MYORG

MYORG (NET37/KIAA1161) is a transmembrane glycoside hydrolase of the GH31 family that resides with its catalytic domain in the lumen of the endoplasmic reticulum/nuclear envelope, where its enzymatic activity supports both myogenic differentiation and the prevention of brain calcification (PMID:19706595, PMID:36129849). It is an α-galactosidase, definitively established by in vitro activity assays and crystal structures solved with substrate and inhibitor, which also map disease-associated mutations onto the active site and explain their loss of catalytic activity (PMID:36129849). In myoblasts, MYORG depletion impairs differentiation and delays myogenin upregulation, and a catalytically dead mutant fails to rescue, demonstrating that the enzymatic function is required; MYORG co-immunoprecipitates with pro-IGF-II, and its loss reduces IGF-II secretion and downstream Akt activation, linking it to IGF-II maturation in the secretory pathway (PMID:19706595). In the brain, MYORG is expressed specifically in S100β-positive astrocytes, and its loss is sufficient to cause brain calcification in vivo in mice and zebrafish, with rescue confirming specificity (PMID:29910000, PMID:35870928). Human MYORG-associated primary familial brain calcification shows calcifications in capillaries and arterioles together with astrocyte foot-process shortening and decreased AQP4, consistent with astrocyte dysfunction and blood-brain barrier disruption (PMID:39180105).

1. 2009 High

   Established MYORG as an ER/nuclear-envelope luminal glycosidase whose catalytic activity is functionally required, answering both its membrane topology and whether its enzymatic function matters biologically.

   Evidence Protease mapping for topology plus RNAi depletion and catalytic-dead rescue in C2C12 myoblasts with myogenin readout

   PMID:19706595

   Open questions at the time

   • Substrate of the glycosidase activity not identified at this stage
   • Mechanism linking luminal enzyme to myogenic gene expression unresolved
2. 2009 Medium

   Provided a candidate molecular route for the differentiation phenotype by tying MYORG to IGF-II maturation and Akt signaling in the secretory pathway.

   Evidence Co-immunoprecipitation with pro-IGF-II, IGF-II secretion assay, and Akt phosphorylation assay in C2C12 cells

   PMID:19706595

   Open questions at the time

   • Single Co-IP without reciprocal validation or reconstitution
   • Whether pro-IGF-II is a direct enzymatic substrate vs. binding partner unresolved
   • Causal chain from IGF-II to myogenin upregulation not dissected
3. 2018 High

   Demonstrated that MYORG loss-of-function is sufficient to cause brain calcification in vivo and localized expression to astrocytes, establishing the cell type and organ-level phenotype.

   Evidence Myorg knockout mouse with calcification phenotyping and S100β co-localization expression analysis

   PMID:29910000

   Open questions at the time

   • Biochemical mechanism connecting astrocytic enzyme loss to mineral deposition unknown
   • Relationship between myogenic role and brain phenotype unclear
4. 2022 High

   Defined MYORG's precise enzymatic identity as an α-galactosidase and provided structural basis for how disease mutations abolish activity, resolving the long-standing substrate-class ambiguity.

   Evidence In vitro α-galactosidase activity assays, X-ray crystal structures with substrate/inhibitor co-complexes, thermal stabilization with a pharmacological chaperone

   PMID:36129849

   Open questions at the time

   • Physiological in vivo substrate(s) in astrocytes not identified
   • How accumulated/unprocessed glycoconjugate leads to calcification not established
5. 2022 Medium

   Confirmed conservation of the calcification phenotype across vertebrates, strengthening causality of MYORG loss.

   Evidence Morpholino knockdown in zebrafish with calcein staining and mRNA rescue

   PMID:35870928

   Open questions at the time

   • Morpholino-based knockdown susceptible to off-target effects despite rescue
   • Cell-type and molecular mechanism in fish not dissected
6. 2024 Medium

   Connected MYORG dysfunction to vascular/astrocyte pathology in patients, indicating blood-brain barrier disruption as a downstream consequence.

   Evidence Post-mortem neuropathology of a MYORG-PFBC patient with AQP4 immunohistochemistry and astrocyte morphology assessment

   PMID:39180105

   Open questions at the time

   • Single autopsy case
   • Whether astrocyte changes are cause or consequence of calcification unresolved
   • Molecular link from enzyme deficiency to AQP4 loss not established

• The physiological glycoconjugate substrate of MYORG in astrocytes and the mechanistic pathway by which its accumulation drives capillary calcification remain unidentified.
• No endogenous substrate characterized in vivo
• Causal pathway from enzymatic deficiency to mineral deposition undefined
• Reconciliation of myogenic and brain-calcification roles not addressed