Quick Summary
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Phenotype: This disorder is characterized by early onset (as early as 7 weeks of age) of signs that include generalized muscle weakness, progressive muscle atrophy, a hoarse bark, episodic collapse, droopy jaw, and difficulty eating. Without the option of intensive supportive care, and because this disease is progressive, affected puppies invariably require euthanasia between 3 and 6 months of age.
Mode of Inheritance: X-linked recessive
Alleles: N = Normal, MTML = X-linked myotubular myopathy
Breeds appropriate for testing: Labrador Retriever
Explanation of Results:
Males only have one X chromosome whereas females have two, therefore possible genotypes will differ by sex.
- Female dogs with N/N genoytpe and male dogs with N genotype will not have X-linked myotubular myopathy, and cannot transmit this variant to their offspring.
- Female dogs with N/MTML genotype will not have X-linked myotubular myopathy, but are carriers. If a carrier female is bred to a normal (N) male, all female puppies will be normal but 50% of them will be carriers. Among male puppies from this type of cross, 50% will be normal and 50% will be affected by X-linked myotubular myopathy.
- Female dogs with MTML/MTML genotype and male dogs with MTML genotype will develop X-linked myotubular myopathy, a progressive muscle disease.
Results of this test can be submitted to the OFA (Orthopedic Foundation for Animals)
Labrador Retriever Health Panel 1
$165 per animal
Labrador Retriever Health Panel 2
$180 per animal
Sample Collection
Dog DNA tests are carried out using cells brushed from your dog's cheeks and gums. The preferred cytology brushes are sent to you by mail, or you may provide your own brushes. For accepted alternative brushes, click here
We recommend waiting until puppies are at least three weeks old before testing.
Step-By-Step:
- Make sure the dog has not had anything to eat or drink for at least 1 hour prior to collecting sample.
- When swabbing puppies, isolate each puppy from the mother, littermates and any shared toys for 1 hour prior to swabbing. Puppies should not have nursed or eaten for 1 hour prior to collecting sample.
- If collecting samples from more than one dog, make sure to sample one dog at a time and wash your hands before swabbing another dog.
- Label brush sleeve with name or ID of dog to be sampled.
- Open brush sleeve by arrow and remove one brush by its handle.
- Place bristle head between the dog’s gums and cheek and press lightly on the outside of the cheek while rubbing or rotating the brush back and forth for 15 seconds.
- Wave the brush in the air for 20 seconds to air dry.
- Insert brush back into sleeve.
- Repeat steps 5 - 8 for each unused brush in sleeve on a fresh area of cheek and gums. Make sure to use and return all brushes sent by the VGL. In most cases, it will be 3 brushes per dog. If using interdental gum brushes, please note that the VGL requires 4 brushes per dog and only moderate or wide interdental gum brushes are accepted.
- Do not seal brushes in sleeve.
- Place all samples in an envelope and return to the address provided.
ATTENTION:
- Do not collect saliva/drool – the key to obtaining a good sample is getting cheek cells on the swab
- Do not rub swab on the dog’s tongue or teeth – this will result in poor quality sample
- Do not collect a sample from a puppy that has recently nursed – the mother’s genetic material can rub off on the puppy’s mouth and contaminate the sample
X-linked myotubular myopathy (XLMTM) is a subtype of centronuclear myopathies, rare genetic muscle disorders named after the central location of the muscle fiber nuclei. This disorder is characterized by early onset (as early as 7 weeks of age) of signs that include generalized muscle weakness, progressive muscle atrophy, a hoarse bark, episodic collapse, droopy jaw, and difficulty eating. Without the option of intensive supportive care, and because this disease is progressive, affected puppies invariably require euthanasia between 3 and 6 months of age.
XLMTM in Labrador Retrievers is caused by a single nucleotide change (c.465C>A) in exon 7 of the Myotubular Myopathy 1 (MTM1) gene located on the X chromosome. This mutation causes altered localization and reduced quantities of the myotubularin protein in affected dogs, leading to an excessive variability in muscle fiber size and the presence of central nuclei (as opposed to peripheral nuclei in normal dogs). According to published information (see reference below), the mutation is thought to have appeared recently and to have a restricted geographic localization in some pedigrees in Canada. Screening of a random set of Labradors Retrievers at the VGL determined that the MTM1 disease allele is rare in this breed, with frequency less than 1%.
The mode of inheritance for this disease is X-linked recessive. Females have two X chromosomes, therefore for a female to be affected she must have two copies of the defective MTM1 gene to develop XLMTM. Clinical signs are predominantly absent in females with one normal and one affected copy of the gene (carriers). Males, on the other hand, only have one X chromosome and therefore if they inherit the defective copy of the MTM1 gene they will develop XLMTM. If the inherited copy is normal, males will not have the disease.
Testing for XLMTM assists veterinarians with diagnosis of MTM1 and helps breeders identify carrier females to avoid matings that can produce affected dogs. When a carrier female is bred to a normal male, all female puppies will be normal but 50% of them will be carriers. Among male puppies, they always inherit their X chromosome from the dam so from this type of cross, 50% will be normal (those that inherit the X chromosome with the N version of MTM1) and 50% will be affected (those that inherit the X chromosome with the altered version of MTM1).
