Xanthinuria Diagnostics – Methods and Findings

What Is Xanthinuria Diagnostics?

Xanthinuria diagnostics refers to the full range of diagnostic procedures used to identify and confirm xanthinuria, a rare inherited metabolic disorder affecting purine metabolism. The condition is caused by a deficiency or complete loss of function of the enzyme xanthine oxidase (also known as xanthine dehydrogenase). As a result, xanthine cannot be converted to uric acid as normal, leading to the accumulation of xanthine in the blood, urine, and various tissues.

Causes and Clinical Background

Xanthinuria is classified into two main subtypes:

• Type I: Isolated xanthine oxidase deficiency due to mutations in the XDH gene.
• Type II: Combined deficiency of xanthine oxidase and aldehyde oxidase, caused by mutations in the MOCOS gene, which encodes an enzyme involved in molybdenum cofactor biosynthesis.

Many individuals are clinically asymptomatic and diagnosed incidentally. Symptomatic cases may present with xanthine kidney stones (nephrolithiasis), muscle complaints (myopathy), or in rare cases, xanthine deposits in muscle tissue.

Diagnostic Methods

Laboratory Testing in Urine and Blood

The first step in xanthinuria diagnostics is the measurement of xanthine and hypoxanthine levels in urine and blood serum, typically using HPLC (high-performance liquid chromatography). Characteristic laboratory findings include:

• Markedly elevated xanthine levels in urine
• Elevated hypoxanthine levels in urine
• Significantly reduced or undetectable uric acid in serum (hypouricemia)
• Reduced urinary uric acid excretion (hypouricosuria)

Enzymatic Testing

To confirm the diagnosis and differentiate between Type I and Type II, xanthine oxidase activity can be measured in liver biopsy tissue or intestinal mucosal samples. In Type II, aldehyde oxidase activity is additionally absent.

Molecular Genetic Testing

Today, the diagnosis is preferably confirmed by molecular genetic analysis. The genes XDH (Type I) and MOCOS (Type II) are analyzed for disease-causing mutations. This method is non-invasive, highly specific, and also enables family screening and prenatal counseling.

Imaging Diagnostics

When kidney stones are suspected, renal ultrasound is performed. Unlike calcium stones, xanthine stones are not visible on conventional X-rays (radiolucent), making ultrasound or CT imaging the preferred modalities.

Differential Diagnosis

Xanthinuria must be distinguished from other causes of hypouricemia, including:

• Renal hypouricemia (URAT1 or GLUT9 transporter defects)
• Molybdenum cofactor deficiency (which also involves sulfite oxidase deficiency)
• Medication-induced hypouricemia (e.g., due to uricosuric agents)

Clinical Relevance of the Diagnosis

Early diagnosis of xanthinuria is important to enable targeted management. This includes a low-purine diet, adequate fluid intake to prevent xanthine stone formation, and avoidance of medications that further inhibit purine metabolism -- for example, allopurinol, which inhibits xanthine oxidase and is contraindicated in xanthinuria.

References

1. Yamamoto T. et al. - Hereditary xanthinuria: a review of the literature with emphasis on clinical aspects. Nucleosides Nucleotides Nucleic Acids, 2016; 35(10-12): 578-584. PubMed PMID: 27906609.
2. Ichida K. et al. - Two families with classical xanthinuria type 1: significance of allopurinol loading test. Journal of Human Genetics, 1998; 43(2): 82-87.
3. Orphanet - Xanthinuria Type I and II. Orphanet Encyclopedia. Available at: https://www.orpha.net (accessed 2024).