Cytogenetics

Cytogenetics is the study of chromosomes and how changes in their structure and number can relate to disease. Cytogenetic tests are often used in the diagnosis of genetic diseases and in prenatal diagnosis. The two major techniques used in cytogenetics are karyotyping and fluorescent in situ hybridization (FISH).

Karyotyping

Karyotyping is used to confirm diagnoses of conditions caused by chromosomal abnormalities. These can include a change in number, such as trisomy (three copies of a chromosome), monosomy (one copy of a chromosome) or triploidy (three copies of every chromosome so that the total number is 69). One of the most common trisomies is trisomy 21, which is also known as Down syndrome. Trisomy 13 (Patau's syndrome) and trisomy 18 (Edwards' syndrome) can also be seen in live-born babies, but all other trisomies are not compatible with life. An example of monosomy is monosomy X, or 45X, which is also known as Turner's syndrome. Monosomy of any other chromosome is not compatible with life.

Translocations are another type of chromosomal abnormality that can be identified through karyotyping. A translocation is the transfer of a part of one chromosome to another chromosome. The end of a chromosome can break off and attach to the end of another chromosome, often at a spot where another end has broken off. Translocations are not always easy to detect, so FISH is often used to confirm a translocation seen on a karyotype.

A doctor may request a karyotype if a patient has some of the features of a condition that is known to be related to a chromosomal abnormality, or if a couple who are planning a pregnancy have a family history of a chromosomal abnormality. In prenatal diagnosis, samples of amniotic fluid removed in amniocentesis or placental tissue removed in chorionic villus sampling contain cells from the baby, which can be karyotyped to look for chromosomal abnormalities.

Chromosomes can be isolated from nearly any cell in the body, but samples of blood, bone marrow or amniotic fluid are usually used. The cells are removed from these liquid samples and encouraged to grow in a specially developed growth medium. Once enough cells have grown, a chemical is used to synchronize the cells so that they are all dividing together. They are then stopped when they reach a step in cell division called metaphase. This is the point where the chromosomes are lined up in the middle of the cell, ready to be separated into two new daughter cells. The chromosomes at this stage are the most condensed and can easily be identified under a microscope. A dye called Giemsa is applied to the chromosomes, which gives them a characteristic black and white banding pattern. This is known as G-banding. The chromosomes are examined under the microscope by a technician who is trained to identify each one and arrange them into a karyotype (an alignment of an organism's chromosome pairs in order of size in chart format. It enables the connecting of chromosomes to symptoms - for example, of genetic diseases in the organism - and traits) using computer software. Technicians who work in this area knows the chromosomes so well that they can tell when small pieces are missing, added or rearranged, and can accurately identify extra or missing whole chromosomes.

FISH

This is a method developed to quickly identify a gene or a region on a chromosome. It involves the hybridization of a DNA fragment with the region of interest. The fragment has a fluorescent-coloured tag attached to it. When the fragment is mixed with the chromosomes prepared on a slide, it will look for the sequence that matches and attach to it. If this sequence is missing, the fragment cannot attach to anything and is washed off when the slide is processed with a certain compound. The slide is then examined under a special microscope that can pick up the fluorescent colour.

Applications

FISH is used to confirm diagnoses of certain genetic conditions, test for chromosomal translocations and prenatally diagnose an at-risk baby. It is specifically designed to identify changes in the chromosomes which cannot be picked up on karyotype, such as small deletions, duplications and insertions.

An example where FISH is used to confirm a diagnosis is with 22q deletion syndrome. FISH can be done using a fragment specific to the region of chromosome 22 where there is a small deletion of bases. A person with the correct sequences on both chromosome 22s will have two fluorescent signals, one for each chromosome. A person with the deletion on one chromosome will have only one fluorescent signal, because the fragment cannot bind to the deleted region.