Spinal Muscular Atrophy

What Your Test Results Mean:

Carriers typically show no symptoms of spinal muscular atrophy; however, carriers are at an increased risk of having a child with spinal muscular atrophy. Risk for the current or future pregnancies is dependent on your partner’s carrier status.

Disease Explained:

Spinal muscular atrophy is an inherited neuromuscular disorder characterized by progressive muscle weakness caused by spinal cord and brain stem motor neuron degeneration. Individuals with spinal muscular atrophy do not produce enough of one of the motor neuron proteins, SMN, needed for proper motor neuron function.

There are several types of spinal muscular atrophy including severe, intermediate, mild, and adult. The clinical spectrum ranges from early infant death in severe type I to normal adult life with only mild weakness in adult type IV. Severe type I spinal muscular atrophy presents with muscle weakness by six months of age and inability to sit independently. Poor weight gain, sleep difficulties, pneumonia, scoliosis, and joint contractures are common. In intermediate type II, and mild type III, some children sit but never walk, whereas others show delayed walking but may be able to maintain walking until adult years. Pulmonary disease is a major complication of the disease in addition to gastrointestinal dysfunction and limited motor function of the trunk and extremities. In adult type IV, onset occurs in the second or third decade of life. Motor impairment is mild without respiratory or gastrointestinal problems.

Treatment of individuals with spinal muscular atrophy is supportive and typically provided by a team of specialists including pulmonology, gastroenterology, orthopedics, and others.

How the Genetics Works:

The clinical features of spinal muscular atrophy can be explained by a deletion in both copies of the SMN1 gene. In general, individuals have two copies of the SMN1 gene. Carriers of spinal muscular atrophy have a deletion in one copy of the SMN1 gene while individuals with spinal muscular atrophy have deletions in both copies of their genes. In addition, it is suspected that the number of copies of the SMN2 gene may play a role in the severity of disease. A higher copy number of SMN2 has been associated with milder disease; however, predicting the severity of disease using SMN2 copy number is not currently recommended as the phenotype can be variable.

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