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Types of Genetic Tests
With the evolution of genome sequencing in recent years, it is quicker and easier than ever to determine the order of our base-pairs which makes up our genes, and identify mutations responsible for particular genetic conditions. In the past, Sanger Sequencing was the primary form of genome sequencing, but is a relatively time-consuming and expensive process which has been replaced by Next Generation Sequencing. This allows for all of our DNA to be analysed (Whole Genome Sequencing) and has become a popular method as we learn that mutations outside our genes and exon regions can also influence gene activity and protein production.
Alternatively, specifically analysing only the protein-making regions in our genes which provide information about how to make these proteins (Whole Exome Sequencing) is also possible. These protein-coding instructions are called exons and because most known mutations exist in these exons, whole exome sequencing is an efficient way of identifying these disease-causing mutations.
In the case that a person is expected by their ophthalmologist to have an IRD, be a carrier, or if there is a family history of IRDs, they will call for a genetic test. There are three main types of genetic tests to investigate each of these scenarios; diagnostic testing, carrier testing, predictive/pre-symptomatic testing.
Types of Genetic Tests
With respect to IRD’s, diagnostic testing is used to confirm a positive or negative diagnosis for a retinal condition which is suspected by the presence of certain physical signs and symptoms. As there are over 260 different genes known to cause IRDs, it is important to try to find the specific mutation causing the condition in an individual, as other family members may also be at risk or affected. Receiving a positive diagnosis for an IRD allows the ophthalmologist to understand the molecular basis of the condition and advise affected individuals about:
- The inheritance pattern of the disease.
- How the condition is likely to develop.
- How this diagnosis will impact other family members.
- The possibility of joining a patient registry.
- Participating in an appropriate clinical trial.
- Accessing treatments should they be available (in the future).
A genetic test provides valuable information to people with and IRD and allows them to accurately and effectively adjust their lifestyle so they can best manage their condition.
Diagnostic panel testing is a genetic technique which examines a set of particular genes which are linked to specific genetic condition (in this case, IRDs) in search of the mutations causing the disease. These genes are sequenced in parallel, using Next Generation Sequencing and can accurately identify which mutations exist in an individual.
Carrier testing is done to identify individuals who have one mutated gene copy for a particular disease, which is inherited in an autosomal recessive manner and requires two faulty copies for the condition to develop. Carrier testing is frequently carried out in individuals with a family history of a genetic disorder, for example, in females to test for 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 as males who receive this mutation will be affected by the disease. Carrier testing is also common for people from families with consanguineous marriages and for people from certain ethnic groups who are at an increased risk of specific genetic conditions. If both parents are tested, the risk of having a child with a genetic condition can be determined.
Predictive and pre-symptomatic 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 mutation. 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.