Muscular hypertrophy (Double Muscling)

Muscular hypertrophy, also known as "double muscularity", is a phenomenon in which affected dogs show a more noticeable and marked muscular development than usual. This disorder is mainly observed in the Whippet breed and is not linked to the dog`s exercise or training, but is an inherited genetic characteristic.

Symptoms

The Whippet breed is characterized by its slender body, long limbs and a weight ranging from 6 to 14 kilograms. However, dogs affected by muscular hypertrophy have exceptionally developed musculature, especially in the neck and limbs, as well as a broader chest, resulting in a phenotype markedly different from the breed standard. Despite these changes, so far no significant health problems associated with this muscular disorder have been reported, beyond some muscle cramps or bite variations. In addition, dogs heterozygous for the variant causing this phenotype appear to exhibit superior athletic performance compared to wild-type homozygotes, as they demonstrate significantly greater speed.

Disease Management

There are no specific preventive measures for this condition, since no specific associated health problems have been reported. However, to ensure the welfare of the animal, it is advisable to maintain regular weight control, provide a balanced diet and ensure an appropriate level of physical activity.

Genetic basis

This disease follows an autosomal recessive mode of inheritance. Autosomal recessive inheritance means that the dog, regardless of sex, must receive two copies of the mutation or pathogenic variant to be at risk of developing the disease. Both parents of an affected dog must carry at least one copy of the mutation. Animals with only one copy of the mutation are not at increased risk of developing the disease, but may pass the mutation on to future generations. Breeding between dogs carrying genetic variants that can cause disease, even if they do not show symptoms, is not recommended.

Technical report

Muscle hypertrophy in the Whippet breed arises due to a mutation in the myostatin (MSTN) gene, a member of the transforming growth factor beta family that negatively regulates skeletal muscle mass by controlling the proliferation and differentiation of precursor muscle cells. The causative variant of this disorder in Whippets involves a base pair deletion in the third exon of the MSTN gene, generating a premature stop codon and resulting in a truncated protein. This event leads to a reduction in the functionality of myostatin, resulting in an increase in muscle fibers.

Most affected breeds

Whippet

Bibliography

Mosher DS, Quignon P, Bustamante CD, et al. A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genet. 2007 May 25;3(5):e79.

Stinckens A, Georges M, Buys N. Mutations in the myostatin gene leading to hypermuscularity in mammals: indications for a similar mechanism in fish? Anim Genet. 2011 Jun;42(3):229-34.