Spinal Muscular Atrophy
Spinal muscular atrophy (SMA) is among the most frequent autosomal recessive diseases with a carrier frequency of 1 in 35 to 1 in 117 depending on ethnicity and an overall incidence of 1 in 10,000 live births. The disease is characterized by the degeneration of alpha motor neurons of the spinal cord anterior horn cells, leading to progressive symmetric weakness, atrophy of the proximal voluntary muscles and infant death. Homozygous deletions of the SMN1 gene on chromosome 5q13 are the most common cause of SMA. Over 95% of SMA patients are missing both copies of SMN1 exon 7 and exon 8 sequences. To test for SMN1 and SMN2 copy number, MLPA analysis is performed.
Common dosage analysis methods are limited by their inability to identify silent (2+0) carriers, with two copies (duplication) of SMN1 on the one chromosome 5 and deletion on the other. Enhanced SMA carrier screening developed at Mount Sinai involves testing for a single nucleaotide polymorphism (SNP) in intron 7 on SMN1, g.27134T>G, which is part of a haplotype specific for SMN1 duplication alleles in the Ashkenazi Jewish and Asian populations an is significantly enriched in individuals with SMN1 duplications in the African American, Hispanic, and Caucasian populations. Testing for this SNP increases detection test performance by providing increased detection rate and more accurate residual risks.
The American College of Medical Genetics recommends that carrier screening for SMA be offered to all couples regardless of ethnicity before or during pregnancy.
Prenatal diagnosis is also available. Prior to ordering prenatal testing, please contact our laboratory at 212-241-7518 to discuss.
1. Pearn J. (1980) Classification of spinal muscular atropies. Lancet; I: 919-922.
2. Hendrickson BC, Donohoe C, Akmaev VR, Sugarman EA, Labrousse P, Boguslavskiy L, Flynn K, Rohlfs EM, Walker A, Allitto B, Sears C, Scholl T (2009) Differences in SMN1 allele frequencies among ethnic groups within North America. J. Med. Genet. 2009; 46:641-644.
3. Ogino S et al. Genetic risk assessment in carrier testing for spinal muscular atrophy testing. Am J Med Genet. 2002;110:301-7.
4. Prior TW et al. Technical standards and guidelines for spinal muscular atrophy testing. Genet Med. 2011; 13:686-694.
5. Luo M et al. An Ashkenazi Jewish SMN1 haplotype specific to duplication alleles improves pan-ethnic carrier screening for spinal muscular atrophy. Genet Med. 2013; 16:149-156.
Post-natal: Two 5-10 mL tubes of anticoagulated blood in EDTA (lavender top) or two 5-10 mL tubes of anticoagulated blood in ACD (yellow top).
- Chorionic Villi: 5-10 mg in conical tube with sterile OR transport media
- Amniotic Fluid: 10 ml in conical tube
- Cultured Cells: two confluent T-25 flasks
Additionally, please send:
- 5-10 ml maternal blood in EDTA (lavender top) required to perform MCC studies on all prenatal samples and in case maternal confirmation studies are necessary
- 5-10 ml paternal blood in EDTA (lavender top) in case paternal confirmation studies are necessary
Shipping: Ship at room temperature.
Turnaround Time: 10-14 days
CPT Code: 81403
Consent and Requisition Forms: Carrier Screening Requisition and Consent [PDF]
Additional Information: Expanded Ashkenazi Jewish Carrier Screening brochure [PDF]