Salla Disease

Salla disease belongs to the group of inborn errors of metabolism characterized by an abnormal accumulation of sialic acid or sialin that eventually causes neuronal and multisystem damage.

Free sialic acid deposition disease (FSASD) or Salla disease is a lysosomal storage disorder. It is a very rare neurodegenerative disease, except in northern Finland, where it is estimated that 1 in 100 people carry a mutation causing this condition. The incidence of Salla disease worldwide is approximately one case per million people.

This disorder is caused by mutations in the SLC17A5 gene that codes for the sialin protein, which is responsible for the transport of free sialic acid out of the lysosome. Pathogenic variants in SLC17A5 disrupt the transport process, causing accumulation of large amounts of sialic acid in the lysosome and altering its function.

Symptoms

Salla disease can manifest in early childhood or appear progressively. When it occurs in infancy it is known as infantile Salla disease, it can develop even before birth and the symptoms are very severe. Only 150 cases have been reported worldwide, mainly in Finland and Sweden.

Progressive onset Salla disease of mild to intermediate severity, with highly variable symptomatology, is characterized by progressive cognitive dysfunction ranging from mild to severe, hypotonia, delayed psychomotor development, spasticity, epileptic seizures, hypomyelination and characteristic facial features. The presence of high levels of free sialic acid in urine may be indicative of the pathology.

Disease management

There is no treatment that can cure Salla patients. The treatments used are aimed at alleviating symptoms and close follow-up by pediatricians and specialists who develop an individualized educational plan is needed. Physiotherapy is also recommended to help them acquire as much autonomy as possible and reduce the risk of injury from motor dysfunction.

Genes analyzed

SLC17A5

Bibliography

Adams D, Wasserstein M . Free Sialic Acid Storage Disorders. 2003 Jun 13 [updated 2020 Jan 23]. In: Adam MP, Mirzaa GM, Pagon RA, Wallace SE, Bean LJH, Gripp KW, Amemiya A, editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2022.

Barmherzig R, Bullivant G, Cordeiro D, et al . A New Patient With Intermediate Severe Salla Disease With Hypomyelination: A Literature Review for Salla Disease. Pediatr Neurol. 2017 Sep;74:87-91.e2.

Huizing M, Hackbarth ME, Adams DR, Wasserstein M, et al . Free sialic acid storage disorder: progress and promise. Neurosci Lett. 2021 Jun 11;755:135896.

Miyaji T, Omote H, Moriyama Y. Functional characterization of vesicular excitatory amino acid transport by human sialin. J Neurochem. 2011 Oct;119(1):1-5.

Verheijen FW, Verbeek E, Aula N, et al . A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases. Nat Genet. 1999 Dec;23(4):462-5.

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