Bully Whippet – Whippet Double Muscling
Bully whippet syndrome is known as double muscling trait found in 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.
Characteristics and Symptoms
The whippet breed was developed in 19th century for purposes of sport of racing. It is considered as very fast dog, despite its relatively small body size. Recently, a mutation within whippet breed has been discovered which causes muscle mass doubling in the affected dogs. Greyhounds and whippets, both developed as racing dogs, share common ancestor, but in greyhounds causative gene was not found. Unaffected whippet is typically similar in conformation to the greyhound, and is a medium sized dog, with average weight 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 broad chest, 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 mutated gene appear more muscular than wild type whippets (not carrying mutated gene) and achieve better results while racing. However, they cannot be phenotypically distinguished from wild type dog, and only way to identify a carrier is to do genetic testing.
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 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 greater number of muscle fibers. In whippets, the mutation causes 20% decrease of 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 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.
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