Spinal Muscular Atrophy is an autosomal recessive neurodegenerative disease, and the most common cause of mortality in infants linked to a genetic mutation. It affects between 1 in 6,000 and 1 in 10,000 people.1
The disease is characterized by progressive muscle weakness caused by a loss of specialized nerve cells (motor neurons) in the spinal cord and the part of the brain that is connected to the spinal cord (brainstem nuclei).2
Causes and classification of SMA
SMA is caused by mutations in the copy of the survival motor neuron 1 (SMN1) gene that is in the free end of the chromosome 5, but all the patients maintain a copy of the gene, SMN2, in the center of the same chromosome.
The absence of the SMN1 exon 7 has been observed in the majority of patients, and is being used as a reliable and sensitive SMA diagnostic test. In most cases, the severity of the condition is inversely correlated with a higher number of copies of SMN2 gene (i.e., the more copies, the more likely the disease will be less severe and occur later in life).
The detection of disease carriers relies on the accurate determination of SMN1 gene copies. Since SMA is one of the most lethal genetic disorders, with a carrier frequency of 1 in 40 to 1 in 60, direct carrier dosage testing has proven beneficial to many families.1
The loss of the motor neurons leads to weakness and atrophy of the muscles used for daily activities, such as crawling, walking, sitting up, and controlling head movement. In severe cases of SMA, the muscles used for breathing and swallowing are also affected.2
The disease is classified on the basis of age of onset and clinical course.1 SMA type 1 is the most severe, and found in infants in their first months of life. Type 2 affects infants under 1 year of age and is marked by serious muscular weakness (assisted walking and sitting), while type 3 is somewhat milder and occurs later in childhood or in a person’s teenage years. Type 4 strikes adults, typically after age 30, and can be mild to moderate.
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