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Spinal muscular atrophy (SMA) causes progressive muscular weakness and degeneration by loss of anterior horn cells in the brain stem and spinal cord. Age of onset can vary from prenatal onset to adult onset as SMA I, with onset before six months of age; SMA II, with onset between six and 12 months; SMA III, with onset in childhood after 12 months; and SMA IV, with adult onset. Approximately 95% of individuals with SMA are homozygous for a deletion of exons 7 & 8 of the SMN1 gene and approximately 5% are compound heterozygotes for the deletion on one allele and a point mutation on the other SMN1 allele. Compound heterozygotes will appear to be carriers by routine deletion testing of exons 7 & 8 by PCR. Dosage testing by MLPA is therefore useful for further testing of patients with high clinical suspicion of SMA but normal result from deletion studies of exons 7 & 8. These individuals would then require DNA sequencing to identify their point mutation.
Couples with a child affected with SMA have approximately a 25% risk of recurrence in a subsequent child. Roughly 2% of cases of SMA are caused by one inherited mutation and one de novo or spontaneous mutation. In this scenario, only one parent is a carrier.
This test is a dosage analysis to asses the SMN1 gene for duplications or deletions and will detect approximately 90% of carriers. About 5% of the general population has three copies of the SMN1 gene. A carrier with an SMN deletion on one chromosome but two normal copies on the other chromosome will have normal results by this assay. Point mutations will also not be detected. Carrier testing will not predict which type of SMA a couple is at risk for. Pre-test genetic counseling is highly recommended.
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