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

Hepatocanalicular transport defects: pathophysiologic mechanisms of rare diseases

The apical membrane of the hepatocyte fulfils a unique function in the formation of primary bile. For all important biliary constituents a primary active transporter is present that extrudes or translocates its substrate toward the canalicular lumen. Most of these transporters are ATP-binding cassette (ABC) transporters. Two types of transporters can be recognized: those having endogenous metabolites as substrates (which could be referred to as "physiologic" transporters) and those involved in the elimination of drugs, toxins, and waste products. It should be emphasized that this distinction cannot be strictly made as some endogenous metabolites can be regarded as toxins as well. The importance of the canalicular transporters has been recognized by the pathologic consequence of their genetic defects. For each of the physiologic transporter genes an inherited disease has now been identified and most of these diseases have a quite serious clinical phenotype. Strikingly, complete defects in drug transporter function have not been recognized (yet) or only cause a mild phenotype. In this review we only briefly discuss the inherited defects in transporter function, and we focus on the pathophysiologic concepts that these diseases have generated.

Benign recurrent intrahepatic cholestasis associated with mutations of the bile salt export pump

A young patient with recurrent attacks of intrahepatic cholestasis is described. On the basis of clinical presentation, laboratory findings and genetic analysis, the diagnosis of benign recurrent intrahepatic cholestasis type 2 (BRIC-2) was established. By the use of BSEP-specific antibodies, almost complete absence of BSEP from the canalicular membrane of liver cells was detected in the patient. Two different BSEP mutations were found. One mutation (E186G) had been described in one BRIC-2 case; the second mutation (V444A) is more frequent and has been linked to intrahepatic cholestasis of pregnancy. It is concluded that this form of compound heterozygosity of the BSEP gene reduces the amount of BSEP protein due to protein instability or mis-targeting, which is the underlying reason for reduced bile salt excretion and cholemia.

Genes and (auto)immunity in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic autoimmune cholestatic liver disease most commonly encountered in postmenopausal women; it is characterized by high-titer serum autoantibodies to mitochondrial antigens, elevated serum IgM, progressive destruction of intrahepatic bile ducts, and ultimately liver cirrhosis and failure. The cytopathic mechanisms leading to the selective destruction of intrahepatic cholangiocytes are still largely unknown. The current theory on the pathogenesis of PBC indicated that environmental factors might trigger autoimmunity in genetically susceptible individuals. In fact, genetic predisposition is critical to disease onset and progression, yet peculiar among autoimmune diseases, as indicated by the lack of a strong association with major histocompatibility complex haplotypes. Further, the recently reported concordance rate among monozygotic twins strengthens the importance of genetic factors, while also indicating that additional factors, possibly infectious agents or xenobiotics, intervene to trigger the disease. In this review, the available data regarding the genetic factors associated with PBC susceptibility and progression, as well as the available evidence regarding the immunomediated pathogenesis of PBC, will be critically illustrated and discussed.

Impaired expression and function of the bile salt export pump due to three novel ABCB11 mutations in intrahepatic cholestasis

BACKGROUND/AIMS

Inherited dysfunction of the bile salt export pump BSEP (ABCB11) causes a progressive and a benign form of familial intrahepatic cholestasis, denominated as PFIC2 and BRIC2, respectively. We functionally characterized novel ABCB11 mutations encountered in two patients with a PFIC2 and a BRIC2 phenotype, respectively.

METHODS

BSEP expression was determined in liver biopsies by immunohistochemistry. ABCB11 mutations were functionally characterized by taurocholate transport in SF9 cells transfected with human ABCB11.

RESULTS

The PFIC2 patient was compound heterozygous for a splicing mutation in intron 4 ((+3)A > C) combined with an early stop codon at position 930 (R930X), while the BRIC2 patient was compound heterozygous for two nonsynonymous mutations in exon 9 (E297G) and exon 12 (R432T), respectively. Hepatic BSEP expression was absent in PFIC2 and preserved in BRIC2. In BRIC2, taurocholate transport was decreased to 13% and 20% of reference levels for R432T and E297G, respectively.

CONCLUSIONS

The intron 4 (+3)A > C, R930X and R432T represent previously undescribed mutations of the ABCB11 gene that confer a PFIC2 and a BRIC2 phenotype, respectively. By combining functional in-vitro characterization with immunohistochemical detection of variant BSEP we provide direct evidence for the role of ABCB11 mutations in the pathogenesis of different forms of intrahepatic cholestasis.

[The MDR3 gene mutation: a rare cause of progressive familial intrahepatic cholestasis (PFIC)]

A 47-year old man complained about persistant pain and cholestasis 12-years after a cholescystectomy. In his family, all his brothers and sisters had cholecystectomy. Genetic explorations revealed a MDR3 gene mutation. All symptoms disappeared with a treatment by ursodesoxycholic acid. MDR3 gene mutation is to be researched in all cases of familial cholestasis.

Sclerosing cholangitis

PURPOSE OF REVIEW

Primary sclerosing cholangitis is a chronic cholestatic liver disease characterized by strictures of the biliary tree. It is immune mediated, although the precise cause remains unknown. Recent reports have shown a higher prevalence and burden of disease than was previously suspected.

RECENT FINDINGS

The research into the etiopathogenesis, epidemiology, diagnosis of cholangiocarcinoma, medical and surgical therapy, and timing and outcome of liver transplantation is discussed.

SUMMARY

Genetic heterogeneity among patients with primary sclerosing cholangitis is supported, and further gene polymorphisms associated with protection against primary sclerosing cholangitis have been elucidated. Bile duct injury seems to be a multistep process. Magnetic resonance cholangiopancreatography is a cost-effective and accurate way of diagnosing primary sclerosing cholangitis in comparison with endoscopic retrograde cholangiopancreatography. Ursodeoxycholic acid may have a role as a colorectal and hepatobiliary cancer chemopreventive agent. Liver transplantation remains the only treatment in end-stage disease. The 5-year and 10-year patient and graft survival rates are comparable with those in patients without primary sclerosing cholangitis, but there is a higher rate of retransplantation for primary sclerosing cholangitis in most centers. Hepatobiliary malignancy is found in a minority of patients at transplantation, although 5-year survival rates for these patients are still promising.

From bases to basis: linking genetics to causation in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a multifactorial autoimmune disease with inherited and environmental components in pathogenesis. It is exceptional among autoimmune diseases in showing strong heritability according to familial occurrence and monozygotic twins concordance, yet with weak associations with the usual genetic risk elements for autoimmunity, such as the HLA alleles. Among the latter, there is risk (at least in some populations) conferred by HLA DRB1*08 and possibly some protection by DRB1*11. However, the inconsistency among studies on HLA is surprising, given that PBC is a relatively homogenous disease entity. Among non-HLA genes, some studies implicate polymorphisms of genes for cytotoxic T-lymphocyte antigen-4, interleukin-2, or interleukin-10; polymorphisms of the vitamin D receptor could synergize with low sunlight exposure to create deficiency of the immunoregulatory factor, activated vitamin D. A new lead is available from the finding in female subjects with PBC of an increase in the degree of monosomy of the X chromosome that is presumed to carry immune response genes. A further suggested source of inquiry is the apparent protection of African-American women from PBC. Finally, data on inheritance should be sought in PBC by descent methodology, rather than by cross-sectional association studies in cases and control subjects, and based on analysis of a large number of families with an affected member through a worldwide effort.

Molecular regulation of hepatobiliary transport systems: clinical implications for understanding and treating cholestasis

Hepatobiliary transport systems are responsible for hepatic uptake and excretion of bile salts and other biliary constituents (eg, bilirubin) into bile. Hereditary transport defects can result in progressive familial and benign recurrent intrahepatic cholestasis. Exposure to acquired cholestatic injury (eg, drugs, hormones, proinflammatory cytokines, biliary obstruction or destruction) also results in altered expression and function of hepatic uptake and excretory systems, changes that may maintain and contribute to cholestasis and jaundice. Recruitment of alternative efflux pumps and induction of phase I and II detoxifying enzymes may limit hepatic accumulation of potentially toxic biliary constituents in cholestasis by providing alternative metabolic and escape routes. These molecular changes are mediated by bile salts, proinflammatory cytokines, drugs, and hormones at a transcriptional and posttranscriptional level. Alterations of hepatobiliary transporters and enzymes are not only relevant for a better understanding of the pathophysiology of cholestatic liver diseases, but may also represent important targets for pharmacotherapy. Drugs (eg, ursodeoxycholic acid, rifampicin) used to treat cholestatic liver diseases and pruritus may counteract cholestasis via stimulation of defective transporter expression and function. In addition, therapeutic strategies may be aimed at supporting and stimulating alternative detoxification pathways and elimination routes for bile salts in cholestasis.

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.

Genetic polymorphisms influencing xenobiotic metabolism and transport in patients with primary biliary cirrhosis

Epidemiological data suggest that environmental factors may trigger autoimmunity in genetically susceptible individuals. In primary biliary cirrhosis (PBC), it has been postulated that halogenated xenobiotics can modify self-molecules, facilitating the breakdown of tolerance to mitochondrial antigens. The transport and metabolism of xenobiotics is highly dependent on key genetic polymorphisms that alter enzymatic phenotype. We analyzed genomic DNA from 169 patients with PBC and 225 geographically and sex-matched healthy subjects for polymorphisms of genes coding for cytochromes P450 (CYPs) 2D6 (CYP2D6*4, CYP2D6*3, CYP2D6*5, and CYP2D6*6) and 2E1 (cl/c2), multidrug resistance 1 (MDR1 C3435T) P-glycoprotein, and pregnane X receptor (PXR C-25385T, C8055T, and A7635G). We compared the genotype frequencies in patients and controls and also correlated polymorphisms with PBC severity. The distributions of the studied genotypes did not significantly differ between patients and controls. However, when clinical characteristics of patients with PBC were compared according to genotype, the CYP2E1 c2 allele was associated with signs of more severe disease. In conclusion, genetic polymorphisms of CYP 2D6 and 2E1, PXR, and MDR1 do not appear to play a role in the onset of PBC.

Chemokine receptor 5 and primary biliary cirrhosis: a two-centre genetic association study

BACKGROUND

Chemokines and their receptors are important mediators of leucocyte trafficking and are suggested to be critical for establishment of inflammatory autoimmune processes. CC chemokine receptor 5 (CCR5) is expressed preferentially by CD4+ T cells. We hypothesised that the CCR5delta(Delta)32 genotype, which impairs surface expression of CCR5 in heterozygotes and is linked to a functional polymorphism of CD45RA expressed on suppressor-inducer-like 'naive' CD4+ T cells, may modulate the inflammatory process in primary biliary cirrhosis (PBC).

METHODS

CCR5Delta32 polymorphism was determined by PCR in 226 Caucasian PBC patients and 197 racially matched controls from two geographical areas, Newcastle, UK and Padua, Italy. (UK: 144 PBC, 105 controls, Italy: 82 PBC, 92 controls).

RESULTS

When the two series were analysed separately, there were no significant differences in the genotype distribution comparing patients and controls (UK: wt/wt 72% vs 76%; wt/Delta32 28% vs 22%; Delta32/Delta32 0% vs 2%, P=0.24; Italy: wt/wt 72% vs 82%; wt/Delta32 27% vs 17%; Delta32/Delta32 0% vs 1%, P=0.14). However, when the data for the two series were pooled and reanalysed, we found an increase in the CCR5Delta32 mutation in PBC patients vs controls (28% vs 21%, OR=1.43, P=0.03), but there was no evidence that this Delta32 polymorphism is associated with less severe disease.

CONCLUSIONS

Although this two-centre genetic association study is large compared with others performed in PBC, taken separately, each geographically based cohort of patients and controls is underpowered to detect a small effect of this functional polymorphism. This emphasises the need for far larger case-control collections to address which polymorphic markers or haplotypes might modify the pathogenesis and clinical course of PBC. We propose that multi-centre collaboration on an international scale in 'orphan' complex liver diseases such as (PBC) is supported by the International Association for the Study of the Liver and promoted via their journal with development of a brief format for web-based publication of studies.

The bile salt export pump: molecular properties, function and regulation

Secretion of bile salts from the hepatocyte into bile is the major driving force for the generation of bile flow. Identification of the bile salt export pump (BSEP, ABCB11) as the main adenosine-triphosphate-dependent bile salt transporter in mammalian liver has led to a greater understanding of the biliary bile salt secretory process and its regulation. The biology and pathobiology of BSEP have been the subject of many recent studies. Thus, it has been recognized that while mutations in the gene encoding BSEP are responsible for a subgroup of progressive familial cholestasis (progressive familial intrahepatic cholestasis subtype 2), a pediatric cholestatic disorder that may progress to cirrhosis, defective expression or function of BSEP may underlie some forms of drug-induced cholestasis. The present review summarizes recent data on the molecular properties and regulation of BSEP, as well as the clinical implications of absent or defective function of this hepatic efflux pump.