The neuromuscular impact of symptomatic SMN restoration in a mouse model of spinal muscular atrophy

Abstract

Background Significant advances in the development of SMN-restoring therapeutics have occurred since 2010 when very effective biological treatments were reported in mouse models of spinal muscular atrophy. As these treatments are applied in human clinical trials, there is pressing need to define quantitative assessments of disease progression, treatment stratification, and therapeutic efficacy. The electrophysiological measures Compound Muscle Action Potential and Motor Unit Number Estimation are reliable measures of nerve function. In both the SMNΔ7 mouse and a pig model of spinal muscular atrophy, early SMN restoration results in preservation of electrophysiological measures. Currently, clinical trials are underway in patients at post-symptomatic stages of disease progression. In this study, we present results from both early and delayed SMN restoration using clinically-relevant measures including electrical impedance myography, compound muscle action potential, and motor unit number estimation to quantify the efficacy and time-sensitivity of SMN-restoring therapy. Methods SMAΔ7 mice were treated via intracerebroventricular injection with antisense oligonucleotides targeting ISS-N1 to increase SMN protein from the SMN2 gene on postnatal day 2, 4, or 6 and compared with sham-treated spinal muscular atrophy and control mice. Compound muscle action potential and motor unit number estimation of the triceps surae muscles were performed at day 12, 21, and 30 by a single evaluator blinded to genotype and treatment. Similarly, electrical impedance myography was measured on the biceps femoris muscle at 12 days for comparison. Results Electrophysiological measures and electrical impedance myography detected significant differences at 12 days between control and late-treated (4 or 6 days) and sham-treated spinal muscular atrophy mice, but not in mice treated at 2 days(p<0.01). EIM findings paralleled and correlated with compound muscle action potential and motor unit number estimation (r=0.61 and r=0.50, respectively, p<0.01). Longitudinal measures at 21 and 30 days show that symptomatic therapy results in reduced motor unit number estimation associated with delayed normalization of compound muscle action potential. Conclusions The incomplete effect of symptomatic treatment is accurately identified by both electrophysiological measures and electrical impedance myography. There is strong correlation between these measures and with weight and righting reflex. This study predicts that measures of compound muscle action potential, motor unit number estimation, and electrical impedance myography are promising biomarkers of treatment stratification and effect for future spinal muscular atrophy trials. The ease of application and simplicity of electrical impedance myography compared with standard electrophysiological measures may be particularly valuable in future pediatric clinical trials.

Antisense Oligonucleotides: Translation from Mouse Models to Human Neurodegenerative Diseases

“…Using a morpholino design, researchers identified a 3-fold increase in exon 7 inclusion and SMN protein expression within the brain and spinal cord after a single ICV injection in SMA model mice, conferring a dose-dependent, 97-day extension in survival (Porensky et al, 2012). Restoration of SMA expression also corrected electrophysiological and motor unit dysfunction in delta7-SMA mutant mice (Arnold et al, 2016; Arnold et al, 2014). Collectively, ISS-N1 targeting studies agree on the early and central delivery of ASOs to not only correct SMN2 splicing but also improve functional outcomes and survival.…”

Natural history of infantile‐onset spinal muscular atrophy

“…CMAP responses were reliably lower in SMA infants than controls and correlated with MFS in SMA infants, consistent with previous reports (22, 40–42). Given its promise in SMA animal model studies (43, 44), future studies in infants who receive effective disease-modifying therapy may indicate that CMAP is, if not a prognostic biomarker, a predictive biomarker of treatment response. This may be particularly important because effective SMN-targeted therapy trials indicate a clear relationship between treatment response and timing of delivery(18).…”

How the discovery of ISS-N1 led to the first medical therapy for spinal muscular atrophy

“…Namely, electrophysiological abnormalities and serum biomarkers in the Δ7 mouse are partially corrected with administration of a 20-mer ISS-N1-targeting PMO. 63,65,66 Finally, these ISS-N1-targeting ASOs may also develop new models of the disease. The Wirth group generated an intermediate SMA mouse model through SC administration of suboptimal doses of the 18-mer ISS-N1-targeting MOE ASO in severe Taiwanese mice.…”