Arylsulfatase A, 24 Hour, Urine
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by a deficiency of the arylsulfatase A (ARSA) enzyme, which leads to the accumulation of galactosyl sulfatide (cerebroside sulfate) in the white matter of the central nervous system and in the peripheral nervous system. Galactosyl sulfatide and, to a smaller extent, lactosyl sulfatide, also accumulate within the kidney, gallbladder, and other visceral organs and are excreted in excessive amounts in the urine.
The 3 clinical forms of MLD are late-infantile, juvenile, and adult, depending on age of onset. All result in progressive neurologic changes and leukodystrophy demonstrated on magnetic resonance imaging. Late-infantile MLD is the most common (50%-60% of cases) and usually presents between age 1 to 2 years with hypotonia, clumsiness, diminished reflexes, and slurred speech. Progressive neurodegeneration occurs and most patients die within 5 years of the diagnosis. Juvenile MLD (20%-30% of cases) is characterized by onset between 4 to 14 years. Presenting features are behavior problems, declining school performance, clumsiness, and slurred speech. Neurodegeneration occurs at a somewhat slower and more variable rate than the late-infantile form. Adult MLD (15%-20% of cases) has an onset after puberty and can be as late as the fourth or fifth decade. Presenting features are often behavior and personality changes, including psychiatric symptoms. Clumsiness, neurologic symptoms, and seizures are also common. The disease course has variable progression and may occur over 2 to 3 decades. The disease prevalence is estimated to be approximately 1 in 100,000.
MLD is an autosomal recessive disorder and is caused by mutations in the ARSA gene coding for the ARSA enzyme. This disorder is distinct from conditions caused by deficiencies of arylsulfatase B (Maroteaux-Lamy disease) and arylsulfatase C (steroid sulfatase deficiency). Saposin B deficiency is a rare autosomal recessive disorder with symptoms that mimic MLD; however ARSA enzyme level is normal. Like MLD, patients with saposin B deficiency can also excrete excessive amounts of sulfatides in their urine.
Extremely low ARSA levels have been found in some clinically normal parents and other relatives of MLD patients. These individuals do not have metachromatic deposits in peripheral nerve tissues, and their urine content of sulfatide is normal. Individuals with this "pseudodeficiency" have been recognized with increasing frequency among patients with other apparently unrelated neurologic conditions as well as among the general population. This has been associated with a fairly common polymorphism in the ARSA gene which leads to low expression of the enzyme (5%-20% of normal). These patients can be difficult to differentiate from actual MLD patients. Additional studies, such as molecular genetic testing of ARSA (ARSAZ / ARSA Gene, Full Gene Analysis), urinary excretion of sulfatides (CTSA / Ceramide Trihexosides and Sulfatides, Urine), and/or histological analysis for metachromatic lipid deposits in nervous system tissue are recommended to confirm a diagnosis.
Current treatment options for MLD are focused on managing disease manifestations such as seizures. Bone marrow transplantation remains controversial, and the effectiveness of enzyme replacement therapy may be limited due to difficulties crossing the blood-brain barrier. Other treatments under ongoing investigation include hematopoietic stem cell transplantation and fetal umbilical cord blood transplantation.
Detection of metachromatic leukodystrophy
Greatly reduced levels of arylsulfatase A in urine (< or =15 nmol/h/mL), as well as in serum and various tissues, is seen in patients with metachromatic leukodystrophy.
Individuals with pseudoarylsulfatase A deficiency can have results in the affected range, but are otherwise unaffected with metachromatic leukodystrophy.
Abnormal results should be confirmed using CTSA / Ceramide Trihexosides and Sulfatides, Urine. If molecular confirmation is desired, consider molecular genetic testing ARSAZ / ARSA Gene, Full Gene Analysis.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Leukocytes are the preferred screening specimen for metachromatic leukodystrophy.
Arylsulfatase A is also deficient in individuals with multiple sulfatase deficiency.
Individuals with pseudodeficiency of arylsulfatase A may have decreased enzyme activity and are not clinically affected with metachromatic leukodystrophy.
Due to the use of artificial substrate, this test does not reliably pick up carriers.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
> or =19 nmol/h/mL
Note: Results from this assay may not reflect carrier status because of individual variation of arylsulfatase A enzyme levels.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. von Figura K, Gieselmann V, Jaeken J: Chapter 148: Metachromatic leukodystrophy. In The Metabolic Basis of Inherited Disease. Eighth edition. Edited by D Valle, AL Beaudet, B Vogelstein. New York, McGraw-Hill Book Company. Accessed 01/21/2015. Available at: www.ommbid.com
2. Fluharty AL: Arylsulfatase A Deficiency. Accessed 01/21/2015. Available at: www.ncbi.nlm.nih.gov/books/NBK1130
3. Mahmood A, Berry J, Wenger D, et al: Metachromatic leukodystrophy: a case of triplets with the late infantile variant and a systematic review of the literature. J Child Neurol 2010 May;25(5):572-580
4. van Rappard DF, Boelens JJ, Wolf NI.: Metachromatic leukodystrophy: Disease spectrum and approaches for treatment. Best Pract Res Clin Endocrinol Metab 2015 Mar;29(2):261-273. DOI: 10.1016/j.beem.2014.10.001.