Chromosome Studies

What type of testing is available to determine whether a child has a genetic birth defect?

Karyotype l Extended banding chromosome studies l Fluorescence in situ hybridization (FISH)Chromosomal microarray analysis

Karyotype

Chromosomes are stained to see them with a microscope. When stained, the chromosomes look like strings with light and dark bands.  A karyotype refers to a photograph of the chomosomes of a cell.  The pairs of chromosomes are called a karyotype. A normal female karyotype is written 46, XX, and a normal male karyotype is written 46, XY.

The standard analysis of the chromosomes evaluates both the number and structure of the chromosomes, with an accuracy of more than 99.9 percent. Chromosome analyses are usually done from a blood sample (white blood cells), prenatal specimen, skin biopsy, or other tissue sample.

Chromosomes are analyzed by specially trained healthcare personnel with advanced cytogenetic technology and genetic degrees. Cytogenetics is the study of chromosomes.

In a karyotype, the chromosomes can look bent or twisted. This is normal because of how they were sitting on the slide when the photograph was taken. Chromosomes are flexible structures that grow or shrink during different stages of cell division. If you unraveled all of the DNA that makes up the 46 chromosomes, you would find over seven feet of DNA from one single cell.

Extended banding chromosome studies

Extended banding or high resolution chromosome studies look at chromosomes at a higher resolution than the standard chromosome analysis mentioned above.

The chromosomes are prepared so that they are a little longer. This means more bands can be seen. This allows  smaller pieces of the chromosome to be seen, helping us identify smaller structural chromosome abnormalities not visible by routine analysis.

Fluorescence in situ hybridization (FISH)

FISH is a laboratory technique used to find out how many copies of a specific segment of DNA is present in a cell. It is also used to identify structurally abnormal chromosomes.

In the lab, a segment of DNA is chemically modified and labeled to look fluorescent (brightly colored) under a special microscope. This DNA is called a probe.

Probes find matching segments of DNA when added to cells with certain conditions. For example, if a baby is suspected of having trisomy 21 Down syndrome and an amniocentesis is done during the pregnancy, then a FISH study can be performed on the cells found in the amniotic fluid.

A probe made for chromosome #21 can determine how many copies the baby has of the #21 chromosome. Under a special microscope, the cells from a baby with trisomy 21 would contain three signals or three brightly colored areas, where the probe matched up with the three #21 chromosomes.

Another area the FISH test can examine is the telomere. Telomere is a term used to describe the very ends of chromosomes. When FISH is used specifically to look for chromosome abnormalities in this area, it is referred to as subtelomeric FISH testing.

A FISH study does not replace a chromosome study, but it is done, in addition to a standard chromosome study, depending on the birth defect. FISH can be used to detect structural chromosome abnormalities, such as submicroscopic deletions, beyond the resolution of extended banding chromosome studies.

Chromosomal microarray analysis

A chromosomal microarray analysis (CMA) is a new laboratory test used to detect chromosomal imbalance. CMA does this at a higher resolution than current standard chromosome or FISH techniques.

A sample of DNA from the patient and a control DNA sample are put together in a particular order (called an array) on a glass slide. Fluorescent dyes are attached to the DNA samples. These slides are then placed in a special scanner, which measures the brightness of each fluorescent area.

This process identifies changes of the DNA copy number. These changes of DNA copy number may represent changes within the general population, which do not cause genetic diseases. However, some changes of DNA copy number may indicate a chromosomal abnormality, such as a chromosomal imbalance, loss, or gain.

Types of chromosomal abnormalities may include small chromosomal rearrangements, small duplications of chromosomal material (trisomy), or small deletion of chromosomal material (monosomy).