Intragenic dystrophin (DMD) duplication variant in Entlebucher Mountain Dogs with Duchenne muscular dystrophy

Muscular dystrophies represent a group of disorders characterized by progressive muscle degeneration and weakness. An important subgroup are the dystrophin-related muscular dystrophies caused by variants in the DMD gene. They can be divided into the more severe Duchenne muscular dystrophy and the milder Becker muscular dystrophy. Here, we characterize the clinical, histopathological and molecular genetic aspects of two male Entlebucher Mountain Dogs with clinical signs of muscular dystrophy. The two dogs presented with marked dysphagia starting at the age of several weeks and in the later course recognizable exercise intolerance with highly increased serum creatine kinase levels. Histopathological signs of a dystrophic myopathy represented by degeneration of muscle fibers and signs of regeneration were present. Whole genome sequencing of one affected dog identified an intragenic 8.6 kb duplication in the X-chromosomal DMD gene, c.7528-4048_7645 + 4450dup. No other protein-changing variants in candidate genes for muscular dystrophy were identified. The duplication includes exon 52 of DMD and is predicted to lead to a frameshift and truncation of 30% of the wild-type open reading frame. Genotyping of the whole family confirmed the presence of the mutant allele in both affected dogs and the unaffected dam. The correct co-segregation of the mutant allele in the affected family as well as knowledge from humans and other species suggest the identified DMD variant as the most likely candidate variant for the muscular dystrophy phenotype in the two investigated dogs.