Types of Genetic Testing
Genetic tests look for changes in a person's genes. They can also look for changes in the amount, function, or structure of key proteins coded by genes. Genetic tests can look at the DNA or RNA that play a role in certain conditions. Abnormal test results could mean that someone has a genetic disorder or be more likely to have a disease.
There are three main types of genetic testing: chromosome studies, DNA studies, and biochemical genetic studies. Tests for cancer risk genes are done by DNA studies.
Chromosomes are the threadlike structures of DNA in every cell. Chromosomes contain your genes. Cytogenetics is the study of chromosomes. The chromosomes need to be stained in order to see them with a microscope. When stained, the chromosomes look like strings with light and dark bands.
A photograph from one cell of all 46 chromosomes is called a karyotype. A normal female karyotype is written 46, XX. A normal male karyotype is written 46, XY.
The standard chromosome study assesses both the number and structure of the chromosomes. It is more 99.9% accurate. Chromosome studies are usually done using a blood sample, prenatal sample, skin biopsy, or other tissue sample. Chromosomes are assessed by healthcare staff who have advanced degrees in cytogenetic technology and genetics.
Chromosome studies may be done when a child is born with multiple birth defects. Chromosome studies may also be done when people have certain types of leukemias and lymphomas. The studies look for specific changes in the chromosomes linked with these types of cancers.
A gene is a section of DNA on a chromosome. The stretch of DNA is a code, or recipe, for making a specific protein the body needs to function correctly. To study genes, medical staff assesses the DNA to find out whether the DNA "alphabet" has any "spelling errors" in it. There are two ways to analyze the DNA:
Direct DNA studies
Direct DNA studies look directly at the gene in question for an error. This technology is called FISH or PCR. Errors in the DNA may include:
Duplication. This is when the gene's DNA is replicated.
Deletion. This is when a piece of the gene's DNA is lost.
Point mutation. This is a change in a single unit (base pair) of the gene's DNA.
Trinucleotide repeat. This is a repeated replication of a small sequence of the gene's DNA.
Different types of errors (mutations) are found in different disorders. It is usually very important to find the mutation that is present in a family by first studying the family member with the genetic disorder such as cancer before testing other relatives without the cancer. When a particular mutation is found in a relative with cancer, other family members can choose to have testing for the mutation. This will tell them if they are at higher risk of developing certain cancers and passing the mutation on to the next generation. The DNA needed for direct DNA studies is usually obtained through a blood sample.
Indirect DNA studies
Sometimes the gene that causes a condition (when mutated) has not yet been identified, but researchers know about where it lies on a particular chromosome. Or other times, the gene is identified, but direct gene studies are not possible because the gene is too large to analyze. In these cases, indirect DNA studies may be done.
Indirect DNA studies involve using markers to find out whether a person has inherited the important region of the genetic code that is passing through the family with the disease. Markers are DNA sequences located close to or even within the gene of interest. Because the markers are so close, they are almost always inherited together with the disease. When markers are this close to a gene, they are said to be linked. If someone in a family has the same set of linked markers as the relative with the disease, this person often also has the disease-causing gene mutation. Because indirect DNA studies involve using linked markers, these types of studies are also called linkage studies.
Indirect studies usually involve blood samples from several family members, including those with and without the disorder in question. This is to see what pattern of markers appear to be linked with the disease. Once the disease-linked pattern of markers is found, it is possible to offer testing to relatives. The testing can determine who inherited this pattern and so is at increased risk of cancer.
The accuracy of linkage studies depends on how close the markers are to the faulty gene. In some cases, a reliable marker is not available. The test can't give any useful information to the healthy family members. In many cases, several family members are needed to establish the most accurate set of markers to determine who is at risk for the disease in the family. Linkage studies may take many weeks to complete because these studies are complex.
Many of the cancer risk genes known today were discovered using linkage studies of families who had multiple family members with cancer.
Biochemical genetic studies
Biochemical genetic testing involves the study of enzymes in the body that may be abnormal in some way. Enzymes are proteins that regulate chemical reactions in the body. The enzymes may be deficient or absent, unstable, or have altered activity that can lead to signs in an adult or child (for example, birth defects). There are hundreds of enzyme defects that can be studied in people. Sometimes it's easier to study the enzyme itself instead of the gene mutation. The approach depends on the disorder. Biochemical genetic studies may be done from a blood sample, urine sample, spinal fluid, or other tissue sample, depending on the disorder.
Protein truncation studies
Another way to look at gene products is through protein truncation studies. Testing involves looking at the protein a gene makes to see if it is shorter than normal. Sometimes a mutation in a gene causes it to make a protein that is shortened (truncated). With the protein truncation test, it is possible to "measure" the length of the protein the gene is making to see if it is the right size or shortened. Protein truncation studies can be done on a blood sample. These types of studies are often done for disorders in which the known mutations most often lead to shortened proteins.