GYG1

GYG1 encodes glycogenin-1, the auto-glucosylating priming enzyme required for glycogen synthesis, whose loss disrupts glycogen homeostasis across multiple tissues (PMID:29143313, PMID:29264399). Disease-associated variants—including a substrate-binding-domain missense mutation (p.Gly135Arg), additional missense (p.Asp102His), frameshift, splice-acceptor, and deep intronic variants—abolish or reduce glycogenin-1 expression and eliminate autoglucosylation activity, as established directly by in vitro autoglucosylation assays and patient-muscle protein analysis (PMID:29143313, PMID:31791869, PMID:29264399). Splice-disrupting alleles act through exon 2 skipping and cryptic exon activation, producing aberrant transcripts that can occur in trans as compound heterozygous lesions (PMID:42023422). Enzymatic failure leads to accumulation of polyglucosan storage material in skeletal muscle fibers and cardiomyocytes (PMID:31791869, PMID:42165248). In GYG1 cardiomyopathy (GSD15), this storage material sequesters glycogen-metabolism enzymes (glycogen synthase, UDP-glucose pyrophosphorylase 2, glycogenin-1, glycogen phosphorylase, debranching enzyme) along with p62 and desmin, accompanied by downregulation of mitochondrial proteins indicating impaired energy metabolism, without increased autophagy (PMID:42165248). Beyond its role in storage disease, glycogenin-1-dependent glycogen supports inflammatory activation of myeloid cells: in vivo Gyg1 knockdown lowers myeloid glycogen, attenuates IL-6 and TNF-α production and neutrophilia, and improves survival in LPS-induced sepsis (PMID:41333476).

1. 2017 Medium

   It was unknown whether specific GYG1 missense variants directly impair catalytic function rather than merely correlating with disease; an in vitro assay established that a substrate-binding-domain mutation abolishes autoglucosylation activity.

   Evidence In vitro autoglucosylation assay with mutant protein plus patient muscle protein expression analysis

   PMID:29143313

   Open questions at the time

   • Single variant tested in a single study
   • Effect on downstream glycogen synthesis not directly measured
   • No structural data on the substrate-binding domain
2. 2017 Medium

   Whether diverse GYG1 mutations converge on a common loss-of-expression/loss-of-glucosylation mechanism was unresolved; protein analysis across multiple families showed absence or reduced glycogenin-1 with impaired glucosylation.

   Evidence Western blot/immunohistochemistry of patient muscle biopsies with exome and direct sequencing

   PMID:29264399

   Open questions at the time

   • Functional inference partly from variant type rather than direct enzymatic assay
   • Genotype-phenotype correlation across tissues not established
3. 2019 Medium

   It was unclear whether residual mutant protein retains function; analysis showed loss-of-function variants yield absent or non-functional glycogenin-1 and drive glycogen/polyglucosan storage in muscle fibers.

   Evidence Western blot, PAS staining, and electron microscopy of patient muscle biopsies

   PMID:31791869

   Open questions at the time

   • Functional status inferred from variant type for several alleles
   • Mechanism linking enzyme loss to polyglucosan structure not resolved
4. 2025 Medium

   Whether glycogenin-1-dependent glycogen has a role beyond storage disease was untested; knockdown showed myeloid glycogen fuels inflammatory cytokine production and that lowering it improves sepsis outcomes.

   Evidence LNP-mediated siRNA Gyg1 knockdown in LPS sepsis mouse model with glycogen, cytokine, and flow cytometry readouts

   PMID:41333476

   Open questions at the time

   • Single in vivo study
   • Cell-intrinsic vs systemic contribution not dissected
   • Mechanism linking glycogen to cytokine output not defined
5. 2026 Medium

   The composition of cardiac storage material in GYG1 cardiomyopathy was unknown; proteomics revealed enrichment of sequestered glycogen-metabolism enzymes, p62, and desmin alongside mitochondrial protein downregulation.

   Evidence Laser capture microdissection mass spectrometry, whole-tissue MS, and immunohistochemistry of patient heart explants vs controls

   PMID:42165248

   Open questions at the time

   • Single lab, limited patient samples
   • Causal link between protein sequestration and mitochondrial dysfunction not established
   • No mechanism for absence of autophagic clearance
6. 2026 Medium

   The molecular consequence of non-coding GYG1 variants was uncharacterized; transcript and genome analysis confirmed exon skipping and cryptic exon activation arising from compound heterozygous in-trans alleles.

   Evidence RNA-seq, long-read genome sequencing, and long-range PCR of patient material

   PMID:42023422

   Open questions at the time

   • Single case
   • Quantitative effect on protein output not measured

• How polyglucosan structure forms in the absence of normal priming, and how glycogen-fueled metabolism is mechanistically coupled to inflammatory activation, remain open.
• No structural model of glycogenin-1 substrate-binding domain in the corpus
• Mechanism linking enzyme loss to abnormal glucan branching unresolved
• Pathway connecting myeloid glycogen to cytokine production undefined