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The 38 kDa survival motor neuron (SMN) protein is encoded by two ubiquitously expressed genes: telomeric SMN (SMN^T) and centromeric SMN (SMN^C). Mutations in SMN^T, but not SMN^C, cause proximal spinal muscular atrophy (SMA), an autosomal recessive disorder that results in loss of motor neurons. SMN is found in the cytoplasm and nucleus. The nuclear form is located in structures termed gems. Using a panel of anti-SMN antibodies, we demonstrate that the SMN protein is expressed from both the SMN^T and SMN^C genes. Western blot analysis of fibroblasts from SMA patients with various clinical severities of SMA showed a moderate reduction in the amount of SMN protein, particularly in type I (most severe) patients. Immunocytochemical analysis of SMA patient fibroblasts indicates a significant reduction in the number of gems in type I SMA patients and a correlation of the number of gems with clinical …