Long-term muscle-specific overexpression of DOK7 in mice using AAV9-tMCK-DOK7

Neuromuscular junction (NMJ) dysfunction underlies several diseases, including congenital myasthenic syndromes (CMS) and motor neuron disease (MND). Molecular pathways governing NMJ stability are therefore of interest from both biological and therapeutic perspectives. Muscle-specific kinase (MuSK) is necessary for the formation and maintenance of postsynaptic elements of the NMJ, and downstream of tyrosine kinases 7 (DOK7) is crucial for activation of the MuSK pathway. Overexpression of DOK7 using AAV9 has been shown to ameliorate neuromuscular pathology in pre-clinical disease models of CMS and MND. However, long-term consequences of DOK7 expression have been sparsely investigated, and targeted overexpression of DOK7 in skeletal muscle yet to be established. Here, we developed and characterised a novel AAV9-DOK7 facilitating forced expression of DOK7 under a skeletal muscle-specific promoter. AAV9-tMCK-DOK7 was systemically delivered to newborn mice which were monitored over 6 months. DOK7 overexpression was restricted to skeletal muscles. Body weight, blood biochemistry and histopathological assessments were unaffected by AAV9-tMCK-DOK7 treatment. In contrast, forced expression of DOK7 resulted in enlargement of both the pre- and post-synaptic components of the NMJ, without causing denervation. We conclude that muscle-specific DOK7 overexpression can be achieved in a safe manner, with the capacity to target NMJs in vivo.