Test ID: MANN    
Alpha-Mannosidase, Leukocytes

Diagnosis of alpha-mannosidosis

This test is not useful for establishing carrier status for alpha-mannosidosis.

Alpha-mannosidosis is an autosomal recessive lysosomal storage disorder caused by reduced or absent acid alpha-mannosidase enzyme activity.

Determining enzymatic activity is the next step of the diagnostic workup for an individual clinically suspicious for an oligosaccharidosis with a positive screening result suggestive of alpha-mannosidosis.

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 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 such as dysostosis multiplex, and severe central nervous system involvement. Although treatment is mostly supportive and aimed at preventing complications, hematopoietic stem cell transplant 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 may include a screening assay for several oligosaccharides in urine, OLIGU / Oligosaccharide Screen, Random, Urine. If the urine oligosaccharide screening assay is suggestive of alpha-mannosidosis, enzyme analysis of acid alpha-mannosidase can confirm the diagnosis.

> or =0.54 nmol/min/mg protein

Values below 0.54 nmol/min/mg protein are consistent with a diagnosis of alpha-mannosidosis.

No significant cautionary statements

1. Malm D, Nilssen O: Alpha-mannosidosis. In: Adam MP, Ardinger HH, Pagon RA, et al, eds. GeneReviews [Internet]. University of Washington, Seattle. 2001. Updated July 18, 2019. Accessed March 29, 2021. Available at www.ncbi.nlm.nih.gov/books/NBK1396/

2. Thomas GH: Disorders of glycoprotein degradation: alpha-mannosidosis, beta-mannosidosis, fucosidosis, and sialidosis. In: Valle DL, Antonarakis S, Ballabio A, Beaudet AL, Mitchell GA eds. The Online Metabolic and Molecular Bases of Inherited Disease.  McGraw-Hill; 2019. Accessed March 18, 2019. Available at https://ommbid.mhmedical.com/content.aspx?sectionid=225545029

3. Mynarek M, Tolar J, Albert MH, et al: Allogeneic hematopoietic SCT for alpha-mannosidosis: an analysis of 17 patients. Bone Marrow Transplant. 2012 Mar;47(3):352-359. doi: 10.1038/bmt.2011.99

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