alpha-Mannosidosis in the guinea pig: cloning of the lysosomal alpha-mannosidase cDNA and identification of a missense mutation causing alpha-mannosidosis

A Homozygous MAN2B1 Missense Mutation in a Doberman Pinscher Dog with Neurodegeneration, Cytoplasmic Vacuoles, Autofluorescent Storage Granules, and an α-Mannosidase Deficiency

A 7-month-old Doberman Pinscher dog presented with progressive neurological signs and brain atrophy suggestive of a hereditary neurodegenerative disorder. The dog was euthanized due to the progression of disease signs. Microscopic examination of tissues collected at the time of euthanasia revealed massive accumulations of vacuolar inclusions in cells throughout the central nervous system, suggestive of a lysosomal storage disorder. A whole genome sequence generated with DNA from the affected dog contained a likely causal, homozygous missense variant in MAN2B1 that predicted an Asp104Gly amino acid substitution that was unique among whole genome sequences from over 4000 dogs. A lack of detectable α-mannosidase enzyme activity confirmed a diagnosis of a-mannosidosis. In addition to the vacuolar inclusions characteristic of α-mannosidosis, the dog exhibited accumulations of autofluorescent intracellular inclusions in some of the same tissues. The autofluorescence was similar to that which occurs in a group of lysosomal storage disorders called neuronal ceroid lipofuscinoses (NCLs). As in many of the NCLs, some of the storage bodies immunostained strongly for mitochondrial ATP synthase subunit c protein. This protein is not a substrate for α-mannosidase, so its accumulation and the development of storage body autofluorescence were likely due to a generalized impairment of lysosomal function secondary to the accumulation of α-mannosidase substrates. Thus, it appears that storage body autofluorescence and subunit c accumulation are not unique to the NCLs. Consistent with generalized lysosomal impairment, the affected dog exhibited accumulations of intracellular inclusions with varied and complex ultrastructural features characteristic of autophagolysosomes. Impaired autophagic flux may be a general feature of this class of disorders that contributes to disease pathology and could be a target for therapeutic intervention. In addition to storage body accumulation, glial activation indicative of neuroinflammation was observed in the brain and spinal cord of the proband.

Biochemical characteristics of point mutated Capra hircus lysosome α-mannosidase

Locoweeds, a type of poisonous weedare, are widely distributed throughout the world and have a significant impact on the development of herbivore animal husbandry. Swainsonine (SW), the main toxin in locoweeds, can competitively inhibit lysosomes α-mannosidase (LAM) in animal cells, resulting in α-mannosidosis. However, the specifics of the interaction between SW and LAM are still unclear. Here, we used molecular docking to predicte the interaction points between SW and LAM, built mutated lysosomes α-mannosidase (LAM^M), and analyzed its biochemical properties changes in presumption points. The Trp at the 28th position and the Tyr at the 599th position of the LAM were interaction point candidates, and the above two amino acids in Capra hircus LAM (chLAM), were successfully mutated to glycine by constructing recombinant yeast GS115/PIC9K- LAM^M. The results showed that the sensitivity of Capra hircus LAM^M (chLAM^M), to SW decreased significantly compared with wild-type LAM, the enzyme activity of LAM decreased approximately threefold, the optimum temperature of LAM^M decreased from 55°C to 50°C, the optimum pH value increased from 4.5 to 5.0, and the effects of Mn²⁺, Fe³⁺, Al³⁺, Co²⁺, Cr³⁺, and ethylenediaminetetraacetic acid (EDTA) on LAM enzyme activity before and after point mutation changed significantly. These findings help us better understanding the molecular mechanism of the interaction mechanism between SW and chLAM, and provide new reference for solving locoweeds poisoning.

Alpha-Mannosidosis: Therapeutic Strategies

Alpha-mannosidosis (α-mannosidosis) is a rare lysosomal storage disorder with an autosomal recessive inheritance caused by mutations in the gene encoding for the lysosomal α-d-mannosidase. So far, 155 variants from 191 patients have been identified and in part characterized at the biochemical level. Similarly to other lysosomal storage diseases, there is no relationship between genotype and phenotype in alpha-mannosidosis. Enzyme replacement therapy is at the moment the most effective therapy for lysosomal storage disease, including alpha-mannosidosis. In this review, the genetic of alpha-mannosidosis has been described together with the results so far obtained by two different therapeutic strategies: bone marrow transplantation and enzyme replacement therapy. The primary indication to offer hematopoietic stem cell transplantation in patients affected by alpha-mannosidosis is preservation of neurocognitive function and prevention of early death. The results obtained from a Phase I⁻II study and a Phase III study provide evidence of the positive clinical effect of the recombinant enzyme on patients with alpha-mannosidosis.

amamutdb.no: A relational database for MAN2B1 allelic variants that compiles genotypes, clinical phenotypes, and biochemical and structural data of mutant MAN2B1 in α-mannosidosis

α-Mannosidosis is an autosomal recessive lysosomal storage disorder caused by mutations in the MAN2B1 gene, encoding lysosomal α-mannosidase. The disorder is characterized by a range of clinical phenotypes of which the major manifestations are mental impairment, hearing impairment, skeletal changes, and immunodeficiency. Here, we report an α-mannosidosis mutation database, amamutdb.no, which has been constructed as a publicly accessible online resource for recording and analyzing MAN2B1 variants (http://amamutdb.no). Our aim has been to offer structured and relational information on MAN2B1 mutations and genotypes along with associated clinical phenotypes. Classifying missense mutations, as pathogenic or benign, is a challenge. Therefore, they have been given special attention as we have compiled all available data that relate to their biochemical, functional, and structural properties. The α-mannosidosis mutation database is comprehensive and relational in the sense that information can be retrieved and compiled across datasets; hence, it will facilitate diagnostics and increase our understanding of the clinical and molecular aspects of α-mannosidosis. We believe that the amamutdb.no structure and architecture will be applicable for the development of databases for any monogenic disorder.

A review of gene therapy in canine and feline models of lysosomal storage disorders

Lysosomal storage disorders (LSDs) are inherited diseases that result from the intracellular accumulation of incompletely degraded macromolecules. The majority of LSDs affect both the peripheral and central nervous systems and are not effectively treated by enzyme replacement therapy, substrate reduction therapy, or bone marrow transplantation. Advances in adeno-associated virus and retroviral vector development over the past decade have resurged gene therapy as a promising therapeutic intervention for these monogenic diseases. Animal models of LSDs provide a necessary intermediate to optimize gene therapy protocols and assess the safety and efficacy of treatment prior to initiating human clinical trials. Numerous LSDs are naturally occurring in large animal models and closely reiterate the lesions, biochemical defect, and clinical phenotype observed in human patients, and whose lifetime is sufficiently long to assess the effect on symptoms that develop later in life. Herein, we review that gene therapy in large animal models (dogs and cats) of LSDs improved many manifestations of disease, and may be used in patients in the near future.

Proton nuclear magnetic resonance spectroscopic detection of oligomannosidic n glycans in alpha-mannosidosis: a method of monitoring treatment

In Alpha-mannosidosis (MIM 248500) the patients accumulate mainly unbranched oligosaccharide chains in the lysosomes in all body tissues, including the brain. With ensuing therapeutic modalities in man (BMT and ERT) non-invasive methods of monitoring the effect of treatment are needed. Paramount is the possible effect of the treatment on the brain, since this organ is regarded as difficult to reach because of the blood-brain barrier. We therefore performed proton nuclear magnetic resonance spectroscopy (MRS) of the brain in two untreated patients, and a 16-year-old patient treated with BMT at the age of 10 to assess whether this non-invasive method could be applied in the monitoring of the accumulation of abnormal chemicals in the brain of patients. We found an abnormal peak that was not present in the treated patient. A similar pattern was also found in MRS of urine from patients, reflecting the concentration of oligosaccharides in serum and tissues. We therefore conclude that MRS can be a useful method to monitor the effect of treatment for Alpha-Mannosidosis.

A multi-species comparative structural bioinformatics analysis of inherited mutations in alpha-D-mannosidase reveals strong genotype-phenotype correlation

Background

Lysosomal α-mannosidase is an enzyme that acts to degrade N-linked oligosaccharides and hence plays an important role in mannose metabolism in humans and other mammalian species, especially livestock. Mutations in the gene (MAN2B1) encoding lysosomal α-D-mannosidase cause improper coding, resulting in dysfunctional or non-functional protein, causing the disease α-mannosidosis. Mapping disease mutations to the structure of the protein can help in understanding the functional consequences of these mutations and thus indirectly, the finer aspects of the pathology and clinical manifestations of the disease, including phenotypic severity as a function of the genotype.

Results

A comprehensive homology modeling study of all the wild-type and inherited mutations of lysosomal α-mannosidase in four different species, human, cow, cat and guinea pig, reveals a significant correlation between the severity of the genotype and the phenotype in α-mannosidosis. We used the X-ray crystallographic structure of bovine lysosomal α-mannosidase as template, containing only two disulphide bonds and some ligands, to build structural models of wild-type structures with four disulfide linkages and all bound ligands. These wild-type models were then used as templates for disease mutations. All the truncations and substitutions involving the residues in and around the active site and those that destabilize the fold led to severe genotypes resulting in lethal phenotypes, whereas the mutations lying away from the active site were milder in both their genotypic and phenotypic expression.

Conclusion

Based on the co-location of mutations from different organisms and their proximity to the enzyme active site, we have extrapolated observed mutations from one species to homologous positions in other organisms, as a predictive approach for detecting likely α-mannosidosis. Besides predicting new disease mutations, this approach also provides a way for detecting mutation hotspots in the gene, where novel mutations could be implicated in disease. The current study has identified five mutational hot-spot regions along the MAN2B1 gene. Structural mapping can thus provide a rational approach for predicting the phenotype of a disease, based on observed genotypic variations.

Morphopathological features in tissues of α-mannosidosis guinea pigs at different gestational ages

Alpha-mannosidosis is an inherited metabolic disorder characterized by a reduction in alpha-D-mannosidase and intralysosomal accumulation of undegraded mannose-containing oligosaccharides. The alpha-mannosidosis guinea pig exhibits pathological similarities to its human counterpart, which make it a valuable animal model. To trace the progression of alpha-mannosidosis during foetal development, brain and visceral organs from affected and unaffected guinea pigs at 30, 36, 38, 51 and 65 days of gestation (dg) were examined by light and electron microscopy (term: approximately 68 dg). In the affected brain, distended lysosomes (vacuoles) were scarce up to 38 dg and were seen in few differentiating neuronal cells but mostly in macrophages, pericytes and endothelial cells. At 51 and 65 dg, several vacuoles were observed in some neurones, in many Purkinje cells, pericytes, endothelial and microglial cells, and in few cerebellar internal granule cells. Myelination had started by 51 dg. Non-myelinated axonal spheroids were detected in the brainstem at 65 dg. In the kidney cortex and liver, an increase in vacuolation was noticed between 36 and 65 dg. Some vacuolated cells were also noticed in the lungs and spleen at 51 and 65 dg. Altogether, these histological observations suggest that alpha-mannosidosis is unlikely to affect ontogenesis before the second half of gestation in guinea pigs; however, the morphopathological features recorded during the last quarter of gestation (which may roughly correspond to the period covering near term to 1-2 years of age in human) were clearly noticeable and may have had some impact.

Developmental Analysis of CNS Pathology in the Lysosomal Storage Disease α-Mannosidosis

The lysosomal storage disease alpha-mannosidosis is due to absence or defective function of lysosomal alpha-mannosidase, resulting in primary storage of undegraded mannose-rich oligosaccharides. Disease has been described in humans, cattle, cats, mice, and guinea pigs and is characterized in all species by progressive neurologic deterioration and premature death. We analyzed the neurodegenerative processes relative to clinical disease in alpha-mannosidosis guinea pigs as a human disease model, from birth to end-stage disease. Before the onset of obvious neurologic abnormalities at 2 months, we observed widespread neuronal lysosomal vacuolation including secondary accumulation of GM3 ganglioside, widespread axonal spheroids, and reduced myelination of white matter. Histopathologic changes subsequently showed rapid progression in severity in a pattern common to a number of different lysosomal storage disorders, with additional abnormalities including accumulation of GM2 ganglioside and cholesterol, astrogliosis, neuron loss particularly in the cerebellum, and activation and infiltration of the CNS with microglia/macrophages. End-stage clinical disease was seen at 10 to 14 months of age. Our findings show that complex neuropathologic changes in alpha-mannosidosis guinea pigs are already present at birth, before clinical changes are evident, and similar events are likely to occur in patients with this disorder.

Funtional characterization of four novel MAN2B1 mutations causing juvenile onset alpha-mannosidosis

Alpha-mannosidosis is a recessively inherited disorder due to the deficiency of the lysosomal alpha-mannosidase. We report the molecular analysis performed in two patients with the late onset form of alpha-mannosidosis. Four new alleles were identified: three missense mutations involving highly conserved residues, c.597 C>A (p.H200N), c.1553 T>C (p.L518P) and c.2746 C>A (p.R916S) and a single nucleotide deletion, c.2660delC. In vitro expression studies in COS-1 cells demonstrated that pH200N, p.L518P and p.R916S proteins are expressed but retained no residual enzyme activity. These data are supported by structural 3D analysis which predicted that both p.L518P and p.R916S could affect the interaction of the small E-domain with the active site domain or the main body of the structure while the pH200N might alter substrate binding or other catalytic properties. Finally, the c.2660delC causes a frameshift introducing a premature stop codon (p.T887SfsX45), presuming to be a severe mutation.

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.

Intracellular transport of human lysosomal alpha-mannosidase and alpha-mannosidosis-related mutants

Human LAMAN (lysosomal a-mannosidase) was synthesized as a 120 kDa precursor in transfected COS cells [African-green-monkey kidney cells], which was partly secreted as a single-chain form and partly sorted to the lysosomes being subsequently cleaved into three peptides of 70, 40 and 15 kDa respectively. Both the secreted and the lysosomal forms contained endo H (endoglucosidase H)-resistant glycans, suggesting a common pathway through the trans-Golgi network. A fraction of LAMAN was retained intracellularly as a single-chain endo H-sensitive form, probably in the ER (endoplasmic reticulum). The inherited lack of LAMAN causes the autosomal recessive storage disease a-mannosidosis. To understand the biochemical consequences of the disease-causing mutations, 11 missense mutations and two in-frame deletions were introduced into human LAMAN cDNA by in vitro mutagenesis and the resulting proteins were expressed in COS cells. Some selected mutants were also expressed in Chinese-hamster ovary cells. T355P (Thr355Pro), P356R, W714R, R750W and L809P LAMANs as well as both deletion mutants were misfolded and arrested in the ER as inactive single-chain forms. Six of the mutants were transported to the lysosomes, either with less than 5% of normal specific activity (H72L, D196E/N and R220H LAMANs) or with more than 30% of normal specific activity (E402K LAMAN). F320L LAMAN resulted in much lower activity in Chinese-hamster ovary cells when compared with COS cells. Modelling into the three-dimensional structure revealed that the mutants with highly reduced specific activities contained substitutions of amino acids involved in the catalysis, either co-ordinating Zn2+ (His72 and Asp196), stabilizing the active-site nucleophile (Arg220) or positioning the active-site residue Asp319 (Phe320).

The structure of bovine lysosomal alpha-mannosidase suggests a novel mechanism for low-pH activation

Lysosomal alpha-mannosidase (LAM: EC 3.2.1.24) belongs to the sequence-based glycoside hydrolase family 38 (GH38). Two other mammalian GH38 members, Golgi alpha-mannosidase II (GIIAM) and cytosolic alpha-mannosidase, are expressed in all tissues. In humans, cattle, cat and guinea pig, lack of lysosomal alpha-mannosidase activity causes the autosomal recessive disease alpha-mannosidosis. Here, we describe the three-dimensional structure of bovine lysosomal alpha-mannosidase (bLAM) at 2.7A resolution and confirm the solution state dimer by electron microscopy. We present the first structure of a mammalian GH38 enzyme that offers indications for the signal areas for mannose phosphorylation, suggests a previously undetected mechanism of low-pH activation and provides a template for further biochemical studies of the family 38 glycoside hydrolases as well as lysosomal transport. Furthermore, it provides a basis for understanding the human form of alpha-mannosidosis at the atomic level. The atomic coordinates and structure factors have been deposited in the Protein Data Bank (accession codes 1o7d and r1o7dsf).