A MANBA mutation resulting in residual beta-mannosidase activity associated with severe leukoencephalopathy: a possible pseudodeficiency variant.

BACKGROUND: beta-Mannosidosis (OMIM 248510) is a rare inborn lysosomal storage disorder caused by the deficient activity of beta-mannosidase, an enzyme encoded by a single gene (MANBA) located on chromosome 4q22-25. To date, only 20 cases of this autosomal recessive disorder have been described and 14 different MANBA mutations were incriminated in the disease. These are all null mutations or missense mutations that abolish beta-mannosidase activity. In this study, we characterized the molecular defect of a new case of beta-mannosidosis, presenting with a severe neurological disorder. METHODS: Genomic DNA was isolated from peripheral blood leukocytes of the patient to allow MANBA sequencing. The identified mutation was engineered by site-directed mutagenesis and the mutant protein was expressed through transient transfection in HEK293T cells. The beta-mannosidase expression and activity were respectively assessed by Western blot and fluorometric assay in both leukocytes and HEK293T cells. RESULTS: A missense disease-associated mutation, c.1922G>A (p.Arg641His), was identified for which the patient was homozygous. In contrast to previously described missense mutations, this substitution does not totally abrogate the enzyme activity but led to a residual activity of about 7% in the patient's leukocytes, 11% in lymphoblasts and 14% in plasma. Expression studies in transfected cells also resulted in 7% residual activity. CONCLUSION: Correlations between MANBA mutations, residual activity of beta-mannosidase and the severity of the ensuing neurological disorder are discussed. Whether the c.1922G>A mutation is responsible for a yet undescribed pseudodeficiency of beta-mannosidase is also discussed.

Beta-mannosidosis: a new cause of spinocerebellar ataxia.

Beta-mannosidosis (OMIM 248510) is an inborn lysosomal storage disorder caused by deficiency of beta-mannosidase activity. This enzyme is encoded by a single gene (MANBA), located on chromosome 4q22-25. This autosomal recessive disorder is characterized by a wide range of symptoms including mental retardation, behavioural problems, hearing loss, recurrent respiratory infections, angiokeratoma, facial dysmorphism, skeletal deformation, seizures, hypotonia, demyelinating polyneuropathy, and hepatosplenomegaly. The age of symptom onset is variable. We describe a 14-year clinical follow-up of a patient with beta-mannosidase deficiency with symptoms of mental retardation, progressive spasticity and cerebellar ataxia, a clinical spectrum that so far has never been reported in beta-mannosidosis. A novel mutation in the MANBA gene was found in our patient. Evoked potentials were in favour of a demyelinating pathology of the central nervous system. Serial MRI showed generalized cortical and subcortical atrophy in the absence of white matter changes suggesting an additional axonal pathophysiological component.

Atypical Gilles de la Tourette Syndrome with beta-mannosidase deficiency.

BACKGROUND: beta-Mannosidosis is a rare inborn error of metabolism with various phenotypes, including mental retardation, behavioral problems, hearing loss, and recurrent airway infections in childhood. To our knowledge, there is no published description of Gilles de la Tourette syndrome in association with this enzymatic deficiency. OBJECTIVE: To describe a unique case of Gilles de la Tourette syndrome associated with beta-mannosidosis. SETTING: University hospital. Patient An 18-year-old man exhibited motor and vocal tics since childhood, attention-deficit/hyperactivity disorder, impulsivity, and aggressiveness compatible with Gilles de la Tourette syndrome. A screen for inborn errors of metabolism was made because of the atypical association with slight mental retardation and bilateral perceptive hypoacousia. RESULTS: Urinary analysis showed disacchariduria, and leukocyte analysis revealed a profound deficit in beta-mannosidase activity. Two novel mutations in the beta-mannosidase gene were found: a new splice mutation in one allele, and a unique 10-base-pair insertion in the other. CONCLUSIONS: This case illustrates the phenotypic variability of inborn errors of metabolism in adults and demonstrates the need to screen inborn errors of metabolism in atypical Gilles de la Tourette syndrome.

Beta-mannosidosis mice: a model for the human lysosomal storage disease.

Beta-mannosidase, a lysosomal enzyme which acts exclusively at the last step of oligosaccharide catabolism in glycoprotein degradation, functions to cleave the unique beta-linked mannose sugar found in all N-linked oligosaccharides of glycoproteins. Deficiency of this enzyme results in beta-mannosidosis, a lysosomal storage disease characterized by the cellular accumulation of small oligosaccharides. In human beta-mannosidosis, the clinical presentation is variable and can be mild, even when caused by functionally null mutations. In contrast, two existing ruminant animal models have disease that is consistent and severe. To further explore the molecular pathology of this disease and to investigate potential treatment strategies, we produced a beta-mannosidase knockout mouse. Homozygous mutant mice have undetectable beta-mannosidase activity. General appearance and growth of the knockout mice are similar to the wild-type littermates. At >1 year of age, these mice exhibit no dysmorphology or overt neurological problems. The mutant animals have consistent cytoplasmic vacuolation in the central nervous system and minimal vacuolation in most visceral organs. Thin-layer chromatography demonstrated an accumulation of disaccharide in epididymis and brain. This mouse model closely resembles human beta-mannosidosis and provides a useful tool for studying the phenotypic variation in different species and will facilitate the study of potential therapies for lysosomal storage diseases.