Suspected congenital centronuclear myopathy in an Arabian-cross foal

X-linked myotubular myopathy associated with an MTM1 variant in a Maine coon cat

Objective

Describe the clinical course and diagnostic and genetic findings in a cat with X‐linked myotubular myopathy.

Case Summary

A 7‐month‐old male Maine coon was evaluated for progressively worsening gait abnormalities and generalized weakness. Neurolocalization was to the neuromuscular system. Genetic testing for spinal muscular atrophy (LIX1) was negative. Given the progressive nature and suspected poor long‐term prognosis, the owners elected euthanasia. Histopathology of skeletal muscle obtained post‐mortem disclosed numerous rounded atrophic or hypotrophic fibers with internal nuclei or central basophilic staining. Using oxidative reactions mediated by cytochrome C oxidase and succinic dehydrogenase, scattered myofibers were observed to have central dark staining structures and a “ring‐like” appearance. Given the cat's age and clinical history, a congenital myopathy was considered most likely, with the central nuclei and “ring‐like” changes consistent with either centronuclear or myotubular myopathy. Whole genome sequencing identified an underlying missense variant in myotubularin 1 (MTM1), a known candidate gene for X‐linked myotubular myopathy.

New or Unique Information Provided

This case is the first report of X‐linked myotubular myopathy in a cat with an MTM1 missense mutation. Maine coon cat breeders may consider screening for this variant to prevent production of affected cats and to eradicate the variant from the breeding population.

Genetics of Equine Muscle Disease

There are 5 single-gene mutations that are known to cause muscle disease in horses. These mutations alter the amino acid sequence of proteins involved in cell membrane electrical conduction, muscle energy metabolism, muscle contraction, and immunogenicity. The clinical signs depend on the pathway affected. The likelihood that an animal with a mutation will exhibit clinical signs depends on the mode of inheritance, environmental influences, and interactions with other genes. Selection of a genetic test for use in diagnostic or breeding decisions requires a knowledge of clinical signs, mode of inheritance, breeds affected, and proper scientific test validation.

Centronuclear Myopathy with Abundant Nemaline Rods in a Japanese Black and Hereford Crossbred Calf

Histopathological examination was performed on skeletal and diaphragmatic muscles from an 8-month-old male crossbred calf showing abnormal gait and tremor of the hindlimbs. There were numerous round fibres with centrally placed nuclei forming nuclear chains in longitudinal sections, associated with interstitial fibrosis or adipose tissue infiltration. On nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) staining, some muscle fibres in severe lesions showed a spoke-like appearance due to a radial arrangement of sarcoplasmic strands. Additionally, increased NADH-TR activity in the subsarcolemmal structures, appearingas ring-like or necklace-like forms, were observed. Transmission electron microscopy revealed dilated sarcoplasmic reticulum and variably shaped electron-dense inclusions consisting of myofibrillar streams. Another prominent feature was the existence of numerous nemaline rods within muscle fibres; these were stained red by Gomori's trichrome stain. Immunohistochemistry revealed that the nemaline rods showed strong immunoreactivity with α-actinin and desmin antibodies. Electron microscopically, these structures were composed of dense-homogeneous material and continuous with the Z disk. The case was diagnosed as centronuclear myopathy with increased nemaline rods.

Neuromyotonia in a horse

This article describes the clinical and electromyographic findings of neuromyotonia in a 19‐month‐old male crossbred Quarter Horse that presented with stiffness and muscle asymmetry in the hind limbs as well as sacrococcygeal, paravertebral, and gluteal myokymia. An electromyographic study showed spontaneous continuous muscle fiber activity with high‐frequency discharges, fibrillations, positive sharp waves, fasciculation potentials, and complex repetitive discharges. Histological examination of the gluteal muscle showed a mixed neurogenic and myopathic pattern. The findings are consistent with neuromyotonia.

X-linked myotubular myopathy in Rottweiler dogs is caused by a missense mutation in Exon 11 of the MTM1 gene

Background

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.

Results

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.

Conclusions

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.