ATP binding cassette G8 T400K polymorphism may affect the risk of gallstone disease among Chinese males

Resistance to Cholesterol Gallstone Disease: Hepatic Cholesterol Metabolism

Cholesterol gallstone disease (CGD) is one of the most common digestive diseases, and it is closely associated with hepatic cholesterol metabolism. Cholesterol gallstones may be caused by abnormal hepatic cholesterol metabolism, such as excessive cholesterol biosynthesis within the liver, interfering with the uptake or export of cholesterol in the liver, and abnormal hepatic cholesterol esterification. In this review, we begin with a brief overview of the clinical diagnosis and treatment of gallstone disease (GSD). Then, we briefly describe the major processes of hepatic cholesterol metabolism and summarize the key molecular expression changes of hepatic cholesterol metabolism in patients with gallstones. We review and analyze the recent advances in elucidating the relationships between these key molecules and CGD, and some targets significantly impacting on CGD via hepatic cholesterol metabolism are also listed. We also provide a significant discussion on the relationship between CGD and nonalcoholic fatty liver disease (NAFLD). Finally, the new discoveries of some therapeutic strategies associated with hepatic cholesterol metabolism to prevent and treat CGD are summarized.

Association of ABCG5 and ABCG8 Transporters with Sitosterolemia

Sitosterolemia is a rare genetic lipid disorder, mainly characterized by the accumulation of dietary xenosterols in plasma and tissues. It is caused by inactivating mutations in either ABCG5 or ABCG8 subunits, a subfamily-G ATP-binding cassette (ABCG) transporters. ABCG5/G8 encodes a pair of ABC half transporters that form a heterodimer (G5G8). This heterodimeric ATP-binding cassette (ABC) sterol transporter, ABCG5/G8, is responsible for the hepatobiliary and transintestinal secretion of cholesterol and dietary plant sterols to the surface of hepatocytes and enterocytes, promoting the secretion of cholesterol and xenosterols into the bile and the intestinal lumen. In this way, ABCG5/G8 function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Here, we review the biological characteristics and function of ABCG5/G8, and how the mutations of ABCG5/G8 can cause sitosterolemia, a loss-of-function disorder characterized by plant sterol accumulation and premature atherosclerosis, among other features.

Risk of gallstones based on ABCG8 rs11887534 single nucleotide polymorphism among Taiwanese men and women

BACKGROUND

Gallstones are abnormal masses caused by impaired metabolism of cholesterol, bilirubin, or bile salts in the gallbladder or biliary tract. ATP-binding cassette subfamily G member 8 (ABCG8) is a protein that regulates cholesterol efflux from the liver. Genome-wide association studies (GWAS) and meta-analyses of GWAS revealed the ABCG8 rs11887534 variant as the most common genetic determinant of gallstones in humans. These findings have not been extensively replicated in Taiwanese. Therefore, we appraised the relationship between gallstones and rs11887534 in a relatively large Taiwanese sample.

METHODS

We retrieved data collected through questionnaires, physical and biochemical tests from the Taiwan Biobank Bank (TWB). The study participants comprised 7388 men and 13,880 women who voluntarily enrolled in the Taiwan Biobank project between 2008 and 2019. Gallstones were self-reported.

RESULTS

The overall sample size was 21,268 comprising 938 gallstone patients and 20,330 non-gallstone individuals. Among the participants, 20,640 had the GG and 628 had the GC + CC genotype. At p-value < 0.05, the baseline genotypes and gallstone status between men and women were not significantly different. The risk of gallstones was higher in participants having the GC + CC compared to the GG genotype: odds ratio (OR); 95% confidence interval (CI) = 1.698; 1.240-2.325), but was lower in men compared to women (OR = 0.763; 95% CI = 0.638-0.913). Compared to men with the rs11887534 GG genotype, women with the GG and GC + CC genotypes had a higher risk of gallstone (OR; 95% CI = 1.304; 1.087-1.565 for GG and 2.291; 1.514-3.467 for GC + CC). The positive association between GC + CC and gallstones was retained after we restricted the analysis to the female participants (OR; 95% CI = 1.789 = 1.208-2.648). Hormone use was associated with an elevated risk of gallstones (OR; 95% CI = 1.359; 1.107-1.668). Relative to GG and no hormone use, we found a significantly high risk among hormone users with the GC + CC genotype (OR; 95% CI = 3.596; 1.495-8.650).

CONCLUSIONS

The rs11887534 GC + CC genotype was independently associated with a higher risk of gallstones. This risk was much higher among women, especially those who used hormones for various gynecological purposes.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

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

Association of Common Single Nucleotide Polymorphisms of Candidate Genes with Gallstone Disease: A Meta-Analysis

Numerous studies have investigated the relationship between various candidate gene polymorphisms and gallbladder stone disease (GSD) across ethnic populations; however, the results are often inconsistent. This meta-analysis aims to comprehensively evaluate the influence of common ABCG8 T400K, ABCG8 D19H, ABCG8 C54Y, ApoB100 EcoRI, ApoB100 XbaI, ApoE HhaI, CETP TaqI, CYP7A1 Bsa, LRPAP1 I/D and TNF-α A308G polymorphisms on the risk of gallbladder stone disease. 33 Full-text articles with 9250 cases and 12,029 healthy controls (total 21,279 subjects) were analyzed using the RevMan software (V5.1) and the Comprehensive Meta-analysis software (Version 2.0, BIOSTAT, Englewood, NJ) a Random-effects model was applied. Begg's funnel plots, Fail-safe number, Egger's regression intercept and Begg and Mazumdar rank correlation tests were performed for the potential publication bias and sensitivity analysis. The studies were also sub-grouped into European and non-European groups to find out role of ethnicity, if any, on GSD risk. Studies included in quantitative synthesis were ABCG8 T400K rs4148217 (cases/controls, n = 671/1416) (4 studies), ABCG8 D19H rs11887534 (n = 1633/2306) (8 studies), ABCG8 C54Y rs4148211 (n = 445/1194) (3 studies), ApoB100 EcoRI rs1042031 (n = 503/390) (4 studies), ApoB100 XbaI rs693 (n = 1214/1389) (9 studies), ApoE HhaI rs429358 (n = 1335/1482) (12 studies), CETP TaqI rs708272 (n = 1038/1025) (5 studies), CYP7A1 Bsa rs3808607 (n = 565/514) (3 studies), LRPAP1 I/D rs11267919 (n = 849/900) (3 studies), TNF-α A308G rs1800629 (n = 997/1413) (3 studies). The combined results displayed significant association of ABCG8 D19H (GC + CC) [OR with 95%CI = 2.2(1.7-2.8); p < 0.00001], ABCG8 Y54C (GA + GG) [OR with 95%CI = 0.65(0.5-0.9); p = 0.01]. APOB100 EcoRI (GG vs. AA) [OR with 95%CI = 0.51(0.3-0.9); p = 0.05], (GG vs. GA) [OR with 95%CI = 0.6(0.4-0.9); p = 0.04], (GA + AA) [OR with 95%CI = 0.6(0.4-0.9); p = 0.006]. APOB Xba I (X^- vs. X⁺) [OR with 95%CI = 0.53(0.3-0.8); p = 0.006. APOE Hha I (E4/E4 vs. E3/E3) [OR with 95%CI = 3.5(1.1-14.9); p = 0.04] and LRPAP1 I/D (ID + II) [OR with 95%CI = 1.27(1.0-1.6); p = 0.03] with the GSD risk. It was found that ABCG D19H was significantly associated with GSD in both European and Non-European populations. While APOB XbaI and LRPAP1 I/D markers were associated with gallstone disease only in Non- European population. Additionally, APOE HhaI and APOB 100 ECoRI were found to be associated with GSD only in European population. The results of quantitative synthesis suggest that the ABCG8 D19H polymorphism was associated with the increased risk of GSD in both European and Non-European populations, APOE Hha I and LRPAP1 I/D polymorphisms were associated with the increased risk of GSD in European and Non-European population respectively. However, no association was found in ABCG8 T400K, CETP Taq1, CYP7A1 Bsa and TNF-A308G polymorphisms with Gallstone Disease.

ABCG5/G8: a structural view to pathophysiology of the hepatobiliary cholesterol secretion

The ABCG5/G8 heterodimer is the primary neutral sterol transporter in hepatobiliary and transintestinal cholesterol excretion. Inactivating mutations on either the ABCG5 or ABCG8 subunit cause Sitosterolemia, a rare genetic disorder. In 2016, a crystal structure of human ABCG5/G8 in an apo state showed the first structural information on ATP-binding cassette (ABC) sterol transporters and revealed several structural features that were observed for the first time. Over the past decade, several missense variants of ABCG5/G8 have been associated with non-Sitosterolemia lipid phenotypes. In this review, we summarize recent pathophysiological and structural findings of ABCG5/G8, interpret the structure-function relationship in disease-causing variants and describe the available evidence that allows us to build a mechanistic view of ABCG5/G8-mediated sterol transport.

Recent Advances in the Critical Role of the Sterol Efflux Transporters ABCG5/G8 in Health and Disease

Cardiovascular disease is characterized by lipid accumulation, inflammatory response, cell death, and fibrosis in the arterial wall and is the leading cause of morbidity and mortality worldwide. Cholesterol gallstone disease is caused by complex genetic and environmental factors and is one of the most prevalent and costly digestive diseases in the USA and Europe. Although sitosterolemia is a rare inherited lipid storage disease, its genetic studies led to identification of the sterol efflux transporters ABCG5/G8 that are located on chromosome 2p21 in humans and chromosome 17 in mice. Human and animal studies have clearly demonstrated that ABCG5/G8 play a critical role in regulating hepatic secretion and intestinal absorption of cholesterol and plant sterols. Sitosterolemia is caused by a mutation in either the ABCG5 or the ABCG8 gene alone, but not in both simultaneously. Polymorphisms in the ABCG5/G8 genes are associated with abnormal plasma cholesterol metabolism and may play a key role in the genetic determination of plasma cholesterol concentrations. Moreover, ABCG5/G8 is a new gallstone gene, LITH9. Gallstone-associated variants in ABCG5/G8 are involved in the pathogenesis of cholesterol gallstones in European, Asian, and South American populations. In this chapter, we summarize the latest advances in the critical role of the sterol efflux transporters ABCG5/G8 in regulating hepatic secretion of biliary cholesterol, intestinal absorption of cholesterol and plant sterols, the classical reverse cholesterol transport, and the newly established transintestinal cholesterol excretion, as well as in the pathogenesis and pathophysiology of ABCG5/G8-related metabolic diseases such as sitosterolemia, cardiovascular disease, and cholesterol gallstone disease.

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.

ABCG5 and ABCG8: more than a defense against xenosterols

The elucidation of the molecular basis of the rare disease, sitosterolemia, has revolutionized our mechanistic understanding of how dietary sterols are excreted and how cholesterol is eliminated from the body. Two proteins, ABCG5 and ABCG8, encoded by the sitosterolemia locus, work as obligate dimers to pump sterols out of hepatocytes and enterocytes. ABCG5/ABCG8 are key in regulating whole-body sterol trafficking, by eliminating sterols via the biliary tree as well as the intestinal tract. Importantly, these transporters keep xenosterols from accumulating in the body. The sitosterolemia locus has been genetically associated with lipid levels and downstream atherosclerotic disease, as well as formation of gallstones and the risk of gallbladder cancer. While polymorphic variants raise or lower the risks of these phenotypes, loss of function of this locus leads to more dramatic phenotypes, such as premature atherosclerosis, platelet dysfunction, and thrombocytopenia, and, perhaps, increased endocrine disruption and liver dysfunction. Whether small amounts of xenosterol exposure over a lifetime cause pathology in normal humans with polymorphic variants at the sitosterolemia locus remains largely unexplored. The purpose of this review will be to summarize the current state of knowledge, but also highlight key conceptual and mechanistic issues that remain to be explored.

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.

Recent advances in understanding and managing cholesterol gallstones

The high prevalence of cholesterol gallstones, the availability of new information about pathogenesis, and the relevant health costs due to the management of cholelithiasis in both children and adults contribute to a growing interest in this disease. From an epidemiologic point of view, the risk of gallstones has been associated with higher risk of incident ischemic heart disease, total mortality, and disease-specific mortality (including cancer) independently from the presence of traditional risk factors such as body weight, lifestyle, diabetes, and dyslipidemia. This evidence points to the existence of complex pathogenic pathways linking the occurrence of gallstones to altered systemic homeostasis involving multiple organs and dynamics. In fact, the formation of gallstones is secondary to local factors strictly dependent on the gallbladder (that is, impaired smooth muscle function, wall inflammation, and intraluminal mucin accumulation) and bile (that is, supersaturation in cholesterol and precipitation of solid crystals) but also to "extra-gallbladder" features such as gene polymorphism, epigenetic factors, expression and activity of nuclear receptors, hormonal factors (in particular, insulin resistance), multi-level alterations in cholesterol metabolism, altered intestinal motility, and variations in gut microbiota. Of note, the majority of these factors are potentially manageable. Thus, cholelithiasis appears as the expression of systemic unbalances that, besides the classic therapeutic approaches to patients with clinical evidence of symptomatic disease or complications (surgery and, in a small subgroup of subjects, oral litholysis with bile acids), could be managed with tools oriented to primary prevention (changes in diet and lifestyle and pharmacologic prevention in subgroups at high risk), and there could be relevant implications in reducing both prevalence and health costs.

Thyroid dysfunction, either hyper or hypothyroidism, promotes gallstone formation by different mechanisms

We have investigated comprehensively the effects of thyroid function on gallstone formation in a mouse model. Gonadectomized gallstone-susceptible male C57BL/6 mice were randomly distributed into three groups each of which received an intervention to induce hyperthyroidism, hypothyroidism, or euthyroidism. After 5 weeks of feeding a lithogenic diet of 15% (w/w) butter fat, 1% (w/w) cholesterol, and 0.5% (w/w) cholic acid, mice were killed for further experiments. The incidence of cholesterol monohydrate crystal formation was 100% in mice with hyperthyroidism, 83% in hypothyroidism, and 33% in euthyroidism, the differences being statistically significant. Among the hepatic lithogenic genes, Trβ was found to be up-regulated and Rxr down-regulated in the mice with hypothyroidism. In contrast, Lxrα, Rxr, and Cyp7α1 were up-regulated and Fxr down-regulated in the mice with hyperthyroidism. In conclusion, thyroid dysfunction, either hyperthyroidism or hypothyroidism, promotes the formation of cholesterol gallstones in C57BL/6 mice. Gene expression differences suggest that thyroid hormone disturbance leads to gallstone formation in different ways. Hyperthyroidism induces cholesterol gallstone formation by regulating expression of the hepatic nuclear receptor genes such as Lxrα and Rxr, which are significant in cholesterol metabolism pathways. However, hypothyroidism induces cholesterol gallstone formation by promoting cholesterol biosynthesis.

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.

Evidence that the adenosine triphosphate-binding cassette G5/G8-independent pathway plays a determinant role in cholesterol gallstone formation in mice

The adenosine triphosphate-binding cassette (ABC) sterol transporter, Abcg5/g8, is Lith9 in mice, and two gallstone-associated variants in ABCG5/G8 have been identified in humans. Although ABCG5/G8 plays a critical role in determining hepatic sterol secretion, cholesterol is still secreted to bile in sitosterolemic patients with a defect in either ABCG5 or ABCG8 and in either Abcg5/g8 double- or single-knockout mice. We hypothesize that in the defect of ABCG5/G8, an ABCG5/G8-independent pathway is essential for regulating hepatic secretion of biliary sterols, which is independent of the lithogenic mechanism of the ABCG5/G8 pathway. To elucidate the effect of the ABCG5/G8-independent pathway on cholelithogenesis, we investigated the biliary and gallstone characteristics in male wild-type (WT), ABCG5(-/-)/G8(-/-), and ABCG8 (-/-) mice fed a lithogenic diet or varying amounts of cholesterol, treated with a liver X receptor (LXR) agonist, or injected intravenously with [(3) H]sitostanol- and [(14) C]cholesterol-labeled high-density lipoprotein (HDL). We found that ABCG5(-/-)/G8(-/-) and ABCG8 (-/-) mice displayed the same biliary and gallstone phenotypes. Although both groups of knockout mice showed a significant reduction in hepatic cholesterol output compared to WT mice, they still formed gallstones. The LXR agonist significantly increased biliary cholesterol secretion and gallstones in WT, but not ABCG5(-/-)/G8(-/-) or ABCG8 (-/-), mice. The 6-hour recovery of [(14) C]cholesterol in hepatic bile was significantly lower in both groups of knockout mice than in WT mice and [(3) H]sitostanol was detected in WT, but not ABCG5(-/-)/G8(-/-) or ABCG8 (-/-), mice.

CONCLUSIONS

The ABCG5/G8-independent pathway plays an important role in regulating biliary cholesterol secretion, the transport of HDL-derived cholesterol from plasma to bile, and gallstone formation, which works independently of the ABCG5/G8 pathway. Further studies are needed to observe whether this pathway is also operational in humans. (Hepatology 2016;64:853-864).

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.

Association of FXR gene variants with cholelithiasis

BACKGROUND AND AIM

Impairment of bile acid homeostasis is the most important risk factor of gallstone disease. Thereby the bile acid sensor farnesoid X receptor (FXR) plays a pivotal role in hepatic and intestinal bile acid metabolism. In this explorative study, the FXR gene was investigated to identify gene variants, associated with gallstone formation in a Caucasian population.

METHODS

Sequencing of the FXR gene was conducted in a randomly selected cohort of gallstone carriers (n=30) and control subjects (n=16) from Stuttgart, Germany. Genomic DNA was obtained from blood leukocytes. Genotype frequencies were established in the total cohort (controls: n=133, gallstone carriers: n=74). For expression analysis, total RNA and protein were isolated from ileal biopsies.

RESULTS

The sequencing showed the sole appearance of 10 SNPs in gallstone carriers. Further genotype analysis revealed significant gender- and weight-dependent frequency differences of 3 SNPs between gallstone carriers and controls in males (rs35724: OR=4.73, P=0.022) and normal weight subjects (rs11110385: OR=3.67, P=0.027; rs11110386: OR=3.67, P=0.027) applying the 11+12<>22 allele model. Furthermore, rs11110385 carriers showed a significantly decreased FXR protein expression (11+12<>22: P=0.003). Significant mRNA expression differences between lean rs11110385 carriers and non-carriers were observed in FXR target genes (decrease: ILBP: P=0.042, OSTalpha: P=0.071, FGF19: P=0.011. Increase: LRH1: P=0.044).

CONCLUSIONS

Three FXR gene variants (rs35724, rs11110385, rs11110386) were identified as potential susceptibility factors for cholelithiasis in a German cohort in gender- and weight-dependent manners. Thereby the tag SNP rs11110385 seemed to influence the activation of the FXR gene.

Recent advances in understanding the STSL locus and ABCG5/ABCG8 biology

PURPOSE OF REVIEW

To provide an update on recent advances made in our mechanistic and pathophysiological understanding of the rare human disease Sitosterolemia, the role of ABCG5/ABCG8 in sterol trafficking and how newer data implicate a more wider role in the body.

RECENT FINDINGS

Sitosterolemia is caused by a genetic defect of sterolins (ABCG5/ABCG8) mapped to the STSL locus. Polymorphic variations in STSL have been linked to lipid levels and gallstone disease in whites. Newer studies now link this locus to a more diverse ethnic group for gallstone disease, susceptibility to biliary cancer, and show variants that alter sterolin function. Intriguingly, carriers of a mutant allele seem to show protection against carotid wall disease. Although the 'promoter' region of the STSL is minimal, regulatory regions responsive to liver X receptor have remained elusive, but no longer; two intronic regions in ABCG8 have now been identified. Xenosterol accumulation leads to loss of abdominal fat, infertility, and premature death. Xenosterol accumulation in mouse platelet membranes leads to platelet hyperactivation, increased microparticle formation, and reduced αIIbβ3 surface expression. In humans, phytosterols may promote liver injury in parenteral nutrition-associated liver disease.

SUMMARY

Progress in understanding sterolin function is beginning to show that xenosterols can be toxic and are involved on pathogenesis, and the role of ABCG5/ABCG8 may extend into other metabolic processes by altering intracellular sterol metabolism.

ABCG5/8 variants are associated with susceptibility to coronary heart disease

ATP-binding cassette sub-family G member 5 (ABCG5) and ABCG8 are members of an ATP-binding cassette transporter superfamily. ABCG5 and ABCG8 variants affected serum levels of cholesterol and were considered as risk factors for coronary heart disease (CHD). The present control study analyzed ABCG5 and ABCG8 variants in a population for association with the risk of CHD. A total of 417 CHD patients and 267 controls were recruited for genotyping of four single nucleotide polymorphisms (SNPs; i.e. i7892T>C in ABCG5 and Tyr54CysA>G, Thr400LysC>A and 5U145A>C in ABCG8) using quantitative PCR high-resolution melting (qPCR-HRM). Serum lipid levels were measured using an automatic biochemical analyzer. The association of ABCG5/8 variants with lipid levels was analyzed using a Chi-square test. The impact of candidate ABCG5/8 SNPs on CHD was evaluated in a dominant genetic model with stepwise multiple regression analysis. Subgroup analyses were performed with regard to these SNPs, tobacco smoking status, alcohol consumption and gender. Genotypic and allelic frequencies of ABCG8 Thr400LysC>A were significantly different (P<0.05) between CHD patients and controls. CC homozygotes of the ABCG8 Thr400LysC>A SNP had greater triglyceride levels than CA/AA carriers with CHD. Logistic analysis revealed CHD risk was significantly higher in CC homozygotes of ABCG8 Thr400LysC>A than in carriers of the A allele (adjusted P=0.048; OR=2.034; 95% CI=0.983-4.207). Furthermore, there was a significant gene-tobacco smoking interaction. CC homozygotes of ABCG8 Thr400LysC>A SNP had significantly higher triglyceride concentrations (P=0.012) and an increased risk of CHD than tobacco smoking carriers of the A allele. The data from the current study suggested that ABCG8 Thr400LysC>A SNP genetic variants modulated plasma triglyceride levels and thereby affected CHD risk in the population studied. The genetic variant of ABCG8 also contributed to CHD risk through interaction with tobacco smoking.

The ABCG5/8 cholesterol transporter and myocardial infarction versus gallstone disease

OBJECTIVES

The study sought to test the hypothesis that genetic variation in ABCG5/8, the transporter responsible for intestinal and hepatobiliary cholesterol efflux, may simultaneously influence plasma and biliary cholesterol levels, and hence risk of myocardial infarction (MI) and gallstone disease in opposite directions.

BACKGROUND

High plasma levels of low-density lipoprotein (LDL) cholesterol are a causal risk factor for MI, whereas high levels of biliary cholesterol promote gallstone formation.

METHODS

A total of 60,239 subjects from Copenhagen were included, including 5,647 with MI and 3,174 with symptomatic gallstone disease. Subjects were genotyped for 6 common, nonsynonymous and functional variants in ABCG5/8, and a combined weighted genotype score was calculated.

RESULTS

Combined, weighted genotype scores were associated with stepwise decreases in LDL cholesterol of up to 5.9% (0.20 mmol/l) for individuals with a score ≥8.0 (prevalence = 11%) versus <2.0 (prevalence = 9%; p for trend across 5 groups = 2 × 10E-35). The cumulative incidences of MI and gallstone disease as a function of age and increasing genotype score were decreased and increased (log-rank ps for trend: 6 × 10E-4 and 9 × 10E-45), respectively. The multifactorially adjusted odds ratios were 0.83 (95% confidence interval: 0.73 to 0.94) for MI and 2.85 (95% confidence interval: 2.39 to 3.39) for symptomatic gallstone disease for individuals with a genotype score ≥8.0 versus <2.0.

CONCLUSIONS

Genetic variation in ABCG5/8, which associates with decreased levels of plasma LDL cholesterol protects against MI, but increases the risk of symptomatic gallstone disease. These results suggest that MI and gallstones, 2 seemingly unrelated diseases, are intrinsically linked via the function of the ABCG5/8 cholesterol transporter.

Association of three common single nucleotide polymorphisms of ATP binding cassette G8 gene with gallstone disease: a meta-analysis

BACKGROUND

In this study, we evaluated the association between these polymorphisms and gallstone disease using meta-analysis and compared the hepatic ABCG5/G8 mRNA expression and biliary lipids composition in patients with different genotypes of T400K and Y54C.

METHODS

Data were analyzed using the Stata/SE 11.0 software and a random- effects model was applied irrespective of between-study heterogeneity. Hepatic mRNA expression of ABCG5/G8 genes in 182 patients with gallstone disease and 35 gallstone-free patients who underwent cholecystectomy were determined using real-time PCR. Genotypes of Y54C and T400K in the ABCG8 gene were determined by allelic discrimination using either genomic DNA or hepatic cDNA as template by Taqman assays. Biliary compostion in gallbladder bile was assayed in these patients as well.

RESULTS

Ten papers including 13 cohorts were included for the final analysis. In the genotype model, the overall association between genotype with gallstone was significant for D19H (OR = 2.43, 95%CI: 2.23-2.64, P<0.001), and for Y54C (OR = 1.36, 95%CI: 1.01-1.83, P = 0.044), or T400K (OR = 1.17, 95%CI: 0.96-1.43. P = 0.110). In allele model, minor alleles of D19H polymorphism (allele D: OR = 2.25, 95%CI: 2.10-2.42, P<0.001) and of T400K polymorphism (allele K: OR = 1.18, 95%CI: 1.06-1.31, P<0.001) were related with an increased risk of gallstone disease. However, minor allele of Y54C polymorphism (allele Y, OR = 1.08, 95%CI: 0.96-1.21, P = 0.146) was not related with gallstone disease. I(2) statistics indicated no significant between-study heterogeneity for all genetic models for any of the three polymorphisms. Funnel plot and Egger's test suggested the absence of publication bias as well. However, no association of T400K and Y54C polymorphism with hepatic ABCG8/G5 mRNA expression or biliary lipids composition was found.

CONCLUSIONS

Our study showed strong association of D19H polymorphism with gallstone disease. T400K and Y54C polymorphism, though to a less extent, may also relate with gallstone disease.

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.

Genetic and functional identification of the likely causative variant for cholesterol gallstone disease at the ABCG5/8 lithogenic locus

The sterolin locus (ABCG5/ABCG8) confers susceptibility for cholesterol gallstone disease in humans. Both the responsible variant and the molecular mechanism causing an increased incidence of gallstones in these patients have as yet not been identified. Genetic mapping utilized patient samples from Germany (2,808 cases, 2,089 controls), Chile (680 cases, 442 controls), Denmark (366 cases, 766 controls), India (247 cases, 224 controls), and China (280 cases, 244 controls). Analysis of allelic imbalance in complementary DNA (cDNA) samples from human liver (n = 22) was performed using pyrosequencing. Transiently transfected HEK293 cells were used for [(3) H]-cholesterol export assays, analysis of protein expression, and localization of allelic constructs. Through fine mapping in German and Chilean samples, an ∼250 kB disease-associated interval could be defined for this locus. Lack of allelic imbalance or allelic splicing of the ABCG5 and ABCG8 transcripts in human liver limited the search to coding single nucleotide polymorphisms. Subsequent mutation detection and genotyping yielded two disease-associated variants: ABCG5-R50C (P = 4.94 × 10(-9) ) and ABCG8-D19H (P = 1.74 × 10(-10) ) in high pairwise linkage disequilibrium (r(2) = 0.95). [(3) H]-cholesterol export assays of allelic constructs harboring these genetic candidate variants demonstrated increased transport activity (3.2-fold, P = 0.003) only for the ABCG8-19H variant, which was also superior in nested logistic regression models in German (P = 0.018), Chilean (P = 0.030), and Chinese (P = 0.040) patient samples.

CONCLUSION

This variant thus provides a molecular basis for biliary cholesterol hypersecretion as the mechanism for cholesterol gallstone formation, thereby drawing a link between "postgenomic" and "pregenomic" pathophysiological knowledge about this common complex disorder. (HEPATOLOGY 2012).

Single nucleotide polymorphism rs3732860 in the 3'-untranslated region of CYP8B1 gene is associated with gallstone disease in Han Chinese

BACKGROUND AND AIMS

Gallstone disease (GD) is a common disease of multigenetic origin; however, the major susceptibility loci for GD in human populations remain unidentified. This study aimed to identify the genetic factors contributing to gallstone development in Chinese.

METHODS

A genome-wide scan was conducted in 12 Han Chinese GD families to identify linkage loci. The linkage region showing the highest logarithm of odds score encompasses the sterol 12α-hydroxylase gene (CYP8B1). Replication analysis with an independent sample of 192 GD patients and 192 unrelated, matched controls was carried out to verify the associations between CYP8B1 polymorphisms and GD.

RESULTS

Three loci (D3S1266, D4S406, and D9S1682) showed suggestive or nominal evidence of linkage in all 12 GD families. The logarithm of odds score of D3S1266 reached 2.71 in the families with late-onset patients. The single nucleotide polymorphism rs3732860 in the 3'-untranslated region of CYP8B1 showed significant association to GD (P = 0.022), and carriers of the A allele had lower risk of GD (odds ratio = 1.46, 95% confidence interval: 1.055-2.034) compared with carriers of the G allele.

CONCLUSIONS

The single nucleotide polymorphism rs3732860 in the 3'-untranslated region of the CYP8B1 gene is associated with risk of GD in Chinese Han and appears to be responsible for the observed linkage with D3S1266.

Plant sterols and stanols: their role in health and disease

Mammalian physiological processes, and likely any organism with a biliary tree, can distinguish between dietary cholesterol and non-cholesterols, retaining very little of the non-cholesterol in their bodies. Historically, the distinction between plant sterols and cholesterol has been known about for a century or more. That plants sterols were not 'absorbed' has been investigated for almost half a century. Indeed, the oral of plant sterols in gram quantities was shown to interfere with cholesterol absorption and is one of the oldest pharmacological therapies for hypercholesterolemia. Although the basis for the latter was shown to be caused by exclusion of cholesterol from intestinal micelles by plant sterols, it was not until the identification of the a rare genetic disease, sitosterolemia, first described in 1974, that led to the hypothesis that specific molecular mechanism(s) governed both the entry and excretion of sterols by the body. This talk will cover the physiology of dietary sterol metabolism, genetics and pathophysiology of sitosterolemia. Additionally, the role of plant sterols in normal and abnormal metabolism in humans as well as selected animal models will be discussed.

Association of ATP binding cassette transporter G8 rs4148217 SNP and serum lipid levels in Mulao and Han nationalities

Background

The association of ATP binding cassette transporter G8 gene (ABCG8) rs4148217 single nucleotide polymorphism (SNP) and serum lipid profiles is still controversial in diverse racial/ethnic groups. Mulao nationality is an isolated minority in China. The aim of this study was to evaluate the association of ABCG8 rs4148217 SNP and several environmental factors with serum lipid levels in the Guangxi Mulao and Han populations.

Methods

A total of 634 subjects of Mulao nationality and 717 participants of Han nationality were randomly selected from our previous samples. Genotyping of the ABCG8 rs4148217 SNP was performed by polymerase chain reaction and restriction fragment length polymorphism combined with gel electrophoresis, and then confirmed by direct sequencing.

Results

The genotypic and allelic frequencies of ABCG8 rs4148217 SNP were different between the two nationalities (P < 0.01 for each), the frequency of A allele was higher in Mulao than in Han. The A allele carriers in Han had lower high-density lipoprotein cholesterol (HDL-C) and apolipoprotein (Apo) A1 levels than the A allele noncarriers (P < 0.05 for each), whereas the A allele carriers in Mulao had lower ApoA1 levels than the A allele noncarriers (P < 0.05). Subgroup analyses showed that the A allele carriers in Han had lower HDL-C and higher triglyceride (TG) levels in females but not in males than the A allele noncarriers (P < 0.05 for each), and the A allele carriers in Mulao had lower ApoA1 levels in females but not in males than the A allele noncarriers (P < 0.05). The levels of TG and HDL-C in Han, and ApoA1 in Mulao were associated with genotypes in females but not in males (P < 0.05-0.01). Serum lipid parameters were also correlated with several environmental factors (P < 0.05-0.001).

Conclusions

The ABCG8 rs4148217 SNP is associated with serum TG, HDL-C and ApoA1 levels in our study populations, but this association is different between the Mulao and Han populations. There is a sex (female)-specific association in both ethnic groups.

Concept of the pathogenesis and treatment of cholelithiasis

Gallstone disease (GD) is a chronic recurrent hepatobiliary disease, the basis for which is the impaired metabolism of cholesterol, bilirubin and bile acids, which is characterized by the formation of gallstones in the hepatic bile duct, common bile duct, or gallbladder. GD is one of the most prevalent gastrointestinal diseases with a substantial burden to health care systems. GD can result in serious outcomes, such as acute gallstone pancreatitis and gallbladder cancer. The epidemiology, pathogenesis and treatment of GD are discussed in this review. The prevalence of GD varies widely by region. The prevalence of gallstone disease has increased in recent years. This is connected with a change in lifestyle: reduction of motor activity, reduction of the physical load and changes to diets. One of the important benefits of early screening for gallstone disease is that ultrasonography can detect asymptomatic cases, which results in early treatment and the prevention of serious outcomes. The pathogenesis of GD is suggested to be multifactorial and probably develops from complex interactions between many genetic and environmental factors. It suggests that corticosteroids and oral contraceptives, which contain hormones related to steroid hormones, may be regarded as a model system of cholelithiasis development in man. The achievement in the study of the physiology of bile formation and the pathogenesis of GD has allowed expanding indications for therapeutic treatment of GD.

Cholesterol metabolism gene polymorphisms and the risk of biliary tract cancers and stones: a population-based case-control study in Shanghai, China

Biliary tract cancers are rare but fatal malignancies, with increasing incidence in Shanghai, China. Gallstones, the primary risk factor for biliary tract cancer, typically result from oversaturation of cholesterol in bile. We examined the association of five variants in three lipid metabolism-related genes (CETP, ABCG8 and LRPAP1) and biliary tract cancers and stones in a population-based case-control study in Shanghai, China. We included 439 biliary tract cancer cases (253 gallbladder, 133 extrahepatic bile duct and 53 ampulla of Vater cancer cases), 429 biliary stone cases and 447 population controls. Carriers of the CG genotype of ABCG8 rs11887534 had higher risk of biliary stones [odds ratio (OR) = 2.3, 95% confidence interval (CI) 0.82-6.5), gallbladder cancer (OR = 4.3, 95% CI 1.7-10.4) and bile duct cancer (OR = 1.94, 95% CI 0.64-5.91), compared with carriers of the GG genotype. Analysis stratified by gender showed both male and female carriers of CG rs11887534 had higher risks of biliary stones and gallbladder cancer, although the association was statistically significant only for women and gallbladder cancer (OR = 6.3, 95% CI 1.86-22.3). Carriers of the ABCG8 haplotype C-C (rs4148217-rs11887534) had a 4.16-fold (95% CI 1.71-10.1) risk of gallbladder cancer compared with those carrying the C-G haplotype. Our findings suggest that ABCG8 rs11887534, identified as a gallstone risk single-nucleotide polymorphism by whole genome scan, is also associated with an increased risk of biliary tract cancer.

Role of ABCG8 D19H (rs11887534) variant in gallstone susceptibility in northern India

BACKGROUND AND AIM

The excretion of cholesterol from the liver is regulated by the ATP-binding cassette transporter ABCG8. A common genetic polymorphism D19H of ABCG8 might be related to the genetic predisposition of gallstone disease, which is causatively related to supersaturation of cholesterol in bile. We aimed to examine the role of the ABCG8 D19H (rs11887534) polymorphism in susceptibility to gallstone disease in the northern Indian population.

METHODS

The study included 220 confirmed gallstone patients and 230 controls. Genotyping for the ABCG8 D19H polymorphism was carried out using the PCR-RFLP method.

RESULTS

We observed that the ABCG8 DH genotype frequency was significantly higher in gallstone patients (P = 0.038; odds ratio [OR] = 2.20; 95% confidence interval [CI] = 1.1-4.6). At allele level also, the ABCG8 variant allele conferred an increased risk for gallstone susceptibility (P = 0.043; OR = 2.12; 95% CI = 1.2-4.3). The risk as a result of ABCG8 D19H variation was more pronounced in female gallstone patients at genotype (P = 0.026; OR = 3.01, 95% CI = 1.1-7.9) as well as allele level (P = 0.030; OR = 2.85; 95% CI = 1.1-7.3). However, the molecular modeling results of the rs11887534 polymorphism showed that the overall configuration of both wild-type and polymorphic ABCG8 protein were similar, with negligible deviation at the site of polymorphism.

CONCLUSION

  Carriers of the DH genotype and H allele of the ABCG8 D19H polymorphism harbor a higher risk for gallstone susceptibility in the northern Indian population.

The effects of ABCG5/G8 polymorphisms on HDL-cholesterol concentrations depend on ABCA1 genetic variants in the Boston Puerto Rican Health Study

BACKGROUND AND AIMS

ATP-binding cassette transporters G5/G8 (ABCG5/G8) are associated with HDL-C concentrations. To assess whether the effect of ABCG5/G8 genetic variants on HDL-C concentrations is dependent on ATP-binding cassette transporters A1 (ABCA1), we studied potential interactions between single nucleotide polymorphisms (SNPs) at ABCG5/G8 (i7892T > C, 5U145A > C, T54CA > G, T400KC > A) and ABCA1 (i27943G > A, i48168G > A, K219RG > A, i125970G > C, 3U8995A > G) genes with HDL-C concentrations.

METHODS AND RESULTS

ABCG5/G8 and ABCA1 SNPs were genotyped in 788 subjects (228 men and 560 women) who participated in the Boston Puerto Rican Health Study. Biochemical measurements were determined by standard procedures. Genotyping was performed using TaqMan assays according to routine laboratory protocols. Significant gene-gene interactions for HDL-C were found between ABCG8 (5U145A > C, T54CA > G, T400KC > A) SNPs and ABCA1_i48168G > A genetic variant (P = 0.009, P = 0.042 and P = 0.036, respectively), in which carriers of the 5U145C and 54C alleles, and homozygotes for the T400 allele at ABCG8 genetic variants displayed lower HDL-C concentrations than homozygotes for the 5U145A and T54 alleles, and heterozygotes for the 400K allele at ABCG8 SNPs, only if they were also homozygous for the minor allele (A) at the aforementioned ABCA1 SNP.

CONCLUSIONS

The gene-gene interactions reported in the present study support the hypothesis that the effect of ABCG5/G8 genetic variants on HDL-C concentrations is dependent on ABCA1 expression. Replication of these analyses to further populations, particularly with low HDL-C, is clearly warranted.

Frequencies of four ATP-binding cassette transporter G8 polymorphisms in patients with ischemic vascular diseases

ATP-binding cassette transporter G8 (ABCG8) was found to participate in plant sterol and cholesterol (CHOL) transport; however, the potential associations of ABCG8 genetic variants and ischemic vascular diseases are largely unknown. Determinations of allele frequencies of four common ABCG8 polymorphisms (D19H, Y54C, T400K, and A632V) were carried out in 241 unrelated patients with ischemic stroke, 148 patients with coronary heart disease, and 191 blood donors (controls). Allele frequencies of the investigated polymorphisms in patient groups showed no significant differences compared with controls. There was a tendency toward reduced 54YY-genotype frequency among male patients with stroke. On stratification by age at disease onset, male patients with stroke under the age of 50 (n = 62) showed significantly reduced 54YY-frequency compared with male controls (n = 92; 24.2% vs. 41.3%; odds ratio: 0.45 [95% confidence intervals: 0.22-0.93]; p = 0.038). No such associations were found among women. In healthy controls, CHOL levels of individuals with the 54YY genotype (n = 71; median: 4.51 mM, 25th-75th percentiles: 4.19-5.43) were significantly reduced compared with 54YC and 54CC individuals combined (n = 120; median: 4.95 mM, 25th-75th percentiles: 4.42-5.88, p = 0.009). Further, we identified a new ABCG8-variant, T401S, in a control subject. In conclusion, ABCG8 54YY-genotype may be a potential protecting factor against ischemic stroke in young men and may influence plasma CHOL levels.

ABCG5/G8 polymorphisms and markers of cholesterol metabolism: systematic review and meta-analysis

Genetic variation at the ABCG5/G8 locus has been associated with markers of cholesterol homeostasis. As data originate from small-scale studies, we performed a meta-analysis to study these associations in a large dataset. We first investigated associations between five common ABCG5/G8 polymorphisms (p.Q604E, p.D19H, p.Y54C, p.T400K, and p.A632V) and plasma sterol levels in 245 hypercholesterolaemic individuals. No significant associations were found. Subsequently, our data were pooled into a meta-analysis that comprised 3,364 subjects from 16 studies (weighted mean age, 46.7 ± 10.5 years; BMI, 23.9 ± 3.5 kg/m(2)). Presence of the minor 632V allele correlated with reduced LDL-C concentrations (n = 367) compared with homozygosity for the 632A variant [n = 614; -0.11 mmol/l (95% CI, range: -0.20 to -0.02 mmol/l); P = 0.01]. The remaining polymorphisms were not associated with plasma lipid levels. Carriers of the 19H allele exhibited lower campesterol/TC (n = 83; P < 0.001), sitosterol/TC (P < 0.00001), and cholestanol/TC (P < 0.00001), and increased lathosterol/TC ratios (P = 0.001) compared with homozygous 19D allele carriers (n = 591). The ABCG8 632V variant was associated with a clinically irrelevant LDL-C reduction, whereas the 19H allele correlated with decreased cholesterol absorption and increased synthesis without affecting the lipid profile. Hence, associations between frequently studied missense ABCG5/G8 polymorphisms and markers of cholesterol homeostasis are modest at best.

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.

Hepatic cholesterol transporter ABCG8 polymorphisms in gallstone disease in an Indian population

BACKGROUND AND AIM

Gallstone formation is characterized by the abnormal regulation of cholesterol trafficking and solubilization. The prevalence of gallstone disease (GSD) differs between ethnic groups sharing the common environment. These differences can be explained by a genetic predisposition to gallstone formation. Studies have identified single nucleotide polymorphisms (SNP) D19H and T400K in the cholesterol transporter gene ATP-binding cassette, subfamily G, member 8 (ABCG8) in patients with cholesterol gallstones. The aim of this study was to analyze the relationship between D19H and T400K polymorphisms in the ABCG8 gene and GSD in an Indian population, and the effects of these polymorphisms on cholesterol levels in sera and bile.

METHODS

A total of 226 patients with GSD were analyzed for their lipid profile in plasma and bile. A total of 289 controls were recruited, and their plasma lipid profile was analyzed by standard protocols. The genotype of SNP D19H and T400K of ABCG8 was analyzed in 226 patients and 222 control samples. SNP D19H was analyzed by direct sequencing, and SNP T400K genotyping was assayed by the amplification refractory mutation system-polymerase chain reaction.

RESULTS

There was no significant difference in the allelic distribution of SNP T400K between the GSD and gallstone-free groups (P > 0.05), but the distribution of the SNP variant, D19H, was significantly higher (P = 0.017, odds ratio = 2.274) in patients compared to controls. The analysis of serum and bile cholesterol followed a strong association with genotypes.

CONCLUSION

SNP D19H, but not SNP T400K, in the ABCG8 gene is significantly associated with GSD in an Indian population.

Pathogenesis of cholesterol gallstone disease

Cholesterol gallstone disease is a common disorder with an incidence rate of about 10%. In recent years, the research on metabolic disorders involving the gut-liver axis and the relation between gallstone disease and metabolic syndrome is attracting more attention. This article focuses on the origin of biliary cholesterol, intestinal cholesterol absorption, regulation of gallbladder motility, cholesterol transporters and metabolic syndromes and their role in the pathogenesis of gallstone disease.

Single nucleotide polymorphism in the ABCG8 transporter gene is associated with gallbladder cancer susceptibility

BACKGROUND

Gallbladder cancer (GBC) usually arises against the background of gallstone disease, which may be causatively related to supersaturation of cholesterol in bile. An imbalance in cholesterol homeostasis because of oversecretion of cholesterol in the gallbladder promotes gallstone formation. The excretion of cholesterol from the liver is regulated by adenosine triphosphate-binding cassette transporter ABCG8. A common genetic polymorphism D19H of ABCG8 associated with gallstone disease may be causatively related to the genetic predisposition of GBC.

AIM

We aimed to examine the role of ABCG8 D19H (rs11887534) polymorphism in susceptibility to GBC.

METHODOLOGY

This study included 171 confirmed GBC patients and 221 controls. Genotyping for the ABCG8 D19H polymorphism was performed by the polymerase chain reaction-restriction fragment length polymorphism method.

RESULTS

We observed that in our population the ABCG8 DH genotype frequency was significantly higher in GBC patients [P=0.011; odds ratio (OR)=1.79; 95% confidence interval (CI)=1.1-2.8]. Also, at the allele level, ABCG8H conferred an increased risk for GBC (P=0.023; OR=1.60; 95% CI=1.0-2.4). The risk was more pronounced in GBC patients with gallstones (P=0.027; OR=1.85; 95% CI=1.0-3.1), and in patients with an early onset of the disease (P=0.013; OR=2.55, 95% CI=1.2-5.3). However, there was no modulation of GBC risk because of the ABCG8 polymorphism in a gender-specific manner.

CONCLUSION

The results suggest that the DH genotype and the H allele of the ABCG8 D19H polymorphism are associated with GBC susceptibility. The GBC patients with gallstone disease harbouring the ABCG8 variant allele are at a higher risk, while the effect of this polymorphism on GBC patients without gallstones appears to be small.

Determinants of transhepatic cholesterol flux and their relevance for gallstone formation

Cholesterol available for bile secretion is controlled by a wide variety of proteins that mediate lipoprotein cholesterol uptake and cholesterol transport and metabolism in the liver. From a disease perspective, abnormalities in the transhepatic traffic of cholesterol from plasma into the bile may influence the risk of cholesterol gallstone formation. This review summarizes some recent progress in understanding the hepatic determinants of biliary cholesterol secretion and its potential pathogenic implications in cholesterol gallstone disease. This information together with new discoveries in this field may lead to improved risk evaluation, novel surrogate markers and earlier diagnosis, better preventive approaches and more effective pharmacological therapies for this prevalent human disease.

Single nucleotide polymorphisms in ABCG5 and ABCG8 genes in Chilean subjects with polygenic hypercholesterolemia and controls

BACKGROUND

The excretion of cholesterol from liver and intestine is regulated by ATP-binding cassette half-transporters ABCG5 and ABCG8. Polymorphisms in their genes have been related to higher levels of plasma lipids. Thus, the aim of the present study was to evaluate the possible association between the 1950C > G (ABCG5) and the 251A > G (ABCG8) polymorphisms and the presence of hypercholesterolemia (HC) in Chilean subjects.

METHODS

A total of 118 non-related individuals with HC (total cholesterol, TC > or = 6.2 mmol/L) and 104 normolipidemic subjects (controls, TC < or = 5.17 mmol/L), aged 18 to 65 years old, were included in this study. The ABCG5 1950C>G and ABCG8 251A>G genotypes were assessed by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

The genotype distribution of the ABCG5/ABCG8 polymorphisms was in Hardy-Weinberg equilibrium in both groups. The frequency of CC homozygous genotype for ABCG5 1950C > G polymorphism was higher in HC patients than in controls (42% vs. 10%, p < 0.001). On the other hand, no significant differences for the 251A > G polymorphism of the ABCG8 gene were observed (p=NS). Nevertheless, HC subjects carrying the GG genotype for the 251A > G variant exhibited higher values of high-density lipoprotein cholesterol when compared to other genotypes (p=0.020).

CONCLUSIONS

Our data showed that the ABCG5 1950C > G polymorphism is associated with HC in the studied population. Nevertheless, this study is limited by its sample size.

Increased NPC1L1 and ACAT2 expression in the jejunal mucosa from Chinese gallstone patients

The incidence of cholesterol gallstones is a very common disease. The aim of this study is to probe for underlying intestinal molecular defects associated with development of gallstones. Twelve Chinese patients with cholesterol gallstone disease (GS) and 31 gallstone-free (GSF) patients were investigated. Quantitation of mRNA levels for individual genes in mucosal biopsies from jejunum was carried out with real-time PCR. The frequency of two SNPs in the ABCG8 gene (Y54C and T400K) was determined by allelic discrimination. The intestinal mRNA expression of NPC1L1 and ACAT2 were significantly higher in GS than GSF (P<0.05). No differences were observed concerning the levels for plasma lipids, plant sterols and 7alpha-hydroxy-4-cholesten-3-one between GS and GSF. No correlations were observed between patients carrying the different genotypes for Y54C or T400K and their mRNA levels for ABCG5 or ABCG8. The increased NPC1L1 and ACAT2 mRNA levels in gallstone patients might indicate an upregulated absorption and esterification of cholesterol in the small intestine.

Cholesterol and non-cholesterol sterol transporters: ABCG5, ABCG8 and NPC1L1: a review

1. Whole-body sterol (cholesterol and xenosterol) balance is delicately regulated by the gastrointestinal tract and liver, which control sterol absorption and excretion, respectively, in addition to the contribution to the cholesterol pool by whole-body cholesterol synthesis. In the past ten years enormous strides have been made not only in establishing that specific transporters mediate the entry and exit of sterols and how these may regulate selective sterol access to the body pools, but also in how these pathways operate to integrate these physiological pathways. 2. The entry of sterols from the gastrointestinal and biliary canalicular lumen into the body is mediated by NPC1L1, which was discovered by a novel method, via a genomics-bioinformatics approach. 3. Identification of the genetic basis responsible for causing sitosterolaemia, characterized by plant sterol accumulation, led to the identification of two half-transporters (ABCG5 and ABCG8) that normally efflux plant sterols (and cholesterol) into the intestinal and biliary lumen for faecal excretion. 4. The objective of this review is to provide up-to-date knowledge on genomics, proteomics and function of these two transporter systems.

The effects of ABCG5/G8 polymorphisms on plasma HDL cholesterol concentrations depend on smoking habit in the Boston Puerto Rican Health Study

Low HDL-cholesterol (HDL-C) is associated with an increased risk for atherosclerosis, and concentrations are modulated by genetic factors and environmental factors such as smoking. Our objective was to assess whether the association of common single-nucleotide polymorphisms (SNPs) at ABCG5/G8 (i18429G>A, i7892T>C, Gln604GluC>G, 5U145A>C, Tyr54CysA>G, Asp19HisG>C, i14222A>G, and Thr400LysC>A) genes with HDL-C differs according to smoking habit. ABCG5/G8 SNPs were genotyped in 845 participants (243 men and 602 women). ABCG5/G8 (i7892T>C, 5U145A>C, Tyr54CysA>G, Thr400LysC>A) SNPs were significantly associated with HDL-C concentrations (P < 0.001-0.013) by which carriers of the minor alleles at the aforementioned polymorphisms and homozygotes for the Thr400 allele displayed lower HDL-C. A significant gene-smoking interaction was found, in which carriers of the minor alleles at ABCG5/G8 (Gln604GluC>G, Asp19HisG>C, i14222A>G) SNPs displayed lower concentrations of HDL-C only if they were smokers (P = 0.001-0.025). Also, for ABCG8_Thr400LysC>A SNP, smokers, but not nonsmokers, homozygous for the Thr400 allele displayed lower HDL-C (P = 0.004). Further analyses supported a significant haplotype global effect on lowering HDL-C (P = 0.002) among smokers. In conclusion, ABCG5/G8 genetic variants modulate HDL-C concentrations, leading to an HDL-C-lowering effect and thereby a potential increased risk for atherosclerosis only in smokers.

The synthesis of MP-CDCA conjugates and dissolution kinetics of model cholesterol gallstones

The comb-like copolymers of polycarboxylic acid were synthesized and then reacted with chenodeoxycholic acid (CDCA) to obtain a series of conjugates, MPn-CDCA, where n is the number of the groups of oxyethylene in each graft chain. This was confirmed by infrared spectroscopy and thin-layer chromatography. We investigated the effects of dissolving model cholesterol gallstones with the MPn-CDCA conjugates in phosphate-buffered saline at pH 7.4. The dissolution rates of CDCA, MP40-CDCA, MP30-CDCA, MP20-CDCA and MP10-CDCA were 5.33, 5.717, 17.59, 6.868 and 9.615x10(-7)kgm(-2)s(-1), micellar solubilities were 0.2431, 3.095, 12.972, 5.248 and 5.790kgm(-3) and total resistances were 5.33, 5.717, 17.59, 6.868 and 9.615x10(-7)kgm(-2)s(-1), respectively. These studies suggested that the interfacial resistance was the dominant rate-determining factor in dissolving model cholesterol gallstones. Model cholesterol gallstones could be more effectively dissolved by increasing the steric interactive potential energy of side chains and ensuring that the hydrophilic-lipophilic properties of MP-CDCA are within an appropriate range. The micellar dissolution rates of model cholesterol gallstones by MP20-CDCA were significantly faster than by the other conjugates.

Polymorphisms in ABCG5/G8 transporters linked to hypercholesterolemia and gallstone disease

ATP-binding cassette (ABC) transporters function in the homeostasis of lipids. Dysfunction of ABC transporters is frequently associated with disease. This review examines links between polymorphisms of ABC G5 (ABCG5) and G8 (ABCG8) transporter genes to hypercholesterolemia and to gallstone disease risk. Various polymorphisms (A632V, T400K, D19H, M429V, and C54Y) in the ABCG8 and ABCG5 (Q604E) gene have been found to be associated with several facets of cholesterol metabolism, including baseline cholesterol level, cholesterol kinetics, individual responsiveness of plasma cholesterol to dietary and pharmaceutical interventions for hypercholesterolemia, and increased risk of gallstones. Clearly, the ABCG5 and ABCG8 genes play an important role in cholesterol homeostasis. However, more research is needed to establish how specific polymorphisms of these genes confer to higher risk of these diseases.

Significant association of ABCG5 604Q and ABCG8 D19H polymorphisms with gallstone disease

BACKGROUND

Adenosine triphosphate-binding cassette (ABC) transporters ABCG5 and ABCG8 are sterol export pumps regulating biliary cholesterol excretion. The formation of gallstones, supersaturated with cholesterol in bile, is determined by genetic and environmental factors. The interaction of susceptible gene polymorphisms with age, sex and body mass index in gallstone disease is unclear.

METHODS

In a cross-sectional study, 979 subjects (880 men and 99 women, mean(s.d.) age 47.7(10.4) years) were recruited from a hospital-based population. Of these, 74 were diagnosed with gallstone disease by abdominal ultrasonography. Five non-synonymous polymorphisms, E604Q (ABCG5), D19H, C54Y, T400K and A632V (ABCG8), were analysed using the TaqMan genotyping assay.

RESULTS

The serum total cholesterol and both low- and high-density lipoprotein cholesterol levels were significantly lower in subjects with gallstones than in those without. 604Q (CC) and D19H (GC) genotypes were significantly associated with gallstone disease, even when adjusted for age, sex and body mass index. The genetic risk of developing gallstone disease was further stratified by age. The risk was greatly increased in subjects younger than 50 years with the D19H genotype and those of 50 years or more with the 604Q genotype.

CONCLUSION

Carriers of ABCG5 604Q or ABCG8 D19H polymorphisms have an increased risk of gallstone disease independent of age, sex and body mass index.

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.