Pathogenesis of Cholestatic Liver Disease and Therapeutic Approaches

Cholestatic liver disorders are caused by genetic defects, mechanical aberrations, toxins, or dysregulations in the immune system that damage the bile ducts and cause accumulation of bile and liver tissue damage. They have common clinical manifestations and pathogenic features that include the responses of cholangiocytes and hepatocytes to injury. We review the features of bile acid transport, tissue repair and regulation, apoptosis, vascular supply, immune regulation, and cholangiocytes that are associated with cholestatic liver disorders. We now have a greater understanding of the physiology of cholangiocytes at the cellular and molecular levels, as well as genetic factors, repair pathways, and autoimmunity mechanisms involved in the pathogenesis of disease. These discoveries will hopefully lead to new therapeutic approaches for patients with cholestatic liver disease. Cholestatic liver disorders are caused by genetic defects, mechanical aberrations, toxins, or dysregulations in the immune system that damage the bile ducts and cause accumulation of bile and liver tissue damage. They have common clinical manifestations and pathogenic features that include the responses of cholangiocytes and hepatocytes to injury. We review the features of bile acid transport, tissue repair and regulation, apoptosis, vascular supply, immune regulation, and cholangiocytes that are associated with cholestatic liver disorders. We now have a greater understanding of the physiology of cholangiocytes at the cellular and molecular levels, as well as genetic factors, repair pathways, and autoimmunity mechanisms involved in the pathogenesis of disease. These discoveries will hopefully lead to new therapeutic approaches for patients with cholestatic liver disease. Cholestatic liver diseases arise from impaired hepatobiliary production and excretion of bile, which cause bile constituents to enter the circulation. In these disorders, injuries to bile ducts or hepatocytes can lead to a range of clinical presentations, from isolated abnormalities in liver biochemistry, to liver failure or hepatobiliary malignancy; congenital, immunologic, structural (obstructive/vascular), and toxic factors can all contribute to disease (Figure 1 and Table 1). In response to injury, mature cholangiocytes and hepatocytes proliferate, which may lead to periductular fibrosis, biliary fibrosis, and cirrhosis. Disease progression and the efficacy of repair depend on etiology and the individual's response to injury. We review mechanisms and biliary pathophysiology of cholestatic liver disease, focusing on primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC).Table 1The Characteristics of Common Cholestatic SyndromesPrimary biliary cirrhosisPrimary sclerosing cholangitisDrug induced cholestasisBiliary atresiaDemographicsFemale predominant; middle age onset; geographic hotspots; co-existent autoimmunity;Asymptomatic screening now most common mode of presentationMen more than women; onset before 40 years old common; high prevalence of co-existent colitis;Identified commonly through asymptomatic screening in inflammatory bowel disease; may present with cholangitis10% of drug induced liver injury is cholestatic; more common in elderly; jaundice and pruritus seenExclusively neonatal; Isolated in 65%–90%; Presents with persistent neonatal conjugated hyperbilirubinemia >14 days oldKey diagnosticsAnti-mitochondrial antibody (AMA) positive >95%; confirm immunofluorescence with specific AMA assayaThe mitochondrial autoantigens have been cloned, sequenced, and identified as members of the 2-oxo-acid dehydrogenase pathway, including the E2 subunits of pyruvate dehydrogenase (PDC-E2), branched-chain 2-oxo-acid dehydrogenase (BCOADC-E2), and 2-oxo-glutarate dehydrogenase (OGDC-E2).Cholangiographic appearance of stricturing and beading of intra- and/or extra-hepatic bile ducts;MRI largely supplanted ERCP as initial diagnostic modalityHistory of offending agentbCholestatic liver injury is reported with many agents including, but not exclusiviely, estrogens, anabolic steroids, chlorpromazine, erythromycin, the oxypenicillins, tamoxifen, macrolides, ticlopidine, terfenadine, terbinafine, nimesulide, irbesartan, fluoroquinolones, cholesterol-lowering ‘statins,’ herbal remedies (greater celandine, glycyrrhizin, chaparral), amoxicillin-clavulanic acid and ibuprofen. and exclusion of alternative etiologiesLiver biopsy differentiates obstructive and hepatocellular causes of cholestasis, with 90% sensitivity and specificity; cholangiography if diagnostic doubt (patent biliary tree proximally and distally excludes biliary atresia)Characteristic pathologyGranulomatous non-suppurative small duct cholangitis, ductopenia, ductal proliferation, interface hepatitisBiopsy is not required routinely for diagnosis or stagingFibro-obliterative cholangitis, periductal fibrosis and inflammation, absence of bile ducts in some portal tracts and ductal proliferation in other portal tractsBiopsy is not required routinely for diagnosis or stagingBland cholestasis or cholestatic hepatitis>Biopsy commonly performed to exclude alternative etiologiesActive inflammation with bile duct degeneration, a chronic inflammatory reaction with proliferation of both ductular and glandular elements, and fibrosisBiopsy commonly performedOverlapping featuresAnti-nuclear antibodies present in 50%cHighly specific anti-nuclear antibodies are detected in ∼ 50% of patients with PBC (gp210 and sp100). and possible overlap with autoimmune hepatitis (AIH) in <10%Autoimmune serology frequently positive; pediatric onset AIH is accompanied by sclerosing cholangitis in 50% of cases; overlap with AIH in adults <10%.Non-specific serology may be presentEmbryonic form in 10%–35% with associated situs inversus or polysplenia/asplenia +/- other congenital anomaliesNatural history and treatmentProgressive biliary cirrhosis, portal hypertension and liver failureRecurrent cholangitis, progressive biliary cirrhosis, portal hypertension and liver failure; malignancy (hepatobiliary/colorectal)Usually self limiting but overt ductopenia may be observedBiliary cirrhosis, liver failureTreatmentUrsodeoxycholic acid (UDCA) 13–15 mg/kg/day effective in majorityNo primary medical intervention presently; liver transplant effective rescue therapy (>50% require transplant within 15 years of symptoms)No proven therapy other than cessation of drugNo primary medical therapy; Kasai porto-enterostomy and/or liver transplantation highly effectiveDisease severity indicesBiochemical response to UDCA predicts disease progression and survival, thus identifying patients at risk of adverse outcomedA number of biochemical predictors of treatment response are associated with histological progression and survival in patients treated with UDCA (eg, Barcelona, Paris and Toronto criteria respectively).No early markers of outcome; Mayo PSC score and MELD of use in late disease“Hy's rule”: if both drug-induced hepatocellular injury and jaundice occur simultaneously, a mortality of at least 10% can be expectedIf bilirubin is less than 2 mg/dL at 3 months post-surgery, then the chance of being transplant-free at 2 years of age is 84%Key unmet needSecond line therapy for UDCA non-respondersEffective primary treatment; effective screening for cholangiocarcinoma; intervention for transplant recurrence; surrogate end points of disease progressionNo effective therapy for progressive injury; predictive tools for preventing exposure to risk-prone patientsEarly diagnosis to facilitate early surgery (<60 days old); no effective medial therapyDiagnostic pitfallsAMA identified in autoimmune hepatitis, acute liver failure and 0.5% of healthy individuals; isolated elevated alkaline phosphatase values occur in NAFLDIgG4 associated autoimmune pancreatitis/sclerosing cholangitis responds to steroid therapy; ∼10% of patients with PSC have elevated IgG4Liver injury can present after the cessation of potentially injurious medicinesDifferential diagnosis include Alagille syndrome, progressive familial intrahepatic cholestasis, alpha-1-antitrypsin deficiency, and cystic fibrosisa The mitochondrial autoantigens have been cloned, sequenced, and identified as members of the 2-oxo-acid dehydrogenase pathway, including the E2 subunits of pyruvate dehydrogenase (PDC-E2), branched-chain 2-oxo-acid dehydrogenase (BCOADC-E2), and 2-oxo-glutarate dehydrogenase (OGDC-E2).b Cholestatic liver injury is reported with many agents including, but not exclusiviely, estrogens, anabolic steroids, chlorpromazine, erythromycin, the oxypenicillins, tamoxifen, macrolides, ticlopidine, terfenadine, terbinafine, nimesulide, irbesartan, fluoroquinolones, cholesterol-lowering ‘statins,’ herbal remedies (greater celandine, glycyrrhizin, chaparral), amoxicillin-clavulanic acid and ibuprofen.c Highly specific anti-nuclear antibodies are detected in ∼ 50% of patients with PBC (gp210 and sp100).d A number of biochemical predictors of treatment response are associated with histological progression and survival in patients treated with UDCA (eg, Barcelona, Paris and Toronto criteria respectively). Open table in a new tab Approximately 5% of cells in the liver are cholangiocytes; these ciliated epithelial cells line the biliary tree, an intricate network of interconnecting bile ducts that increase in diameter from the ducts of Hering to the extrahepatic bile ducts.1Alpini G. McGill J.M. Larusso N.F. The pathobiology of biliary epithelia.Hepatology. 2002; 35: 1256-1268Google Scholar, 2Sirica A.E. Nathanson M.H. Gores G.J. et al.Pathobiology of biliary epithelia and cholangiocarcinoma: proceedings of the Henry M. and Lillian Stratton Basic Research Single-Topic Conference.Hepatology. 2008; 48: 2040-2046Google Scholar Cholangiocytes that line the large interlobular and major ducts are predominantly involved in secretory functions, whereas cholangiocytes that line smaller bile duct branches, cholangioles, and ducts of Hering have roles in inflammatory and proliferative responses.3Alpini G. Roberts S. 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Banales J.M. et al.Cholangiocyte primary cilia are chemosensory organelles that detect biliary nucleotides via P2Y12 purinergic receptors.Am J Physiol Gastrointest Liver Physiol. 2008; 295: G725-G734Google Scholar Cholangiocyte stimulation through paracrine and endocrine routes leads to secretion of water and biliary alkalinization with a net luminal flux of chloride and bicarbonate.7Strazzabosco M. Mennone A. Boyer J.L. Intracellular pH regulation in isolated rat bile duct epithelial cells.J Clin Invest. 1991; 87: 1503-1512Google Scholar The process of integrating prosecretory (secretin, glucagon, vasoactive intestinal polypeptide, acetylcholine, bombesin) and antisecretory (somatostatin, endothelin-1) stimuli involves transmembrane adenylyl cyclases that regulate the concentration of intracellular 3′, 5′-cyclic monophosphate.8Fiorotto R. Spirli C. 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It's all about bile.Hepatology. 2009; 49: 711-723Google Scholar Nuclear receptors regulate transcription of genes that encode proteins involved in hepatobiliary transport systems, bile acid synthesis, bile acid detoxification,13Wagner M. Zollner G. Trauner M. Nuclear receptor regulation of the adaptive response of bile acid transporters in cholestasis.Semin Liver Dis. 2010; 30: 160-177Google Scholar and fibrogenesis.14Fiorucci S. Antonelli E. Rizzo G. et al.The nuclear receptor SHP mediates inhibition of hepatic stellate cells by FXR and protects against liver fibrosis.Gastroenterology. 2004; 127: 1497-1512Google Scholar, 15Fickert P. Fuchsbichler A. Moustafa T. et al.Farnesoid X receptor critically determines the fibrotic response in mice but is expressed to a low extent in human hepatic stellate cells and periductal myofibroblasts.Am J Pathol. 2009; 175: 2392-2405Google Scholar These nuclear receptors include the farnesoid X receptor, pregnane X receptor, vitamin D receptor, and constitutive androstane receptor. Bile acids are internalized across the basolateral membranes of hepatocytes by the Na+/taurocholate cotransporter and organic anion transporting proteins (OATP2/OATP1B1); this transporter also mediates the hepatic uptake of many drugs.16Konig J. Seithel A. Gradhand U. et al.Pharmacogenomics of human OATP transporters.Naunyn Schmiedebergs Arch Pharmacol. 2006; 372: 432-443Google Scholar Genetic polymorphisms in the gene encoding this transporter, SLCO1B1, associate with total bilirubin levels in healthy individuals17Johnson A.D. Kavousi M. Smith A.V. et al.Genome-wide association meta-analysis for total serum bilirubin levels.Hum Mol Genet. 2009; 18: 2700-2710Google Scholar and development of statin-induced myopathy.18Link E. Parish S. Armitage J. et al.SLCO1B1 variants and statin-induced myopathy--a genomewide study.N Engl J Med. 2008; 359: 789-799Google Scholar Active export into bile is mediated by the canalicular bile salt export pump (ABCB11)19Lam P. Soroka C.J. Boyer J.L. The bile salt export pump: clinical and experimental aspects of genetic and acquired cholestatic liver disease.Semin Liver Dis. 2010; 30: 125-133Google Scholar and the canalicular conjugate export pump (MRP2).20Nies A.T. Keppler D. The apical conjugate efflux pump ABCC2 (MRP2).Pflugers Arch. 2007; 453: 643-659Google Scholar MRP2 regulates canalicular excretion of organic anions, such as bilirubin. Formation of mixed micelles in bile results from the presence of bile acids, cholesterol, and phosphatidylcholine, and the phospholipid export pump, multi-drug– resistant 3 protein (MDR3), is actively involved in controlling the process.21Davit-Spraul A. Gonzales E. Baussan C. et al.The spectrum of liver diseases related to ABCB4 gene mutations: pathophysiology and clinical aspects.Semin Liver Dis. 2010; 30: 134-146Google Scholar The cholangiocytes in the smaller bile ducts, cholangioles, and ducts of Hering express receptors that allow proliferation in response to liver damage and participation in inflammatory responses.22Chen X.M. O'Hara S.P. LaRusso N.F. The immunobiology of cholangiocytes.Immunol Cell Biol. 2008; 86: 497-505Google Scholar, 23Chuang Y.H. Lan R.Y. Gershwin M.E. The immunopathology of human biliary cell epithelium.Semin Immunopathol. 2009; 31: 323-331Google Scholar These receptors mediate protection from pathogens (via signals from antimicrobial peptides or the Toll-like receptors 2, 3, 4, and 5, which bind to bacterial molecules, double-stranded RNA, gram-negative bacteria, and lipopolysaccharide24Chen X.M. O'Hara S.P. Nelson J.B. et al.Multiple TLRs are expressed in human cholangiocytes and mediate host epithelial defense responses to Cryptosporidium parvum via activation of NF-kappaB.J Immunol. 2005; 175: 7447-7456Google Scholar), antigen presentation (HLA molecules and costimulatory molecules25Leon M.P. Bassendine M.F. Wilson J.L. et al.Immunogenicity of biliary epithelium: investigation of antigen presentation to CD4+ T cells.Hepatology. 1996; 24: 561-567Google Scholar, 26Spengler U. Leifeld L. 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Randhawa S. et al.CD40 activation-induced, Fas-dependent apoptosis and NF-kappaB/AP-1 signaling in human intrahepatic biliary epithelial cells.FASEB J. 2001; 15: 2345-2354Google Scholar), and leukocyte apoptosis. Reactive cholangiocytes also produce growth factors such as vascular endothelial growth factor, endothelin-1, platelet-derived growth factor BB, transforming growth factor β2, and connective tissue growth factor. Biliary proliferation contributes to the initiation and progression of liver fibrosis.30Glaser S.S. Gaudio E. Miller T. et al.Cholangiocyte proliferation and liver fibrosis.Expert Rev Mol Med. 2009; 11: e7Google Scholar, 31Marzioni M. Fava G. Alvaro D. et al.Control of cholangiocyte adaptive responses by visceral hormones and neuropeptides.Clin Rev Allergy Immunol. 2009; 36: 13-22Google Scholar Cholangiocytes normally do not proliferate because they constitutively express the cyclin-dependent kinase inhibitors p27, Bbcl2, Bcl-xL, and Mcl-1. In response to liver damage, gastrointestinal (GI) and neuroendocrine hormones, and autocrine and paracrine signaling factors, cholangiocytes proliferate and acquire a neuroendocrine secretory phenotype. In the “ductular reaction,” an expanded population of epithelial cells accumulates at the interface of the biliary tree and hepatocytes, along with proliferation of preexisting ductules, activation of progenitor cells, and the appearance of intermediate hepatocytes. The integrin αvβ6 is up-regulated in proliferating bile duct epithelia and promotes fibrogenesis via adhesion to fibronectin and autocrine/paracrine activation of transforming growth factor β1.32Patsenker E. Popov Y. Stickel F. et al.Inhibition of integrin alphavbeta6 on cholangiocytes blocks transforming growth factor-beta activation and retards biliary fibrosis progression.Gastroenterology. 2008; 135: 660-670Google Scholar Reactive cholangiocytes express receptors, such as the β1 and β2 adrenergic receptors, the M3 acetylcholine receptor, and serotonin 1A and 1B receptors, that bind neurotransmitters. Cholangiocytes can also directly secrete serotonin, which limits the growth of bile ducts through an inhibitory, autocrine loop.33Marzioni M. Glaser S. Francis H. et al.Autocrine/paracrine regulation of the growth of the biliary tree by the neuroendocrine hormone serotonin.Gastroenterology. 2005; 128: 121-137Abstract Full Text Full Text PDF Scopus (109) Google Scholar Epithelial-mesenchymal transition is a process that might also occur during cholangiocyte proliferation and fibrogenesis.34Omenetti A. Porrello A. Jung Y. et al.Hedgehog signaling regulates epithelial-mesenchymal transition during biliary fibrosis in rodents and humans.J Clin Invest. 2008; 118: 3331-3342Google Scholar, 35Choi S.S. Diehl A.M. Epithelial-to-mesenchymal transitions in the liver.Hepatology. 2009; 50: 2007-2013Google Scholar, 36Scholten D. Osterreicher C.H. Scholten A. et al.Genetic labeling does not detect epithelial-to-mesenchymal transition (EMT) of cholangiocytes in liver fibrosis in mice.Gastroenterology. 2010; 139: 987-998Google Scholar Some epithelia-derived mesenchymal cells undergo a mesenchymal-epithelial transition, reverting to epithelial cells that ultimately become hepatocytes or cholangiocytes. The hedgehog signaling pathway appears to promote and regulate hepatic accumulation of immune cells that interact with cholangiocytes. An unbalanced equilibrium between cholangiocyte death (via apoptosis or necrosis) and proliferation leads to duct loss and fibrosis. In advanced biliary disease, cholangiocytes lose the ability to proliferate37Kahraman A. Gerken G. Canbay A. Apoptosis in immune-mediated liver diseases.Dig Dis. 2010; 28: 144-1449Google Scholar; in progressive ductopenia, there is more apoptosis than proliferation. Apoptosis contributes to duct loss and is induced by signals such as activation of death receptors, immune-mediated injury, oxidative stress, infections, and toxins. Apoptosis also promotes fibrogenesis, with apoptotic debris contributing to activation of hepatic stellate cells. Cholangiocytes are the primary epithelial source of tumor necrosis factor α in the liver, a proinflammatory mediator that activates caspase cleavage and apoptosis but also mediates cell survival via activation of the transcription factor nuclear factor κB. In vitro studies have shown that a combination of tumor necrosis factor α, interleukin (IL)-1, IL-6, and interferon gamma inhibits 3′, 5′-cyclic monophosphate–dependent ductal secretion. Nuclear factor κB, which is regulated by I-κB and its kinases (IKKs), regulates immune and inflammatory responses; it protects against cytokine-induced death and oxidative damage. Mice with disruptions in IKK1 and IKK2 or IKK1 and NEMO (nuclear factor κB essential modulator) develop jaundice and a fatal cholangitis, characterized by inflammatory destruction of small portal bile ducts.38Luedde T. Heinrichsdorff J. de Lorenzi R. et al.IKK1 and IKK2 cooperate to maintain bile duct integrity in the liver.Proc Natl Acad Sci U S A. 2008; 105: 9733-9738Google Scholar The combined loss of IKK1-specific functions leads to biliary disease, a process proposed to result from changes to the regulation of tight junctions in biliary epithelial cells. Activation of the receptor tumor necrosis factor–related apoptosis-inducing ligand 2 and death receptor 5 induce cell death and mediate cholestatic liver injury.39Takeda K. Kojima Y. Ikejima K. et al.Death receptor 5 mediated-apoptosis contributes to cholestatic liver disease.Proc Natl Acad Sci U S A. 2008; 105: 10895-10900Google Scholar Healthy cholangiocytes do not express tumor necrosis factor–related apoptosis-inducing ligand, but diseased cholangiocytes up-regulate it, which might control inflammatory responses by killing leukocytes that express the death receptors. In healthy liver, lymphocytes are scattered throughout the parenchyma and portal tracts and include subpopulations from the innate and adaptive immune systems. Less than 10% of human intrahepatic lymphocytes are B cells; most are CD5+ B1 cells that control innate and adaptive immunity.40Selmi C. Podda M. Gershwin M.E. Old and rising stars in the lymphoid liver.Semin Immunopathol. 2009; 31: 279-282Google Scholar, 41Dienes H.P. Drebber U. Pathology of immune-mediated liver injury.Dig Dis. 2010; 28: 57-62Google Scholar T-cell populations, primarily CD4+ helper (Th1 and Th2), CD8+ cytotoxic, T-regulatory, and Th17 cells, have important roles in pathogenesis, along with hepatic macrophages. Nonparenchymal liver cells involved in immune system tolerance to liver include resident dendritic cells, liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells. These cells mediate immunosuppression by producing anti-inflammatory cytokines such as IL-10 and transforming growth factor β; they also express the inhibitor of T-cell activation, programmed cell death 1. IL-10, transforming growth factor β, programmed cell death 1, and cytotoxic T lymphocyte antigen can contribute to the immunosuppressive mechanisms of CD4+CD25+Foxp3+ regulatory T cells, which appear to be converted in the liver from infiltrating naive CD4+ T cells and/or effector CD4+ T cells. In response to inflammation, cholangiocytes secrete cytokines and chemokines (eg, tumor necrosis factor α, IL-1, or interferon gamma) that recruit and activate immune cells, including T cells, macrophages, and natural killer (NK) cells.23Chuang Y.H. Lan R.Y. Gershwin M.E. The immunopathology of human biliary cell epithelium.Semin Immunopathol. 2009; 31: 323-331Google Scholar, 42Borchers A.T. Shimoda S. Bowlus C. et al.Lymphocyte recruitment and homing to the liver in primary biliary cirrhosis and primary sclerosing cholangitis.Semin Immunopathol. 2009; 31: 309-322Google Scholar Human cholangiocytes constitutively express and secrete chemotactic agents for neutrophils, monocytes, and T cells, including IL-8, IL-6, and MCP-1; under basal conditions, cholangiocytes express low levels of lymphocyte adhesion molecules. In normal livers, some cholangiocytes express HLA class I but not class II molecules. However, cholangiocytes from patients with PBC express HLA class II, but it is not known if costimulatory molecules for T-cell activation are present. The presence of the adhesion molecule leukocyte factor antigen 3 on the cholangiocyte cell surface allows their interaction with CD2 on cytotoxic T lymphocytes and NK cells. T cells are also activated by CD40, which is expressed on cholangiocytes.43Alabraba E.B. Lai V. Boon L. et al.Coculture of human liver macrophages and cholangiocytes leads to CD40-dependent apoptosis and cytokine secretion.Hepatology. 2008; 47: 552-562Google Scholar The CD40-CD40L and leukocyte factor antigen 2 (CD2)/leukocyte factor antigen 3 (CD58) complexes induce production of IL-12. Cholangiocytes also secrete and transport protective immunoglobulins. In bile, immunoglobulin (Ig) A has a role in biliary mucosal immune defense; by preventing the attachment of pathogens or their toxins to the cholangiocyte surface, it protects biliary ducts.44Mantis N.J. Forbes S.J. Secretory IgA: arresting microbial pathogens at epithelial borders.Immunol Invest. 2010; 39: 383-406Google Scholar IgA is synthesized by plasma cells around bile ducts and secreted into bile after it binds to the polymeric Ig receptor located on the basolateral membranes of cholangiocytes. Bile ducts are supplied with blood only from hepatic arteries; in contrast to hepatocytes, the biliary epithelium receives blood from a network of capillaries near the intrahepatic bile ducts, the peribiliary vascular plexus, which originate from the terminal branches of the hepatic artery. This specific vascular supply, lacking in canals of Hering and terminal cholangioles, accounts for the prevalent involvement of the interlobular bile ducts in ischemic injury.45Strazzabosco M. Fabris L. Functional anatomy of normal bile ducts.Anat Rec (Hoboken). 2008; 291: 653-660Google Scholar From the many genetic insights recognized in both common and uncommon cholestatic liver diseases, it is possible to appreciate a number of important pathophysiologic pathways likely relevant to cholestasis more generally. Alagille syndrome (arteriohepatic dysplasia or congenital deficiency in interlobular bile ducts) is characterized by chronic cholestasis, posterior ocular embryotoxon, butterfly-like vertebral arch defects, peripheral pulmonary artery hypoplasia or stenosis, and dysmorphic facial features.46Kamath B.M. Loomes K.M. Piccoli D.A. Medical management of Alagille syndrome.J Pediatr Gastroenterol Nutr. 2010; 50: 580-586Google Scholar This autosomal dominant multi-system disorder varies in phenotypic expression and is largely associated with mutations in JAG1, which encodes a ligand in the Notch signaling pathway. Less than 1% of patients have mutations in the receptor NOTCH2. By altering the expression of liver-enriched transcription factors, Notch signaling contributes to biliary tree development during ductal plate remodeling and controls transdifferentiation of hepatoblasts and mature hepatocytes into cholangiocytes.47Kodama Y. Hijikata M. Kageyama R. et al.The role of notch signaling in the development of intrahepatic bile ducts.Gastroenterology. 2004; 127: 1775-1786Google Scholar In the course of cholestatic liver disease more generally, changes in the expression of Jagged1 and Notch have been described. Changes in cell-cell tight junction function and membrane fusion are also associated with cholestasis, leading to changes in the cytoskeletons and tight junctions of hepatocytes. Neonatal ichthyosis-sclerosing cholangitis syndrome, a rare autosomal recessive condition characterized by hypotrichosis of the scalp, scarring alopecia, ichthyosis, and sclerosing cholangitis, has been associated with changes in the gene that encodes the tight junction protein claudin-1,48Hadj-Rabia S. Baala L. Vabres P. et al.Claudin-1 gene mutations in neonatal sclerosing cholangitis associated with ichthyosis: a tight junction disease.Gastroenterology. 2004; 127: 1386-1390Google Scholar whereas other even rarer multi-system disorders, including neonatal cholestasis, have been associated with defects in vesical fusion (eg, exocytosis).49Gissen P. Johnson C.A. Morgan