Functional rescue of an ABCB11 mutant by ivacaftor: A new targeted pharmacotherapy approach in bile salt export pump deficiency

Structure-based discovery of CFTR potentiators and inhibitors

The cystic fibrosis transmembrane conductance regulator (CFTR) is a crucial ion channel whose loss of function leads to cystic fibrosis, whereas its hyperactivation leads to secretory diarrhea. Small molecules that improve CFTR folding (correctors) or function (potentiators) are clinically available. However, the only potentiator, ivacaftor, has suboptimal pharmacokinetics and inhibitors have yet to be clinically developed. Here, we combine molecular docking, electrophysiology, cryo-EM, and medicinal chemistry to identify CFTR modulators. We docked ∼155 million molecules into the potentiator site on CFTR, synthesized 53 test ligands, and used structure-based optimization to identify candidate modulators. This approach uncovered mid-nanomolar potentiators, as well as inhibitors, that bind to the same allosteric site. These molecules represent potential leads for the development of more effective drugs for cystic fibrosis and secretory diarrhea, demonstrating the feasibility of large-scale docking for ion channel drug discovery.

Emerging drugs for the treatment of progressive familial intrahepatic cholestasis: a focus on phase II and III trials

INTRODUCTION

Progressive familial intrahepatic cholestasis (PFIC) is a group of disorders characterized by inappropriate bile formation, causing hepatic accumulation of bile acids and, subsequently, liver injury. Until recently, no approved treatments were available for these patients.

AREAS COVERED

Recent clinical trials for PFIC treatment have focused on intestine-restricted ileal bile acid transporter (IBAT) inhibitors. These compounds aim to reduce the pool size of bile acids by interrupting their enterohepatic circulation. Other emerging treatments in the pipeline include systemic IBAT inhibitors, synthetic bile acid derivatives, compounds targeting bile acid synthesis via the FXR/FGF axis, and chaperones/potentiators that aim to enhance the residual activity of the mutated transporters.

EXPERT OPINION

Substantial progress has been made in drug development for PFIC patients during the last couple of years. Although data concerning long-term efficacy are as yet only scarcely available, new therapies have demonstrated robust efficacy in a considerable fraction of patients at least on the shorter term. However, a substantial fraction of PFIC patients do not respond to these novel therapies and thus still requires surgical treatment, including liver transplantation before adulthood. Hence, there is still an unmet medical need for long-term effective medical, preferably non-surgical, treatment for all PFIC patients.

EASL Clinical Practice Guidelines on genetic cholestatic liver diseases

Genetic cholestatic liver diseases are caused by (often rare) mutations in a multitude of different genes. While these diseases differ in pathobiology, clinical presentation and prognosis, they do have several commonalities due to their cholestatic nature. These Clinical Practice Guidelines (CPGs) offer a general approach to genetic testing and management of cholestatic pruritus, while exploring diagnostic and treatment approaches for a subset of genetic cholestatic liver diseases in depth. An expert panel appointed by the European Association for the Study of the Liver has created recommendations regarding diagnosis and treatment, based on the best evidence currently available in the fields of paediatric and adult hepatology, as well as genetics. The management of these diseases generally takes place in a tertiary referral centre, in order to provide up-to-date approaches and expertise. These CPGs are intended to support hepatologists (for paediatric and adult patients), residents and other healthcare professionals involved in the management of these patients with concrete recommendations based on currently available evidence or, if not available, on expert opinion.

Structural basis of bile salt extrusion and small-molecule inhibition in human BSEP

BSEP (ABCB11) is an ATP-binding cassette transporter that is expressed in hepatocytes and extrudes bile salts into the canaliculi of the liver. BSEP dysfunction, caused by mutations or induced by drugs, is frequently associated with severe cholestatic liver disease. We report the cryo-EM structure of glibenclamide-bound human BSEP in nanodiscs, revealing the basis of small-molecule inhibition. Glibenclamide binds the apex of a central binding pocket between the transmembrane domains, preventing BSEP from undergoing conformational changes, and thus rationalizing the reduced uptake of bile salts. We further report two high-resolution structures of BSEP trapped in distinct nucleotide-bound states by using a catalytically inactivated BSEP variant (BSEPE1244Q) to visualize a pre-hydrolysis state, and wild-type BSEP trapped by vanadate to visualize a post-hydrolysis state. Our studies provide structural and functional insight into the mechanism of bile salt extrusion and into small-molecule inhibition of BSEP, which may rationalize drug-induced liver toxicity.

Transmembrane Helices 7 and 8 Confer Aggregation Sensitivity to the Cystic Fibrosis Transmembrane Conductance Regulator

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a large multi-spanning membrane protein that is susceptible to misfolding and aggregation. We have identified here the region responsible for this instability. Temperature-induced aggregation of C-terminally truncated versions of CFTR demonstrated that all truncations up to the second transmembrane domain (TMD2), including the R region, largely resisted aggregation. Limited proteolysis identified a folded structure that was prone to aggregation and consisted of TMD2 and at least part of the Regulatory Region R. Only when both TM7 (TransMembrane helix 7) and TM8 were present, TMD2 fragments became as aggregation-sensitive as wild-type CFTR, in line with increased thermo-instability of late CFTR nascent chains and in silico prediction of aggregation propensity. In accord, isolated TMD2 was degraded faster in cells than isolated TMD1. We conclude that TMD2 extended at its N-terminus with part of the R region forms a protease-resistant structure that induces heat instability in CFTR and may be responsible for its limited intracellular stability.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

ABC transporters: human disease and pharmacotherapeutic potential

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are a 48-member superfamily of membrane proteins that actively transport a variety of biological substrates across lipid membranes. Their functional diversity defines an expansive involvement in myriad aspects of human biology. At least 21 ABC transporters underlie rare monogenic disorders, with even more implicated in the predisposition to and symptomology of common and complex diseases. Such broad (patho)physiological relevance places this class of proteins at the intersection of disease causation and therapeutic potential, underlining them as promising targets for drug discovery, as exemplified by the transformative CFTR (ABCC7) modulator therapies for cystic fibrosis. This review will explore the growing relevance of ABC transporters to human disease and their potential as small-molecule drug targets.

Ivacaftor-Mediated Potentiation of ABCB4 Missense Mutations Affecting Critical Motifs of the NBDs: Repositioning Perspectives for Hepatobiliary Diseases

ABCB4 (ATP-binding cassette subfamily B member 4) is a hepatocanalicular floppase involved in biliary phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene give rise to several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), an autosomal recessive disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ten ABCB4 missense variations in NBD1:NBD2 homologous positions (Y403H/Y1043H, K435M/K1075M, E558K/E1200A, D564G/D1206G and H589Y/H1231Y) all localized at the conserved and functionally critical motifs of ABC transporters, six of which are mutated in patients. By combining structure analysis and in vitro studies, we found that all ten mutants were normally processed and localized at the canalicular membrane of HepG2 cells, but showed dramatically impaired PC transport activity that was significantly rescued by treatment with the clinically approved CFTR potentiator ivacaftor. Our results provide evidence that functional ABCB4 mutations are rescued by ivacaftor, paving the way for the repositioning of this potentiator for the treatment of selected patients with PFIC3 caused by mutations in the ATP-binding sites of ABCB4.

Genotype-phenotype relationships of truncating mutations, p.E297G and p.D482G in bile salt export pump deficiency

Background & Aims

Bile salt export pump (BSEP) deficiency frequently necessitates liver transplantation in childhood. In contrast to two predicted protein truncating mutations (PPTMs), homozygous p.D482G or p.E297G mutations are associated with relatively mild phenotypes, responsive to surgical interruption of the enterohepatic circulation (siEHC). The phenotype of patients with a compound heterozygous genotype of one p.D482G or p.E297G mutation and one PPTM has remained unclear. We aimed to assess their genotype-phenotype relationship.

Methods

From the NAPPED database, we selected patients with homozygous p.D482G or p.E297G mutations (BSEP1/1; n = 31), with one p.D482G or p.E297G, and one PPTM (BSEP1/3; n = 30), and with two PPTMs (BSEP3/3; n = 77). We compared clinical presentation, native liver survival (NLS), and the effect of siEHC on NLS.

Results

The groups had a similar median age at presentation (0.7-1.3 years). Overall NLS at age 10 years was 21% in BSEP1/3 vs. 75% in BSEP1/1 and 23% in BSEP3/3 (p <0.001). Without siEHC, NLS in the BSEP1/3 group was similar to that in BSEP3/3, but considerably lower than in BSEP1/1 (at age 10 years: 38%, 30%, and 71%, respectively; p = 0.003). After siEHC, BSEP1/3 and BSEP3/3 were associated with similarly low NLS, while NLS was much higher in BSEP1/1 (10 years after siEHC, 27%, 14%, and 92%, respectively; p <0.001).

Conclusions

Individuals with BSEP deficiency with one p.E297G or p.D482G mutation and one PPTM have a similarly severe disease course and low responsiveness to siEHC as those with two PPTMs. This identifies a considerable subgroup of patients who are unlikely to benefit from interruption of the enterohepatic circulation by either surgical or ileal bile acid transporter inhibitor treatment.

Impact and implications

This manuscript defines the clinical features and prognosis of individuals with BSEP deficiency involving the combination of one relatively mild and one very severe BSEP deficiency mutation. Until now, it had always been assumed that the mild mutation would be enough to ensure a relatively good prognosis. However, our manuscript shows that the prognosis of these patients is just as poor as that of patients with two severe mutations. They do not respond to biliary diversion surgery and will likely not respond to the new IBAT (ileal bile acid transporter) inhibitors, which have recently been approved for use in BSEP deficiency.

In Vitro Rescue of the Bile Acid Transport Function of ABCB11 Variants by CFTR Potentiators

ABCB11 is responsible for biliary bile acid secretion at the canalicular membrane of hepatocytes. Variations in the ABCB11 gene cause a spectrum of rare liver diseases. The most severe form is progressive familial intrahepatic cholestasis type 2 (PFIC2). Current medical treatments have limited efficacy. Here, we report the in vitro study of Abcb11 missense variants identified in PFIC2 patients and their functional rescue using cystic fibrosis transmembrane conductance regulator potentiators. Three ABCB11 disease-causing variations identified in PFIC2 patients (i.e., A257V, T463I and G562D) were reproduced in a plasmid encoding an Abcb11-green fluorescent protein. After transfection, the expression and localization of the variants were studied in HepG2 cells. Taurocholate transport activity and the effect of potentiators were studied in Madin–Darby canine kidney (MDCK) clones coexpressing Abcb11 and the sodium taurocholate cotransporting polypeptide (Ntcp/Slc10A1). As predicted using three-dimensional structure analysis, the three variants were expressed at the canalicular membrane but showed a defective function. Ivacaftor, GLP1837, SBC040 and SBC219 potentiators increased the bile acid transport of A257V and T463I and to a lesser extent, of G562D Abcb11 missense variants. In addition, a synergic effect was observed when ivacaftor was combined with SBC040 or SBC219. Such potentiators could represent new pharmacological approaches for improving the condition of patients with ABCB11 deficiency due to missense variations affecting the function of the transporter.

Gene Therapy for Acquired and Genetic Cholestasis

Cholestatic diseases can be caused by the dysfunction of transporters involved in hepatobiliary circulation. Although pharmacological treatments constitute the current standard of care for these diseases, none are curative, with liver transplantation being the only long-term solution for severe cholestasis, albeit with many disadvantages. Liver-directed gene therapy has shown promising results in clinical trials for genetic diseases, and it could constitute a potential new therapeutic approach for cholestatic diseases. Many preclinical gene therapy studies have shown positive results in animal models of both acquired and genetic cholestasis. The delivery of genes that reduce apoptosis or fibrosis or improve bile flow has shown therapeutic effects in rodents in which cholestasis was induced by drugs or bile duct ligation. Most studies targeting inherited cholestasis, such as progressive familial intrahepatic cholestasis (PFIC), have focused on supplementing a correct version of a mutated gene to the liver using viral or non-viral vectors in order to achieve expression of the therapeutic protein. These strategies have generated promising results in treating PFIC3 in mouse models of the disease. However, important challenges remain in translating this therapy to the clinic, as well as in developing gene therapy strategies for other types of acquired and genetic cholestasis.

Review article: epidemiology, pathogenesis and management of liver disease in adults with cystic fibrosis

BACKGROUND

Cystic fibrosis-related liver disease (CFLD) is the leading cause of death in cystic fibrosis (CF), after pulmonary disease. To improve identification and management of this condition requires an understanding of the underlying disease mechanism.

AIMS

This review summarises the current understanding of CFLD epidemiology, pathology, diagnosis and management.

METHODS

Relevant reports on cystic fibrosis liver disease were identified using a literature search and summarised.

RESULTS

CFLD is a heterogeneous condition with several different co-existent pathologies, including environmental and genetic factors. Incidence of clinically significant CFLD continues at a linear rate into early adulthood and has been described in up to 25% of CF patients. Diagnosis strategies lack precision and patient risk stratification needs to look beyond Childs-Pugh scoring. Efficacious therapies are lacking and, at present, newer modulator therapies lack data in CFLD and carry an increased risk of hepatotoxicity. Outcomes of liver transplant are comparable to non-CF transplant indications.

CONCLUSIONS

The incidence of CFLD increases with age and hence is increasingly important to adult patients with CF. Effective therapies are lacking. For progress to be made a better understanding of pathogenesis and disease detection are required.

Recent updates on progressive familial intrahepatic cholestasis types 1, 2 and 3: Outcome and therapeutic strategies

Recent evidence points towards the role of genotype to understand the phenotype, predict the natural course and long term outcome of patients with progressive familial intrahepatic cholestasis (PFIC). Expanded role of the heterozygous transporter defects presenting late needs to be suspected and identified. Treatment of pruritus, nutritional rehabilitation, prevention of fibrosis progression and liver transplantation (LT) in those with end stage liver disease form the crux of the treatment. LT in PFIC has its own unique issues like high rates of intractable diarrhoea, growth failure; steatohepatitis and graft failure in PFIC1 and antibody-mediated bile salt export pump deficiency in PFIC2. Drugs inhibiting apical sodium-dependent bile transporter and adenovirus-associated vector mediated gene therapy hold promise for future.

Induction of Bile Canaliculi-Forming Hepatocytes from Human Pluripotent Stem Cells

Cell polarity and formation of bile canaliculi can be achieved in hepatocytes which are generated from patient-derived induced pluripotent stem cells. This allows for the study of endogenous mutant proteins, patient-specific pathogenesis, and drug responses for diseases where hepatocyte polarity and bile canaliculi play a key role. Here, we describe a step-by-step protocol for the generation of bile canaliculi-forming hepatocytes from induced pluripotent stem cells and their evaluation.

Binding mode analysis of ABCA7 for the prediction of novel Alzheimer's disease therapeutics

The adenosine-triphosphate-(ATP)-binding cassette (ABC) transporter ABCA7 is a genetic risk factor for Alzheimer's disease (AD). Defective ABCA7 promotes AD development and/or progression. Unfortunately, ABCA7 belongs to the group of 'under-studied' ABC transporters that cannot be addressed by small-molecules. However, such small-molecules would allow for the exploration of ABCA7 as pharmacological target for the development of new AD diagnostics and therapeutics. Pan-ABC transporter modulators inherit the potential to explore under-studied ABC transporters as novel pharmacological targets by potentially binding to the proposed 'multitarget binding site'. Using the recently reported cryogenic-electron microscopy (cryo-EM) structures of ABCA1 and ABCA4, a homology model of ABCA7 has been generated. A set of novel, diverse, and potent pan-ABC transporter inhibitors has been docked to this ABCA7 homology model for the discovery of the multitarget binding site. Subsequently, application of pharmacophore modelling identified the essential pharmacophore features of these compounds that may support the rational drug design of innovative diagnostics and therapeutics against AD.

In vitro functional rescue by ivacaftor of an ABCB11 variant involved in PFIC2 and intrahepatic cholestasis of pregnancy

Background

ABCB11 variations are responsible for a spectrum of rare liver diseases, including progressive familial intrahepatic cholestasis type 2 (PFIC2) and intrahepatic cholestasis of pregnancy (ICP). Current medical treatment of these conditions mostly relies on ursodeoxycholic acid with limited efficacy. We report on the in vitro study of the p.A257V missense variant of ABCB11 identified in a PFIC2 patient and in her mother who experienced ICP.

Results

The Ala257 residue is located outside the ATP-binding site of ABCB11. We show that the p.A257V variant of ABCB11 is correctly expressed at the canalicular membrane of HepG2 cells but that its function significantly decreased when studied in MDCK cells. This functional defect can be fully rescued by Ivacaftor.

Conclusion

Ivacaftor could be considered as a new pharmacological tool able to respond to an unmet medical need for patients with ICP and PFIC2 due to ABCB11 variations affecting ABCB11 function, even when the residue involved is not located in an ATP-binding site of ABCB11.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-02125-4.

Role of inflammation and oxidative stress in tissue damage associated with cystic fibrosis: CAPE as a future therapeutic strategy

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, responsible for the synthesis of the CFTR protein, a chloride channel. The gene has approximately 2000 known mutations and all of them affect in some degree the protein function, which makes the pathophysiological manifestations to be multisystemic, mainly affecting the respiratory, gastrointestinal, endocrine, and reproductive tracts. Currently, the treatment of the disease is restricted to controlling symptoms and, more recently, a group of drugs that act directly on the defective protein, known as CFTR modulators, was developed. However, their high cost and difficult access mean that their use is still very restricted. It is important to search for safe and low-cost alternative therapies for CF and, in this context, natural compounds and, mainly, caffeic acid phenethyl ester (CAPE) appear as promising strategies to assist in the treatment of the disease. CAPE is a compound derived from propolis extracts that has antioxidant and anti-inflammatory activities, covering important aspects of the pathophysiology of CF, which points to the possible benefit of its use in the disease treatment. To date, no studies have effectively tested CAPE for CF and, therefore, we intend with this review to elucidate the role of inflammation and oxidative stress for tissue damage seen in CF, associating them with CAPE actions and its pharmacologically active derivatives. In this way, we offer a theoretical basis for conducting preclinical and clinical studies relating the use of this molecule to CF.

The spectrum of Progressive Familial Intrahepatic Cholestasis diseases: Update on pathophysiology and emerging treatments

The Progressive Familial Intrahepatic Cholestasis (PFIC) disease spectrum encompasses a variety of genetic diseases that affect the bile production and the secretion of bile acids. Typically, the first presentation of these diseases is in early childhood, frequently followed by a severe course necessitating liver transplantation before adulthood. Except for transplantation, treatment modalities have been rather limited and frequently only aim at the symptoms of cholestasis, such as cholestatic pruritus. In recent years, progress has been made in understanding the pathophysiology of these diseases and new treatment modalities have been emerging. Herewith we summarize the latest developments in the field and formulate the current key questions and opportunities for further progress.

Uncurtaining the pivotal role of ABC transporters in diabetes mellitus

The metabolic disorders are the edge points for the initiation of various diseases. These disorders comprised of several diseases including diabetes, obesity, and cardiovascular complications. Worldwide, the prevalence of these disorders is increasing day by day. The world's population is at higher threat of developing metabolic disease, especially diabetes. Therefore, there is an impregnable necessity of searching for a newer therapeutic target to reduce the burden of these disorders. Diabetes mellitus (DM) is marked with the dysregulated insulin secretion and resistance. The lipid and glucose transporters portray a pivotal role in the metabolism and transport of both of these. The excess production of lipid and glucose and decreased clearance of these leads to the emergence of DM. The ATP-binding cassette transporters (ABCT) are important for the metabolism of glucose and lipid. Various studies suggest the key involvement of ABCT in the pathologic process of different diseases. In addition, the involvement of other pathways, including IGF signaling, P13-Akt/PKC/MAPK signaling, and GLP-1 via regulation of ABCT, may help develop new treatment strategies to cope with insulin resistance dysregulated glucose metabolism, key features in DM.

Systems Biology and Bile Acid Signalling in Microbiome-Host Interactions in the Cystic Fibrosis Lung

The study of the respiratory microbiota has revealed that the lungs of healthy and diseased individuals harbour distinct microbial communities. Imbalances in these communities can contribute to the pathogenesis of lung disease. How these imbalances occur and establish is largely unknown. This review is focused on the genetically inherited condition of Cystic Fibrosis (CF). Understanding the microbial and host-related factors that govern the establishment of chronic CF lung inflammation and pathogen colonisation is essential. Specifically, dissecting the interplay in the inflammation–pathogen–host axis. Bile acids are important host derived and microbially modified signal molecules that have been detected in CF lungs. These bile acids are associated with inflammation and restructuring of the lung microbiota linked to chronicity. This community remodelling involves a switch in the lung microbiota from a high biodiversity/low pathogen state to a low biodiversity/pathogen-dominated state. Bile acids are particularly associated with the dominance of Proteobacterial pathogens. The ability of bile acids to impact directly on both the lung microbiota and the host response offers a unifying principle underpinning the pathogenesis of CF. The modulating role of bile acids in lung microbiota dysbiosis and inflammation could offer new potential targets for designing innovative therapeutic approaches for respiratory disease.

Targeting the Four Pillars of Enterohepatic Bile Salt Cycling; Lessons From Genetics and Pharmacology

Bile salts play a pivotal role in lipid homeostasis, are sensed by specialized receptors, and have been implicated in various disorders affecting the gut or liver. They may play a role either as culprit or as potential panacea. Four very efficient transporters mediate most of the hepatic and intestinal bile salt uptake and efflux, and are each essential for the efficient enterohepatic circulation of bile salts. Starting from the intestinal lumen, conjugated bile salts cross the otherwise impermeable lipid bilayer of (primarily terminal ileal) enterocytes through the apical sodium-dependent bile acid transporter (gene SLC10A2) and leave the enterocyte through the basolateral heteromeric organic solute transporter, which consists of an alpha and beta subunit (encoded by SLC51A and SLC51B). The Na+ -taurocholate cotransporting polypeptide (gene SLC10A1) efficiently clears the portal circulation of bile salts, and the apical bile salt export pump (gene ABCB11) pumps the bile salts out of the hepatocyte into primary bile, against a very steep concentration gradient. Recently, individuals lacking either functional Na+ -taurocholate cotransporting polypeptide or organic solute transporter have been described, completing the quartet of bile acid transport deficiencies, as apical sodium-dependent bile acid transporter and bile salt export pump deficiencies were already known for years. Novel pathophysiological insights have been obtained from knockout mice lacking functional expression of these genes and from pharmacological transporter inhibition in mice or humans. Conclusion: We provide a concise overview of the four main bile salt transport pathways and of their status as possible targets of interventions in cholestatic or metabolic disorders.

Medically Important Alterations in Transport Function and Trafficking of ABCG2

Several polymorphisms and mutations in the human ABCG2 multidrug transporter result in reduced plasma membrane expression and/or diminished transport function. Since ABCG2 plays a pivotal role in uric acid clearance, its malfunction may lead to hyperuricemia and gout. On the other hand, ABCG2 residing in various barrier tissues is involved in the innate defense mechanisms of the body; thus, genetic alterations in ABCG2 may modify the absorption, distribution, excretion of potentially toxic endo- and exogenous substances. In turn, this can lead either to altered therapy responses or to drug-related toxic reactions. This paper reviews the various types of mutations and polymorphisms in ABCG2, as well as the ways how altered cellular processing, trafficking, and transport activity of the protein can contribute to phenotypic manifestations. In addition, the various methods used for the identification of the impairments in ABCG2 variants and the different approaches to correct these defects are overviewed.

The Bile Salt Export Pump: Molecular Structure, Study Models and Small-Molecule Drugs for the Treatment of Inherited BSEP Deficiencies

The bile salt export pump (BSEP/ABCB11) is responsible for the transport of bile salts from hepatocytes into bile canaliculi. Malfunction of this transporter results in progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2) and intrahepatic cholestasis of pregnancy (ICP). Over the past few years, several small molecular weight compounds have been identified, which hold the potential to treat these genetic diseases (chaperones and potentiators). As the treatment response is mutation-specific, genetic analysis of the patients and their families is required. Furthermore, some of the mutations are refractory to therapy, with the only remaining treatment option being liver transplantation. In this review, we will focus on the molecular structure of ABCB11, reported mutations involved in cholestasis and current treatment options for inherited BSEP deficiencies.

Case report: progressive familial intrahepatic cholestasis type 3 with compound heterozygous ABCB4 variants diagnosed 15 years after liver transplantation

BACKGROUND

Progressive familial intrahepatic cholestasis (PFIC) type 3 is an autosomal recessive disorder arising from mutations in the ATP-binding cassette subfamily B member 4 (ABCB4) gene. This gene encodes multidrug resistance protein-3 (MDR3) that acts as a hepatocanalicular floppase that transports phosphatidylcholine from the inner to the outer canalicular membrane. In the absence of phosphatidylcholine, the detergent activity of bile salts is amplified and this leads to cholangiopathy, bile duct loss and biliary cirrhosis. Patients usually present in infancy or childhood and often progress to end-stage liver disease before adulthood.

CASE PRESENTATION

We report a 32-year-old female who required cadaveric liver transplantation at the age of 17 for cryptogenic cirrhosis. When the patient developed chronic ductopenia in the allograft 15 years later, we hypothesized that the patient's original disease was due to a deficiency of a biliary transport protein and the ductopenia could be explained by an autoimmune response to neoantigen that was not previously encountered by the immune system. We therefore performed genetic analyses and immunohistochemistry of the native liver, which led to a diagnosis of PFIC3. However, there was no evidence of humoral immune response to the MDR3 and therefore, we assumed that the ductopenia observed in the allograft was likely due to chronic rejection rather than autoimmune disease in the allograft.

CONCLUSIONS

Teenage patients referred for liver transplantation with cryptogenic liver disease should undergo work up for PFIC3. An accurate diagnosis of PFIC 3 is key for optimal management, therapeutic intervention, and avoidance of complications before the onset of end-stage liver disease.

Current Treatment Options for Cystic Fibrosis-Related Liver Disease

Cystic Fibrosis-related liver disease (CFLD) has become a leading cause of morbidity and mortality in patients with Cystic Fibrosis (CF), and affects children and adults. The understanding of the pathogenesis of CFLD is key in order to develop efficacious treatments. However, it remains complex, and has not been clarified to the last. The search for a drug might be additionally complicated due to the diverse clinical picture and lack of a unified definition of CFLD. Although ursodeoxycholic acid has been used for decades, its efficacy in CFLD is controversial, and the potential of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) modulators and targeted gene therapy in CFLD needs to be defined in the near future. This review focuses on the current knowledge on treatment strategies for CFLD based on pathomechanistic viewpoints.

ABCB11 deficiency presenting as transient neonatal cholestasis: Correlation with genotypes and BSEP expression

BACKGROUND & AIMS

ABCB11 deficiency presenting in infancy is believed generally to manifest as persistent/progressive cholestasis. We describe a group of patients with biallelic ABCB11 variants whose disorder manifested as transient neonatal cholestasis (TNC).

METHODS

Neonatal intrahepatic cholestasis in 68 children (31 males) with biallelic predictedly pathogenic variants (PPV) in ABCB11 was classified as transient (TNC group, n = 23, 11 males), intermittent (benign recurrent intrahepatic cholestasis [BRIC] group, n = 3, 1 male) or persistent/ progressive (progressive familial intrahepatic cholestasis [PFIC] group, n = 42, 19 males). Clinical, genetic and bile salt export pump (BSEP) expression information was correlated with outcomes.

RESULTS

The median onset age of jaundice was 3 days (birth to 2 months) for the TNC group and 10.5 days (birth to 3 months) for the PFIC group (P = .034). The median length of follow-up of TNC patients was 44 months (12 months-168 months). At presentation, hepatobiliary-injury biomarker values were similar between the groups (P > .05). TNC patients (17/23) more often than PFIC patients (20/42, P = .041) harboured biallelic non-null variants (predicted not to terminate translation prematurely). TNC patient livers (7/7) more often than PFIC patient livers (5/16, P = .005) expressed immunohistochemically detectable BSEP. Kaplan-Meier analysis showed better prognosis for patients with BSEP expression (P = .009). Too few BRIC patients were available for statistical study.

CONCLUSIONS

Neonatal cholestasis associated with biallelic PPV in ABCB11 can resolve temporarily or persistently in one third of cases. Resolution is more likely in patients with biallelic non-null PPV or with liver BSEP expression.