Molecular defects in the alpha-N-acetylglucosaminidase gene in Italian Sanfilippo type B patients

Sanfilippo syndrome type B (mucopolysaccharidosis IIIB) is a rare autosomal recessive disorder characterized by the inability to degrade heparan sulfa te because of a deficiency of the lysosomal enzyme alpha -N-acetylglucosami nidase (NAGLU). We performed mutation screening in a group of 20 patients, identyifing 28 mutations, 14 of which were novel (L35F, 204delC, 22linsGCGC G, G82D, W156C, 507delC, IVS3+1G-->A, E336X, V501G, R520W, S534Y, W649C, 19 53insGCCA, 2185delAGA). Four of these mutations were found in homozygosity and only one was seen in two different patients, showing the remarkable mol ecular heterogeneity of the disease. Mutation IVS3+1G-->A produces aberrant RNA splicing: it represents a base substitution from G to A of the invaria nt GT dinucleotides at the splicing donor site of intron 3 resulting in the skipping of exon 3 and both exons 2 and 3. Transient transfection of COS c ells, by DNA mutagenized with NAGLU mutations, produced enzymatic molecules without activity, demonstrating the deleterious nature of the defects. Met abolic labeling of transfected mutants suggested a normal synthesis of the involved polypeptide for missense alterations, whereas increased protein or mRNA instability was shown for nonsense and most of the frameshift mutatio ns.