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Xanthine Dehydrogenase Deficiency

Overview

Xanthine dehydrogenase deficiency (also called xanthinuria) is a rare inherited metabolic disorder in which the enzyme xanthine dehydrogenase (XDH) – also known as xanthine oxidoreductase – is either absent or markedly reduced. XDH catalyzes the conversion of hypoxanthine to xanthine and then xanthine to uric acid, the final step of purine degradation. When the enzyme is deficient, hypoxanthine and xanthine accumulate in the blood and urine, while uric acid levels are abnormally low.

The condition is inherited in an autosomal recessive pattern (type I) or, less commonly, as an autosomal dominant disorder (type II) caused by a mutation in the *MOCOS* gene that secondarily impairs XDH activity. Worldwide prevalence is estimated at 1–2 per 1 million people, though exact numbers vary because many cases are asymptomatic and go undiagnosed [NIH]. Both sexes are equally affected, and symptoms usually appear in childhood or early adulthood, but some individuals remain completely asymptomatic.

Symptoms

Clinical presentation ranges from silent to severe renal disease. The most common findings include:

• Low serum uric acid (hypouricemia) – often the first laboratory clue.
• Elevated urinary xanthine and hypoxanthine – detected by stone analysis or metabolic screening.
• Kidney stones composed of xanthine – may cause flank pain, hematuria, or urinary obstruction.
• Recurrent urinary tract infections (UTIs) – due to stone fragments or impaired urine flow.
• Renal insufficiency or chronic kidney disease – progressive if stones are not managed.
• Muscle weakness or fatigue – occasionally reported, likely secondary to metabolic disturbances.
• Neurological symptoms (rare) – seizures or developmental delay have been described in a few case reports, possibly related to severe hypouricemia.

Many patients are diagnosed incidentally during work‑up for kidney stones or routine blood tests showing very low uric acid.

Causes and Risk Factors

Genetic Basis

• Type I Xanthinuria – Caused by biallelic loss‑of‑function mutations in the XDH gene on chromosome 2p23.1.
• Type II Xanthinuria – Caused by biallelic mutations in the MOCOS gene, which encodes the molybdenum cofactor sulfurase needed for XDH activity.

Inheritance Pattern

Both types are inherited in an autosomal recessive manner, meaning a child must receive a pathogenic variant from each parent to manifest the disease. Carriers (heterozygotes) are usually asymptomatic but may have mildly reduced XDH activity.

Non‑Genetic Influences

• Consanguineous marriage increases the chance of inheriting two defective copies.
• Certain medications (e.g., allopurinol) that inhibit XDH can mimic biochemical findings, but they do not cause true deficiency.

Diagnosis

Clinical Suspicion

Physicians consider xanthinuria when a patient presents with:

• Recurrent xanthine kidney stones.
• Unexplained low serum uric acid (< 2 mg/dL) with normal renal function.
• Family history of similar stones or the condition.

Laboratory Tests

• Serum uric acid – markedly low (often < 1–2 mg/dL).
• Urine analysis – high concentrations of xanthine and hypoxanthine; stone analysis shows pure xanthine crystals (yellow‑white, hexagonal).
• Blood and urine purine profile via high‑performance liquid chromatography (HPLC) or mass spectrometry.
• Genetic testing – sequencing of XDH and MOCOS genes confirms the diagnosis and guides counseling.

Imaging

Non‑contrast CT of the abdomen/pelvis is the gold standard for detecting radiolucent xanthine stones, which may be missed on plain X‑ray.

Differential Diagnosis

Other causes of hypouricemia include:

• Renal tubular disorders (e.g., Fanconi syndrome).
• Medication‑induced (e.g., allopurinol, febuxostat).
• Hereditary renal hypouricemia (SLC22A12 or SLC2A9 mutations).

Treatment Options

There is no cure for the enzymatic defect; management focuses on preventing stone formation and preserving kidney function.

Dietary Measures

• Low‑purine diet – limit red meat, organ meats, sardines, and high‑purine legumes.
• High fluid intake – aim for >2.5 L/day of urine‑producing fluids (water, low‑calorie beverages) to keep urinary xanthine concentration below stone‑forming levels.
• Avoid fructose‑rich drinks – fructose can increase purine synthesis.

Pharmacologic Strategies

• Alkalinizing agents (e.g., potassium citrate) – raise urinary pH, slightly improving xanthine solubility.
• Allopurinol – paradoxically can reduce xanthine production by inhibiting upstream steps, but is not routinely recommended because it may worsen stone burden; only used under specialist supervision.

Surgical/Procedural Interventions

• Extracorporeal shock wave lithotripsy (ESWL) – effective for small xanthine stones.
• Ureteroscopic laser lithotripsy – for stones resistant to ESWL.
• Percutaneous nephrolithotomy (PCNL) – for large or staghorn stones.

Monitoring

Regular follow‑up every 6–12 months with serum uric acid, renal function tests, and ultrasound or low‑dose CT to assess stone burden.

Living with Xanthine Dehydrogenase Deficiency

Daily Management Tips

• Carry a water bottle and set reminders to drink throughout the day.
• Track fluid intake with a smartphone app; aim for at least 30 mL/kg body weight daily.
• Plan meals that emphasize low‑purine foods such as dairy, eggs, most vegetables, and fruits.
• Limit intake of high‑purine snacks (e.g., dried meats, anchovies).
• Maintain a urinary pH between 6.5–7.0; some home test strips are available.
• Inform your dentist and surgeon that you have low uric acid; certain anesthesia drugs can affect purine metabolism.

Psychosocial Support

Because the condition is rare, connecting with patient groups (e.g., Rare Kidney Stone Foundation) can reduce isolation. Genetic counseling is recommended for affected individuals and their families.

Prevention

Since XDH deficiency is congenital, primary prevention is not possible. However, secondary prevention of complications is achievable:

• Early detection through family screening when a mutation is known.
• Adherence to hydration and dietary recommendations to prevent stone formation.
• Prompt treatment of urinary infections to avoid ascending kidney damage.
• Avoid medications that further reduce uric acid (e.g., high‑dose allopurinol) unless specifically prescribed.

Complications

If left unmanaged, the following can occur:

• Recurrent xanthine stones leading to hydronephrosis, chronic pain, or acute obstruction.
• Chronic kidney disease (CKD) – progressive decline in glomerular filtration rate (GFR) in up to 30 % of documented cases.
• Urinary tract infections – may become recurrent or resistant.
• Renal failure – rare but reported in adult patients with longstanding stone disease.
• Severe hypouricemia‑related complications – rare events such as exercise‑induced acute kidney injury.

When to Seek Emergency Care

References

• Mayo Clinic. “Xanthinuria.” https://www.mayoclinic.org
• National Institutes of Health (NIH). “Xanthinuria Type I and II.” https://www.ncbi.nlm.nih.gov
• Cleveland Clinic. “Kidney Stones – Types and Treatment.” https://my.clevelandclinic.org
• World Health Organization. “Guidelines for the Management of Rare Metabolic Disorders.” 2021.
• American Urological Association. “Guideline for the Management of Staghorn Calculi.” 2022.

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