When someone is first diagnosed with Stargardt Disease, often their first question is “how did I get it?” Young adults diagnosed may soon follow up that question with the next one on their minds, “will my kids have it?” This blog post is meant to help answer those questions.
What is Stargardt’s?
Stargardt Macular Distrophy, or Stargardt Disease, is an inherited juvenile form of macular degeneration. It causes severe vision loss, especially of the central vision, but not total blindness. For a little more information along with a description and visuals of what it is like to live with Stargardt’s, see my previous blog post, How I see With Stargardt’s.
The primary symptom of Stargardt’s is vision loss, primarily of the central vision. Other symptoms can include wavy vision, blind spots, blurriness, impaired color vision, sensitivity to bright light and glare, and difficulty adapting to dim lighting. Because Stargardt’s is inherited, patients are “born with it.” However, that doesn’t mean they are born with bad vision. Patients typically start to experience vision loss during their teenage years. Some patients start experiencing these symptoms at a younger age, some as young as 6 years old. Although uncommon, some patients don’t experience symptoms and aren’t diagnosed until much later in life, some well into their 70’s or 80’s. Either way, the root cause of the symptoms for everyone comes down to genetics. So let’s go into the details of that a little bit.
Genes are part of your DNA. They carry the information that determine a person’s traits, characteristics, and features. Genes come in pairs. Each of your parents has two copies of each of their genes, and each parent passes along just one copy to make up the genes you have. For example, if both of your parents have green eyes, you probably have green eyes too because you would have inherited those genes from them.
Sometimes there can be problems with the genes. There can be missing genes, extra genes, or genes that have changed (mutations). Scientists believe that these problematic genes are partly responsible for many problems and illnesses.
A mutation of the ABCA4 gene is the most common cause of Stargardt’s, accounting for about 95% of those diagnosed. The ABCA4 gene is passed along to offspring in an autosomal recessive manner, meaning it has to be inherited from both parents in order for the disease to appear.
There is also a rare dominant form of Stargardt Disease caused by mutations in the ELOVL4 gene. An even more rare form of Stargardt’s associated with the PROM1 gene.
Since the ABCA4 mutation is by far the most common, that’s the condition I’ll focus on when trying to help answer the questions “how did I get Stargardt’s?” and “will my kids have it?” in the rest of this article.
How Did I Get Stargardt’s?
If you’ve read this far, then by now you know you inherited it from your parents. Both parents! You may be saying to yourself, “but both of my parents have pretty good vision, and definitely don’t have Stargardt’s!” Right. So let’s take a look at exactly how this happens.
So to illustrate this, let’s say [N] represents a normal ABCA4 gene and [s] represents the Stargardt-causing mutation in the ABCA4 gene. So [N] is a good gene and [s] is a bad one. Remember, genes come in pairs, and those pairs determine your features. So most people with normal vision would have [NN]. Also mentioned earlier, Stargardt’s is inherited through autosomal recessive manner, meaning a bad gene ([s]) has to be inherited from both parents in order for the disease to appear. This would result in [ss]. The other two possible gene combinations, [Ns] and [sN] would both produce normal vision as well. But those people are considered to be “carriers” since they carry at least one of the Stargardt’s genes (ABCA4 mutation).
If you have the ABCA4 mutation form of Stargardt’s, then you are [ss]. How did you get it? You got a [s] from each of your parents. If neither of your parents has Stargardt’s symptoms, then they are carriers, either [Ns] or [sN], but each of them passed on their [s] to you. The following image illustrates this, showing two parent carriers and the four possible gene combinations for their offspring. Each offspring gene combination is equally likely.
Your parents both inherited a [s] from one of their parents, and so on. This is how it is possible to not have any prior family history of Stargardt’s – a single [s] keeps getting passed down. Obviously, if one of your parents has Stargardt’s, they too are [ss] and would definitely pass down an [s] to you.
Will My Kids Have Stargardts?
The only way to know this for sure is through genetic testing. Get yourself tested to verify that you have the ABCA4 form of Stargardt Disease, and then have your partner tested as well. If your partner does not have a mutation on their ABCA4, then there’s zero chance your kids will have it (i.e. your partner is [NN], so all of your kids would be [Ns]). You could get your child genetically tested. Some people question whether you should do that or not. The thought there is – why risk having to tell them at a young age that they will lose their vision eventually? It may take away their ability to live out a normal childhood if they have that dark cloud weighing on them. Now, if there were treatments available to prevent Stargardt’s, that would obviously change that decision.
If genetic testing isn’t feasible, you can still calculate the odds of your kids having Stargardt’s. It is estimated that only about 1 in 20 people are carriers for Stargardt’s. So 5% of people carry it, and 95% do not.
Let’s say that your partner is one of the 95% that do not carry it. Your partner is [NN] and you are [ss].
Each of you passes down one gene to your kids. Your partner only has [N] to pass down and you only have [s] to pass down, so all of your kids will be [Ns], which means they will be carriers, but not have any Stargardt’s symptoms.
Now let’s assume that your partner is in that 5% and is a carrier. They are either [Ns] or [sN], it doesn’t matter for the purpose of these calculations.
You will certainly pass down [s] to your kids. Your partner has a 50% chance of passing down [N], resulting in [Ns] for the child, giving them normal vision. There’s also a 50% chance of your partner passing down [s], resulting in [ss], in which case the child will have Stargardt’s.
So, without genetic testing, just accounting for probabilities and statistics among the human population, the odds that your kids will have Stargardt’s is simply the odds of your partner being a carrier (5%) multiplied by the odds that they pass down their [s] gene (50%). 0.05 x 0.50 = 0.025. So even with you having Stargardt’s, the odds are pretty slim, just 2.5%, that your child will also have it.
I hope that helps clear up some of the confusion about how a person could get Stargardt’s when there have been no family history of it reported. I also hope it sheds some light on the risk of (or lack thereof) passing it on to your kids – it is pretty slim. Being a father myself, I’d encourage anyone with Stargardt’s to not be afraid of having kids because of this disease. The emotional reward of being a parent is too awesome to pass up!