Dual polymorphisms in UDP-glucuronosyltransferases 1A1 and 1A6: a novel mechanism for hyperserotoninaemia in Gilbert's syndrome mimicking carcinoid syndrome?

Gilbert's syndrome is a common inherited disorder, in which genetic defects in uridine diphosphate-glucuronosyltransferase 1A1 lead to deficient glucuronidation of bilirubin and hence hyperbilirubinaemia. Although usually considered asymptomatic, Gilbert's syndrome can be associated with gastrointestinal and psychiatric symptoms unexplained by the metabolic defect. Genetic polymorphism of a closely related enzyme, uridine diphosphate-glucuronosyltransferase 1A6, results in altered metabolism and elimination of serotonin. On the basis of a case of hyperserotoninaemia in the absence of a detectable carcinoid tumour in a patient with Gilbert's syndrome, who presented with a history of night sweats, flushing, abdominal discomfort and intermittent diarrhoea, we propose that in a subgroup of Gilbert's syndrome patients, homozygocity for dual uridine diphosphate-glucuronosyltransferase 1A1 and uridine diphosphate-glucuronosyltransferase 1A6 polymorphisms may lead to combined hyperbilirubinaemia and hyperserotoninaemia. The latter may account for symptoms experienced by patients with Gilbert's syndrome hitherto considered unrelated to, or unexplainable by, the known defect in bilirubin metabolism.