Infantile free sialic acid storage disease (ISSD) is a
lysosomal storage disease.
ISSD occurs when
sialic acid Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone.
The term "sialic acid" (from the Greek for saliva, - ''sÃalon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this gr ...
is unable to be transported out of the lysosomal membrane and instead accumulates in the tissue, causing free sialic acid to be excreted in the urine. Mutations in the SLC17A5 (solute carrier family 17 (anion/sugar transporter), member 50) gene cause all forms of sialic acid storage disease. The SLC17A5 gene is located on the long (q) arm of
chromosome 6
Chromosome 6 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 6 spans more than 170 million base pairs (the building material of DNA) and represents between 5.5 and 6% of the total ...
between positions 14 and 15. This gene provides instructions for producing a protein called sialin that is located mainly on the membranes of lysosomes, compartments in the cell that digest and recycle materials.
ISSD is the most severe form of the sialic acid storage diseases. Babies with this condition have severe developmental delay, weak muscle tone (
hypotonia), and failure to gain weight and grow at the expected rate (
failure to thrive). They may have unusual facial features that are often described as "coarse," seizures, bone malformations, enlarged liver and spleen (
hepatosplenomegaly), and an enlarged heart (
cardiomegaly
Cardiomegaly (sometimes megacardia or megalocardia) is a medical condition in which the heart is enlarged. As such, it is more commonly referred to simply as "having an enlarged heart". It is usually the result of underlying conditions that make t ...
).
ISSD is a rare
autosomal recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
disorder and affects 1 in 528,000 live births worldwide.
Symptoms and signs
Symptoms present by eight months of age and are marked by developmental delay followed by neurological complications such as
seizures, involuntary eye movements, and
ataxia, involuntary muscle movements and failure to gain weight and grow at the expected rate (
failure to thrive). Babies with this condition also have and enlarged liver and spleen (
hepatosplenomegaly) and enlarged heart (
cardiomegaly
Cardiomegaly (sometimes megacardia or megalocardia) is a medical condition in which the heart is enlarged. As such, it is more commonly referred to simply as "having an enlarged heart". It is usually the result of underlying conditions that make t ...
).
Cause
Diagnosis
A diagnosis can be made by measuring cultured tissue samples for increased levels of free sialic acid. Prenatal testing is also available for known carriers of this disorder
Treatment
There is no treatment for ISSD. Treatment is limited to controlling the symptoms of this disorder such as administering anti-convulsant medication to control
seizure episodes.
References
External links
GeneReviews/NCBI/NIH/UW entry on Free Sialic Acid Storage Disorders{{Medical resources
, ICD10 = E77.8
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, OMIM = 269920
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, GeneReviewsNBK =
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, Orphanet = 309324
Rare diseases
Autosomal recessive disorders