A mutation in the human canalicular multispecific organic anion transporter gene causes the Dubin-Johnson syndrome

Interplay of OATP1A/1B/2B1 uptake transporters and ABCB1 and ABCG2 efflux transporters in the handling of bilirubin and drugs

Transmembrane drug transporters can be important determinants of the pharmacokinetics, efficacy, and safety profiles of drugs. To investigate the potential cooperative and/or counteracting interplay of OATP1A/1B/2B1 uptake transporters and ABCB1 and ABCG2 efflux transporters in physiology and pharmacology, we generated a new mouse model (Bab12), deficient for Slco1a/1b, Slco2b1, Abcb1a/1b and Abcg2. Bab12 mice were viable and fertile. We compared wild-type, Slco1a/1b/2b1-/-, Abcb1a/1b;Abcg2-/- and Bab12 strains. Endogenous plasma conjugated bilirubin levels ranked as follows: wild-type = Abcb1a/1b;Abcg2-/- << Slco1a/1b/2b1-/- < Bab12 mice. Plasma levels of rosuvastatin and fexofenadine were elevated in Slco1a/1b/2b1-/- and Abcb1a/1b;Abcg2-/- mice compared to wild-type, and dramatically increased in Bab12 mice. Although systemic exposure of larotrectinib and repotrectinib was substantially increased in the separate multidrug transporter knockout strains, no additive effects were observed in the combination Bab12 mice. Significantly higher plasma exposure of fluvastatin and pravastatin was only found in Slco1a/1b/2b1-deficient mice. However, noticeable transport by Slco1a/1b/2b1 and Abcb1a/1b and Abcg2 across the BBB was observed for fluvastatin and pravastatin, respectively, by comparing Bab12 mice with Abcb1a/1b;Abcg2-/- or Slco1a/1b/2b1-/- mice. Quite varying behavior in plasma exposure of erlotinib and its metabolites was observed among these strains. Bab12 mice revealed that Abcb1a/1b and/or Abcg2 can contribute to conjugated bilirubin elimination when Slco1a/1b/2b1 are absent. Our results suggest that the interplay of Slco1a/1b/2b1, Abcb1a/1b, and Abcg2 could markedly affect the pharmacokinetics of some, but not all drugs and metabolites. The Bab12 mouse model will represent a useful tool for optimizing drug development and clinical application, including efficacy and safety.

Decoding triancestral origins, archaic introgression, and natural selection in the Japanese population by whole-genome sequencing

We generated Japanese Encyclopedia of Whole-Genome/Exome Sequencing Library (JEWEL), a high-depth whole-genome sequencing dataset comprising 3256 individuals from across Japan. Analysis of JEWEL revealed genetic characteristics of the Japanese population that were not discernible using microarray data. First, rare variant-based analysis revealed an unprecedented fine-scale genetic structure. Together with population genetics analysis, the present-day Japanese can be decomposed into three ancestral components. Second, we identified unreported loss-of-function (LoF) variants and observed that for specific genes, LoF variants appeared to be restricted to a more limited set of transcripts than would be expected by chance, with PTPRD as a notable example. Third, we identified 44 archaic segments linked to complex traits, including a Denisovan-derived segment at NKX6-1 associated with type 2 diabetes. Most of these segments are specific to East Asians. Fourth, we identified candidate genetic loci under recent natural selection. Overall, our work provided insights into genetic characteristics of the Japanese population.

Rapid in vivo evaluation system for cholestasis-related genes in mice with humanized bile acid profiles

BACKGROUND

Pediatric cholestatic liver diseases (Ped-CLD) comprise many ultrarare disorders with a genetic basis. Pharmacologic therapy for severe cases of Ped-CLD has not been established. Species differences in bile acid (BA) metabolism between humans and rodents contribute to the lack of phenocopy of patients with Ped-CLD in rodents and hinder the development of therapeutic strategies. We aimed to establish an efficient in vivo system to understand BA-related pathogenesis, such as Ped-CLD.

METHODS

We generated mice that express spCas9 specifically in the liver (L-Cas9Tg/Tg [liver-specific Cas9Tg/Tg] mice) and designed recombinant adeno-associated virus serotype 8 encoding small-guide RNA (AAV8 sgRNA) targeting Abcc2, Abcb11, and Cyp2c70. In humans, ABCC2 and ABCB11 deficiencies cause constitutional hyperbilirubinemia and most severe Ped-CLD, respectively. Cyp2c70 encodes an enzyme responsible for the rodent-specific BA profile. Six-week-old L-Cas9Tg/Tg mice were injected with this AAV8 sgRNA and subjected to biochemical and histological analysis.

RESULTS

Fourteen days after the injection with AAV8 sgRNA targeting Abcc2, L-Cas9Tg/Tg mice exhibited jaundice and phenocopied patients with ABCC2 deficiency. L-Cas9Tg/Tg mice injected with AAV8 sgRNA targeting Abcb11 showed hepatomegaly and cholestasis without histological evidence of liver injury. Compared to Abcb11 alone, simultaneous injection of AAV8 sgRNA for Abcb11 and Cyp2c70 humanized the BA profile and caused higher transaminase levels and parenchymal necrosis, resembling phenotypes with ABCB11 deficiency.

CONCLUSIONS

This study provides proof of concept for efficient in vivo assessment of cholestasis-related genes in humanized bile acid profiles. Our platform offers a more time- and cost-effective alternative to conventional genetically engineered mice, increasing our understanding of BA-related pathogenesis such as Ped-CLD and expanding the potential for translational research.

Genetics of Gallstone Disease and Their Clinical Significance: A Narrative Review

Gallstone (GS) disease is common and arises from a combination of genetic and environmental factors. Although genetic abnormalities specifically leading to cholesterol GSs are rare, there are clinically significant gene variants associated with cholesterol GSs. In contrast, most bilirubin GSs can be attributed to genetic defects. The pathogenesis of cholesterol and bilirubin GSs differs greatly. Cholesterol GSs are notably influenced by genetic variants within the ABC protein superfamily, including ABCG8, ABCG5, ABCB4, and ABCB11, as well as genes from the apolipoprotein family such as ApoB100 and ApoE (especially the E3/E3 and E3/E4 variants), and members of the MUC family. Conversely, bilirubin GSs are associated with genetic variants in highly expressed hepatic genes, notably UGT1A1, ABCC2 (MRP2), ABCC3 (MRP3), CFTR, and MUC, alongside genetic defects linked to hemolytic anemias and conditions impacting erythropoiesis. While genetic cases constitute a small portion of GS disease, recognizing genetic predisposition is essential for proper diagnosis, treatment, and genetic counseling.

Structural basis for the modulation of MRP2 activity by phosphorylation and drugs

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is a polyspecific efflux transporter of organic anions expressed in hepatocyte canalicular membranes. MRP2 dysfunction, in Dubin-Johnson syndrome or by off-target inhibition, for example by the uricosuric drug probenecid, elevates circulating bilirubin glucuronide and is a cause of jaundice. Here, we determine the cryo-EM structure of rat Mrp2 (rMrp2) in an autoinhibited state and in complex with probenecid. The autoinhibited state exhibits an unusual conformation for this class of transporter in which the regulatory domain is folded within the transmembrane domain cavity. In vitro phosphorylation, mass spectrometry and transport assays show that phosphorylation of the regulatory domain relieves this autoinhibition and enhances rMrp2 transport activity. The in vitro data is confirmed in human hepatocyte-like cells, in which inhibition of endogenous kinases also reduces human MRP2 transport activity. The drug-bound state reveals two probenecid binding sites that suggest a dynamic interplay with autoinhibition. Mapping of the Dubin-Johnson mutations onto the rodent structure indicates that many may interfere with the transition between conformational states.

Transport mechanism of human bilirubin transporter ABCC2 tuned by the inter-module regulatory domain

Bilirubin is mainly generated from the breakdown of heme when red blood cells reach the end of their lifespan. Accumulation of bilirubin in human body usually leads to various disorders, including jaundice and liver disease. Bilirubin is conjugated in hepatocytes and excreted to bile duct via the ATP-binding cassette transporter ABCC2, dysfunction of which would lead to Dubin-Johnson syndrome. Here we determine the structures of ABCC2 in the apo, substrate-bound and ATP/ADP-bound forms using the cryo-electron microscopy, exhibiting a full transporter with a regulatory (R) domain inserted between the two half modules. Combined with substrate-stimulated ATPase and transport activity assays, structural analysis enables us to figure out transport cycle of ABCC2 with the R domain adopting various conformations. At the rest state, the R domain binding to the translocation cavity functions as an affinity filter that allows the substrates of high affinity to be transported in priority. Upon substrate binding, the R domain is expelled from the cavity and docks to the lateral of transmembrane domain following ATP hydrolysis. Our findings provide structural insights into a transport mechanism of ABC transporters finely tuned by the R domain.

Molecular and Clinical Links between Drug-Induced Cholestasis and Familial Intrahepatic Cholestasis

Idiosyncratic Drug-Induced Liver Injury (iDILI) represents an actual health challenge, accounting for more than 40% of hepatitis cases in adults over 50 years and more than 50% of acute fulminant hepatic failure cases. In addition, approximately 30% of iDILI are cholestatic (drug-induced cholestasis (DIC)). The liver's metabolism and clearance of lipophilic drugs depend on their emission into the bile. Therefore, many medications cause cholestasis through their interaction with hepatic transporters. The main canalicular efflux transport proteins include: 1. the bile salt export pump (BSEP) protein (ABCB11); 2. the multidrug resistance protein-2 (MRP2, ABCC2) regulating the bile salts' independent flow by excretion of glutathione; 3. the multidrug resistance-1 protein (MDR1, ABCB1) that transports organic cations; 4. the multidrug resistance-3 protein (MDR3, ABCB4). Two of the most known proteins involved in bile acids' (BAs) metabolism and transport are BSEP and MDR3. BSEP inhibition by drugs leads to reduced BAs' secretion and their retention within hepatocytes, exiting in cholestasis, while mutations in the ABCB4 gene expose the biliary epithelium to the injurious detergent actions of BAs, thus increasing susceptibility to DIC. Herein, we review the leading molecular pathways behind the DIC, the links with the other clinical forms of familial intrahepatic cholestasis, and, finally, the main cholestasis-inducing drugs.

Neonatal Dubin-Johnson Syndrome and its Differentiation from Biliary Atresia

BACKGROUND AND AIMS

The aim was to determine if liver biochemistry indices can be used as biomarkers to help differentiate patients with neonatal Dubin-Johnson syndrome (nDJS) from those with biliary atresia (BA).

METHODS

Patients with genetically-confirmed nDJS or cholangiographically confirmed BA were retrospectively enrolled and randomly assigned to discovery or verification cohorts. Their liver chemistries, measured during the neonatal period, were compared. Predictive values were calculated by receiver operating characteristic curve analysis.

RESULTS

A cohort of 53 nDJS patients was recruited, of whom 13 presented with acholic stools, and 14 underwent diagnostic cholangiography or needle liver biopsy to differentiate from BA. Thirty-five patients in the cohort, with complete biochemical information measured during the neonatal period, were compared with 133 infants with cholangiographically confirmed BA. Total and direct bilirubin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bile acids, alkaline phosphatase, and gamma-glutamyl transferase were significantly lower in nDJS than in BA. In the discovery cohort, the areas under the curve for ALT and AST were 0.908 and 0.943, respectively. In the validation cohort, 13/15 patients in the nDJS group were classified as nDJS, and 10/53 in the BA control group were positive (p<0.00001) with an ALT biomarker cutoff value of 75 IU/L. Thirteen of 15 patients were classified as nDJS and none were classified positive in the BA group (13/15 vs. 0/53, p<0.00001) with an AST cutoff of 87 IU/L.

CONCLUSIONS

Having assembled and investigated the largest cohort of nDJS patients reported to date, we found that nDJS patients could be distinguished from BA patients using the serum AST level as a biomarker. The finding may be clinically useful to spare cholestatic nDJS patients unnecessary invasive procedures.

Urinary coproporphyrins as a diagnostic biomarker of Dubin-Johnson syndrome in neonates: A diagnostic pathway is proposed

BACKGROUND

Dubin-Johnson syndrome (DJS) presents during the neonatal period with a phenotype that overlaps with a broad list of causes of neonatal cholestasis (NC), which makes the identification of DJS challenging for clinicians. We conducted a case-controlled study to investigate the utility of urinary coproporphyrins (UCP) I% as a potential diagnostic biomarker.

METHODS

We reviewed our database of 533 cases of NC and identified 28 neonates with disease-causing variants in ATP-binding cassette-subfamily C member 2 (ABCC2) gene "Cases" (Study period 2008-2019). Another 20 neonates with cholestasis due to non-DJS diagnoses were included as "controls." Both groups underwent UCP analysis to measure CP isomer I percentage (%).

RESULTS

Serum alanine aminotransferase (ALT) levels were within the normal range in 26 patients (92%) and mildly elevated in 2 patients. ALT levels were significantly lower in neonates with DJS than in NC from other causes (P < 0.001). The use of normal serum ALT levels to predict DJS among neonates with cholestasis had a sensitivity of 93%, specificity 90%, positive predictive value (PPV) 34%, and negative predictive value (NPV) 99.5%. The median UCPI% was significantly higher in DJS patients [88%, interquartile range (IQR) 1-IQR3, 84.2%-92.7%] than in NC from other causes [67%, (IQR1-IQR3, 61%-71.5%; Confidence interval 0.18-0.28; P< 0.001)]. The use of UCPI% >80% to predict DJS had a sensitivity, specificity, PPV, and NPV of 100%.

CONCLUSION

Based on the results from our study, we propose sequencing of the ABCC2 gene in neonates with normal ALT, presence of cholestasis and UCP1% >80%.

A historical review of jaundice: May the golden oriole live forever

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Case Report: Three novel pathogenic ABCC2 mutations identified in two patients with Dubin–Johnson syndrome

Background: Dubin–Johnson syndrome (DJS) is a rare autosomal recessive genetic disease which is caused by mutations in the ABCC2 gene; it is characterized by chronic hyperbilirubinemia. Here, we report two pedigrees affected with DJS which were caused by three novel pathogenic ABCC2 mutations.

Case summary: The two patients exhibited intermittent low-grade, predominantly conjugated hyperbilirubinemia and showed no other abnormalities. They were diagnosed clinically with DJS. Three novel pathogenic ABCC2 mutations—c.2980delA, c.1834C&gt;T, and c.4465_4473delinsGGCCCACAG—were identified by whole-exome sequencing. These mutations could be responsible for DJS in the two pedigrees. The genetic test confirmed the diagnosis of DJS.

Conclusion: These results contributed to the genetic diagnosis of the two patients with DJS and expanded the variant database for the ABCC2 gene.

Case Report: Dubin-Johnson Syndrome Presenting With Infantile Cholestasis: An Overlooked Diagnosis in an Extended Family

Dubin-Johnson syndrome (DJS) is an often-missed diagnosis of neonatal cholestasis. We report two patients with DJS, who presented with neonatal cholestasis. The first patient underwent extensive investigations for infantile cholestasis with no definitive etiology reached; the diagnosis of DJS was missed until the age of 14 years old. The diagnosis was confirmed genetically with c.2273G > T, p.G758V mutation in exon 18 of the ABCC2 gene. The 2nd patient is a 7-day-old baby, the son of the 1st patient who gave birth to him at the age of 21 years old. He was diagnosed with DJS at the age of 2 weeks based on normal clinical and laboratory workup apart from direct hyperbilirubinemia. He had the same mutation as his mother in homozygous status. The husband was heterozygous for the same mutation. DJS is one of the often-missed differential diagnoses of neonatal cholestasis. It should be suspected in patients of infantile cholestasis, who have an, otherwise, normal physical examination, and laboratory investigations to avoid unnecessary lengthy, invasive, and expensive workups.

Hemolysis in Early Infancy: Still a Cause of Cholestatic Neonatal Giant Cell Hepatitis

Before the prophylactic use of anti-D antibodies in pregnancy, hemolytic anemia of the newborn was the most common cause of hyperbilirubinemia. Nowadays, given the rarity of hemolytic anemia of the newborn, hepatobiliary abnormalities, perinatal infections, and metabolic disorders have become the most common conditions in the differential diagnosis of neonatal cholestasis. Here, we report 3 instances of cholestatic giant cell hepatitis in 3 infants who had Coombs' positive hemolysis due to ABO incompatibility in 1, Rh incompatibility in another, and combined ABO and Rh incompatibility in the third. Although rare, cholestatic neonatal giant cell hepatitis associated with hemolysis still needs to be considered in patients with neonatal cholestasis. A marked elevation of aspartate aminotransferase over alanine aminotransferase can be a helpful clue to an early diagnosis.

The Genetics of Inherited Cholestatic Disorders in Neonates and Infants: Evolving Challenges

Many inherited conditions cause cholestasis in the neonate or infant. Next-generation sequencing methods can facilitate a prompt diagnosis in some of these cases; application of these methods in patients with liver diseases of unknown cause has also uncovered novel gene-disease associations and improved our understanding of physiological bile secretion and flow. By helping to define the molecular basis of certain cholestatic disorders, these methods have also identified new targets for therapy as well patient subgroups more likely to benefit from specific therapies. At the same time, sequencing methods have presented new diagnostic challenges, such as the interpretation of single heterozygous genetic variants. This article discusses those challenges in the context of neonatal and infantile cholestasis, focusing on difficulties in predicting variant pathogenicity, the possibility of other causal variants not identified by the genetic screen used, and phenotypic variability among patients with variants in the same genes. A prospective, observational study performed between 2010-2013, which sequenced six important genes (ATP8B1, ABCB11, ABCB4, NPC1, NPC2 and SLC25A13) in an international cohort of 222 patients with infantile liver disease, is given as an example of potential benefits and challenges that clinicians could face having received a complex genetic result. Further studies including large cohorts of patients with paediatric liver disease are needed to clarify the spectrum of phenotypes associated with, as well as appropriate clinical response to, single heterozygous variants in cholestasis-associated genes.

Characterization of a novel ABCC2 mutation in infantile Dubin Johnson syndrome

BACKGROUND AND AIMS

The Dubin Johnson Syndrome (DJS) occurs mostly in young adults but an early-onset of the disease has been reported in less common forms (Neonatal DJS and Infantile DJS). In this case, the clinical findings are of limit for the DJS diagnosis. Hence, the genetic testing remains the method of choice to provide an accurate diagnosis. In our study, we aimed to perform a genetic analysis for two siblings presented with an intrahepatic cholestasis before the age of 1 year to provide a molecular explanation for the developed phenotype.

PATIENTS & METHODS

A Tunisian family, having two siblings, manifesting signs of a hepatopathy, was enrolled in our study. A molecular analysis was performed, using a panel-based next generation sequencing, supplying results that were the subject of computational analysis. Then, a clinical follow-up was carried out to assess the evolution of the disease.

RESULTS

The genetic analysis revealed the presence of a novel missense c.4179G > T, (p.M1393I) mutation in ABCC2 gene associated with a substitution c.2789G > A (R930Q) in ATP8B1 gene. Predictive results consolidated the pathogenic effect of both variants. These results confirmed the DJS diagnosis in the studied patients. The clinical course of both patients fit well with the benign nature of DJS.

CONCLUSION

We described here a novel ABCC2 mutation associated with a putative ATP8B1 modifier variant. This finding constituted the first report of a complex genotype in DJS. Hence, genetic analysis by a panel-based next generation sequencing permits an accurate diagnosis and the identification of putative variants that could influence the developed phenotype.

Clinical characteristics and ABCC2 genotype in Dubin-Johnson syndrome: A case report and review of the literature

BACKGROUND

Dubin-Johnson syndrome (DJS) is a benign autosomal recessive liver disease involving mutations of the ABCC2 gene. It is characterized by chronic or intermittent conjugated hyperbilirubinemia, with chronic idiopathic jaundice as the main clinical manifestation. Genetic alterations of the ABCC2 gene are commonly used for diagnosing DJS; however, the causative ABCC2 point mutation in Chinese patients remains unknown. Research on ABCC2 mutations in Chinese DJS patients is extremely rare, and the diagnosis of DJS remains limited. The routine analysis of ABCC2 mutations is helpful for the diagnosis of DJS. Here, we report the clinical characteristics and ABCC2 genotype of an adult female DJS patient. This article is to expound the discovery of more potentially pathogenic ABCC2 variants will that contribute to DJS identification.

CASE SUMMARY

This study investigated a woman referred for DJS and involved clinical and genetic analyses. ABCC2 mutations were identified by next-generation sequencing (NGS). The patient showed intermittent jaundice and conjugated hyper-bilirubinemia. Histopathological examinations were consistent with the typical phenotype of DJS. Genetic diagnostic analysis revealed an ABCC2 genotype exhibiting a pathogenic variant, namely c.2443C>T (p.Arg815*), which has not been reported previously in the domestic or foreign literature.

CONCLUSION

Pathogenic ABCC2 mutations play an important role in the diagnosis of DJS, especially in patients with atypical presentations. Currently, NGS is used in the routine analysis of DJS cases and such tests of further cases will better illuminate the relationship between various genotypes and phenotypes of DJS.

Clinical, Biochemical, and Molecular Characterization of Neonatal-Onset Dubin-Johnson Syndrome in a Large Case Series From the Arabs

Background: There are only a few case reports and small case series on neonatal-onset Dubin-Johnson syndrome (DJS), particularly from Far-East Asia, Iranian and Moroccan Jews, and Europe. Objectives: In this first study from the Arabs and the largest series reported to date, we characterized the clinical, laboratory, and molecular features and outcome of gene-confirmed neonatal-onset DJS. Methods: We reviewed our database of 533 cases of neonatal cholestasis that presented to our center during the period from 2008 to 2019. We identified neonates with a disease-causing mutation in ABCC2 gene. Results: Twenty-eight neonates with DJS were diagnosed (5.3%). All of the 28 were full-term, well looking neonates without hepatosplenomegaly, with cholestasis, and normal liver synthetic function since the 1 week of life that resolved within 3-6 months of age, followed by a benign course punctuated by recurrent episodes of jaundice in 43% during a median follow up period of 9.25 (range 2.5-14 years). Alanine aminotransferase levels were within normal range in 26 patients (92%) and mildly elevated in two patients. ALT levels were significantly lower in neonates with DJS than in other cases with neonatal cholestasis from other causes (p < 0.001). The median urinary coproporphyrin I% was 88% (IQ1-IQ3 = 84.2-92.7%). We identified four homozygous variants in the ABCC2 gene (from 22 unrelated families), one splicing variant (c.3258+1G>A; p.?), and three were missense variants; two of which were novel missense variants [c.1594G>A (p.Glu532Lys) and c.2439G>C (p.Lys813Asn)]. The p.Gly758Val mutation has occurred in 23 patients (from 19 unrelated families). Conclusions: Our study suggests that normal ALT-cholestasis in a well-looking neonate should trigger evaluation for DJS. The p.Gly758Val variant in ABCC2 is the most predominant mutation among Arabs with "founder effects." Identification of the predominant ABCC2 variant in any population is likely to facilitate rapid molecular analysis by future targeting of that specific mutation.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

Current progress in identifying endogenous biomarker candidates for drug transporter phenotyping and their potential application to drug development

Drug transporters play important roles in the elimination of various compounds from the blood. Genetic variation and drug-drug interactions underlie the pharmacokinetic differences for the substrates of drug transporters. Some endogenous substrates of drug transporters have emerged as biomarkers to assess differences in drug transporter activity-not only in animals, but also in humans. Metabolomic analysis is a promising approach for identifying such endogenous substrates through their metabolites. The appropriateness of metabolites is supported by studies in vitro and in vivo, both in animals and through pharmacogenomic or drug-drug interaction studies in humans. This review summarizes current progress in identifying such endogenous biomarkers and applying them to drug transporter phenotyping.

Isolated Elevated Bilirubin

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Genetic contribution of ABCC2 to Dubin-Johnson syndrome and inherited cholestatic disorders

BACKGROUND AND AIMS

The ABCC2 gene is implicated in Dubin-Johnson syndrome (DJS), a rare autosomal recessive liver disorder. The primary aim of this study was to determine the diagnostic value of ABCC2 genetic testing in the largest cohort of DJS reported to date. The high number of patients with cholestatic manifestations in this series prompted us to evaluate the genetic contribution of rare, potentially pathogenic ABCC2 variants to other inherited cholestatic disorders.

METHODS

The cohort study included 32 patients with clinical DJS diagnosis, and 372 patients referred for the following disorders: low phospholipid-associated cholelithiasis (LPAC) syndrome, intrahepatic cholestasis of pregnancy (ICP) and benign recurrent intrahepatic cholestasis (BRIC). ABCC2 was screened by next-generation sequencing.

RESULTS

Most patients with clinical DJS had positive genetic diagnosis (n = 30; 94%), with a great diversity of point mutations and copy number variations in ABCC2. Strikingly, eight (27%) of these patients showed transient cholestatic features at presentation: four neonatal cholestasis, two ICP, one contraceptive-induced cholestasis and one sporadic cholestasis. Conversely, the frequency of rare, heterozygous, potentially pathogenic ABCC2 variants in patients with LPAC, ICP or BRIC did not differ significantly from that of the general population.

CONCLUSIONS

This large series reveals that DJS is a highly homogeneous Mendelian disorder involving a large spectrum of ABCC2 variants. Genetic testing is crucial to establish early DJS diagnosis in patients with atypical presentations, such as neonatal cholestasis. This study also provides no evidence for the contribution of rare, potentially pathogenic ABCC2 variants to other inherited cholestatic disorders.

Roles of Hepatic Drug Transporters in Drug Disposition and Liver Toxicity

Hepatic drug transporters are mainly distributed in parenchymal liver cells (hepatocytes), contributing to drug's liver disposition and elimination. According to their functions, hepatic transporters can be roughly divided into influx and efflux transporters, translocating specific molecules from blood into hepatic cytosol and mediating the excretion of drugs and metabolites from hepatic cytosol to blood or bile, respectively. The function of hepatic transport systems can be affected by interspecies differences and inter-individual variability (polymorphism). In addition, some drugs and disease can redistribute transporters from the cell surface to the intracellular compartments, leading to the changes in the expression and function of transporters. Hepatic drug transporters have been associated with the hepatic toxicity of drugs. Gene polymorphism of transporters and altered transporter expressions and functions due to diseases are found to be susceptible factors for drug-induced liver injury (DILI). In this chapter, the localization of hepatic drug transporters, their regulatory factors, physiological roles, and their roles in drug's liver disposition and DILI are reviewed.

[Clinical features and ABCC2 genotypic analysis of an infant with Dubin-Johnson syndrome]

Dubin-Johnson syndrome (DJS) is an autosomal recessive disorder resulting from biallelic mutations of ABCC2 gene, with long-term or intermittent conjugated hyperbilirubinemia being the main clinical manifestation. This paper aims to report the clinical features and ABCC2 genotypes of an infant with DJS. A 9.5-month-old male infant was referred to the hospital due to abnormal liver function discovered over 9 months. The major clinical presentation was prolonged jaundice since neonatal period. A series of biochemistry analysis revealed markedly elevated total bilirubin, conjugated bilirubin and total bile acids. The patient had been managed in different hospitals, but the therapeutic effects were unsatisfactory due to undetermined etiology. Physical examination revealed jaundiced skin and sclera, and a palpable liver 3 cm below the right subcostal margin with medium texture. The spleen was not enlarged. Genetic analysis revealed a splice-site variant c.3988-2A>T and a nonsense variant c.3825C>G (p.Y1275X) in the ABCC2 gene of the infant, which were inherited from his mother and father respectively. The former had not been previously reported. Then ursodeoxycholic acid and phenobarbital were given orally. Half a month later, as a result, his jaundice disappeared and the biochemistry indices improved. However, the long-term outcome needs to be observed. Literature review revealed that neonates/infants with DJS presented with cholestatic jaundice soon after birth as the major clinical feature, and the ABCC2 variants exhibited marked heterogeneity.

A Case of Dubin-Johnson Syndrome in Pregnancy

Dubin-Johnson syndrome is an autosomal recessive condition characterized by recurrent episodes of jaundice and conjugated hyperbilirubinemia. It exacerbates during pregnancy and needs to be differentiated from other causes of jaundice. A 30-year-old patient presented to us with jaundice in her fourth pregnancy. She had intermittent episodes of jaundice earlier, with exacerbation in each pregnancy during the second trimester. The diagnosis of Dubin-Johnson syndrome was made on detailed evaluation along with histopathological confirmation on liver biopsy tissue. The patient was managed conservatively and had a good perinatal outcome.

Review article: therapeutic bile acids and the risks for hepatotoxicity

BACKGROUND

Bile acids play important roles in cholesterol metabolism and signal through farnesoid X receptor and G protein-coupled receptors. Given their importance in liver biology, bile acid therapy enables therapeutic applications beyond the treatment of cholestatic liver disease. However, predicting hepatotoxicity of bile acids in humans is obscured due to inconsistent extrapolations of animal data to humans.

AIM

To review the evidence that could explain discordant bile acids hepatotoxicity observed in humans and animals.

METHOD

Literature search was conducted in PubMed using keywords "bile acid," "transporter," "hepatotoxicity," "clinical study," "animal study," "species difference," "mechanism," "genetic disorder." Relevant articles were selected for review.

RESULTS

Clinically significant hepatotoxicity was reported in response to certain bile acids, namely chenodeoxycholic acid, which was given a boxed warning for potential hepatotoxicity. The chemical structure, specifically the number and orientation of hydroxyl groups, significantly affects their hydrophobicity, an important factor in bile acid toxicity. Experimental studies show that hydrophobic bile acids can lead to liver injury through various mechanisms, such as death receptor signalling, mitochondrial dysfunction and inflammation. Although animal studies play a central role in investigating bile acid safety, there are considerable differences in bile acid composition, metabolism and hepatobiliary disposition across species. This does not allow appropriate safety inference, especially for predicting hepatotoxicity in humans. Exploring evidences stemming from inborn errors, genetic models of disease and toxicology studies further improves an understanding of bile acid hepatotoxicity.

CONCLUSION

Species differences should be considered in the development of bile acid related therapeutics. Although the mechanism of bile acid hepatotoxicity is still not fully understood, continued mechanistic studies will deepen our understanding.

Clinical, Pathologic, and Genetic Features of Neonatal Dubin-Johnson Syndrome: A Multicenter Study in Japan

OBJECTIVE

To clarify the clinical, pathologic, and genetic features of neonatal Dubin-Johnson syndrome.

STUDY DESIGN

Ten patients with neonatal Dubin-Johnson syndrome were recruited from 6 pediatric centers in Japan between September 2013 and October 2016. Clinical and laboratory course, macroscopic and microscopic liver findings, and molecular genetic findings concerning ATP-binding cassette subfamily C member 2 (ABCC2) were retrospectively and prospectively examined.

RESULTS

All neonates exhibited cholestasis, evident as prolonged jaundice with or without acholic stools and elevations of serum direct bilirubin as well as γ-glutamyltransferase or total bile acids. Only 38% (3 of 8) of patients who underwent liver biopsy showed a grossly black liver or melanin-like pigment deposits in hepatocytes; their biopsies were performed in early infancy. Immunohistochemically, all liver specimens showed no expression of multidrug resistance-associated protein 2 but increased expression of the bile salt export pump protein. Homozygous or compound heterozygous pathogenic variants of ABCC2 were identified in all patients, representing 11 distinct pathogenic variants including 2 not previously reported.

CONCLUSIONS

Immunohistochemical staining of the liver for multidrug resistance-associated protein 2 and molecular genetic analysis of ABCC2 are crucial for accurate diagnosis of neonatal Dubin-Johnson syndrome.

Beyond Competitive Inhibition: Regulation of ABC Transporters by Kinases and Protein-Protein Interactions as Potential Mechanisms of Drug-Drug Interactions

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters mediating the extrusion of an array of substrates ranging from amino acids and lipids to xenobiotics, and many therapeutic compounds, including anticancer drugs. The ABC transporters are also recognized as important contributors to pharmacokinetics, especially in drug-drug interactions and adverse drug effects. Drugs and xenobiotics, as well as pathologic conditions, can influence the transcription of ABC transporters, or modify their activity or intracellular localization. Kinases can affect the aforementioned processes for ABC transporters as do protein interactions. In this review, we focus on the ABC transporters ABCB1, ABCB11, ABCC1, ABCC4, and ABCG2 and illustrate how kinases and protein-protein interactions affect these transporters. The clinical relevance of these factors is currently unknown; however, these examples suggest that our understanding of drug-drug interactions will benefit from further knowledge of how kinases and protein-protein interactions affect ABC transporters.

Pharmacogenetic analysis of high-dose methotrexate treatment in children with osteosarcoma

Inter-individual differences in toxic symptoms and pharmacokinetics of high-dose methotrexate (MTX) treatment may be caused by genetic variants in the MTX pathway. Correlations between polymorphisms and pharmacokinetic parameters and the occurrence of hepato- and myelotoxicity were studied. Single nucleotide polymorphisms (SNPs) of the ABCB1, ABCC1, ABCC2, ABCC3, ABCC10, ABCG2, GGH, SLC19A1 and NR1I2 genes were analyzed in 59 patients with osteosarcoma. Univariate association analysis and Bayesian network-based Bayesian univariate and multilevel analysis of relevance (BN-BMLA) were applied. Rare alleles of 10 SNPs of ABCB1, ABCC2, ABCC3, ABCG2 and NR1I2 genes showed a correlation with the pharmacokinetic values and univariate association analysis. The risk of toxicity was associated with five SNPs in the ABCC2 and NR1I2 genes. Pharmacokinetic parameters were associated with four SNPs of the ABCB1, ABCC3, NR1I2, and GGH genes, and toxicity was shown to be associated with ABCC1 rs246219 and ABCC2 rs717620 using the univariate and BN-BMLA method. BN-BMLA analysis detected relevant effects on the AUC_0-48 in the following SNPs: ABCB1 rs928256, ABCC3 rs4793665, and GGH rs3758149. In both univariate and multivariate analyses the SNPs ABCB1 rs928256, ABCC3 rs4793665, GGH rs3758149, and NR1I2 rs3814058 SNPs were relevant. These SNPs should be considered in future dose individualization during treatment.

From Classical Toxicology to Tox21: Some Critical Conceptual and Technological Advances in the Molecular Understanding of the Toxic Response Beginning From the Last Quarter of the 20th Century

Toxicology has made steady advances over the last 60+ years in understanding the mechanisms of toxicity at an increasingly finer level of cellular organization. Traditionally, toxicological studies have used animal models. However, the general adoption of the principles of 3R (Replace, Reduce, Refine) provided the impetus for the development of in vitro models in toxicity testing. The present commentary is an attempt to briefly discuss the transformation in toxicology that began around 1980. Many genes important in cellular protection and metabolism of toxicants were cloned and characterized in the 80s, and gene expression studies became feasible, too. The development of transgenic and knockout mice provided valuable animal models to investigate the role of specific genes in producing toxic effects of chemicals or protecting the organism from the toxic effects of chemicals. Further developments in toxicology came from the incorporation of the tools of "omics" (genomics, proteomics, metabolomics, interactomics), epigenetics, systems biology, computational biology, and in vitro biology. Collectively, the advances in toxicology made during the last 30-40 years are expected to provide more innovative and efficient approaches to risk assessment. A goal of experimental toxicology going forward is to reduce animal use and yet be able to conduct appropriate risk assessments and make sound regulatory decisions using alternative methods of toxicity testing. In that respect, Tox21 has provided a big picture framework for the future. Currently, regulatory decisions involving drugs, biologics, food additives, and similar compounds still utilize data from animal testing and human clinical trials. In contrast, the prioritization of environmental chemicals for further study can be made using in vitro screening and computational tools.

Can Tangier disease cause male infertility? A case report and an overview on genetic causes of male infertility and hormonal axis involved

Tangier disease is an autosomal recessive disorder caused by mutations in the ABCA1 gene and characterized by the accumulation of cholesteryl ester in various tissues and a near absence of high-density lipoprotein. The subject in this investigation was a 36-year-old Italian man with Tangier disease. He and his wife had come to the In Vitro Fertilization Unit, Pesaro Hospital (Azienda Ospedaliera Ospedali Riuniti Marche Nord) seeking help regarding fertility issues. The man was diagnosed with severe oligoasthenoteratozoospermia. Testosterone is the sex hormone necessary for spermatogenesis and cholesterol is its precursor; hence, we hypothesized that the characteristic cholesterol deficiency in Tangier disease patients could compromise their fertility. The aim of the study was to therefore to determine if there is an association between Tangier disease and male infertility. After excluding viral, infectious, genetic and anatomical causes of the subject's oligoasthenoteratozoospermia, we performed a hormonal analysis to verify our hypothesis. The patient was found to be negative for frequent bacteria and viruses. The subject showed a normal male karyotype and tested negative for Yq microdeletions and Cystic Fibrosis Transmembrane Conductance Regulator gene mutations. A complete urological examination was performed, and primary hypogonadism was also excluded. Conversely, hormonal analyses showed that the subject had a high level of follicle stimulating hormone and luteinizing hormone, low total testosterone and a significant decline in inhibin B. We believe that the abnormally low cholesterol levels typically found in subjects with Tangier disease may result in a reduced testosterone production which in turn could affect the hormonal axis responsible for spermatogenesis leading to a defective maturation of spermatozoa.

Genetics of primary sclerosing cholangitis and pathophysiological implications

Primary sclerosing cholangitis (PSC) is a chronic disease leading to fibrotic scarring of the intrahepatic and extrahepatic bile ducts, causing considerable morbidity and mortality via the development of cholestatic liver cirrhosis, concurrent IBD and a high risk of bile duct cancer. Expectations have been high that genetic studies would determine key factors in PSC pathogenesis to support the development of effective medical therapies. Through the application of genome-wide association studies, a large number of disease susceptibility genes have been identified. The overall genetic architecture of PSC shares features with both autoimmune diseases and IBD. Strong human leukocyte antigen gene associations, along with several susceptibility genes that are critically involved in T-cell function, support the involvement of adaptive immune responses in disease pathogenesis, and position PSC as an autoimmune disease. In this Review, we survey the developments that have led to these gene discoveries. We also elaborate relevant interpretations of individual gene findings in the context of established disease models in PSC, and propose relevant translational research efforts to pursue novel insights.

Prediction of hepatotoxicity for drugs using human pluripotent stem cell-derived hepatocytes

Drug-induced liver toxicity is a main reason for withdrawals of new drugs in late clinical phases and post-launch of the drugs. Thus, hepatotoxicity screening of drug candidates in pre-clinical stage is important for reducing drug attrition rates during the clinical development process. Here, we show commercially available hepatocytes that could be used for early toxicity evaluation of drug candidates. From our hepatic differentiation technology, we obtained highly pure (≥98%) hepatocytes from human embryonic stem cells (hESCs) having mature phenotypes and similar gene expression profiles with those of primary human tissues. Furthermore, we optimized 96-well culture condition of hESC-derived hepatocytes suitable for toxicity tests in vitro. To this end, we demonstrated the efficacy of our optimized hepatocyte model for predicting hepatotoxicity against the Chinese herbal medicines and showed that toxicity patterns from our hepatocyte model was similar to those of human primary cultured hepatocytes. We conclude that toxicity test using our hepatocyte model could be a good alternative cell source for pre-clinical study to predict potential hepatotoxicity in drug discovery industries.

A disintegrin and metalloprotease 10 (ADAM10) is a central regulator of murine liver tissue homeostasis

A Disintegrin And Metalloprotease (ADAM) 10 exerts essential roles during organ development and tissue integrity in different organs, mainly through activation of the Notch pathway. However, only little is known about its implication in liver tissue physiology. Here we show that in contrast to its role in other tissues, ADAM10 is dispensable for the Notch2-dependent biliary tree formation. However, we demonstrate that expression of bile acid transporters is dependent on ADAM10. Consequently, mice deficient for Adam10 in hepatocytes, cholangiocytes and liver progenitor cells develop spontaneous hepatocyte necrosis and concomitant liver fibrosis. We furthermore observed a strongly augmented ductular reaction in 15-week old ADAM10(Δhep/Δch) mice and demonstrate that c-Met dependent liver progenitor cell activation is enhanced. Additionally, liver progenitor cells are primed to hepatocyte differentiation in the absence of ADAM10. These findings show that ADAM10 is a novel central node controlling liver tissue homeostasis.

HIGHLIGHTS

Loss of ADAM10 in murine liver results in hepatocyte necrosis and concomitant liver fibrosis. ADAM10 directly regulates expression of bile acid transporters but is dispensable for Notch2-dependent formation of the biliary system. Activation of liver progenitor cells is enhanced through increased c-Met signalling, in the absence of ADAM10. Differentiation of liver progenitor cells to hepatocytes is augmented in the absence of ADAM10.

CFTR-associated ligand is a negative regulator of Mrp2 expression

The multidrug resistance-associated protein 2 (Mrp2) is an ATP-binding cassette transporter that transports a wide variety of organic anions across the apical membrane of epithelial cells. The expression of Mrp2 on the plasma membrane is regulated by protein-protein interactions. Cystic fibrosis transmembrane conductance regulator (CFTR)-associated ligand (CAL) interacts with transmembrane proteins via its PDZ domain and reduces their cell surface expression by increasing lysosomal degradation and intracellular retention. Our results showed that CAL is localized at the trans-Golgi network of rat hepatocytes. The expression of CAL is increased, and Mrp2 expression is decreased, in the liver of mice deficient in sodium/hydrogen exchanger regulatory factor-1. To determine whether CAL interacts with Mrp2 and is involved in the posttranscriptional regulation of Mrp2, we used glutathione S-transferase (GST) fusion proteins with or without the COOH-terminal PDZ binding motif of Mrp2 as the bait in GST pull-down assays. We demonstrated that Mrp2 binds to CAL via its COOH-terminal PDZ-binding motif in GST pull-down assays, an interaction verified by coimmunoprecipitation of these two proteins in cotransfected COS-7 cells. In COS-7 and LLC-PK1 cells transfected with Mrp2 alone, only a mature, high-molecular-mass band of Mrp2 was detected. However, when cells were cotransfected with Mrp2 and CAL, Mrp2 was expressed as both mature and immature forms. Biotinylation and streptavidin pull-down assays confirmed that CAL dramatically reduces the expression level of total and cell surface Mrp2 in Huh-7 cells. Our findings suggest that CAL interacts with Mrp2 and is a negative regulator of Mrp2 expression.

How to apply clinical cases and medical literature in the framework of a modified "failure mode and effects analysis" as a clinical reasoning tool--an illustration using the human biliary system

BACKGROUND

Clinicians use various clinical reasoning tools such as Ishikawa diagram to enhance their clinical experience and reasoning skills. Failure mode and effects analysis, which is an engineering methodology in origin, can be modified and applied to provide inputs into an Ishikawa diagram.

METHOD

The human biliary system is used to illustrate a modified failure mode and effects analysis. The anatomical and physiological processes of the biliary system are reviewed. Failure is defined as an abnormality caused by infective, inflammatory, obstructive, malignancy, autoimmune and other pathological processes. The potential failures, their effect(s), main clinical features, and investigation that can help a clinician to diagnose at each anatomical part and physiological process are reviewed and documented in a modified failure mode and effects analysis table. Relevant medical and surgical cases are retrieved from the medical literature and weaved into the table.

RESULTS

A total of 80 clinical cases which are relevant to the modified failure mode and effects analysis for the human biliary system have been reviewed and weaved into a designated table. The table is the backbone and framework for further expansion. Reviewing and updating the table is an iterative and continual process. The relevant clinical features in the modified failure mode and effects analysis are then extracted and included in the relevant Ishikawa diagram.

CONCLUSIONS

This article illustrates an application of engineering methodology in medicine, and it sows the seeds of potential cross-pollination between engineering and medicine. Establishing a modified failure mode and effects analysis can be a teamwork project or self-directed learning process, or a mix of both. Modified failure mode and effects analysis can be deployed to obtain inputs for an Ishikawa diagram which in turn can be used to enhance clinical experiences and clinical reasoning skills for clinicians, medical educators, and students.

Inherited disorders of bilirubin clearance

Inherited disorders of hyperbilirubinemia may be caused by increased bilirubin production or decreased bilirubin clearance. Reduced hepatic bilirubin clearance can be due to defective (i) unconjugated bilirubin uptake and intrahepatic storage, (ii) conjugation of glucuronic acid to bilirubin (e.g., Gilbert syndrome, Crigler-Najjar syndrome, Lucey-Driscoll syndrome, breast milk jaundice), (iii) bilirubin excretion into bile (Dubin-Johnson syndrome), or (iv) conjugated bilirubin re-uptake (Rotor syndrome). In this review, the molecular mechanisms and clinical manifestations of these conditions are described, as well as current approaches to diagnosis and therapy.

Molecular changes in hepatic metabolism and transport in cirrhosis and their functional importance

Liver cirrhosis is the common endpoint of many hepatic diseases and represents a relevant risk for liver failure and hepatocellular carcinoma. The progress of liver fibrosis and cirrhosis is accompanied by deteriorating liver function. This review summarizes the regulatory and functional changes in phase I and phase II metabolic enzymes as well as transport proteins and provides an overview regarding lipid and glucose metabolism in cirrhotic patients. Interestingly, phase I enzymes are generally downregulated transcriptionally, while phase II enzymes are mostly preserved transcriptionally but are reduced in their function. Transport proteins are regulated in a specific way that resembles the molecular changes observed in obstructive cholestasis. Lipid and glucose metabolism are characterized by insulin resistance and catabolism, leading to the disturbance of energy expenditure and wasting. Possible non-invasive tests, especially breath tests, for components of liver metabolism are discussed. The heterogeneity and complexity of changes in hepatic metabolism complicate the assessment of liver function in individual patients. Additionally, studies in humans are rare, and species differences preclude the transferability of data from rodents to humans. In clinical practice, some established global scores or criteria form the basis for the functional evaluation of patients with liver cirrhosis, but difficult treatment decisions such as selection for transplantation or resection require further research regarding the application of existing non-invasive tests and the development of more specific tests.

Structural and functional hepatocyte polarity and liver disease

Hepatocytes form a crucially important cell layer that separates sinusoidal blood from the canalicular bile. They have a uniquely organized polarity with a basal membrane facing liver sinusoidal endothelial cells, while one or more apical poles can contribute to several bile canaliculi jointly with the directly opposing hepatocytes. Establishment and maintenance of hepatocyte polarity is essential for many functions of hepatocytes and requires carefully orchestrated cooperation between cell adhesion molecules, cell junctions, cytoskeleton, extracellular matrix and intracellular trafficking machinery. The process of hepatocyte polarization requires energy and, if abnormal, may result in severe liver disease. A number of inherited disorders affecting tight junction and intracellular trafficking proteins have been described and demonstrate clinical and pathophysiological features overlapping those of the genetic cholestatic liver diseases caused by defects in canalicular ABC transporters. Thus both structural and functional components contribute to the final hepatocyte polarity phenotype. Many acquired liver diseases target factors that determine hepatocyte polarity, such as junctional proteins. Hepatocyte depolarization frequently occurs but is rarely recognized because hematoxylin-eosin staining does not identify the bile canaliculus. However, the molecular mechanisms underlying these defects are not well understood. Here we aim to provide an update on the key factors determining hepatocyte polarity and how it is affected in inherited and acquired diseases.

SLC transporters as therapeutic targets: emerging opportunities

Solute carrier (SLC) transporters - a family of more than 300 membrane-bound proteins that facilitate the transport of a wide array of substrates across biological membranes - have important roles in physiological processes ranging from the cellular uptake of nutrients to the absorption of drugs and other xenobiotics. Several classes of marketed drugs target well-known SLC transporters, such as neurotransmitter transporters, and human genetic studies have provided powerful insight into the roles of more-recently characterized SLC transporters in both rare and common diseases, indicating a wealth of new therapeutic opportunities. This Review summarizes knowledge on the roles of SLC transporters in human disease, describes strategies to target such transporters, and highlights current and investigational drugs that modulate SLC transporters, as well as promising drug targets.

Genetics of liver disease: From pathophysiology to clinical practice

Paralleling the first 30 years of the Journal of Hepatology we have witnessed huge advances in our understanding of liver disease and physiology. Genetic advances have played no small part in that. Initial studies in the 1970s and 1980s identified the strong major histocompatibility complex associations in autoimmune liver diseases. During the 1990 s, developments in genomic technologies drove the identification of genes responsible for Mendelian liver diseases. Over the last decade, genome-wide association studies have allowed for the dissection of the genetic susceptibility to complex liver disorders, in which also environmental co-factors play important roles. Findings have allowed the identification and elaboration of pathophysiological processes, have indicated the need for reclassification of liver diseases and have already pointed to new disease treatments. In the immediate future genetics will allow further stratification of liver diseases and contribute to personalized medicine. Challenges exist with regard to clinical implementation of rapidly developing technologies and interpretation of the wealth of accumulating genetic data. The historical perspective of genetics in liver diseases illustrates the opportunities for future research and clinical care of our patients.

History of hepatic bile formation: old problems, new approaches

Studies of hepatic bile formation reported in 1958 established that it was an osmotically generated water flow. Intravenous infusion of sodium taurocholate established a high correlation between hepatic bile flow and bile acid excretion. Secretin, a hormone that stimulates bicarbonate secretion, was also found to increase hepatic bile flow. The sources of the water entering the biliary system with these two stimuli were differentiated by the use of mannitol. An increase in its excretion parallels the increase in bile flow in response to bile acids but not secretin, which led to a quantitative distinction between canalicular and ductular water flow. The finding of aquaglyceroporin-9 in the basolateral surface of the hepatocyte accounted for the rapid entry of mannitol into hepatocytes and its exclusion from water movement in the ductules where aquaporin-1 is present. Electron microscopy demonstrated that bile acids generate the formation of vesicles that contain lecithin and cholesterol after their receptor-mediated canalicular transport. Biophysical studies established that the osmotic effect of bile acids varies with their concentration and also with the proportion of mono-, di-, and trihydroxy bile acids and provides a basis for understanding their physiological effects. Because of the varying osmotic effect of bile acids, it is difficult to quantify bile acid independent flow generated by other solutes, such as glutathione, which enters the biliary system. Monohydroxy bile acids, by markedly increasing aggregation number, severely reduce water flow. Developing biomarkers for the noninvasive assessment of normal hepatic bile flow remains an elusive goal that merits further study.

Role of OATP1B3 in the transport of bile acids assessed using first-trimester trophoblasts

AIMS

The aim of this study was to investigate the transport of two kinds of bile acids by organic anion transporting polypeptide 1B3 (OATP1B3) using first-trimester trophoblasts. The mechanisms of damage to fetuses with intrahepatic cholestasis of pregnancy were investigated, providing new potential strategies for targeted therapies aimed at reducing fetal risk.

MATERIAL AND METHODS

The expression of OATP1B3 was knocked down by lentiviral vector-mediated RNA interference, and silencing efficiency was assessed using real-time polymerase chain reaction and Western blotting. The cytotoxicity of two bile acids (glycocholic acid [GCA] and glycochenodeoxycholic acid [GCDCA]) was assessed using the MTT method. Transport of bile acids was assessed by establishing an in vitro trophoblast monolayer model using polyester Transwell-clear inserts, and the concentration of bile acids in the upper compartment was assessed using high-pressure liquid chromatography.

RESULTS

GCA and GCDCA (10 and 20 μM) were not cytotoxic to the SWAN cell line (P > 0.05). RNAi treatment decreased the mRNA and protein expressions of OATP1B3 by 94.42% and 49.51%, respectively (P < 0.05). The bile acid transport curves were similar in the control and negative RNAi groups, whereas those in the RNAi group differed significantly from those in the control and negative RNAi groups. The concentration of GCA and GCDCA in the upper compartment was significantly lower in the RNAi group than in the control and negative RNAi groups.

CONCLUSIONS

OATP1B3 expression in trophoblasts was confirmed indirectly by its ability to transport the bile acids GCA and GCDCA.

Effect of drug transporter pharmacogenetics on cholestasis

INTRODUCTION

The liver is the central place for the metabolism of drugs and other xenobiotics. In the liver cell, oxidation and conjugation of compounds take place, and at the same time, bile formation helps in extrusion of these compounds via the biliary route. A large number of transporters are responsible for drug uptake into the liver cell and excretion into bile or efflux to the sinusoidal blood.

AREAS COVERED

Genetic variants of these transporters and their transactivators contribute to changes in drug handling and are also responsible for cholestatic syndromes of different severity. This review summarizes the current knowledge regarding the influence of these genetic changes. The review covers progressive hereditary cholestatic syndromes as well as recurrent or transient cholestatic syndromes such as drug-induced liver injury, intrahepatic cholestasis of pregnancy, and benign recurrent intrahepatic cholestasis.

EXPERT OPINION

Polymorphisms in transporter genes are frequent. For clinically relevant cholestatic syndromes, it often requires a combination of genetic variants or acquired triggers such as pregnancy or drug treatment. In combination with other pathogenetic aspects, genetic variants in drug transporters may contribute to our understanding of not only cholestatic diseases such as primary sclerosing cholangitis or primary biliary cirrhosis, but also the natural course of chronic liver disease in general.

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.