Alpha-mannosidosis

Alpha-mannosidosis is a rare inherited disorder that causes problems in many organs and tissues of the body. Signs and symptoms of alpha-mannosidosis include intellectual disability; distinctive facial features; and skeletal abnormalities such as reduced bone density (osteopenia), thickening of the bones at the top of the skull (calvaria), deformations of the bones in the spine (vertebrae), bowed legs or knock knees, and deterioration of the bones and joints. Characteristic facial features include a large head, prominent forehead, low hairline, rounded eyebrows, large ears, flattened bridge of the nose, protruding jaw, widely spaced teeth, overgrown gums, and large tongue.

Affected individuals may also experience difficulty in coordinating movements (ataxia); muscle weakness (myopathy); delay in developing motor skills such as sitting and walking; speech impairments; increased risk of infections; enlargement of the liver and spleen (hepatosplenomegaly); a buildup of fluid in the brain (hydrocephalus); hearing loss; a clouding of the lens of the eye (cataract); and psychiatric symptoms such as depression, anxiety, or hallucinations.

Signs of alpha-mannosidosis may appear in infancy with rapid progression and severe neurological deterioration. Early-onset alpha-mannosidosis is typically fatal during childhood. In the most severe cases, an affected fetus may die before birth.

Some individuals with alpha-mannosidosis experience milder signs and symptoms that appear later and progress more slowly. People with later-onset alpha-mannosidosis may survive into their fifties. The mildest cases may be detected only through laboratory testing and result in few if any symptoms.

Alpha-mannosidosis is believed to occur in approximately 1 in 500,000 people worldwide.

Mutations in the MAN2B1 gene cause alpha-mannosidosis.

The MAN2B1 gene provides instructions for making the enzyme alpha-mannosidase. This enzyme works in the lysosomes, which are compartments that digest and recycle materials in the cell. Within lysosomes, the enzyme helps break down complexes of sugar molecules (oligosaccharides) attached to certain proteins (glycoproteins). In particular, alpha-mannosidase helps break down oligosaccharides containing a sugar molecule called mannose.

Mutations in the MAN2B1 gene interfere with the ability of the alpha-mannosidase enzyme to perform its role in breaking down mannose-containing oligosaccharides. These oligosaccharides accumulate in the lysosomes and cause cells to malfunction and eventually die, resulting in the signs and symptoms of alpha-mannosidosis.

Read more about the MAN2B1 gene.

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.