A gene encoding a P-type ATPase mutated in two forms of hereditary cholestasis

Correction of liver disease by hepatocyte transplantation in a mouse model of progressive familial intrahepatic cholestasis

BACKGROUND & AIMS

Patients with progressive familial intrahepatic cholestasis (PFIC) type 3 have a mutation in the MDR3 gene, encoding the hepatocanalicular phospholipid translocator. In general, liver failure develops within the first decade of life in these patients. Previous studies have shown that in the mdr2-knockout mouse, the animal model for this disease, the absence of phospholipids in bile causes chronic bile salt-induced damage to hepatocytes. We aimed to test the efficacy of hepatocyte transplantation and liver repopulation in this disease model.

METHODS

Transgenic MDR3-expressing hepatocytes as well as normal mdr2(+/+) hepatocytes were transplanted in mdr2(-/-) mice, and liver repopulation was assessed by immunohistochemistry and measurement of biliary lipid secretion.

RESULTS

Transplanted hepatocytes partially repopulated the liver, restored phospholipid secretion, and diminished liver pathology. Repopulation was stronger when hepatocellular damage was enhanced by a bile salt-supplemented diet. After 1 year, however, these animals developed multiple hepatic tumors, and biliary phospholipid secretion decreased. In transplanted animals receiving a control diet, repopulation was slower but eventually remained stable at 21%, while liver pathology was completely abrogated and tumor formation was prevented.

CONCLUSIONS

These results suggest that moderate liver pathology is a safe condition for the induction of effective hepatocyte repopulation and that this therapy is potentially applicable to patients with PFIC type 3.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

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.

[Benign recurrent intrahepatic cholestasis: a seven-year follow-up report]

Benign recurrent intrahepatic cholestasis is a rare autosomal recessive disorder characterized by repeated episodes of intense pruritus and jaundice. Patients are completely asymptomatic for months to years between symptomatic periods. We report a case of a patient with a 7-year history of benign recurrent intrahepatic cholestasis. During the follow-up period the patient has suffered three attacks of cholestasis, confirmed by biochemical tests and histological exam. Liver enzymes were normal between the cholestasis episodes. Despite multiple attacks of cholestasis, no permanent liver damage has occurred. Although the diagnosis of benign recurrent intrahepatic cholestasis is rare, it should be included in the evaluation of a patient with cholestasis. The patients should be reassured of the benign course of this disorder.

The pathophysiology of cholestasis with special reference to primary biliary cirrhosis

Cholestasis in primary biliary cirrhosis results from impairment of bile flow either by reduced transport at the level of the canaliculi or by disturbed bile flow through damaged intrahepatic bile ductules. Whatever its cause, the expression of hepatic transport proteins will be affected. In cholestatic rats: the expression of the multispecific organic anion transporter mrp2 is decreased; the bile salt export pump bsep and the phospholipid transporter mdr2 are less affected; the carrier protein for hepatic uptake of bile salts ntcp is sharply down-regulated; Mrp3, a basolateral ATP-dependent transporter for glucuronides and bile salts, is upregulated. Thus, bile salts that cannot exit the hepatocyte because of the cholestasis are effectively removed across the basolateral membrane. These may be adaptive responses in defence against overloading of hepatocytes with cytotoxic bile salts. These responses show that the expression of hepatic transporter proteins is highly regulated. This occurs by transcriptional and post-transcriptional mechanisms. Primary biliary cirrhosis starts as a disease of the small intrahepatic bile ducts and therefore the experimental evidence for 'cross-talk' between hepatocytes and cholangiocytes is of great interest for this disease and needs to be further investigated. New insights in bile physiology may enable the development of new therapies for cholestatic liver diseases as primary biliary cirrhosis.

Abnormal hepatic sinusoidal bile acid transport in an Amish kindred is not linked to FIC1 and is improved by ursodiol

BACKGROUND & AIMS

The mechanism for abnormal hepatic bile acid transport was investigated in an 18-month-old Amish boy who presented with pruritus, poor growth, and severe bleeding episodes. Serum bilirubin, gamma-glutamyltranspeptidase, and cholesterol levels were normal, but prothrombin time and partial thromboplastin time were prolonged and bone alkaline phosphatase level was elevated.

METHODS AND RESULTS

Cholic acid plus chenodeoxycholic acid levels measured by capillary gas-chromatography were 32 times higher than control in serum (34.7 vs. 1.1+/-0.4 microg/dL) but were not detected in liver and were reduced in gallbladder bile. Treatment with ursodiol, a more hydrophilic bile acid, improved pruritus, produced 37% weight gain, and after 2 years reduced serum primary bile acid concentrations about 85%, while accounting for 71% of serum and 24% of biliary bile acid conjugates. On ursodiol therapy, hepatic bile acid synthesis was enhanced 2-fold compared with controls, and microscopy revealed chronic hepatitis without cholestasis. Three younger sisters with elevated serum bile acids responded positively to ursodiol. Microsatellite markers for the FIC1 (gene for Byler's disease) region in these 4 children were inconsistent with linkage to FIC1.

CONCLUSIONS

Conjugated cholic acid and chenodeoxycholic acid were synthesized in the liver and secreted into bile but could not reenter the liver from portal blood and accumulated in serum. In contrast, unconjugated ursodiol entered the liver and was conjugated and secreted into bile. Thus, the enterohepatic circulation of all conjugated bile acids was interrupted at the hepatic sinusoidal basolateral membrane. Unconjugated ursodiol bypassed the hepatic uptake block to enlarge the biliary and intestinal bile acid pools. A mutation in FIC1 recognized among the Amish and linkage of the disorder to FIC1 were excluded.

Localization of a recessive gene for North American Indian childhood cirrhosis to chromosome region 16q22-and identification of a shared haplotype

North American Indian childhood cirrhosis (NAIC, or CIRH1A) is an isolated nonsyndromic form of familial cholestasis reported in Ojibway-Cree children and young adults in northwestern Quebec. The pattern of transmission is consistent with an autosomal recessive mode of inheritance. To map the NAIC locus, we performed a genomewide scan on three DNA pools of samples from 13 patients, 16 unaffected siblings, and 22 parents from five families. Analysis of 333 highly polymorphic markers revealed 3 markers with apparent excess allele sharing among affected individuals. Additional mapping identified a chromosome 16q segment shared by all affected individuals. When the program FASTLINK/LINKAGE was used and a completely penetrant autosomal recessive mode of inheritance was assumed, a maximum LOD score of 4.44 was observed for a recombination fraction of 0, with marker D16S3067. A five-marker haplotype (D16S3067, D16S752, D16S2624, D16S3025, and D16S3106) spanning 4.9 cM was shared by all patients. These results provide significant evidence of linkage for a candidate gene on chromosome 16q22.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

Mechanisms of cholestasis

From the multiple mechanisms of cholestasis presented in this article, a unifying hypothesis may be deduced by parsimony. The disturbance of the flow of bile must inevitably lead to the intracellular retention of biliary constituents. Alternatively, the lack of specific components of bile unmasks the toxic potential of other components, as in the case of experimental mdr2 deficiency. In the sequence of events that leads to liver injury, the cytotoxic action of bile salts is pivotal to all forms of cholestasis. The inhibition of the bsep by drugs, sex steroids, or monohydroxy bile salts is an example of direct toxicity to the key mediator in canalicular bile salt excretion. In other syndromes, the dysfunction of distinct hepatocellular transport systems is the primary pathogenetic defect leading to cholestasis. Such dysfunctions include the genetic defects in PFIC and the direct inhibition of gene transcription by cytokines. Perturbations in the short-term regulation of transport protein function are exemplified by the cholestasis of endotoxinemia. The effect of bile salts on signal transduction, gene transcription, and transport processes in hepatocytes and cholangiocytes has become the focus of intense research in recent years. The central role of bile salts in the pathogenesis of cholestasis has, ironically, become all the more evident from the improvement of many cholestatic syndromes with oral bile salt therapy.

Hepatobiliary transport

The alterations of hepatobiliary transport that occur in cholestasis can be divided into primary defects, such as mutations of transporter genes or acquired dysfunctions of transport systems that cause defective canalicular or cholangiocellular secretion, and secondary defects, which result from biliary obstruction. The dysfunction of distinct biliary transport systems as a primary cause of cholestasis is exemplified by the genetic defects in progressive familial intrahepatic cholestasis or by the direct inhibition of transporter gene expression by cytokines. In both, the hepatocellular accumulation of toxic cholephilic compounds causes multiple alterations of hepatocellular transporter expression. In addition, lack of specific components of bile caused by a defective transporter, as in the case of mdr2/MDR3 deficiency, unmasks the toxic potential of other components. The production of bile is critically dependent upon the coordinated regulation and function of sinusoidal and canalicular transporters, for instance of Na+-taurocholate cotransporting polypeptide (NTCP) and bile salt export pump (BSEP). Whereas the downregulation of the unidirectional sinusoidal uptake system NTCP protects the hepatocyte from further intracellular accumulation of bile salts, the relative preservation of canalicular BSEP expression serves to uphold bile salt secretion, even in complete biliary obstruction. Conversely, the strong downregulation of canalicular MRP2 (MRP, multidrug resistance protein) in cholestasis forces the hepatocyte to upregulate basolateral efflux systems such as MRP3 and MRP1, indicating an inverse regulation of basolateral and apical transporters The regulation of hepatocellular transporters in cholestasis adheres to the law of parsimony, since many of the cellular mechanisms are pivotally governed by the effect of bile salts. The discovery that bile salts are the natural ligand of the farnesoid X receptor has shown us how the major bile component is able to regulate its own enterohepatic circulation by affecting transcription of the genes critically involved in transport and metabolism.

Chronic cholestatic diseases

Chronic cholestatic diseases, whether occurring in infancy, childhood or adulthood, are characterized by defective bile acid transport from the liver to the intestine, which is caused by primary damage to the biliary epithelium in most cases. In this article, approaches to diagnosis and management of the main specific disorders are provided and some of the recent developments in this field are discussed. Major advances in the understanding of the cellular and molecular physiology of bile secretion have led to identification of genetic defects responsible for the different types of progressive familial intrahepatic cholestasis (PFIC). The potential role of the genes involved in PFIC in some adult cholestatic disorders remains to be determined. The majority of adult patients with chronic cholestasis have primary biliary cirrhosis (PBC) or primary sclerosing cholangitis (PSC). Recently, variant forms of PBC have been described. The term autoimmune cholangitis is used to describe patients having chronic non-suppurative cholangitis with negative antimitochondrial antibodies (AMA) but positive antinuclear and/or antismooth muscle antibodies. Autoimmune cholangitis and AMA-positive PBC are quite similar in terms of clinical presentation, survival and response to ursodeoxycholic acid (UDCA) therapy. In contrast, autoimmune cholangitis must be distinguished from PBC-autoimmune hepatitis (AIH) overlap syndrome in which biochemical and histological characteristics of both PBC and AIH coexist. Combination of UDCA and corticosteroids is required in most patients with overlap syndrome to obtain a complete clinical and biochemical response. Long-term UDCA treatment improves survival without liver transplantation in PBC patients. Among the putative mechanisms of the beneficial effects of UDCA, description of anti-apoptotic properties and effect on endotoxin disposal in biliary cells have provided new insights. In patients with incomplete response to UDCA, combination of UDCA with antiinflammatory or immunosuppressive drugs is under evaluation. Variant forms of PSC have also been described, including PSC-AIH overlap syndrome, especially in children or young adults, and small-duct PSC, which is characterized by normal cholangiogram in patients having chronic cholestasis, histologic features compatible with PSC and inflammatory bowel disease. Development of cholangiocarcinoma (CC) is a major feature of PSC, occurring in 10-15% of patients. Early diagnosis of CC is a difficult challenge, although positron emission tomography seems a promising tool. Unlike PBC, effective medical therapy is not yet available in PSC, reflecting the lack of knowledge about the exact pathogenesis of the disease. Currently, liver transplantation is the only effective therapy for patients with advanced disease, although recurrence of PSC in the graft may occur.

Lipid traffic: the ABC of transbilayer movement

Membrane lipids do not spontaneously exchange between the two leaflets of lipid bilayers because the polar headgroups cannot cross the hydrophobic membrane interior. Cellular membranes, notably eukaryotic plasma membranes, are equipped with special proteins that actively translocate lipids from one leaflet to the other. In addition, cellular membranes contain proteins that facilitate a passive equilibration of lipids between the two membrane halves. In recent years, a growing number of proteins have been put forward as lipid translocators or facilitators. Unexpectedly, some of these appear to be required for efficient translocation of lipids lacking bulky headgroups, like cholesterol and fatty acids. The candidate lipid translocators identified so far belong to large protein families whose other members include pumps for amphiphilic molecules like bile salts and drugs.

Advances in familial and congenital cholestatic diseases. Clinical and diagnostic implications

Recent progress in liver cell biology and molecular genetics revealed that a number of familial and congenital cholestatic disorders are caused by mutations in genes coding for hepatobiliary-transporter or for signalling proteins involved in morphogenesis. The status of the field is reviewed in the light of its impact on current diagnostic and clinical practice. The heterogeneous progressive familial intrahepatic cholestasis can now be separated into different genetic diseases. FIC1-defective progressive familial intrahepatic cholestasis (previously Byler disease) is determined by mutations in the FIC1 gene, coding for P-type ATPases of unknown physiological function, while a second form (bile salt export pump defective progressive familial intrahepatic cholestatis) is caused by a defective function of the canalicular bile salt export pump. Furthermore, a group of progressive familial intrahepatic cholestasis patients with high serum gamma glutamyltranspeptidase have mutations in the gene (PGY3) coding for the MDR3 protein, a canalicular ATP-dependent phopshatidylcholine translocator. Recurrent intrahepatic cholestasis (previously benign recurrent cholestasis), is also linked to specific mutations in the FIC1 gene. Finally, in Alagille syndrome, mutations in the JAG1 gene cause deficiency Jagged 1, a ligand for Notch 1, a receptor determining cell fate during early embryogenesis. Diagnosis of Alagille syndrome, a condition that should be suspected in all patients with unexplained cholestasis, will thus be confirmed by genetic analysis for mutations of JAG1. In children with cholestasis and low serum bile acid levels, an inborn error of bile acid synthesis should be excluded by urinary bile acid analysis by means of fast atom bombardment-ionization mass-spectrometry. In contrast, in children with cholestasis and high serum bile acid concentrations, a high serum gamma glutamyltranspeptidase value would indicate MDR3 deficiency, which should be excluded through biliary phospholipid determination and genetic analysis of PGY3 gene. Finally, in those children with cholestasis, high serum bile acids and low gamma glutamyltranspeptidase activity, analysis of mutation in FIC1 and bile salt export pump genes may lead to the diagnosis of progressive familial intrahepatic cholestasis either from bile salt export pump or FIC1 deficiency.

Pediatric hepatology. A half-century of progress

Treating a pediatric patient offers a unique opportunity to develop effective strategies to prevent progressive liver injury and to develop novel therapeutic regimens to reduce the need for OLT. Universal vaccination against hepatitis viruses will prevent cirrhosis and liver cancer. Education and counseling may reduce the incidence of alcoholic liver disease. Precise and early screening for metabolic liver disease and genetic or targeted therapy may prevent disease progression. A retrospective look at the 1983 National Institutes of Health Consensus Conference on Liver Transplantation, after more than 15 years of experience among many centers, indicates that liver transplantation can be effectively used to childhood liver disease. Projections 10 years into the future offer hope that liver transplantation may not be needed in the treatment of certain diseases such as metabolic liver disease and fulminant hepatic failure. Focusing on prevention or treatment of liver disease in early life, thoughtful medical management, precise decision making, and conscientious, creative, and courageous use of nontransplant options, can save both livers and lives.

Expression of the bile salt export pump is maintained after chronic cholestasis in the rat

BACKGROUND & AIMS

This study assessed the expression of the recently identified adenosine triphosphate-dependent bile salt export pump and the functional ability to excrete bile salts in cholestatic models in the rat.

METHODS

The effects of common bile duct ligation, endotoxin, and ethinylestradiol on bile salt export pump messenger RNA levels, protein expression, and tissue localization were determined. Changes in the expression of 3 other hepatocyte membrane transporters (Na(+) taurocholate cotransporter, multispecific organic anion transporter, and P-glycoprotein) were also determined for comparison. Functional assessment of bile salt excretion was determined after bile duct ligation.

RESULTS

Expression of the bile salt export pump was diminished but relatively preserved compared with other membrane transporters. Tissue localization of the bile salt export pump persisted at the canalicular domain in all 3 models. In contrast, expressions of the Na(+) taurocholate cotransporter and multispecific organic anion transporter were more profoundly diminished. P-glycoprotein levels increased severalfold with common bile duct ligation but were unchanged with either endotoxin or ethinylestradiol. The capacity to excrete bile salts was relatively maintained 3 and even 14 days after bile duct ligation.

CONCLUSIONS

Alterations in expression of the bile salt export pump may account for the functional alterations of bile salt secretion observed in cholestasis. However, relative preservation of expression is associated with persistent bile salt excretion and may lessen the extent of liver injury produced by bile salt retention.

Hepatocanalicular bile salt export pump deficiency in patients with progressive familial intrahepatic cholestasis

BACKGROUND & AIMS

Progressive familial intrahepatic cholestasis (PFIC), an inherited liver disease of childhood, is characterized by cholestasis and either normal or increased serum gamma-glutamyltransferase activity. Patients with normal gamma-glutamyltransferase activity have mutations of the FIC1 locus on chromosome 18q21 or mutations of the BSEP gene on chromosome 2q24. Also, patients with bile acid synthesis defects have low gamma-glutamyltransferase activity. We investigated expression of the bile salt export pump (BSEP) in liver samples from patients with a PFIC phenotype and correlated this with BSEP gene mutations.

METHODS

BSEP and multidrug resistance protein 2 (MRP2) expressions were studied by immunohistochemistry in liver specimens of 28 patients and BSEP gene mutation analysis in 19 patients. Bile salt kinetics were studied in 1 patient.

RESULTS

Sixteen of 28 liver samples showed no canalicular BSEP staining. Staining for MRP2 showed a normal canalicular pattern in all but 1 of these samples. Ten of 19 patients showed BSEP gene mutations; BSEP protein expression was lacking in all 10 patients. No mutations were found in 9 of 19 patients, and in all except 1, BSEP protein expression was normal. Bile salt concentration in bile of BSEP-negative/MRP2-positive PFIC patients was 0.2 +/- 0.2 mmol/L (n = 9; <1% of normal) and in BSEP-positive PFIC patients 18.1 +/- 9.9 mmol/L (n = 3; 40% of normal). The kinetic study confirmed the dramatic decrease of bile salt secretion in BSEP-negative patients.

CONCLUSIONS

The findings show a close correlation between BSEP gene mutations and canalicular BSEP expression. Biliary secretion of bile salts is greatly reduced in BSEP-negative patients.

Ursodeoxycholic acid treatment in children with Byler disease

BACKGROUND

There have been a few reports of patients with Byler disease and the best medical treatment is not known. The aim of the present study is to show the effect of ursodeoxycholic acid (UDCA) on clinical, laboratory and histologic findings in children with Byler disease.

METHODS

Nine children aged between 1.5 and 9 years with Byler disease were administered UDCA orally at doses of 15-20 mg/kg per day. They were followed for at least 12 months. Clinical, laboratory and histologic outcomes were evaluated after 12 months of treatment.

RESULTS

Seven children presented in the first 6 months of life with itching and/or jaundice. Gamma-glutamyl transpeptidase and cholesterol levels were normal in all patients, despite severe cholestasis. With UDCA therapy, pruritus disappeared/diminished in four (44.4%) patients. The mean serum concentrations of alanine aminotransferase, aspartate aminotransferase (AST), total and conjugated bilirubin decreased, although it was significant only for AST (P = 0.01). Before treatment, all biopsy materials showed cellular/canalicular cholestasis and fibrosis. After UDCA therapy cholestasis was ameliorated. Two patients died during follow up.

CONCLUSIONS

The results suggest that administration of UDCA leads to clinical and biochemical improvement in children with Byler disease. The UDCA ameliorates symptoms partially, improves the life quality of patients and may be given for as long as the disease continues.

Genes for jaundice

The inheritable causes of jaundice comprise a large group of conditions of varying frequency, from Gilbert's syndrome which is relatively common, to the very rare Crigle-Najjar syndrome. Although these conditions have been well characterized clinically and in some cases biochemically, the underlying molecular defects were unknown because of a lack of knowledge about the process of bile secretion by hepatocytes. The recent cloning of several transporters for bile acids and other organic anions has enabled a greater understanding of this process and allowed correlation of the malfunction of these genes with specific disease processes. This new knowledge will provide for precision in diagnosis, allow antenatal testing and provide opportunities for gene therapy for some of the more serious disorders.

Differential expression of putative transbilayer amphipath transporters

The aminophospholipid translocase transports phosphatidylserine and phosphatidylethanolamine from one side of a bilayer to another. Cloning of the gene encoding the enzyme identified a new subfamily of P-type ATPases, proposed to be amphipath transporters. As reported here, mammals express as many as 17 different genes from this subfamily. Phylogenetic analysis reveals the genes to be grouped into several distinct classes and subclasses. To gain information on the functions represented by these groups, Northern analysis and in situ hybridization were used to examine the pattern of expression of a panel of subfamily members in the mouse. The genes are differentially expressed in the respiratory, digestive, and urogenital systems, endocrine organs, the eye, teeth, and thymus. With one exception, all of the genes are highly expressed in the central nervous system (CNS); however, the pattern of expression within the CNS differs substantially from gene to gene. These results suggest that the genes are expressed in a tissue-specific manner, are not simply redundant, and may represent isoforms that transport a variety of different amphipaths.

Benign recurrent intrahepatic cholestasis

Benign recurrent intrahepatic cholestasis is a rare autosomal recessive disorder characterized by repeated episodes of intense pruritus and jaundice. Each attack lasts from several weeks to months before resolving spontaneously. Patients are completely asymptomatic for months to years between symptomatic periods. The disorder does not lead to progressive liver disease. Although attacks seem to be associated with a viral prodrome, an inciting viral agent or toxin has not been defined. Genetic studies have mapped the defect of this disorder to the long arm of chromosome 18 and a gene that codes for a P-type ATPase, which appears to be involved in aminophospholipid transport. Therapy during symptomatic periods is supportive and aimed at relief of severe pruritus until the episode resolves spontaneously.

Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency

Genes have a major role in the control of high-density lipoprotein (HDL) cholesterol (HDL-C) levels. Here we have identified two Tangier disease (TD) families, confirmed 9q31 linkage and refined the disease locus to a limited genomic region containing the gene encoding the ATP-binding cassette transporter (ABC1). Familial HDL deficiency (FHA) is a more frequent cause of low HDL levels. On the basis of independent linkage and meiotic recombinants, we localized the FHA locus to the same genomic region as the TD locus. Mutations in ABC1 were detected in both TD and FHA, indicating that TD and FHA are allelic. This indicates that the protein encoded by ABC1 is a key gatekeeper influencing intracellular cholesterol transport, hence we have named it cholesterol efflux regulatory protein (CERP).

Intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy is one of the primary disorders of the liver that adversely affects maternal well-being and fetal outcome. Early identification of this condition, careful interdisciplinary monitoring, and prompt delivery at fetal maturity can improve outcomes in the mother and child. Although the cause is unclear, IHCP probably arises from a genetic predisposition for increased sensitivity to estrogens and progestogens and altered membrane composition and expression of bile ducts, hepatocytes, and canalicular transport systems. As a result, the elevations in maternal levels of bile acids and their molar ratios seen in healthy pregnancy rise further in IHCP patients. Also, as the normal fetal-to-maternal transfer of bile acids across the trophoblast is impaired, the excess bile acids with abnormal profiles accumulate and are toxic to the fetus. The management of IHCP is dictated by the increased risks of fetal distress, spontaneous preterm delivery, and sudden death, as well as by alleviating pruritus in the mother. These risks to the fetus rise progressively to delivery, regardless of serum levels of bile acids and ALT. Close monitoring of these markers is essential but does not prevent sudden fetal distress and death. Provision should be made to induce labor as soon as fetal lung maturity has been established. Ursodeoxycholic acid is the only therapy that has proven effective, albeit in small studies, in alleviating pruritus and restoring towards normal the abnormal profiles of bile acids and sulfated steroids in serum and other body fluids. Ursodeoxycholic acid seems to have no obvious adverse effects on the fetus, but experience is insufficient to draw conclusions regarding teratogenicity and prevention of adverse outcomes.

Benign recurrent intrahepatic cholestasis in a Saudi child

We report a case of benign recurrent intrahepatic cholestasis (BRIC) in an 11-year-old Saudi girl who developed three episodes of pruritus and jaundice at the ages of 4, 8, and 9 years. These episodes were almost stereotypic and lasted 5-8 weeks. Although she had elevated liver enzymes and serum bile acids in her blood during the attacks, they returned to normal between attacks. Thorough investigation excluded other causes of liver disease and her recurrent attacks were shortened by cholestyramine therapy. A diagnosis of BRIC should be kept in mind in patients with cholestasis.

Linkage-disequilibrium mapping of disease genes by reconstruction of ancestral haplotypes in founder populations

Linkage disequilibrium (LD) mapping may be a powerful means for genome screening to identify susceptibility loci for common diseases. A new statistical approach for detection of LD around a disease gene is presented here. This method compares the distribution of haplotypes in affected individuals versus that expected for individuals descended from a common ancestor who carried a mutation of the disease gene. Simulations demonstrate that this method, which we term "ancestral haplotype reconstruction" (AHR), should be powerful for genome screening of phenotypes characterized by a high degree of etiologic heterogeneity, even with currently available marker maps. AHR is best suited to application in isolated populations where affected individuals are relatively recently descended (< approximately 25 generations) from a common disease mutation-bearing founder.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC), also known as Byler disease, is an inherited disorder of childhood in which cholestasis of hepatocellular origin often presents in the neonatal period and leads to death from liver failure before adolescence. The pattern of appearance of affected children within families is consistent with autosomal recessive inheritance. Several studies have provided support for the heterogeneity of this clinical entity suggesting the existence of different types due to different disorders affecting the hepatocyte and related to defects of bile acid secretion or bile acid metabolism. Recent molecular and genetic studies have identified genes responsible for three types of PFIC and have shown that PFIC was related to mutations in hepatocellular transport system genes involved in bile formation. These findings now provide specific diagnostic tools for the investigation of children with PFIC and should allow prenatal diagnosis in the future. Genotype-phenotype correlations performed in patients treated with ursodeoxycholic acid or biliary diversion should allow those PFIC patients who could benefit from these therapies to be precisely identified. In the future, other therapies, such as cell and gene therapies, might be considered and could also represent an alternative to liver transplantation.

Clinical aspects of familial cholestasis (with molecular explanations)

Familial cholestatic diseases exhibit familial patterns of occurrence and result from known or presumed gene defects. Until recently, these diseases were all considered syndromes, with recognizable patterns of clinical characteristics. Moreover, little was know about the molecular pathophysiology of cholestasis in general, and nothing was known about any of these diseases. The recent discovery and characterization of the genes involved in five of these diseases has led to improved understanding of the diseases and of the physiologic function and importance of the gene products. Furthermore, as has happened many times in the past, these patients with genetic disease have served as human models of disease pathophysiology. Study of the course of the disease in these patients has rapidly increased our understanding of the molecular mechanisms of bile formation, cholestasis, and liver injury caused by cholestasis.

Cholestatic syndromes

Recent advances in cholestatic liver disease have occurred in several areas. Molecular cloning of hepatobiliary transport systems has resulted in the identification of the molecular basis of hereditary and acquired cholestatic syndromes. Apoptosis has been identified as an important mechanism of cholestatic liver injury and bile duct loss. New insights into the pathogenesis of pruritus and fatigue have resulted in new treatment strategies for these debilitating symptoms. Important new studies have been published about pathogenesis, clinical features, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, and drug-induced cholestasis.

Pediatric liver disease

Pediatric hepatology has advanced greatly over the past few years. Many more children with severe liver disease are now surviving into adulthood. There is a number of conditions not seen in adult practice that have been the focus of research efforts, and many of these efforts have borne fruit in the past year. Disorders characterized by intrahepatic cholestasis in particular have now been substantially unravelled, and this work has elucidated a great deal about hepatic physiology. Liver-directed gene therapy is on the threshold of human application. This research has been facilitated by excellent animal models and the advent of isolated hepatocyte transplantation. Relatively common conditions such as biliary atresia, however, remain largely unexplained and the viral hepatitides have no effective form of treatment. It remains a hope that our increase in knowledge in the fields of genetics and immunology will translate into advances in therapy.

Cloning, expression, and chromosomal mapping of a human ATPase II gene, member of the third subfamily of P-type ATPases and orthologous to the presumed bovine and murine aminophospholipid translocase

Recently, a P-type ATPase was cloned from bovine chromaffin granules (b-ATPase II) and a mouse teratocarcinoma cell line (m-ATPase II) and was shown to be homologous to the Saccharomyces cerevisiae DRS2 gene, the inactivation of which resulted in defective transport of phosphatidylserine. Here, we report the cloning from a human skeletal muscle cDNA library of a human ATPase II (h-ATPase II), orthologous to the presumed bovine and mouse aminophospholipid translocase (95.3 and 95.9% amino acid identity, respectively). Compared with the bovine and mouse counterparts, the cloned h-ATPase II polypeptide exhibits a similar membrane topology, but contains 15 additional amino acids (1163 vs 1148) located in the second intracytoplasmic loop, near the DKTGTLT-phosphorylation site. However, RT-PCR analysis performed with RNA from different human tissues and cell lines revealed that the coding sequence for these 15 residues is sometimes present and sometimes absent, most likely as a result of a tissue-specific alternative splicing event. The h-ATPase II gene, which was mapped to chromosome 4p14-p12, is expressed as a 9.5-kb RNA species in a large variety of tissues, but was not detected in liver, testis, and placenta, nor in the erythroleukemic cell line K562.

Bile acid transport

Bile acids undergo a unique enterohepatic circulation, which allows them to be efficiently reused with minimal loss. With the cloning of key bile acid transporter genes in the liver and intestine, clinicians now have a detailed understanding of how the different components in the enterohepatic circulation operate. These advances in basic knowledge of this process have directly led to a rapid and highly detailed understanding of rare genetic disorders of bile acid transport, which usually present as pediatric cholestatic disorders. Mutations in specific bile acid or lipid transporters have been identified within specific cholestatic disorders, which allows for genetic tests to be established for specific diseases and provides a unique opportunity to understand how these genes operate together. These same transporters may also prove useful for development of novel drug delivery systems, which can either enhance intestinal absorption of drugs or be used to target delivery to the liver or biliary system. Knowledge gained from these transporters will provide new therapeutic modalities to treat cholestatic disorders caused by common diseases.

Inborn errors of bile acid biosynthesis and transport. Novel forms of metabolic liver disease

Defective bile acid biosynthesis, metabolism, and transport can now be delineated in a wide variety of disease states. This ability to recognize specific aberrations, such as the documented inborn errors in bile acid biosynthesis manifesting as neonatal cholestasis, offers new opportunities for therapeutic intervention. Future studies should determine the incidence of bile acid biosynthetic and transport defects in patients with enigmatic and unexplained liver diseases.

Physical and metabolic factors in gallstone pathogenesis

Gallstones form when the tenuous balance of solubility of biliary lipids tips in favor of precipitation of cholesterol, unconjugated bilirubin, or bacterial degradation products of biliary lipids. For cholesterol gallstones, metabolic alterations in hepatic cholesterol secretion combine with changes in gallbladder motility and intestinal bacterial degradation of bile salts to destabilize cholesterol carriers in bile and produce cholesterol crystals. For black pigment gallstones, changes in heme metabolism or bilirubin absorption lead to increased bilirubin concentrations and precipitation of calcium bilirubinate. In contrast, mechanical obstruction of the biliary tract is the major factor leading to bacterial degradation and precipitation of biliary lipids in brown pigment stones. Further understanding of the physical and metabolic factors of cholesterol and black pigment formation is likely to provide interventions to interrupt the earliest stages of gallstone formation.

The mechanism of biliary lipid secretion and its defects

Biliary lipid secretion is an important physiological event; not only for the disposal of cholesterol from the body, but also for the protection of cells lining the biliary tree against bile salts. Insight into the (patho)physiological role of biliary lipid secretion has been recently expanded through the study of a generation of mice with a disruption of the Mdr2 gene, who do not secrete lipids into bile. Mdr2 P-glycoprotein translocates phospholipids across the hepatocanalicular membrane. These animals suffer from progressive liver disease caused by the toxic detergent action of bile salts. Very recently, it has become clear that an analogous inherited human liver disease exists, which is caused by the absence of biliary lipid secretion. Patients with this disease, Progressive Familial Intrahepatic Cholestasis (PFIC) type 3, have a mutation in the MDR3 gene, which is the human homologue of the murine Mdr2 gene.

Recurrent familial intrahepatic cholestasis in the Faeroe Islands. Phenotypic heterogeneity but genetic homogeneity

Recurrent familial intrahepatic cholestasis is an autosomal recessive disorder characterized by episodes of severe pruritus and jaundice lasting for weeks to months without extrahepatic bile duct obstruction. Symptom-free intervals may last for months to years, and chronic liver damage does not develop. We recently studied four of the five patients from the Faeroe Islands described by us 30 years ago (one had recently died) and an additional five patients that were identified after the initial report. The episodes of cholestasis were more frequent and severe in patients with early onset, but tended to reduce in frequency with age. The youngest patient, aged 25 years, who had had 16 episodes each lasting about 6 months, had a liver transplant after which no further episodes were recorded (1 year after surgery). Signs of chronic liver disease were absent in all patients. The FIC1 gene was investigated for mutations in the surviving patients. A single mutation (I661T) was found on both chromosomes in all nine patients, indicating that they are genetically identical for the disease-causing defect. Nevertheless, considerable differences among patients were observed clinically.

Sensorineural hearing loss associated with Byler disease

Progressive familial intrahepatic cholestasis, sometimes described as Byler disease, is a lethal liver disease and its inheritance is autosomal recessive. There is a previous report on the occasional association between this disease and sensorineural hearing loss without any audiological findings. We report here two siblings, an 18-year-old female and a 16-year-old male, suffering from Byler disease and hearing loss. Pure tone, Bekesy and speech audiometries and auditory brain stem response examination were performed. Audiometric data showed hearing characteristics of cochlear origin, high-frequency loss and progressiveness. This sensorineural hearing loss possibly results from a genetic mutation. The mechanism of cochlear disorder in patients with Byler disease is unknown, however, a novel gene responsible for deafness might be found to be related to Byler disease.

A gene encoding a liver-specific ABC transporter is mutated in progressive familial intrahepatic cholestasis

The progressive familial intrahepatic cholestases (PFIC) are a group of inherited disorders with severe cholestatic liver disease from early infancy. A subgroup characterized by normal serum cholesterol and gamma-glutamyltranspeptidase (gammaGT) levels is genetically heterogeneous with loci on chromosomes 2q (PFIC2) and 18q. The phenotype of the PFIC2-linked group is consistent with defective bile acid transport at the hepatocyte canalicular membrane. The PFIC2 gene has now been identified by mutations in a positional candidate, BSEP, which encodes a liver-specific ATP-binding cassette (ABC) transporter, sister of p-glycoprotein (SPGP). The product of the orthologous rat gene has been shown to be an effective bile acid transporter in vitro. These data provide evidence that SPGP is the human bile salt export pump (BSEP).

The role of dietary choline in the beneficial effects of lecithin on the secretion of biliary lipids in rats

Earlier studies showed that dietary soybean lecithin increases biliary lipid secretion, which mainly comes from the contribution of high density lipoprotein (HDL) and hepatic microsomal pools of phosphatidylcholine and cholesterol. In addition, a lecithin diet enhances bile secretion and prevents bile acid-induced cholestasis. This study evaluated the contribution of choline, a component of lecithin, to the observed effect of lecithin on biliary secretory function. Rats were fed either a control diet (CD), a choline diet (ChD) or a lecithin-enriched diet (LD) for 2 weeks. Results showed that like LD, ChD induced an increase in bile flow and bile acid secretion rate when compared with the control diet. However, unlike LD, ChD did not significantly increase biliary phospholipids and cholesterol output. An increase of hydrophilic bile acids (i.e. ursodeoxycholic and muricholic acids) in bile of rats fed choline could explain why the biliary phospholipid and cholesterol secretion was not increased. During taurocholic acid infusion, both experimental diets increased bile flow and the bile acid secretion rate maximum (BASRm). The cholestasis usually observed after the BASRm is reached was inhibited by ChD and LD. Both diets induced a decrease in plasma cholesterol (total and HDL), however, only LD induced statistically significant changes. Analysis of total cholesterol and phospholipid content of microsomes and canalicular membranes indicated no statistically significant difference between control and experimental groups either under basal conditions or after bile acid infusion. Similarly, the phospholipid classes and fatty acid composition of biliary phosphatidylcholine were not altered by feeding ChD and LD. We conclude that choline contributes to the beneficial effect of a lecithin diet on bile secretion. It is postulated that this effect may be attributed to modulation of HDL and an enhancement of the cholesterol and phospholipid pools destined for biliary secretion.