Identification of a compound heterozygous mutation of ABCC2 in a patient with hyperbilirubinemia

ABCC2p.R393W variant contributes to Dubin-Johnson syndrome by targeting MRP2 to proteasome degradation

Background

Dubin-Johnson syndrome (DJS), a rare autosomal recessive liver condition, is caused by biallelic loss-of-function mutations of theABCC2gene. This study aimed to investigate genetic variations in the drug efflux transporterABCC2(MRP2) gene in patients with DJS and to characterise the expression and mechanism of theABCC2gene variant.

Methods

Trio whole exome sequencing was performed in the family to identify the genetic causes. Bioinformatics analysis was performed to assess pathogenicity. Inin vitroexperiments, site-directed mutagenesis was used to introduceABCC2variants in constructs then expressed in HEK293T, HuH-7 and HepG2 cell lines. The expression of total and cell membrane MRP2 was quantified in cells expressing the wild-type or variant forms. Chloroquine and MG132 were used to evaluate the effects of p.R393W on lysosomal and/or proteasomal degradation.

Results

The twin probands carry DJS-associated variants c.1177C>T (rs777902199) in theABCC2gene inherited from the father and the c.3632T>C mutation in the other allele inherited from the mother. TheABCC2variant, c.1177C>T, results in a p.R393W substitution in MRP2 that is highly conserved among vertebrates, drastically decreasing the expression of mutant protein by promoting proteasomal degradation. Another variant c.3632T>C results in a p.L1211P substitution in MRP2, decreasing the expression of membrane MRP2 but not changing the expression of total protein.

Conclusion

These results strongly suggest that the p.R393W variant affects the stability of the MRP2 protein and decreases its expression by ubiquitin-mediated proteasomal degradation, and the p.L1211P decreases the expression of membrane MRP2, indicating that these two variants, respectively, cause a loss-of-function of the MRP2 protein and membrane MRP2 ultimately leading to DJS development.

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>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.

A novel homozygous frameshift variant in the ABCC2-gene in Dubin-Johnson syndrome may predispose to chronic liver disease

Dubin-Johnson syndrome (DJS), an autosomal recessive disorder presenting with conjugated hyperbilirubinemia, is not associated with progression to chronic liver disease (CLD). Next-generation sequencing, application of bioinformatics pipeline, and segregation analysis were performed on 8 members of a consanguineous family with DJS and CLD. A novel variant, c.4406_4407delTA (p.Leu1469fs), in the ABCC2-gene in a homozygous state was found to be associated with DJS and CLD in proband and afflicted family members. DJS may not be a benign entity and novel genetic variants may be associated with progressive liver disease.

High-Throughput Screening to Evaluate Inhibition of Bile Acid Transporters Using Human Hepatocytes Isolated From Chimeric Mice

Cholestasis resulting from hepatic bile acid efflux transporter inhibition may contribute to drug-induced liver injury (DILI). This condition is a common safety-related reason for drug attrition and withdrawal. To screen for safety risks associated with efflux transport inhibition, we developed a high-throughput cellular assay for different drug discovery phases. Hepatocytes isolated from chimeric mice with humanized livers presented gene expression resembling that of the human liver and demonstrated apical membrane polarity when sandwiched between Matrigel and collagen. The fluorescent bile acid-derivative cholyl-l-lysyl-fluorescein (CLF) was used to quantify drug-induced efflux transport inhibition in hepatocytes. Cyclosporine inhibited CLF accumulation in the apical bile canalicular lumen in a concentration-dependent manner. The assay had equivalent predictive power to a primary human hepatocyte-based assay and greater predictive power than an assay performed with rat hepatocytes. Predictive power was tested using 45 pharmaceutical compounds, and 91.3% of the compounds with cholestatic potential (21/23) had margins (IC50/Cmax) < 20. In contrast, 90.9% (20/22) of compounds without cholestatic potential had IC50/Cmax>20. Assay sensitivity and specificity were 91.3% and 90.9%, respectively. We suggest that this improved assay performance could result from higher expression of efflux transporters, metabolic pathways, and/or species differences. Given the long-term supply of cells from the same donor, the humanized mouse-derived hepatocyte-based CLF efflux assay could be a valuable tool for predicting cholestatic DILI.

Mutation analysis of the ABCC2 gene in Chinese patients with Dubin-Johnson syndrome

Dubin-Johnson syndrome (DJS) is a rare, autosomal recessive disorder characterized by predominantly conjugated hyperbilirubinemia, caused by a mutation in the adenosine triphosphate-binding cassette subfamily C member 2 (ABCC2) gene coding the multidrug resistance-associated protein 2 (MRP2) protein. ABCC2 mutations have been identified in patients with DJS worldwide; however, the mutation pattern of ABCC2 in China is not well studied. In the present study, the mutation pattern of the ABCC2 gene in Chinese patients with DJS was investigated. A total of 7 clinically confirmed patients with DJS were enrolled, and mutation analysis of the ABCC2 gene was performed by Sanger sequencing of genomic DNA extracted from whole blood. All 32 exons and the adjacent splice junction areas were sequenced. All cases were identified to harbor at least one non-synonymous variant in the ABCC2 gene, including three known mutations in 3 cases and three novel variants (p.G693R, p.G808V and p.E647X) in the other 4 cases, with the known p.R393W and the novel p.G693R and p.E647X variants identified in 2 of the 7 cases (28.6%), respectively. All the identified mutations were heterozygous, and 1 case presented with a compound heterozygous mutation, namely p.G693R/p.G808V, while the other cases carried only one single mutation. The loss of membrane expression of MRP2 caused by the novel nonsense variant, p.E647X, was confirmed by immunohistochemical analysis of liver biopsy. The present study provided the first report on the mutation patterns of the ABCC2 gene in Chinese patients with DJS, and the clinical association of these mutations with the syndrome.

Neuroinflammation and ER-stress are key mechanisms of acute bilirubin toxicity and hearing loss in a mouse model

Hyperbilirubinemia (jaundice) is caused by raised levels of unconjugated bilirubin in the blood. When severe, susceptible brain regions including the cerebellum and auditory brainstem are damaged causing neurological sequelae such as ataxia, hearing loss and kernicterus. The mechanism(s) by which bilirubin exerts its toxic effect have not been completely understood to date. In this study we investigated the acute mechanisms by which bilirubin causes the neurotoxicity that contributes to hearing loss. We developed a novel mouse model that exhibits the neurological features seen in human Bilirubin-Induced Neurological Dysfunction (BIND) syndrome that we assessed with a behavioural score and auditory brainstem responses (ABR). Guided by initial experiments applying bilirubin to cultured cells in vitro, we performed whole genome gene expression measurements on mouse brain tissue (cerebellum and auditory brainstem) following bilirubin exposure to gain mechanistic insights into biochemical processes affected, and investigated further using immunoblotting. We then compared the gene changes induced by bilirubin to bacterial lipopolysaccharide (LPS), a well characterized inducer of neuroinflammation, to assess the degree of similarity between them. Finally, we examined the extent to which genetic perturbation of inflammation and both known and novel anti-inflammatory drugs could protect hearing from bilirubin-induced toxicity. The in vitro results indicated that bilirubin induces changes in gene expression consistent with endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). These gene changes were similar to the gene expression signature of thapsigargin–a known ER stress inducer. It also induced gene expression changes associated with inflammation and NF-κB activation. The in vivo model showed behavioural impairment and a raised auditory threshold. Whole genome gene expression analysis confirmed inflammation as a key mechanism of bilirubin neurotoxicity in the auditory pathway and shared gene expression hallmarks induced by exposure to bacterial lipopolysaccharide (LPS) a well-characterized inducer of neuroinflammation. Interestingly, bilirubin caused more severe damage to the auditory system than LPS in this model, but consistent with our hypothesis of neuroinflammation being a primary part of bilirubin toxicity, the hearing loss was protected by perturbing the inflammatory response. This was carried out genetically using lipocalin-2 (LCN2)-null mice, which is an inflammatory cytokine highly upregulated in response to bilirubin. Finally, we tested known and novel anti-inflammatory compounds (interfering with NF-κB and TNFα signalling), and also demonstrated protection of the auditory system from bilirubin toxicity. We have developed a novel, reversible, model for jaundice that shows movement impairment and auditory loss consistent with human symptoms. We used this model to establish ER-stress and inflammation as major contributors to bilirubin toxicity. Because of the rapid and reversible onset of toxicity in this novel model it represents a system to screen therapeutic compounds. We have demonstrated this by targeting inflammation genetically and with anti-inflammatory small molecules that offered protection against bilirubin toxicity. This also suggests that anti-inflammatory drugs could be of therapeutic use in hyperbilirubinemia.