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Alpha-mannosidosis is an autosomal recessive lysosomal storage disorder caused by reduced or absent acid alpha-mannosidase enzyme activity. This enzyme is involved in glycoprotein catabolism, with absent or reduced activity resulting in the accumulation of undigested mannose-containing complex oligosaccharides in the lysosomes, disrupting the normal functioning of cells.
Clinical features and severity of symptoms are widely variable within alpha-mannosidosis, but in general, the disorder is characterized by skeletal abnormalities, immune deficiency, hearing impairment, and mental retardation. Three clinical subtypes of the disorder have been described and they vary with respect to age of onset and clinical presentation. Type 1 is generally classified by a mild presentation and slow progression with onset after 10 years of age and absence of skeletal abnormalities. Type 2 is generally a more moderate form with slow progression and onset prior to 10 years of age with skeletal abnormalities and myopathy. Type 3 is the most severe form with onset in early infancy, skeletal abnormalities (dysostosis multiplex), and severe central nervous system involvement. Although treatment is mostly supportive and aimed at preventing complications, hematopoietic stem cell transplant (HSCT) has been reported to be a feasible therapeutic option. The incidence of alpha-mannosidosis is estimated at 1 in 500,000 live births.
An initial diagnostic workup for alpha-mannosidosis includes a multienzyme screening assay for several oligosaccharidoses, including mannosidosis in leukocytes or fibroblasts (OLIWB / Oligosaccharidoses Screen, Leukocytes or OLITC / Oligosaccharidoses Screen, Fibroblasts). If the screening assay is suggestive of alpha-mannosidosis, enzyme analysis of acid alpha-mannosidase can confirm the diagnosis. Sequencing of MAN2B1 allows for detection of disease-causing mutations in affected patients and identification of familial mutations allows for testing of at-risk family members.
Diagnosis of alpha-mannosidosis
Deficient activity of this enzyme is consistent with a diagnosis of alpha-mannosidosis.
This test cannot be used to establish carrier status for alpha-mannosidosis.
Interfering factors include lack of viable cells, bacterial contamination, failure to transport tissue in an appropriate media, excessive transport time, and exposure of the specimen to temperature extremes (freezing or >30 degrees C).
> or =0.53 nmol/min/mg protein
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2. Thomas GH: Disorders of Glycoprotein Degradation: α-Mannosidosis, β-Mannosidosis, Fucosidosis, and Sialidosis. Chapter 40 In The Online Metabolic and Molecular Bases of Inherited Disease. Edited by D Valle, AL Beaudet, B Volgelstein, et al: New York, NY: McGraw-Hill;2014 Available from: http://ommbid.mhmedical.com/content.aspx?bookid=971&Sectionid=62642875 Accessed 3/17/2015
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