Mutations in the MTM1 gene encoding myotubularin cause X-linked myotubular myopathy (XLMTM), a well-defined subtype of human centronuclear myopathy. Seven male Labrador Retrievers, age 14–26 wk, were clinically evaluated for generalized weakness and muscle atrophy. Muscle biopsies showed variability in fiber size, centrally placed nuclei resembling fetal myotubes, and subsarcolemmal ringed and central dense areas highlighted with mitochondrial specific reactions. Ultrastructural studies confirmed the centrally located nuclei, abnormal perinuclear structure, and mitochondrial accumulations. Wild-type triads were infrequent, with most exhibiting an abnormal orientation of T tubules. MTM1 gene sequencing revealed a unique exon 7 variant in all seven affected males, causing a nonconservative missense change, p.N155K, which haplotype data suggest derives from a recent founder in the local population. Analysis of a worldwide panel of 237 unaffected Labrador Retrievers and 59 additional control dogs from 25 other breeds failed to identify this variant, supporting it as the pathogenic mutation. Myotubularin protein levels and localization were abnormal in muscles from affected dogs, and expression of GFP-MTM1 p.N155K in COS-1 cells showed that the mutant protein was sequestered in proteasomes, where it was presumably misfolded and prematurely degraded. These data demonstrate that XLMTM in Labrador Retrievers is a faithful genetic model of the human condition.

Congenital and inherited myopathies in dogs are faithful models of human muscle diseases and are being recognized with increasing frequency. In fact, canine models of dystrophin deficient muscular dystrophy and X-linked myotubular myopathy are of tremendous value in the translation of new and promising therapies for the treatment of these diseases. We have recently identified a family of Australian Rottweilers in which male puppies were clinically affected with severe muscle weakness and atrophy that resulted in early euthanasia or death. X-linked myotubular myopathy was suspected based on the early and severe clinical presentation and histopathological changes within muscle biopsies. The aim of this study was to determine the genetic basis for myopathy in these dogs and compare and contrast the clinical presentation, histopathology, ultrastructure, and mutation in this family of Rottweiler dogs with the previously described myotubular myopathy in Labrador retrievers. Histopathology, histochemistry, and ultrastructural examination of muscle biopsies from affected Rottweiler puppies were consistent with an X-linked myotubular myopathy. An unusual finding that differed from the previously reported Labradors and similar human cases was the presence of excessive autophagy and prominent autophagic vacuoles. Molecular investigations confirmed a missense mutation in exon 11 of MTM1 that was predicted to result in a non-functional phosphatase activity. Although the clinical presentations and histopathology were similar, the MTM1 p.(Q384P) mutation is different from the p.(N155K) mutation in exon 7 affecting Labrador retrievers with X-linked myotubular myopathy. Here we describe a second pathogenic mutation in MTM1 causing X-linked myotubular myopathy in dogs. Our findings suggest a variety of MTM1 mutations in dogs as seen in human patients. The number of MTM1 mutations resulting in similar severe and progressive clinical myopathy and histopathological changes are likely to increase as canine myopathies are further characterized.

Globoid cell leukodystrophy (GLD), or Krabbe disease, is an inherited, neurologic disorder that results from deficiency of a lysosomal enzyme, galactosylceramidase (GALC). Most commonly, deficits of GALC result in widespread central and peripheral nervous system (CNS, PNS) demyelination and death in affected infants typically by 2 years of age. Hematopoietic stem cell transplantation is the current standard of care in children diagnosed prior to symptom onset; however, disease correction is incomplete. Herein we present the first adeno-associated virus (AAV) gene therapy experiments in a naturally occurring canine model of GLD that closely recapitulates the clinical disease progression, neuropathological alterations, and biochemical abnormalities observed in human patients. Adapted from studies in the twitcher mice, GLD dogs were treated by combination intravenous (IV) and intracerebroventricular (ICV) injections of AAVrh10 to target both PNS and CNS. Combination of IV and ICV AAV gene therapy had a clear dose response and resulted in delayed onset of clinical signs, extended lifespan, correction of biochemical defects, and attenuation of neuropathology. For the first time we have established therapeutic effect in the canine model of GLD by targeting both peripheral and central nervous system impairments with potential clinical implications for GLD patients.

Axonal dysfunction is the major phenotypic change in many neurodegenerative diseases, but the processes underlying this impairment are not clear. Modifier of cell adhesion (MOCA) is a presenilin binding protein that functions as a guanine nucleotide exchange factor for Rac1. The loss of MOCA in mice leads to axonal degeneration and causes sensorimotor impairments by decreasing cofilin phosphorylation and altering its upstream signaling partners LIM kinase and p21-activated kinase, an enzyme directly downstream of Rac1. The dystrophic axons found in MOCA-deficient mice are associated with abnormal aggregates of neurofilament protein, the disorganization of the axonal cytoskeleton, and the accumulation of autophagic vacuoles and polyubiquitinated proteins. Furthermore, MOCA deficiency causes an alteration in the actin cytoskeleton and the formation of cofilin-containing rod-like structures. The dystrophic axons show functional abnormalities, including impaired axonal transport. These findings demonstrate that MOCA is required for maintaining the functional integrity of axons and define a model for the steps leading to axonal degeneration.