Ssa Type 2 Essay

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SMA is largely an inherited autosomal recessive disease caused by mutations in chromosome 5q that lead to a deficiency in SMN1-related proteins. In rare instances (2-3% of SMA), SMA can occur de novo rather than inheriting a defective copy of the gene from each parent. This deficiency results in degeneration of motor neurons causing muscle atrophy, particularly in the limbs and the muscles that control the mouth, throat and respiration. There are four types of SMA, types I, II, III, and IV which are defined based on the severity of muscle weakness and the age of symptom onset. SMA type I (Werdnig-Hoffmann disease) is the most severe. SMA type I-affected infants represent approximately 60% of SMA diagnoses and present with the disease by 6 months of age. These infants are profoundly hypotonic and often succumb to complications of the disease by their second year of life. SMA type II affected children (intermediate form) present with symptoms prior to 18 months of age and develop the ability to sit unaided but not the ability to stand or walk. Individuals affected by SMA type III (Kugelberg-Welander disease) are also generally diagnosed by 18 months but are able to stand and walk. SMA type III affected individuals may live into their thirties and …show more content…

Therefore, although the role of SMN protein in motor neurons is not completely understood and the amount for normal functioning undefined, the phenotype of spinal muscular atrophy (type I, II, III, or IV) is largely related to the number of SMN2 gene copies present. The number of copies of SMN2 in individuals diagnosed with SMA has been found to negatively correlate with disease severity. For instance, infants diagnosed with SMA type I, are likely to have two copies or less of SMN2 and individuals with SMA type III and IV are likely to have three copies or more (Mailman, et al., 2002)

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