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Bully Whippet – Whippet Double Muscling
| Acronym: | “Bully Whippet” |
| Gene: | MSTN |
| Mutation: | c.939_940delTG |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
Bully whippet syndrome is known as a double muscling trait found in the Whippet dog breed. This condition is relatively newly recognized and variants of muscular hypertrophy phenotype have been reported also in cattle, sheep, mice, cats, and human beings. The whippet breed was developed in the 19th century for the purposes of the sport of racing. It is considered a very fast dog, despite its relatively small body size. Recently, a mutation within the whippet breed has been discovered that causes muscle mass doubling in the affected dogs. Greyhounds and whippets, both developed as racing dogs, share a common ancestor, but in greyhounds, causative genes were not found. Unaffected whippet is typically similar in conformation to the greyhound, and is a medium-sized dog, with an average weight of about 9 kg, slim body build, long neck, small head, and pointed snout. Double muscling affected dogs are homozygous for the mutation (they have two copies of mutated genes), and appear as heavily muscled dogs, with a broad chest, and strongly developed leg and neck musculature. Based on their appearance, bully whippets are easily distinguished from unaffected whippets. No health abnormalities have been reported in affected dogs, except muscle cramping in the shoulder and thigh. About 50% of bully whippets have a distinguishable overbite.
Heterozygous whippets carrying one copy of the mutated gene appear more muscular than wild-type whippets (not carrying the mutated gene) and achieve better results while racing. However, they cannot be phenotypically distinguished from wild-type dogs, and the only way to identify a carrier is to do genetic testing. The mutation causing bully whippet syndrome, or double muscling, is a 2-bp deletion in the whippet MSTN gene (myostatin) located on canine chromosome 37. The mutation causes premature stop codon, resulting in a truncated protein with 17% of the protein removed. The myostatin gene is a member of the transforming growth factor β family, encodes the myostatin protein, and is a negative regulator of skeletal muscle mass. MSTN signaling prevents myoblast cell progression within the cell cycle and therefore regulates the total number of muscle fibers. When the named protein is deficient, it results in a greater number of muscle fibers. In whippets, the mutation causes a 20% decrease in the functional protein. Double muscling or bully whippet syndrome is inherited as an autosomal recessive disorder. Healthy parents of an affected puppy are obligate heterozygotes and therefore carry one mutant allele. Heterozygotes have no symptoms. Dogs homozygous for the mutation will show a double muscling phenotype. At conception, when mating two carrier dogs, each cub has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Mosher DS, Quignon P, Bustamante CD, Sutter NB, Mellersh CS, et al. (2007) A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genet 3(5): e79. doi:10.1371/journal.pgen.0030079