Development of a single vector system that enhances trans-splicing of SMN2 transcripts.

RNA modalities are developing as a powerful means to re-direct pathogenic pre-mRNA splicing events. Improving the efficiency of these molecules in vivo is critical as they move towards clinical applications. Spinal muscular atrophy (SMA) is caused by loss of SMN1. A nearly identical copy gene called SMN2 produces low levels of functional protein due to alternative splicing. We previously reported a trans-splicing RNA (tsRNA) that re-directed SMN2 splicing. Now we show that reducing the competition between endogenous splices sites enhanced the efficiency of trans-splicing. A single vector system was developed that expressed the SMN tsRNA and a splice-site blocking antisense (ASO-tsRNA). The ASO-tsRNA vector significantly elevated SMN levels in primary SMA patient fibroblasts, within the central nervous system of SMA mice and increased SMN-dependent in vitro snRNP assembly. These results demonstrate that the ASO-tsRNA strategy provides insight into the trans-splicing mechanism and a means of significantly enhancing trans-splicing activity in vivo.