Characterization of the most frequent ATP7B mutation causing Wilson disease in hepatocytes from patient induced pluripotent stem cells

A new Caenorhabditis elegans model to study copper toxicity in Wilson disease

Wilson disease (WD) is caused by mutations in the ATP7B gene that encodes a copper (Cu) transporting ATPase whose trafficking from the Golgi to endo-lysosomal compartments drives sequestration of excess Cu and its further excretion from hepatocytes into the bile. Loss of ATP7B function leads to toxic Cu overload in the liver and subsequently in the brain, causing fatal hepatic and neurological abnormalities. The limitations of existing WD therapies call for the development of new therapeutic approaches, which require an amenable animal model system for screening and validation of drugs and molecular targets. To achieve this objective, we generated a mutant Caenorhabditis elegans strain with a substitution of a conserved histidine (H828Q) in the ATP7B ortholog cua-1 corresponding to the most common ATP7B variant (H1069Q) that causes WD. cua-1 mutant animals exhibited very poor resistance to Cu compared to the wild-type strain. This manifested in a strong delay in larval development, a shorter lifespan, impaired motility, oxidative stress pathway activation, and mitochondrial damage. In addition, morphological analysis revealed several neuronal abnormalities in cua-1 mutant animals exposed to Cu. Further investigation suggested that mutant CUA-1 is retained and degraded in the endoplasmic reticulum, similarly to human ATP7B-H1069Q. As a consequence, the mutant protein does not allow animals to counteract Cu toxicity. Notably, pharmacological correctors of ATP7B-H1069Q reduced Cu toxicity in cua-1 mutants indicating that similar pathogenic molecular pathways might be activated by the H/Q substitution and, therefore, targeted for rescue of ATP7B/CUA-1 function. Taken together, our findings suggest that the newly generated cua-1 mutant strain represents an excellent model for Cu toxicity studies in WD.

Functional characterization of novel or yet uncharacterized ATP7B missense variants detected in patients with clinical Wilson's disease

Wilson's disease (WD, MIM#277900) is an autosomal recessive disorder resulting in copper excess caused by biallelic variants in the ATP7B gene (MIM#606882) encoding a copper transporting P-type ATPase. ATP7B variants of unknown significance (VUS) are detected frequently, sometimes impeding a clear diagnosis. Functional analyses can help to classify these variants as benign or pathogenic. Additionally, variants already classified as (likely) pathogenic benefit from functional analyses to understand their pathomechanism, thus contribute to the development of personalized treatment approaches in the future. We described clinical features of six WD patients and functionally characterized five ATP7B missense variants (two VUS, three yet uncharacterized likely pathogenic variants), detected in these patients. We determined the protein level, copper export capacity, and cellular localization in an in vitro model and potential structural consequences using an ATP7B protein model based on AlphaFold. Our analyses give insight into the pathomechanism and allowed reclassification for the two VUS to likely pathogenic and for two of the three likely pathogenic variants to pathogenic.

Experimental evidence and clinical implications of Warburg effect in the skeletal muscle of Fabry disease

Skeletal muscle (SM) pain and fatigue are common in Fabry disease (FD). Here, we undertook the investigation of the energetic mechanisms related to FD-SM phenotype. A reduced tolerance to aerobic activity and lactate accumulation occurred in FD-mice and patients. Accordingly, in murine FD-SM we detected an increase in fast/glycolytic fibers, mirrored by glycolysis upregulation. In FD-patients, we confirmed a high glycolytic rate and the underutilization of lipids as fuel. In the quest for a tentative mechanism, we found HIF-1 upregulated in FD-mice and patients. This finding goes with miR-17 upregulation that is responsible for metabolic remodeling and HIF-1 accumulation. Accordingly, miR-17 antagomir inhibited HIF-1 accumulation, reverting the metabolic-remodeling in FD-cells. Our findings unveil a Warburg effect in FD, an anaerobic-glycolytic switch under normoxia induced by miR-17-mediated HIF-1 upregulation. Exercise-intolerance, blood-lactate increase, and the underlying miR-17/HIF-1 pathway may become useful therapeutic targets and diagnostic/monitoring tools in FD.

Missing heritability of Wilson disease: a search for the uncharacterized mutations

Wilson disease (WD), a copper metabolism disorder caused by mutations in ATP7B, manifests heterogeneous clinical features. Interestingly, in a fraction of clinically diagnosed WD patients, mutations in ATP7B appears to be missing. In this review we discuss the plausible explanations of this missing heritability and propose a workflow that can identify the hidden mutations. Mutation analyses of WD generally includes targeted sequencing of ATP7B exons, exon-intron boundaries, and rarely, the proximal promoter region. We propose that variants in the distal cis-regulatory elements and/or deep intronic variants that impact splicing might well represent the hidden mutations. Heterozygous del/ins that remain refractory to conventional PCR-sequencing method may also represent such mutations. In this review, we also hypothesize that mutations in the key copper metabolism genes, like, ATOX1, COMMD1, and SLC31A1, could possibly lead to a WD-like phenotype. In fact, WD does present overlapping symptoms with other rare genetic disorders; hence, the possibility of a misdiagnosis and thus adding to missing heritability cannot be excluded. In this regard, it seems that whole-genome analysis will provide a comprehensive and rapid molecular diagnosis of WD. However, considering the associated cost for such a strategy, we propose an alternative customized screening schema of WD which include targeted sequencing of ATP7B locus as well as other key copper metabolism genes. Success of such a schema has been tested in a pilot study.

A Century of Progress on Wilson Disease and the Enduring Challenges of Genetics, Diagnosis, and Treatment

Wilson disease (WD) is a rare, inherited metabolic disorder manifested with varying clinical presentations including hepatic, neurological, psychiatric, and ophthalmological features, often in combination. Causative mutations in the ATP7B gene result in copper accumulation in hepatocytes and/or neurons, but clinical diagnosis remains challenging. Diagnosis is complicated by mild, non-specific presentations, mutations exerting no clear effect on protein function, and inconclusive laboratory tests, particularly regarding serum ceruloplasmin levels. As early diagnosis and effective treatment are crucial to prevent progressive damage, we report here on the establishment of a global collaboration of researchers, clinicians, and patient advocacy groups to identify and address the outstanding challenges posed by WD.

From a Single Cell to a Whole Human Liver: Disease Modeling and Transplantation

Although the underlying cause may vary across countries and demographic groups, liver disease is a major cause of morbidity and mortality globally. Orthotopic liver transplantation is the only definitive treatment for liver failure but is limited by the lack of donor livers. The development of drugs that prevent the progression of liver disease and the generation of alternative liver constructs for transplantation could help alleviate the burden of liver disease. Bioengineered livers containing human induced pluripotent stem cell (iPSC)-derived liver cells are being utilized to study liver disease and to identify and test potential therapeutics. Moreover, bioengineered livers containing pig hepatocytes and endothelial cells have been shown to function and survive after transplantation into pig models of liver failure, providing preclinical evidence toward future clinical applications. Finally, bioengineered livers containing human iPSC-derived liver cells have been shown to function and survive after transplantation in rodents but require considerable optimization and testing prior to clinical use. In conclusion, bioengineered livers have emerged as a suitable tool for modeling liver diseases and as a promising alternative graft for clinical transplantation. The integration of novel technologies and techniques for the assembly and analysis of bioengineered livers will undoubtedly expand future applications in basic research and clinical transplantation.

Advanced human developmental toxicity and teratogenicity assessment using human organoid models

Tremendous progress has been made in the field of toxicology leading to the advance of developmental toxicity assessment. Conventional animal models and in vitro two-dimensional models cannot accurately describe toxic effects and predict actual in vivo responses due to obvious inter-species differences between humans and animals, as well as the lack of a physiologically relevant tissue microenvironment. Human embryonic stem cell (hESC)- and induced pluripotent stem cell (iPSC)-derived three-dimensional organoids are ideal complex and multicellular organotypic models, which are indispensable in recapitulating morphogenesis, cellular interactions, and molecular processes of early human organ development. Recently, human organoids have been used for drug discovery, chemical toxicity and safety in vitro assessment. This review discusses the recent advances in the use of human organoid models, (i.e., brain, retinal, cardiac, liver, kidney, lung, and intestinal organoid models) for developmental toxicity and teratogenicity assessment of distinct tissues/organs following exposure to pharmaceutical compounds, heavy metals, persistent organic pollutants, nanomaterials, and ambient air pollutants. Combining next-generation organoid models with innovative engineering technologies generates novel and powerful tools for developmental toxicity and teratogenicity assessment, and the rapid progress in this field is expected to continue.

Retinoids rescue ceruloplasmin secretion and alleviate oxidative stress in Wilson's disease-specific hepatocytes

Wilson’s disease (WD) is a copper metabolic disorder caused by a defective ATP7B function. Conventional therapies cause severe side effects and significant variation in efficacy, according to cohort studies. Thus, exploring new therapeutic approaches to prevent progression to liver failure is urgent. To study the physiology and pathology of WD, immortalized cell lines and rodent WD models have been used conventionally; however, a large gap remains among different species as well as in genetic backgrounds among individuals. We generated induced pluripotent stem cells (iPSCs) from four WD patients carrying compound heterozygous mutations in the ATP7B gene. ATP7B loss- and gain-of-functions were further manifested with ATP7B-deficient iPSCs and heterozygously corrected R778L WD patient-derived iPSCs using CRISPR-Cas9-based gene editing. Although the expression of ATP7B protein varied among WD-specific hepatocytes differentiated from these iPSCs, the expression and secretion of ceruloplasmin (Cp), a downstream copper carrier in plasma, were consistently decreased in WD patient-derived and ATP7B-deficient hepatocytes. A transcriptome analysis detected abnormalities in the retinoid signaling pathway and lipid metabolism in WD-specific hepatocytes. Drug screening using WD patient-derived hepatocytes identified retinoids as promising candidates for rescuing Cp secretion. All-trans retinoic acid also alleviates reactive oxygen species production induced by lipid accumulation in WD-specific hepatocytes treated with oleic acid. These patient-derived iPSC-based hepatic models function as effective platforms for the development of potential therapeutics for hepatic steatosis in WD and other fatty liver diseases.

Advancements in Disease Modeling and Drug Discovery Using iPSC-Derived Hepatocyte-like Cells

Serving as the metabolic hub of the human body, the liver is a vital organ that performs a variety of important physiological functions. Although known for its regenerative potential, it remains vulnerable to a variety of diseases. Despite decades of research, liver disease remains a leading cause of mortality in the United States with a multibillion-dollar-per-year economic burden. Prior research with model systems, such as primary hepatocytes and murine models, has provided many important discoveries. However, progress has been impaired by numerous obstacles associated with these models. In recent years, induced pluripotent stem cell (iPSC)-based systems have emerged as advantageous platforms for studying liver disease. Benefits, including preserved differentiation and physiological function, amenability to genetic manipulation via tools such as CRISPR/Cas9, and availability for high-throughput screening, make these systems increasingly attractive for both mechanistic studies of disease and the identification of novel therapeutics. Although limitations exist, recent studies have made progress in ameliorating these issues. In this review, we discuss recent advancements in iPSC-based models of liver disease, including improvements in model system construction as well as the use of high-throughput screens for genetic studies and drug discovery.

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.

Induced Pluripotent Stem Cells for the Treatment of Liver Diseases: Novel Concepts

Millions of people worldwide with incurable liver disease die because of inadequate treatment options and limited availability of donor organs for liver transplantation. Regenerative medicine as an innovative approach to repairing and replacing cells, tissues, and organs is undergoing a major revolution due to the unprecedented need for organs for patients around the world. Induced pluripotent stem cells (iPSCs) have been widely studied in the field of liver regeneration and are considered to be the most promising candidate therapies. This review will conclude the current state of efforts to derive human iPSCs for potential use in the modeling and treatment of liver disease.

Wilson's disease: update on pathogenesis, biomarkers and treatments

Wilson's disease is an autosomal-recessive disorder of copper metabolism caused by mutations in ATP7B and associated with neurological, psychiatric, ophthalmological and hepatic manifestations. Decoppering treatments are used to prevent disease progression and reduce symptoms, but neurological outcomes remain mixed. In this article, we review the current understanding of pathogenesis, biomarkers and treatments for Wilson's disease from the neurological perspective, with a focus on recent advances. The genetic and molecular mechanisms associated with ATP7B dysfunction have been well characterised, but despite extensive efforts to identify genotype-phenotype correlations, the reason why only some patients develop neurological or psychiatric features remains unclear. We discuss pathological processes through which copper accumulation leads to neurodegeneration, such as mitochondrial dysfunction, the role of brain iron metabolism and the broader concept of selective neuronal vulnerability in Wilson's disease. Delayed diagnoses continue to be a major problem for patients with neurological presentations. We highlight limitations in our current approach to making a diagnosis and novel diagnostic biomarkers, including the potential for newborn screening programmes. We describe recent progress in developing imaging and wet (fluid) biomarkers for neurological involvement, including findings from quantitative MRI and other neuroimaging studies, and the development of a semiquantitative scoring system for assessing radiological severity. Finally, we cover the use of established and novel chelating agents, paradoxical neurological worsening, and progress developing targeted molecular and gene therapy for Wilson's disease, before discussing future directions for translational research.

Liver Organoids: Updates on Disease Modeling and Biomedical Applications

Liver organoids are stem cell-derived 3D structures that are generated by liver differentiation signals in the presence of a supporting extracellular matrix. Liver organoids overcome low complexity grade of bidimensional culture and high costs of in vivo models thus representing a turning point for studying liver disease modeling. Liver organoids can be established from different sources as induced pluripotent stem cells (iPSCs), embryonic stem cells (ESCs), hepatoblasts and tissue-derived cells. This novel in vitro system represents an innovative tool to deeper understand the physiology and pathological mechanisms affecting the liver. In this review, we discuss the current advances in the field focusing on their application in modeling diseases, regenerative medicine and drug discovery.

From Cells to Organs: The Present and Future of Regenerative Medicine

Regenerative medicine promises a bright future where damaged body parts can be restored, rejuvenated, and replaced. The application of regenerative medicine is interdisciplinary and covers nearly all fields of medical sciences and molecular engineering. This review provides a road map on how regenerative medicine is applied on the levels of cell, tissue, and organ and summarizes the advantages and limitation of human pluripotent stem cells in disease modeling and regenerative application.

Inhibition of ASIC1a-Mediated ERS Improves the Activation of HSCs and Copper Transport Under Copper Load

Hepatolenticular degeneration (HLD) is an autosomal recessive genetic disease caused by the toxic accumulation of copper in the liver. Excessive copper will disrupt the redox balance in cells and tissues, causing ischemia, hypoxia, and inflammation. Acid-sensitive ion channel 1a is a cationic channel activated by extracellular acid and allowing Ca²⁺ and Na⁺ to flow into cells. Its expression appears in inflammation, arthritis, fibrotic tissue, and damaged environment, but its role in hepatolenticular degeneration has not been studied. This study established a Wistar rat model of high copper accumulation and used CuSO₄ to induce the activation of HSC-T6 in an in vitro experiment. In vivo, Wistar rats were examined to determine the serum copper concentration, serum ALT and AST activities, and liver copper accumulation, and liver tissue HE staining and immunohistochemical analyses were conducted. The expression of ASIC1a, α-SMA, Collagen-Ι, GRP78, XBP1, ATP7B, and CCS were detected. Besides, immunofluorescence technology can detect the expression of the phosphorylated protein in vitro. It is suggested that ASIC1a is involved in the quality control of the endoplasmic reticulum, which degrades mutant ATP7B and increases the accumulation of copper. After blocking or silencing the expression of ASIC1a, ELISA can detect the level of inflammatory factors, the expression of endoplasmic reticulum stress-related factors, and ATP7B was improved in a higher copper environment reduction of copper deposition was observed in liver Timm’s staining. Collectively, we conclude that ASIC1a is involved in the HSC activation induced by copper accumulation and promotes the occurrence of hepatolenticular fibrosis.

Synthetic human livers for modeling metabolic diseases

PURPOSE OF REVIEW

In this review, we will explore recent advances in human induced pluripotent stem cell (iPSC)-based modeling of metabolic liver disease and biofabrication of synthetic human liver tissue while also discussing the emerging concept of synthetic biology to generate more physiologically relevant liver disease models.

RECENT FINDING

iPSC-based platforms have facilitated the study of underlying cellular mechanisms and potential therapeutic strategies for a number of metabolic liver diseases. Concurrently, rapid progress in biofabrication and gene editing technologies have led to the generation of human hepatic tissue that more closely mimic the complexity of the liver.

SUMMARY

iPSC-based liver tissue is rapidly becoming available for modeling liver physiology due to its ability to recapitulate the complex three-dimensional architecture of the liver and recapitulate interactions between the different cell types and their surroundings. These mini livers have also been used to recapitulate liver disease pathways using the tools of synthetic biology, such as gene editing, to control gene circuits. Further development in this field will undoubtedly bolster future investigations not only in disease modeling and basic research, but also in personalized medicine and autologous transplantation.

Therapeutic strategies in Wilson disease: pathophysiology and mode of action

Wilson disease is a copper overload disease treatable with the chelators D-penicillamine and trientine to enhance urinary excretion or with zinc which predominantly inhibits absorption. By lifelong treatment a normal life expectancy and significant improvement of hepatic injury as well as neurologic manifestation is achievable. Here we evaluate the mode of action for effective therapy of Wilson disease. We postulate that there is no quantitative removal of copper from the liver possible. The therapeutic goal is the removal of toxic free copper (non-ceruloplasmin, but albumin bound copper). This is achievable by the induction of metallothionein which is accomplished by chelators and in particular by zinc. For control of therapy the option of a direct measurement of free copper would be preferable over the less reliable calculation of this fraction. A therapeutic challenge is still the full restoration of neurological deficits which can hardly be reached by the available chelators. Whether bis-choline-tetrathiomolybdate as intracellular copper chelator is an option has to be awaited. It is concluded that the goal of actual drug therapy in Wilson disease is the normalization of free copper in serum.

Pharmacoproteomics pinpoints HSP70 interaction for correction of the most frequent Wilson disease-causing mutant of ATP7B

Pathogenic mutations in the copper transporter ATP7B have been hypothesized to affect its protein interaction landscape contributing to loss of function and, thereby, to hepatic copper toxicosis in Wilson disease. Although targeting mutant interactomes was proposed as a therapeutic strategy, druggable interactors for rescue of ATP7B mutants remain elusive. Using proteomics, we found that the frequent H1069Q substitution promotes ATP7B interaction with HSP70, thus accelerating endoplasmic reticulum (ER) degradation of the mutant protein and consequent copper accumulation in hepatic cells. This prompted us to use an HSP70 inhibitor as bait in a bioinformatics search for structurally similar Food and Drug Administration-approved drugs. Among the hits, domperidone emerged as an effective corrector that recovered trafficking and function of ATP7B-H1069Q by impairing its exposure to the HSP70 proteostatic network. Our findings suggest that HSP70-mediated degradation can be safely targeted with domperidone to rescue ER-retained ATP7B mutants and, hence, to counter the onset of Wilson disease.

p.P1379S, a benign variant with reduced ATP7B protein level in Wilson Disease

BACKGROUND

Wilson disease (WD) is an autosomal recessive disorder of copper transport caused by inherited defects in the ATP7B gene and results in toxic accumulation of copper in various organs. We previously reported a family with three consecutive generations affected by WD that carries the variant, p.P1379S, which was classified at the time as likely pathogenic. However, recent investigations of the p.P1379S variant indicate a possible conflict of interpretations regarding its pathogenicity. This led us to explore the quantification of ATP7B in dried blood spots (DBS) using a surrogate peptide to study the effects of the p.P1379S variant on ATP7B concentrations in two unrelated families with the common p.P1379S variant.

METHODS AND RESULTS

ATP7B was quantified using the peptide immunoaffinity enrichment coupled with selected reaction monitoring mass spectrometry (immuno-SRM) method which utilizes antibody-mediated peptide capture from DBS. Two patients affected with WD had undetectable ATP7B level while four compound heterozygous children with one known pathogenic variant and the p.P1379S had significantly reduced ATP7B levels. Of note, all four children remain asymptomatic without abnormal laboratory consequences despite being untreated for WD.

CONCLUSION

These two families demonstrated that p.P1379S, when compounded with two known pathogenic variants, resulted in significantly reduced protein levels but retained enough function to maintain normal copper homeostasis. This implies that p.P1379S is benign in nature. A better understanding of the nature and consequences of variants in WD will help in informing patient care and avoiding unnecessary treatments.

Human Embryonic Stem Cell-Derived Wilson's Disease Model for Screening Drug Efficacy

Human pluripotent stem cells (hPSCs) including human embryonic stem cells (hESCs) and human-induced pluripotent stem cells (hiPSCs) have been extensively studied as an alternative cellular model for recapitulating phenotypic and pathophysiologic characters of human diseases. Particularly, hiPSCs generated from the genetic disease somatic cells could provide a good cellular model to screen potential drugs for treating human genetic disorders. However, the patient-derived cellular model has a limitation when the patient samples bearing genetic mutations are difficult to obtain due to their rarity. Thus, in this study, we explored the potential use of hPSC-derived Wilson's disease model generated without a patient sample to provide an alternative approach for modeling human genetic disease by applying gene editing technology. Wilson's disease hPSCs were generated by introducing a R778L mutation in the ATP7B gene (c.2333G>T) using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 system into wildtype hESCs. Established Wilson's disease hESCs were further differentiated into hepatocyte-like cells (HLCs) and analyzed for disease phenotypes and responses against therapeutic agent treatment. R778L mutation in the ATP7B gene was successfully introduced into wildtype hESCs, and the introduction of the mutation neither altered the self-renewal ability of hESCs nor the differentiation capability into HLCs. However, R778L mutation-introduced HLCs exhibited higher vulnerability against excessive copper supplementation than wildtype HLCs. Finally, the applicability of the R778L mutation introduced HLCs in drug screening was further demonstrated using therapeutic agents against the Wilson's diseases. Therefore, the established model in this study could effectively mimic the Wilson's disease without patient's somatic cells and could provide a reliable alternative model for studying and drug screening of Wilson's disease.

Computing the Pathogenicity of Wilson's Disease ATP7B Mutations: Implications for Disease Prevalence

Genetic variations in the gene encoding the copper-transport protein ATP7B are the primary cause of Wilson's disease. Controversially, clinical prevalence seems much smaller than the prevalence estimated by genetic screening tools, causing fear that many people are undiagnosed, although early diagnosis and treatment is essential. To address this issue, we benchmarked 16 state-of-the-art computational disease-prediction methods against established data of missense ATP7B mutations. Our results show that the quality of the methods varies widely. We show the importance of optimizing the threshold of the methods used to distinguish pathogenic from nonpathogenic mutations against data of clinically confirmed pathogenic and nonpathogenic mutations. We find that most methods use thresholds that predict too many ATP7B mutations to be pathogenic. Thus, our findings explain the current controversy on Wilson's disease prevalence because meta-analysis and text search methods include many computational estimates that lead to higher disease prevalence than clinically observed. As proteins and diseases differ widely, a one-size-fits-all threshold cannot distinguish pathogenic and nonpathogenic mutations efficiently, as shown here. We also show that amino acid changes with small evolutionary substitution probability, mainly due to amino acid volume, are more associated with the disease, implying a pathological effect on the conformational state of the protein, which could affect copper transport or adenosine triphosphate recognition and hydrolysis. These findings may be a first step toward a more quantitative genotype-phenotype relationship of Wilson's disease.

Wilson disease missense mutations in ATP7B affect metal-binding domain structural dynamics

Wilson disease (WD) is caused by mutations in the gene for ATP7B, a copper transport protein that regulates copper levels in cells. A large number of missense mutations have been reported to cause WD but genotype-phenotype correlations are not yet established. Since genetic screening for WD may become reality in the future, it is important to know how individual mutations affect ATP7B function, with the ultimate goal to predict pathophysiology of the disease. To begin to assess mechanisms of dysfunction, we investigated four proposed WD-causing missense mutations in metal-binding domains 5 and 6 of ATP7B. Three of the four variants showed reduced ATP7B copper transport ability in a traditional yeast assay. To probe mutation-induced structural dynamic effects at the atomic level, molecular dynamics simulations (1.5 μs simulation time for each variant) were employed. Upon comparing individual metal-binding domains with and without mutations, we identified distinct differences in structural dynamics via root-mean square fluctuation and secondary structure content analyses. Most mutations introduced distant effects resulting in increased dynamics in the copper-binding loop. Taken together, mutation-induced long-range alterations in structural dynamics provide a rationale for reduced copper transport ability.

From and to the Golgi - defining the Wilson disease protein road map

Recent studies highlight the continued growth in the identification of a variety of cellular functions that involve the Golgi apparatus. Apart from well-known membrane sorting/trafficking and glycosylation machineries, the Golgi harbors molecular platforms operating in intracellular signaling, cytoskeleton organization, and protein quality control mechanisms. One of new emerging Golgi functions consists in the regulation of copper homeostasis by coordinating the relocation and activity of copper transporters. Of these, the Cu-transporting ATPase ATP7B (known as Wilson disease protein) plays a key role in the maintenance of the Cu balance in the body via the supply of essential Cu to the systemic circulation and via elimination of excess Cu into the bile. These activities require tightly regulated shuttling of ATP7B between the Golgi and different post-Golgi compartments. Despite significant progress over recent years, a number of issues regarding ATP7B trafficking remain to be clarified. This review summarizes current views on ATP7B trafficking pathways from and to the Golgi and underscores the challenges that should be addressed to define the ATP7B trafficking routes and mechanisms in health and disease.

Pluripotent stem cell-derived bile canaliculi-forming hepatocytes to study genetic liver diseases involving hepatocyte polarity

BACKGROUND & AIMS

Hepatocyte polarity is essential for the development of bile canaliculi and for safely transporting bile and waste products from the liver. Functional studies of autologous mutated proteins in the context of the polarized hepatocyte have been challenging because of the lack of appropriate cell models. The aims of this study were to obtain a patient-specific hepatocyte model that recapitulated hepatocyte polarity and to employ this model to study endogenous mutant proteins in liver diseases that involve hepatocyte polarity.

METHODS

Urine cell-derived pluripotent stem cells, taken from a patient with a homozygous mutation in ATP7B and a patient with a heterozygous mutation, were differentiated towards hepatocyte-like cells (hiHeps). HiHeps were also derived from a patient with MEDNIK syndrome.

RESULTS

Polarized hiHeps that formed in vivo-like bile canaliculi could be generated from embryonic and patient urine cell-derived pluripotent stem cells. HiHeps recapitulated polarized protein trafficking processes, exemplified by the Cu²⁺-induced redistribution of the copper transporter protein ATP7B to the bile canalicular domain. We demonstrated that, in contrast to the current dogma, the most frequent yet enigmatic Wilson disease-causing ATP7B-H1069Q mutation per se did not preclude trafficking of ATP7B to the trans-Golgi Network. Instead, it prevented its Cu²⁺-induced polarized redistribution to the bile canalicular domain, which could not be reversed by pharmacological folding chaperones. Finally, we demonstrate that hiHeps from a patient with MEDNIK syndrome, suffering from liver copper overload of unclear etiology, showed no defect in the Cu²⁺-induced redistribution of ATP7B to the bile canaliculi.

CONCLUSIONS

Functional cell polarity can be achieved in patient pluripotent stem cell-derived hiHeps, enabling, for the first time, the study of the endogenous mutant proteins, patient-specific pathogenesis and drug responses for diseases where hepatocyte polarity is a key factor.

LAY SUMMARY

This study demonstrates that cells that are isolated from urine can be reprogrammed in a dish towards hepatocytes that display architectural characteristics similar to those seen in the intact liver. The application of this methodology to cells from patients diagnosed with inherited copper metabolism-related liver diseases (that is, Wilson disease and MEDNIK syndrome) revealed unexpected and novel insights into patient mutation-specific disease mechanisms and drug responses.

Age and Sex but Not ATP7B Genotype Effectively Influence the Clinical Phenotype of Wilson Disease

Wilson disease (WD) is an inherited disorder of hepatic copper metabolism with considerable variation in clinical presentations, the most common ones being liver disease and neuropsychiatric disturbances. This study investigated the clinical presentation in relation to mutations in a large cohort of patients with WD. A total of 1,357 patients (702 children, 655 adults; 1,172 index patients, 185 siblings, all with a Leipzig score ≥4, male/female: 679/678) were studied. The age and the symptoms at presentation were used as key phenotypic markers. Index patients were clinically classified as having either hepatic (n = 711) or neurologic disease (n = 461). Seven hundred fifteen (52.7%) patients had a liver biopsy at diagnosis. DNA was sequenced by the Genetic Analyzers ABI Prism 310 (Perkin Elmer) or 3500 (Applied Biosystems). Three hundred ninety-four different mutation combinations were detected. The most frequent mutation was H1069Q (c.3207C>A; allele frequency: 46.9%), followed by P767P-fs (c.2304dupC; 2.85%), P1134P-fs (c.3402delC; 2.8%), and R969Q (c.2755C>T; 2.18%). There was no correlation between mutations and individual clinical manifestation. There was a gender effect in index patients: Hepatic presentation was more common in females (male/female: 328/383) and neurologic presentation in males (259/202; P < 0.001). At diagnosis, 39.5% of children/adolescents (≤18 years) and 58% of adults already had cirrhosis. The presence of cirrhosis did not correlate with the genotype. Conclusion: These findings refine and extend our understanding of the natural history and individual spectrum/manifestations of WD. Initially, there is asymptomatic hepatic involvement, which may progress and become symptomatic. Neurologic symptoms present many years later.

Complex ATP7B mutation patterns in Wilson disease and evaluation of a yeast model for functional analysis of variants

Wilson disease (WD) is a rare autosomal recessive genetic disorder that is associated with various mutations in the ATP7B gene. Although ATP7B variants are frequently identified, the exact mutation patterns remain unknown because of the absence of pedigree studies, and the functional consequences of individual ATP7B variants remain to be clarified. In this study, we recruited 65 clinically diagnosed WD patients from 60 unrelated families. Pedigree analysis showed that besides several ATP7B homozygous variants (8/65, 12.3%), compound heterozygous variants (43/65, 66.2%) were present in the majority of WD patients. There were 20% of the patients had one (12/65, 18.5%) or multiple (1/65, 1.5%) variants in only a single allele, characterized by a high ratio of splicing or frameshift variants. Nine ATP7B variants were cloned into the pAG426GPD yeast expression vector to evaluate their functional consequences, and the results suggested different degrees of functional disruption from mild or uncertain to severe, consistent with the corresponding phenotypes. Our study revealed the complex ATP7B mutation patterns in WD patients and the applicability of a yeast model system to the evaluation of the functional consequences of ATP7B variants, which is essential for WD cases that are difficult to interpret.

Characterization of mutation spectrum and identification of novel mutations in ATP7B gene from a cohort of Wilson disease patients: Functional and therapeutic implications

Wilson disease (WD), a copper metabolism disorder, occurs due to the presence of mutations in the gene encoding ATP7B, a protein that primarily facilitates hepatic copper excretion. A better understanding of spectrum and functional significance of ATP7B variants is critical to formulating targeted and personalized therapies. Henceforth, we screened and sequenced 21 exons of ATP7B gene from 50 WD patients and 60 healthy subjects. We identified 28 variants comprising, seven novels in 20% alleles, while eight variations affecting 23% alleles were first time reported in Indian cohort. The c.813C>A, p.(Cys271*) (10%) was the most frequent mutation. Bioinformatics analysis revealed five of seven novel variants viz. c.1600C>A, p.(Pro534Thr); c.1616C>A, p.(Pro539His); c.1924G>T, p.(Asp642Tyr); c.2168G>C, p.(Arg723Thr); c.2174G>C, p.(Arg725Thr) resulted in protein misfolding. Sequence conservation analysis of ATP7B regions containing novel variants documented an evolutionarily conserved nature. Functional analysis of these novel variants in five different cell lines lacking inherent ATP7B expression demonstrated sensitivity to CuCl₂ -treatment, experiencing augmented cellular copper retention and decreased copper excretion as well as ceruloplasmin secretion to that of wildtype-ATP7B expressing cells. Interestingly, pharmacological chaperone 4-phenylbutyrate, a clinically approved compound, partially restored protein function of ATP7B mutants. These findings might enable novel treatment strategies in WD by clinically enhancing the protein expression of mutant ATP7B with residual copper export activity.

An αB-Crystallin Peptide Rescues Compartmentalization and Trafficking Response to Cu Overload of ATP7B-H1069Q, the Most Frequent Cause of Wilson Disease in the Caucasian Population

The H1069Q substitution is the most frequent mutation of the Cu transporter ATP7B that causes Wilson disease in the Caucasian population. ATP7B localizes to the Golgi complex in hepatocytes, but, in the presence of excessive Cu, it relocates to the endo-lysosomal compartment to excrete Cu via bile canaliculi. In contrast, ATP7B-H1069Q is strongly retained in the ER, does not reach the Golgi complex and fails to move to the endo-lysosomal compartment in the presence of excessive Cu, thus causing toxic Cu accumulation. We have previously shown that, in transfected cells, the small heat-shock protein αB-crystallin is able to correct the mislocalization of ATP7B-H1069Q and its trafficking in the presence of Cu overload. Here, we first show that the α-crystallin domain of αB-crystallin mimics the effect of the full-length protein, whereas the N- and C-terminal domains have no such effect. Next, and most importantly, we demonstrate that a twenty-residue peptide derived from the α-crystallin domain of αB-crystallin fully rescues Golgi localization and the trafficking response of ATP7B-H1069Q in the presence of Cu overload. In addition, we show that this peptide interacts with the mutant transporter in the live cell. These results open the way to attempt developing a pharmacologically active peptide to specifically contrast the Wilson disease form caused by the ATP7B-H1069Q mutant.