Gain of toxic function by long-term SMN overexpression in the sensory-motor circuit.

The neurodegenerative disease spinal muscular atrophy (SMA) is a leading genetic cause of infant death caused by deficiency in the survival motor neuron (SMN) protein. Currently approved SMA treatments aim to restore SMN, but the potential for expression of SMN beyond physiological levels is a unique feature of 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 (snRNPs), leading to splicing dysregulation and widespread transcriptome abnormalities with prominent signatures of neuroinflammation and innate immune response. Thus, long-term SMN overexpression can interfere with its normal activity in RNA regulation and trigger SMA-like pathogenic events through toxic gain of function mechanisms. These unanticipated, SMN-dependent and neuron-specific liabilities of AAV9-SMN warrant further evaluation of the long-term safety of gene therapy in SMA. To determine the effects of AAV9-mediated long-term SMN overexpression on the transcriptome of dorsal root ganglia (DRGs) of wild type mice, we performed transcriptome analysis of DRGs from 300-day old wild type mice that were either uninjected or injected with AAV9-GFP or AAV9-SMN at P1 by deep sequencing in triplicates.