Alpha-mannosidosis is a rare genetic disorder characterized by a deficiency of the enzyme alpha-D-mannosidase. Alpha-mannosidosis is best thought of as a continuum of diseases that is generally broken down into three forms: a mild, slowly progressive form (type 1); a moderate form (type 2); and a severe, often rapidly progressive, and potentially life-threatening form (type 3). The symptoms and severity of the disorder are highly variable. Symptoms may include distinctive facial features, skeletal abnormalities, hearing loss, intellectual disability, and dysfunction of the immune system. Alpha-mannosidosis is caused by mutations of the MAN2B1 gene. This genetic mutation is inherited as an autosomal recessive trait.
Alpha-mannosidosis belongs to a group of diseases known as the lysosomal storage disorders. Lysosomes are particles bound in membranes within cells that function as the primary digestive units. Enzymes within the lysosomes break down or digest particular nutrients, such as complex molecules composed of a sugar attached to a protein (glycoproteins). Low levels or inactivity of the alpha-mannosidase enzyme leads to the abnormal accumulation of compounds upstream in the metabolic pathway in the cells of affected individuals with unwanted consequences. Affected individuals may have intellectual disability, distinctive facial features, and skeletal abnormalities. Characteristic facial features can 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. The skeletal abnormalities that can occur in this disorder include reduced bone density (osteopenia), thickening of the bones at the top of the skull (calvaria), and deformations of the bones in the spine (vertebrae), knock knees, and deterioration of the bones and joints.
Causes
Alpha-mannosidosis is caused by changes (mutations) in the MAN2B1 gene. The MAN2B1 gene contains instructions for producing the enzyme lysosomal alpha-mannosidase (LAMAN). This enzyme is essential for breaking down (metabolizing) certain glycoproteins. Without proper levels of the functional version of this enzyme, these glycoproteins abnormally accumulate in and damage various tissues and organs of the body. Mutations of the MAN2B1 gene result in the lack of production of the alpha-D-mannosidase enzyme or the production of a defective, inactive form of the enzyme.
Alpha-mannosidosis is inherited in an autosomal recessive pattern. Recessive genetic disorders occur when an individual inherits an abnormal gene from each parent. If an individual receives one normal gene and one abnormal gene for the disease, the person will be a carrier of the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the abnormal gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier, like the parents, is 50% with each pregnancy. The chance for a child to receive normal genes from both parents is 25%. The risk is the same for males and females.
Mutations in the MAN2B1 gene cause alpha-mannosidosis. This 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. Tissues and organs are damaged by the abnormal accumulation of oligosaccharides and the resulting cell death, leading to the characteristic features of alpha-mannosidosis.
Diagnosis
A diagnosis of alpha-mannosidosis is suspected based upon the identification of characteristic findings, a thorough clinical evaluation, a detailed patient history, and specialized tests that can detect deficient levels or activity of the enzyme alpha-mannosidase in white blood cells (leukocytes) or cultured tissue cells (fibroblasts).
- Medical history including evidence of hearing loss, irritability, depression; change in social, domestic, school- or work-related activities or in ability to walk distances; diarrhea or incontinence, muscle pain, joint aches, reduced range of movement, and bone pain
- Physical examination including otoscopy, ophthalmoscopy, assessment of liver and spleen size, auscultation of heart and lungs, neurologic status including gait, and orthopedic evaluation including joint range of motion. In children, attention to growth (plot height, weight, and especially head circumference using standardized growth charts)
- Examination by an otolaryngologist to detect impaired hearing and middle-ear infections
- Audiometry. If intellectual disability or young age makes cooperation difficult, brain stem evoked response testing
- Ophthalmologic examination to evaluate for corneal opacities, myopia, hyperopia, and strabismus
- Neuropsychological testing to establish the functional level and learning capacity
- Blood tests. Clinical examination and immunologic tests (e.g., antinuclear antibodies, anti-ds-DNA antibodies) to exclude systemic lupus erythematosus (SLE)
- Skeletal assessment. Plain radiographs of the head, knees (anterior-posterior view), spine (lateral view), and any symptomatic sites
- Bone densitometry to detect osteopenia or osteoporosis in older individuals
- CT scan of the brain to evaluate the size of the ventricles and shape and size of the cerebellum, particularly if signs and symptoms of hydrocephalus are present (e.g., headache, increasing gait ataxia, nausea, papilledema)
- A. Oligosaccharides in urine – A preliminary investigation may be performed to measure mannose-rich oligosaccharide concentrations in urine. Elevated urinary excretion of mannose-rich oligosaccharides is suggestive, but not diagnostic of the disease.
- B. Acid alpha-mannosidase activity – Diagnosis is confirmed by measuring residual alpha-mannosidase activity in leukocytes or other nucleated cells via a fluorometric assay.[rx] This is the most reliable diagnostic method, along with genetic testing
- C. Genetic testing – Identification of disease-causing mutations is achieved using DNA from peripheral blood cells, by polymerase chain reaction (PCR) amplification of all 24 MAN2B1 exons, followed by DNA sequencing.[rx]
A diagnosis of alpha-mannosidosis can be confirmed through molecular genetic testing, which can reveal the characteristic mutation of the MAN2B1 gene that causes the disorder. Molecular genetic testing is available on a clinical basis. Elevated levels of certain mannose-rich oligosaccharides (a complex carbohydrate) may be found through urinary analysis. Although this finding is considered suggestive of alpha-mannosidosis, it is not diagnostic.
Treatment
Treatment of alpha-mannosidosis is symptomatic and supportive. Therapy is directed at preventing and treating the complications of the disorder. Thus, it is important to be proactive. Antibiotics are used to suppress bacterial infections. Hearing aids and pressure equalizing tubes are used to improve hearing. Physiotherapy for muscle weakness is often prescribed.
Orthopedic interventions including surgery or the use of assistive devices (e.g., special shoes or orthosis) may be necessary to treat associated skeletal abnormalities. Some individuals may require the use of a wheelchair.
Hydrocephalus may be treated by the insertion of a tube (shunt) to drain excess cerebrospinal fluid (CSF) away from the brain and into another part of the body where the CSF can be absorbed.
Medical measures
- Early use of antibiotics for bacterial infections. A clinical consequence of immunodeficiency is that bacterial and viral infections must be treated with vigilance.
- Hearing aids as early as possible for individuals with sensorineural hearing loss to improve hearing and enhance speech development and social functioning
- Insertion of PE (pressure-equalizing) tubes to reduce the conductive/mechanical component of hearing loss from fluid in the middle ear
- Glasses (spectacles) to correct refractive error to improve vision. Although lens replacement for cataracts is a standard procedure in alpha-mannosidosis, corneal transplantation can be difficult; postoperative complications include astigmatism (which may be correctable with repeat surgery, laser treatment, or optical devices).
- Physiotherapy includes hydrotherapy to avoid strain on the joints
- Use of a wheelchair if necessary
- Treatment of osteoporosis or osteopenia identified on bone densitometry with pamidronate (Aredia®) monthly or zoledronic acid (Aclasta®) once a year
- Orthopedic intervention if necessary. Special shoes may help with ankle and foot support.
- Ventriculocaval shunt for communicating hydrocephalusNote: Ventriculoperitoneal shunts may cause ascites because of the reduced absorptive capacity of the peritoneal cavity [Malm, personal communication]. Therefore, ventriculocaval shunts are preferred.
- Hematopoietic stem cell transplantation (HSCT)- can be a treatment option for some patients, however, the risk-benefit profile is more favorable in younger patients, therefore ensuring an early diagnosis is critical for this to be a viable option. The rationale is that enzyme-producing donor cells repopulate the host tissue and transfer enzyme to nearby enzyme-deficient host cells.[rx] Despite early reports to the contrary,[rx][rx][rx] The possible benefits of HSCT must be weighed against the overall risk of procedure-related morbidity and mortality. The benefits are greater in younger patients before complications have developed, and also transplant-related complications are more frequent and severe in older patients.
- Enzyme replacement therapy (ERT) – is a therapeutic alternative in a number of lysosomal storage diseases.[rx][rx] The overall principle of ERT is that a recombinantly produced version of the deficient enzyme is introduced into the bloodstream, from where it is internalized by the cells and reaches the lysosomes by mannose-6-phosphate receptor-mediated uptake, thus replacing the missing endogenous enzyme.[rx] An ERT is approved for use in the European Union.[rx]
Educational opportunities / social considerations
- Use of sign language in individuals with significant hearing loss
- Early educational intervention for the development of social skills
- Speech therapy to improve speech
- Special education to maximize learning
- Planning to house for possible future wheelchair use
Early intervention is important in ensuring that children with alpha-mannosidosis reach their highest potential. Services that may be beneficial may include special education, speech therapy, special services for children with hearing loss, and other medical, social, and/or vocational services. Genetic counseling may be of benefit for patients and their families.
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