I wrote this essay as part of my Genetics course assesment at UCL.

Dogs as models of genetic neuromuscular diseases

Dogs as models of genetic neuromuscular diseases

In recent years more and more discoveries are made in the area of genetic neuromuscular diseases. Thanks to this, DNA based diagnosis is now available (Reilly et al, 2002). The term ‘neuromuscular diseases’ encompasses an immense variety of diseases and for some of them dogs are models. Animal models are very important in research as they allow us to study various mechanisms and test new therapies. The animals used range from tiny rodents to huge dogs, each of them having their faults and their strengths. What is most important for us is that they resemble humans in the highest degree.

Neuromuscular diseases

The term neuromuscular disease encompasses a broad variety of diseases which damage the functioning of muscles. A motor unit is a single motor neuron which innervates several muscle fibers. Connecting the two together are the neuromuscular junctions. Neuromuscular diseases affect all three of these. A myopathy is a disease or disorder of the skeletal muscle tissue itself (Tortora, Derrickson 2009). Some of the most common neuromuscular diseases are: Muscular Dystrophy, Myasthenia Gravis, Amyotrophic Lateral Sclerosis (the famous physicist Stephen Hawking suffers from this disease), myotonia congenita, Charcot – Marie – Tooth disease, Spinal muscular atrophy and lots more. Nearly all of them share symptoms like paralysis, muscle weakness, muscle pain and spasticity. There are many causes, these include autoimmune disorders, abnormal myelin, poisoning (e.g. heavy metals), tumors, prions, viruses and of course genetic disorders.

Why dogs??

Rats and mice have their obvious advantages: they are small, easy to keep, can be genetically manipulated (which is very useful with these disorders) and are available in large quantities. Mice and rats provide information on the effects of gene manipulation and cell transplantation. They are also used to study molecular pathogenesis. Cats and dogs however, are more useful from the anatomical point of view, especially in medicine (Shelton, Engvall 2005). Canine organs are of comparable size to humans’. Dogs are not genetically modified to have the diseases but must be naturally born with them. The most important advantage of dogs is that they have around 450 genetic diseases and approximately half of them have amazing clinical similarities to corresponding diseases which occur in humans. Another important factor is the complete dog genome sequence which is available for researchers. As we often say: dogs are mans best friends, which is why people are also more interested in research carried out on dogs and they visit the vet on regular check ups. Just as in human genetics, pedigrees are vastly used, because they enable to establish the linkage of diseases. In the case of purebred dogs this is typically easy as pedigrees are well kept for breeding. There is another immensely important characteristic, which isn’t found in other animal models: dogs generally live with humans so the difference in environmental effects is minimized (Ostrander et al, 2000). Of course they will have different reactions to stress or other such factors, but in comparison with other animals which are normally kept in laboratories, human environment is more advantageous. This may also be useful when studying gene-environment interactions (Tsais et al, 2007). In the area of genetics dogs have yet another advantage: the general dog population consists of over 300 partially inbred genetic breeds and a heterogeneous population of mixed-breed dogs. The pedigree barrier restricts from gene flow between breeds (e.g. for a dog to be registered as a Labrador his parents must also be registered). Nearly half of the identified genetic diseases in dogs occur only in one (or a couple) breed. The development of a canine genome map increases the possibility that genes of interest for human medicine might be most quickly identified by mapping the corresponding illness in dogs. This has very interesting prospects when thinking about rare human diseases which don’t have many well documented families with the disease (Ostrander et al, 2000). Breeds used for research include Golden Retrievers, Dalmatians, Labradors, Rottweiler’s, Chow Chow and Great Dames.

Muscular dystrophy

Muscular dystrophy is a group of genetic diseases which cause muscle weakness. The number of types is quite big, but not all of them have animal models. The most common and severe type of muscle dystrophy is Duchenne muscular dystrophy, known as DMD. It is caused by a mutation in the gene which encodes for dystrophin – a protein which is situated in the sarcolemma, linking the cytoskeleton to the extracellular matrix and stabilizing it (Tsai et al, 2007). The lack of dystrophin has serious consequences: leads to muscle necrosis, paralysis and finally death by around the age of 20. It is an X-linked recessive disease and occurs in around 1 in 3500 males across the world. A similar but milder type is known as Becker muscular dystrophy (BMD). Unfortunately there still isn’t any treatment available (Ostrander et al, 2000). Golden Retrievers have been found to have a naturally occurring form of DMD. The symptoms appear around the 8-10 weeks. One of the first teams to carry out research on Golden Retriever Muscular Dystrophy (GRMD) was lead by Kornegay (1988). As he proved, the dogs disease was also X-linked and characterized by elevated serum creatinine kinase activity. Other similarities include muscle fiber necrosis and regeneration. However some differences were also noted. Pseudomyotonia (muscle weakness which resembles myotonia) is present in patients with Duchenne muscle dystrophy however it isn’t as prominent a feature as in the dogs. Also preliminary histochemical findings suggest that the syndrome in golden retrievers is a hypertrophic (enlargement) myopathy without fiber type predominance and in Duchenne patients mean fiber diameter is normal and there is type 1 fiber predominance (Kornegay et al. 1988). As DMD is so common and severe, gene therapy has been at the centre of interest. ”Recently, the Golden Retriever mutation was transferred on to the Beagle, which is a more suitable canine model because of its smaller size and extensive prior use in research”(Shelton et al, 2005).Other dogs have also been found to have dystrophinopathies. The German shorthaired pointer, the Rottweiler and the Golden Retriever, have all had genetic mutations characterized in them. For DMD, dogs are most useful in testing new therapies as there is no known treatment. They have been used in trials of gene therapies focused on the dystrophin gene. An additional method involved modified antisense oligonucleotides (AOs) that cause exon-skipping. By changing the splicing pattern, AOs can cause a mutated exon to be removed from the pre-mRNA, leading to a functional protein. McClorey et al. (2006) successfully used AOs to restore dystrophin expression in dogs. Human clinical AO trials have been started (Tsai et al, 2007).

Myotonia Congenita

Myotonia congenita is a hereditary disease characterized by myotonia, which is a delayed relaxation of muscles. Its’ main cause is a mutation on chromosome 7, in a gene which codes for skeletal muscle chloride channels (CLCN1). There have been two types classified: Thomsen’s, which is a dominant disorder and Bekcer’s – recessive (Colding-Jrrgensen, 2005). The ‘warm up’ effect is an improvement in myotonia severity after exercise. The phenotypes of this disease vary immensely. Myotonia congenital has been reported in some dog types, e.g. in the Schnauzers. Similarities to the human disease include muscle hypertrophy, myotonia (severe and moderate), the ‘warm up’ effect and low sarcolemmal chloride conductance (Rhodesa et al, 1999). In dogs the phenotype is passed on as an autosomal recessive missense mutation in the canine C1C-1 chloride channel. This mutation causes a dramatic shift in the voltage-dependence of its open probability. Although myotonia congenita is a very rare disease with mild morbidity the tests carried out on dogs have improved our knowledge on muscle excitation and contraction (Rhodesa et al, 1999). This might lead to further research and discoveries.

Charcot – Marie – Tooth disease

Charcot – Marie – Tooth disease is a group of disorders which share a similar phenotype: wasted and weak distal limb muscles, with or without distal sensory loss, abnormal tendon reflexes and skeletal deformations (Pareyson, 1999). First symptoms are visible during the first decades of life. The disease progresses very slowly, rarely leading to severe cases. CMT is usually passed on as an autosomal dominant, but the X-linked dominant form is also common as opposed to the recessive subtypes. Charcot – Marie – Tooth disease has been divided into two main categories based on electrophysiological and pathological findings. In CMT type 1 segmental demyelination of axons (or abnormal myelination which forms bulbs) leads to a decrease in nerve conduction velocities which are the same or mildly decreased in type 2 CMT. In CMT type 2 chronic axonal degeneration and regeneration is evident (Shelton et al, 2003). The Leonberger dogs have been found to have distal polyneuropathy with several syndromes similar to those of patients with Charcot – Marie – Tooth disease. In most of the dogs the first problems arose at an early onset. Problems with distal limb muscles, general weakness and abnormal tendon reflexes were all symptoms. The affected dogs had a high-stepping gait which is a striking similarity to humans. Also, laryngeal weakness and paralysis (a common feature in the dogs) and less common facial weakness in dogs are present in humans. Muscle biopsies carried out in all the dogs indicated denervartion and axonal loss as severe atrophy and fatty replacement of muscle fibers was visible. However there is a big difference: in dogs the trait is X-linked recessive. In addition no diaphragm abnormalities have been reported in dogs (Shelton et al, 2003). Research teams are hoping that thanks to these dog models there will be new genes associated with X-linked neuropathies discovered which might correlate to human ones.

Summary

Dogs are the best choice of animal models for neuromuscular diseases as they are relatively big, live in the same environment as humans, have over 250 diseases which share symptoms with us and their genome has been fully sequenced. Neuromuscular diseases affect at least one of the three parts of the motor unit causing muscle weakness, paralysis, myotonia etc. Some of them have got treatment while others don’t. Dogs are used for testing the different treatments before clinical trials are possible. They also help us understand the pathways underlying diseases from which millions of people suffer each year. In the area of genetics, dogs lead to discoveries of new genes which underlie the diseases. This may also be useful when considering treatments as new gene therapies may become available. However we most always remember that there will definitely also be differences. Caution is advised when comparing the results to human tests. I hope more people will learn to appreciate our good companions, thanks to whom many lives have already been saved and still more are to be.

Bibliography:

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