Spinal Muscular Atrophy: SMN1 Sequencing

Clinical Information

Spinal Muscular Atrophy (SMA) is a neuromuscular disorder characterized by progressive muscle weakness resulting from the degeneration of motor neurons in the brain and spinal cord. The incidence of SMA is 1 in 10,000 births, and the age of onset and severity are variable. While specific types of SMA have been historically described, phenotypic overlap does occur from one type to another.

Congenital SMA (SMA 0) may present prenatally with reduced fetal movement or at birth and is characterized by severe hypotonia, respiratory failure, and absent developmental milestones. Feeding difficulties are common, and death typically occurs by the age of 6 months. SMA I, also known as Werdnig-Hoffman disease, presents in individuals less than six months of age, and associated symptoms include proximal muscle weakness, swallowing issues, and labored breathing. Tongue fasciculations are present in a majority of affected individuals, and death typically occurs by 2 years of age.

SMA II appears at 6-12 months of age, and it is characterized by delayed motor development due to muscle weakness, scoliosis, and lung disease. Affected individuals are unable to walk without assistance, but most patients survive to adulthood. Individuals SMA II show delayed motor development due to muscle weakness, scoliosis, and lung disease, and are unable to walk without assistance; however, most patients survive to adulthood. In individuals with onset at greater than 18 months of age (SMA III, or Kugelberg-Welander disease), the majority maintain the ability to walk and have a normal lifespan. The onset of muscle weakness in adulthood (SMA IV) commonly occurs with a course similar to type III. These forms of spinal muscular atrophy are inherited in an autosomal recessive pattern, and most are due to homozygous deletions of at least exon 7 of the SMN1 gene. Typically, the number of copies of SMN2 can modify the phenotype of SMA.

Technical Information

• This test is performed by Sanger sequencing.
• Genomic DNA is used to perform SMN1 gene-specific sequencing by long-PCR enrichment of the SMN1 locus using allele-specific primers followed by independent exon-scale amplification and sequencing of exons 1-7.
• Five nucleotides distinguish SMN1 from SMN2, at least four of which are analyzed as controls to insure the absence of SMN2 sequence contamination.

Specimen Requirements

• The preferred sample type is 3-4 ml of peripheral blood collected in an EDTA (purple top) tube. Dried blood spots, extracted DNA, and saliva are also accepted for this test. Blood and saliva kits are available by request.
• Approximately 75 µL of blood should be applied to each of the five circles on a filter paper dried blood spot card. Allow blood to dry for at least 4 hours before shipping.
• Send approximately 5µg of extracted DNA at a requested concentration of 90-130 ng/µl.
• Saliva samples must be submitted in an approved saliva kit.

Transport Instructions

• The blood specimen should be kept at room temperature and delivered via overnight shipping. If shipment is delayed by one or two days, the specimen should be refrigerated and shipped at room temperature. Do not freeze the specimen. Samples collected on Friday can be safely designated for Monday delivery.
• DBS cards should be shipped at room temperature.
• Extracted DNA should be sent at room temperature via overnight delivery.
• Saliva is stable at room temperature and can be delivered via overnight or ground shipping.