Abstract

Salla disease (SD), also called sialic acid storage disease or Finnish type sialuria, is an autosomal recessive lysosomal storage disease characterized by early physical impairment and mental retardation. It was first described in 1979, after Salla, a municipality in Finnish Lapland. Salla disease is one of 40 Finnish heritage diseases and affects approximately 130 individuals, mainly from Finland and Sweden.

Characteristics

Individuals with Salla disease may present with nystagmus in the first months of life as well as hypotonia, reduced muscle tone and strength, and cognitive impairment. The most severely impaired children do not walk or acquire language, but the typical patient learns to walk and speak and has normal life expectancy. The MRI shows arrested or delayed myelination.

Cause and Genetics

SD is caused by a mutation in the SLC17A5 gene, located at human chromosome 6q14-15. This gene codes for sialin, a lysosomal membrane protein that transports the charged sugar, N-acetylneuraminic acid (sialic acid), out of lysosomes. The mutation causes sialic acid to build up in the cells.

The disease is inherited in an autosomal recessive manner. This means the defective gene responsible for the disorder is located on an autosome (chromosome 6 is an autosome), and two copies of the defective gene (one inherited from each parent) are required in order to be born with the disorder. The parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but usually do not experience any signs or symptoms of the disorder.

Diagnosis and Testing

A diagnosis of this disorder can be made by measuring urine to look for elevated levels of free sialic acid. Prenatal testing is also available for known carriers of this disorder.

Treatment

There is no cure for Salla disease. Treatment is limited to controlling the symptoms of this disorder. Anti-convulsant medication may control seizure episodes. Physical therapists can assist an affected individual to build muscle strength and coordination.

Prognosis

The life expectancy for individuals with Salla disease is between the ages of 50 and 60.