Dissection of the molecular pathology of aspartylglucosammuria provides the basis for DNA diagnostics and future therapeutic interventions

Abstract

Aspartylglucosaminuria (AGU) is exceptional among lysosomal storage diseases since it represents the only known amidase deficiency in man, being caused by an inadequate function of aspartylglucosaminidase (AGA, E.C. 3.5.1.26.). This amidase is essential in one of the final steps in the ordered breakdown of glycoproteins since it cleaves Asn from the residual N-acetylglucosamines (for reviews see 1, 2). The deficiency of the enzyme activity results in the typical lysosomal accumulation of the abnormal degradation products (mainly aspartylglucosamine, 2-acetamido-1-β-L-aspartamido-1,2-dideoxyglucose) in patients' cells and tissues [3,4]. The diagnosis of AGU has so far been based on the detection of abnormal metabolites in urine and decreased enzyme activity in the cultured fibroblasts or isolated lymphocytes [3,5]. Prenatal diagnosis has been possible by demonstrating the deficient enzyme activity of amniocytes or chorion villus biopsies [6,7]. Identification of carriers has been difficult and unreliable due to the high individual variation in AGA activity and prerequisite for isolated blood lymphocytes.

During the past few years we have purified the human enzyme into homogeneity, isolated the full length cDNA and characterized the majority of AGU mutations in this cDNA. This work facilitated the development of a reliable DNA diagnostic test suitable also for large scale carrier screening. The molecular pathology of the most common AGU mutation was unravelled, this being a prerequisite for the oncoming developments for therapy.

Although AGU is a relatively rare disease, characterization of the AGU mutations and their cellular consequences have revealed highly interesting new phenomena in the biosynthesis of this lysosomal enzyme, some of which carry general biological significance. Further, the reliable DNA test developed has proved its importance in clinical diagnostics, and also demonstrated its wide suitability to answer questions related to quantitation of specific DNA or RNA sequences in different areas of research from population genetics to expression analyses in vitro and in vivo.

Key Words:

• Lysosomal disease
• DNA diagnostics
• population screening