DMD pseudoexon insertion in canine MD
X-linked recessive dystrophinopathies are the most common muscular dystrophies (MDs) in humans and dogs. To date, 20 breed-specific MD-associated variants are described in the dystrophin gene (DMD), including one associated with dystrophin-deficient MD in the Border Collie mixed breed. Here, we report the diagnosis and follow-up of mild dystrophin-deficient MD in a 5-month-old male Border Collie, associated with a novel DMD variant. Diagnosis was based on the neurological examination, laboratory evaluations including creatine kinase activity, electromyography and muscle biopsies with immunofluorescent staining. Inspection of the Sashimi plots of the RNA-seq data from the affected muscle biopsy, led to the discovery of a 162-bp L1 pseudoexon in DMD intron 63, introducing a frameshift and a premature stop codon (NC_051843.1(NM_001003343.1): c.9271_9272insN[162] p.(Ala3091fs*21)). Reduced DMD mRNA levels were detected for both the non-pseudoexon (50x less) and pseudoexon (3x less) containing transcripts in the affected muscle, compared to the level of the non-pseudoexon containing transcript in a non-affected muscle, resulting in very low dystrophin protein levels and upregulation of utrophin. Because the variant was only found in the affected dog, not in the healthy mother and grandmother, or in 108 unrelated Border Collies from the Belgian population (46 males and 62 females), it was considered a de novo variant. Although the prognosis of a dystrophinopathy is generally guarded to poor, the dog stabilized at the age of 6 months and is still clinically stable at the age of 2 years.