β-Mannosidosis is an autosomal recessive disorder that belongs to a family of disorders identified as lysosomal storage diseases. This disorder is characterized by the lysosomal accumulation of the disaccharide moiety Man(β1,4)GlcNAc (mannose and N-acetylglucosamine linked via a β1,4 glycosidic bond) as a consequence of defects in the lysosomal hydrolase, β-mannosidase.
The β-mannosidase gene (MANBA: mannosidosis, beta A) is located on chromosome 4q24 comprising 19 exons encoding an 879 amino acid glycoprotein.
The most consistent clinical findings in β-mannosidosis are mental retardation, hearing loss and respiratory infections. Unlike the related disorder, α-mannosidosis, patients with β-mannosidosis do not show corneal opacities, hepatosplenomegaly nor dystosis multiplex. Almost all patients appear to be normal at birth, symptoms are mild by comparison to other lysosomal storage diseases and appear between 1 and 6 years of age. A definitive diagnosis of β-mannosidosis can be made by the measurement of β-mannosidase activity in leukocytes or fibroblasts.