Hawkinsinuria

Hawkinsinuria is an inborn error of tyrosine metabolism characterized by failure to thrive, persistent metabolic acidosis, fine and sparse hair, and excretion of the unusual cyclic amino acid metabolite, hawkinsin ((2-l-cystein-S-yl, 4-dihydroxycyclohex-5-en-1-yl)acetic acid), in the urine.

Epidemiology

The prevalence is unknown, but the disease appears to be very rare with only a small number of affected families reported in the literature.

Clinical description

Symptoms manifest in infants fed on formula or cow's milk or after weaning from breast milk.

Etiology

The disorder is transmitted as an autosomal dominant trait and is caused by an A33T mutation in 4-hydroxyphenylpyruvic acid dioxygenase (4-HPPD), an enzyme that catalyses the conversion of hydroxyphenylpyruvate to homogentisate.

Diagnostic methods

The diagnosis is confirmed by detection of characteristic tyrosine metabolites by organic acid analysis of the urine.

Management and treatment

Patients are treated with ascorbic acid and a low-protein diet (in particular, restricted phenylalanine and tyrosine intake). On this diet, the patients grow normally and the metabolic acidosis resolves.

Prognosis

The prognosis for hawkinsinuria patients is good: although patients continue to excrete hawkinsin in their urine, the symptoms improve significantly after the first year of life and the children appear to be asymptomatic by the time they reach late childhood.