Both copies of the SMN1 exon7 gene are missing in about 95 percent of spinal muscular atrophy cases. Since SMA patients have no detectable SMN1 genes, testing for SMN1 exon7 is a reliable and powerful diagnostic test for most cases where the disease is suspected.
The first genetic diagnostic test should be the SMN1 deletion test. The molecular diagnosis of SMA consists of the detection of the absence of exon7 in the SMN1 gene.
Genetic testing is not only the most rapid and sensitive method to confirm an SMA diagnosis, but also helps avoid further invasive investigations such as the electromyography and/or muscle biopsy, which can cause pain and discomfort to the patient.
SMN1 dosage testing is used to determine the number of SMN1 copies and detect SMA carriers; carriers will have one SMN1 copy and non-carriers will have two SMN1 copies.
There are different methods used for the determination of the number of SMN1 copies, but quantitative polymerase chain reaction (PCR) is the most common because it is extremely sensitive, detecting less than five SMN1 copies) relatively quickly and has minimal cross-contamination.
Any positive results in clinically diagnosed people leads to further genetic counseling and other at-risk family members should also be tested as SMA carriers.
It is important that couples who undergo carrier testing are aware that the carrier test does not provide genotype/phenotype information: SMA type 1 occurs in approximately 60 percent of cases, where SMA types 2 and 3 account for the remaining 40 percent.
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