Identification of two novel beta-mannosidosis-associated sequence variants: biochemical analysis of beta-mannosidase (MANBA) missense mutations.

Beta-mannosidosis (OMIM # 248510) is an autosomal-recessive lysosomal storage disorder caused by deficiency of the lysosomal enzyme beta-mannosidase (MANBA, E.C. 3.2.1.25). The disorder has been reported in goat, cattle and man. The human disorder is rare and only 20 cases in 16 families have been reported. We have sequenced the exons and exon-intron borders in a European patient with infantile onset of beta-mannosidosis. The patient was compound heterozygous for a silent mutation (c.375A>G) in exon 3 causing alternative splicing, and a missense mutation (c.1513T>C, p.Ser505Pro) in exon 12. The alternative splicing event deleted four nucleotides from the transcript and was predicted to result in premature termination of translation. In order to evaluate the consequence of the missense mutation, we inserted the human beta-mannosidase gene into an expression vector, performed site-directed mutagenesis and expressed the normal and mutant enzyme in COS-7 cells. We also included the previously reported beta-mannosidosis-associated missense mutations c.544C>T (p.Arg182Trp) and c.1175G>A (p.Gly392Glu), which were found in patients presenting a milder phenotype. Cells transfected with the wild-type construct showed a 33-fold increase in beta-mannosidase activity compared to mock-transfected cells, whereas cells transfected with the mutant constructs showed no detectable increase in activity. We propose that the milder phenotype described in some beta-mannosidosis patients with missense mutations in the MANBA gene is not due to residual beta-mannosidase activity, but rather caused by epigenetic and/or environmental factors.

Molecular cloning and structural organization of the gene encoding the mouse lysosomal di-N-acetylchitobiase (ctbs).

The lysosomal enzyme di-N-acetylchitobiase hydrolyzes N-acetylglucosamine from the reducing-end of the N,N' diacetylchitobiose core of N-linked-oligosaccharides. The presence of chitobiase in the tissues of different species is probably responsible for differences in the structure of oligosaccharides accumulated in the lysosomal storage disease beta-mannosidosis. The disease has so far been described in humans, cats, cattle and goats. Low chitobiase activity has been observed in the tissues of ruminants and it has been hypothesized that in cattle this low level of expression is due to evolutionary changes in the promoter region. A cDNA encoding the mouse chitobiase has been isolated, sequenced and its identity confirmed by expression in COS-7 cells. Comparison of the mouse genomic sequence with the cDNA sequence revealed the presence of seven exons within the chitobiase gene. The gene spans about 15 kb and a single transcription initiation site was determined by 5'RACE. Chitobiase is differentially and ubiquitously expressed in mouse tissues as demonstrated by qRT-PCR analysis. Chitobiase is differentially expressed at lower levels in bovine tissues. In two bovine tissues (heart and muscle) mRNA was not detectable. Mouse and bovine promoters have been isolated and sequenced and their activities compared. The activity of the bovine promoter is very low and might explain the low activity of chitobiase observed in cattle.

Identification and characterization of five novel MAN2B1 mutations in Italian patients with alpha-mannosidosis.

Mutation analysis performed on six Italian families with alpha-mannosidosis type II allowed the identification of five new mutations in the MAN2B1 gene: c.157G>T, c.562C>T, c.599A>T, c.293dupA, c.2402G>A (p.E53X, p.R188X, p.H200L, p.Y99VfsX61, p.G801D). Protein residues G801 and H200 are conserved among the four mammalian alpha-mannosidases cloned to date: human, cattle, cat and mouse. In vitro expression studies demonstrated that both missense mutations expressed no residual alpha-mannosidase activity indicating that they are disease-causing mutations. Modelling into the three-dimensional structure revealed that the p.H200L could involve the catalytic mechanism, whereas p.G801D would affect the correct folding of the enzyme.