Gain of toxic function by long-term AAV9-mediated SMN overexpression in the sensorimotor circuit

AAV9-SMN is used to treat SMA. This study shows that AAV9-mediated SMN overexpression in mice causes late-onset motor dysfunction and synaptic and neuronal loss through protein aggregation, suggesting caution on the long-term safety of SMN gene therapy. The neurodegenerative disease spinal muscular atrophy (SMA) is caused by deficiency in the survival motor neuron (SMN) protein. Currently approved SMA treatments aim to restore SMN, but the potential for SMN expression beyond physiological levels is a unique feature of adeno-associated virus serotype 9 (AAV9)-SMN gene therapy. Here, we show that long-term AAV9-mediated SMN overexpression in mouse models induces dose-dependent, late-onset motor dysfunction associated with loss of proprioceptive synapses and neurodegeneration. Mechanistically, aggregation of overexpressed SMN in the cytoplasm of motor circuit neurons sequesters components of small nuclear ribonucleoproteins, leading to splicing dysregulation and widespread transcriptome abnormalities with prominent signatures of neuroinflammation and the innate immune response. Thus, long-term SMN overexpression interferes with RNA regulation and triggers SMA-like pathogenic events through toxic gain-of-function mechanisms. These unanticipated, SMN-dependent and neuron-specific liabilities warrant caution on the long-term safety of treating individuals with SMA with AAV9-SMN and the risks of uncontrolled protein expression by gene therapy.

Automated summary

This study demonstrates that long-term use of AAV9-SMN gene therapy in mice can lead to motor dysfunction and neuronal loss through protein aggregation. Overexpression of SMN interferes with RNA regulation and triggers SMA-like pathogenic events through toxic gain-of-function mechanisms. These findings suggest caution in the long-term safety of treating individuals with SMA with AAV9-SMN.