Alkaptonuria

Alkaptonuria is an inherited disorder in which deficient homogentisate 1,2-dioxygenase causes homogentisic acid to accumulate. Its oxidation darkens urine and can deposit pigment in connective tissues, damaging joints and heart valves.

Homogentisate 1,2-dioxygenase: An enzyme that breaks down homogentisic acid in the tyrosine degradation pathway. Reduced activity of this enzyme causes homogentisic acid to accumulate.
Urine organic acid analysis: Laboratory measurement of organic acids in urine, commonly using gas or liquid chromatography with mass spectrometry. It can quantify the markedly elevated homogentisic acid that confirms alkaptonuria.
Alkaptonuria in childhood: The early-life presentation of alkaptonuria, often marked by urine that darkens after standing. Urine discoloration may be the first noticeable sign, despite few other symptoms.
Autosomal recessive inheritance: A pattern in which a condition occurs when both copies of a gene carry disease-causing variants. Alkaptonuria typically develops when pathogenic HGD variants are inherited from both parents.
Spinal stenosis: Narrowing of the spinal canal that can compress nerves or the spinal cord. Ochronotic degeneration of spinal structures can contribute to nerve compression and pain.
Tyrosine metabolism: The biochemical pathways that synthesize and degrade the amino acid tyrosine. Alkaptonuria interrupts tyrosine breakdown downstream of homogentisic acid production.
Nitisinone: A drug that inhibits 4-hydroxyphenylpyruvate dioxygenase and reduces homogentisic acid production. It lowers the precursor supply responsible for tissue accumulation.
Ochronotic arthropathy: Degenerative joint disease caused by ochronotic pigment deposition in cartilage and other connective tissues. It explains the characteristic premature spinal and large-joint damage.
HGD gene: The gene encoding homogentisate 1,2-dioxygenase. Pathogenic variants in HGD account for the enzyme deficiency underlying alkaptonuria.
Chronic pain: Pain that persists or recurs over an extended period. Progressive joint and spinal damage can impose a lasting pain burden.

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