CHARACTERIZATION OF A POINT MUTATION IN ASPARTYLGLUCOSAMINIDASE GENE - EVIDENCE FOR A READTHROUGH OF A TRANSLATIONAL STOP CODON

We have identified a novel aspartylglucosaminuria (AGU) mutation in th e second exon of the aspartyl-glucosaminidase (AGA) gene resulting in a lysosomal storage disease in a Puerto Rican pedigree. This T-192-->A transversion causes replacement of Cys(64) With a premature translati onal stop codon and the patients' fibroblasts exhibit dramatically dec reased steady-state levels of AGA mRNA. Immunofluorescence analysis an d analysis of immunoprecipitated metabolically labelled AGA polypeptid es from patient fibroblasts unexpectedly revealed traces of normally s ized inactive AGA precursor polypeptide instead of the predicted short polypeptide of 40 amino acids, thus demonstrating readthrough due to suppression of the premature translational stop codon. The translated AGA precursor is not processed further and remains inactive. The Cys(6 4) substitution evidently disturbs the folding of the nascent polypept ide in the endoplasmic reticulum, thus preventing activation by proteo lytic cleavage.