Evaluation of the dystrophin-glycoprotein complex, alpha-actinin, dysferlin and calpain 3 in an autosomal recessive muscular dystrophy in Labrador retrievers

Progressive Structural Defects in Canine Centronuclear Myopathy Indicate a Role for HACD1 in Maintaining Skeletal Muscle Membrane Systems

Mutations in HACD1/PTPLA cause recessive congenital myopathies in humans and dogs. Hydroxyacyl-coA dehydratases are required for elongation of very long chain fatty acids, and HACD1 has a role in early myogenesis, but the functions of this striated muscle-specific enzyme in more differentiated skeletal muscle remain unknown. Canine HACD1 deficiency is histopathologically classified as a centronuclear myopathy (CNM). We investigated the hypothesis that muscle from HACD1-deficient dogs has membrane abnormalities in common with CNMs with different genetic causes. We found progressive changes in tubuloreticular and sarcolemmal membranes and mislocalized triads and mitochondria in skeletal muscle from animals deficient in HACD1. Furthermore, comparable membranous abnormalities in cultured HACD1-deficient myotubes provide additional evidence that these defects are a primary consequence of altered HACD1 expression. Our novel findings, including T-tubule dilatation and disorganization, associated with defects in this additional CNM-associated gene provide a definitive pathophysiologic link with these disorders, confirm that dogs deficient in HACD1 are relevant models, and strengthen the evidence for a unifying pathogenesis in CNMs via defective membrane trafficking and excitation-contraction coupling in muscle. These results build on previous work by determining further functional roles of HACD1 in muscle and provide new insight into the pathology and pathogenetic mechanisms of HACD1 CNM. Consequently, alterations in membrane properties associated with HACD1 mutations should be investigated in humans with related phenotypes.

Centronuclear myopathy in Labrador retrievers: a recent founder mutation in the PTPLA gene has rapidly disseminated worldwide

Centronuclear myopathies (CNM) are inherited congenital disorders characterized by an excessive number of internalized nuclei. In humans, CNM results from ∼70 mutations in three major genes from the myotubularin, dynamin and amphiphysin families. Analysis of animal models with altered expression of these genes revealed common defects in all forms of CNM, paving the way for unified pathogenic and therapeutic mechanisms. Despite these efforts, some CNM cases remain genetically unresolved. We previously identified an autosomal recessive form of CNM in French Labrador retrievers from an experimental pedigree, and showed that a loss-of-function mutation in the protein tyrosine phosphatase-like A (PTPLA) gene segregated with CNM. Around the world, client-owned Labrador retrievers with a similar clinical presentation and histopathological changes in muscle biopsies have been described. We hypothesized that these Labradors share the same PTPLA^cnm mutation. Genotyping of an international panel of 7,426 Labradors led to the identification of PTPLA^cnm carriers in 13 countries. Haplotype analysis demonstrated that the PTPLA^cnm allele resulted from a single and recent mutational event that may have rapidly disseminated through the extensive use of popular sires. PTPLA-deficient Labradors will help define the integrated role of PTPLA in the existing CNM gene network. They will be valuable complementary large animal models to test innovative therapies in CNM.

Canine and feline models of human inherited muscle diseases

Animal models are of immense importance for studying mechanisms of disease and testing new therapies, and rodents have been used extensively in the field of neuromuscular disorders. Mice and rats can be genetically manipulated to over-express or not express genes that are important to muscle function, and these animals can be available in large numbers for analysis. Other species, such as cats and dogs, cannot be manipulated in the same ways or be used in large numbers, but they have spontaneously occurring muscle diseases with clinical presentations more closely resembling those of the human disorders. Therefore, cats and dogs may become valuable as intermediate disease models. This review focuses on canine and feline models of human inherited muscle diseases with comparisons to rodent models and an emphasis on the muscular dystrophies.

Genetic aspects of Labrador Retriever myopathy

Labrador Retriever myopathy (LRM) has become a relatively common muscular disease. The objective of our prospective study was to determine by segregation analyses a plausible mode of inheritance within a Labrador Retriever population. Therefore we performed neurological examinations, as well as electromyographic and histopathological evaluations of 58 closely related dogs. Seven dogs with an average age of 27.8 months had clinical signs consistent with LRM including exercise intolerance or fatigue. The diagnosis was based on neurological deficits and confirmed by histopathological results of muscle biopsy. We found in all cases obvious differences in fiber calibre size associated with texture disturbances. In addition, we found 41 clinically normal dogs with histological findings consistent with LRM. Three genetic models, the major gene, the mixed inheritance as well as the environmental model, were evaluated by segregation analyses. They were applied to an extended pedigree including 164 non-randomly ascertained related Labradors. According to phenotype the clinically examined dogs were divided into two different data sets. One data set distinguished between clinically normal and abnormal dogs, the second data set between histopathologically normal and abnormal dogs. We concluded that the clinical form of LRM is transmitted by a major gene and controlled by an autosomal recessive mode of inheritance. Furthermore, for expression of the subclinical form an additional gene or an environmental factor is responsible. Our findings suggest that LRM is similar to limb-girdle muscular dystrophy in man and therefore, may be used in the future as an animal model.

Dystrophin-deficient muscular dystrophy in a Labrador retriever

Sex-linked muscular dystrophy associated with dystrophin deficiency has been reported in several breeds of dogs and is best characterized in the golden retriever. In this case report, a young, male Labrador retriever with dystrophin-deficient muscular dystrophy is presented. Clinical signs included generalized weakness, lingual hypertrophy, and dysphagia. Electromyographic abnormalities including complex repetitive discharges were present. Serum creatine kinase concentration was dramatically elevated. Histopathological changes within a muscle biopsy specimen confirmed a dystrophic myopathy, and dystrophin deficiency was demonstrated by immunohistochemical staining. While X-linked muscular dystrophy has not previously been reported in the Labrador retriever, a hereditary myopathy with an autosomal recessive mode of inheritance has been characterized. A correct diagnosis and classification of these two disorders are critical for breeders and owners since both the mode of inheritance and the prognosis differ.

Muscular dystrophies and other inherited myopathies

It is certain that more inherited neuromuscular disorders of dogs and cats will be identified as the ability of practicing veterinarians to recognize disorders of muscle, nerve, and neuromuscular junction improves and newer diagnostic tests become available. Two specific points are critical. Before DNA-based genetic tests and specific therapies can be developed, an accurate description of the problem, clinically and histopathologically, must be performed. This is particularly important for the accuracy of a pedigree analysis, because inclusion of dogs with unrelated problems would alter the interpretation. Second, animals with inherited breed-associated disease should not be bred for generation of companion animals.