SMA is the most common genetic cause of infant death

  • Universal: SMA carrier screening is recommended for all patients, all ethnicities1
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About Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is the most common genetic cause of infant death and is the second most common inherited disorder, after cystic fibrosis. SMA causes progressive muscle wasting and weakness due to loss of motor neurons in the spinal cord. Patients with SMA are generally grouped into one of four types (I, II, III, IV) based on their highest level of motor function. The most common form, SMA type I, causes death by two years of age. About 1 in every 6,000 to 10,000 babies is born with SMA.

SMA is caused by a deletion of the gene known as survival motor neuron gene 1 (SMN1). In a healthy person, this gene produces a protein called survival motor neuron (SMN) protein. In an affected person, this protein is absent or significantly decreased. Since SMN protein is critical to the health and survival of motor neurons, nerve cells may shrink and die without this protein, resulting in muscle atrophy.

SMA is an autosomal recessive disorder. When both parents are carriers, there is a 1 in 4, or 25%, chance the child will be affected. Because SMA is present in all populations, the American College of Medical Genetics and Genomics (ACMG) recommends that carrier screening be offered to all couples, regardless of race or ethnicity.1

Carrier Frequency and Detection Rates

Table: SMAnxt: Spinal Muscular Atrophy Carrier Frequency and Detection Rates

Residual Risk

A finding of two or more copies of the SMN1 gene significantly reduces the risk that the individual is a carrier of SMA. A negative carrier screening result greatly reduces, but does not completely eliminate, the chance of being a SMA carrier and having affected children. A patient who receives a negative test result still has a residual risk of being a carrier.
Table: SMAnxt: Spinal Muscular Atrophy Residual Risk Comparison

Screening Guidelines

Because SMA is present in all populations, carrier testing should be offered to all couples regardless of race or ethnicity. Ideally, the testing should be offered before conception or early in pregnancy. The primary goal is to allow carriers to make informed reproductive choices.1  

References
1. Carrier screening for spinal muscular atrophy. ACMG Practice Guidelines. American College of Medical Genetics and Genomics. Genet Med 2008 10:11:840-842