Identification of a novel RBCK1 splice site donor variant in Basset Hounds with glycogen storage disease myopathy

Glycogen storage diseases (GSDs) are rare, typically inherited, disorders caused by various defects in glycogen metabolism enzymes, generally resulting in the accumulation of glycogen in several tissues. Recently, two young adult Basset Hound (BH) littermates were diagnosed with GSD via postmortem histopathology, with excess glycogen manifesting in both cardiac and smooth muscle. Using whole genome sequencing, a homozygous splice site donor variant was identified in exon 8 of RBCK1, a gene which encodes an E3 ubiquitin ligase, in both littermates, suggesting an autosomal recessive mode of inheritance. The presumptive loss of the splice site donor is predicted to result in premature termination in the mid-domain of the protein. Screening for the variant in related (n = 21) and unrelated (n = 124) BHs identified one additional affected littermate and nine familial heterozygous carriers. No variant alleles were present in the unrelated BH population, establishing the novelty of the identified mutation. RBCK1 variants have previously been associated with polyglucosan body myopathy type 1 (PGBM1), a type of GSD characterized by skeletal muscle myopathy, cardiomyopathy, and polyglucosan accumulation in humans. To date, no reported variants in RBCK1 have been identified in dogs or other large animals associated with GSD, making this the first naturally occurring large animal model of PGBM1 due to an RBCK1 defect.