Cytogenetics is the study of chromosomes, which are thread-like structures inside all of our cells. Chromosomes contain thousands of genes that are stored in DNA, the basic genetic material. A healthy cell contains 46 chromosomes (23 pairs).
Why is cytogenetics used?
In many cases of leukemia, changes occur to chromosomes inside leukemic cells. Identifying chromosome changes helps determine the molecular subtype of your child’s leukemia.
Cytogenetics is a way to find out if chromosomal changes have occurred in your child's leukemic cells. Knowing the type of chromosome change helps doctors select the best treatment plan and intensity for your child.
It is important to remember that:
- these chromosome changes only occur inside blood cells that are leukemic. They do not occur in the rest of your child's healthy cells.
- the changes are not inherited. A mother or father cannot pass these chromosome changes to their child. They occur when the leukemia first develops in your child.
Chromosome translocations
Inside leukemic cells, a part of a chromosome can separate and stick itself onto another unrelated chromosome. This produces a new chromosome that expresses genes in different ways. When chromosomes spontaneously rearrange themselves this way, it is called a translocation.
Problems arise when the translocation produces a new gene that instructs the cell to do things it normally would not – for instance, tell the cell to divide uncontrollably.
Changes in the number of chromosomes
Some leukemic cells can have too few (hypodiploid) or too many (hyperdiploid) chromosomes.
For more information, see Genes and ALL.
Cytogenetic techniques
Cytogenetics combine different techniques to determine the number of chromosome copies inside the tumour cells.
Karyotyping
A karyotype is a photograph of all the chromosomes in the leukemia cell, which shows:
- how the chromosomes are arranged inside cells
- the number of chromosomes inside cells
A sample of your child’s bone marrow aspirate is placed into a special dish and allowed to grow in the laboratory. The cells are then extracted from the growing sample and stained with a dye.
The laboratory specialist uses a microscope to study the size, shape, and number of chromosomes in the cell sample. They then photograph the stained sample to provide the karyotype.
This technique gives doctors an idea of which chromosomes appear abnormal in the leukemic cells.
Karyotyping is sometimes not sensitive enough to detect all chromosome changes in the leukemic cells. Since some of these chromosome changes affect how well your child’s leukemia responds to treatment, we use a more sensitive technique called FISH.
FISH (Fluorescence In Situ Hybridization)
FISH is a sensitive technique that confirms if:
- a chromosome has translocated inside leukemic cells
- cells are hypodiploid or hyperdiploid
In this technique, particular chromosomes in your child's leukemic cells are labelled with fluorescent dyes. To find chromosome changes, the labelled cells are placed under a special microscope that detects the fluorescence.
- If the cell is hyperdiploid, extra chromosomes will emit extra fluorescent signals.
- If a translocation has occurred, a particular pattern of different fluorescent signals will be seen inside the leukemic cells.