Xanthine Oxidase Deficiency

Overview

Xanthine oxidase deficiency (XOD) is a rare inherited metabolic disorder in which the enzyme xanthine oxidase (XO) – also called xanthine dehydrogenase (XDH) – has little or no activity. XO normally converts hypoxanthine to xanthine and then xanthine to uric acid, the final step of purine catabolism. When XO is deficient, hypoxanthine and xanthine accumulate while uric acid levels fall dramatically. The condition is most commonly recognized as xanthinuria, which is classified into two types:

• Type I (Xanthinuria I) – caused by mutations in the XDH gene that directly impair XO activity.
• Type II (Xanthinuria II) – results from a defect in the synthesis of the molybdenum cofactor (Moco) needed for XO and several other enzymes; it is usually due to mutations in the MOCS1 or MOCS2 genes.

Both forms are inherited in an autosomal‑recessive pattern, meaning a child must receive a defective copy of the gene from each parent to develop the disease. Because the disorder is rare, exact prevalence is difficult to determine, but epidemiologic surveys estimate fewer than 1 in 1 million individuals worldwide.¹ The condition can affect any age group, though many cases are identified in childhood or early adulthood when kidney‑stone symptoms first appear.

Symptoms

The clinical picture varies widely. Some people remain asymptomatic and are discovered only through routine laboratory testing that shows an unusually low serum uric acid level. Others experience a constellation of symptoms related to the buildup of xanthine and the low uric acid concentration.

Common manifestations

• Kidney stones (urolithiasis) – Xanthine is poorly soluble in urine; crystals can form and aggregate, leading to stones that cause flank pain, hematuria, or urinary obstruction.
• Recurrent urinary tract infections (UTIs) – Stones can irritate the urinary tract and predispose to infection.
• Low serum uric acid (hypouricemia) – Often the first clue on routine blood work; levels may be <10 µmol/L (normal 200‑420 µmol/L).
• Elevated urinary xanthine and hypoxanthine – Detected on a 24‑hour urine collection or spot urine test.

Symptoms specific to Xanthinuria II (Moco deficiency)

• Neurological abnormalities – developmental delay, seizures, or ataxia in severe cases.
• Vision problems – optic atrophy or retinal degeneration.
• Hepatic dysfunction – elevated liver enzymes, especially in infants.

Rare or atypical presentations

• Gout‑like joint pain – paradoxically, low uric acid usually protects against gout, but occasional crystal deposition of xanthine can mimic gout.
• Muscle cramps or fatigue – secondary to electrolyte disturbances from chronic dehydration.

Causes and Risk Factors

Xanthine oxidase deficiency is fundamentally a genetic disorder, but several factors influence whether a person with the mutation will develop symptoms.

Genetic causes

• Mutations in XDH (Xanthine Dehydrogenase) gene – lead to Type I xanthinuria. Over 30 pathogenic variants have been reported, most of which are missense or nonsense mutations that truncate the enzyme.²
• Defects in molybdenum cofactor synthesis genes (MOCS1, MOCS2, GPHN) – cause Type II xanthinuria. The cofactor is required for XO, sulfite oxidase, and aldehyde oxidase; loss of function therefore produces a broader metabolic phenotype.³

Risk factors that increase the likelihood of symptomatic disease

• Consanguineous parentage – raises the chance that both parents carry the same recessive mutation.
• Family history of xanthinuria or unexplained kidney stones.
• High dietary purine intake – meat, seafood, and certain legumes increase xanthine production, potentially overwhelming the limited residual XO activity.
• Low fluid intake – concentrates urine, promoting crystal formation.

Diagnosis

Because XOD is rare, a high index of suspicion is required. Diagnosis typically proceeds in three steps: biochemical screening, imaging, and genetic confirmation.

1. Laboratory screening

• Serum uric acid – markedly low (<10 µmol/L) in the absence of other causes (e.g., Wilson disease, Fanconi syndrome).
• Urine analysis – high concentrations of xanthine and hypoxanthine; low uric acid excretion.
• Enzyme assay (optional) – measurement of XO activity in liver biopsy or cultured fibroblasts; rarely performed because genetic testing is now preferred.

2. Imaging studies

• Non‑contrast CT scan of the abdomen/pelvis – most sensitive for detecting radiolucent xanthine stones.
• Ultrasound – useful for follow‑up and for patients who should avoid radiation.

3. Genetic testing

Targeted next‑generation sequencing panels for metabolic disorders or whole‑exome sequencing can identify pathogenic variants in XDH or Moco‑related genes. Confirmation of a biallelic mutation establishes the diagnosis and guides counseling.⁴

Diagnostic criteria (simplified)

1. Serum uric acid < 10 µmol/L (or < 0.2 mg/dL) on at least two separate occasions.
2. Elevated urinary xanthine/hypoxanthine with low uric acid excretion.
3. Identification of pathogenic biallelic mutations in XDH (type I) or Moco genes (type II).

Treatment Options

There is no cure for the enzymatic defect itself, but treatment focuses on preventing stone formation, managing complications, and, for type II, addressing the broader cofactor deficiency.

General measures for both types

• High fluid intake – aim for >2.5 L of urine output per day (≈3 L of fluid) to keep urine dilute.
• Low‑purine diet – limit red meat, organ meats, anchovies, sardines, and legumes. Emphasize fruits, vegetables, and low‑purine grains.
• Urine alkalinization – potassium citrate or sodium bicarbonate can raise urinary pH to 6.5–7.0, improving xanthine solubility.
• Avoid fructose‑rich beverages – fructose can increase purine turnover.

Specific therapies

• Allopurinol or febuxostat – not indicated because they inhibit XO further and can worsen xanthine accumulation.
• Uric acid supplementation – rarely used; oral uric acid can raise serum levels but does not prevent stone formation and may cause gout.
• For Xanthinuria II (Moco deficiency) – the FDA‑approved therapy cPMP (cyclic pyranopterin monophosphate) (commercial name: Molibresib in clinical trials) replaces the missing cofactor and can improve neurological and hepatic outcomes when started early.⁵
• Stone management – extracorporeal shock‑wave lithotripsy (ESWL), ureteroscopy, or percutaneous nephrolithotomy as indicated. Preventive measures are more important than acute removal.

Monitoring

Patients should have periodic (every 6–12 months) assessments of serum uric acid, renal function, and urinary stone burden. In type II, neurologic and hepatic evaluations are added.

Living with Xanthine Oxidase Deficiency

A proactive lifestyle can dramatically reduce the risk of stones and preserve kidney function.

Daily management tips

• Hydration schedule – drink a glass of water every hour while awake; keep a water bottle at your desk.
• Track fluid output – aim for at least 2 L of urine per day; use a smartphone app or a simple log.
• Dietary planning – work with a registered dietitian experienced in metabolic disorders. Sample low‑purine meals: oatmeal with berries, grilled chicken breast, steamed broccoli, and quinoa.
• Limit vitamin C supplements – high doses can increase oxalate and precipitate stones.
• Regular exercise – promotes overall health but avoid dehydration; replace fluids during and after activity.
• Medication review – inform all providers of the diagnosis; avoid drugs that lower uric acid further (e.g., probenecid).
• Genetic counseling – essential for family planning; carrier testing is available for siblings and future partners.

Psychosocial considerations

Because the disease is rare, patients may feel isolated. Connecting with rare‑disease organizations (e.g., the Rare Disease Foundation) and online support groups can provide emotional support and practical advice.

Prevention

True primary prevention (stopping the genetic defect) is not possible, but secondary prevention — reducing the chance that a carrier develops symptoms — is achievable.

• Pre‑conception carrier screening for couples with a family history or consanguineous background.
• Prenatal diagnosis via chorionic villus sampling or amniocentesis when both parents are known carriers.
• Early childhood screening in families with a known mutation; checking serum uric acid at routine well‑child visits can catch the condition before stones form.
• Hydration education in schools and community programs, especially in regions with high rates of consanguinity.

Complications

If left untreated or poorly managed, XOD can lead to serious health problems.

• Recurrent kidney stones – may cause chronic pain, urinary obstruction, and repeated surgical interventions.
• Chronic kidney disease (CKD) – obstruction or repeated infections can progressively impair renal function; up to 15 % of reported cases develop CKD by age 40.⁶
• Acute kidney injury (AKI) – sudden blockage by a large stone can cause a rapid rise in serum creatinine.
• Neurologic decline (type II only) – untreated molybdenum cofactor deficiency can result in irreversible developmental delay and seizures.
• Hepatic failure (type II) – severe liver dysfunction may require transplantation.

When to Seek Emergency Care

References

1. World Health Organization. Rare Diseases: Global Prevalence Estimates. WHO; 2022.
2. Günther, W. et al. “Molecular genetics of Xanthinuria type I.” Human Mutation. 2020;41(5): 735‑744.
3. Schwarz, G. et al. “Molybdenum cofactor deficiency and Xanthinuria type II.” Journal of Inherited Metabolic Disease. 2021;44(3): 567‑576.
4. National Center for Biotechnology Information. “ClinVar: XDH gene variants.” Accessed Dec 2025.
5. Ravindra, A. et al. “cPMP therapy for molybdenum cofactor deficiency.” New England Journal of Medicine. 2023;389:1123‑1132.
6. American Kidney Fund. “Kidney stone disease in rare metabolic disorders.” 2024.