Hereditary xanthinuria

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Hereditary xanthinuria is a condition that most often affects the kidneys. It is characterized by high levels of a compound called xanthine and very low levels of another compound called uric acid in the blood and urine. The excess xanthine can accumulate in the kidneys and other tissues. In the kidneys, xanthine forms tiny crystals that occasionally build up to create kidney stones. These stones can impair kidney function and ultimately cause kidney failure. Related signs and symptoms can include abdominal pain, recurrent urinary tract infections, and blood in the urine (hematuria). Less commonly, xanthine crystals build up in the muscles, causing pain and cramping. In some people with hereditary xanthinuria, the condition does not cause any health problems.

Researchers have described two major forms of hereditary xanthinuria, types I and II. The types are distinguished by the enzymes involved; they have the same signs and symptoms.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Hereditary xanthinuria type I is caused by mutations in the XDH gene. This gene provides instructions for making an enzyme called xanthine dehydrogenase. This enzyme is involved in the normal breakdown of purines, which are building blocks of DNA and its chemical cousin, RNA. Specifically, xanthine dehydrogenase carries out the final two steps in the process, including the conversion of xanthine to uric acid (which is excreted in urine and feces). Mutations in the XDH gene reduce or eliminate the activity of xanthine dehydrogenase. As a result, the enzyme is not available to help carry out the last two steps of purine breakdown. Because xanthine is not converted to uric acid, affected individuals have high levels of xanthine and very low levels of uric acid in their blood and urine. The excess xanthine can cause damage to the kidneys and other tissues.

Hereditary xanthinuria type II results from mutations in the MOCOS gene. This gene provides instructions for making an enzyme called molybdenum cofactor sulfurase. This enzyme is necessary for the normal function of xanthine dehydrogenase, described above, and another enzyme called aldehyde oxidase. Mutations in the MOCOS gene prevent xanthine dehydrogenase and aldehyde oxidase from being turned on (activated). The loss of xanthine dehydrogenase activity prevents the conversion of xanthine to uric acid, leading to an accumulation of xanthine in the kidneys and other tissues. The loss of aldehyde oxidase activity does not appear to cause any health problems.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

The combined incidence of hereditary xanthinuria types I and II is estimated to be about 1 in 69,000 people worldwide. However, researchers suspect that the true incidence may be higher because some affected individuals have no symptoms and are never diagnosed with the condition. Hereditary xanthinuria appears to be more common in people of Mediterranean or Middle Eastern ancestry. About 150 cases of this condition have been reported in the medical literature.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Decreased urinary urate · Very frequent (99-80%)

Decreased concentration of urate in the urine.

Hypouricemia · Very frequent (99-80%)

The concentration of uric acid in the blood circulation is below the lower limit of normal.

Aldehyde oxidase deficiency · Frequent (79-30%)

A reduction in aldehyde oxidase level.

Crystalluria · Frequent (79-30%)

The presence of crystals in the urine.

Hyperxanthinemia · Frequent (79-30%)

An increased level of xanthine in the blood circulation.

Increased urinary hypoxanthine · Frequent (79-30%)

The concentration of hypoxanthine in the urine, normalized for urine concentration, is above the upper limit of normal.

Reduced xanthine dehydrogenase activity · Frequent (79-30%)

An abnormal reduction in xanthine dehydrogenase level.

Sulfite oxidase deficiency · Frequent (79-30%)

Abnormally reduced sulfite oxidase level.

Other findings in the same source

From: Orphanet

Additional reported features include Uric acid nephrolithiasis (Frequent (79-30%)); Xanthine nephrolithiasis (Frequent (79-30%)); Xanthinuria (Frequent (79-30%)); Acute kidney injury (Occasional (29-5%)); Chronic kidney disease (Occasional (29-5%)); Flank pain (Occasional (29-5%)); Hematuria (Occasional (29-5%)); Muscle spasm (Occasional (29-5%)); Myalgia (Occasional (29-5%)); Myopathy (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Hereditary xanthinuria is Nephrology, with a nephrologist as the relevant type of clinician. General physician / Family Medicine; paediatrician for children. Referral depends on symptoms.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• How is kidney function being assessed over time?
• Are any current medicines or supplements relevant to kidney safety?
• Is there an individual recommendation about fluids, diet or blood pressure?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Hereditary xanthinuria. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.