Henning Wittenburg

FXR and ABCG5/ABCG8 as determinants of cholesterol gallstone formation from quantitative trait locus mapping in mice

BACKGROUND & AIMS Cholesterol gallstone formation is a complex genetic trait. To identify additional cholesterol gallstone susceptibility loci, we performed a quantitative trait locus analysis using an intercross of PERA/Ei and I/LnJ inbred strains of mice. METHODS Mice of both sexes were examined for gallstone weight and evaluated according to a scoring system for the physical chemistry of cholelithiasis during feeding of a lithogenic diet. Intercross offspring were genotyped, and linkage analysis was performed by interval mapping. Differences in messenger RNA expression of positional candidate genes were determined using reverse-transcription and real-time polymerase chain reaction. RESULTS We identified significant loci associated with gallstone weight on chromosomes 10 and 4, named Lith7 and Lith8, respectively (both susceptibility alleles conferred by strain I/LnJ). Positional candidate genes with higher expression in I/LnJ mice are Fxr (official symbol, Nr1h4), encoding the nuclear bile salt receptor, on chromosome 10 and Shp1 (official symbol, Nr0b2), encoding the small heterodimer partner 1, on chromosome 4. A significant locus associated with gallstone score on chromosome 17, named Lith9 (susceptibility allele conferred by strain PERA/Ei), colocalizes with the genes Abcg5 and Abcg8 that encode the canalicular cholesterol transporter. Higher hepatic messenger RNA expression of Abcg5 and Abcg8 in strain PERA/Ei correlates positively with higher biliary cholesterol levels. CONCLUSIONS Our findings suggest a primary role of the nuclear bile salt receptor FXR and the canalicular cholesterol transporter ABCG5/ABCG8 in the genetic susceptibility and pathogenesis of cholesterol cholelithiasis in these strains of inbred mice.

Biliary cholesterol secretion by the twinned sterol half-transporters ABCG5 and ABCG8

The long wait for a cellular and functional definition of canalicular transporters for biliary cholesterol appears to be over. Articles in this issue of the JCI provide a fascinating account of the cellular itinerary of two murine ATP-binding cassette (ABC) half-transporters from the ribosome to the hepatocyte apical membranes (1) and show that the corresponding human genes, when overexpressed in transgenic mice, can greatly augment cholesterol secretion into bile (2). This field was opened with the genetic solution (3, 4) to the enigma of a disease commonly known as sitosterolemia (5, 6). This autosomal recessive disorder of lipid metabolism is characterized by an accumulation of plant sterols, including sitosterol, in the plasma of patients with this disease and is therefore also known as phytosterolemia. Normally, the intestinal absorption of sitosterol and other unwanted plant sterols is dwarfed by the absorption of cholesterol, and the absorbed phytosterols are cleared from the body by highly efficient secretion into bile (5). However, patients with sitosterolemia display intestinal hyperabsorption of plant sterols and impaired biliary sterol secretion, leading to accumulation of phytosterols in the body (6). The metabolism of cholesterol is altered in the same manner, but to a lesser extent (5), rendering many patients hypercholesterolemic (6) and prone to xanthomas and premature atherosclerosis (5, 6). The dramatic consequences of this rare disease demonstrate the importance of a finely tuned mechanism for restricting the absorption and accumulation of ingested sterols. The underlying pathophysiological defects indicated that unraveling the molecular basis of sitosterolemia might shed light on important principles concerning intestinal sterol absorption as well as cholesterol secretion into bile. Two independent groups (3, 4) identified mutations in sitosterolemia patients in either member of an adjacent pair of genes, ABCG5 and ABCG8, encoding ABC transporters expressed in the liver and intestine (3, 4, 7). Functional ABC transporters consist of 12 membrane-spanning domains and two ATP-binding sites, whereas ABCG5 and ABCG8 are examples of half-transporters containing only six transmembrane domains and one ATP-binding site. Therefore, both teams of investigators (3, 7) suggested that ABCG5 and ABCG8 function as a heterodimer to limit intestinal sterol absorption by effluxing sterols from enterocytes to small intestinal lumen and by promoting hepatic sterol secretion into bile. Still, in the absence of direct evidence, the function of ABCG5 and ABCG8 and their subcellular localization remained hypothetical until now.

A randomised, double-blind trial comparing budesonide formulations and dosages for short-term treatment of eosinophilic oesophagitis

Objective To investigate the efficacy and safety of two different budesonide formulations (effervescent tablet for orodispersible use (BET) and viscous suspension (BVS)) with different daily dosages for short-term treatment of eosinophilic oesophagitis (EoE). Design Adults with active EoE (n=76) randomly received 14 days’ treatment with either BET 2×1 mg/day (BET1, n=19) or BET 2×2 mg/day (BET2, n=19), or BVS 2×5 mL (0.4 mg/mL)/day (BVS, n=19) or placebo (n=19) in a double-blind, double-dummy fashion, with a 2-week follow-up. Primary end point was histological remission (mean of <16 eosinophils/mm2 hpf). Secondary end points included endoscopy score, dysphagia score, drug safety and patients preference for drug formulation. Results Histological remission occurred in 100%, 94.7% and 94.7% of budesonide (BET1, BET2, BVS, respectively) and in 0% of placebo recipients (p<0.0001). The improvement in total endoscopic intensity score was significantly higher in the three budesonide groups compared with placebo. Dysphagia improved in all groups at the end of treatment; however, improvement of dysphagia persisted only in those treated with BET1 (p=0.0196 vs placebo). There were no serious adverse events. Local fungal infection (stained fungi) occurred in two patients of each budesonide group (10.5%). The effervescent tablet was preferred by 80% of patients. Conclusions BET or BVS was highly effective and safe for short-term treatment of EoE. The 1 mg (twice daily) dosage was equally effective as the 2 mg twice daily dosage. The majority of patients preferred the effervescent tablet formulation. ClinicalTrials.gov number NCT02280616; EudraCT number, 2009-016692-29.

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 [3H]‐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 (r2 = 0.95). [3H]‐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)

Association of a lithogenic Abcg5/Abcg8 allele on Chromosome 17 (Lith9) with cholesterol gallstone formation in PERA/EiJ mice

To examine further the genetic determinants of cholesterol gallstone susceptibility in inbred mice, we performed quantitative trait locus (QTL) analysis of an intercross of gallstone-susceptible PERA/EiJ and gallstone-resistant DBA/2J inbred mice. Three hundred twenty-four F2 offspring were phenotyped for cholelithiasis during consumption of a lithogenic diet and genotyped using microsatellite markers. Linkage analysis was performed by interval mapping. In addition, we analyzed the combined datasets from this cross and from an independent cross of strain PERA and gallstone-resistant I/Ln mice. QTL mapping detected one significant new gallstone susceptibility (Lith) locus on Chromosome 13 (Lith15). A second significant QTL on Chr 6 (Lith16) confirmed a previous QTL. Furthermore, suggestive QTLs confirmed Lith loci from previous crosses on Chromosomes 1, 2, 5, 16 and X. QTL analysis of the dataset derived from the combined crosses increased the detection power and narrowed confidence intervals of Lith loci on Chromosomes 2, 6, 13, and 16. Moreover, the analysis of combined datasets revealed a shared QTL between both crosses on Chromosome 17 (Lith9). Significantly higher mRNA expression of Abcg5 and Abcg8 in strain PERA compared with strains I/Ln and DBA/2 further substantiated that the PERA allele of Abcg5/Abcg8 was responsible for lithogenicity underlying Lith9.

A Variant of the SLC10A2 Gene Encoding the Apical Sodium-Dependent Bile Acid Transporter Is a Risk Factor for Gallstone Disease

Background Cholelithiasis is a multifactorial process and several mechanisms of gallstone formation have been postulated. As one of these mechanisms, a decreased expression of the ileal apical sodium-dependent bile acid transporter gene SLC10A2 in gallstone carriers was described previously. In this study the SLC10A2 gene was investigated to identify novel genetic variants and their association with gallstone formation. Methodology/Principal Findings Study subjects were selected with the presence or absence of gallstones confirmed by ultrasound and medical history. Genomic DNA was obtained from blood leukocytes. Sequence analysis was performed of all six exonic and flanking regions as well as of 2,400 base pairs of the SLC10A2 promoter in a cohort of gallstone carriers and control subjects from Stuttgart, Germany. Genotype frequencies of newly identified genetic variants (n = 6) and known single nucleotide polymorphisms (n = 24) were established using MALDI-TOF mass spectrometry. Six new genetic variants were found within the SLC10A2 gene. Although none of the variants was linked to gallstone disease in the Stuttgart cohort overall, the minor allele of SNP rs9514089 was more prevalent in male non-obese gallstone carriers (p = 0.06680, OR = 11.00). In a separate population from Aachen, Germany, the occurrence of rs9514089 was two-fold higher in gallstone patients (22%) than in corresponding controls (11%) (p = 0.00995, OR = 2.19). In the pooled Aachen/Stuttgart cohort rs9514089 was highly significantly linked to cholelithiasis (p = 0.00767, OR = 2.04). A more frequent occurrence was observed for male gallstone carriers (22%) compared to controls (9%) (p = 0.01017, OR = 2.99), for the total normal weight group (p = 0.00754, OR = 2.90), and for male non-obese gallstone patients (p = 0.01410, OR = 6.85). Moreover, for the minor allele of rs9514089 an association with low plasma cholesterol levels was found especially in gallstone carriers (p = 0.05). Conclusions/Significance We have identified SLC10A2 as a novel susceptibility gene for cholelithiasis in humans. Comprehensive statistical analysis provides strong evidence that rs9514089 is a genetic determinant especially in male non-obese gallstone carriers. The minor allele of rs9514089 is related to differences in plasma cholesterol levels among the subjects.

Hereditary liver disease: gallstones.

Gallstones are common in Western countries and due to pain and complications pose a substantial burden on health care systems. In general, cholesterol gallstones are distinguished from bilirubin gallstones. Bilirubin gallstones form if the ion product of unconjugated bilirubin and calcium in gallbladder bile exceeds the solubilisation capacities of mixed micelles and vesicles. Cholesterol gallstones develop if the amount of cholesterol in gallbladder bile exceeds the maximum concentration that is soluble at the given concentration of bile salts and phospholipids. In addition, cholesterol gallstone formation requires hypomotility of the gallbladder and a mucin gel as nucleation matrix for monohydrate crystals. The individual risk of gallstone formation is determined by interactions of lithogenic alleles of gallstone susceptibility genes and multiple environmental factors. For asymptomatic gallstones, expectant management is recommended, whereas an episode of gallstone-associated pain substantially increases the risk of complications such as cholecystitis, cholangitis and pancreatitis and therefore necessitates cholecystectomy.

Lith6 a new QTL for cholesterol gallstones from an intercross of CAST/Ei and DBA/2J inbred mouse strains,,

A complex genetic basis determines the individual predisposition to develop cholesterol gallstones in response to environmental factors. We employed quantitative trait locus/loci (QTL) analyses of an intercross between inbred strains CAST/Ei (susceptible) and DBA/2J (resistant) to determine the subset of gallstone susceptibility (Lith) genes these strains possess. Parental and first filial generation mice of both genders and male intercross offspring were evaluated for gallstone formation after feeding a lithogenic diet. Linkage analysis was performed using a form of multiple interval mapping. One significant QTL colocalized with Lith1 [chromosome (chr) 2, 50 cM], a locus identified previously. Significantly, new QTL were detected and named Lith10 (chr 6, 4 cM), Lith6 (chr 6, 54 cM), and Lith11 (chr 8, 58 cM). Statistical and genetic analyses suggest that Lith6 comprises two QTL in close proximity. Our molecular and genetic data support the candidacy of peroxisome proliferator-activated receptor γ (Pparg) and Slc21a1, encoding Pparg, and the basolateral bile acid transporter SLC21A1 (Slc21a1/Oatp1), respectively, as genes underlying Lith6.

QTL mapping for genetic determinants of lipoprotein cholesterol levels in combined crosses of inbred mouse strains.

To identify additional loci that influence lipoprotein cholesterol levels, we performed quantitative trait locus (QTL) mapping in offspring of PERA/EiJ×I/LnJ and PERA/EiJ×DBA/2J intercrosses and in a combined data set from both crosses after 8 weeks of consumption of a high fat-diet. Most QTLs identified were concordant with homologous chromosomal regions that were associated with lipoprotein levels in human studies. We detected significant new loci for HDL cholesterol levels on chromosome (Chr) 5 (Hdlq34) and for non-HDL cholesterol levels on Chrs 15 (Nhdlq9) and 16 (Nhdlq10). In addition, the analysis of combined data sets identified a QTL for HDL cholesterol on Chr 17 that was shared between both crosses; lower HDL cholesterol levels were conferred by strain PERA. This QTL colocalized with a shared QTL for cholesterol gallstone formation detected in the same crosses. Haplotype analysis narrowed this QTL, and sequencing of the candidate genes Abcg5 and Abcg8 confirmed shared alleles in strains I/LnJ and DBA/2J that differed from the alleles in strain PERA/EiJ. In conclusion, our analysis furthers the knowledge of genetic determinants of lipoprotein cholesterol levels in inbred mice and substantiates the hypothesis that polymorphisms of Abcg5/Abcg8 contribute to individual variation in both plasma HDL cholesterol levels and susceptibility to cholesterol gallstone formation.

A common variant of PNPLA3 (p.I148M) is not associated with alcoholic chronic pancreatitis.

Background Chronic pancreatitis (CP) is an inflammatory disease that in some patients leads to exocrine and endocrine dysfunction. In industrialized countries the most common aetiology is chronic alcohol abuse. Descriptions of associated genetic alterations in alcoholic CP are rare. However, a common PNPLA3 variant (p.I148M) is associated with the development of alcoholic liver cirrhosis (ALC). Since, alcoholic CP and ALC share the same aetiology PNPLA3 variant (p.I148M) possibly influences the development of alcoholic CP. Methods Using melting curve analysis we genotyped the variant in 1510 patients with pancreatitis or liver disease (961 German and Dutch alcoholic CP patients, 414 German patients with idiopathic or hereditary CP, and 135 patients with ALC). In addition, we included in total 2781 healthy controls in the study. Results The previously published overrepresentation of GG-genotype was replicated in our cohort of ALC (p-value <0.0001, OR 2.3, 95% CI 1.6–3.3). Distributions of genotype and allele frequencies of the p.I148M variant were comparable in patients with alcoholic CP, idiopathic and hereditary CP and in healthy controls. Conclusions The absence of an association of PNPLA3 p.I148M with alcoholic CP seems not to point to a common pathway in the development of alcoholic CP and alcoholic liver cirrhosis.