Introduction:
Free sialic acid storage disorders are a group of diseases that occurs due to the accumulation of sialic acid in various tissues and cells of the body. They are divided broadly into three depending on the degree of severity. It is also called lysosomal storage disorder. Lysosomes are membrane-bound organelles of a cell. They have enzymes that help to break down large substances such as carbohydrates, proteins, and fats into smaller parts. Sialin is responsible for transporting sialic acid out of lysosomes. If this transport protein is inactive or has low levels of activity, it leads to the accumulation of sialic acid in tissues. It leads to a sialic acid storage disorder.
What Are the Causes of Free Sialic Acid Storage Disorder?
The free sialic acid disorder occurs due to a gene mutation of the gene SLC17A5. The SLC17A5 gene has the coding for a protein called sialin. It helps in transporting free sialic acid, which is found in lysosomes. The defect in the gene causes sialin functioning impairment, which leads to the accumulation of free sialic acid in lysosomes. Sialic acid is produced by the lysosomes when it breaks down sugar-containing proteins (glycoprotein) or fats (glycolipids). It is an autosomal recessive disorder. That is, both pairs of chromosomes are to be affected for the illness to be inherited.
What Are the Symptoms of Free Sialic Acid Storage Disorder?
Free sialic acid storage disorder is a term given to describe a group of disorders. They vary in severity and symptoms. It may be life-threatening in some individuals while milder in others. Based on this, they have been divided into:
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Infantile Sialic Acid Storage Disease (ISSD): It is the most severe form. They exhibit symptoms at birth or within five months of birth.
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Intermediate Lysosomal Free Sialic Acid Storage Disease: It is not as severe as ISSD but more serious than Salla disease.
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Salla Disease: Mildest form.
There are symptoms specific to each form, but all forms of the disease are characterized by some degree of nerve cell degeneration and impairment of cognitive ability.
Infantine Free Sialic Acid Storage Disease (ISSD):
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This is the most severe form of free sialic storage disorder, with symptoms usually being apparent at birth or within six months of birth. In some rare cases, it can appear before birth. Affected infants can have a premature birth.
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There is an accumulation of fluid in the abdominal cavity called ascites.
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Coarse facial features.
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Enlargement of liver and spleen.
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Diminished muscle tone (hypotonia) in some infants is seen, and it is referred to as "floppy babies."
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Some infants fail to gain weight, have a delay in meeting developmental milestones, or even lose milestones that were acquired previously.
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Seizures and cognitive deficits may also occur.
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Skeletal abnormalities can occur, including malformation (dysplasia) at the end of long bones, short femur (thigh bones), clubbed feet, hip malformation, and underdeveloped bones such as the toes and fingers.
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ISSD can progress to serious complications that can be life-threatening, such as abnormal enlargement of the heart called cardiomegaly and respiratory infections.
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Nephrotic syndrome is seen in some infants in which protein is found in urine. Damage to the kidney leads to protein loss through urine. In these patients, swelling is seen in arms, legs, around the eyes, or other areas due to fluid accumulation.
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Weight gain, high blood pressure, and swollen abdomen are other symptoms.
Intermediate Salla Disease:
Intermediate Salla disease, as the name suggests, is less severe than ISSD but more severe than Salla disease. It is less common when compared to the other two groups of free sialic acid storage disorders. Symptoms are similar to that of infantile sialic storage disease and Salla disease.
Salla Disease:
Symptoms of Salla disease vary from one individual to another, but it is considered to be the mildest form of free sialic acid storage disorder. Some individuals live up to 70 years of age; however, some forms of Salla disease can have life-threatening complications.
Affected individuals do not exhibit symptoms at birth and develop them during the first year of life. Such as:
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Hypotonia: Diminished muscle tone.
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Nystagmus: Involuntary and rapid eye movements.
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Ataxia: Difficulty in coordinating voluntary movements.
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Delayed Developmental Milestones: Like sitting, walking, or talking. About two-thirds of children with Salla disease eventually learn to walk but may have some degree of speech impairment.
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Trouble Forming Speech: Affected children may learn single words or short sentences but may lose this ability as they age. The patients do not have difficulty understanding speech as much as they have trouble forming speech.
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Cognitive Impairment: It is also seen in some children along with other neurological symptoms.
In some cases, individuals develop symptoms much later in life. The disease is recognized when neurological findings become evident, such as:
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Seizures: Sudden and electrical disturbances are seizures.
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Spasticity: Stiff movements of the legs.
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Athetosis: Writhing movements of the arms and legs, which is repetitive and involuntary.
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Change In Facial Features- Gradual coarsening of facial features may be noticed in some individuals.
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Regression: Individuals who could previously talk and walk may lose these skills.
Who Are Affected by Free Sialic Acid Storage Disorder?
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Free sialic acid storage disorder affects both men and women equally. It is a rare disorder and is easily misdiagnosed.
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It may be confused with sialuria. It is characterized by elevated levels of free sialic acid in the cytoplasm, unlike in free sialic acid storage disorders, where it accumulates in lysosomes. It occurs due to mutation of the GNE gene and can have similar symptoms such as enlarged liver, delay in developmental milestones, coarse facial features, and seizures. However, it is inherited in a dominant pattern.
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Other disorders which result in the accumulation of fats and sugars include sialidosis, galactosialidosis, galactosemia, Gaucher disease, and other lysosomal storage disorders.
How Is Free Sialic Acid Storage Disorder Diagnosed?
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Free sialic acid storage disorder can be diagnosed based on clinical signs and symptoms that are characteristic of the disease. The patient's medical and family history and clinical symptoms can be evaluated.
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Genetic testing can be done to identify the SLC17A5 gene to confirm the diagnosis.
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Prenatal genetic testing can be done through chorionic villus sampling (CVS). During this, the fetal cells are subjected to enzyme assay tests to determine elevated levels of free sialic acid.
How to Treat Free Sialic Acid Storage Disorder?
Since the symptoms of this group of disorders vary from individual to individual, there is no specific treatment. Treatment depends on the symptom expressed by the patient.
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Anticonvulsants: To treat seizures.
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Special Therapy: For children diagnosed with the free sialic acid disorder, early intervention can make sure they do not regress, special education, physical therapy to improve coordination and strength, speech therapy, and other methods can be used to prevent symptoms from worsening and improve the quality of life.
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Genetic Counseling: For families that have a history of free sialic acid storage disorders, genetic counseling is recommended.
Conclusion:
The accumulation of sialic acid in lysosomes causes a number of different neurological and other symptoms with varying degrees of severity. Based on the difference in severity and the age of onset, it is categorized into three different diseases. This group of disorders is called free sialic acid storage disorder. It can cause seizures, neurological impairments, involuntary muscle movements, and developmental milestones delays. It is treated based on the symptoms, and genetic counseling is recommended for families with a history of the disease.
