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SMA Carrier Screen

SMA Carrier Screen

Test Summary

SMA Carrier Screen

  

Clinical Use

  • Detect carriers of spinal muscular atrophy (SMA)

  • Determine a couple’s risk of having a child with SMA

Clinical Background

Spinal muscular atrophy (SMA) is an autosomal recessive disease that causes progressive muscle wasting and weakness due to loss of motor neurons in the spinal cord. The disease is often fatal and is the leading genetic cause of death in infants.1 Childhood SMA is subdivided into 3 clinical groups based on age at onset and degree of motor function (Table). Type I is the most common (60% to 70% of cases) and the most severe.3 Adult-onset SMA (type IV) is less common and much milder than the childhood types.

Table. Classification of Spinal Muscular Atrophy1,2

SMA Type

Age of Onset

Clinical Severity Outcome

Childhood

I

<6 months

Severe: generalized
weakness and hypotonia; unable to sit

Patients typically die of
respiratory failure before
2 years of age

II

7–18 months

Intermediate: patients
typically able to sit but not
stand

Survival beyond 4 years into adulthood depending on degree of respiratory distress

III

>18 months

Mild: patients can typically
stand and walk

Normal lifespan

Adult onset

IV

>20 years

Very mild: patients have motor impairment without respiratory or gastrointestinal problems

Normal lifespan

SMA incidence is approximately 1 in 10,000 live births and the carrier rate is 1 in 50.2 According to the American College of Medical Genetics (ACMG), SMA meets the criteria for population-based carrier screening.4 However, recommendations for SMA carrier screening vary. The ACMG recommends offering SMA screening to all couples before conception or early in pregnancy regardless of race or ethnicity.4 The American College of Obstetricians and Gynecologists (ACOG), on the other hand, recommends carrier screening for individuals with a family history of SMA or SMA-like disease and for patients who request the test and have completed genetic counseling.1

SMA is caused by mutations in the survival motor neuron 1 gene (SMN1) on chromosome 5q13. Roughly 95% of cases are caused by deletion of the entire exon 7.4 Whereas normal individuals have 1 or more full-length SMN1 copies on each chromosome, affected patients have mutated copies on both chromosomes. SMA carriers typically have 1 normal copy and 1 mutated copy, and have no symptoms. Gene dosage analysis is the method of choice to identify SMA carriers.4 The SMA Carrier Screen determines the copy number, or gene dosage, of exon 7 of the SMN1 gene.

Individuals Suitable for Testing

  • Individuals with a family history of SMA, SMN1 deletions, or SMA-like disease1

  • Reproductively active individuals or couples4

Method

  • Real-time allele-specific PCR of exon 7 of the SMN1 gene and the CFTR control gene

  • Comparison between fluorescent signals of target and control PCR products to determine gene copy number

  • SMN1 gene copy number is reported, along with interpretive information

  • Results reviewed, interpreted, and reported by a board-certified geneticist

Interpretive Information

The presence of 2 or more full-length SMN1 gene copies significantly reduces the risk of being a SMA carrier. Presence of only 1 SMN1 full-length gene copy is consistent with being a carrier, whereas 0 copies are consistent with a diagnosis of SMA. When both partners are carriers, there is a 1 in 4 chance of having a child with SMA.

False-negative results are possible. A small percentage of the general population may have point mutations and other nondeletion mutations that will not be detected with this test.5 In addition, carriers with 2 SMN1 gene copies on 1 chromosome and none on the other cannot be distinguished from a normal noncarrier, and therefore the test result would incorrectly indicate noncarrier status.1 Thus, genetic counseling should include discussion of the possibility of false-negative results.

Assistance with the interpretation of results is available by calling Quest Diagnostics Genetic Counselors at 866-GENE-INFO (866-436-3463).

References

  1. ACOG Committee Opinion No. 432: Spinal muscular atrophy. Obstet Gynecol. 2009;113:1194-1196.

  2. Lunn MR, Wang CH. Spinal muscular atrophy. Lancet. 2008;371:2120-2133.

  3. Prior TW. Perspectives and diagnostic considerations in spinal muscular atrophy. Genet Med. 2010;12:145-152.

  4. Prior TW. ACMG Practice Guidelines: Carrier screening for spinal muscular atrophy. Genet Med. 2008;10:840-842.

  5. Prior TW, Nagan N, Sugarman EA, et al. Technical standards and guidelines for spinal muscular atrophy testing. Genet Med. 2011;13:686-694.
     

This test was developed and its performance characteristics have been determined by Athena Diagnostics. Performance characteristics refer to the analytical performance of the test.

Content reviewed 06/2013
 

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