The familial unconjugated hyperbilirubinemias

Assessment of UGT polymorphisms and neonatal jaundice

Elevation of the serum bilirubin level is a common, if not universal, finding during the first week of life. This can be a transient phenomenon that resolves spontaneously or can signify a serious or even life-threatening condition. There are many causes of hyperbilirubinemia and related therapeutic and prognostic implications. The diseases in which there is a primary disorder of the metabolism of bilirubin will be reviewed regarding their clinical presentation, pathophysiology, diagnosis, and treatment. These disorders-Gilbert's syndrome and Crigler-Najjar Syndrome-both involve abnormalities in bilirubin conjugation secondary to deficiency of bilirubin uridine diphosphate glucuronosyltransferase. The purpose of this article is to review the current understanding of the genetic polymorphisms that result in these diseases and discuss recent advances in diagnosis and treatment.

Combined test for UGT1A1 -3279T-->G and A(TA)nTAA polymorphisms best predicts Gilbert's syndrome in Italian pediatric patients

Gilbert's syndrome is a common hereditary chronic or recurrent, mild unconjugated hyperbilirubinemia. Polymorphisms in the bilirubin uridine diphosphate glucuronosyl transferase gene (UGT1A1) causing a decreased enzyme activity are associated with susceptibility to the syndrome. Homozygosity for TA(7) allele of the A(TA)(n)TAA promoter polymorphism is found in the majority of Caucasian patients. We sought to investigate the role of three UGT1A1 polymorphisms (A[TA](n)TAA, -3279T-->G, and G71R) in the susceptibility to Gilbert's syndrome in 53 Italian pediatric subjects compared to 83 unaffected controls. Carriage of two TA(n) risk alleles (TA(7) and TA(8)) and -3279G homozygosity were similarly associated with hyperbilirubinemia (odds ratio [OR] = 11.59, 95% confidence interval [CI] = 4.80-27.98; p < 0.001, and OR = 11.51, 95% CI = 5.06-26.19; p < 0.001, respectively). Homozygosity for both TA7 and -3279G was associated with the highest relative risk estimate (OR = 19.23, 95% CI = 7.34-50.4; p < 0.001), but a significant association was found also for TA7 heterozygosity combined with -3279G/G genotype (OR = 7.98, 95% CI = 2.54-25.06; p < 0.001). The G71R variant was found only in two controls. Our results demonstrate that genotyping of both UGT1A1 A(TA)(n)TAA and -3279T-->G polymorphisms best defines genetic susceptibility to Gilbert's syndrome in Caucasian pediatric patients, and the TA7 heterozygous genotype combined with homozygosity for the -3279G allele can also be associated with pediatric mild hyperbilirubinemia.

Review article: new insights into the mechanisms of hepatic transport and bile secretion

The transport processes responsible for bile flow are reviewed. Canalicular bile acid-dependent flow is the result of active transport of bile acids by the hepatocyte into bile canaliculi. Bile acids are taken up by at least two transport systems whose mRNA have been expressed in Xenopus oocytes: (i) a Na(+)-dependent system, named NTCP, and (ii) a Na(+)- independent system, named OATP. Bile acids are then secreted into bile by two other transport systems, an ATP-dependent system and an 'electrogenic' voltage-dependent system. It is not known whether these two systems are mediated by the same protein or by two different proteins. Canalicular bile acid-independent flow is mainly the result of the secretion of glutathione into bile. The canalicular membrane also contains several proteins of the multi drug resistance (MDR) family. MDRI is responsible for biliary secretion of cationic drugs. MDR3 (mdr 2 in mice) plays a major role in the secretion of phospholipids. A third MDR-related protein has been shown recently to be the canalicular carrier of organic anions, such as bilirubin and dyes (the canalicular multiple organic anion transporter, or cMOAT). Biliary epithelial cells secrete a bicarbonate-rich solution, mostly in response to secretin. This secretion depends upon the presence, on the apical membrane of these cells of the CFTR, a chloride channel activated by cAMP and of a chloride/bicarbonate exchanger. Knowledge of these transport systems should allow a better understanding of the mechanisms involved in cholestasis.