PTEN Depletion Decreases Disease Severity and Modestly Prolongs Survival in a Mouse Model of Spinal Muscular Atrophy

Daniel Little,Chiara F Valori,Chantal A Mutsaers,Ellen J Bennett,Matthew Wyles,Basil Sharrack,Pamela J Shaw,Thomas H Gillingwater,Mimoun Azzouz,Ke Ning

doi:10.1038/mt.2014.209

Abstract

Spinal muscular atrophy (SMA) is the second most common genetic cause of death in childhood. However, no effective treatment is available to halt disease progression. SMA is caused by mutations in the survival motor neuron 1 (SMN1) gene. We previously reported that PTEN depletion leads to an increase in survival of SMN-deficient motor neurons. Here, we aimed to establish the impact of PTEN modulation in an SMA mouse model in vivo. Initial experiments using intramuscular delivery of adeno-associated vector serotype 6 (AAV6) expressing shRNA against PTEN in an established mouse model of severe SMA (SMNΔ7) demonstrated the ability to ameliorate the severity of neuromuscular junction pathology. Subsequently, we developed self-complementary AAV9 expressing siPTEN (scAAV9-siPTEN) to allow evaluation of the effect of systemic suppression of PTEN on the disease course of SMA in vivo. Treatment with a single injection of scAAV9-siPTEN at postnatal day 1 resulted in a modest threefold extension of the lifespan of SMNΔ7 mice, increasing mean survival to 30 days, compared to 10 days in untreated mice. Our data revealed that systemic PTEN depletion is an important disease modifier in SMNΔ7 mice, and therapies aimed at lowering PTEN expression may therefore offer a potential therapeutic strategy for SMA.

Highlights

The characteristic neuromuscular defects observed in spinal muscular atrophy (SMA) result primarily from the death of motor neurons in the anterior horn of the spinal cord
associated vector serotype 6 (AAV6)-mediated PTEN silencing improves neuromuscular junction (NMJ) innervation Given our previous encouraging data showing that PTEN depletion leads to an increase in survival of SMN-deficient motor neurons in vitro and that injection of adeno-associated virus serotype 6 (AAV6) expressing siPTEN into hind limb muscles of mice is capable of targeting PTEN expression in spinal cord motor neurons,[16] we first wanted to examine whether local targeting of PTEN with AAV6 could rescue effects on muscle denervation in Spinal muscular atrophy (SMA)
AAV6-siPTEN treatment had no effect on the remaining healthy NMJs located in the rostral band of the muscle (Figure 1b), suggesting that reducing PTEN expression was well tolerated by healthy motor neurons

Summary

Introduction

The characteristic neuromuscular defects observed in spinal muscular atrophy (SMA) result primarily from the death of motor neurons in the anterior horn of the spinal cord. SMA is caused by mutations or deletion of the telomeric copy of the survival motor neuron 1 gene (SMN1) which results in reduced SMN protein levels.[1,2] SMN is ubiquitously expressed and is involved in many aspects of RNA metabolism.[3] It remains unclear exactly why SMN deficiency predominantly affects motor neurons.[4] One distinctive feature of SMN-deficient motor neurons is an axon elongation defect, which has been reported in cultured cells,[5] as well as in zebrafish embryos with reduced SMN levels.[6] In the growth cones of Smn-deficient murine motor neurons, reduced β-actin protein and mRNA levels were observed.[7] This suggests that SMN may be necessary for mRNA transport along axons. Neuromuscular junction (NMJ) defects have been widely reported in a range of animal models of SMA.[7,8,9,10]

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