Clinical and biochemical analysis of two families with type I and type II mannosidosis

Retinal and optic nerve degeneration in α-mannosidosis

Background

α-mannosidosis is a rare, autosomal-recessive, lysosomal storage disease caused by a deficient activity of α-mannosidase. Typical symptoms include intellectual, motor and hearing impairment, facial coarsening, and musculoskeletal abnormalities. Ocular pathologies reported previously were mainly opacities of the cornea and lens, strabismus, and ocular motility disorders. However, retinal and optic nerve degeneration have been rarely described.

Methods

We report ocular findings of 32 patients with α-mannosidosis. We particularly concentrated on retinal abnormalities which we supported by posterior segment examination, fundus photography, and Spectral-Domain optical coherence tomography (SD-OCT) imaging.

Results

Tapeto-retinal degeneration with bone spicule formations in the peripheral retina or macular changes were seen in three patients (9.4%) on funduscopy; of these, two with optic nerve atrophy. Eight retinal images could be obtained by OCT or fundus photography; of these, six showed thinning of the outer retinal layers on OCT. Overall, optic nerve atrophy was seen in six patients (18.8%); of these, four with partial atrophy. Two patients had partial optic nerve atrophy with no retinal abnormalities on funduscopy. Cataract was seen in two (6.3%), corneal haze also in two patients (6.3%). Six patients (18.8%) had manifest strabismus, four (12.5%) nystagmus, and in five patients (15.6%) impaired smooth pursuit eye movements were seen.

Conclusion

Ocular pathologies are not exclusively confined to opacities of the cornea and lens or strabismus and ocular motility disorders but tapeto-retinal degeneration and optic nerve atrophy may be a common feature in α-mannosidosis. OCT technology helps detecting early outer retinal thinning which can progress with age and potentially leads to vision loss over time.

Alpha-mannosidosis: correlation between phenotype, genotype and mutant MAN2B1 subcellular localisation

Background

Alpha-mannosidosis is caused by mutations in MAN2B1, leading to loss of lysosomal alpha-mannosidase activity. Symptoms include intellectual disabilities, hearing impairment, motor function disturbances, facial coarsening and musculoskeletal abnormalities.

Methods

To study the genotype-phenotype relationship for alpha-mannosidosis 66 patients were included. Based on the predicted effect of the mutations and the subcellular localisation of mutant MAN2B1 in cultured cells, the patients were divided into three subgroups.

Clinical and biochemical data were collected. Correlation analyses between each of the three subgroups of genotype/subcellular localisation and the clinical and biochemical data were done to investigate the potential relationship between genotype and phenotype in alpha-mannosidosis.

Statistical analyses were performed using the SPSS software. Analyses of covariance were performed to describe the genotype-phenotype correlations. The phenotype parameters were modelled by the mutation group and age as a covariate. P values of <0.05 were considered as statistically significant.

Results

Complete MAN2B1 genotypes were established for all patients. We found significantly higher scores in the Leiter-R test, lower concentrations of CSF-oligosaccharides, higher point scores in the Bruininks-Oseretsky Test of Motor Proficiency subtests (BOT-2); Upper limb coordination and Balance, and a higher FVC% in patients in subgroup 3, harbouring at least one variant that allows localisation of the mutant MAN2B1 protein to the lysosomes compared to subgrou 2 and/or subgroup 1 with no lysosomal localization of the mutant MAN2B1 protein.

Conclusion

Our results indicate a correlation between the MAN2B1 genotypes and the cognitive function, upper limb coordination, balance, FVC% and the storage of oligosaccharides in CSF. This correlation depends on the subcellular localisation of the mutant MAN2B1 protein.

Exclusion of NEU1 and PPGB from candidate genes for a lysosomal storage disease in Japanese Black cattle

A case of lysosomal storage disease has been reported in a calf of Japanese Black cattle. Lysosomal storage diseases are hereditary diseases caused by deficiency of lysosomal hydrolases. The clinical and pathological features and accumulated substrates of the affected animal indicated a possibility of sialidosis or galactosialidosis caused by deficiency of neuraminidase (NEU1) or protective protein for beta-galactosidase (PPGB). In the present study, we investigated nucleotide sequences of the genes encoding these two proteins to evaluate whether mutation of these genes is involved in this disease. We determined cattle genomic sequences of these two genes by using bovine EST sequences and the nucleotide sequences of all exons of these genes were compared between affected and normal animals. The results showed several nucleotide substitutions, but none of them was a functional mutation or specific to the affected animal. Furthermore, genotyping of the microsatellite markers in the vicinity of these two genes revealed no homozygosity of the chromosomal regions including these genes in the affected animal. These findings indicated that neither NEU1 nor PPGB gene is responsible for the lysosomal storage disease of Japanese Black cattle and therefore the disease is neither sialidosis nor galactosialidosis.

Impaired lysosomal trimming of N-linked oligosaccharides leads to hyperglycosylation of native lysosomal proteins in mice with alpha-mannosidosis

Alpha-mannosidosis is caused by the genetic defect of the lysosomal alpha-d-mannosidase (LAMAN), which is involved in the breakdown of free alpha-linked mannose-containing oligosaccharides originating from glycoproteins with N-linked glycans, and thus manifests itself in an extensive storage of mannose-containing oligosaccharides. Here we demonstrate in a model of mice with alpha-mannosidosis that native lysosomal proteins exhibit elongated N-linked oligosaccharides as shown by two-dimensional difference gel electrophoresis, deglycosylation assays, and mass spectrometry. The analysis of cathepsin B-derived oligosaccharides revealed a hypermannosylation of glycoproteins in mice with alpha-mannosidosis as indicated by the predominance of extended Man3GlcNAc2 oligosaccharides. Treatment with recombinant human alpha-mannosidase partially corrected the hyperglycosylation of lysosomal proteins in vivo and in vitro. These data clearly demonstrate that LAMAN is involved not only in the lysosomal catabolism of free oligosaccharides but also in the trimming of asparagine-linked oligosaccharides on native lysosomal proteins.

Early onset alpha-mannosidosis with slow progression in three Hispanic males

Alpha-mannosidosis (AMS) is an autosomal recessive lysosomal storage disorder which results from a deficiency of lysosomal alpha-mannosidase [corrected] activity and displays a wide range of clinical phenotypes. Patients have traditionally been divided into type I, a more severe form that presents in infancy, and type II, a milder form that typically presents in later childhood. We describe three Hispanic males who presented in infancy with relatively mild forms of AMS. They were aged between 6 and 24 years at their last assessment. Homozygous mutations in the MAN2B1 gene were found in all three patients, one of which is a newly reported mutation. Two of the patients were brothers who were homozygous for the same MAN2B1 mutation. Despite being homozygous for the same mutation, the older brother had more severe developmental delay, hearing loss, and growth retardation. This report illustrates the difficulty in determining a strict genotype-phenotype correlation in AMS, and supports screening for oligosaccharides in children with neurodevelopmental delay with mild phenotypic signs and symptoms.

[Adult leukoencephalopathy caused by alpha-mannosidosis deficiency]

Adult leukoencephalopathy caused by alpha-mannosidosis deficiency (MIM248500) is a recessive inherited lysosomal storage disease associated with decreased activity of alpha-mannosidase. This enzyme degrades oligosaccharides and glycoproteins in neural and visceral tissues. There are two different disease phenotypes, type-I or severe infantile phenotype and type 2, which progresses more slowly and is compatible with survival into adulthood. We report the case of a 51-year-old man with gait disorders beginning at the age of 40 years associated with leukoencephalopathy due to alpha-mannosidosis deficiency.

Multivariate neurocognitive and emotional profile of a mannosidosis murine model for therapy assessment

alpha-Mannosidosis is a lysosomal storage disorder caused by lysosomal alpha-mannosidase (LAMAN) deficiency that leads to neurocognitive dysfunctions, psychotic symptoms and emotional changes in human patients. A murine mannosidosis model, LAMAN-deficient mice, was examined on a behavioral task battery that included test for neuromotor, exploratory and neurocognitive (spatial learning and memory) abilities, and multivariate statistical analyses were used to identify behavioral and neurocognitive domains that are most heavily affected by LAMAN deficiency. In addition, we further investigated synaptic plasticity recordings on hippocampal slices that may relate to these behavioral alterations. Correlation analysis revealed significant intra- and intertask correlations and factor analysis that included all 21 behavioral variables identified three main factors (exploration/emotionality, locomotion and learning/memory abilities). Significant correlations were observed between genotype, and factor 1 (exploration/emotionality) and factor 3 (learning/memory abilities). Discriminant function analysis showed that "path length in the open field test" and "time spent in the target quadrant during the water maze probe trial" were the most decisive variables to distinguish between the genotypes. We therefore suggest that these variables would be especially important in forthcoming therapy assessment experiments using this murine mannosidosis model. LAMAN-deficient mice displayed severe changes in synaptic plasticity, which may have contributed to the neurocognitive impairments observed. The present report further shows that targeted deletion of the LAMAN gene in mice mimics many aspects of human alpha-mannosidosis, and these data provide a basis for future therapeutic experiments.

Neurocognitive and psychotiform behavioral alterations and enhanced hippocampal long-term potentiation in transgenic mice displaying neuropathological features of human alpha-mannosidosis

Mice with alpha-mannosidase gene inactivation provide an experimental model for alpha-mannosidosis, a lysosomal storage disease with severe neuropsychological and psychopathological complications. Neurohistological alterations in these mice were similar to those in patients and included vacuolations and axonal spheroids in the CNS and peripheral nervous system. Vacuolation was most prominent and evenly distributed in neuronal perikarya of the hippocampal CA2 and CA3 regions, whereas CA1 and dentate gyrus were weakly or not affected. Field potential recordings from CA1 region in hippocampal slices showed enhanced theta burst-induced long-term potentiation (LTP) in alpha-mannosidase-deficient mice. Longitudinal assessment in age-matched alpha-mannosidase-deficient and wild-type littermates, using an extended test battery, demonstrated a neurocognitive and psychotiform profile that may relate to the psychopathological alterations in clinical alpha-mannosidosis. Brainstem auditory-evoked potentials and basic neuromotor abilities were not impaired and did not deteriorate with age. Exploratory and conflict tests revealed consistent decreases in exploratory activity and emotional blunting in the knock-out group. alpha-Mannosidosis mice were also impaired in aversively motivated learning and acquisition of signal-shock associations. Acquisition and reversal learning in the water maze task, passive avoidance learning in the step-through procedure, as well as emotional response conditioning in an operant procedure were all impaired. Acquisition or shaping of an appetitive instrumental conditioning task was unchanged. Appetitive odor discrimination learning was only marginally impaired during shaping, whereas both the discrimination and reversal subtasks were normal. We propose that prominent storage and enhanced LTP in hippocampus have contributed to these specific behavioral alterations in alpha-mannosidase-deficient mice.

Late-onset retinal dystrophy in α-mannosidosis

alpha-Mannosidosis is a rare lysosomal storage disease that is caused by an inherited deficiency of the lysosomal alpha-mannosidase. Clinical symptoms include coarse facial features, skeletal involvement (dysostosis multiplex), hearing disabilities, mental retardation and hepatosplenomegaly. Only few cases with ocular symptoms have been reported, mainly with lenticular opacities. We report on two brothers with complex neurological symptoms who presented with late-onset retinal dystrophy and were followed up for 6 years.

Unique metal dependency of cytosolic alpha-mannosidase from Thermotoga maritima, a hyperthermophilic bacterium

A putative cytosolic alpha-mannosidase gene from a hyperthermophilic marine bacterium Thermotoga maritima was cloned and expressed in Escherichia coli. The purified recombinant enzyme appeared to be a homodimer of a 110-kDa subunit. The enzyme showed metal-dependent ability to hydrolyze p-nitrophenyl-alpha-D-mannopyranoside. In the absence of a metal, the enzyme was inactive. Cobalt and cadmium supported high activity (60 U/mg at 70 degrees C), while the activity with zinc and chromium was poor. Cobalt (0.8 mol) bound to 1 mol monomer with a K(d) of 70 microM. The optimum pH and temperature were 6.0 and 80 degrees C, respectively. The activity was inhibited by swainsonine, but not by 1-deoxymannojirimycin, which is in agreement with the features of cytosolic alpha-mannosidase.

Identification of the catalytic residue of Thermococcus litoralis 4-alpha-glucanotransferase through mechanism-based labeling

Thermococcus litoralis 4-alpha-glucanotransferase (TLGT) belongs to family 57 of glycoside hydrolases and catalyzes the disproportionation and cycloamylose synthesis reactions. Family 57 glycoside hydrolases have not been well investigated, and even the catalytic mechanism involving the active site residues has not been studied. Using 3-ketobutylidene-beta-2-chloro-4-nitrophenyl maltopentaoside (3KBG5CNP) as a donor and glucose as an acceptor, we showed that the disproportionation reaction of TLGT involves a ping-pong bi-bi mechanism. On the basis of this reaction mechanism, the glycosyl-enzyme intermediate, in which a donor substrate was covalently bound to the catalytic nucleophile, was trapped by treating the enzyme with 3KBG5CNP in the absence of an acceptor and was detected by matrix-assisted laser desorption ionization time-of-flight mass spectrometry after peptic digestion. Postsource decay analysis suggested that either Glu-123 or Glu-129 was the catalytic nucleophile of TLGT. Glu-123 was completely conserved between family 57 enzymes, and the catalytic activity of the E123Q mutant enzyme was greatly decreased. On the other hand, Glu-129 was a variable residue, and the catalytic activity of the E129Q mutant enzyme was not decreased. These results indicate that Glu-123 is the catalytic nucleophile of TLGT. Sequence alignment of TLGT and family 38 enzymes (class II alpha-mannosidases) revealed that Glu-123 of TLGT corresponds to the nucleophilic aspartic acid residue of family 38 glycoside hydrolases, suggesting that family 57 and 38 glycoside hydrolases may have had a common ancestor.

Targeted disruption of the lysosomal alpha-mannosidase gene results in mice resembling a mild form of human alpha-mannosidosis

Alpha-mannosidosis is a lysosomal storage disease with autosomal recessive inheritance caused by a deficiency of the lysosomal alpha-mannosidase, which is involved in the degradation of asparagine-linked carbohydrate cores of glycoproteins. An alpha-mannosidosis mouse model was generated by targeted disruption of the gene for lysosomal alpha-mannosidase. Homozygous mutant animals exhibit alpha-mannosidase enzyme deficiency and elevated urinary secretion of mannose-containing oligosaccharides. Thin-layer chromatography revealed an accumulation of oligosaccharides in liver, kidney, spleen, testis and brain. The cellular alterations were characterized by multiple membrane-limited cytoplasmic vacuoles as seen for instance in liver, exocrine pancreas, kidney, thyroid gland, smooth muscle cells, osteocytes and in various neurons of the central and peripheral nervous systems. The morphological lesions and their topographical distribution, as well as the biochemical alterations, closely resemble those reported for human alpha-mannosidosis. This mouse model will be a valuable tool for studying the pathogenesis of inherited alpha-mannosidosis and may help to evaluate therapeutic approaches for lysosomal storage diseases.

Isolation and characterization of the gene encoding the mouse broad specificity lysosomal alpha-mannosidase1

A genomic clone encoding the mouse lysosomal alpha-mannosidases was isolated and the gene was found to be encoded by 24 exons spanning approximately 14.5 kb of genomic DNA. The intron-exon boundaries were conserved between the mouse, human, and bovine lysosomal alpha-mannosidase genes as well as being partially conserved in several other species. In order to define the promoter of the mouse mannosidase gene, >1 kb of DNA sequence was obtained upstream from the respective initiation codon. The transcription start site was identified by a 5'-RACE procedure and putative promoter elements were identified by expression of promoter/reporter constructs. Fluorescence in situ hybridization analysis using the mouse and human mannosidase genomic clones as probes, localized the mouse gene to chromosome 8, at band position 8C2, and the human gene to chromosome 19p13.2, a region syntenic to the lysosomal mannosidase gene on mouse chromosome 8.

Spectrum of mutations in alpha-mannosidosis

alpha-Mannosidosis is an autosomal recessive disorder caused by deficiency of lysosomal alpha-mannosidase (LAMAN). The resulting intracellular accumulation of mannose-containing oligosaccharides leads to mental retardation, hearing impairment, skeletal changes, and immunodeficiency. Recently, we reported the first alpha-mannosidosis-causing mutation affecting two Palestinian siblings. In the present study 21 novel mutations and four polymorphic amino acid positions were identified by the screening of 43 patients, from 39 families, mainly of European origin. Disease-causing mutations were identified in 72% of the alleles and included eight splicing, six missense, and three nonsense mutations, as well as two small insertions and two small deletions. In addition, Southern blot analysis indicated rearrangements in some alleles. Most mutations were private or occurred in two or three families, except for a missense mutation resulting in an R750W substitution. This mutation was found in 13 patients, from different European countries, and accounted for 21% of the disease alleles. Although there were clinical variations among the patients, no significant LAMAN activity could be detected in any of the fibroblast cultures. In addition, no correlation between the types of mutations and the clinical manifestations was evident.

Missense and nonsense mutations in the lysosomal alpha-mannosidase gene (MANB) in severe and mild forms of alpha-mannosidosis

alpha-Mannosidosis is an autosomal recessive lysosomal-storage disorder caused by a deficiency of lysosomal alpha-mannosidase activity. This disease shows a wide range of clinical phenotypes, from a severe, infantile form (type I), which is fatal at <3-8 years of age, to a less severe, late-onset form (type II), which ultimately may involve hearing loss, coarse face, mental retardation, and hepatosplenomegaly. To elucidate the molecular mechanism underlying this disease in both types of patients, we have used PCR, followed by either SSCP analysis or direct sequencing, to analyze the 24 exons and intron/exon boundaries of the alpha-mannosidase gene (MANB) from five patients. Two amino acid substitutions-H72L and R750W, in exons 2 and 18, respectively-and two nonsense mutations-Q639X and R760X, in exons 15 and 19, respectively-were identified in four type II patients. One amino acid substitution, P356R, was identified in exon 8 from a type I patient. This patient and three of the type II patients were homozygous for their mutations (H72L, P356R, R750W, and R760X) and one type II patient was heterozygous for the Q639X and R750W mutations. Transfection experiments of COS 7 cells, using the alpha-mannosidase cDNA containing one of the missense mutations-H72L, P356R, or R750W-revealed that each of these mutations dramatically reduces the enzymatic activity of alpha-mannosidase. These data demonstrate that widely heterogeneous missense or nonsense mutations of the MANB gene are the molecular basis underlying alpha-mannosidosis.

Labeling and identification of the postulated acid/base catalyst in the alpha-glucosidase from Saccharomyces cerevisiae using a novel bromoketone C-glycoside

alpha-Glucosidase from Saccharomyces cerevisiae is a member of a sequence-related family of alpha-glycosidases (family 13) that includes digestive alpha-amylases and commercially important cyclodextrin glucanotransferases. These enzymes catalyze the hydrolysis of alpha-linked oligosaccharides by a two-step mechanism involving a glycosyl-enzyme intermediate. A novel bromoketone C-glycoside inactivator, 1'-bromo-3'-(alpha-D-mannopyranosyl)-2'-propanone, has been synthesized and used to label the putative acid/base catalyst (Glu-276) of yeast alpha-glucosidase. Electrospray ionization mass spectrometry was used to demonstrate stoichiometric labeling of the protein. The labeled residue was identified by comparative liquid chromatographic/mass spectrometric analysis of peptic digests of labeled and unlabeled enzyme samples, which confirmed the unique presence of two labeled peptides of m/z 745 and 694. Subsequent tandem mass spectrometric analysis in the daughter-ion scan mode showed the two peptides to have an overlapping sequence in which Glu-276 was the labeled residue. Together with active-site-directed protection against inactivation with deoxynojirimycin, these results prove that Glu-276 is located within the active site of yeast alpha-glucosidase and, thus, provide further evidence for this residue playing an important role in catalysis.

Identification of the active site nucleophile in jack bean alpha-mannosidase using 5-fluoro-beta-L-gulosyl fluoride

Mannosidases play a key role in the processing of glycoproteins and thus are of considerable pharmaceutical interest and indeed have emerged as targets for the development of anti-cancer therapies. Access to useful quantities of the mammalian enzymes has not yet been achieved; therefore, jack bean mannosidase, a readily available enzyme, has become the model system. However, the relevance of this enzyme has not been demonstrated, nor is anything known about the active site structure of this, or any other, mannosidase. Hydrolysis by this enzyme occurs with net retention of sugar anomeric configuration; thus, a double displacement mechanism involving a mannosyl-enzyme intermediate is presumably involved. Two new mechanism-based inhibitors, 5-fluoro-alpha-D-mannosyl fluoride and 5-fluoro-beta-L-gulosyl fluoride, which function by the steady state trapping of such an intermediate, have been synthesized and tested. Both show high affinity for jack bean alpha-mannosidase (Ki' = 71 and 86 microM, respectively), and the latter has been used to label the active site nucleophile. The labeled peptide present in a peptic digest of this trapped glycosyl-enzyme intermediate was identified by neutral loss scans on an electrospray ionization triple quadrupole mass spectrometer. Comparative liquid chromatographic/mass spectrometric analysis of peptic digests of labeled and unlabeled enzyme samples confirmed the unique presence of this peptide of m/z 1180.5 in the labeled sample. The label was cleaved from the peptide by treatment with ammonia, and the resultant unlabeled peptide was purified and sequenced by Edman degradation. The peptide identified contained only one candidate for the catalytic nucleophile, an aspartic acid. This residue was contained within the sequence Gly-Trp-Gln-Ile-Asp-Pro-Phe-Gly-His-Ser, which showed excellent sequence similarity with regions in mammalian lysosomal and Golgi alpha-mannosidase sequences. These mammalian alpha-mannosidases belong to family 38 (or class II alpha-mannosidases) in which the Asp in the above sequence is totally conserved. This finding therefore assigns jack bean alpha-mannosidase to family 38, validating it as a model for other pharmaceutically interesting enzymes and thereby identifying the catalytic nucleophile within this family.

Purification of feline lysosomal alpha-mannosidase, determination of its cDNA sequence and identification of a mutation causing alpha-mannosidosis in Persian cats

alpha-Mannosidosis is a lysosomal storage disorder that is caused by the deficiency of lysosomal alpha-mannosidase. Feline alpha-mannosidosis is a well-characterized animal model used for studying pathological and therapeutic aspects of lysosomal storage disorders. We here report the purification of feline liver lysosomal alpha-mannosidase and determination of its cDNA sequence. The active enzyme consisted of three polypeptides, with molecular masses of 72, 41 and 12 kDa, joined by non-covalent forces. The cDNA sequence of feline lysosomal alpha-mannosidase was determined from reverse transcriptase PCR products obtained from skin fibroblast mRNA. The deduced amino acid sequence contained the N-terminal sequences of the 72 and 41 kDa peptides. This indicated that the enzyme is synthesized as a single-chain precursor with a putative signal peptide of 50 amino acids followed by a polypeptide chain of 957 amino acids, which is cleaved into the three polypeptides of the mature enzyme. The deduced amino acid sequence was 81.1 and 83.2% identical with the human and bovine lysosomal alpha-mannosidases sequences respectively. A 4 bp deletion was identified in an alpha-mannosidosis-affected Persian cat by DNA sequencing of reverse transcriptase PCR products. The deletion resulted in a frame shift from codon 583 and premature termination at codon 645. No lysosomal alpha-mannosidase activity could be detected in the liver of this cat. A domestic long-haired cat expressing a milder alpha-mannosidosis phenotype than the Persian cat had a lysosomal alpha-mannosidase activity of 2% of normal. This domestic long-haired cat did not possess the 4 bp deletion, proving molecular heterogeneity for feline alpha-mannosidosis.

Human lysosomal and jack bean alpha-mannosidases are retaining glycosidases

The stereochemical course of the hydrolyses catalysed by two alpha-mannosidases has been determined directly by 1H NMR. Synthetic substrates were incubated with the enzymes and the anomeric configuration of the initially formed product was ascertained in each case by observation of the chemical shift of the anomeric proton at the hemiacetal centre. Both mannosidases were found to catalyse hydrolysis with retention of stereochemistry at the anomeric position. Human lysosomal alpha-mannosidase (a class II mannosidase) is a member of the glycosidase family 38 and thus has sequence similarity with several alpha-mannosidases responsible for glycoprotein biosynthesis. Jack bean alpha-mannosidase was shown to be mechanistically similar to the lysosomal enzyme and will provide a useful model system in mechanistic studies and inhibitor design.

Cloning, expression, purification, and characterization of the murine lysosomal acid alpha-mannosidase

Catabolism of alpha-linked mannose residues on eukaryotic glycoproteins is accomplished by a broad specificity lysosomal alpha-mannosidase (EC 3.2.1.24). Based on regions of protein sequence conservation between the lysosomal alpha-mannosidase from Dictyostelium discoideum and the murine Golgi glycoprotein processing alpha 1,3/1,6-mannosidase, alpha-mannosidase II, we have cloned a cDNA encoding the murine lysosomal alpha-mannosidase. The longest of the clones was 3.1 kb in length and encoded a polypeptide of 992 amino acids containing a putative NH2-terminal signal sequence and 11 potential N-glycosylation sites. The deduced amino acid sequence was 76.5% identical to the human lysosomal alpha-mannosidase and 38.1% identical to the lysosomal alpha-mannosidase from D. discoideum. Expression of the cDNA in Pichia pastoris resulted in the secretion of an alpha-mannosidase activity into the culture medium. This recombinant expression product was purified and was shown to have enzymatic characteristics highly similar to the enzyme purified from mammalian sources and to the human lysosomal alpha-mannosidase cDNA expressed in Pichia. These characteristics include a similar pH optimum, Km, Vmax, inhibition by swainsonine, and activity toward natural substrates. Northern blots identified a major 3.5 kb RNA transcript in all murine tissues tested. A minor transcript of 5.4 kb was also detected in some murine tissues similar to the alternatively spliced transcripts that have been previously identified in human tissues.

alpha-Mannosidosis: functional cloning of the lysosomal alpha-mannosidase cDNA and identification of a mutation in two affected siblings

a-Mannosidosis (MIM 248500) is an autosomal recessive lysosomal storage disorder resulting from deficient activity of lysosomal alpha-mannosidase (LAMAN) (EC 3.2.1.24). The disease is characterized by massive intracellular accumulation of mannose-rich oligosaccharides with resulting mental retardation, hearing loss, immune deficiency and skeletal changes. We report here the purification and characterization of human placenta LAMAN. The enzyme is synthesized as a single-chain precursor which is processed into three glycopeptides of 70, 42 and 15 kDa. The 70 kDa peptide is further partially proteolysed into three more peptides that are joined by disulfide bridges. The laman cDNA sequence was assembled from overlapping fragments obtained by PCR on human fibroblast and human lung cDNA. The deduced amino acid sequence contains a putative signal peptide of 48 amino acids followed by a polypeptide sequence of 962 amino acids. Northern blot analyses revealed a single transcript of approximately 3.5 kb present in all tissues examined but at varying levels. Two affected siblings of Palestinian origin were homozygous for a mutation that causes a His-->Leu replacement at a position which is conserved among class 2 alpha-mannosidases from several species.

Cloning, expression, purification, and characterization of the human broad specificity lysosomal acid alpha-mannosidase

We have cloned and expressed two cDNAs encoding the human lysosomal alpha-mannosidase (EC 3.2.1.24) by RT-PCR of human spleen mRNA. This enzyme is required for the degradation of N-linked carbohydrates during glycoprotein catabolism in eucaryotic cells. The shorter of the two cDNAs (3 kilobases (kb)) was found to encode an open reading frame of 2964 base pairs and, when expressed in Pichia pastoris, was found to encode an enzyme that could cleave high mannose oligosaccharides, oligosaccharides isolated from alpha-mannosidosis fibroblasts, and p-nitrophenyl-alpha-D-mannopyranoside substrates. In addition, the Pichia-expressed enzyme was inhibited by swainsonine, and had a pH optimum, Km, and Vmax characteristic of the enzyme purified previously from human liver. The second, larger RT-PCR product (3.6 kb) was found to contain an insertion and a deletion relative to the 3-kb spleen amplimer and encoded a truncated coding region, indicating that it resulted from alternate transcript splicing. No alpha-mannosidase activity could be detected in Pichia transformants containing this coding region, indicating that it did not encode a functional enzyme. Antiserum raised to the recombinant product of the 3-kb alpha-mannosidase cDNA immunoprecipitated lysosomal alpha-mannosidase activity from human fibroblast extracts. Northern blots identified a 3-kb RNA transcript in all human tissues tested, including alpha-mannosidosis fibroblasts, while minor transcripts of 3.6 kb were also present in several adult tissues. Human chromosome mapping of the mannosidase gene confirmed that the functional gene maps to the MANB locus on chromosome 19. Sequence comparisons were made to previously published human cDNA sequences encoding a putative lysosomal alpha-mannosidase (Nebes, V. L., and Schmidt, M. C. (1994) Biochem. Biophys. Res. Commun. 200, 239-245) and several differences were found relative to the functional lysosomal alpha-mannosidase encoded by the 3-kb spleen cDNA.

Neurology of adult alpha-mannosidosis

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