Correcting Neuromuscular Deficits With Gene Therapy in Pompe Disease.

Gene therapy strategies have the potential to advance treatment options for pediatric neuromuscular disorders. The capacity to restore or preserve integrity and functionality of the neuromuscular junction (NMJ) is presumably limited by multiple factors, including bioavailability of tissue composing the NMJ at the time of intervention and efficacy of transgene replacement to both pre- and postsynaptic components. For these reasons, evaluating adeno-associated virus (AAV) serotypes with favorable tissue transduction characteristics and establishing early markers of NMJ deterioration to define an optimal therapeutic window are essential to preclinical development of gene therapy strategies to treat neuromuscular disorders. Maximum therapeutic benefit is presumably conferred by intervention at a prepathological stage; however, in the clinical setting, presymptomatic intervention is not always possible without newborn screening or clinical suspicion owing to an affected sibling. In the advanced stage of disease, the reversibility of pathology or plasticity of NMJ components may be limited. Examples from studies in spinal muscular atrophy have defined a narrow therapeutic window for scAAV9 therapy to impart functional benefit by restoration of survival motor neuron expression. Loss of therapeutic efficacy is related to rapid degeneration of the NMJ.1 Therefore, understanding the influence of NMJ pathology on the optimal therapeutic window is critical in evaluating a therapeutic strategy for patients challenged with a variety of neuromuscular disorders. Pompe disease is a neuromuscular disorder characterized by a deficiency in degradation of lysosomal glycogen resulting from reduced or absent acid alpha-glucosidase (GAA).2 Severe glycogen accumulation causes muscle atrophy and weakness and leads to loss of muscle function and cardiorespiratory failure before 12 months of age in early-onset patients.2,3 We have previously characterized contractile dysfunction and weakness of the diaphragm in the murine model of Pompe disease4–6 that mirrors the progressive phenotype of human patients.7 More recently, we demonstrated NMJ dysfunction in both the diaphragm and tibialis anterior (TA) muscles in Gaa-deficient mice, confirming that both muscle and nerve pathophysiology are likely contributors to disease progression.8–10 These observations justify the development of AAV-based gene therapy as a viable therapeutic candidate for Pompe, based on the intrinsic properties of AAV9 to target transgene replacement in both muscle and motor neurons.4–6,9,11,12 Supporting this hypothesis, correction of the Pompe phenotype, as demonstrated by previous studies, is likely attributable to both muscle and neuronal targeting of AAV9 expressing GAA (AAV9-hGAA).1,4,5 In this study, we used the Pompe mouse model to evaluate the therapeutic benefit of AAV9 vectors by direct intramuscular injection at early, mid, and advanced stages of NMJ pathology. We demonstrate that direct intramuscular administration of AAV9-hGAA is sufficient to promote glycogen clearance in gene-corrected muscle at all disease stages; however, normalizing muscle glycogen is ineffective in restoring motor endplate gene expression or neuromuscular functional profiles in the setting of end-stage disease. Overall, these data have important implications to guide the preclinical development of AAV9 vectors for treatment of Pompe disease and also provide potential markers of NMJ dysfunction that may be used to interpret therapeutic outcomes.