Benign recurrent intrahepatic cholestasis type 2 is caused by mutations in ABCB11

A patient with novel ABCB11 gene mutations with phenotypic transition between BRIC2 and PFIC2

We describe a PFIC2 patient with a good response to ursodeoxycholic acid for 9 years. We found two novel ABCB11 gene mutations in the patient, i.e. I498T and 2098delA. The correlation of the patient's genotypes with the clinical course supports the existence of a phenotypic continuum between BRIC2 and PFIC2.

Gallstone disease. Pathogenesis of gallstones: A genetic perspective

Cholelithiasis is one of the most prevalent gastroenterological diseases, imposing a huge economic burden on health-care systems. Gallbladder stones form when the concentration of cholesterol or bilirubin exceeds the solubility in the bile salt and phospholipid-rich bile. The physiology of biliary lipid secretion by a number of specialized transport proteins has recently been elucidated, and underlying genetic defects in these proteins have been identified as susceptibility factors for gallstone disease. Recent studies of identical twins and family strongly support the idea of a genetic component to gallstone disease. Epidemiological studies in high-risk populations indicate that gallstone formation is caused by multiple environmental influences and common genetic factors and their interactions. Monogenic subtypes of cholelithiasis, such as biliary lipid transporter deficiencies, appear to be rare. The characterization of lithogenic genes in knockout and transgenic mice, and the identification of many gallstone susceptibility loci in inbred mice, provide the basis for studies of the corresponding genes in patients with gallstones. The transfer of findings from mouse genetics to the bedside might lead to new strategies for individual risk assessment and reveal molecular targets for the development of new treatment strategies.

Mechanisms of disease: the genetic epidemiology of gallbladder stones

Cholelithiasis is one of the most prevalent and most expensive gastroenterologic diseases. It belongs to the group of complex metabolic disorders that affect humans, and its critical pathogenic mechanisms are not well defined. As a result, primary or secondary prevention strategies are sparse, and the only effective treatment is cholecystectomy. Here we provide an update on the molecular pathogenesis of gallbladder stones, evidence supporting the hypothesis that genetic factors are key elements predisposing to gallstones, and progress in human genetic studies of cholesterol stones. Data from recent identical twin, family and linkage studies provide conclusive evidence for a strong genetic component to gallstone disease. Furthermore, epidemiologic studies in at-risk populations indicate that gallstone formation is caused by multiple environmental influences and common genetic factors and their interactions. By contrast, monogenic subtypes of cholelithiasis, such as ATP-binding-cassette transporter deficiencies, appear to be rare. The summary of human association studies illustrates that distinct common gene variants might contribute to gallstone formation in different ethnic groups. The characterization of lithogenic genes in knockout and transgenic mice and the identification of many gallstone-susceptibility loci in inbred mice provide the basis for studies of the corresponding genes in patients with gallstones. The transfer of findings from mouse genetics to the bedside might lead to new strategies for individual risk assessment and reveal novel molecular targets for prevention and medical therapies.

Treatment options for chronic cholestasis in infancy and childhood

Altered bile flow physiology leads to many complications commonly seen in patients with cholestatic liver disease, regardless of the etiology. For each individual patient, a coordinated and effective treatment strategy must address the presence and the severity spectrum of malabsorption, malnutrition, vitamin and micronutrient deficiencies, pruritus, xanthomata, ascites, and liver failure, which are attributed directly or indirectly to diminished bile flow. An aggressive approach to maximizing the nutritional status of the child is vital to ensure optimal growth and development. Protein-calorie and/or fat supplementation is best discussed early. Decreasing the percentage of dietary long-chain triglycerides, providing medium-chain triglycerides, and ensuring adequate essential fatty acid and adequate protein intake may be helpful. Fat-soluble vitamin (A, D, E, and K) levels and micronutrient levels must be carefully and serially monitored and supplemented as necessary. Because the mechanisms that mediate pruritus of cholestasis remain to be determined, the use of empirical therapies continues to be standard practice. Ursodeoxycholic acid may ameliorate pruritus. Antihistamines and rifampicin may also provide temporary relief for some children. Based on the evidence that increased central opioidergic tone is present in chronic cholestasis, the use of opiate antagonists is promising but has not been evaluated in children. Selected patients with refractory pruritus that have failed maximal medical therapy have benefited from partial external biliary diversion. Ongoing dialogue with the family regarding the indications for liver transplantation is reasonable. Optimization and adherence with the pretransplant medical management enhance the chances for a successful outcome from liver transplantation. Specific to the pediatric patient, optimizing growth, development and nutrition, minimizing discomfort and disability, and aiding the child and family in coping with the stress, social, and emotional effects of chronic liver disease remain paramount.

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

BACKGROUND/AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Colestasis infantil y transportadores biliares

Identification of the transport systems involved in bile secretion and of the genes codifying these systems has allowed the etiology of familial intrahepatic cholestasis to be determined in most affected children. Mutations in ATP8B1 cause a defect in FIC1, an aminophospholipid flipase, and give rise to a variable spectrum of disease, ranging from progressive intrahepatic cholestasis to benign recurrent cholestasis, due to alterations in the lipid composition of the membranes and decreased expression of the nuclear factor FXR. Mutations in ABCB11 cause a defect of the canalicular bile salt export pump (BSEP), with early clinical manifestations and progression to hepatocellular failure in childhood. Mutations in ABCB4 cause an alteration in the MDR3 phospholipid transporter, and a variable spectrum of disease from progressive ductal injury to cirrhosis in children, and gallstones, cholestasis of pregnancy, or late cirrhosis in adults.

Hepatobiliary pathology

PURPOSE OF REVIEW

This review highlights recent publications on hepatobiliary pathology concerning several unusual types of hepatitis, fatty liver disease, disorders of the biliary tree and other topics that have a substantial impact on liver biopsy interpretation.

RECENT FINDINGS

In the outbreak of severe acute respiratory syndrome (SARS), many patients had abnormalities in liver function tests. Liver biopsy findings in three cases were reported that showed a generic picture of hepatitis, with exceptionally increased mitotic activity. The role of portal myofibroblasts in cirrhosis was examined in several studies. A newly described lesion, isolated ductular hyperplasia (IDH) was found in patients with prolonged abnormalities of liver function tests of uncertain origin. Hyperplastic, well-differentiated bile ductules were seen on liver biopsy in the absence of any identifiable biliary disease. Hereditary hemochromatosis is now a complex entity with various clinicopathological forms based on mutations in the HFE gene and other iron-homeostatic genes such as transferrin receptor 2 and ferroportin 1. In some of these heritable forms of primary iron overload, stainable iron is present in both hepatocytes and Kupffer cells. After liver transplantation, differentiating recurrent HCV infection from acute rejection on liver biopsy is problematic, with exceptionally low inter- and intra-observer reliability shown in one study.

SUMMARY

The hepatitis associated with the SARS coronavirus, Isolated Ductular Hyperplasia in patients with liver function test abnormalities and other topics with pathologic relevance are reviewed.

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

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

Hepatocellular transporters and cholestasis

The secretion of bile is the result of active hepatocellular transport processes, most of which occur across the canalicular membrane of liver cells. Disturbance of the function and/or expression of these transporters leads to the intracellular accumulation of toxic bile acids, thereby promoting cholestatic liver cell injury. Genetically determined alterations of hepatobiliary transporter function are increasingly recognized as important risk factors for an individual's susceptibility to develop cholestasis. It has become evident that, besides the established pathogenic role of mutations in canalicular transporter genes in progressive and benign forms of familial intrahepatic cholestasis, genetics may also play an important role in acquired cholestatic syndromes, such as intrahepatic cholestasis of pregnancy or drug-induced cholestasis. This overview summarizes the physiologic function and regulation of human hepatobiliary transport systems and discusses the impact of their genetic variations for the pathophysiology of different cholestatic syndromes.

Inborn errors of biliary canalicular transport systems

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