Multidrug resistance 3 gene mutation 1712delT and estrogen receptor alpha gene polymorphisms in Finnish women with obstetric cholestasis

Functional Characterization of ABCB4 Mutations Found in Low Phospholipid-Associated Cholelithiasis (LPAC)

Multidrug resistance 3 (MDR3) is expressed on the canalicular membrane of the hepatocytes and plays an important role in protecting the liver from bile acids. Altered ABCB4 gene expression can lead to a rare hepatic disease, low phospholipid-associated cholelithiasis (LPAC). In this study, we characterized 3 ABCB4 mutations in LPAC patients using various in vitro assay systems. We first measured the ability of each mutant to transport paclitaxel and then the mechanisms by which these mutations might change MDR3 transport activity were determined using immunoblotting, cell surface protein biotinylation, and immunofluorescence. Through a membrane vesicular transport assay, we observed that the uptake of paclitaxel was significantly reduced in membrane vesicles expressing 2 ABCB4 mutations, F165I and S320F. Both mutants showed significantly decreased total and cell surface MDR3 expression. These data suggest two missense mutations of ABCB4 may alter function of MDR3 and ultimately can be determined as LPAC-causing mutations.

Hepatocellular transporters and cholestasis

The secretion of bile is the result of active hepatocellular transport processes, most of which occur across the canalicular membrane of liver cells. Disturbance of the function and/or expression of these transporters leads to the intracellular accumulation of toxic bile acids, thereby promoting cholestatic liver cell injury. Genetically determined alterations of hepatobiliary transporter function are increasingly recognized as important risk factors for an individual's susceptibility to develop cholestasis. It has become evident that, besides the established pathogenic role of mutations in canalicular transporter genes in progressive and benign forms of familial intrahepatic cholestasis, genetics may also play an important role in acquired cholestatic syndromes, such as intrahepatic cholestasis of pregnancy or drug-induced cholestasis. This overview summarizes the physiologic function and regulation of human hepatobiliary transport systems and discusses the impact of their genetic variations for the pathophysiology of different cholestatic syndromes.

Sequence analysis of bile salt export pump (ABCB11) and multidrug resistance p-glycoprotein 3 (ABCB4, MDR3) in patients with intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy (ICP) is a liver disorder associated with increased risk of intrauterine fetal death and prematurity. There is increasing evidence that genetically determined dysfunction in the canalicular ABC transporters bile salt export pump (BSEP, ABCB11) and multidrug resistance protein 3 (MDR3, ABCB4) might be risk factors for ICP development. This study aimed to (i). describe the extent of genetic variability in BSEP and MDR3 in ICP and (ii). identify new disease-causing mutations. Twenty-one women with ICP and 40 women with uneventful pregnancies were recruited between April 2001 and April 2003. Sequencing of BSEP and MDR3 spanned 8-10 kb per gene and comprised the promoter region and 100-350 bp of the flanking intronic region around each exon. DNA sequencing of polymerase chain reaction fragments was performed on an ABI3700 capillary sequencer. MDR3 promoter activity of promoter constructs carrying different ICP-specific mutations was studied using reporter assays. A total of 37 and 51 variant sites were detected in BSEP and MDR3, respectively. Three non-synonymous sites in codons for evolutionarily conserved amino acids were specific for the ICP collective (BSEP, N591S; MDR3, S320F and G762E). Furthermore, four ICP-specific splicing mutations were detected in MDR3 [intron 21, G(+1)A; intron 25, G(+5)C and C(-3)G; and intron 26, T(+2)A]. Activity of the mutated MDR3 promoter was similar to that observed for the wild-type promoter. Our data further support an involvement of MDR3 genetic variation in the pathogenesis of ICP, whereas analysis of BSEP sequence variation indicates that this gene is probably less important for the development of pregnancy-associated cholestasis.

Cholestatic syndromes

Further insights into the cellular and molecular mechanisms underlying hepatobiliary transport function and its regulation now permit a better understanding of the pathogenesis and treatment options of cholestatic liver diseases. Identification of the molecular basis of hereditary cholestatic syndromes will result in an improved diagnosis and management of these conditions. New insights into the pathogenesis of extrahepatic manifestations of cholestasis (eg, pruritus) have facilitated new treatment strategies. Important new studies have been published about the pathogenesis, clinical features, diagnosis, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, total parenteral nutrition-induced cholestasis, drug-induced cholestasis, and viral cholestatic syndromes.

Pharmacogenetics of hepatocellular transporters

One of the main functions of the liver is the production of bile and the biliary secretion of endogenous and exogenous substances, including drugs and drug metabolites. Bile formation is a complex sequence of cellular events, which involves uptake of bile constituents and xenobiotics on the basolateral (sinusoidal) plasma membrane of hepatocytes and secretion of cholephilic compounds across the apical (canalicular) membrane. These uptake and efflux processes are maintained by distinct transport systems expressed at the two polar surface domains of liver cells. Any functional disturbance of these canalicular transport systems can lead to cholestatic liver disease, which is associated with intracellular accumulation of toxic bile constituents and consecutive cholestatic liver cell damage. Interaction of drugs with hepatobiliary transport systems is increasingly recognized as cause of acquired cholestatic syndromes. Thereby, genetically determined alterations of hepatobiliary transporter functions are important risk factors for an individual's susceptibility to develop cholestasis. Especially, mutations in canalicular transporter genes can cause certain forms of hereditary cholestatic liver disease, including progressive familial intrahepatic cholestasis or intrahepatic cholestasis of pregnancy. In addition, systematic genetic screenings have discovered numerous single nucleotide polymorphisms in hepatobiliary transporter genes that lead to amino acid exchanges in the encoded proteins. However, the functional consequences and the clinical relevance of most of these polymorphisms remain to be defined. This overview summarizes the physiological function of human hepatobiliary transport systems and discusses the impact of their genetic variations for the pathophysiology of cholestatic syndromes and the pharmacogenetics of drug-induced cholestasis.

Intrahepatic Cholestasis of Pregnancy

Intrahepatic cholestasis of pregnancy (or obstetric cholestasis) is a liver disorder that occurs in late pregnancy. Despite the potential adverse maternal and fetal/neonatal outcomes, cholestasis of pregnancy is often neglected and treated expectantly. More research is needed to improve the molecular and genetic understanding of the disease and to define a safe and effective medical treatment that improves clinical outcome. Ursodeoxycholic acid is considered to be a safe treatment option in the third trimester, but further randomized controlled trials are needed before ursodeoxycholic acid treatment can be generally recommended. Ursodeoxycholic acid is preferentially administered to patients with severe cholestasis (onset before week 33 or serum bile acid levels > 70 mmol/L) or to patients with a history of sudden fetal death, while maintaining close obstetric and regular biochemical surveillance (transaminases, bilirubin, and bile acid levels). Ursodeoxycholic acid can decrease pruritus and ameliorate liver tests, but effects on obstetric complications are ambiguous. S-Adenosylmethionine, dexamethasone, and cholestyramine can provide some relief of itching. Because none of these drugs have been shown to be harmful to mother or fetus, the individual woman and her clinician may decide whether to try one of the treatments described.