The molecular bases of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by the absence of a functional Survival of Motor Neuron 1 gene (SMN1). The nearly identical paralog, SMN2, cannot compensate for the loss of SMN1 because exon 7 is aberrantly skipped from most SMN2 transcripts, a process mediated by synergistic activities of Sam68/KHDR1 and hnRNP A1. This results in the production of a truncated, non-functional protein that is rapidly degraded. Here we present several crystal structures of Sam68 RNA-binding domain (RBD). Sam68-RBD forms stable symmetric homodimers by antiparallel association of helices α3 from two monomers. However, the details of domain organization and the dimerization interface differ significantly from previously characterized homologs. We demonstrate that Sam68 and hnRNP A1 bind proximal but distinct motifs within the central region of SMN2(ex7). Our findings have important implications for the etiology of SMA and open new avenues for the design of novel therapeutics to treat splicing diseases.