Generation and characterization of a mouse model for the achalasia, alacrima and mental retardation (AAMR) syndrome

The AAMR syndrome is characterized by mental retardation and gait abnormalities as well as achalasia and alacrima. This disorder is inherited as an autosomal recessive trait and is caused by mutations in the Guanosine-diphosphate-(GDP)-mannose-pyrophosphorylase A (GMPPA) gene. GMPPA encodes the 420 aa protein GMPPA. Its homolog, the GDP-mannose-pyrophosphorylase B (GMPPB), converts mannose-1-phosphate and guanosine triphosphate (GTP) to GDP-mannose, which is an essential substrate for glycosylation. Up to date, 18 patients with inactivating mutations in the GMPPA gene have been reported worldwide. To elucidate the function of GMPPA in more detail we generated a Gmppa knockout (KO) mouse model. Importantly, these mice recapitulate many features of human AAMR syndrome patients, e.g. homozygous Gmppa KO mice show structural brain alterations. Moreover, Gmppa KO mice show a progressive gait disorder with muscle weakness accompanied by centralization of nuclei, alterations of the mean fiber diameter, the distribution of extracellular matrix (ECM) proteins and of the Z-disc related protein α-Actinin. In immunoblot analysis we found hyperglycosylation of proteins, especially hyperglycosylation of alpha-Dystroglycan (α-DG). Hyperglycosylated α-DG shows decreased protein stability and an increased binding to ECM proteins. Suggesting that the observed muscle phenotype is muscle intrinsic, sciatic nerve structure and nerve conduction velocities are normal in Gmppa KO mice. Mechanistically, elevated GDP-mannose levels and a direct interaction of GMPPA with GMPPB support a role of GMPPA as an allosteric feedback inhibitor of GMPPB. Gmppa knockdown studies in myoblasts revealed an increased α-DG turnover and activation of ERK signaling. In mice, a mannose-depleted diet dramatically improved the motor phenotype and almost normalized glycosylation of α-DG and ERK signaling. Thus, we propose that AAMR syndrome caused by GMPPA mutations is at least in part a treatable condition.