Mutations in the SMN1 gene cause spinal muscular atrophy (SMA), a disorder characterised by progressive symmetric muscle weakness that can be complicated by other features including joint contractures, scoliosis, growth failure and restrictive lung disease. SMA is classified into clinical subtypes depending on severity and age of onset. Inheritance of SMA is autosomal recessive.
This test detects deletions of the SMN1 gene which cause more than 95% of cases of SMA. The result can have diagnostic and familial implications.
Interpretation of SMA test results will depend on the purpose of testing (diagnostic or carrier assessment), the patient’s family history and test results for other family members; however generally speaking test results are interpreted in the following way:
Homozygous deletion (0 copies of SMN1 gene): confirms a diagnosis of SMA.
Heterozygous deletion (1 copy of SMN1 gene detected): is supportive of but does not confirm a diagnosis of SMA. Patients with this result and a clinical diagnosis of SMA can have rare sequence level mutations on the non-deleted copy of the SMN1 gene, and additional testing may be recommended.
No deletion (2 copies of SMN1 gene detected): a diagnosis of SMA is unlikely.
1 copy of the SMN1 gene detected: patient is a carrier of SMA.
2 copies of the SMN1 gene detected: patient is unlikely to be a carrier of SMA, but the possibility cannot be excluded.
This is an assay for heritable mutations. It may raise issues of ethics or consent that are different from most other investigations ordered in the routine care of a patient. Use of the Sonic Genetics consent form (below) is not mandated, but could be used to guide the discussion with the patient.
In the case of carrier testing of an unaffected child, national regulations require that the laboratory has evidence of pre-test counselling by a genetics professional and written consent.
This test can be requested by any medical practitioner.
4 mL blood in EDTA (separate tube required). Specimens may be collected by the requesting practitioner or at any Sonic Healthcare collection centre (see link below).
To help ensure the quality of the test, a genetic test should be done with a dedicated sample whenever possible i.e. a sample collected specifically for that test rather than a sample that is used for multiple tests.
We recommend that the patient or another adult check the labelling of request forms and sample tubes.
Please note whether or not the patient has a family history of SMA on the referral form. If there is a family history of SMA, please provide as much information as possible about the family pedigree and any previous testing that has been performed for other family members.