Intrahepatic Cholestasis

A heterogeneous subset of cholestatic diseases, characterized by intrahepatic cholestasis with or without bile duct alterations (paucity), represents specific syndromes with different prognostic implications. There are multiple forms with varying clinical features and with a high degree of variability in presentation and prognosis. Certain progressive, familial forms, such as Byler disease, are often fatal; however, in patients with syndromic paucity of the ducts (Alagille syndrome), the prognosis is much more favorable. The current system of nomenclature for the various syndromes in which intrahepatic cholestasis is present is imperfect. The terminology used to describe the clinical syndromes and the attendant hepatic pathology has mutated rapidly, with no evidence of consensus. These syndromes are of great theoretical interest—detailed study may enhance our understanding of hepatic excretory function and bile acid metabolism. The pathogenetic basis for these diseases has been defined only partially, and the techniques of molecular genetics have been applied only recently to these disorders. Precise terminology of the intrahepatic cholestatic disorders, based on the documented genetic defect, should be a high priority. This not only will provide clues to the pathophysiology but will also help establish registries, promote development of new treatments, and allow the institution of valid clinical therapeutic trials. Table 1 reflects a proposed classification scheme. The term bile duct paucity indicates an absence or marked decrease in the number of interlobular bile ducts within the portal triad, defined as fewer than 0.5 bile ducts for each portal triad (BD/T), despite the presence of normal-sized branches of the portal venule and hepatic arteriole. In full-term infants, children, and adults, the normal ratio is 0.9 to 1.8 BD/T; a value of less than 0.9 is seen commonly in infants before 30 weeks of gestation. Alagille et al. (1) described the most commonly occurring constellation of findings to incorporate intrahepatic bile duct paucity. Patients with this syndrome characteristically have cholestasis, a decreased number of interlobular ducts, and various congenital malformations (e.g., peripheral pulmonic stenosis, vertebral arch defects, renal disease, and posterior embryotoxon). Serum levels of alkaline phosphatase, γ-GTP, and bile acids are increased, indicating a defect in biliary excretion. The mechanism of cholestasis is not known. Affected family members may manifest only one or two of these “syndromic” features. Therapy is highly ineffective. Despite the generally nonprogressive nature of the liver disease, liver transplantation has been performed in desperation. The term nonsyndromic paucity has been used in patients whose liver biopsy tissue shows an absence or decreased number of interlobular bile ducts, yet in whom none of the classic features of Alagille syndrome are present. This finding probably represents an eclectic group of diseases that is now more readily understood. Patients with intrahepatic cholestasis associated with paucity of interlobular bile ducts without associated abnormalities must be distinguished from patients with syndromic paucity because the prognoses are disparate. A familial tendency is noted frequently in the nonsyndromic form, suggesting that the underlying pathophysiology is a spate of genetic/metabolic aberrations (2, 3). These cases may represent either late manifestation of “infantile obstructive cholangiopathy” or ongoing obliteration of an unknown nature (4-6). Severe forms of intrahepatic cholestasis with progressive hepatocellular damage occur sporadically or on a familial basis. The clinical and pathologic features and natural progression described in case reports vary, implying significant heterogeneity among this group of patients. Therefore, the term progressive familial intrahepatic cholestasis (PFIC) is used to refer to the group of disorders with the following: the presence of chronic, unremitting hepatocellular cholestasis; the exclusion of identifiable metabolic or anatomic disorders; an occurrence pattern consistent with autosomal recessive inheritance; and a characteristic combination of clinical, biochemical (e.g., subgroups have decreased/normal serum γ-GTP levels in the presence of cholestasis), and histologic features (7-9). Progressive familial intrahepatic cholestasis typically presents in the first 6 months of life as cholestasis, hepatomegaly, severe pruritus, growth failure, pancreatic insufficiency, or fat-soluble vitamin deficiency. The pathophysiology remains largely undefined. Currently treatment is based on liver replacement, or biliary diversion. Paucity syndromes may have a disparate genesis, representing congenital absence, partial failure to form, atrophy secondary to diminished bile flow, or progressive injury (immune, viral, ischemic) with disappearance. The histopathologic changes may relate to presumed physiologic alterations; the ductular abnormalities associated with intrahepatic cholestasis may represent a primary functional or enzymatic defect or a change secondary to the toxic effects of retained compounds, such as bile acids. The pathogenesis of bile duct paucity is unknown. However, the progressive nature (segmental destructive changes or progressive decrease in the number of bile ducts per portal tract seen in serial sectioning of biopsy specimens), from the early features of bile duct inflammation to the later observation of paucity, suggests immunologic injury to existing ducts—similar to other syndromes of “disappearing” intrahepatic bile ducts—rather than failure of ducts to develop. Other postulated mechanisms include alterations in bile acid metabolism, chromosomal abnormalities, and intrauterine or postnatal infection. The characterization of immunologic and biologic processes that lead to paucity in the neonate and during childhood must be established by carefully studying the biliary epithelial cell physiology during development and studying its interaction with growth factors, cytokines, and other effectors of the immune system. Developing isolated cholangiocyte models and relevant animal models would greatly enhance our understanding of these disorders. Further characterization of the responsible gene(s) and molecular testing must be performed to understand the pathophysiology and to precisely diagnose Alagille syndrome. These studies would be of great clinical benefit in avoiding unnecessary surgery, in initiating proper therapy, and in instituting genetic counseling. Alagille syndrome exhibits a complex phenotype and inheritance pattern. Alagille syndrome has been reported in successive generations of single kindreds, strongly supporting an autosomal dominant mode of inheritance, with decreased penetrance and variable expressivity. Siblings and parents of probands often have mild expression of the disease gene, with only one or two abnormalities. Systematic screening for these specific features should be performed to improve the accuracy of diagnosis and genetic counseling. A segregation analysis of 33 families collected through 43 probands corroborated the theory of autosomal dominant inheritance; penetrance was 94%, and 15% of the cases were sporadic (10). Expressivity of the Alagille phenotype was variable—relatives (considered affected if they exhibited any one of the major features) were identified as presenting minor forms of the disease. The frequency of new mutations seems to be high (15–50%) (11). Therefore, in the absence of definitive clinical or genetic markers for carriers, counseling for recurrence risks is inaccurate. Cytogenetic abnormalities in multiple Alagille syndrome patients in whom the disease gene has been mapped beg the issue of the role of chromosome 20 in regulating morphogenesis (12). Partial deletion of the short arm of chromosome 20 (del 20p) has been noted in only a small proportion (h7%) of affected individuals (13). This low frequency of del 20p suggests that Alagille syndrome is not a contiguous gene syndrome but is caused by the alteration of a single gene (14). The candidate region for the Alagille gene has been narrowed to a 250 kb segment within 20p12; within this region JAG1 has been identified (15, 16). Mutations in human JAG1 (Jagged 1), which encodes a ligand for the Notch receptor, have been linked to Alagille syndrome (15, 16). Members of the Notch gene family that encode evolutionarily conserved transmembrane receptors are involved in cell fate specification during embryonic development. The Notch locus encodes a receptor that mediates cell–cell interactions. Loss of Notch function leads to embryonic lethality in Drosophila, female flies carrying only a single copy of the gene have notched wings. Future studies must focus on the Notch signaling pathway, which seems to define a fundamental mechanism controlling cell fate in early development, perhaps controlling the ability of non–terminally differentiated cells to respond to differentiation/proliferation signals. Therefore, the broad spectrum of organ involvement in Alagille syndrome could be explained by human JAG1 mutations. Two studies have shown that patients with Alagille syndrome have mutations in the JAG1 gene that are capable of disrupting the gene product and potentially altering cell-to-cell signaling during development (15, 16). Therefore, patients with cytogenetically detectable deletions that include JAG1 have Alagille syndrome, supporting the hypothesis that haploinsufficiency for this gene is one of the mechanisms that cause the Alagille syndrome phenotype. The exact functional consequences of these mutations remain to be defined. How does the JAG1 mutation cause multiorgan disease? How are the development of intrahepatic bile ducts and bile secretion affected by abnormalities in JAG1 function? Future studies must define the frequency and type of mutations and their association to phenotype. It is also not known whether all or only a subset of patients with Alagille syndrome produce mutant forms of human JAG1. It also remains uncertain how this defective gene leads to the loss of interlobular bile ducts and in some (but not all) cases to progressive cholestatic liver disease and other organ disease, including pancreatic disease. Multicenter collaborative studies could aid in dissection of the functional genomics. In patients with any form of intrahepatic cholestasis, the clinical consequences of prolonged cholestasis attributable directly or indirectly to diminished bile flow must be addressed: 1) decreased delivery of bile acids to the proximal intestine, with decreased intraluminal bile acid concentrations, resulting in malabsorption of fat and fat-soluble vitamins; 2) retention by the liver of substances normally excreted in bile—namely, bile acids, cholesterol, and trace elements; and 3) progressive liver damage, with biliary cirrhosis, portal hypertension, and liver failure. Cholestasis, liver damage, and malnutrition interact to alter the metabolism of factors required for growth, resulting in marked growth failure. Studies of hormonal interactions and a search for receptor defects may uncover the mechanisms of growth failure. The impact of protein-calorie malnutrition is significant because nutritional status predicts outcome before and after liver transplantation. Therefore, for patients with intrahepatic cholestasis, treatment becomes an effort to minimize growth failure and to decrease discomfort, often as they await liver transplantation. The limiting factors are the residual functional capacity of the liver and the rate of liver disease progression. Recommendations for managing cholestasis are currently empiric; individualization and careful monitoring serve as the most reliable guides. The factors that contribute to the decision for transplantation in patients with Alagille syndrome are bone fractures, pruritus, and xanthomas; cirrhosis is not common and transplantation often is performed to improve the quality of life (17). The major clinical issue in patients with Alagille syndrome is intractable itching. To effectively treat patients with Alagille syndrome, it is important to understand the mechanism of pruritus. How can the “itch signal” be altered (medically, surgically, or otherwise) in patients with Alagille syndrome? Can currently available antipruritic agents, such as ursodeoxycholic acid (UDCA), be made more effective (18)? Partial external biliary diversion (PEBD) has been attempted in several patients who were unresponsive to all medical therapy, especially older, larger patients (19). The procedure is not universally effective and may not benefit young patients (infants). Partial external biliary diversion may decrease the intensity of pruritus and the degree of hypercholesterolemia. Should concomitant UDCA therapy and other antipruritic therapies be continued to maximize the beneficial effect of PEBD? Criteria should be developed to define subgroups of patients who may benefit from this approach. Many other questions remain about effectively managing pruritus. What is the mechanism of the beneficial effect of rifampin? Are central opioid antagonists effective and safe in children with pruritus? To understand the pathophysiology of PFIC, molecular and physiologic investigation of basic mechanisms involved in bile formation must be performed. Generation of bile flow is a regulated, adenosine triphosphate–dependent process attributed to the coordinated activity of a variety of transporter proteins localized to the sinusoidal and canalicular domains of the liver cell and to ion channels of the cholangiocyte (20). Therefore, inherited or acquired dysfunction of any of these proteins may result in substrate retention and may be manifest as cholestasis. The phenotype of PFIC presumably reflects defects in any of several genes that express proteins important in bile formation (e.g., MDR3, FIC1, BSEP) (Table 2). With the identification, cloning, and functional characterization of transport proteins involved in bile formation, correlation of genetic mutations and hepatocellular dysfunction with specific subtypes of PFIC must be performed. Assessing morphologic findings, analyzing bile composition, and dissecting molecular genetics will allow more precise categorizing of PFIC. Proper categorization of these disorders will allow the development of accurate diagnostic and therapeutic strategies. The first detailed description of PFIC involved members of an Amish kindred, descended from Jacob Byler; therefore the term Byler disease was used to describe this variant. Since that initial report, several patients and families in many populations with disease that phenotypically, but not genotypically, resembles that of affected members of the Byler family have been described. Thus, PFIC is a syndrome; children who have a disorder similar to Byler disease, but are not members of the Amish kindred in which this disorder was described, are said to have Byler syndrome. Although subtypes of PFIC represent more than one clinicopathologic entity, controversy exists as to the proper nomenclature. These diseases that have been characterized by phenotype, often eponymously, must be subjected to a strict nosologic categorization through linkage to specific genetic loci and, ultimately, on the basis of the identified pathophysiologic defect. Currently, a combination of clinical and pathologic features is used to define subsets of the disease. For example, genetic analysis and light and electron microscopy of the liver may help distinguish Byler disease from other forms of Byler syndrome (21). The responsible gene must be identified, followed by functional characterization, mutation and of other genes involved in liver disease. The gene for Byler disease has been mapped to a region of chromosome The disease caused by a in this region has been for intrahepatic cholestasis have been mapped to the locus on chromosome Although PFIC and are diseases because they to the chromosomal they are diseases caused by different mutations in the of the gene responsible for PFIC and and of the gene product will provide important the of bile secretion analysis narrowed the candidate region for diseases to the of less than 1 in which a gene mutated in patients with and was identified This gene is the first identified human of a recently described of The gene is in several epithelial and, more strongly in the and the small than in the characterization of its product is probably will an role in of bile acids. Further characterization of and the physiologic function of in liver and other must be performed to understanding of normal bile formation and cholestasis In PFIC linked to FIC1, cholestasis and pruritus may and but may microscopy findings on liver biopsy in are in from in but portal tract and are seen with in patients with microscopy in patients with shows of the canalicular which is with a decreased number of and of the canalicular (21). The bile in with electron microscopy in has not been identified in or in other forms of PFIC. The phenotype is of a defect in bile acid transport the canalicular To no specific in has been however, a in of the bile acids seems to occur among and The biliary bile acid are with a of However, acid in the serum and in the Further study of this proposed defect in canalicular which for the paucity of in should be performed. of this transport defect by may be an effective treatment for patients with A form of PFIC not linked to chromosome was recently described, in and populations The gene responsible for this form of PFIC has been mapped to chromosome this disorder has been during the first weeks of and serum γ-GTP levels are of the liver shows paucity of intrahepatic bile ducts, cholestasis, or Therapy is and progression to cirrhosis is the et al. liver biopsy and bile from members of one one in which studies no evidence for involvement of and for a chromosomal segment in affected children presentation in liver biopsy findings with change than the cholestasis of In and with inflammation and was The phenotype suggests an isolated transport defect in bile acid because in bile were as in patients with the bile seen by in was not present. Mutations in the gene of now bile or important in canalicular secretion of bile acids may be present in patients with to altered bile acid transport of the mutated gene and characterization of the gene must be performed to allow and to aid in understanding bile and must be of patients with cholestasis and may also aid in understanding hepatic excretory function of bile acid should be strongly cholestatic liver disease with clinical features to Byler disease is associated with normal serum γ-GTP normal serum bile acid by the absence of pruritus, and to normal liver function during UDCA However, patients with PFIC without either serum γ-GTP activity and serum bile acid concentrations, or normal serum γ-GTP activity but pruritus and serum bile acid The genes responsible for defects in bile acid must be to each and family to a specific of transporter genes or genes involved in metabolic in animal models should be a and may produce similar to in human This has been for one relevant deficiency. transport of the canalicular of the liver cell bile is by a of primary that to the of this genes and genes is a that is in the canalicular of the The function of has been by of its in the the gene are of bile and progressive liver disease characterized by inflammation of the portal bile duct and the disease is caused by the of bile that are not in by The of liver disease in can be by of the of the bile through of Mutations in the human gene to be responsible for a of PFIC associated with serum γ-GTP levels This disease phenotype is similar to that seen in with of the A with PFIC and γ-GTP levels was reported to have a decreased the was normal in a with et al. later that mutations in the human gene lead to a form of PFIC, the important physiologic role of It seems that progressive intrahepatic cholestasis that becomes manifest in the first after and hepatic failure. In patients described, the mutation to a no could be of with the disease phenotype with was reported to biliary but to have a normal biliary bile Further studies must be performed to that the absence of in leads to a defect in models must be developed to produce defects in the genes for and This not only will understanding of the physiology of the and of PFIC but also will provide models in which to treatment strategies. proper is valid and clinical will be Precise characterization of disorders will be a A diagnostic genetic or biochemical for Alagille syndrome and for each specific form of PFIC will be The of diagnosis must be The development of should be performed in with the A in basic is to the by patients with intrahepatic cholestasis; this must be a high priority. The basic and clinical of these disorders have been in with hepatic should in that questions relevant to intrahepatic cholestasis. to of investigation for each two to for each and collaborative clinical or genetic studies because the number of affected patients is and to per per for and to per for clinical should help and that result from this in and regulating and bile acid metabolism, will have important for the of for in liver also is should be developed to or in this of existing disease may be to liver in the have been in to study disease. These disorders, which are are common are and most to liver transplantation. effective treatment would decrease the this is an of great for and the pathogenesis of many liver disorders that infants and and treatment to this group probably will be especially a is to the for the first of to the of with the prolonged of patients. A disease or in a can a in the for many has been made in the of diseases that manifest as intrahepatic cholestasis in infants and These chronic, progressive liver diseases may be or may normal and development. understanding of the mechanisms that these forms of disease is that effective treatment and for can be will only from a in basic and clinical