SMA (spinal muscular atrophy) is a severe, often fatal, genetic disorder in which muscles involved in many essential functions, such as breathing, eating, and movement, become progressively weaker and ultimately waste away (atrophy) and die. Every year, about one in every 10,000 babies is born with SMA.
In most cases, SMA is caused by errors, or mutations, in a gene called SMN1. SMN1 carries the genetic blueprint for a protein (SMN, or survival of motor neuron) that is critical to the survival and proper functioning of specialized nerve cells, called motor neurons, in the spinal cord that are needed for muscles to be able to function properly.
There are three types of SMA. The most common form, type I, which affects about 70% of patients, is the most severe. Children with type 1 SMA usually die from respiratory failure before the age of two. In fact, SMA is the leading genetic cause of death in early childhood.
Children with type II SMA may be able to sit unaided, but cannot stand or walk unaided. These children typically live past age four. Although they face many challenges, children with type III SMA are able to walk unaided and have a normal lifespan.
About one in every 40 to 60 adults worldwide carries a mutated SMN1 gene; in the United States alone, roughly 7.5 million people are carriers.
SMA is an autosomal recessive disorder, which means that a child can only have the disease if both parents carry the mutated SMN1 gene. A person with a mutation in the SMN1 gene does not have and will not develop SMA. SMA is the second most common fatal autosomal recessive disorder after cystic fibrosis.
When both partners are carriers, there is a 25% (one in four) chance with each pregnancy of having a child with SMA. (Of course, this means that there is a 75% chance that each pregnancy will not result in a child with SMA.) There is a 50% chance of having a child who does not have the disease, but is a carrier of the mutated gene, and a 25%
chance of having a baby who does not have the disease and is also not a carrier.
Fortunately for individuals planning a family, a blood test can determine with a very high degree of certainty whether a couple carries the mutation responsible for SMA. It is now recommended by the American College of Medical Genetics that all couples, regardless of their race or ethnicity, undergo carrier testing for SMA. Couples who learn that they are both SMA carriers and are, therefore, at high risk of having a baby with SMA, have many options.
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