Spinal muscular atrophy: the RNP connection

Degenerated motor neurons in the spinal cord are the pathological hallmark of spinal muscular atrophy (SMA). SMA is caused by mutations in the ubiquitously expressed survival motor neuron 1 (SMN1) gene, which lead to reduced levels of functional SMN protein. Many different functions have been assigned to SMN, including assembly of ribonucleoproteins (RNPs), splicing, transcription and axonal mRNA transport. Recently, tissue from SMA patients and animal models has been used to determine which function of SMN is affected in SMA patients. A surprising picture has emerged: the impaired assembly of RNP subunits of the spliceosome seems to be responsible for SMA pathogenesis. Here, we present a model of how this defect might cause motor-neuron degeneration and consider potential therapies.