A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs

Abstract

Double muscling is a trait previously described in several mammalian species including cattle and sheep and is caused by mutations in the myostatin (MSTN) gene (previously referred to as GDF8). Here we describe a new mutation in MSTN found in the whippet dog breed that results in a double-muscled phenotype known as the "bully" whippet. Individuals with this phenotype carry two copies of a two-base-pair deletion in the third exon of MSTN leading to a premature stop codon at amino acid 313. Individuals carrying only one copy of the mutation are, on average, more muscular than wild-type individuals (p = 7.43 x 10(-6); Kruskal-Wallis Test) and are significantly faster than individuals carrying the wild-type genotype in competitive racing events (Kendall's nonparametric measure, tau = 0.3619; p approximately 0.00028). These results highlight the utility of performance-enhancing polymorphisms, marking the first time a mutation in MSTN has been quantitatively linked to increased athletic performance.

Figure 1. Comparison of Whippets with Each of the Three Potential Genotypes

(A) Dogs have two copies of the wild-type allele (+/+). (B) Dogs are heterozygous with one wild-type allele and one mutant cys → stop allele (mh/+). (C) Dogs are homozygous for the mutant allele with two copies of the cys → stop mutation (mh/mh). All photos represent unique individuals except for the top and middle panels in the righthand column.

Figure 2. A 2-bp Deletion at Nucleotides 939 and 940 of the Canine MSTN Coding Sequence

This deletion results in a cysteine→stop codon change at amino acid 313. The top panel shows the sequence trace of a wild-type (+/+) individual in the region of the mutation. The middle panel shows the sequence trace of a mh/+ individual with a single copy of both the wild-type and mutant alleles. The bottom panel shows the sequence trace from a homozygote “bully” dog of the mh/mh genotype. The amino acid sequences for +/+ and mh/mh individuals are located above each trace.

Figure 3. Variation in Musculature among Whippets of the Three Potential Genotypes

Whippets homozygous for the cys → stop mutation have a higher mass-to-height ratio (A), a larger neck girth (B), and larger chest girth (C) than wild-type or heterozygous individuals. Males and females are shown separately. +/+, wild-type individuals; mh/+, individuals heterozygous for the cys → stop mutation; and mh/mh, individuals homozygous for the cys → stop mutation. The center bar indicates the median value and the top and bottom edges of the box delimit the 75th and 25th percentiles, respectively.

Figure 4. Frequency of Whippets Carrying the Mutation among the Dogs Sampled in Each Racing Grade

Dotted lines above and below each point represent 95% confidence intervals on the frequency (± 1.96 standard error on the mean).

Figure 5. Haplotypes Spanning the MSTN Gene Interval from 15 Dog Breeds

SNPs and indels were genotyped from sequence reads aligned by Phrap. Insertions and deletions are indicated by I and D, respectively. Haplotypes are shown on the left side with alleles colored yellow if they match the golden jackal, or blue otherwise. The golden jackal is missing data for markers at positions 3,698,667 and 3,698,824 so the most common allele was assumed to be the ancestral allele and was colored yellow. Marker positions and the MSTN gene are shown to scale along Chromosome 37 in the plot at the top left. The table to the right shows the number of chromosomes inferred to carry the haplotype in any given breed. Haplotypes were inferred by PHASE independently for each breed and the most likely haplotype pair was selected for each individual. The mh mutation (nucleotide position 3,692,430 circled in red) occurs on just one haplotype and is observed only in the whippet breed.