Myostatin inhibition in combination with antisense oligonucleotide therapy improves outcomes in spinal muscular atrophy

BACKGROUND Spinal muscular atrophy (SMA) is caused by genetic defects in the survival motor neuron 1 (SMN1) gene that lead to SMN deficiency. Different SMN‐restoring therapies substantially prolong survival and function in transgenic mice of SMA. However, these therapies do not entirely prevent muscle atrophy and restore function completely. To further improve the outcome, we explored the potential of a combinatorial therapy by modulating SMN production and muscle‐enhancing approach as a novel therapeutic strategy for SMA. METHODS The experiments were performed in a mouse model of severe SMA. A previously reported 25‐mer morpholino antisense oligomer PMO25 was used to restore SMN expression. The adeno‐associated virus‐mediated expression of myostatin propeptide was used to block the myostatin pathway. Newborn SMA mice were treated with a single subcutaneous injection of 40 μg/g (therapeutic dose) or 10 μg/g (low‐dose) PMO25 on its own or together with systemic delivery of a single dose of adeno‐associated virus‐mediated expression of myostatin propeptide. The multiple effects of myostatin inhibition on survival, skeletal muscle phenotype, motor function, neuromuscular junction maturation, and proprioceptive afferences were evaluated. RESULTS We show that myostatin inhibition acts synergistically with SMN‐restoring antisense therapy in SMA mice treated with the higher therapeutic dose PMO25 (40 μg/g), by increasing not only body weight (21% increase in male mice at Day 40), muscle mass (38% increase), and fibre size (35% increase in tibialis anterior muscle in 3 month female SMA mice), but also motor function and physical performance as measured in hanging wire test (two‐fold increase in time score) and treadmill exercise test (two‐fold increase in running distance). In SMA mice treated with low‐dose PMO25 (10 μg/g), the early application of myostatin inhibition prolongs survival (40% increase), improves neuromuscular junction maturation (50% increase) and innervation (30% increase), and increases both the size of sensory neurons in dorsal root ganglia (60% increase) and the preservation of proprioceptive synapses in the spinal cord (30% increase). CONCLUSIONS These data suggest that myostatin inhibition, in addition to the well‐known effect on muscle mass, can also positively influence the sensory neural circuits that may enhance motor neurons function. While the availability of the antisense drug Spinraza for SMA and other SMN‐enhancing therapies has provided unprecedented improvement in SMA patients, there are still unmet needs in these patients. Our study provides further rationale for considering myostatin inhibitors as a therapeutic intervention in SMA patients, in combination with SMN‐restoring drugs

TIA1 is a gender-specific disease modifier of a mild mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by deletions or mutations of Survival Motor Neuron 1 (SMN1) gene. The nearly identical SMN2 cannot compensate for SMN1 loss due to exon 7 skipping. The allele C (C+/+) mouse recapitulates a mild SMA-like phenotype and offers an ideal system to monitor the role of disease-modifying factors over a long time. T-cell-restricted intracellular antigen 1 (TIA1) regulates SMN exon 7 splicing. TIA1 is reported to be downregulated in obese patients, although it is not known if the effect is gender-specific. We show that female Tia1-knockout (Tia1−/−) mice gain significant body weight (BW) during early postnatal development. We next examined the effect of Tia1 deletion in novel C+/+/Tia1−/− mice. Underscoring the opposing effects of Tia1 deletion and low SMN level on BW gain, both C+/+ and C+/+/Tia1−/− females showed similar BW gain trajectory at all time points during our study. We observed early tail necrosis in C+/+/Tia1−/−females but not in males. We show enhanced impairment of male reproductive organ development and exacerbation of the C+/+/Tia1−/−testis transcriptome. Our findings implicate a protein factor as a gender-specific modifier of a mild mouse model of SMA

Oligonucleotide therapies for disorders of the nervous system

Oligonucleotide therapies are currently experiencing a resurgence driven by advances in backbone chemistry and discoveries of novel therapeutic pathways that can be uniquely and efficiently modulated by the oligonucleotide drugs. A quarter of a century has passed since oligonucleotides were first applied in living mammalian brain to modulate gene expression. Despite challenges in delivery to the brain, multiple oligonucleotide-based compounds are now being developed for treatment of human brain disorders by direct delivery inside the blood brain barrier (BBB). Notably, the first new central nervous system (CNS)-targeted oligonucleotide-based drug (nusinersen/Spinraza) was approved by US Food and Drug Administration (FDA) in late 2016 and several other compounds are in advanced clinical trials. Human testing of brain-targeted oligonucleotides has highlighted unusual pharmacokinetic and pharmacodynamic properties of these compounds, including complex active uptake mechanisms, low systemic exposure, extremely long half-lives, accumulation and gradual release from subcellular depots. Further work on oligonucleotide uptake, development of formulations for delivery across the BBB and relevant disease biology studies are required for further optimization of the oligonucleotide drug development process for brain applications