26638 S-Sulfocysteine Panel, Urine (SSCTU)

Hypoxanthine, S-Sulfocysteine, Uric Acid, Xanthine, Hereditary xanthinuria, Isolated sulfite oxidase deficiency, Lesch-Nyhan syndrome, Molybdenum cofactor deficiency, Xanthine dehydrogenase and xanthine aldehyde oxidase dual deficiency, Xanthine dehydrogenase deficiency, Xanthine dehydrogenase/xanthine aldehyde oxidase/sulfite oxidase combined deficiency​

​This test provides a quantitative report of S-sulfocysteine, xanthine, hypoxanthine, and uric acid in urine identified via liquid chromatography-mass spectrometry​

Diagnosis of molybdenum cofactor deficiency, isolated sulfite oxidase deficiency, and hereditary xanthinuria

Monitoring patients with molybdenum cofactor deficiency or isolated sulfite oxidase deficiency who are on treatment

​This is the recommended test when clinical features are suggestive of, or when molecular testing results suggest, molybdenum cofactor deficiency, isolated sulfite oxidase deficiency, and hereditary xanthinuria. This test includes measurement of relevant purines in addition to urine S-sulfocysteine and uric acid. If the clinical features are suggestive of a purine and pyrimidine metabolism disorder or are nonspecific, order PUPYUSO / Purine and Pyrimidines Panel, Random, Urine.

THIS TEST WILL BE CANCELED IF ORDERED WITH PUPYUSO (Purines and Pyrimidines Panel, Urine (PUPYU))

​Collect a random urine specimen.

All specimens will be evaluated at Mayo Clinic Laboratories for test suitability

​Additional confirmatory testing via enzyme assays and molecular genetic testing is required for follow-up of abnormal results. 

Urine S-sulfocysteine is elevated in 2 disorders with similar clinical phenotypes, molybdenum cofactor deficiency (MoCD) and isolated sulfite oxidase deficiency. Molybdenum is an important trace element that is biosynthesized into an important cofactor, which is essential for the proper functioning of the enzymes, xanthine oxidase, sulfite oxidase, and aldehyde oxidase in addition to nitrogenases and nitrate reductase. Four genes are important in mediating the biosynthetic pathway to create molybdenum cofactor, MOCS1, MOCS2, MOCS3, and GPHN (gephyrin). The 3 clinical types of MoCD are autosomal recessive diseases resulting from 2 disease-causing variants in the respective causative gene. MoCDs result in a progressive neurodegenerative disease that manifests with seizures and brain abnormalities in the first weeks to months of life. The most common type of MoCD is MoCD A, caused by variants in MOCS1 and resulting in neonatal or infantile onset seizures and postnatal encephalopathy with rapidly progressive neurodegeneration. Infants with MoCD B (MOCS2 or MOCS3), and C (GPHN) have all been reported but are rare. Infants with MoCD have increased S-sulfocysteine and hypoxanthine and decreased uric acid concentrations in urine. The treatment for MoCD A only is cyclic pyranopterin monophosphate infusion and is most effective when initiated early.

Isolated sulfite oxidase deficiency (ISOD) is an autosomal recessive disorder caused by deficiency of the enzyme sulfite oxidase, which results in progressive neurodegenerative disease in most cases. ISOD is the result of disease-causing variants in the SUOX gene. ISOD is a spectrum of disease ranging from severe, early onset disease that appears in the first days of life with seizures, feeding issues, and neurologic issues causing abnormal muscle tone, to mild, later onset disease manifesting after 6 months of age with developmental delay or regression, movement issues, which can be episodic, and ectopia lentis in some cases. Infants with ISOD have increased S-sulfocysteine and normal hypoxanthine concentrations in urine. Treatment is largely symptomatic, with medication for seizures and movement/neurologic issues. Unfortunately, no treatment for the underlying metabolic defect is currently available. Prevalence is unknown, but ISOD is likely underdiagnosed.

​Hereditary xanthinuria results in kidney stones and, less commonly, muscle pain and cramping caused by accumulation of xanthine that forms crystals in the kidneys and muscle tissue. There are 2 types of hereditary xanthinuria: type I caused by deficiency of xanthine dehydrogenase resulting from disease-causing variants in the XDH gene, and type II caused by deficiency of molybdenum cofactor sulfurase resulting from variants in the MOCOS gene. Individuals with xanthinuria have increased xanthine and decreased uric acid concentrations in urine. The incidence of both types of hereditary xanthinuria is about 1 in 69,000 individuals.

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All results reported as mmol/mol creatinine

​Abnormal concentrations of measurable compounds will be reported along with an interpretation. The interpretation of an abnormal metabolite pattern includes an overview of the results and of their significance, a correlation to available clinical information, possible differential diagnosis, recommendations for additional biochemical testing and confirmatory studies (enzyme assay, molecular analysis), name, and phone number of contacts who may provide these studies, and a phone number of the laboratory directors in case the referring physician has additional questions.​