Beta-mannosidosis, also called lysosomal beta-mannosidase deficiency,is a disorder of oligosaccharide metabolism caused by decreased activity of the enzyme beta-mannosidase. This enzyme is coded for by the gene MANBA, located at 4q22-25. Beta-mannosidosis is inherited in an autosomal recessive manner.[1] Affected individuals appear normal at birth, and can have a variable clinical presentation. Infantile onset forms show severe neurodegeneration, while some children have intellectual disability. Hearing loss and angiokeratomas are common features of the disease, however because it is so rare, the full phenotype associated with the disease is not fully understood.

Beta-mannosidosis was identified as a neurodegenerative disorder in goats in 1981, five years before the first case was described in a human patient. The initial patient described in 1986 had a complex phenotype, and was later found to have both beta-mannosidosis and Sanfilippo syndrome.[1] Patients have been described with a wide spectrum of clinical presentations from infants and children with intellectual disability to adults who present with isolated skin findings (angiokeratomas).[1] Most cases are identified in the first year of life with respiratory infections, hearing loss and intellectual disability. Affected patients with beta-mannosidosis differ from those affected with other lysosomal storage disorders in the absence of coarse facies, organomegaly, and dysostosis multiplex. Because of its rarity, and non-specific clinical findings, beta-mannosidosis can go undiagnosed until adulthood, where it can present with intellectual disability and behavioral problems, including aggression. A diagnosis of beta-mannosidosis is suspected based on the patients clinical presentation. Urine testing to identify abnormal oligosaccharides is a useful screening test, and enzymatic analysis or molecular testing can be used for confirmation. Currently, there is no treatment for individuals affected with beta-mannosidosis.

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