Role of Genetic Tests in IRDs and RED
There are various different types of genetic tests that can be ordered by a physician depending on the purpose – these can be:
- diagnostic testing
- predictive testing
- carrier testing
- Sequencing of one gene, usually by Sanger (capillary) sequencing
- Sequencing of a panel of genes, usually by NGS, (next generation sequencing)
- Sequencing of the Exome, which is only possible through NGS
- Sequencing of the Whole Genome, which is only possible through NGS
For information on inheritance click here: http://www.retina-international.org/patients/your-eyes/inheritance-patterns
With respect to IRDs diagnostic tests are used when a person is already experiencing symptoms and the Eye Doctor suspects there is a genetic cause. With over 260 different genes causing IRDs it is important to try to find the one affecting a particular patient and their family. An eye doctor may already suspect that the symptoms are caused by an IRD and so the use of genetic testing confirms if this is the case and identifies the genetic fault causing the condition.
In the case of IRDs the doctor may suspect that one of a large number of genes may be causing a particular set of symptoms and wants to rule out a specific genetic condition based on physical signs and symptoms, this is known as a phenotype. Patients can have a genetic test at any time in life including during pregnancy.
The genetic test tries to find out which gene and which mutation in this gene is responsible for the particular patient’s disease.
Knowing the molecular basis of a condition can help physicians to advise those affected about:
- the inheritance pattern of a gene.
- how the disease is likely to develop.
- the possibility of joining a register.
- participating in an appropriate clinical trial.
These options are opened up by knowing your gene type as is what the likelihood of treatment for your particularly IRD is.
Predictive and presymptomatic tests are considered by physicians only when individuals have a family member with a genetic disorder, but who have no symptoms themselves at that time. In the case of IRDs a predictive test is only recommended when a specific disease-causing mutation has been identified in an affected family member.
Huntington's disease and breast cancer are examples of diseases for which pre-symptomatic predictive testing is often performed as when mutations in the relevant genes are present in a healthy individual, symptoms are most likely to occur if the person lives long enough. Even in the absence of medical interventions, predictive testing can influence life-planning decisions.
If a late onset IRD runs in a family, then a decision may be made to take a test to establish whether other family members have inherited this gene.
There are many factors to consider when deciding whether or not to have a predictive test. Ultimately predictive testing provides information on the risk of developing a specific condition and can be helpful with making decisions about medical care and life choices.
Pre-clinical test is designed to test someone that has a related family member with an IRD but has no signs or symptoms of the disease.
Some considerations a person needs to make if they are concerned about taking a pre-clinical test are
- If the disease has a treatment option
- That early treatment can lead to a better outcome
- If the particular IRD looks like it could be a disease of incomplete penetrance or variable expressivity that may skip a generation.
- The psychological impact
- Family planning
Carrier testing is used to identify individuals (males and females) who have a single gene mutation for a disorder inherited in an autosomal recessive manner. Carrier testing is also done for females carrying a single recessive mutation in one of their two X-chromosomes. Usually the carrier does not have symptoms of the disease itself but seeks the information to make informed reproductive choices.
Some female carriers of an X-linked disease may have clinical signs of the disease. These are mild in most cases but can be severe in some cases. But a molecular test to detect this carrier status may be more efficient. After this test genetic counselling should be provided in order to help the individual to plan for their future lives.
Although the issues differ significantly from predictive testing, pre-test and post-test counselling are vital, this is because results can have serious often life changing implications.
The importance of appropriate genetic counselling cannot be overemphasised. Before the test the geneticist confirms the diagnosis and draws the family pedigree tree.
A genetic variation of a gene (mutation) can cause disease. Some genes can cause the disease with a recessive pattern with the loss of its function. Other genes can manifest as a dominant disease when the mutated protein causes disruption of the function of the normal protein, or when it does not produce enough protein from just one normal copy of the gene.
For some genetic conditions that cause IRDs two copies of a faulty gene must be inherited to cause a condition, this is called a recessive condition. If someone has only one copy of the gene they are called a carrier. (http://www.retina-international.org/)
Carriers of most recessive inherited mutations are themselves unaffected because the normal copy of the gene overrides the faulty copy, but they can pass on a copy of the faulty gene to their children. Carrier testing allows you to know whether or not you and your partner carry a faulty copy of a gene, in order to make informed choices about family planning in the future. This is very important to consanguineous families or consanguineous populations.
Carrier testing is used to identify people who carry one copy of a gene mutation that, when present in two copies, causes a genetic disorder. This type of testing is offered to individuals who have a family history of a genetic disorder, families with consanguineous marriages and to people in certain ethnic groups with an increased risk of specific genetic conditions. If both parents are tested, the test can provide information about a couple's risk of having a child with a genetic condition.
Prenatal testing is used to detect changes in a foetus’s genes or chromosomes before birth. This type of testing is offered during pregnancy if there is an increased risk that the baby will have a genetic or chromosomal disorder. In some cases, prenatal testing can lessen a couple's uncertainty or help them make decisions about a pregnancy. It cannot identify all possible inherited disorders and birth defects, however.
This kind of test is not considered appropriate for diseases that affect vision even in a very severe manner. This type of test is mostly prescribed where there is a concern that a genetic disease that is incompatible with life exists.