Prenatal SMN-dependent defects in translation uncover a reversible primary ciliopathy in spinal muscular atrophy
Spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of SMN protein resulting from mutations in the SMN1 gene. Several therapeutic approaches capable of boosting SMN are now approved for use in human patients, delivering remarkable improvements in lifespan and symptom severity. However, new and unexpected phenotypes are being reported in treated SMA patients, including growing evidence for significant neurodevelopmental comorbidities in some individuals, indicative of alterations in brain development. Here, using a mouse model of severe SMA, we reveal an underlying neurodevelopmental phenotype in SMA where prenatal SMN-dependent defects in translation drive a primary ciliopathy in the CNS. Cell proliferation was significantly reduced in the hippocampus of SMA mice at E14.5, accompanied by widespread perturbations in translation, disrupting genes associated with primary cilia. The density of primary cilia in vivo, as well as cilial length in vitro, was significantly decreased in prenatal SMA mice, revealing core morphological hallmarks of a primary ciliopathy. Prenatal transplacental therapeutic intervention with an SMN-restoring small molecule (Risdiplam) rescued primary cilia defects in SMA mouse embryos. We conclude that SMN protein is required for normal cellular and molecular development of the CNS, with low levels in SMA driving a primary ciliopathy. Early, systemic treatment with SMN-restoring therapies is likely to be required to target neurodevelopmental comorbidities. Overall design: Ribosome profiling was obtained from control (Smn+/-; SMN2tg/0) and SMA Taiwanese mouse model (Smn-/-; SMN2tg/0, strain 005058) at embryonic day 14.5 from brain and spinal cord. Experiments were performed in biological triplicates for each tissue.