Heterozygous non-sense mutation of the MDR3 gene in familial intrahepatic cholestasis of pregnancy

Progressive familial intrahepatic cholestasis: genetic disorders of biliary transporters

Progressive familial intrahepatic cholestasis types 1, 2 and 3 are childhood diseases of the liver. Benign recurrent intrahepatic cholestasis is predominantly an adult form with similar clinical symptoms that spontaneously resolve. These genetic disorders have significantly helped to unravel the basic mechanisms of the canalicular bile transport processes. Progressive familial intrahepatic cholestasis type 1 involves a gene also linked to benign recurrent intrahepatic cholestasis. The gene codes for an aminophospholipid translocase protein that maintains the integrity of the membrane. How a mutation in this protein causes cholestasis is unknown but is thought to involve the enterohepatic recirculation of bile acids. Progressive familial intrahepatic cholestasis types 2 and 3 involve the canalicular bile salt export pump and a phospholipid translocase, respectively, both of which are fundamental to bile secretion. This review covers the clinical manifestations, genetics, treatment and mechanism of each disease.

Expression and localization of hepatobiliary transport proteins in progressive familial intrahepatic cholestasis

Mutations of the bile salt export pump (BSEP) or the multidrug resistance P-glycoprotein 3 (MDR3) are linked to impaired bile salt homeostasis and lead to progressive familial intrahepatic cholestasis (PFIC)-2 and -3, respectively. The regulation of bile salt transporters in PFIC is not known. Expression of hepatobiliary transporters in livers of ten patients with a PFIC phenotype was studied by quantitative reverse transcription polymerase chain reaction, Western blotting, and immunofluorescence microscopy. PFIC was diagnosed by clinical and laboratory findings. All patients could be assigned to PFIC-2 or PFIC-3 by the use of BSEP- and MDR3-specific antibodies and by MDR3 gene-sequencing. Whereas in all PFIC-2 patients, BSEP immunoreactivity was absent from the canalicular membrane, in three PFIC-3 livers, canalicular MDR3 immunoreactivity was detectable. Serum bile salts were elevated to 276 +/- 233 and to 221 +/- 109 micromol/L in PFIC-2 and PFIC-3, respectively. Organic anion transporting polypeptide OATP1B1, OATP1B3, and MRP2 mRNA and protein levels were reduced, whereas sodium taurocholate cotransporting polypeptide (NTCP) was only reduced at the protein level, suggesting a posttranscriptional NTCP regulation. Whereas MRP3 mRNA and protein were not significantly altered, MRP4 messenger RNA and protein were significantly increased in PFIC. In conclusion, PFIC-2 may be reliably diagnosed by immunofluorescence, whereas the diagnosis of PFIC-3 requires gene-sequencing. Several mechanisms may contribute to elevated plasma bile salts in PFIC: reduced bile salt uptake via NTCP, OATP1B1, and OATP1B3, decreased BSEP-dependent secretion into bile, and increased transport back into plasma by MRP4. Upregulation of MRP4, but not of MRP3, might represent an important escape mechanism for bile salt extrusion in PFIC.

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.

Applications of cytotoxicity assays and pre-lethal mechanistic assays for assessment of human hepatotoxicity potential

While drug toxicity (especially hepatotoxicity) is the most frequent reason cited for withdrawal of an approved drug, no simple solution exists to adequately predict such adverse events. Simple cytotoxicity assays in HepG2 cells are relatively insensitive to human hepatotoxic drugs in a retrospective analysis of marketed pharmaceuticals. In comparison, a panel of pre-lethal mechanistic cellular assays hold the promise to deliver a more sensitive approach to detect endpoint-specific drug toxicities. The panel of assays covered by this review includes steatosis, cholestasis, phospholipidosis, reactive intermediates, mitochondria membrane function, oxidative stress, and drug interactions. In addition, the use of metabolically competent cells or the introduction of major human hepatocytes in these in vitro studies allow a more complete picture of potential drug side effect. Since inter-individual therapeutic index (TI) may differ from patient to patient, the rational use of one or more of these cellular assay and targeted in vivo exposure data may allow pharmaceutical scientists to select drug candidates with a higher TI potential in the drug discovery phase.

Causality assessment in drug-induced hepatotoxicity

Drugs are currently an important cause of liver disease, ranked as the most frequent reason for acute liver failure. Despite recent advances in knowledge of the mechanisms implicated in drug-induced hepatocellular damage and cholestasis, as well as the identification of several risk factors, the diagnosis of hepatotoxicity remains a difficult task because specific tests are not available. In a step-by-step approach, the incrimination of a drug in liver symptoms requires a high degree of suspicion on the part of the physician, temporal eligibility, awareness of the drug's hepatotoxic potential, the exclusion of alternative causes of liver damage, and the ability to detect the presence of subtle data that favour a toxic aetiology. Ultimately, the use of diagnostic algorithms may add consistency to the diagnostic process either by translating the suspicion into a quantitative score or by providing a framework that emphasises the features that merit attention in cases of suspected hepatic adverse reactions.

Inborn errors of biliary canalicular transport systems

Cholestatic syndromes are inborn or acquired disorders of bile formation. In recent years, several inherited cholestatic syndromes were characterized at the molecular level: progressive familial intrahepatic cholestasis (PFIC) and benign recurrent intrahepatic cholestasis (BRIC). Both PFIC and BRIC were divided phenotypically in distinct subtypes; however, at the genotype level, these clinical entities overlap. PFIC starts in early childhood and progresses toward liver cirrhosis, which often requires liver transplantation within the first decade of life. The diagnosis of PFIC is usually made on the basis of clinical and laboratory findings but needs to be confirmed by genetic and histological analysis. Only recently was it recognized that BRIC, which was estimated as a milder form of PFIC-1, may be caused by more than one gene.

Liver disease in pregnancy

Liver dysfunction during pregnancy can be caused by conditions that are specific to pregnancy or by liver diseases that are not related to pregnancy itself. This review attempts to summarize the epidemiology, pathophysiology, and management of the different pregnancy-related liver diseases, and to review different liver diseases not related to pregnancy and how they may affect or be effected by pregnancy. Some of the liver diseases specific to pregnancy can cause significant morbidity and mortality both to the mother and to the fetus, while most of the liver diseases not specific to pregnancy do not have a deleterious effect on the pregnancy itself.

Progressive intrahepatic cholestasis: mechanisms, diagnosis and therapy

Progressive intrahepatic cholestasis (PIHC, also known as progressive familial intrahepatic cholestasis) is a general term encompassing a devastating group of illnesses manifest by severe morbidity and potential mortality. By definition these diseases are characterized by persistent cholestasis that is the result of intra-hepatic rather than extra-hepatic pathology. Recent scientific advances have begun to clarify the molecular basis of many of these disorders. The morbidities of these diseases are primarily the result of profound cholestasis. This cholestasis is often associated with intractable pruritus, which leads to a very poor quality of life. Normal development and sleep are not possible for the affected individual and family dynamics are sometimes irreparably damaged. The cholestasis also leads to complications of fat soluble vitamin malabsorption including osteopenia and pathologic bone fractures, xeropthalmia, and peripheral neuropathy. End-stage liver disease and all of its attendant problems may develop in affected individuals by young adulthood. Optimal therapeutic approaches to PIHC are not well established and disease-specific approaches may be required.

Preeclampsia with abnormal liver function tests is associated with cholestasis in a subgroup of cases

OBJECTIVE

To investigate whether women with preeclampsia and abnormal liver function tests have raised serum bile acids.

DESIGN

Measurement of serum bile acids in serum specimens collected at the John Radcliffe Hospital, Oxford.

SETTING

Imperial College School of Medicine.

SAMPLE

Stored sera from 37 women with preeclampsia and abnormal liver function tests and from 19 controls.

METHODS

Enzymic total bile acid assay.

MAIN OUTCOME MEASURES

Total bile acid levels.

RESULTS

Women with preeclampsia and abnormal liver function tests had higher median bile acid levels than controls (5.7 vs. 3.2, p = 0.01). The reason for the raised median serum bile acid levels in the patient group is that three (8%) women with preeclampsia had markedly raised serum bile acids levels. There were no obvious clinical or biochemical features specific to these patients.

CONCLUSIONS

The pathological mechanisms causing hepatic impairment in some women with preeclampsia may predispose to cholestasis. As some women with preeclampsia and abnormal liver function complain of pruritus, we recommend checking the serum bile acids in this group of women. If these acids are raised the fetal prognosis may be adversely affected.

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.

BSEP and MDR3 haplotype structure in healthy Caucasians, primary biliary cirrhosis and primary sclerosing cholangitis

Primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC) are characterized by a cholestatic pattern of liver damage, also observed in hereditary or acquired dysfunction of the canalicular membrane transporters bile salt export pump (BSEP, ABCB11) and multidrug resistance protein type 3 (MDR3, ABCB4). Controversy exists whether a genetically determined dysfunction of BSEP and MDR3 plays a pathogenic role in PBC and PSC. Therefore, 149 healthy Caucasian control individuals (control group) were compared to 76 PBC and 46 PSC patients with respect to genetic variations in BSEP and MDR3. Sequencing spanned approximately 10,000 bp including promoter and coding regions as well as 50-350 bp of flanking intronic regions. In all, 46 and 45 variants were identified in BSEP and MDR3, respectively. No differences between the groups were detected either in the total number of variants (BSEP: control group: 37, PBC: 37, PSC: 31; and MDR3: control group: 35; PBC: 32, PSC: 30), or in the allele frequency of the common variable sites. Furthermore, there were no significant differences in haplotype distribution and linkage disequilibrium. In conclusion, this study provides an analysis of BSEP and MDR3 variant segregation and haplotype structure in a Caucasian population. Although an impact of rare variants on BSEP and MDR3 function cannot be ruled out, our data do not support a strong role of BSEP and MDR3 genetic variations in the pathogenesis of PBC and PSC.

Intrahepatic cholestasis of pregnancy

Patients with ICP should be considered to have a high-risk pregnancy. Once the diagnosis of ICP is suspected, usually because of generalized pruritus, it should be confirmed by liver function tests, and other causes of cholestasis should be ruled out. Treatment with UDCA is effective in ameliorating the cholestasis and is especially useful in severe forms or when there is a history of sudden fetal death in a previous pregnancy. The understanding of the pathogenesis of ICP has recently progressed as the result of the discovery of several defects in the MDR3 gene in isolated affected patients. More studies of this and other genes that regulate bile flow, linked with careful clinical observations to rule out unsuspected chronic liver disease not related to pregnancy, should lead to the discovery of the pathogenesis of this enigmatic disorder.

Enterohepatic bile salt transporters in normal physiology and liver disease

The vectorial transport of bile salts from blood into bile is essential for the generation of bile flow, solubilization of cholesterol in bile, and emulsification of lipids in the intestine. Major transport proteins involved in the enterohepatic circulation of bile salts include the hepatocellular bile salt export pump (BSEP, ABCB11), the apical sodium-dependent bile salt transporter (ASBT, SLC10A2) in cholangiocytes and enterocytes, the sodium-dependent hepatocyte bile salt uptake system NTCP (SLC10A1), the organic anion transporting polypeptides OATP-C (SLC21A6), OATP8 (SLC21A8) and OATP-A (SLC21A3), and the multidrug resistance protein MRP3 (ABCC3). Synthesis and transport of bile salts are intricately linked processes that undergo extensive feedback and feed-forward regulation by transcriptional and posttranscriptional mechanisms. A key regulator of hepatocellular bile salt homeostasis is the bile acid receptor/farnesoid X receptor FXR, which activates transcription of the BSEP and OATP8 genes and of the small heterodimer partner 1 (SHP). SHP is a transcriptional repressor that mediates bile acid-induced repression of the bile salt uptake systems rat Ntcp and human OATP-C. A nuclear receptor that activates rodent Oatp2 (Slc21a5) and human MRP2 (ABCC2) is the pregnane X receptor/steroid X receptor PXR/SXR. Intracellular trafficking and membrane insertion of bile salt transporters is regulated by lipid, protein, and extracellular signal-related kinases in response to physiologic stimuli such as cyclic adenosine monophosphate or taurocholate. Finally, dysfunction of individual bile salt transporters such as BSEP, on account of genetic mutations, steric inhibition, suppression of gene expression, or disturbed signaling, is an important cause of cholestatic liver disease.

Obstetric cholestasis with elevated gamma glutamyl transpeptidase: incidence, presentation and treatment

BACKGROUND

Obstetric cholestasis (OC) may cause severe pruritus in the mother and lead to fetal distress and stillbirth. The etiology of OC is multifactorial, but includes inherited dysfunction of bile canalicular transporters. One of these, multidrug resistant protein 3 (MDR3), a phospholipid transporter, when dysfunctional is associated with elevated levels of gamma glutamyl transpeptidase (GGT). The aim of the present study was to assess the incidence of OC associated with elevated GGT. We compared the natural history of a cholestatic pregnancy and the efficacy of ursodeoxycholic acid (URSO) in OC patients grouped according to a normal or raised GGT level.

METHODS

Eighty-one patients with OC were analyzed. OC was diagnosed in patients with pruritus and elevated serum bile acids (SBA). Fifty-seven consenting volunteer patients (70%) were treated with URSO.

RESULTS

Elevated GGT at presentation was found in 21 patients (30%) and was associated with significantly higher serum levels of aspartate transaminase (AST), bilirubin (BIL) and SBA. OC presented at approximately the same gestation week in both groups of patients. In patients not treated with URSO, liver function tests (LFT) showed no significant change from the time of diagnosis to delivery. Patients from both groups responded to URSO with significant improvement in their AST and alanine aminotransferase (ALT) levels, but SBA fell significantly only in the normal GGT group.

CONCLUSIONS

An elevated GGT occurs in less than one-third of patients with OC in the UK and, when present, is associated with greater impairment of LFT, but no difference in gestational age at onset. Treatment with URSO appears to be safe and significantly improves LFT in patients with OC, with the exception of SBA in the high GGT group.

Genetic cholestasis, causes and consequences for hepatobiliary transport

Bile salts take part in an efficient enterohepatic circulation in which most of the secreted bile salts are reclaimed by absorption in the terminal ileum. In the liver, the sodium-dependent taurocholate transporter at the basolateral (sinusoidal) membrane and the bile salt export pump at the canalicular membrane mediate hepatic uptake and hepatobiliary secretion of bile salts. Canalicular secretion is the driving force for the enterohepatic cycling of bile salts and most genetic diseases are caused by defects of canalicular secretion. Impairment of bile flow leads to adaptive changes in the expression of transporter proteins and enzymes of the cytochrome P-450 system involved in the metabolism of cholesterol and bile acids. Bile salts act as ligands for transcription factors. As such, they stimulate or inhibit the transcription of genes encoding transporters and enzymes involved in their own metabolism. Together these changes appear to serve mainly a hepatoprotective function. Progressive familial intrahepatic cholestasis (PFIC) results from mutations in various genes encoding hepatobiliary transport proteins. Mutations in the FIC1 gene cause relapsing or permanent cholestasis. The relapsing type of cholestasis is called benign recurrent intrahepatic cholestasis, the permanent type of cholestasis PFIC type 1. PFIC type 2 results from mutations in the bile salt export pump (BSEP) gene. This is associated with permanent cholestasis since birth. Serum gamma-glutamyltransferase (gamma-GT) activity is low to normal in PFIC types 1 and 2. Bile diversion procedures, causing a decreased bile salt pool, have a beneficial effect in a number of patients with these diseases. However, liver transplantation is often necessary. PFIC type 3 is caused by mutations in the MDR3 gene. MDR3 is a phospholipid translocator in the canalicular membrane. Because of the inability to secrete phospholipids, patients with PFIC type 3 produce bile acid-rich toxic bile that damages the intrahepatic bile ducts. Serum gamma-GT activity is elevated in these patients. Ursodeoxycholic acid therapy is useful for patients with a partial defect. Liver transplantation is a more definitive therapy for these patients.

Disorders of bile formation and biliary transport

A wide range of cholestatic liver diseases result from various primary defects in bile formation. Clinical features include jaundice, pruritus, failure to thrive, fat malabsorption, cholelithiasis, and variably progressive cirrhosis. Accurate diagnosis of these disorders is essential for determination of prognosis and selection of the most appropriate therapies. Severe genetic defects in canalicular bile acid and phospholipid excretion lead to progressive liver disease that often requires liver transplantation. Defects in bile acid biosynthesis and aminophospholipid transport may be responsive to medical or non-transplant surgical approaches.

ABCB4 gene mutation-associated cholelithiasis in adults

BACKGROUND & AIMS

We recently put forward arguments in favor of ABCB4 gene (adenosine triphosphate-binding cassette, subfamily B, member 4) defects as a risk factor for symptomatic cholelithiasis in adults. In this study, we characterized ABCB4 gene mutations in a series of patients with symptomatic cholelithiasis to determine the genetic basis and the clinical phenotype of ABCB4 gene mutation-associated cholelithiasis.

METHODS

We analyzed the entire ABCB4 gene coding sequences in a first group of 32 patients who had a clinical history compatible with the syndrome previously described, in a second group of 28 patients who presented with a classic gallstone disease that justified a cholecystectomy, and in a third group of 33 patients without a history of cholelithiasis.

RESULTS

We identified both heterozygous and homozygous ABCB4 gene point mutations in 18 of 32 (56%) patients who presented with clinical criteria of the syndrome, whereas no mutation was detected in the 2 other groups of patients (P < 0.001). Three independent clinical features were strongly associated with point mutations: recurrence of symptoms after cholecystectomy (odds ratio, 8.5); intrahepatic hyperechoic foci, intrahepatic sludge, or microlithiasis (odds ratio, 6.1); and age <40 years at the onset of symptoms (odds ratio, 3.0). ABCB4 gene point mutations were detected exclusively in the patients who showed 2 or 3 of these clinical features.

CONCLUSIONS

Our results show that ABCB4 gene mutations represent a major genetic risk factor in a symptomatic and recurring form of cholelithiasis in young adults.

A multidrug resistance 3 gene mutation causing cholelithiasis, cholestasis of pregnancy, and adulthood biliary cirrhosis

We describe a 47-year-old patient who developed cholelithiasis in adolescence, followed by recurrent intrahepatic cholestasis of pregnancy, and finally biliary cirrhosis in adulthood. In our patient, the consecutive presentation of the 3 mentioned disorders raised the suspicion of a defect of MDR3, the canalicular protein involved in the transport of phospatidylcholine to bile. Mutational analysis in our patient showed a heterozygous missense mutation of the MDR3 gene that has not been described previously, which occurs in exon 14 at codon 535, and results in the substitution of glycine for aspartic acid. Further analysis of 7 members of the family showed the same mutation in her daughter who, on follow-up, developed cholestasis of pregnancy and persisting high serum levels of gamma-glutamyl transpeptidase and alkaline phosphatase after delivery. Although biliary cirrhosis associated with MDR3 deficiency typically appears before the age of 25 years, in our case, the relatively mild MDR3 dysfunction allowed for a slower progression of the disease with established, well-advanced cirrhosis in the fifth decade of life. The present case, which accumulates the 3 clinical disorders assocaited with MDR3 deficiency, shows that this condition should be suspected not only in children or young people with high gamma-glutamyl transpeptidase cholestasis but also in middle-aged or older patients with chronic idiopathic cholestasis, especially when there is a previous history of cholestasis of pregnancy or juvenile cholelithiasis.

Specific dermatoses of pregnancy: an evidence-based systematic review

OBJECTIVE

We conducted an evidence-based systematic analysis of the literature on specific dermatoses of pregnancy.

STUDY DESIGN

The bibliographic databases MEDLINE and EMBASE were screened for studies and reports in all languages about herpes gestationis, pruritic urticarial papules and plaques of pregnancy, pruritic folliculitis of pregnancy, and prurigo of pregnancy from January 1962 to January 2002. As main index terms, including analogs and derivatives, we used the names of specific dermatoses of pregnancy. Intrahepatic cholestasis of pregnancy, not a primary dermatosis, was included herein because this disorder is associated with pregnancy and its secondary skin manifestations must be differentiated from specific dermatoses of pregnancy. Other sources were abstract books of symposia and congresses, theses, textbooks, monographs, reviews, editorials, letters to the editor, free or rapid communications, and the reference lists from all the articles that were retrieved. All articles selected for inclusion in this review were evaluated critically with regard to their impact factor and evidence-based contribution to this field, as measured by their citation index and impact factor of the journal in which they were published. Approximately 39% of articles met the selection criteria.

RESULTS

The clinical features and prognosis of the specific dermatoses of pregnancy have been delineated through a number of retrospective and cohort studies. The molecular biologic and immunogenetic properties of herpes gestationis, pruritic urticarial papules and plaques of pregnancy, and intrahepatic cholestasis of pregnancy have been further clarified. A meta-analysis in this review reveals a higher prevalence of multiple gestation pregnancy (11.7%) among patients with pruritic urticarial papules and plaques of pregnancy. Several investigations have unraveled the fetal complications in intrahepatic cholestasis of pregnancy and herpes gestationis. New treatment modalities in intrahepatic cholestasis of pregnancy (cholestyramine, ursodeoxycholic acid) and herpes gestationis (cyclosporin, intravenous immunoglobulin, and tetracyclines postpartum) have shown promise and warrant further evaluation.

CONCLUSION

During the past few decades, a significant amount of new data has provided new insights into the classification, pathogenesis, treatment, prognosis, and fetal risks that are associated with the specific dermatoses of pregnancy.

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.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

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.

Genetic background of cholesterol gallstone disease

Cholesterol gallstone formation is a multifactorial process involving a multitude of metabolic pathways. The primary pathogenic factor is hypersecretion of free cholesterol into bile. For people living in the Western Hemisphere, this is almost a normal condition, certainly in the elderly, which explains the very high incidence of gallstone disease. It is probably because the multifactorial background genes responsible for the high incidence have not yet been identified, despite the fact that genetic factors clearly play a role. Analysis of the many pathways involved in biliary cholesterol secretion reveals many potential candidates and considering the progress in unraveling the regulatory mechanisms of the responsible genes, identification of the primary gallstone genes will be successful in the near future.

Semi quantitative expression analysis of MDR3, FIC1, BSEP, OATP-A, OATP-C,OATP-D, OATP-E and NTCP gene transcripts in 1st and 3rd trimester human placenta

Using real time RT-PCR, we have detected expression of seven genes that influence bile acid transport,MDR3, FIC1, BSEP, OATP-A, OATP-C, OATP-D and OATP-E, in normal human placenta. With the exception of OATP-C and OATP-E these genes were found to be differentially expressed in 1st trimester and 3rd trimester placentae. MDR3 gene expression was found to be up regulated four fold in 3rd trimester placentae compared to 1st trimester, OATP-A gene expression was down regulated eight fold, OATP-D was down regulated 17 fold, while FIC1 expression was reduced by 33 fold in the 3rd trimester. OATP-C and BSEP gene expression was not detected in the 3rd trimester placenta, while low levels of transcripts were detected in the 1st trimester placentae. Transcripts of the hepatic sinusoidal bile acid transporter, NTCP, were not detected in placenta.

Understanding and controlling hepatobiliary function

Over the past decade, enormous progress has been made in identifying the mechanisms that underlie hepatobiliary excretion. A set of transport proteins mediates the canalicular transport of most important bile constituents. With the discovery of these transporter genes, the mechanism of bile formation could be partly elucidated and genetic defects caused by mutations in these genes identified. This progress is crucial not only for paediatric and adult hepatology, but also for pharmacology, because the characterization of these transport systems provides tools for the prediction of the pharmacokinetics of drugs. Indeed, there is a growing interest on the part of the pharmaceutical industry for research into transport systems in general and hepatobiliary secretion in particular. For all of these transporter genes, knockout mice have been bred that allow one to assess the in vivo function of each of these transporters with regard to their role in physiology and drug elimination.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

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

OBJECTIVE

To investigate the contribution of the estrogen receptor alpha (ERalpha) polymorphism in the development of obstetric cholestasis and to determine whether multidrug resistance 3 (MDR3) gene 1712delT mutation detected in French patients is also present in Finnish women with obstetric cholestasis.

STUDY DESIGN

In a retrospective case-control study, two ERalpha polymorphisms and MDR3 gene mutation 1712delT were investigated in healthy control women (N=47) and in women with diagnosis of obstetric cholestasis (N=57). PvuII and XbaI polymorphisms in ERalpha gene were evaluated in genomic DNA by using the polymerase chain reaction (PCR). In addition, the frequencies in the general population in our area are presented for comparison.

RESULTS

None of the ERalpha genotypes or alleles was significantly over-represented in obstetric cholestasis. When the two ERalpha gene polymorphisms were analyzed in parallel, six genotype combinations were recognized, and the distribution of these genotype combinations did not reveal statistically significant differences between the cases and controls (P=0.612). No patient or control was heterozygous or homozygous for the mutant allele in the MDR3 gene.

CONCLUSION

The present data indicate that polymorphism of the ERalpha and MDR3 genes 1712delT mutation are unlikely to play any significant role in obstetric cholestasis in affected Finnish women. Further work to identify explanatory factors is of particular interest.

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

OBJECTIVE

To investigate the contribution of the estrogen receptor alpha (ERalpha) polymorphism in the development of obstetric cholestasis and to determine whether multidrug resistance 3 (MDR3) gene 1712delT mutation detected in French patients is also present in Finnish women with obstetric cholestasis.

STUDY DESIGN

In a retrospective case-control study, two ERalpha polymorphisms and MDR3 gene mutation 1712delT were investigated in healthy control women (N = 47) and in women with diagnosis of obsteric cholestasis (N = 57). PvuII and XbaI polymorphisms in ERalpha gene were evaluated in genomic DNA by using the polymerase chain reaction (PCR). In addition, the frequencies in the general population in our area are presented for comparison.

RESULTS

None of the ERalpha genotypes or alleles was significantly over-represented in obstetric cholestasis. When the two ERalpha gene polymorphisms were analyzed in parallel, six genotype combinations were recognized, and the distribution of these genotype combinations did not reveal statistically significant differences between the cases and controls (P = 0.612). No patient or control was heterozygous or homozygous for the mutant allele in the MDR3 gene.

CONCLUSION

The present data indicate that polymorphism of the ERalpha gene and MDR3 gene 1712delT mutation are unlikely to play any significant role in obstetric cholestasis in affected Finnish women. Further work to identify explanatory factors is of particular interest.

Hereditary forms of intrahepatic cholestasis

Several genes that are mutated in hereditary forms of intrahepatic cholestasis have been identified or mapped, providing new insights into the process of enterohepatic bile acid circulation in health and disease and new tools with which to study this process. Murine models of several of these disorders have been generated. Unanticipated genetic heterogeneity has been identified.

Genetic defects in hepatobiliary transport

Bile formation, the exocrine function of the liver, represents a process that is unique to the hepatocyte as a polarized epithelial cell. The generation of bile flow is an osmotic process and largely depends on solute secretion by primary active transporters in the apical membrane of the hepatocyte. In recent years an impressive progress has been made in the discovery of these proteins, most of which belong to the family of ABC transporters. The number of identified ABC transporter genes has been exponentially increasing and the mammalian subfamily now counts at least 52. This development has been of crucial importance for the elucidation of the mechanism of bile formation, and it is therefore not surprising that the development in this field has run in parallel with the discovery of the ABC genes. With the identification of these transporter genes, the background of a number of inherited diseases, which are caused by mutations in these solute pumps, has now been elucidated. We now know that at least six primary active transporters are involved in canalicular secretion of biliary components (MDR1, MDR3, BSEP, MRP2, BCRP and FIC1). Four of these transporter genes are associated with inherited diseases. In this minireview we will shortly describe our present understanding of bile formation and the associated inherited defects.

Hepatic transport systems

The identification of the genes responsible for various genetic liver disorders lead to a better understanding of basic physiology of hepatic transport systems. In this review we focus on transport systems involved in the generation of bile and in the maintenance of copper homeostasis. Abnormal function of these transporters results in diseases like Wilson's disease, progressive familial cholestasis syndromes, Dubin-Johnson syndrome and cystic fibrosis. Beyond these well defined diseases, functional impairments of transport proteins may predispose to non-genetic diseases ranging from intrahepatic cholestasis of pregnancy to neurodegenerative disorders including Alzheimer's disease.

Progressive familial intrahepatic cholestasis with high gamma-glutamyltranspeptidase levels in Taiwanese infants: role of MDR3 gene defect?

MDR3 P-glycoprotein mediates canalicular phospholipid transport in hepatocytes. Defects in the MDR3 gene have been found to cause a subtype of progressive familial intrahepatic cholestasis (PFIC) with high gamma-glutamyltranspeptidase (GGT) levels. Affected children develop proliferation of biliary epithelium, portal inflammation, and biliary cirrhosis. The frequency of MDR3 mutations in patients with high GGT-PFIC is unclear. There have been no Asian patients reported to carry MDR3 mutations. To determine the role of MDR3 defects in chronic cholestatic patients, we studied six Taiwanese children from five families who presented high GGT-PFIC among 47 patients with infantile onset chronic intrahepatic cholestasis. Sequence analysis of MDR3 cDNA from liver tissues was performed. Only one patient had mutation in the MDR3 gene. This patient had a homozygous 719-bp deletion (nucleotide 287 to 1005) of liver cDNA encompassing exon 5 to 9 and leading to protein truncation. The onset age was 1 y in contrast with the other five patients who presented neonatal cholestasis. Four patients without mutation, including one sibling pair, exhibited histologic features of prominent portal fibrosis leading to advanced biliary cirrhosis that were indistinguishable from the case of MDR3 mutation. We concluded that mutations in MDR3 accounted for approximately 2% (1/47) of infantile onset chronic cholestasis in Taiwan. Those patients presenting high GGT-PFIC with early onset cholestasis but without MDR3 mutation probably had inheritable disorders remaining to be clarified.

Risk of obstetric cholestasis in sisters of index patients

The aim of the present study was to evaluate the rate of intrahepatic cholestasis of pregnancy in first-degree relatives of index patients. Index patients (n=65) with singleton pregnancies complicated by intrahepatic cholestasis were identified among the women (n=11 984) who gave birth at Kuopio University Hospital in 1994-1998. The pregnancy histories of relatives of 56 index patients were reviewed and the rate of cholestasis in first-degree relatives was compared with that in the general obstetric population. Obstetric cholestasis was experienced by 9% of the parous sisters and 11% of the mothers of the index patients. The risk per delivery was 6% in the first-degree relatives. The rate in the general obstetric population was 0.54%. The odds ratios and 95% confidence intervals were 12.6 (5.6-28.1) for the sisters and 12.2 (6.2-24.2) for the mothers. Obstetric cholestasis clusters within some families and is under strong genetic influence, although the precise genetic pattern remains obscure. The sisters of index patients are at an increased risk of the disorder and may benefit from close obstetric care.

Protein kinase C-mediated down-regulation of MDR3 mRNA expression in Chang liver cells

MDR3 is a phospholipid translocator homologous to MDR1 P-glycoprotein. MDR3 localizes to the canalicular membrane and contributes to the secretion of bile. To elucidate the role of protein kinase C in the regulation of MDR3 gene expression, we investigated the effect of phorbol 12-myristate 13-acetate (PMA) on the level of MDR3 mRNA in human Chang liver cells by a reverse transcription-polymerase chain reaction method. The steady-state expression of MDR3 mRNA was decreased by PMA after treatment for 8-20 hr and at concentrations of 1-100 nM. PMA also decreased the doxorubicin-induced expression of MDR3 mRNA. 4alpha-Phorbol 12,13-didecanoate, a negative control compound, did not decrease the expression at these concentrations. The down-regulatory effect of PMA was partially suppressed by the protein kinase C inhibitors 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide (GF109203X) and calphostin C. Furthermore, cycloheximide, a protein synthesis inhibitor, antagonized the effect of PMA. From these results, it was suggested that the level of MDR3 mRNA was negatively regulated by a protein kinase C- and protein synthesis-dependent system and that the system regulated both the stable and inducible expression of MDR3 mRNA.

MDR3 gene defect in adults with symptomatic intrahepatic and gallbladder cholesterol cholelithiasis

BACKGROUND & AIMS

Many studies indicate that gallstone susceptibility has genetic components. MDR3 is the phosphatidylcholine translocator across the hepatocyte canalicular membrane. Because phospholipids are a carrier and a solvent of cholesterol in hepatic bile, we hypothesized that a defect in the MDR3 gene could be the genetic basis for peculiar forms of cholesterol gallstone disease, in particular those associated with symptoms and cholestasis without evident common bile duct stone.

METHODS

We studied 6 adult patients with a peculiar form of cholelithiasis. MDR3 gene sequence was determined by reverse-transcription polymerase chain reaction amplification of mononuclear cell RNAs followed by direct sequencing. Hepatic bile was analyzed in 2 patients.

RESULTS

All patients shared the following features: at least 1 episode of biliary colic, pancreatitis, or cholangitis; biochemical evidence of chronic cholestasis; recurrence of symptoms after cholecystectomy; presence of echogenic material in the intrahepatic bile ducts; and prevention of recurrence by ursodeoxycholic acid therapy. Hepatic bile composition showed a high cholesterol/phospholipid ratio and cholesterol crystals. In all patients, we found MDR3 gene mutations involving a conserved amino acid region.

CONCLUSIONS

These preliminary observations suggest that MDR3 gene mutations represent a genetic factor involved in this peculiar form of cholesterol gallstone disease in adults. They require further studies to assess the prevalence of MDR3 gene defects in symptomatic and silent cholesterol gallstone disease.

The wide spectrum of multidrug resistance 3 deficiency: from neonatal cholestasis to cirrhosis of adulthood

BACKGROUND & AIMS

We have specified the features of progressive familial intrahepatic cholestasis type 3 and investigated in 31 patients whether a defect of the multidrug resistance 3 gene (MDR3) underlies this phenotype.

METHODS

MDR3 sequencing, liver MDR3 immunohistochemistry, and biliary phospholipid dosage were performed.

RESULTS

Liver histology showed a pattern of biliary cirrhosis with patency of the biliary tree. Age at presentation ranged from the neonatal period to early adulthood. Sequence analysis revealed 16 different mutations in 17 patients. Mutations were identified on both alleles in 12 patients and only on 1 allele in 5. Four mutations lead to a frame shift, 2 are nonsense, and 10 are missense. An additional missense mutation probably representing a polymorphism was found in 5 patients. MDR3 mutations were associated with abnormal MDR3 canalicular staining and a low proportion of biliary phospholipids. Gallstones or episodes of cholestasis of pregnancy were found in patients or parents. Children with missense mutations had a less severe disease and more often a beneficial effect of ursodeoxycholic acid therapy.

CONCLUSIONS

At least one third of the patients with a progressive familial intrahepatic cholestasis type 3 phenotype have a proven defect of MDR3. This gene defect should also be considered in adult liver diseases.

Progressive familial intrahepatic cholestasis. Genetic basis and treatment

Major advances in the understanding of the molecular mechanisms of bile formation and genetic studies of children with chronic cholestasis uncovered the molecular basis of PFIC. Specific defects in the FIC1, BSEP, and MDR3 genes are responsible for distinct PFIC phenotypes. These findings have confirmed the autosomal recessive inheritance of the disease and now provide specific diagnostic tools for the investigation of children with PFIC. This understanding should also allow prenatal diagnosis in the future. Identification of mutations in these genes will allow genotype-phenotype correlations to be defined within the spectrum of PFIC. These correlations performed in patients previously treated by UDCA or biliary diversion should identify those PFIC patients who could benefit from these therapies. In the future, other therapies, such as cell and gene therapies, might represent an alternative to liver transplantation. It remains to be determined if defects in the FIC1, BSEP, and MDR3 genes are responsible for all types of PFIC, or if other yet undiscovered genes, possibly involved in bile formation or its regulation, may be involved in the pathogenesis of PFIC.

Mapping of the locus for cholestasis-lymphedema syndrome (Aagenaes syndrome) to a 6.6-cM interval on chromosome 15q

Patients with cholestasis-lymphedema syndrome (CLS) suffer severe neonatal cholestasis that usually lessens during early childhood and becomes episodic; they also develop chronic severe lymphedema. The genetic cause of CLS is unknown. We performed a genome screen, using DNA from eight Norwegian patients with CLS and from seven unaffected relatives, all from an extended pedigree. Regions potentially shared identical by descent in patients were further characterized in a larger set of Norwegian patients. The patients manifest extensive allele and haplotype sharing over the 6.6-cM D15S979-D15S652 region: 30 (83.3%) of 36 chromosomes of affected individuals carry a six-marker haplotype not found on any of the 32 nontransmitted parental chromosomes. All Norwegian patients with CLS are likely homozygous for the same disease mutation, inherited from a shared ancestor.

MDR3 P-glycoprotein, a phosphatidylcholine translocase, transports several cytotoxic drugs and directly interacts with drugs as judged by interference with nucleotide trapping

The human MDR3 gene is a member of the multidrug resistance (MDR) gene family. The MDR3 P-glycoprotein is a transmembrane protein that translocates phosphatidylcholine. The MDR1 P-glycoprotein related transports cytotoxic drugs. Its overexpression can make cells resistant to a variety of drugs. Attempts to show that MDR3 P-glycoprotein can cause MDR have been unsuccessful thus far. Here, we report an increased directional transport of several MDR1 P-glycoprotein substrates, such as digoxin, paclitaxel, and vinblastine, through polarized monolayers of MDR3-transfected cells. Transport of other good MDR1 P-glycoprotein substrates, including cyclosporin A and dexamethasone, was not detectably increased. MDR3 P-glycoprotein-dependent transport of a short-chain phosphatidylcholine analog and drugs was inhibited by several MDR reversal agents and other drugs, indicating an interaction between these compounds and MDR3 P-gp. Insect cell membranes from Sf9 cells overexpressing MDR3 showed specific MgATP binding and a vanadate-dependent, N-ethylmaleimide-sensitive nucleotide trapping activity, visualized by covalent binding with [alpha-(32)P]8-azido-ATP. Nucleotide trapping was (nearly) abolished by paclitaxel, vinblastine, and the MDR reversal agents verapamil, cyclosporin A, and PSC 833. We conclude that MDR3 P-glycoprotein can bind and transport a subset of MDR1 P-glycoprotein substrates. The rate of MDR3 P-glycoprotein-mediated transport is low for most drugs, explaining why this protein is not detectably involved in multidrug resistance. It remains possible, however, that drug binding to MDR3 P-glycoprotein could adversely affect phospholipid or toxin secretion under conditions of stress (e.g. in pregnant heterozygotes with one MDR3 null allele).

Prophylaxis for opportunistic infections in an era of effective antiretroviral therapy

Potent antiretroviral treatment is associated with dramatic improvements in immune function in many human immunodeficiency virus-infected patients. This has led to new US Public Health Service/Infectious Diseases Society of America guidelines that suggest that in certain circumstances (primary prophylaxis for Pneumocystis carinii pneumonia and disseminated Mycobacterium avium complex infection, and secondary prophylaxis for cytomegalovirus retinitis), antimicrobial prophylaxis can be discontinued for patients whose CD4 T-cell counts rise above threshold levels for at least 3-6 months. The new guidelines are probably too conservative, and effective antiretroviral treatment almost certainly provides protection against all major opportunistic pathogens. Therefore, in the future, specific prophylaxis will be needed only for those patients who do not benefit from or fail to adhere to the current more effective treatment of human immunodeficiency virus infection.