You may have wondered if your child would have brown or blue eyes, but you might not have thought about whether or not they’re at risk for a severe and debilitating genetic disorder. Our genetic code makes up much of who we are, from the color of our eyes to how outgoing we may be, and our genes also play an important role in determining whether or not we inherit many serious conditions. One such condition is spinal muscular atrophy (SMA), affecting approximately 1 in 100,000 people during their lifetime.
SMA is caused by a genetic defect that causes the death of motor neurons in the portion of the brain which connects to the spinal cord. The death of these cells leads to muscular atrophy in muscle groups that are used for a number of day-to-day activities including walking, controlling the head, crawling, and in severe cases even breathing or swallowing. SMA is a progressive condition, causing a slow decay in muscle strength, potentially culminating in paralysis. While most people who suffer from SMA don’t live past the age of two, there are several more mild that allow for an average lifespan.
Types and Symptoms of SMA
There are four types of SMA, distinguished by the severity of the symptoms as well as at which age they occur. With SMA type I, symptoms can be seen at birth, and include trouble breathing and swallowing. Type II manifests itself between 6 and 12 months, usually preventing children from walking or standing without support. Type III develops in early childhood, and causes walking to become increasingly difficult with age. Finally, type IV can present as late as age 30, with patients suffering from muscle weakness, twitching, tremors, and breathing problems.
The Genetic Basis of SMA
Usually, spinal muscular atrophy inheritance is caused by a mutation in what’s known as the SMN1 gene. The SMN1 mutation that causes SMA is what’s known as autosomal recessive. That means inheritance of SMA occurs when a two copies of a faulty SMN1 gene is contributed — one by each parent. Individuals who carry just one copy of the SMA mutation are asymptomatic, but are capable of passing on that mutation to their children. When both parents are carriers of the mutated SMA gene, there’s roughly a 25% chance that the mutated genes will be passed on from both parents, causing their child to suffer from SMA.
Spinal Muscular Atrophy Genetic Testing
The fact the SMN1 gene is recessive means that carriers are typically unaware of the dangers lurking within their own genetic code. That’s why it’s a good idea to test for the mutated SMN1 gene before you consider having children. This is particularly true for individuals with a family member who suffers from SMA. However, just because you don’t have a family history of SMA does not mean you’re not at risk for being a carrier. In fact, the mutated gene that can cause SMA exists in about 2% of all people across the globe.
Fortunately, a blood test can allow for a genetic screening to determine with 90% probability whether or not you have the SMA mutation. Even if just one parent does not carry the mutated SMN1 gene, the chances of their child having SMA is drastically reduced. While there are cases where only one parent was a carrier, and there are mutations that can lead to SMA which are not covered by a blood test, these cases are exceedingly rare. Even in cases where both parents are an SMA carrier, options are available. Thanks to the latest advances in genetic research, it’s possible to do prenatal testing to determine whether or not your child will be a carrier of SMA between 10 and 20 weeks after pregnancy.
Genetic diseases can be downright scary. But with responsible testing and preparation, you can rest assured that your own children will be spared from serious genetic conditions like spinal muscular atrophy. Even if you have no family history, you should strongly consider being tested to ensure that you’re not part of the one in fifty people who carry the mutated SMN1 gene. If you are, take the time to have your partner tested as well. Although there’s no certainty that you will or will not pass on a mutated gene to your children, it’s still a possibility that every parent should carefully consider.