Case reportMuscle pathology and whole-body MRI in a polyglucosan myopathy associated with a novel glycogenin-1 mutation

•A polyglucosan myopathy with a novel GYG1 mutation.•A unique pathological pattern with PAS-positive inclusions predominantly in type I fibers.•The GYG1 variant (p.His212Tyr) did not affect protein expression but impair the autoglucosylating ability.•The first report of whole-body MRI in GYG1 myopathy.

We report a 46-year-old female with late-onset skeletal myopathy affecting proximal limb muscles. Muscle biopsy revealed a polyglucosan myopathy with PAS-positive inclusions predominantly in glycogen-depleted fibers, which were demonstrated as type I fibers by ATPase staining. Whole-body magnetic imaging disclosed that the paravertebral, scapular, and pelvic girdle muscles, the anterior compartment of the arms, and the posterior compartment of the thighs were preferentially involved. Genetic analysis revealed a homozygous novel mutation in exon 6 of the glycogenin-1 gene (GYG1) (c.634C>T, p.His212Tyr). Protein analysis revealed normal levels of glycogenin-1 even before alpha-amylase digestion indicating preserved protein expression but impaired glucosylation. In vitro functional assay demonstrated that this variant impaired the autoglucosylating ability resulting in a non-functional protein. We report a glycogenin-1 related myopathy with a distinct histopathology and unique muscle imaging pattern.