Systemic Gene Therapy Effectively Improves Canine X-Linked Myotubular Myopathy

Single-dose intravenous gene therapy was well tolerated and demonstrated dose-dependent improvements in canine X-linked myotubular myopathy (XLMTM), according to a study recently published in Molecular Therapy. The study’s findings have “widespread implications for the treatment of other congenital muscle diseases,” the authors wrote.

XLMTM is a monogenic disease caused by an MTM1 gene mutation and subsequent myotubularin protein deficiency or dysfunction. Affecting about 1 in 50,000 male infants, XLMTM is usually fatal; despite intensive supportive care, nearly half of male babies with XLMTM succumb to severe muscle weakness and respiratory failure by 2 years of age.

Gene therapy with an adeno-associated virus (AAV) vector holds promise for treating monogenic skeletal muscle disorders. A previous study reported the efficacy of single-dose recombinant serotype 8 AAV (rAAV8) vector therapy in canine and murine XLMTM models.

The current study evaluated the safety and efficacy of rAAV8-mediated gene therapy in 21 male and female laboratory dogs. Dogs with XLMTM (n = 15) were randomized into 4 treatment groups and received a single intravenous infusion at 10 weeks of age:

• Low-, mid-, or high-dose rAAV8-canine myotubularin (rAAV8-cMTM1; n = 3 per dose)
• Saline (n = 6)

Normal littermates (n = 6) served as untreated controls. Clinical assessments and muscle biopsies were conducted at specific time points. In addition to safety, vector biodistribution and MTM1 expression were analyzed.

Clinical Efficacy

The authors observed dose-dependent improvements in efficacy for all clinical assessments (survival, neurologic, strength, gait, respiratory). Notably, at the 17-week time point, mid- and high-dose gene therapy resulted in assessment scores comparable to those observed in the control dogs.

The strength improvements were particularly important, given that severe muscle weakness is the clinical hallmark of XLMTM. Muscle strength was measured using nerve stimulation to the muscles responsible for paw flexion and extension; forelimb and hindlimb isometric torque were also evaluated. Strength improvement results, the authors noted, “[represented] the most important readout” of the study’s clinical assessments.

Muscle Pathology Changes

XLMTM-affected myofibers typically undergo several pathologic changes, such as small size and abnormal organelle arrangement. Pre- and post-infusion skeletal muscle biopsies revealed a near-complete correction of myofiber pathology with mid- and high-dose gene therapy; low-dose gene therapy did not produce such improvement.

Vector Biodistribution and MTM1 Protein Expression

The authors observed rAAV8-cMTM1 distribution throughout most organs, excluding the brain, pancreas, and thymus. In muscles, the distribution was homogeneous and dose-dependent. Transgenic MTM1 expression levels in the muscles generally mirrored vector distribution and were also dose-dependent.

Safety

No adverse events with intravenous vector delivery were reported. Variations in left ventricular wall thickness were detected in all dogs using echocardiography, a finding the authors did not believe was dose-dependent. Whether the wall thickness variations were due to MTM1 overexpression in heart muscle, though, was unclear.

Microscopic evaluation revealed dose-independent mild inflammation of heart muscle in several control and treated dogs.

Immune Response

Gene therapy did not induce an immune response against the vector capsid or MTM1, indicated by undetectable levels of MTM1-specific IgG titers and capsid- and MTM1-specific T cells. Notably, dogs receiving gene therapy did not require immunosuppressive treatment. Although anti-rAAV8 neutralizing antibody levels increased in dogs receiving gene therapy, antibody levels did not appear to be dose-dependent.

Given the study’s overall findings, the authors noted there may be a ceiling effect for rAAV8-cMTM1 therapy that is related to skeletal muscle MTM1 levels following vector transduction. Taken together, the study results demonstrate the potential of systemic gene therapy for monogenic skeletal muscle disorders and provide support for XLMTM gene therapy clinical trials.

Dr. JoAnna Pendergrass received her doctorate in veterinary medicine from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, LLC.