Genetic Testing for Inherited Retinal Diseases

Genetic testing for inherited retinal diseases (IRD) has significantly increased in the last decade. Scientists have discovered over 300 genes that cause IRDs. Alongside these developments, genetic testing quality and speed has improved, while costs of testing have reduced. For many years, genetic testing for IRDs was limited to research purposes. However, as new treatments are moving to human trials, provincial governments have recognized the importance and value of genetic testing for IRDs (and all provinces now provide funding for IRD genetic testing).

At Fighting Blindness Canada, we receive many inquiries on genetic testing.  To help answer some of these common questions, on this page we dive deeper into ‘what is a gene’, ‘what is an IRD’, how to seek genetic testing, provincial genetic testing guidelines (at the bottom), and more.

What is a gene?

Genes are the basic building blocks of life and heredity. Genes are made from DNA and provide the instructions that the body needs to function. Each gene contains the instructions to make a specific protein, which are the molecules that do most of the work in the cell and the body.

A mutation is a change in a gene (and in the instructions) which can affect how a protein is made and works. Some changes may spread through the population and become relatively common. At this point, they are called genetic variations and they are responsible for everything, from our hair color to our height. However, some mutations can disrupt the instructions enough that they can cause a disease. If the mutations are passed down from our parents this is called a genetic or inherited disease.

Our cells have two copies of each gene, one from the father and one from the mother.

Dominant Mutation: If a disease is caused by a mutation in only one gene copy, this is called a dominant mutation.

Recessive Mutation: If a disease only occurs when there are mutations in both copies of the gene this is called a recessive mutation. Parents can be carriers, which means they do not have the disease, but each parent has one copy of the mutated recessive gene. Parents who are carriers may have a child who has the disease if the child gets two copies of the mutated gene.

X-Linked Recessive: Some diseases are X-linked recessive, such as X-linked retinitis pigmentosa. In this case, the mutation is found on the X-chromosome. Females have two X-chromosomes, one from each parent. Males only have one X-chromosome from their mother and a different Y chromosome from their father. An X-linked recessive disease is usually only found in males since a mutation on the single copy of the X-chromosome will cause the disease while most females will have a second unmutated X-chromosome.

What is an inherited retinal disease?

An inherited retinal disease (IRD) is a condition that leads to vision loss and is caused by a specific genetic mutation. There are differences between IRDs, but they share some things in common:

1. The diseases are inherited, meaning you are born with the specific genetic mutation. Depending on the disease you may have blindness or vision loss at birth, or it may occur later in life.
2. Vision loss ultimately results from damage to the retina at the back of the eye. The retina is made up of cells that sense and transmit light signals to the brain to form images.
3. Retinal damage is caused by specific genetic mutations that affect how retinal cells work and survive. Researchers have found over 300 genes that cause IRDs. Some IRDs, such as choroideremia or X-linked retinoschisis are caused by one or only a small number of genes. Other diseases can be caused by many different mutations in many different genes. For example, over 64 genes have been identified that cause retinitis pigmentosa. In addition, for many people the gene that causes their IRD has not been identified yet.

What is genetic testing?

Genetic testing involves taking a sample of DNA (usually from blood or saliva) and examining it in a laboratory to try and identify changes or mutations that may be responsible for vision loss. Genetic testing involves a technique called DNA sequencing where a machine “reads” the gene instructions and identifies mutations, the same way a spell checker can recognize a typo. Most genetic testing looks at a “panel” of a few hundred genes that are known to be mutated in IRDs.

It is important to work with a vision care team because genetic testing does not always deliver a clear result. Even though there have been great advances in genetics over the past 20 years, many genes that cause IRDs have not been found yet.

Why should I get genetic testing?

Successful genetic testing can tell you a lot about your condition. It may help you:

• Confirm your IRD diagnosis. Sometimes it is hard to definitively confirm an IRD through eye tests alone. Some IRDs cause similar symptoms and retinal damage. Genetic testing may help your vision care team understand your diagnosis and how your vision loss might progress.
• Understand your inheritance. Gene mutations have specific inheritance patterns such as dominant, recessive or x-linked (as discussed above). Knowing more about the inheritance pattern of your gene can be helpful in determining how the gene is passed on and if other members of your family should get tested for the mutation.
• Determine your next steps. Many clinical trials and treatments currently available are gene specific. If you are interested in participating in a clinical trial, your first step to determining your eligibility may be genetic testing or knowing your gene mutation.

What does “variant of unknown significance” mean?

Researchers have not identified all the genes that cause IRDs. If your disease is caused by a mutation or a gene that has not been identified before, genetic testing may not be able to identify it. Up to 30% of people who get tested for an IRD will get a negative genetic test result.

There is also a chance that the laboratory will identify a genetic mutation that has not been previously shown to cause a disease. This result is known as a variant of unknown significance. This does not mean you do not have an IRD. A variant of unknown significance means the genetic mutation that was identified has not yet been connected to a specific disease. It is possible that in the future the variant may be identified in other people or shown to cause an IRD.

In the case you receive this genetic result, your vision care team will talk to you about your diagnosis. Your IRD diagnosis may be confirmed using other information, like your medical and family history.

Do I have to pay for genetic testing?

The cost of genetic testing for IRDs is covered by each provincial health department. If genetic testing is appropriate for you, your vision care team will apply for this test. If genetic testing is not appropriate for you, discuss your options with your vision care team.

Can I get my genetic testing done privately?

Access to private genetic testing should be discussed with your doctor. There are many considerations if you decide to get your genetic testing done privately, including if you will receive genetic counseling during the process.

How do I get genetic testing in my province?

Below is a list of handouts that describe the process of genetic testing in each province.

• Alberta and Northwest Territories
• British Columbia and Yukon Territories
• Manitoba | Manitoba (en français)
• New Brunswick | New Brunswick (en français)
• Nova Scotia
• Ontario | Ontario (en français) and Nunavut
• Prince Edward Island
• Quebec | Quebec (en français)
• Saskatchewan

Don’t see your province? If you are in Newfoundland, contact the Health Information Line to connect with your local genetic counsellor.

---

Content last reviewed October, 2020