Hawkinsinuria

Hawkinsinuria is an extremely rare inherited disorder of tyrosine metabolism caused by a defect in the enzyme 4-hydroxyphenylpyruvate dioxygenase (4-HPPD), encoded by the HPD gene. Unlike most other inborn errors of tyrosine catabolism, hawkinsinuria follows an autosomal dominant inheritance pattern. The condition is named after the Hawkins family, in whom it was first described. The enzymatic defect leads to the accumulation of an unusual amino acid metabolite called hawkinsin (2-L-cysteinyl-S-yl-1,4-dihydroxycyclohex-5-en-1-yl acetic acid) in the urine, along with other abnormal metabolites including 4-hydroxycyclohexylacetic acid. The disorder primarily affects the metabolic system, specifically the tyrosine degradation pathway in the liver. In infancy, affected individuals may present with failure to thrive, metabolic acidosis, and elevated blood tyrosine levels, particularly when exposed to a high-protein diet or standard infant formula. Symptoms in infancy can include poor weight gain, lethargy, and a characteristic body odor resembling a swimming pool (due to abnormal metabolites). Interestingly, many affected adults are clinically asymptomatic, with the biochemical abnormality (hawkinsinuria) persisting but without significant clinical consequences. Treatment during infancy primarily involves dietary management, specifically the restriction of phenylalanine and tyrosine intake. Breastfeeding has been noted to be protective, as breast milk has a relatively low protein content compared to standard infant formulas. With appropriate dietary management during the vulnerable infantile period, affected children typically improve and may become clinically well, though they continue to excrete hawkinsin in their urine. Supplementation with vitamin C (ascorbic acid) has also been reported to be beneficial, as it may help support residual enzyme activity. Long-term prognosis appears to be favorable, with most individuals leading normal lives after the critical infantile period.

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