Abstract
Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder caused by reduced expression of the survival motor neuron (SMN) protein. SMN has key functions in multiple RNA pathways, including the biogenesis of small nuclear ribonucleoproteins that are essential components of both major (U2-dependent) and minor (U12-dependent) spliceosomes. Here we investigated the specific contribution of U12 splicing dysfunction to SMA pathology through selective restoration of this RNA pathway in mouse models of varying phenotypic severity. We show that virus-mediated delivery of minor snRNA genes specifically improves select U12 splicing defects induced by SMN deficiency in cultured mammalian cells, as well as in the spinal cord and dorsal root ganglia of SMA mice without increasing SMN expression. This approach resulted in a moderate amelioration of several parameters of the disease phenotype in SMA mice, including survival, weight gain, and motor function. Importantly, minor snRNA gene delivery improved aberrant splicing of the U12 intron-containing gene Stasimon and rescued the severe loss of proprioceptive sensory synapses on SMA motor neurons, which are early signatures of motor circuit dysfunction in mouse models. Taken together, these findings establish the direct contribution of U12 splicing dysfunction to synaptic deafferentation and motor circuit pathology in SMA.
Highlights
Removal of introns from precursor mRNAs is a finely regulated nuclear process carried out by the spliceosome, a highly dynamic RNA protein machine [1]
This study aimed to address these issues by investigating the contribution of reduced assembly of minor small nuclear ribonucleoproteins (snRNPs) and dysregulated U12 splicing to Spinal muscular atrophy (SMA) pathology in mouse models of the disease, which provide the best recapitulation of the human condition both genetically and phenotypically [26, 60]
Motor function independent of any alteration of survival motor neuron (SMN) levels. These effects were associated with a robust rescue of the loss of proprioceptive sensory synapses impinging on SMA motor neurons, an early signature of motor dysfunction in mouse models of the disease [42, 43, 62]
Summary
Removal of introns from precursor mRNAs (pre-mRNAs) is a finely regulated nuclear process carried out by the spliceosome, a highly dynamic RNA protein machine [1]. There are 2 functionally distinct spliceosomes comprising different subsets of small nuclear ribonucleoproteins (snRNPs) that are dedicated to the excision of different types of introns based on their specific splicing consensus sequences [2]. U12-type introns and their relative positions are evolutionarily conserved and not randomly distributed across the genome [14,15,16,17]. Rather, they are enriched in information processing genes implicated in DNA and RNA metabolism as well as vesicular transport and voltage-gated ion channels that have especially important functions in neurons.
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