Interaction between neuromuscular junction metabolic requirements in fragile X syndrome and glycogen storage disease models

A classic human patient comorbidity of fragile X syndrome and glycogen storage disease type IX has symptoms far more severe than those for either disease alone. Causal variants result in loss of the translational regulator fragile X ribonucleoprotein 1 (FMRP) and metabolic regulator phosphorylase kinase regulatory subunit alpha 2 (PHKA2), respectively. We hypothesized FMRP-PHKA2 interaction based on unsustainably elevated metabolic demand. In Drosophila disease models, single null mutants were viable, but double knockout (DKO) animals exhibited 100% synthetic lethality, showing an essential interaction. Metabolically, dFMRP and dPHKA2 loss alone caused opposing changes in glycogen and fat stores, but DKO animals had both energy stores returned to normal. Regulatory Fat body protein 1 was elevated in single disease models but likewise returned to normal in the DKO animals. In tests of neurological phenotypes, neuromuscular junction mitochondrial function, synapse architecture and neurotransmission strength were all elevated by dFMRP loss, but these synaptic properties were restored to normal levels by co-removal of dPHKA2 in DKO animals. Thus, dFMRP and dPHKA2 strongly interact in metabolic and neuromuscular mechanisms, without explaining the DKO lethal interaction.