (APPLIED
IMAGING – CYTOVISION KARYOTYPING SYSTEM)
Description
Includes
PHA-stimulated culture of peripheral blood lymphocytes and preparation of
Giemsa-banded chromosomes. Chromosome analysis
includes two banded karyotypes, detailed examination of ten cells, and 20 cells
total counted.
Reference Value 46,XX or 46,XY
Turn Around Time: 2-7 days
(generally 3 days)
Description
Special stains may be performed, in addition to a routine chromosome
analysis, when a chromosome abnormality is discovered that requires further examination
to more definitively describe the aberration. Special stains may include C-banding, Q-banding, NOR, or DAPI. The particular stain(s)
depends upon the chromosome abnormality. Special stains are
performed at the discretion of the cytogenetic laboratory director.
Indications
Structural
chromosome rearrangements such as translocations, inversions, rings, or marker
chromosomes.
Specimen
Requirements
Special stains are performed on chromosome spreads that have already been
prepared from a submitted specimen. Usually no further specimen is required
Reference
Value 46,XX or 46,XY
Turn Around Time: Depends upon the type of
specimen. May increase the usual turn around time by one or
two days.
Description
Includes
special synchronized PHA-stimulated culture of peripheral blood lymphocytes and
preparation of Giemsa-banded chromosomes.
Banding is at the 650 or higher band level of resolution. Chromosome analysis
includes partial karyotypes of at least three different pairs of homologous
chromosomes for each chromosome, two banded karyotypes, detailed examination of
ten cells, and 20 cells total counted.
Indications
High resolution banding
chromosome analysis is helpful to identify subtle chromosome rearrangements
such as microdeletions and cryptic translocations that may go undetected by
routine chromosome analysis. This test is often ordered by
clinical geneticists who suspect a patient may have a specific chromosomal
syndrome.
Reference
Value 46,XX or 46,XY
Turn Around
Time / Reporting Time: 2-3 weeks
SISTER CHROMATID EXCHANGE STUDIES(SCE)
To study chromosomal instability in cases of Fanconi Anemia,
Bloom syndrome and other chromosomal instability syndromes.
escription:
Two requirements for cytogenetic analysis of SCE in peripheral
blood lymphocyte cultures are: a) a population of actively proliferating cells
that will provide an adequate number of metaphases and b) sister chromatids
that in some way are differentially labelled or stained in the metaphases. This
protocol uses PHA stimulated blood cultures. The cells are
incubated in presence of BrdU, a thymidine analog for a minimum of two
cycles. Sliedes prepared from fixed cells are treated with Hoechst 33258,
exposed to light and heat, and then Giemsa stained to produce differentially
stained chromosomes. The chromatids with bifilar substitution exhibit a lighter
purple stain than their unifilarly substituted sister chromatids.
Reference
Value 46,XX or 46,XY The mean SCE per cell in normal human control
blood lymphocutes is <10 with a range from 1-14. In chromosomal instability syndromes the SCE frequency will be considerably higher.
Reporting
Time: 3-4 weeks
Description
Extra karyotypes may be prepared in addition
to a routine chromosome analysis when more than two cell lines have been discovered, or when multiple complex chromosome
rearrangements have been found.
Indications
• mosaicism with more than two abnormal cell
lines
• bone marrow or unstimulated peripheral blood with multiple abnormal clones
and/or multiple complex chromosome rearrangements
Specimen
Requirements
Additional karyotypes are prepared from
chromosome spreads that have already been prepared from a submitted specimen.
Usually no further specimen is required.
Reference
Value 46,XX or 46,XY
Turn
Around Time/Reporting time: Chromosome analysis with complex chromosome abnormalities
requiring additional karyotypes usually increases the average turn around time
by a few days.
Description:
The fragile X chromosome is usually
visualized only after appropriate manipulation of the tissue culture
medium used for chromosome preparation has been performed. The blood cultures
are set up in Folic acid depleted medium. The fragile site is visible in only
some cells, usually a minority. The proportion is reasonably constant for any
individual, in clinically affected males and females it ranges from less than 5
percent to greater than 60 percent, but is mostly in the 10-40% range. Most of
the male and female carriers who are intellectually normal do not show fragile
X on chromosome analysis, but confirmed by molecular analysis.
Reference
Value:46,XX/46,XY – In suspected
cases a fragility is expresedd on the X chromosomes (5-60%).
Reporting
Time: 3-4
weeks
