Cholesterol gallstone susceptibility loci: a mouse map, candidate gene evaluation, and guide to human LITH genes

Association of education with cholelithiasis and mediating effects of cardiometabolic factors: A Mendelian randomization study

BACKGROUND

Education, cognition, and intelligence are associated with cholelithiasis occurrence, yet which one has a prominent effect on cholelithiasis and which cardiometabolic risk factors mediate the causal relationship remain unelucidated.

AIM

To explore the causal associations between education, cognition, and intelligence and cholelithiasis, and the cardiometabolic risk factors that mediate the associations.

METHODS

Applying genome-wide association study summary statistics of primarily European individuals, we utilized two-sample multivariable Mendelian randomization to estimate the independent effects of education, intelligence, and cognition on cholelithiasis and cholecystitis (FinnGen study, 37041 and 11632 patients, respectively; n = 486484 participants) and performed two-step Mendelian randomization to evaluate 21 potential mediators and their mediating effects on the relationships between each exposure and cholelithiasis.

RESULTS

Inverse variance weighted Mendelian randomization results from the FinnGen consortium showed that genetically higher education, cognition, or intelligence were not independently associated with cholelithiasis and cholecystitis; when adjusted for cholelithiasis, higher education still presented an inverse effect on cholecystitis [odds ratio: 0.292 (95%CI: 0.171-0.501)], which could not be induced by cognition or intelligence. Five out of 21 cardiometabolic risk factors were perceived as mediators of the association between education and cholelithiasis, including body mass index (20.84%), body fat percentage (40.3%), waist circumference (44.4%), waist-to-hip ratio (32.9%), and time spent watching television (41.6%), while time spent watching television was also a mediator from cognition (20.4%) and intelligence to cholelithiasis (28.4%). All results were robust to sensitivity analyses.

CONCLUSION

Education, cognition, and intelligence all play crucial roles in the development of cholelithiasis, and several cardiometabolic mediators have been identified for prevention of cholelithiasis due to defects in each exposure.

Relationship between triglyceride-glucose index and gallstones risk: a population-based study

Background

Globally, gallstones represented a prevalent condition of the digestive system, heavily affected by metabolic dysfunctions such as obesity, dyslipidemia, insulin resistance, and diabetes. The triglyceride-glucose (TyG) index served as an accessible novel indicator for evaluating insulin resistance, offering a precise reflection of metabolic conditions. However, no studies have yet explored their relationship. The link between the TyG and gallstone risk was the primary purpose of this study.

Methods

Utilized data from the public database, the National Health and Nutrition Examination Survey, for the years 2017-2020. The logit model was utilized to elucidate the connection between the TyG and the gallstones risk. The restricted cubic spline (RCS) analysis served to verify any non-linear relationships existing between them. Sensitivity analyses, encompassing both stratified and interaction analyses, were conducted to identify populations of particular interest and assess potential interactions between covariates and the TyG index.

Results

A total of 4544 individuals were included. The risk of gallstones in high group was 1.6 times that of the low group. The potential cut-off value for the TyG index was 6.19. Above this threshold, there was a 40% heightened risk of gallstones with each one-unit increment in the TyG. The RCS analysis revealed the absence of a non-linear association between them. The populations warranting particular focus included those over 60 years, non-White people, individuals with a body mass index ≥25, smokers, drinkers, those with hypertension, and diabetes. Apart from smoking history, alcohol consumption, and history of diabetes, there were no interactions between other variables and the TyG index.

Conclusion

The current study represented the inaugural investigation into the link between TyG index and the risk of gallstones. A positive correlation existed between them, signifying that an increase in TyG paralleled an elevated risk of gallstones. No non-linear relationship has been found between them. Besides, a 40% increase in gallstone risk accompanied each unit rise in TyG. Considering the convenience and accessibility of TyG in clinical settings, it has a promising potential for clinical application.

The Impact of Metabolic Factors and Lipid-Lowering Drugs on Common Bile Duct Stone Recurrence after Endoscopic Sphincterotomy with Following Cholecystectomy

BACKGROUND

Recurrent common bile duct stone after endoscopic retrograde cholangiopancreatography is an undesirable problem, even when a following cholecystectomy is carried out. Important factors are the composition and properties of stones; the most significant etiology among these is the lipid level. While numerous studies have established the association between serum lipid levels and gallstones, no study has previously reported on recurrent common bile duct stones after endoscopic sphincterotomy with following cholecystectomy.

MATERIALS AND METHODS

We retrospectively collected 2016 patients underwent endoscopic sphincterotomy from 1 January 2015 to 31 December 2017 in Linkou Chang Gung Memorial Hospital. Finally, 303 patients whose serum lipid levels had been checked following a cholecystectomy after ERCP were included for analysis. We evaluated if metabolic factors including body weight, BMI, HbA1C, serum lipid profile, and lipid-lowering drugs may impact the rate of common bile duct stone recurrence. Furthermore, we tried to find if there is any factor that may impact time to recurrence.

RESULTS

A serum HDL level ≥ 40 (p = 0.000, OR = 0.207, 95% CI = 0.114-0.376) is a protective factor, and a total cholesterol level ≥ 200 (p = 0.004, OR = 4.558, 95% CI = 1.625-12.787) is a risk factor of recurrent common bile duct stones after endoscopic sphincterotomy with cholecystectomy. Lipid-lowering drugs, specifically statins, have been shown to reduce the risk of recurrence significantly (p = 0.003, OR = 0.297, 95% CI = 0.132-0.665). No factors were found to impact the time to recurrence in this study.

CONCLUSIONS

The serum lipid level could influence the recurrence of common bile duct stones after endoscopic sphincterotomy followed by cholecystectomy, and it appears that statins can reduce the risk of recurrence.

Genetic Analysis of ABCB4 Mutations and Variants Related to the Pathogenesis and Pathophysiology of Low Phospholipid-Associated Cholelithiasis

Clinical studies have revealed that the ABCB4 gene encodes the phospholipid transporter on the canalicular membrane of hepatocytes, and its mutations and variants are the genetic basis of low phospholipid-associated cholelithiasis (LPAC), a rare type of gallstone disease caused by a single-gene mutation or variation. The main features of LPAC include a reduction or deficiency of phospholipids in bile, symptomatic cholelithiasis at <40 years of age, intrahepatic sludge and microlithiasis, mild chronic cholestasis, a high cholesterol/phospholipid ratio in bile, and recurrence of biliary symptoms after cholecystectomy. Needle-like cholesterol crystals, putatively “anhydrous” cholesterol crystallization at low phospholipid concentrations in model and native bile, are characterized in ABCB4 knockout mice, a unique animal model for LPAC. Gallbladder bile with only trace amounts of phospholipids in these mice is supersaturated with cholesterol, with lipid composition plotting in the left two-phase zone of the ternary phase diagram, consistent with “anhydrous” cholesterol crystallization. In this review, we summarize the molecular biology and physiological functions of ABCB4 and comprehensively discuss the latest advances in the genetic analysis of ABCB4 mutations and variations and their roles in the pathogenesis and pathophysiology of LPAC in humans, based on the results from clinical studies and mouse experiments. To date, approximately 158 distinct LPAC-causing ABCB4 mutations and variants in humans have been reported in the literature, indicating that it is a monogenic risk factor for LPAC. The elucidation of the ABCB4 function in the liver, the identification of ABCB4 mutations and variants in LPAC patients, and the exploration of gene therapy for ABCB4 deficiency in animal models can help us to better understand the cellular, molecular, and genetic mechanisms underlying the onset of the disease, and will pave the way for early diagnosis and prevention of susceptible subjects and effective intervention for LPAC in patients.

Factors Influencing Gallstone Formation: A Review of the Literature

Gallstone disease is a common pathology of the digestive system with nearly a 10–20% incidence rate among adults. The mainstay of treatment is cholecystectomy, which is commonly associated with physical pain and may also seriously affect a patient’s quality of life. Clinical research suggests that cholelithiasis is closely related to the age, gender, body mass index, and other basic physical characteristics of patients. Clinical research further suggests that the occurrence of cholelithiasis is related to obesity, diabetes, non-alcoholic fatty liver, and other diseases. For this reason, we reviewed the following: genetic factors; excessive liver cholesterol secretion (causing cholesterol supersaturation in gallbladder bile); accelerated growth of cholesterol crystals and solid cholesterol crystals; gallbladder motility impairment; and cardiovascular factors. Herein, we summarize and analyze the causes and mechanisms of cholelithiasis, discuss its correlation with the pathogenesis of related diseases, and discuss possible mechanisms.

The Need for Standardizing Diagnosis, Treatment and Clinical Care of Cholecystitis and Biliary Colic in Gallbladder Disease

Gallstones affect 20% of the Western population and will grow in clinical significance as obesity and metabolic diseases become more prevalent. Gallbladder removal (cholecystectomy) is a common treatment for diseases caused by gallstones, with 1.2 million surgeries in the US each year, each costing USD 10,000. Gallbladder disease has a significant impact on the logistics and economics of healthcare. We discuss the two most common presentations of gallbladder disease (biliary colic and cholecystitis) and their pathophysiology, risk factors, signs and symptoms. We discuss the factors that affect clinical care, including diagnosis, treatment outcomes, surgical risk factors, quality of life and cost-efficacy. We highlight the importance of standardised guidelines and objective scoring systems in improving quality, consistency and compatibility across healthcare providers and in improving patient outcomes, collaborative opportunities and the cost-effectiveness of treatment. Guidelines and scoring only exist in select areas of the care pathway. Opportunities exist elsewhere in the care pathway.

Maternal smoking during pregnancy is risk factor for gallbladder disease in offspring during adulthood: a prospective study from UK Biobank

INTRODUCTION AND OBJECTIVES

Gallbladder disease is a common disease with high prevalence. Majority of gallbladder disease is due to gallstone. Though genetics are believed to play a role in its pathogenesis, the contribution of environmental pressures in early life to the development of this disease in adulthood has not been ever investigated. This study aimed to clarify the risk of maternal smoking exposure in association with gallbladder disease in adulthood. The interaction of maternal smoking and own smoking during adulthood on this association was studied as well.

PATIENTS AND METHODS

A total of 286,731 eligible participants from the UK Biobank population-based cohort were included. Multivariable Cox regression analysis were used to examine the HR and 95% CI with adjustment for covariates.

RESULT

During a median of 8.8 years follow-up, 7110 incident cases of gallbladder disease including 6800 (95.6%) gallstone were identified. Maternal smoking was associated with increased risk of incident total gallbladder disease (HR = 1.13; 95%CI: 1.06 - 1.21; P = 0.0002) as well as gallstones (HR = 1.13; 95%CI: 1.06 -1.21; P = 0.0003) in adulthood. Compared with those who were neither exposed to maternal smoking nor own smoking, subjects adherence to no smoking during adulthood but having maternal smoking exposure still had increased risk of total gallbladder disease (HR = 1.21; 95%CI: 1.1-1.34, P=0.0001) and gallstones (HR = 1.21; 95%CI: 1.1-1.35, P=0.0001).

CONCLUSION

The present study using large prospective cohort data from UK Biobank, for the first time, demonstrated maternal smoking exposure bringing elevated risk of incident total gallbladder disease/gallstone in adulthood.

Activation of Estrogen Receptor G Protein-Coupled Receptor 30 Enhances Cholesterol Cholelithogenesis in Female Mice

BACKGROUND AND AIMS

Estrogen is an important risk factor for cholesterol gallstone disease because women are twice as likely as men to form gallstones. The classical estrogen receptor α (ERα), but not ERβ, in the liver plays a critical role in the formation of estrogen-induced gallstones in female mice. The molecular mechanisms underlying the lithogenic effect of estrogen on gallstone formation have become more complicated with the identification of G protein-coupled receptor 30 (GPR30), an estrogen receptor.

APPROACH AND RESULTS

We investigated the biliary and gallstone phenotypes in ovariectomized female GPR30^-/- , ERα^-/- , and wild-type mice injected intramuscularly with the potent GPR30-selective agonist G-1 at 0 or 1 μg/day and fed a lithogenic diet for 8 weeks. The activation of GPR30 by G-1 enhanced cholelithogenesis by suppressing expression of cholesterol 7α-hydroxylase, the rate-limiting enzyme for the classical pathway of bile salt synthesis. These metabolic abnormalities led to an increase in biliary cholesterol concentrations in company with hepatic hyposecretion of biliary bile salts, thereby inducing cholesterol-supersaturated gallbladder bile and accelerating cholesterol crystallization. G-1 also impairs gallbladder emptying, leading to sluggish gallbladder motility and promoting the development of biliary sludge in the early stage of gallstone formation. The prevalence rates of gallstones were 80% in wild-type and ERα^-/- mice treated with G-1 compared to 10% in wild-type mice receiving no G-1. However, no gallstones were formed in GPR30^-/- mice treated with G-1.

CONCLUSIONS

GPR30 produces additional lithogenic actions, working independently of ERα, to increase susceptible to gallstone formation in female mice; both GPR30 and ERα are potential therapeutic targets for cholesterol gallstone disease, particularly in women and patients exposed to high levels of estrogen.

An Update on the Lithogenic Mechanisms of Cholecystokinin a Receptor (CCKAR), an Important Gallstone Gene for Lith13

The cholecystokinin A receptor (CCKAR) is expressed predominantly in the gallbladder and small intestine in the digestive system, where it is responsible for CCK's regulation of gallbladder and small intestinal motility. The effect of CCKAR on small intestinal transit is a physiological response for regulating intestinal cholesterol absorption. The Cckar gene has been identified to be an important gallstone gene, Lith13, in inbred mice by a powerful quantitative trait locus analysis. Knockout of the Cckar gene in mice enhances cholesterol cholelithogenesis by impairing gallbladder contraction and emptying, promoting cholesterol crystallization and crystal growth, and increasing intestinal cholesterol absorption. Clinical and epidemiological studies have demonstrated that several variants in the CCKAR gene are associated with increased prevalence of cholesterol cholelithiasis in humans. Dysfunctional gallbladder emptying in response to exogenously administered CCK-8 is often found in patients with cholesterol gallstones, and patients with pigment gallstones display an intermediate degree of gallbladder motility defect. Gallbladder hypomotility is also revealed in some subjects without gallstones under several conditions: pregnancy, total parenteral nutrition, celiac disease, oral contraceptives and conjugated estrogens, obesity, diabetes, the metabolic syndrome, and administration of CCKAR antagonists. The physical-chemical, genetic, and molecular studies of Lith13 show that dysfunctional CCKAR enhances susceptibility to cholesterol gallstones through two primary mechanisms: impaired gallbladder emptying is a key risk factor for the development of gallbladder hypomotility, biliary sludge (the precursor of gallstones), and microlithiasis, as well as delayed small intestinal transit augments cholesterol absorption as a major source for the hepatic hypersecretion of biliary cholesterol and for the accumulation of excess cholesterol in the gallbladder wall that further worsens impaired gallbladder motor function. If these two defects in the gallbladder and small intestine could be prevented by the potent CCKAR agonists, the risk of developing cholesterol gallstones could be dramatically reduced.

G Protein-Coupled Estrogen Receptor, GPER1, Offers a Novel Target for the Treatment of Digestive Diseases

There are gender differences between men and women in many physiological functions and diseases, which indicates that female sex hormones may be important. Traditionally, estrogen exerts its biological activities by activating two classical nuclear estrogen receptors, ESR1 and ESR2. However, the roles of estrogen in the regulation of physiological functions and the pathogenesis of diseases become more complicated with the identification of the G protein-coupled estrogen receptor (GPER1). Although many GPER1-specific ligands have been developed, the therapeutic mechanisms of exclusively targeting GPER1 are not yet well understood. Translational applications and clinical trial efforts for the identified GPER1 ligands have been focused primarily on the reproductive, cardiovascular, nervous, endocrine, and immune systems. More recently, research found that GPER1 may play an important role in regulating the digestive system. Cholesterol gallstone disease, a major biliary disease, has a higher prevalence in women than in men worldwide. Emerging evidence implies that GPER1 could play an important role, independent of the classical ESR1, in the pathophysiology of cholesterol gallstones in women. This review discusses the complex signaling pathways of three estrogen receptors, highlights the development of GPER1-specific ligands, and summarizes the latest advances in the role of GPER1 in the pathogenesis of gallstone formation.

Cholesterol cholelithiasis: part of a systemic metabolic disease, prone to primary prevention

Cholesterol gallstone disease have relationships with various conditions linked with insulin resistance, but also with heart disease, atherosclerosis, and cancer. These associations derive from mechanisms active at a local (i.e. gallbladder, bile) and a systemic level and are involved in inflammation, hormones, nuclear receptors, signaling molecules, epigenetic modulation of gene expression, and gut microbiota. Despite advanced knowledge of these pathways, the available therapeutic options for symptomatic gallstone patients remain limited. Therapy includes oral litholysis by the bile acid ursodeoxycholic acid (UDCA) in a small subgroup of patients at high risk of postdissolution recurrence, or laparoscopic cholecystectomy, which is the therapeutic radical gold standard treatment. Cholecystectomy, however, may not be a neutral event, and potentially generates health problems, including the metabolic syndrome. Areas covered: Several studies on risk factors and pathogenesis of cholesterol gallstone disease, acting at a systemic level have been reviewed through a PubMed search. Authors have focused on primary prevention and novel potential therapeutic strategies. Expert commentary: The ultimate goal appears to target the manageable systemic mechanisms responsible for gallstone occurrence, pointing to primary prevention measures. Changes must target lifestyles, as well as experimenting innovative pharmacological tools in subgroups of patients at high risk of developing gallstones.

Mouse models of gallstone disease

PURPOSE OF REVIEW

The establishment of mouse models of gallstones, and the contribution of mouse models to genetic studies of gallstone disease, as well as the latest advances in the pathophysiology of gallstones from mouse experiments are summarized.

RECENT FINDINGS

The combined uses of genomic strategies and phenotypic studies in mice have successfully led to the identification of many Lith genes, which pave the way for the discovery of human LITH genes. The physical-chemical, genetic, and molecular biological studies of gallstone disease in mice with knockout or transgene of specific target genes have provided many novel insights into the complex pathophysiological mechanisms of this very common hepatobiliary disease worldwide, showing that interactions of five primary defects play a critical role in the pathogenesis of cholesterol gallstones. Based on mouse studies, a new concept has been proposed that hepatic hypersecretion of biliary cholesterol is induced by multiple Lith genes, with insulin resistance as part of the metabolic syndrome interacting with cholelithogenic environmental factors to cause the phenotype.

SUMMARY

The mouse model of gallstones is crucial for elucidating the physical-chemical and genetic mechanisms of cholesterol crystallization and gallstone formation, which greatly increase our understanding of the pathogenesis of this disease in humans.

An update on the pathogenesis of cholesterol gallstone disease

PURPOSE OF REVIEW

Gallstone disease is a major epidemiologic and economic burden worldwide, and the most frequent form is cholesterol gallstone disease.

RECENT FINDINGS

Major pathogenetic factors for cholesterol gallstones include a genetic background, hepatic hypersecretion of cholesterol, and supersaturated bile which give life to precipitating cholesterol crystals that accumulate and grow in a sluggish gallbladder. Additional factors include mucin and inflammatory changes in the gallbladder, slow intestinal motility, increased intestinal absorption of cholesterol, and altered gut microbiota. Mechanisms of disease are linked with insulin resistance, obesity, the metabolic syndrome, and type 2 diabetes. The role of nuclear receptors, signaling pathways, gut microbiota, and epigenome are being actively investigated.

SUMMARY

Ongoing research on cholesterol gallstone disease is intensively investigating several pathogenic mechanisms, associated metabolic disorders, new therapeutic approaches, and novel strategies for primary prevention, including lifestyles.

Cd36 knockout mice are protected against lithogenic diet-induced gallstones

The scavenger receptor and multiligand transporter CD36 functions to promote cellular free fatty acid uptake and regulates aspects of both hepatic and intestinal cholesterol metabolism. However, the role of CD36 in regulating canalicular and biliary cholesterol transport and secretion is unknown. Here, we show that germline Cd36 knockout (KO) mice are protected against lithogenic diet (LD)-induced gallstones compared with congenic (C57BL6/J) controls. Cd36 KO mice crossed into congenic L-Fabp KO mice (DKO mice) demonstrated protection against LD-induced gallstones, reversing the susceptibility phenotype observed in L-Fabp KO mice. DKO mice demonstrated reduced biliary cholesterol secretion and a shift into more hydrophophilic bile acid species, without changes in either BA pool size or fecal excretion. In addition, we found that the mean and maximum force of gallbladder contraction was increased in germline Cd36 KO mice, and gallbladder lipid content was reduced compared with wild-type controls. Finally, whereas germline Cd36 KO mice were protected against LD-induced gallstones, neither liver- nor intestine-specific Cd36 KO mice were protected. Taken together, our findings show that CD36 plays an important role in modifying gallstone susceptibility in mice, at least in part by altering biliary lipid composition, but also by promoting gallbladder contractility.

New insights into the role of Lith genes in the formation of cholesterol-supersaturated bile

Cholesterol gallstone formation represents a failure of biliary cholesterol homeostasis in which the physical-chemical balance of cholesterol solubility in bile is disturbed. Lithogenic bile is mainly caused by persistent hepatic hypersecretion of biliary cholesterol and sustained cholesterol-supersaturated bile is an essential prerequisite for the precipitation of solid cholesterol monohydrate crystals and the formation of cholesterol gallstones. The metabolic determinants of the supply of hepatic cholesterol molecules that are recruited for biliary secretion are dependent upon the input-output balance of cholesterol and its catabolism in the liver. The sources of cholesterol for hepatic secretion into bile have been extensively investigated; however, to what extent each cholesterol source contributes to hepatic secretion is still unclear both under normal physiological conditions and in the lithogenic state. Although it has been long known that biliary lithogenicity is initiated by hepatic cholesterol hypersecretion, the genetic mechanisms that cause supersaturated bile have not been defined yet. Identification of the Lith genes that determine hepatic cholesterol hypersecretion should provide novel insights into the primary genetic and pathophysiological defects for gallstone formation. In this review article, we focus mainly on the pathogenesis of the formation of supersaturated bile and gallstones from the viewpoint of genetics and pathophysiology. A better understanding of the molecular genetics and pathophysiology of the formation of cholesterol-supersaturated bile will undoubtedly facilitate the development of novel, effective, and noninvasive therapies for patients with gallstones, which would reduce the morbidity, mortality, and costs of health care associated with gallstones, a very prevalent liver disease worldwide.

PKCβ: Expanding role in hepatic adaptation of cholesterol homeostasis to dietary fat/cholesterol

Cholesterol homeostasis relies on an intricate network of cellular processes whose deregulation in response to Western type high-fat/cholesterol diets can lead to several life-threatening pathologies. Significant advances have been made in resolving the molecular identity and regulatory function of transcription factors sensitive to fat, cholesterol, or bile acids, but whether body senses the presence of both fat and cholesterol simultaneously is not known. Assessing the impact of a high-fat/cholesterol load, rather than an individual component alone, on cholesterol homeostasis is more physiologically relevant because Western diets deliver both fat and cholesterol at the same time. Moreover, dietary fat and dietary cholesterol are reported to act synergistically to impair liver cholesterol homeostasis. A key insight into the role of protein kinase C-β (PKCβ) in hepatic adaptation to high-fat/cholesterol diets was gained recently through the use of knockout mice. The emerging evidence indicates that PKCβ is an important regulator of cholesterol homeostasis that ensures normal adaptation to high-fat/cholesterol intake. Consistent with this function, high-fat/cholesterol diets induce PKCβ expression and signaling in the intestine and liver, while systemic PKCβ deficiency promotes accumulation of cholesterol in the liver and bile. PKCβ disruption results in profound dysregulation of hepatic cholesterol and bile homeostasis and imparts sensitivity to cholesterol gallstone formation. The available results support involvement of a two-pronged mechanism by which intestine and liver PKCβ signaling converge on liver ERK1/2 to dictate diet-induced cholesterol and bile acid homeostasis. Collectively, PKCβ is an integrator of dietary fat/cholesterol signal and mediates changes to cholesterol homeostasis.

Gallstone Etiopathogenesis, Lith and Mucin Genes and New Treatment Approaches

Gallstones constitute one of the more common and relatively costly conditions of the gastrointestinal system and are a major risk factor for gallbladder cancer. Most gallstone cases involve individuals younger than 60 years of age, those older representing 9% of the total in one series. There are many risk factors for gallstones and Lith and Mucin genes, for example, play important roles in their formation. Surgery is one therapeutic approach but in the future it is to be expected that drugs for prevention of gallstones will be developed in the future. This will have clear implications for gallbladder cancer control.

Four Susceptibility Loci for Gallstone Disease Identified in a Meta-analysis of Genome-Wide Association Studies

BACKGROUND & AIMS

A genome-wide association study (GWAS) of 280 cases identified the hepatic cholesterol transporter ABCG8 as a locus associated with risk for gallstone disease, but findings have not been reported from any other GWAS of this phenotype. We performed a large-scale, meta-analysis of GWASs of individuals of European ancestry with available prior genotype data, to identify additional genetic risk factors for gallstone disease.

METHODS

We obtained per-allele odds ratio (OR) and standard error estimates using age- and sex-adjusted logistic regression models within each of the 10 discovery studies (8720 cases and 55,152 controls). We performed an inverse variance weighted, fixed-effects meta-analysis of study-specific estimates to identify single-nucleotide polymorphisms that were associated independently with gallstone disease. Associations were replicated in 6489 cases and 62,797 controls.

RESULTS

We observed independent associations for 2 single-nucleotide polymorphisms at the ABCG8 locus: rs11887534 (OR, 1.69; 95% confidence interval [CI], 1.54-1.86; P = 2.44 × 10(-60)) and rs4245791 (OR, 1.27; P = 1.90 × 10(-34)). We also identified and/or replicated associations for rs9843304 in TM4SF4 (OR, 1.12; 95% CI, 1.08-1.16; P = 6.09 × 10(-11)), rs2547231 in SULT2A1 (encodes a sulfoconjugation enzyme that acts on hydroxysteroids and cholesterol-derived sterol bile acids) (OR, 1.17; 95% CI, 1.12-1.21; P = 2.24 × 10(-10)), rs1260326 in glucokinase regulatory protein (OR, 1.12; 95% CI, 1.07-1.17; P = 2.55 × 10(-10)), and rs6471717 near CYP7A1 (encodes an enzyme that catalyzes conversion of cholesterol to primary bile acids) (OR, 1.11; 95% CI, 1.08-1.15; P = 8.84 × 10(-9)). Among individuals of African American and Hispanic American ancestry, rs11887534 and rs4245791 were associated positively with gallstone disease risk, whereas the association for the rs1260326 variant was inverse.

CONCLUSIONS

In this large-scale GWAS of gallstone disease, we identified 4 loci in genes that have putative functions in cholesterol metabolism and transport, and sulfonylation of bile acids or hydroxysteroids.

Gallstones

Gallstones grow inside the gallbladder or biliary tract. These stones can be asymptomatic or symptomatic; only gallstones with symptoms or complications are defined as gallstone disease. Based on their composition, gallstones are classified into cholesterol gallstones, which represent the predominant entity, and bilirubin ('pigment') stones. Black pigment stones can be caused by chronic haemolysis; brown pigment stones typically develop in obstructed and infected bile ducts. For treatment, localization of the gallstones in the biliary tract is more relevant than composition. Overall, up to 20% of adults develop gallstones and >20% of those develop symptoms or complications. Risk factors for gallstones are female sex, age, pregnancy, physical inactivity, obesity and overnutrition. Factors involved in metabolic syndrome increase the risk of developing gallstones and form the basis of primary prevention by lifestyle changes. Common mutations in the hepatic cholesterol transporter ABCG8 confer most of the genetic risk of developing gallstones, which accounts for ∼25% of the total risk. Diagnosis is mainly based on clinical symptoms, abdominal ultrasonography and liver biochemistry tests. Symptoms often precede the onset of the three common and potentially life-threatening complications of gallstones (acute cholecystitis, acute cholangitis and biliary pancreatitis). Although our knowledge on the genetics and pathophysiology of gallstones has expanded recently, current treatment algorithms remain predominantly invasive and are based on surgery. Hence, our future efforts should focus on novel preventive strategies to overcome the onset of gallstones in at-risk patients in particular, but also in the population in general.

The deletion of the estrogen receptor α gene reduces susceptibility to estrogen-induced cholesterol cholelithiasis in female mice

Compelling evidence has demonstrated that estrogen is a critical risk factor for gallstone formation and enhances cholesterol cholelithogenesis through the hepatic estrogen receptor α (ERα), but not ERβ. To study the lithogenic mechanisms of estrogen through ERα, we investigated whether the deletion of Erα protects against gallstone formation in ovariectomized (OVX) female mice fed a lithogenic diet and treated with 17β-estradiol (E2) at 0 or 6μg/day for 56days. Our results showed that the prevalence of gallstones was reduced from 100% in OVX ERα (+/+) mice to 30% in OVX ERα (-/-) mice in response to high doses of E2 and the lithogenic diet for 56days. Hepatic cholesterol secretion was significantly diminished in OVX ERα (-/-) mice compared to OVX ERα (+/+) mice even fed the lithogenic diet and treated with E2 for 56days. These alterations decreased bile lithogenicity by reducing cholesterol saturation index of gallbladder bile. Immunohistochemical studies revealed that ERα was expressed mainly in the gallbladder smooth muscle cells. High levels of E2 impaired gallbladder emptying function mostly through the ERα and cholecystokinin-1 receptor pathway, leading to gallbladder stasis in OVX ERα (+/+) mice. By contrast, gallbladder emptying function was greatly improved in OVX ERα (-/-) mice. This markedly retarded cholesterol crystallization and the growth and agglomeration of solid cholesterol crystals into microlithiasis and stones. In conclusion, the deletion of Erα reduces susceptibility to the formation of E2-induced gallstones by diminishing hepatic cholesterol secretion, desaturating gallbladder bile, and improving gallbladder contraction function in female mice.

Estrogen induces two distinct cholesterol crystallization pathways by activating ERα and GPR30 in female mice

To distinguish the lithogenic effect of the classical estrogen receptor α (ERα) from that of the G protein-coupled receptor 30 (GPR30), a new estrogen receptor, on estrogen-induced gallstones, we investigated the entire spectrum of cholesterol crystallization pathways and sequences during the early stage of gallstone formation in gallbladder bile of ovariectomized female wild-type, GPR30((-/-)), ERα((-/-)), and GPR30((-/-))/ERα((-/-)) mice treated with 17β-estradiol (E2) at 6 µg/day and fed a lithogenic diet for 12 days. E2 disrupted biliary cholesterol and bile salt metabolism through ERα and GPR30, leading to supersaturated bile and predisposing to the precipitation of cholesterol monohydrate crystals. In GPR30((-/-)) mice, arc-like and tubular crystals formed first, followed by classical parallelogram-shaped cholesterol monohydrate crystals. In ERα((-/-)) mice, precipitation of lamellar liquid crystals, typified by birefringent multilamellar vesicles, appeared earlier than cholesterol monohydrate crystals. Both crystallization pathways were accelerated in wild-type mice with the activation of GPR30 and ERα by E2. However, cholesterol crystallization was drastically retarded in GPR30((-/-))/ERα((-/-)) mice. We concluded that E2 activates GPR30 and ERα to produce liquid crystalline versus anhydrous crystalline metastable intermediates evolving to cholesterol monohydrate crystals from supersaturated bile. GPR30 produces a synergistic lithogenic action with ERα to enhance E2-induced gallstone formation.

Ezetimibe prevents the formation of oestrogen-induced cholesterol gallstones in mice

BACKGROUND

Oestrogen is an important risk factor for cholesterol cholelithiasis not only in women of childbearing age taking oral contraceptives and postmenopausal women undergoing hormone replacement therapy, but also in male patients receiving oestrogen therapy for prostatic cancer. In women, hormonal changes occurring during pregnancy markedly increase the risk of developing gallstones. We investigated whether the potent cholesterol absorption inhibitor ezetimibe could prevent the formation of oestrogen-induced cholesterol gallstones in mice.

DESIGN

Following ovariectomy, female AKR mice were implanted subcutaneously with pellets releasing 17β-estradiol at 6 μg/day and fed a lithogenic diet supplemented with ezetimibe in doses of 0 or 8 mg/kg/day for 8 weeks. Cholesterol crystallization and gallstone prevalence, lipid concentrations and composition in bile, and biliary lipid output were analysed by physical-chemical methods. Intestinal cholesterol absorption efficiency was determined by faecal dual-isotope ratio methods.

RESULTS

Ezetimibe inhibited intestinal cholesterol absorption, while significantly reducing hepatic secretion of biliary cholesterol. Consequently, bile was desaturated through the formation of numerous unsaturated micelles and gallstones were prevented by ezetimibe in mice exposed to high doses of oestrogen and fed the lithogenic diet. Ezetimibe did not influence mRNA levels of the classical oestrogen receptors α (ERα) and ERβ, as well as a novel oestrogen receptor the G protein-coupled receptor 30 (GPR30) in the liver.

CONCLUSIONS

Ezetimibe protects against the oestrogen-mediated lithogenic actions on gallstone formation in mice. Our finding may provide an efficacious novel strategy for the prevention of cholesterol gallstones in high-risk subjects, especially those exposed to high levels of oestrogen.

Hepatic Mttp deletion reverses gallstone susceptibility in L-Fabp knockout mice

Previous studies demonstrated that L-Fabp KO mice are more susceptible to lithogenic diet (LD)-induced gallstones because of altered hepatic cholesterol metabolism and increased canalicular cholesterol secretion. Other studies demonstrated that liver-specific deletion of microsomal triglyceride transfer protein (Mttp-LKO) reduced LD-induced gallstone formation by increasing biliary phospholipid secretion. Here we show that mice with combined deletion (i.e., DKO mice) are protected from LD-induced gallstone formation. Following 2 weeks of LD feeding, 73% of WT and 100% of L-Fabp KO mice developed gallstones versus 18% of Mttp-LKO and 23% of DKO mice. This phenotype was recapitulated in both WT and L-Fabp KO mice treated with an Mttp antisense oligonucleotide (M-ASO). Biliary cholesterol secretion was increased in LD-fed L-Fabp KO mice and decreased in DKO mice. However, phospholipid secretion was unchanged in LD-fed Mttp-LKO and DKO mice as well as in M-ASO-treated mice. Expression of the canalicular export pump ABCG5/G8 was reduced in LD-fed DKO mice and in M-ASO-treated L-Fabp KO mice. We conclude that liver-specific Mttp deletion not only eliminates apical lipoprotein secretion from hepatocytes but also attenuates canalicular cholesterol secretion, which in turn decreases LD-induced gallstone susceptibility.

Genetics of gallstone disease

Gallstone disease (GSD) is one of the most common biliary tract disorders worldwide. The prevalence, however, varies from 5.9-21.9% in Western society to 3.1-10.7% in Asia. Most gallstones (75%) are silent. Approximately half of symptomatic gallstone carriers experience a second episode of biliary pain within 1 year. These individuals are at increased risk of developing acute cholecystitis, acute cholangitis, and biliary pancreatitis. As can be expected, these complications burden health care systems because of their invasive nature and surgical cost. Factors that contribute to gallstone formation include supersaturation of cholesterol in bile, gallbladder hypomotility, destabilization of bile by kinetic protein factors, and abnormal mucins. Epidemiologic studies have implicated multiple environmental factors and some common genetic elements in gallstone formation. Genetic factors that influence gallstone formation have been elaborated from linkage studies of twins, families, and ethnicities. Accumulating evidence suggests that genetic factors play a role in GSD.

Transport and biological activities of bile acids

Bile acids have emerged as important biological molecules that support the solubilization of various lipids and lipid-soluble compounds in the gut, and the regulation of gene expression and cellular function. Bile acids are synthesized from cholesterol in the liver and eventually released into the small intestine. The majority of bile acids are recovered in the distal end of the small intestine and then returned to the liver for reuse. The components of the mechanism responsible for the recycling of bile acids within the enterohepatic circulation have been identified whereas the mechanism for intracellular transport is less understood. Recently, the ileal lipid binding protein (ILBP; human gene symbol FABP6) was shown to be needed for the efficient transport of bile acids from the apical side to the basolateral side of enterocytes in the distal intestine. This review presents an overview of the transport of bile acids between the liver and the gut as well as within hepatocytes and enterocytes. A variety of pathologies is associated with the malfunction of the bile acid transport system.

Gender-specific association of the interleukin 18 gene with symptomatic gallstone disease

BACKGROUND AND AIM

Symptomatic gallstone disease (SGSD) induced several inflammatory responses and affected extrahepatic bile ducts. Although the pathology and environmental risk factors of gallstone disease are well documented, immune or inflammatory responses in SGSD development are still inconclusive. Interleukin 18 (IL18) is a pro-inflammatory cytokine that plays an important role in immune, infectious, and inflammatory diseases because of the induction of interferon-γ. In this study, we investigated whether polymorphisms of the IL18 gene were associated with SGSD susceptibility.

METHODS

Genomic DNA was isolated from the whole blood samples of 445 patients with SGSD and 1121 gallstone-free controls. The IL18 rs549908T>G, rs5744247C>G, rs187238G>C, rs1946518T>G, and rs360719A>G polymorphisms were genotyped using predeveloped TaqMan allelic discrimination assay.

RESULTS

We found IL18 rs5744247G allele conferred protection against SGSD in female patients (odds ratio = 0.75, corrected P-value = 0.015). Haplotype analysis revealed that TGGTA protected females from SGSD development (odds ratio = 0.75, corrected P-value = 0.02).

CONCLUSIONS

Based on our findings, IL18 rs5744247C>G polymorphism could be a potential genetic marker to predict SGSD susceptibility in Han Chinese women.

Augmented cholesterol absorption and sarcolemmal sterol enrichment slow small intestinal transit in mice, contributing to cholesterol cholelithogenesis

Cholesterol gallstones are associated with slow intestinal transit in humans as well as in animal models, but the molecular mechanism is unknown. We investigated in C57L/J mice whether the components of a lithogenic diet (LD; 1.0% cholesterol, 0.5% cholic acid and 17% triglycerides), as well as distal intestinal infection with Helicobacter hepaticus, influence small intestinal transit time. By quantifying the distribution of 3H-sitostanol along the length of the small intestine following intraduodenal instillation,we observed that, in both sexes, the geometric centre (dimensionless) was retarded significantly (P <0.05) by LD but not slowed further by helicobacter infection (males, 9.4±0.5 (uninfected), 9.6±0.5 (infected) on LD compared with 12.5±0.4 and 11.4±0.5 on chow). The effect of the LD was reproduced only by the binary combination of cholesterol and cholic acid. We inferred that the LD-induced cholesterol enrichment of the sarcolemmae of intestinal smooth muscle cells produced hypomotility from signal-transduction decoupling of cholecystokinin (CCK), a physiological agonist for small intestinal propulsion in mice. Treatment with ezetimibe in an amount sufficient to block intestinal cholesterol absorption caused small intestinal transit time to return to normal. In most cholesterol gallstone-prone humans, lithogenic bile carries large quantities of hepatic cholesterol into the upper small intestine continuously, thereby reproducing this dietary effect in mice. Intestinal hypomotility promotes cholelithogenesis by augmenting formation of deoxycholate, a pro-lithogenic secondary bile salt, and increasing the fraction of intestinal cholesterol absorbed.

Transporters in cholelithiasis

Gallstones are a common and costly disease with a projected increase in prevalence due to the increasing ageing population. Numerous endogenous and environmental factors are aetiologically related to this multifactorial disease, and genetic studies continue to unravel the pathobiological mechanisms related to gallstone formation. In particular, variants of genes encoding hepatobiliary transporters have been implicated in gallstone disease and, given their ability to influence biliary lipid composition, have undergone considerable investigation. Here we summarize the role of enterohepatic transporters in cholelithogenesis with a particular focus on pertinent ATP-binding cassette transporters (ABCB4, ABCB11, ABCC7, and ABCG5/G8).

Risk factors in patients with hereditary gallstones in Chinese pedigrees

OBJECTIVE

We aimed to define the risk factors and to evaluate the impact of family background on the prevalence of gallstones in China.

SUBJECTS AND METHODS

Thirty-eight gallstone pedigrees were collected and a case-control study was conducted. This study consisted of 272 first-degree relatives and 201 non-first-degree relatives of index patients. The participants completed a questionnaire and underwent physical and ultrasonographic examinations. The risk factors examined included age, sex, body mass index (BMI), smoking status, alcohol consumption, pregnancy, fat content in dietary meat, history of gastrointestinal surgery, hypertension, hyperlipidemia, fatty liver, coronary heart disease and diabetes.

RESULTS

The prevalence of gallstones in first-degree and non-first-degree relatives of index patients was 38.2 and 10.9%, respectively. Age, pregnancy and BMI significantly differed between cases and controls (p < 0.05). The relative risks were: consumption of meat with a high fat content 1.4 (95% CI 1.1-1.8); hyperlipidemia 2.4 (95% CI 1.3-4.6); diabetes 1.9 (95% CI 1.1-3.2); fatty liver 4.9 (95% CI 1.0-24); coronary heart disease 2.5 (95% CI 1.7-3.9).

CONCLUSION

Data showed that age, overweight, more consumption of high-fat food, high frequency of pregnancy, fatty liver, hyperlipidemia, coronary heart disease and diabetes could increase the risk of gallstones in the first-degree relatives of index patients.

Effects of SLC10A2 variant rs9514089 on gallstone risk and serum cholesterol levels- meta-analysis of three independent cohorts

Background

Recently, a single nucleotide polymorphism (SNP) rs9514089 in SLC10A2 (apical sodium-dependent bile acid transporter gene) has been identified as a susceptibility variant for cholelithiasis in humans.

Methods

Here we assessed the effects of rs9514089 on gallstone risk and related phenotypes of the metabolic syndrome in the self-contained population of Sorbs (183 cases with gallstones/826 controls). Furthermore, we performed a meta-analysis for effects of rs9514089 on susceptibility for cholelithiasis in three independent cohorts (Stuttgart: 56 cases/71 controls, Aachen: 184 cases/184 controls and Sorbs).

Results

There was no significant association of rs9514089 with gallstone risk, serum lipid parameters and BMI in the Sorbs and in the meta-analysis of all three cohorts (p > 0.05). There was an effect trend in the subgroup of lean subjects but based on different effect directions in the three cohorts there was no significant association in the meta-analysis.

Conclusions

We were not able to replicate the effect of rs9514089 on gallstone risk in the Sorbs. Further analyses in larger cohorts are required to finally assess the role of genetic variants in SLC10A2 in human gallstone development and lipid metabolism.

Hyperhomocysteinemia from trimethylation of hepatic phosphatidylethanolamine during cholesterol cholelithogenesis in inbred mice

Because hyperhomocysteinemia can occur in cholesterol gallstone disease, we hypothesized that this may result from trimethylation of phosphatidylethanolamine (PE), which partakes in biliary phosphatidylcholine (PC) hypersecretion during cholesterol cholelithogenesis. We fed murine strains C57L/J, C57BL/6J, SWR/J, AKR/J, PE N-methyltransferase (PEMT) knockout (KO), PEMT heterozygous (HET), and wildtype (WT) mice a cholesterol/cholic acid lithogenic diet (LD) for up to 56 days and documented biliary lipid phase transitions and secretion rates. We quantified plasma total homocysteine (tHcy), folate, and vitamin B12 in plasma and liver, as well as biliary tHcy and cysteine secretion rates. Rate-limiting enzyme activities of PC synthesis, PEMT and cytidine triphosphate: phosphocholine cytidylyltransferase (PCT), S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) were measured in liver homogenates. Other potential sources of plasma tHcy, glycine N-methyltransferase (GNMT) and guanidinoacetate N-methyltransferase (GAMT), were assayed by gene expression. Plasma tHcy and PEMT activities became elevated during cholelithogenesis in gallstone-susceptible C57L, C57BL/6, and SWR mice but not in the gallstone-resistant AKR mice. Persisting in C57L mice, which exhibit the greatest Lith gene burden, these increases were accompanied by elevated hepatic SAM/SAH ratios and augmented biliary tHcy secretion rates. Counter-regulation included remethylation of Hcy to methionine concurrent with decreased folate and vitamin B12 levels and Hcy transsulfuration to cysteine. Concomitantly, methylenetetrahydrofolate reductase (Mthfr), betaine-homocysteine methyltransferase (Bhmt), and cystathionine-β-synthase (Cbs) were up-regulated, but Gnmt and Gamt genes were down-regulated. PEMT KO and HET mice displayed biliary lipid secretion rates and high gallstone prevalence rates similar to WT mice without any elevation in plasma tHcy levels.

CONCLUSION

This work implicates up-regulation of PC synthesis by the PEMT pathway as a source of elevated plasma and bile tHcy during cholesterol cholelithogenesis.

Disruption of the murine protein kinase Cbeta gene promotes gallstone formation and alters biliary lipid and hepatic cholesterol metabolism

The protein kinase C (PKC) family of Ca(2+) and/or lipid-activated serine-threonine protein kinases is implicated in the pathogenesis of obesity and insulin resistance. We recently reported that protein kinase Cβ (PKCβ), a calcium-, diacylglycerol-, and phospholipid-dependent kinase, is critical for maintaining whole body triglyceride homeostasis. We now report that PKCβ deficiency has profound effects on murine hepatic cholesterol metabolism, including hypersensitivity to diet-induced gallstone formation. The incidence of gallstones increased from 9% in control mice to 95% in PKCβ(-/-) mice. Gallstone formation in the mutant mice was accompanied by hyposecretion of bile acids with no alteration in fecal bile acid excretion, increased biliary cholesterol saturation and hydrophobicity indices, as well as hepatic p42/44(MAPK) activation, all of which enhance susceptibility to gallstone formation. Lithogenic diet-fed PKCβ(-/-) mice also displayed decreased expression of hepatic cholesterol-7α-hydroxylase (CYP7A1) and sterol 12α-hydroxylase (CYP8b1). Finally, feeding a modified lithogenic diet supplemented with milk fat, instead of cocoa butter, both increased the severity of and shortened the interval for gallstone formation in PKCβ(-/-) mice and was associated with dramatic increases in cholesterol saturation and hydrophobicity indices. Taken together, the findings reveal a hitherto unrecognized role of PKCβ in fine tuning diet-induced cholesterol and bile acid homeostasis, thus identifying PKCβ as a major physiological regulator of both triglyceride and cholesterol homeostasis.

PXR prevents cholesterol gallstone disease by regulating biosynthesis and transport of bile salts

BACKGROUND & AIMS

Cholesterol gallstone disease (CGD) results from a biochemical imbalance of lipids and bile salts in the gallbladder bile. We investigated whether the xenobiotic receptor pregnane X receptor (PXR) has a role in pathogenesis of CGD.

METHODS

Wild-type, PXR-null (PXR-/-), and CGD-sensitive C57L mice were placed on a lithogenic diet and then analyzed for CGD at the biochemical, histological, and gene-regulation levels.

RESULTS

Loss of PXR sensitized mice to lithogenic diet-induced CGD, characterized by decreases in biliary concentrations of bile salts and phospholipids and an increases in the cholesterol saturation index and formation of cholesterol crystals. The decreased bile acid pool size in PXR-/- mice that received lithogenic diets was associated with reduced expression of cholesterol 7α-hydroxylase, the rate-limiting enzyme of cholesterol catabolism and bile acid formation. The reduced expression of cholesterol 7α-hydroxylase most likely resulted from activation of farnesoid X receptor and induction of fibroblast growth factor 15 in the intestine. In C57L mice given the PXR agonist, pregnenolone-16α-carbonitrile, or the herbal medicine, St John's wort, cholesterol precipitation was prevented by increases in concentrations of biliary bile salt and a reduced cholesterol saturation index. PXR prevented CGD via its coordinate regulation of the biosynthesis and transport of bile salts in the liver and intestine.

CONCLUSIONS

PXR maintains biliary bile acid homeostasis and may be developed as a therapeutic target for CGD.

Genome-wide association studies and genetic risk assessment of liver diseases

Genetic tests can help clinicians to diagnose rare monogenic liver diseases. For most common liver diseases, however, multiple gene variants that have small to moderate individual phenotypic effects contribute to the overall risk of disease. An individual's level of risk depends on interactions between environmental factors and a wide range of modifier genes, which are yet to be identified systematically. The latest genome-wide association studies in large cohorts of patients with gallstones, fatty liver disease, viral hepatitis, chronic cholestatic liver diseases or drug-induced liver injury have provided new insights into the pathophysiology of these illnesses and have suggested the contribution of previously unsuspected pathogenic pathways. Studies in mouse models have identified further susceptibility genes for several complex liver diseases. As a result, in the future polygenic risk scores might help to define subgroups of patients at risk of developing liver diseases who would benefit from preventative measures and/or personalized therapy. Now that whole-genome sequencing is possible, comprehensive strategies for integrating genomic data and counseling of patients need to be developed.

Liver fatty acid-binding protein and obesity

While low levels of unesterified long chain fatty acids (LCFAs) are normal metabolic intermediates of dietary and endogenous fat, LCFAs are also potent regulators of key receptors/enzymes and at high levels become toxic detergents within the cell. Elevated levels of LCFAs are associated with diabetes, obesity and metabolic syndrome. Consequently, mammals evolved fatty acid-binding proteins (FABPs) that bind/sequester these potentially toxic free fatty acids in the cytosol and present them for rapid removal in oxidative (mitochondria, peroxisomes) or storage (endoplasmic reticulum, lipid droplets) organelles. Mammals have a large (15-member) family of FABPs with multiple members occurring within a single cell type. The first described FABP, liver-FABP (L-FABP or FABP1), is expressed in very high levels (2-5% of cytosolic protein) in liver as well as in intestine and kidney. Since L-FABP facilitates uptake and metabolism of LCFAs in vitro and in cultured cells, it was expected that abnormal function or loss of L-FABP would reduce hepatic LCFA uptake/oxidation and thereby increase LCFAs available for oxidation in muscle and/or storage in adipose. This prediction was confirmed in vitro with isolated liver slices and cultured primary hepatocytes from L-FABP gene-ablated mice. Despite unaltered food consumption when fed a control diet ad libitum, the L-FABP null mice exhibited age- and sex-dependent weight gain and increased fat tissue mass. The obese phenotype was exacerbated in L-FABP null mice pair fed a high-fat diet. Taken together with other findings, these data suggest that L-FABP could have an important role in preventing age- or diet-induced obesity.

Gallstone disease in Swedish twins: risk is associated with ABCG8 D19H genotype

OBJECTIVE

Recently, variants of the hepatocanalicular cholesterol hemitransporters ABCG5/8 were linked to gallstone disease; ABCG8 D19H in Caucasians and ABCG5 Q604E in Chinese. We investigated these polymorphisms in Swedish twins by merging the Swedish Twin Registry with the Hospital Discharge and Causes of Death Registries for gallstone disease-related diagnoses.

DESIGN

All monozygotic (MZ) twins with gallstone disease alive in the Stockholm area were invited to participate. Gallstone disease was defined by entry in all above mentioned registries, questionnaire or abdominal ultrasound.

SUBJECTS

ABCG5 Q604E and ABCG8 D19H genotyping was performed in 24 unique MZ and eight dizygotic (DZ) twins from concordant pairs. Screening of the TwinGene database for gallstone disease resulted in an additional 20 concordant MZ and 54 twins from concordant DZ pairs. We included 109 concordantly stone-free MZ and 126 stone-free independent DZ twins as controls.

RESULTS

Amongst the 341 twins, 20.8% carried at least one D19H allele as compared to 9.4% of stone-free controls. The association analysis showed that D19H positivity significantly increased the risk of gallstone disease [odds ratio (OR), 2.54; 95% confidence interval (CI), 1.33-4.82; P = 0.004]. We also found a trend for a positive association between gallstone disease and the Q604E variant (OR, 1.47; 95% CI, 1.00-2.16; P = 0.052).

CONCLUSION

Twins carrying a heterozygous or homozygous ABCG8 D19H genotype have a significantly increased risk of gallstone disease. Our study confirms the ABCG8 D19H genotype as a major risk factor for gallstone disease.

Pathophysiological preconditions promoting mixed "black" pigment plus cholesterol gallstones in a DeltaF508 mouse model of cystic fibrosis

Gallstones are frequent in patients with cystic fibrosis (CF). These stones are generally "black" pigment (i.e., Ca bilirubinate) with an appreciable cholesterol admixture. The pathophysiology and molecular mechanisms for this "mixed" gallstone in CF are unknown. Here we investigate in a CF mouse model with no overt liver or gallbladder disease whether pathophysiological changes in the physical chemistry of gallbladder bile might predict the occurrence of "mixed" cholelithiasis. Employing a DeltaF508 mouse model with documented increased fecal bile acid loss and induced enterohepatic cycling of bilirubin (Am J Physiol Gastrointest Liver Physiol 294: G1411-G1420, 2008), we assessed gallbladder bile chemistry, morphology, and microscopy in CF and wild-type mice, with focus on the concentrations and compositions of the common biliary lipids, bilirubins, Ca(2+), and pH. Our results demonstrate that gallbladder bile of CF mice contains significantly higher levels of all bilirubin conjugates and unconjugated bilirubin with lower gallbladder bile pH values. Significant elevations in Ca bilirubinate ion products in bile of CF mice increase the likelihood of supersaturating bile and forming black pigment gallstones. The risk of potential pigment cholelithogenesis is coupled with higher cholesterol saturations and bile salt hydrophobicity indexes, consistent with a proclivity to cholesterol phase separation during pigment gallstone formation. This is an initial step toward unraveling the molecular basis of CF gallstone disease and constitutes a framework for investigating animal models of CF with more severe biliary disease, as well as the human disease.

The genetic background of gallstone formation: an update

Gallstone disease is one of the most prevalent gastrointestinal diseases with a substantial burden to health care systems that is expected to increase in ageing populations at risk. This review summarizes recent data on the genetic background of cholesterol gallstones and the role of biliary lipid composition. Three previously unknown non-synonymous mutations in the ABCB4 gene encoding the hepatobiliary phospholipid-flippase MDR3 are presented.

Lith genes and genetic analysis of cholesterol gallstone formation

Epidemiologic investigations, clinical observations, and family and twin studies in humans, as well as gallstone prevalence investigations in inbred mouse models, support the concept that cholesterol cholelithiasis could result from a complex interaction of environmental factors and the effects of multiple undetermined genes. Quantitative trait locus (QTL) analysis is a powerful genetic method for identifying primary rate-limiting genetic defects and discriminating them from secondary downstream lithogenic effects caused by mutations of the primary genes, and the subsequent positional cloning of such genes responsible for QTLs, followed by the use of manufactured mouse strains with "knockout" or "knockin" of the genes, could lead to the discovery of lithogenic actions of gallstone (LITH) genes. The combined use of genomic strategies and phenotypic studies in inbred strains of mice has successfully resulted in the identification of many candidate LITH genes. Because there is exceptionally close homology between mouse and human genomes, the orthologous human LITH genes can be identified from the mouse study. The discovery of LITH genes and more fundamental knowledge concerning the genetic determinants and molecular mechanisms underlying the formation of cholesterol gallstones in humans will pave the way for critical diagnostic and prelithogenic preventive measures for this exceptionally prevalent digestive disease.

Transgenic overexpression of Abcb11 enhances biliary bile salt outputs, but does not affect cholesterol cholelithogenesis in mice

BACKGROUND

Cholesterol gallstone disease is a complex genetic trait and induced by multiple but as yet unknown genes. A major Lith gene, Lith1 was first identified on chromosome 2 in gallstone-susceptible C57L mice compared with resistant AKR mice. Abcb11, encoding the canalicular bile salt export pump in the hepatocyte, co-localizes with the Lith1 QTL region and its hepatic expression is significantly higher in C57L mice than in AKR mice.

MATERIAL AND METHODS

To investigate whether Abcb11 influences cholesterol gallstone formation, we created an Abcb11 transgenic strain on the AKR genetic background and fed these mice with a lithogenic diet for 56 days.

RESULT

We excluded functionally relevant polymorphisms of the Abcb11 gene and its promoter region between C57L and AKR mice. Overexpression of Abcb11 significantly promoted biliary bile salt secretion and increased circulating bile salt pool size and bile salt-dependent bile flow rate. However, biliary cholesterol and phospholipid secretion, as well as gallbladder size and contractility were comparable in transgenic and wild-type mice. At 56 days on the lithogenic diet, cholesterol saturation indexes of gallbladder biles and gallstone prevalence rates were essentially similar in these two groups of mice.

CONCLUSION

Overexpression of Abcb11 augments biliary bile salt secretion, but does not affect cholelithogenesis in mice.

Decreased expression of cholesterol 7alpha-hydroxylase and altered bile acid metabolism in Apobec-1-/- mice lead to increased gallstone susceptibility

Quantitative trait mapping in mice identified a susceptibility locus for gallstones (Lith6) spanning the Apobec-1 locus, the structural gene encoding the RNA-specific cytidine deaminase responsible for production of apolipoprotein B48 in mammalian small intestine and rodent liver. This observation prompted us to compare dietary gallstone susceptibility in Apobec-1(-/-) mice and congenic C57BL/6 wild type controls. When fed a lithogenic diet (LD) for 2 weeks, 90% Apobec-1(-/-) mice developed solid gallstones in comparison with 16% wild type controls. LD-fed Apobec-1(-/-) mice demonstrated increased biliary cholesterol secretion as well as increased cholesterol saturation and bile acid hydrophobicity indices. These changes occurred despite a relative decrease in cholesterol absorption in LD-fed Apobec-1(-/-) mice. Among the possible mechanisms to account for this phenotype, expression of Cyp7a1 mRNA and protein were significantly decreased in chow-fed Apobec-1(-/-) mice, decreasing further in LD-fed animals. Cyp7a1 transcription in hepatocyte nuclei, however, was unchanged in Apobec-1(-/-) mice, excluding transcriptional repression as a potential mechanism for decreased Cyp7a1 expression. We demonstrated that APOBEC-1 binds to AU-rich regions of the 3'-untranslated region of the Cyp7a1 transcript, containing the UUUN(A/U)U consensus motif, using both UV cross-linking to recombinant APOBEC-1 and in vivo RNA co-immunoprecipitation. In vivo Apobec-1-dependent modulation of Cyp7a1 expression was further confirmed following adenovirus-Apobec-1 administration to chow-fed Apobec-1(-/-) mice, which rescued Cyp7a1 gene expression. Taken together, the findings suggest that the AU-rich RNA binding-protein Apobec-1 mediates post-transcriptional regulation of murine Cyp7a1 expression and influences susceptibility to diet-induced gallstone formation.

Increased susceptibility to diet-induced gallstones in liver fatty acid binding protein knockout mice

Quantitative trait mapping identified a locus colocalizing with L-Fabp, encoding liver fatty acid binding protein, as a positional candidate for murine gallstone susceptibility. When fed a lithogenic diet (LD) for 2 weeks, L-Fabp(-/-) mice became hypercholesterolemic with increased hepatic VLDL cholesterol secretion. Seventy-five percent of L-Fabp(-/-) mice developed solid gallstones compared with 6% of wild-type mice with an increased gallstone score (3.29 versus 0.62, respectively; P < 0.01). Hepatic free cholesterol content, biliary cholesterol secretion, and the cholesterol saturation index of hepatic bile were increased in LD-fed L-Fabp(-/-) mice. Chow-fed L-Fabp(-/-) mice demonstrated increased fecal bile acid (BA) excretion accompanied by decreased ileal Asbt expression. By contrast, there was an increased BA pool and decreased fecal BA excretion in LD-fed L-Fabp(-/-) mice, associated with increased proximal intestinal Asbt mRNA expression, suggesting that intestinal BA absorption was enhanced in LD-fed L-Fabp(-/-) mice. The increase in biliary BA secretion and enterohepatic pool size in LD-fed L-Fabp(-/-) mice was accompanied by downregulation of Cyp7a1 mRNA and increased intestinal mRNA abundance of Fgf-15, Fxr, and Fabp6. These findings suggest that changes in hepatic cholesterol metabolism and biliary lipid secretion as well as changes in enterohepatic BA metabolism increase gallstone susceptibility in LD fed L-Fabp(-/-) mice.

Biliary lipids and cholesterol gallstone disease

Biliary lipids are a family of four dissimilar molecular species consisting of a mixture of bile salts (substituted cholanoic acids), phospholipids, mostly (>96%) diacylphosphatidylcholines, unesterified cholesterol, and bilirubin conjugates known trivially as lipopigments. The primary pathophysiological defect in cholesterol gallstone disease is hypersecretion of hepatic cholesterol into bile with less frequent hyposecretion of bile salts and/or phospholipids. Several other gallbladder abnormalities contribute and include hypomotility, immune-mediated inflammation, hypersecretion of gelling mucins, and accelerated phase transitions; there is also reduced intestinal motility that augments "secondary" bile salt synthesis by the anaerobic microflora. Cholesterol nucleation is initiated when unilamellar vesicles of cholesterol plus biliary phospholipids fuse to form multilamellar vesicles. From these "plate-like" cholesterol monohydrate crystals, the building blocks of macroscopic stones are nucleated heterogeneously by mucin gel. Multiple Lith gene loci have been identified in inbred mice, paving the way for discovery of an ever-increasing number of LITH genes in humans. Because of the frequency of the metabolic syndrome today, insulin resistance and LITH genes all interact with a number of environmental cholelithogenic factors to cause the gallstone phenotype. This review summarizes current concepts of the physical-chemical state of biliary lipids in health and in lithogenic bile and outlines the molecular, genetic, hepatic, and cholecystic factors that underlie the pathogenesis of cholesterol gallstones.

The G protein-coupled receptor G2A: involvement in hepatic lipid metabolism and gallstone formation in mice

The G2A receptor is a member of the ovarian cancer G protein-coupled receptor 1 family of stress-inducible G protein-coupled receptors. In this study, we examined the hepatobiliary effects of loss of function of G2A in mice fed either a chow or lithogenic diet. G2A-deficient (G2A(-/-)) mice fed chow had a 25% reduction in biliary phosphatidylcholine content, reduced hepatic gene expression of the phosphatidylcholine transporter adenosine triphosphate-binding cassette B4, and an 8-fold increase in expression of the nuclear receptor liver X receptor (LXR). Despite the increased expression of LXR, transcription of several LXR target genes was reduced. G2A(-/-) mice fed a lithogenic diet had rapid gallstone formation, an increased cholesterol saturation index, a 2.5-fold increase in farnesoid X receptor expression, a 5-fold increase in LXR expression, and a 90% reduction in cholesterol 7alpha-hydroxylase expression in comparison with wild-type mice. There were no changes in gallbladder volume.

CONCLUSION

These data demonstrate that the G2A receptor is important for hepatobiliary bile salt, cholesterol, and phospholipid homeostasis and for the pathogenesis of cholesterol gallstone formation.

Gallstone disease: from genes to evidence-based therapy

The number of gallstone patients is increasing in ageing populations with a high prevalence of metabolic syndrome and obesity. Recently variants of hepatic ATP binding cassette transporters have been identified as genetic susceptibility factors for gallstone disease, pointing to novel means for risk assessment and prevention. Although laparoscopic cholecystectomy is the mainstay of therapy for symptomatic gallbladder stones, the clinical management of gallstone disease is changing rapidly, with an increase in day case surgery and the advent of transluminal endoscopic surgery. Here, we summarize the molecular and genetic mechanisms of gallstone formation as well as the current evidence-based algorithms for diagnosis and therapy of gallbladder and bile duct stones.

Genetics of biliary tract diseases: new insights into gallstone disease and biliary tract cancers

PURPOSE OF REVIEW

Chronic biliary diseases are due to complex interactions between environmental and genetic factors. Here we summarize the current knowledge of genetic factors that contribute to common biliary diseases, focusing on gallstones and carcinogenesis, and review the recent association studies.

RECENT FINDINGS

Since most studies were based on small sample sizes, replication of the findings is mandatory. Recently a large twin study confirmed a genetic predisposition to gallstones and a genome-wide association scan identified the hepatocanalicular cholesterol transporter ABCG8 as the common susceptibility factor for gallstone disease. Genetic studies in patients with cholangiocarcinoma indicate that genes controlling the metabolism and transport of xenobiotics or modulating chronic inflammation may determine individual susceptibility.

SUMMARY

Genetic studies have identified the first susceptibility factors for gallstones and biliary tract cancers, but most results have yet to be replicated. In the future, genome-wide studies in different populations are likely to identify the entire set of genes contributing to chronic biliary diseases. Since the disease phenotypes result from the manifestation of susceptibility factors under the influence of environmental triggers, the discovery of these genes will open avenues to control environmental challenges and lead to novel strategies for risk assessment ('gene signatures') and prevention.