Sanfilippo syndrome

Sanfilippo syndrome
Structure of heparan sulfate, one of the molecules that builds up in the tissues of people with Sanfilippo syndrome
Specialty	Endocrinology
Symptoms	Deafness; intellectual disability; dementia; loss of mobility
Prognosis	Lifespan is reduced; most patients survive until the teenage years, but some may reach their 30s
Frequency	1 in 70,000[1]

Sanfilippo syndrome, or mucopolysaccharidosis III (MPS-III) is a rare autosomal recessive lysosomal storage disease. It is caused by a deficiency in one of the enzymes needed to break down the glycosaminoglycan (GAG) heparan sulfate (which is found in the extra-cellular matrix and on cell surface glycoproteins).

Affected children generally do not show any signs or symptoms at birth. In early childhood, they begin to develop developmental disability and loss of previously learned skills. In later stages of the disorder, they may develop seizures and movement disorders. Patients with Sanfilippo syndrome usually live into adolescence or early adulthood.^[2]

History

The condition is named after Sylvester Sanfilippo, the pediatrician who first described the disease.^[3][4]

Signs and Symptoms

The disease manifests in young children. Affected infants are apparently normal, although some mild facial dysmorphism may be noticeable. The stiff joints, hirsuteness and coarse hair typical of other mucopolysaccharidoses are usually not present until late in the disease. After an initial symptom-free interval, patients usually present with a slowing of development and/or behavioral problems, followed by progressive intellectual decline resulting in severe dementia and progressive motor disease.^[5] Acquisition of speech is often slow and incomplete. The disease progresses to increasing behavioural disturbance including temper tantrums, hyperactivity, destructiveness, aggressive behaviour, pica and sleep disturbance. As affected children have normal muscle strength and mobility, the behavioural disturbances are very difficult to manage. The disordered sleep in particular presents a significant problem to care providers. In the final phase of the illness, children become increasingly immobile and unresponsive, often require wheelchairs, and develop swallowing difficulties and seizures. The life-span of an affected child does not usually extend beyond late teens to early twenties.

Individuals with MPS III tend to have mild skeletal abnormalities; osteonecrosis of the femoral head may be present in patients with the severe form. Optical nerve atrophy, deafness, otitis can be seen in moderate to severe individuals. Other characteristics include coarse facial features, thick lips, synophrys, and stiff joints. Chronic diarrhea, enlarged liver and spleen are also common. It is difficult to clinically distinguish differences among the four types of Sanflippo syndrome. However, MPS IIIA is usually the most severe subtype, characterized by earliest onset, rapid clinical progression with severe symptoms, and short survival.^[6] The median age of death for children afflicted with MPS III-A is 15.4 years, +-4.1 years. ^[7]

Of all the MPS diseases, MPS III produces the mildest physical abnormalities. It is important, however, that simple and treatable conditions such as ear infections and toothaches not be overlooked because of behavior problems that make examination difficult. Children with MPS III often have an increased tolerance to pain. Bumps, bruises, or ear infections that would be painful for other children often go unnoticed in children with MPS III. Some children with MPS III may have a blood-clotting problem during and after surgery.

Pathophysiology and genetics

Glycosaminoglycans (GAGs) are polysaccharides that contain repeating disaccharides and sulfate groups. GAGs are attached to serine and threonine at the surface of proteoglycans, which are found in the extracellular matrix and the cell membrane, or stored in the secretory granules. GAGs are stored in the cell lysosome, and degraded by glycosidases, sulfatases and acetyltransferases. Deficiency in these enzyme leads to the four subtypes of MPS III.

All four subtypes of Sanfilippo syndrome have autosomal recessive inheritance. Impaired enzymatic activities are due to multiple mutations. The SGSH gene, which is located on chromosome 17q25.3, encodes heparan-N-sulfatase; deficiency of the enzyme is responsible for MPS IIIA. A total of 137 mutations that causes this form of Sanfilippo syndrome have been found so far. MPS IIIA is more common in Northern Europe. The NAGLU gene, located on chromosome 17q21.2, encodes alpha-N-acetylglucosaminidase, deficiency of which results in MPS IIIB. To date, 152 mutations were identified to cause MPS IIIB. This form of the syndrome is more common in Southern Europe. The HGSNAT gene which encodes CoA:alpha-glucosaminidase N-acetyltransferase is located on chromosome 8p11.21, causing MPS IIIC. 64 mutations associated with this form have been described so far. Deficiency of N-acetylglucosamine 6-sulfatase that leads to MPS IIID is due to mutations in the GNS gene, which is located on chromosome 12q14.3. 23 mutations have been identified. In general, MPS IIIA and B are the most common form of Sanfilippo syndrome.^[8]

Diagnosis

MPS-III A, B, C and D are considered to be clinically indistinguishable, although mutations in different genes are responsible for each disease. The following discussion is therefore applicable to all four conditions.

The diagnosis may be confirmed by assay of enzyme levels in tissue samples and gene sequencing. Prenatal diagnosis is possible.

Treatment

Treatment remains largely supportive. The behavioral disturbances of MPS-III respond poorly to medication. If an early diagnosis is made, bone marrow replacement may be beneficial. Although the missing enzyme can be manufactured and given intravenously, it cannot penetrate the blood–brain barrier and therefore cannot treat the neurological manifestations of the disease. Along with many other lysosomal storage diseases, MPS-III exists as a model of a monogenetic disease involving the central nervous system.

Several promising therapies are in development. Gene therapy in particular is under Phase I/II clinical trial in France since October 2011 under the leadership of Paris-based biotechnology company Lysogene. ^[9] Other potential therapies include chemical modification of deficient enzymes to allow them to penetrate the blood–brain barrier, stabilisation of abnormal but active enzyme to prevent its degradation, and implantation of stem cells strongly expressing the missing enzyme. For any future treatment to be successful, it must be administered as early as possible. Currently MPS-III is mainly diagnosed clinically, by which stage it is probably too late for any treatment to be very effective. Neonatal screening programs would provide the earliest possible diagnosis.

The flavonoid genistein decreases the pathological accumulation of glycosaminoglycans in Sanfilippo syndrome.^[10] In vitro, animal studies and clinical experiments suggest that the symptoms of the disease may be alleviated by an adequate dose of genistein.^[11] Despite its reported beneficial properties, genistein also has toxic side effects.^[12]

Several support and research groups have been established to speed the development of new treatments for Sanfilippo syndrome.^[13][14][15]

Incidence

Incidence of Sanfilippo syndrome varies geographically, with approximately 1 case per 280,000 live births in Northern Ireland,^[16] 1 per 66,000 in Australia,^[17] and 1 per 50,000 in the Netherlands.^[18]

The Australian study estimated the following incidences for each subtype of Sanfilippo syndrome:

See also

• Mucopolysaccharidosis
• List of cutaneous conditions

References

External links

• Sanfilippo syndrome (MPS-III) at the National MPS Society