Abstract
AIM: To investigate the contribution of polymorphisms in nuclear receptors to risk of inflammatory bowel disease (IBD).
METHODS: Genotypes of nuclear factor (NF)-κB (NFKB1) NFκB -94ins/del (rs28362491); peroxisome proliferator-activated receptor (PPAR)-γ (PPARγ) PPARγ Pro12Ala (rs 1801282) and C1431T (rs 3856806); pregnane X receptor (PXR) (NR1I2) PXR A-24381C (rs1523127), C8055T (2276707), and A7635G (rs 6785049); and liver X receptor (LXR) (NR1H2) LXR T-rs1405655-C and T-rs2695121-C were assessed in a Danish case-control study of 327 Crohn’s disease patients, 495 ulcerative colitis (UC) patients, and 779 healthy controls. Odds ratio (OR) and 95% CI were estimated by logistic regression models.
RESULTS: The PXR A7635G variant, the PPARγ Pro12Ala and LXR T-rs2695121-C homozygous variant genotypes were associated with risk of UC (OR: 1.31, 95% CI: 1.03-1.66, P = 0.03, OR: 2.30, 95% CI: 1.04-5.08, P = 0.04, and OR: 1.41, 95% CI: 1.00-1.98, P = 0.05, respectively) compared to the corresponding homozygous wild-type genotypes. Among never smokers, PXR A7635G and the LXR T-rs1405655-C and T-rs2695121-C variant genotypes were associated with risk of IBD (OR: 1.41, 95% CI: 1.05-1.91, P = 0.02, OR: 1.63, 95% CI: 1.21-2.20, P = 0.001, and OR: 2.02, 95% CI: 1.36-2.99, P = 0.0005, respectively) compared to the respective homozygous variant genotypes. PXR A7635G (rs6785049) variant genotype was associated with a higher risk of UC diagnosis before the age of 40 years and with a higher risk of extensive disease (OR: 1.34, 95% CI: 1.03-1.75 and OR: 2.49, 95% CI: 1.24-5.03, respectively).
CONCLUSION: Common PXR and LXR polymorphisms may contribute to risk of IBD, especially among never smokers.
Keywords: Crohn’s disease, Genetic susceptibility, Single nucleotide polymorphisms, Smoking status, Transcription factors, Ulcerative colitis
INTRODUCTION
Chronic inflammatory bowel diseases (IBDs), ulcerative colitis (UC), and Crohn’s disease (CD) are complex diseases that result from the interaction of numerous genetic and environmental factors[1,2]. Recent studies have increased dramatically the number of genes known to be involved in IBD[3-7]. However, the contribution of NOD2 gene polymorphisms to IBD etiology in populations of Northern Europe is relatively small[8-10], which has heightened interest in resolving the genetic determinants of IBD in these countries.
The rising incidence of IBD in the West suggests that environmental factors play a major role in its pathogenesis. The intestinal lumen contains a vast array of different substances that may interact with the host, such as dietary factors, microbial components, and environmental pollutants. Many of these stimuli interact with the transcription factor nuclear factor (NF)-κB via activation of Toll-like receptors (TLRs) such as TLR4[11,12]. Nuclear receptors are intracellular transcription factors that are activated by ligands[13], which constitute a link between environmental factors and the regulation of many cellular processes, including inflammation[14-16]. Thus, genetic variation in certain transcription factors may modify the regulation of relevant environmental factors and the associated risk of IBD.
Activation of NF-κB leads to the induction of pro-inflammatory signal cascades[13,17] and the resolution of intestinal inflammation[18-20]. Studies on animal models of colitis[21,22] and IBD patients[23,24] suggest that impaired NF-κB function leads to IBD. A polymorphism that involves deletion of four nucleotides in the NFκB promoter region, named -94ATTG ins/del, has been associated with attenuated promoter activity in luciferase reporter studies[25]. The variant allele has been investigated as an IBD risk gene, but the results of these studies have been inconsistent[24-31].
Activation of the nuclear receptors peroxisome proliferator-activated receptor (PPAR)γ, pregnane X receptor (PXR), and liver X receptor (LXR) leads to transcriptional regulation of pro-inflammatory target genes[14,32,33] and inhibition of NF-κB activity[15,16,34,35], which results in a decrease in inflammation.
Studies of animal colitis models[36-38] and IBD patients[39] have suggested that impaired PPARγ expression may confer IBD. The PPARγ Pro12Al variant allele is in tight linkage with the PPARγ C1431T variant allele[40], and the Pro to Ala substitution results in decreased transcriptional activation of target genes[41]. Studies on the association of the PPARγ C1431T and Pro12Ala polymorphisms with a risk for IBD have demonstrated varying results[42-44].
Loss of PXR function has been associated with intestinal inflammation in animal studies[15], and low levels of PXR expression have been found in the intestine of UC patients[45]. The PXR A7635G (rs6785049) homozygous variant genotypes and PXR C8055T (rs2276707) variant genotypes have been associated with a pronounced induction of a PXR target gene, CYP3A4, after treatment with rifampin[46]. However, studies of PXR polymorphisms in relation to the risk for IBD have been inconsistent[47-50].
Loss of LXR function compromised innate immunity in an animal model, which was attenuated after LXR administration[14]. The LXR tag polymorphisms in intron 7 rs1405655 and intron 2 rs2695121 have been previously investigated as candidate gene targets involved in Alzheimer’s disease[51-53].
Tobacco smoke is a source of many exogenous compounds and induces inflammation[54]. Moreover, smoking differentially affects the risk of CD and UC[55], and the underlying mechanisms behind these effects are poorly understood[56].
Accordingly, altered responses of NFκB, PPARγ, PXR, and LXR to environmental pathogens may be involved in susceptibility to IBD. Hence, genetic variations in the transcription factors may modify the inflammatory response to environmental stimuli and affect the risk for IBD.
In the present study, we determined the allele and haplotype frequencies of polymorphisms in the genes that encode the transcription factors NFκB (NFKB1) -94ins/del (rs28362491); PPARγ (PPARG) Pro12Ala (rs 1801282) and C1431T (rs 3856806); PXR (NR1I2) A-24381C (rs1523127), C8055T (rs2276707-T), and A7635G (rs 6785049); and LXR-β (NR1H2) T-rs1405655-C and T-rs2695121-C. These polymorphisms were investigated together with the smoking status in a Danish cohort of 327 patients with CD, 495 patients with UC, and 779 healthy controls.
MATERIALS AND METHODS
Ethics
All subjects received written and oral information and provided written informed consent. The study was performed in accordance with the Declaration of Helsinki and was approved by the local Scientific Ethical Committees (VN2003/124).
Patients and controls
Diagnosis of CD or UC was based on clinical, radiological, endoscopic and histological examinations (infectious and other cases of IBD were excluded)[56-58]. Patients were recruited from Viborg, Aalborg, and Herning Regional Hospitals from January 2004 to March 2005. Healthy blood donors recruited from Viborg Hospital served as controls. All subjects were Caucasian and older than 18 years of age. Data on the extent of the disease (CD: L1, L2, L3, UC: E1, E2, E3), family history, surgical treatment, advanced medical treatment, age at diagnosis (under or over 40 years of age), and information on smoking habits at the time of diagnosis (patients) and at study entry (healthy controls) were collected.
Genotyping
Functional single nucleotide polymorphisms (SNPs) were selected based on the literature, except in the case of LXR with tag SNPs selected based on previous disease association[51-53] because there were no available data on the functional effects. DNA was extracted from EDTA-stabilized peripheral blood samples from all patients and healthy controls using either a PureGene (Gentra Systems, Minneapolis, MN, USA) or Wizard Genomic (Promega, Madison, WI, USA) DNA purification kit, according to the manufacturers’ recommendations.
Genotypes were determined by Taqman allelic discrimination (ABI 7500/7900HT, Applied Biosystems). DNA (20 ng) was analyzed in volumes of 4 μL. Samples from cases and sub-cohort members were mixed during genotyping, and laboratory staff were blinded to the case or control status during analysis. Known genotype controls were included in each run. To confirm reproducibility, 10% of the samples were genotyped again. The genotypes exhibited 100 % identity.
NFκB (NFKB1) ATTG ins/del (rs28362491) and PPARγ (PPARG) Pro12Ala were genotyped as previously described ([59] and [60], respectively). PPARγ (PPARG) C1431T[61], PXR (NR1I2) A-24381C (rs1523127), C8055T (rs2276707), and A7635G (rs6785049); and LXR-β (NR1H2) T-rs1405655C and T-rs2695121C were assessed using developed assays (Applied Biosystems).
Statistical analysis
Logistic regression was utilized to analyze the relationship between the investigated polymorphisms and IBD. The statistical analysis included only subjects with all necessary information available. Age was entered linearly in the model after verifying these data using a linear spline[62]. Subgroup analyses were performed on polymorphisms in relation to the extent of the disease (CD: L1, L2, L3, UC: E1, E2, E3), family history, surgical treatment, advanced medical treatment, and age at diagnosis (above or below 40 years of age) for all cases. The haplotypes were inferred manually as described previously[63].
Power analysis
The Genetic Power Calculator for case-control was utilized for power analysis of discrete traits[64]. This study had greater than 80% power to detect a dominant effect with an odds ratio (OR) of 1.5 in either CD or UC, or 1.4 if CD and UC were combined.
RESULTS
Study population description
Characteristics of the Danish IBD patients and controls are shown in Table 1. Current smoking was more common among CD than UC patients, with incidences of 51% and 17%, respectively. The genotype distributions among the controls did not deviate from Hardy-Weinberg equilibrium. The variant allele frequencies of the studied polymorphisms are shown in Table 2.
Table 1.
Description of study participants n (%)
| CD (n = 327) | UC (n = 495) | Controls (n = 779) | |
| Sex | |||
| Male | 129 (39) | 239 (48) | 400 (51) |
| Female | 198 (61) | 256 (51) | 379 (49) |
| Age (yr) | |||
| Median (5%-95%) | 43 (23-76) | 49 (24-76) | 43 (23-60) |
| Age at diagnosis (yr) | |||
| Median (5%-95%) | 30 (15-64) | 35 (17-68) | |
| Smoking habits | |||
| Smokers | 167 (51) | 86 (17) | 205 (26) |
| Never smokers | 115 (35) | 226 (46) | 391 (50) |
| Former smokers | 45 (14) | 183 (37) | 183 (23) |
| Location of UC | |||
| Proctitis (E1) | 207 (42) | ||
| Left side (E2) | 183 (37) | ||
| Extensive (E3) | 93 (19) | ||
| Data not available | 12 (2) | ||
| Location of CD | |||
| Colonic (L2) | 151 (46) | ||
| Ileal (L1) | 74 (23) | ||
| Ileocolonic (L3) | 89 (27) | ||
| Data not available | 13 (4) | ||
| Medication | |||
| Advanced1 | 140 (43) | 103 (21) | |
| No advanced medication2 | 182 (56) | 389 (79) | |
| Data not available | 5 (2) | 3 (1) | |
| Operation | |||
| Yes | 149 (46) | 14 (3) | |
| No | 171 (52) | 472 (95) | |
| Data not available | 7 (2) | 9 (2) |
Disease location was classified according to the WGO Montreal classification. Statistical analyses included subjects for whom all information was available.
Azathioprine, 6-mercaptopurine, tumor necrosis factor inhibitors, or methrotrexate;
5-aminosalicylic acid, prednisolone. CD: Crohn’s disease; UC: Ulcerative colitis.
Table 2.
Allele frequencies for the gene polymorphisms in Crohn’s disease and ulcerative colitis patients n (%)
| CD | UC | Controls | |
| NF-κB -94ins/del | |||
| I | 379 (58) | 583 (59) | 919 (59) |
| D | 275 (42) | 407 (41) | 639 (41) |
| PPARγ Pro12Ala | |||
| C | 564 (86) | 844 (85) | 1315 (84) |
| G | 90 (14) | 146 (15) | 243 (16) |
| PPARγ C1431T | |||
| C | 560 (86) | 832 (84) | 1327 (85) |
| T | 94 (14) | 158 (16) | 231 (15) |
| PXR rs1523127 | |||
| A | 395 (60) | 570 (58) | 926 (59) |
| C | 259 (40) | 420 (42) | 632 (41) |
| PXR rs2276707 | |||
| C | 540 (83) | 825 (83) | 1275 (82) |
| T | 114 (17) | 165 (17) | 283 (18) |
| PXR rs6785049 | |||
| A | 426 (65) | 615 (62) | 1011 (65) |
| G | 228 (35) | 375 (38) | 547 (35) |
| LXR rs1405655 | |||
| T | 435 (67) | 675 (68) | 1079 (69) |
| C | 219 (33) | 315 (32) | 479 (31) |
| LXR rs2695121 | |||
| T | 292 (45) | 430 (43) | 727 (47) |
| C | 362 (55) | 560 (57) | 831 (53) |
NF-κB: Nuclear factor κB; PPARγ: Peroxisome proliferator-activated receptor γ; CD: Crohn’s disease; UC: Ulcerative colitis; PXR: Pregnane X receptor; LXR: Liver X receptor.
Associations between polymorphisms and disease phenotypes
The association between genotypes and the disease risk was analyzed separately for CD and UC (Table 3). The PXR A7635G (rs6785049) variant genotypes, PPARγ Pro12Ala homozygous variant, and LXR T-rs2695121-C homozygous genotypes were associated with a higher risk of UC, as compared to the homozygous wild-type genotype (OR: 1.31, 95% CI: 1.03-1.66, P = 0.03, OR: 2.30, 95% CI: 1.04-5.08, P = 0.04, and OR: 2.41, 95% CI: 1.00-1.98, P = 0.05, respectively). No association was found between risk of CD and any genotype. Furthermore, no association was found between NFκB -94 ins/del or PPARγ C1431T polymorphisms and disease risk (Table 3).
Table 3.
Odds ratio for the studied gene polymorphisms in Crohn’s disease and ulcerative colitis patients
| CD | UC | Control | ORCD | 95% CI1 | P value | ORUC | 95% CI1 | P value | |
| NF-κB -94ins/del | |||||||||
| II | 107 | 175 | 267 | 1.00 | - | 1.00 | - | ||
| ID | 165 | 233 | 385 | 1.08 | 0.80-1.46 | 0.62 | 0.94 | 0.72-1.21 | 0.62 |
| DD | 55 | 87 | 127 | 1.21 | 0.81-1.81 | 0.36 | 1.04 | 0.73-1.47 | 0.83 |
| ID and DD | 220 | 320 | 512 | 1.11 | 0.83-1.48 | 0.48 | 0.96 | 0.75-1.23 | 0.76 |
| PPARγ Pro12Ala | |||||||||
| CC | 240 | 364 | 549 | 1.00 | - | 1.00 | - | ||
| CG | 84 | 116 | 217 | 0.88 | 0.65-1.20 | 0.43 | 0.83 | 0.63-1.09 | 0.17 |
| GG | 3 | 15 | 13 | 0.48 | 0.13-1.77 | 0.27 | 2.30 | 1.04-5.08 | 0.04 |
| CG and GG | 87 | 131 | 230 | 0.86 | 0.64-1.16 | 0.33 | 0.90 | 0.69-1.17 | 0.42 |
| PPARγ C1431T | |||||||||
| CC | 241 | 352 | 561 | 1.00 | - | 1.00 | - | ||
| CT | 78 | 128 | 205 | 0.81 | 0.59-1.12 | 0.20 | 1.00 | 0.76-1.31 | 0.99 |
| TT | 8 | 15 | 13 | 1.36 | 0.54-3.42 | 0.52 | 1.95 | 0.90-4.27 | 0.09 |
| CT and TT | 86 | 143 | 218 | 0.85 | 0.62-1.15 | 0.29 | 1.05 | 0.81-1.37 | 0.69 |
| PXR rs1523127 | |||||||||
| AA | 114 | 160 | 280 | 1.00 | - | 1.00 | - | ||
| AC | 167 | 250 | 366 | 1.06 | 0.79-1.43 | 0.71 | 1.15 | 0.89-1.50 | 0.29 |
| CC | 46 | 85 | 133 | 0.89 | 0.59-1.35 | 0.59 | 1.11 | 0.78-1.56 | 0.57 |
| AC and CC | 213 | 335 | 499 | 1.02 | 0.77-1.35 | 0.91 | 1.14 | 0.89-1.46 | 0.30 |
| PXR rs2276707 | |||||||||
| CC | 223 | 339 | 517 | 1.00 | - | 1.00 | - | ||
| CT | 94 | 147 | 241 | 0.92 | 0.68-1.24 | 0.57 | 0.97 | 0.75-1.26 | 0.84 |
| TT | 10 | 9 | 21 | 1.25 | 0.56-2.76 | 0.58 | 0.67 | 0.30-1.51 | 0.33 |
| CT and TT | 104 | 156 | 262 | 0.94 | 0.71-1.26 | 0.69 | 0.95 | 0.74-1.22 | 0.68 |
| PXR rs6785049 | |||||||||
| AA | 137 | 184 | 334 | 1.00 | - | 1.00 | - | ||
| AG | 152 | 247 | 343 | 1.12 | 0.84-1.49 | 0.46 | 1.35 | 1.05-1.74 | 0.02 |
| GG | 38 | 64 | 102 | 0.91 | 0.58-1.40 | 0.66 | 1.18 | 0.81-1.71 | 0.39 |
| AG and GG | 190 | 311 | 445 | 1.07 | 0.81-1.40 | 0.65 | 1.31 | 1.03-1.66 | 0.03 |
| LXR rs1405655 | |||||||||
| TT | 143 | 229 | 383 | 1.00 | - | 1.00 | - | ||
| CT | 149 | 217 | 313 | 1.26 | 0.95-1.68 | 0.11 | 1.22 | 0.95-1.57 | 0.11 |
| CC | 35 | 49 | 83 | 1.12 | 0.71-1.78 | 0.62 | 1.01 | 0.67-1.51 | 0.97 |
| CT and CC | 184 | 266 | 396 | 1.23 | 0.94-1.62 | 0.13 | 1.18 | 0.93-1.49 | 0.17 |
| LXR rs2695121 | |||||||||
| TT | 62 | 88 | 170 | 1.00 | - | 1.00 | - | ||
| CT | 168 | 254 | 387 | 1.28 | 0.90-1.83 | 0.17 | 1.30 | 0.95-1.77 | 0.10 |
| CC | 97 | 153 | 222 | 1.21 | 0.82-1.79 | 0.34 | 1.41 | 1.00-1.98 | 0.05 |
| CT and CC | 265 | 407 | 609 | 1.26 | 0.89-1.76 | 0.19 | 1.34 | 0.99-1.79 | 0.06 |
Statistical analyses included subjects for whom all information was available.
Adjusted for age, sex and smoking status. NF-κB: Nuclear factor κB; PPARγ: Peroxisome proliferator-activated receptor γ; CD: Crohn’s disease; UC: Ulcerative colitis; OR: Odds ratio; PXR: Pregnane X receptor; LXR: Liver X receptor.
Interaction between gene polymorphisms and smoking
The association between genotypes and disease risk was analyzed for current smokers, previous smokers, and never smokers. There was no interaction between smoking status and gene polymorphisms in relation to the risk of CD or UC (data not shown). In general, there was an association between smoking status and the risk of CD and UC. The OR for risk of CD was high among smokers and low among former smokers, regardless of genotype status. In contrast, the OR for UC was high among former smokers and low among current smokers, regardless of genotype.
The ORs for associations between genotypes and the risk of CD, UC and combined IBD among individuals that had never smoked are shown in Table 4. The ORs were analyzed separately for CD and UC and for the combined groups to describe the risk of IBD because there was no heterogeneity between the two groups. The PXR A7635G (rs6785049) and LXR T-rs1405655-C and T-rs2695121-C variant genotypes were associated with a higher risk for IBD, as compared to the homozygous wild-type genotypes (OR: 1.41, 95% CI: 1.05-1.91, P = 0.02 and OR: 1.63, 95% CI: 1.21-2.20, P = 0.001, OR: 2.02, 95% CI: 1.36-2.99, P = 0.0005, respectively).
Table 4.
Odds ratio for the gene polymorphisms among Crohn’s disease and ulcerative colitis never smokers
| NS-CD | NS-UC | NS-control | ORNS-CD | 95% CI1 | P value | ORNS-UC | 95% CI1 | P value | ORNS-IBD | 95% CI1 | P value | |
| NF-κB -94ins/del | ||||||||||||
| II | 40 | 79 | 136 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| ID | 56 | 109 | 194 | 0.99 | 0.62-1.57 | 0.97 | 0.98 | 0.68-1.42 | 0.93 | 0.98 | 0.71-1.36 | 0.92 |
| DD | 19 | 38 | 61 | 1.07 | 0.57-2.00 | 0.83 | 1.09 | 0.67-1.79 | 0.72 | 1.09 | 0.70-1.68 | 0.71 |
| ID and DD | 75 | 147 | 255 | 1.01 | 0.65-1.56 | 0.97 | 1.01 | 0.72-1.43 | 0.95 | 1.01 | 0.74-1.37 | 0.96 |
| PPARγ Pro12Ala | ||||||||||||
| CC | 83 | 167 | 270 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| CG | 31 | 50 | 117 | 0.86 | 0.54-1.38 | 0.54 | 0.71 | 0.48-1.04 | 0.08 | 0.75 | 0.54-1.05 | 0.09 |
| GG | 1 | 9 | 4 | 0.80 | 0.09-7.29 | 0.84 | 3.99 | 1.20-13.32 | 0.02 | 2.77 | 0.85-9.00 | 0.09 |
| CG and GG | 32 | 59 | 121 | 0.86 | 0.54-1.37 | 0.53 | 0.81 | 0.56-1.17 | 0.26 | 0.82 | 0.59-1.13 | 0.21 |
| PPARγ C1431T | ||||||||||||
| CC | 85 | 163 | 285 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| CT | 26 | 56 | 100 | 0.88 | 0.53-1.44 | 0.60 | 0.98 | 0.67-1.44 | 0.93 | 0.93 | 0.67-1.31 | 0.70 |
| TT | 4 | 7 | 6 | 2.16 | 0.59-7.90 | 0.24 | 2.05 | 0.67-6.26 | 0.21 | 2.06 | 0.75-5.67 | 0.16 |
| CT and TT | 30 | 63 | 106 | 0.95 | 0.59-1.53 | 0.83 | 1.04 | 0.72-1.51 | 0.82 | 1.00 | 0.72-1.39 | 0.99 |
| PXR rs1523127 | ||||||||||||
| AA | 43 | 74 | 149 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| AC | 51 | 103 | 176 | 1.01 | 0.64-1.60 | 0.97 | 1.15 | 0.79-1.67 | 0.46 | 1.11 | 0.80-1.53 | 0.54 |
| CC | 21 | 49 | 66 | 1.10 | 0.60-1.99 | 0.77 | 1.52 | 0.95-2.41 | 0.08 | 1.36 | 0.90-2.05 | 0.15 |
| AC and CC | 72 | 152 | 242 | 1.03 | 0.67-1.59 | 0.89 | 1.25 | 0.88-1.77 | 0.21 | 1.17 | 0.87-1.59 | 0.30 |
| PXR rs2276707 | ||||||||||||
| CC | 73 | 150 | 260 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| CT | 36 | 70 | 119 | 1.08 | 0.69-1.70 | 0.74 | 1.03 | 0.72-1.47 | 0.89 | 1.04 | 0.76-1.43 | 0.81 |
| TT | 6 | 6 | 12 | 1.76 | 0.64-4.86 | 0.27 | 0.86 | 0.31-2.34 | 0.76 | 1.16 | 0.51-2.63 | 0.73 |
| CT and TT | 42 | 76 | 131 | 1.14 | 0.74-1.76 | 0.55 | 1.01 | 0.71-1.43 | 0.96 | 1.05 | 0.77-1.43 | 0.75 |
| PXR rs6785049 | ||||||||||||
| AA | 42 | 77 | 168 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| AG | 52 | 119 | 176 | 1.19 | 0.75-1.88 | 0.47 | 1.49 | 1.04-2.13 | 0.03 | 1.38 | 1.01-1.89 | 0.05 |
| GG | 21 | 30 | 47 | 1.79 | 0.97-3.31 | 0.06 | 1.40 | 0.82-2.39 | 0.22 | 1.53 | 0.97-2.43 | 0.07 |
| AG and GG | 73 | 149 | 223 | 1.31 | 0.85-2.02 | 0.21 | 1.47 | 1.04-2.07 | 0.03 | 1.41 | 1.05-1.91 | 0.02 |
| LXR rs1405655 | ||||||||||||
| TT | 43 | 95 | 203 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| CT | 55 | 106 | 154 | 1.69 | 1.07-2.65 | 0.02 | 1.54 | 1.08-2.18 | 0.02 | 1.58 | 1.16-2.16 | 0.004 |
| CC | 17 | 25 | 34 | 2.32 | 1.19-4.55 | 0.01 | 1.66 | 0.93-2.95 | 0.09 | 1.85 | 1.12-3.07 | 0.02 |
| CT and CC | 72 | 131 | 188 | 1.80 | 1.18-2.77 | 0.01 | 1.56 | 1.11-2.17 | 0.01 | 1.63 | 1.21-2.20 | 0.001 |
| LXR rs2695121 | ||||||||||||
| TT | 15 | 30 | 90 | 1.00 | - | 1.00 | - | 1.00 | - | |||
| CT | 59 | 126 | 197 | 1.82 | 0.98-3.39 | 0.06 | 1.98 | 1.23-3.17 | 0.005 | 1.93 | 1.28-2.92 | 0.002 |
| CC | 41 | 70 | 104 | 2.37 | 1.23-4.57 | 0.01 | 2.09 | 1.25-3.49 | 0.005 | 2.18 | 1.39-3.41 | 0.0007 |
| CT and CC | 100 | 196 | 301 | 2.01 | 1.11-3.64 | 0.02 | 2.01 | 1.28-3.17 | 0.002 | 2.02 | 1.36-2.99 | 0.0005 |
Statistical analyses included subjects for whom all information was available.
Adjusted for age and sex. NS: Never smoker; NF-κB: Nuclear factor κB; PPARγ: Peroxisome proliferator-activated receptor γ; CD: Crohn’s disease; UC: Ulcerative colitis; OR: Odds ratio; PXR: Pregnane X receptor; LXR: Liver X receptor.
Haplotype analysis
Haplotype analysis among the healthy controls demonstrated that the PXR C8055T variant genotype was more frequent in carriers of the PXR A7635G variant allele than among carriers of the A7635G wild-type, which indicated that these two polymorphisms were linked. Moreover, the presence of the A-24381C variant allele seemed to be independent of the PXR C8055T and A7635G genotypes. No significant association of PXR haplotypes and disease risk was determined (data not shown). Tables 5 and 6 show the minor allele frequencies of the PXR polymorphisms compared to those in other studies, and published associations between PXR polymorphisms and risk of IBD[47-59].
Table 5.
Minor allele frequencies of pregnane X receptor polymorphisms in studied populations
| Controls | C-rs3814055-T | A-rs1523127-C | C-rs2276707-T | A-rs6785049-G | ||
| Danish | 779 | 0.41 | 0.18 | 0.35 | Present study | |
| Irish | 336 | 0.433 | 0.452 | 0.142 | 0.406 | Dring et al[47] |
| Scottish | 334 | 0.394 | Ho et al[49] | |||
| Spanish | 550 | 0.382 | 0.192 | Martínez et al[50] |
Rs3814055 and rs1523127 are closely linked.
Table 6.
Published associations between pregnane X receptor polymorphisms and inflammatory bowel disease risk
| Cases | Controls | C-25385T (rs3814055) | A-24381C (rs1523127) | C8055T (rs2276707) | A7635G (rs6785049) | ||
| Danish1 | 822 | 779 | Neg | Neg | Variant | Present study | |
| Irish2 | 422 | 336 | Wild-type | Wild-type | Variant | Wild-type | Dring et al[47] |
| Scottish2 | 715 | 334 | Neg | Ho et al[49] | |||
| Spanish2 | 696 | 550 | Variant | Wild-type | Martínez et al[50] | ||
| Canadian3 | 270 | 336 | Neg | Neg | Amre et al[48] |
No association is indicated by “neg”. The risk allele is indicated for positive associations between the pregnane X receptor polymorphisms and inflammatory bowel disease risk.
Associations adjusted for smoking status;
Associations not adjusted for smoking status,
Children with Crohn’s disease.
Haplotype analysis in the healthy controls demonstrated that carriage of the LXR rs1405655 C variant allele was linked to the presence of the LXR rs2695121 C variant allele. Carriage of the LXR rs1405655 C allele in this instance did not add to the risk of IBD, compared to carriage of only the rs2695121 C allele. The OR for the association between the LXR haplotype that encompassed the T-rs2695121-C and the T-rs1405655-C variant allele was 1.17, 95% CI: 1.00-1.36 and 1.23, 95% CI: 1.00-1.52, compared to the compound wild-type haplotype, respectively (data not shown).
Haplotype analysis was not performed for the closely linked PPARγ Pro12Ala and C1431T polymorphisms.
Subgroup analysis
Subgroup analysis revealed that the PXR A7635G (rs6785049) variant genotype was associated with a higher risk of UC diagnosis before the age of 40 years and with a higher risk of extensive disease (OR: 1.34, 95% CI: 1.03-1.75 and OR: 2.49, 95% CI: 1.24-5.03, respectively), and the LXR T-rs2695121-C variant genotype was associated with a higher risk of advanced medical treatment for UC (OR: 1.80, 95% CI: 1.08-2.99) as compared to the homozygous wild-type genotype (data not shown).
DISCUSSION
In the present case-control study of 822 IBD patients (327 CD and 495 UC) and 773 healthy controls, we determined that PXR and LXR variant allele carriers were at higher risk of UC than the homozygous wild-type carriers, and that the association was strongest among individuals that had never smoked and those with severe UC. An association between PPARγ Pro12Ala and the risk of UC was determined based on only a few subjects. No associations were determined between gene polymorphisms and risk for CD or UC among previous or current smokers. Furthermore, no associations were found between the NFκB gene polymorphism and risk of CD or UC. The association between LXR C-rs1405655-T and T-rs2695121-C variant genotypes and the risk of IBD among individuals that had never smoked withstood Bonferroni correction for multiple testing, whereas the other associations were not validated by these analyses. The strengths and weaknesses of the present study must be considered[65]. For instance, one strength of the present study is the well-characterized study subjects with information that included smoking status. There are various methods used to determine the control group with associated advantages and disadvantages[66]. In this study, the control group consisted of blood donors, who were not a random sample of the population. However, confounding data is not a likely explanation of the association because both cases and controls were not aware of their genotypes, and genotyping was performed blindly. Furthermore, stratification could theoretically result in the determined associations. However, this possibility is considered unlikely because the cohort was recruited from an area of Denmark with a homogeneous population[67]. Minor allele frequencies of PXR polymorphisms in the present study and in other published studies on Caucasian populations are shown in Table 5. The allele frequencies of the present study did not deviate from previously determined frequencies[47,49,50]. Therefore, heterogeneity or stratification in the control group is not a likely explanation for the determined associations in our study (Table 5).
The present study included 1600 participants, and power analysis determined that this study had more than 80% power to detect a dominant effect with an OR of 1.5 in relation to either CD or UC, and 1.4 when CD and UC were combined. Moreover, genetic determinants may be stronger among patients with extensive development of the disease[68,69] and disease onset at a younger age. However, the obtained results cannot be excluded as false positive.
An association of the NFκB -94 ins/del with UC, CD, or IBD was not determined in the present study. The variant allele has been associated with a risk of UC in a study that used the family-based association test and the transmission disequilibrium test in 131 IBD pedigrees with UC offspring, which was replicated in a second set of 258 UC and 653 healthy controls with an OR for the combined studies of 1.57 (1.14-2.16)[25]. This study was further replicated in a small study of 127 UC patients and 155 healthy controls[26], whereas larger studies have not indicated any association between the polymorphism and IBD[27-29], UC[30,31], or CD[24]. Our results are in accordance with the latter studies[27-31].
In the present study, a statistically significant (although modest) association was determined between the homozygous PPARγ Pro12Ala variant genotype and an increased risk of IBD. This result cannot be excluded as random because of the small sample size. In a combined Dutch and Chinese study, the PPARγ C1431T variant allele was associated with UC in the Chinese study group but not in the Dutch study group, and no associations were indicated with CD[42]. No associations between PPARγ Pro12Ala polymorphism and UC[43] or CD[44] have been demonstrated in two small studies. Therefore, these collective studies have not yielded consistent data that supported involvement of PPARγ in IBD.
PXR A7635G (rs6785049) variant allele carriers were at a higher risk of UC and IBD than homozygous wild-type carriers were. Furthermore, risk was highest among individuals that had never smoked. Table 6 shows the results of published association studies of PXR polymorphisms in IBD. The risk allele is indicated for positive associations, whereas a null result is indicated as “neg” in Table 6. These results were inconsistent. No association was determined between the PXR A-24381C (rs1523127) polymorphism and IBD in the present study or in a previous Scottish study[49]. In contrast, Irish and Spanish studies have indicated opposite associations between IBD and the closely linked PXR C-25385T (rs3814055) polymorphism[47,50]. Furthermore, the A7635G (rs6785049) variant genotype was found to be associated with risk for UC in the present study, whereas this allele was indicated to be protective for IBD in the Irish study[47]. Collectively, these results suggest that variable linkage disequilibrium between the investigated and biologically functional SNPs, and population heterogeneity may contribute to the inconsistent results.
Low levels of PXR were expressed in the intestine of UC patients, and high PXR activity ameliorated colitis in an animal IBD model[70]. Thus, impaired PXR function may fail to suppress NF-κB-induced intestinal inflammation[13,71]. Moreover, attenuated activation of PXR target genes, such as the xenobiotic transporters MDR1 (ABCB1) and MRP2 (ABCC2), may lead to a less proficient epithelial barrier. Several lines of evidence support the role of impaired xenobiotic transport in IBD, including the development of colitis in mdr1a-deficient mice[72], low MDR1 expression levels in UC patients[73], and a meta-analysis that indicated an association between an MDR1 (ABCB1) polymorphism and the risk of UC[74]. Therefore, impaired PXR function may lead to less effective induction of MDR1 and export of harmful substances that originate from bacteria, diet, and pollutants.
The present investigation yielded strong associations between the LXR T-rs2695121-C homozygous variant allele and the risk of UC, and between both of the studied LXR variants and the risk of IBD among individuals that had never smoked. Haplotype analysis suggested a strong linkage between the two polymorphisms, and that carriage of the LXR T-rs1405655-C variant genotype coupled to the other LXR polymorphism does not add to the risk of IBD, compared to carriage of only the LXR T-rs2695121-C variant genotype. These polymorphisms have only been previously investigated in relation to Alzheimer’s disease[53]. LXR seems to have anti-inflammatory properties, and LXR represses a set of inflammatory genes after activation by bacterial components or cytokines[32]. Furthermore, LXR has been recently demonstrated to upregulate xenobiotic transport proteins, such as MDR1 (ABCB1)[75] and MRP2 (ABCC2)[76]. Therefore, our results suggest the involvement of LXR in UC etiology.
Finally, the present study suggested that the associations between the PXR A7635G (rs6785049) and both of the studied LXR variant genotypes and UC were stronger among never smokers than among previous or current smokers. Therefore, the impact of the PXR and LXR gene polymorphisms on population disease risk may be larger in population with low frequencies of smokers than in those with many smokers. None of the associations indicated in the previously mentioned studies were adjusted for smoking status. Therefore, differences in relevant exposure may have contributed to the inconsistent results. We have previously found that inclusion of smoking status may be essential for evaluation of genetic predisposition to IBD (unpublished data, V. Andersen), and the present study is in accordance with our former study. Moreover, recently, passive smoking has been suggested to confer risk of IBD in children[77,78].
Tobacco smoke contains > 3000 different chemical substances that have an impact on many biological pathways in relation to IBD[55]. However, no interaction between smoking status and the studied polymorphisms was determined in the present study. Tobacco smoke suppresses NF-κB activation in blood mononuclear cells[58], and a similar mechanism may occur in the intestine.
In summary, the present study of 1600 individuals suggests that PXR and LXR are implicated in determining individual susceptibility to UC in the Danish high-incidence population. Furthermore, the conferred risk seems to be strongest among individuals that have never smoked. Clearly, further research is necessary to assess the overall role of inborn variants in PXR and LXR on UC susceptibility and the underlying biological mechanisms in relation to IBD etiology. Our results suggest that inclusion of smoking status may be essential for the evaluation of the role of genetic predisposition to IBD.
COMMENTS
Background
Environmental and genetic factors are involved in the etiology of the chronic inflammatory bowel diseases (IBDs), ulcerative colitis (UC), and Crohn’s disease. Furthermore, gene-environment interactions may result from variants in genes involved in the handling of environmental factors.
Research frontiers
The rising incidence of IBD in the West suggests that environmental factors play a major role in its pathogenesis. Nuclear receptors are intracellular transcription factors that constitute a link between environmental factors and the regulation of many cellular processes, including inflammation. In this study, the authors demonstrated that genetic variants in the nuclear receptors pregnane X receptor (PXR) and liver X receptor (LXR) may confer risk of UC. Furthermore, the conferred risk seems to be strongest among individuals that have never smoked.
Innovations and breakthroughs
Recent reports have highlighted the importance of genetic variations in the etiology in IBD. This study explores the contribution of genetic variations in nuclear factors to risk of IBD. This is the first study to suggest that LXR may confer risk of UC, and moreover, add to our knowledge of risk of UC associated with PXR variants. Next, this study substantiated the authors’ previous findings that inclusion of smoking status may be essential for the evaluation of the role of genetic predisposition to IBDs.
Applications
By understanding the genetic contribution to risk of IBDs, this study adds further to our knowledge about the biological pathways that lead to disease, which is considered a prerequisite for development of new molecular targets for treatment.
Terminology
PXR, LXR and peroxisome proliferator-activated receptor γ (PPARγ) are nuclear receptors, i.e. sensors of the environment, because they are activated by the binding of various compounds termed ligands, and next, in similarity with nuclear factor (NF)-κB, they are transcription factors, i.e. they regulate transcription of their target genes. Thereby, nuclear factors may constitute a link between environmental factors and the regulation of inflammation.
Peer review
The authors examined the contribution of genetic variants in the nuclear receptors PXR, LXR and PPARγ and the transcription factor NF-κB to the risk of IBDs. The study revealed that variants in genes that coded for PXR and LXR confer risk of UC, especially among never smokers. Furthermore, the study demonstrates that inclusion of smoking status may be essential for the evaluation of the role of genetic predisposition to IBDs.
Footnotes
Supported by This project was supported by the “Familien Erichsen Mindefond”, the Lundbeck Foundation, the Danish Research Council, the Western Danish Research Forum for Health Science, the County of Viborg, the Danish Colitis-Crohn Association, “John M Klein og hustrus mindelegat”, and the A.P. Møller Foundation for the Advancement of Medical Science
Peer reviewer: María IT López, Professor, Experimental Biology, University of Jaen, araje de las Lagunillas s/n, Jaén 23071, Spain
S- Editor Sun H L- Editor Kerr C E- Editor Zheng XM
References
- 1.Xavier RJ, Podolsky DK. Unravelling the pathogenesis of inflammatory bowel disease. Nature. 2007;448:427–434. doi: 10.1038/nature06005. [DOI] [PubMed] [Google Scholar]
- 2.Strober W, Fuss I, Mannon P. The fundamental basis of inflammatory bowel disease. J Clin Invest. 2007;117:514–521. doi: 10.1172/JCI30587. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Franke A, Balschun T, Karlsen TH, Hedderich J, May S, Lu T, Schuldt D, Nikolaus S, Rosenstiel P, Krawczak M, et al. Replication of signals from recent studies of Crohn's disease identifies previously unknown disease loci for ulcerative colitis. Nat Genet. 2008;40:713–715. doi: 10.1038/ng.148. [DOI] [PubMed] [Google Scholar]
- 4.Østergaard M, Ernst A, Labouriau R, Dagiliené E, Krarup HB, Christensen M, Thorsgaard N, Jacobsen BA, Tage-Jensen U, Overvad K, et al. Cyclooxygenase-2, multidrug resistance 1, and breast cancer resistance protein gene polymorphisms and inflammatory bowel disease in the Danish population. Scand J Gastroenterol. 2009;44:65–73. doi: 10.1080/00365520802400826. [DOI] [PubMed] [Google Scholar]
- 5.A Catalog of Published Genome-Wide Association Studies. 2008. Available from: http://www.genome.gov/26525384.
- 6.Andersen V, Ernst A, Christensen J, Østergaard M, Jacobsen BA, Tjønneland A, Krarup HB, Vogel U. The polymorphism rs3024505 proximal to IL-10 is associated with risk of ulcerative colitis and Crohns disease in a Danish case-control study. BMC Med Genet. 2010;11:82. doi: 10.1186/1471-2350-11-82. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Ernst A, Andersen V, Østergaard M, Jacobsen BA, Dagiliene E, Pedersen IS, Drewes AM, Okkels H, Krarup HB. Genetic variants of glutathione S-transferases mu, theta, and pi display no susceptibility to inflammatory bowel disease in the Danish population. Scand J Gastroenterol. 2010;45:1068–1075. doi: 10.3109/00365521.2010.490594. [DOI] [PubMed] [Google Scholar]
- 8.Törkvist L, Noble CL, Lördal M, Sjöqvist U, Lindforss U, Nimmo ER, Russell RK, Löfberg R, Satsangi J. Contribution of CARD15 variants in determining susceptibility to Crohn's disease in Sweden. Scand J Gastroenterol. 2006;41:700–705. doi: 10.1080/00365520500395245. [DOI] [PubMed] [Google Scholar]
- 9.Medici V, Mascheretti S, Croucher PJ, Stoll M, Hampe J, Grebe J, Sturniolo GC, Solberg C, Jahnsen J, Moum B, et al. Extreme heterogeneity in CARD15 and DLG5 Crohn disease-associated polymorphisms between German and Norwegian populations. Eur J Hum Genet. 2006;14:459–468. doi: 10.1038/sj.ejhg.5201576. [DOI] [PubMed] [Google Scholar]
- 10.Ernst A, Jacobsen B, Østergaard M, Okkels H, Andersen V, Dagiliene E, Pedersen IS, Thorsgaard N, Drewes AM, Krarup HB. Mutations in CARD15 and smoking confer susceptibility to Crohn's disease in the Danish population. Scand J Gastroenterol. 2007;42:1445–1451. doi: 10.1080/00365520701427102. [DOI] [PubMed] [Google Scholar]
- 11.Becker CE, O'Neill LA. Inflammasomes in inflammatory disorders: the role of TLRs and their interactions with NLRs. Semin Immunopathol. 2007;29:239–248. doi: 10.1007/s00281-007-0081-4. [DOI] [PubMed] [Google Scholar]
- 12.Chen F, Castranova V, Shi X, Demers LM. New insights into the role of nuclear factor-kappaB, a ubiquitous transcription factor in the initiation of diseases. Clin Chem. 1999;45:7–17. [PubMed] [Google Scholar]
- 13.Wang K, Wan YJ. Nuclear receptors and inflammatory diseases. Exp Biol Med (Maywood) 2008;233:496–506. doi: 10.3181/0708-MR-231. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Bensinger SJ, Tontonoz P. Integration of metabolism and inflammation by lipid-activated nuclear receptors. Nature. 2008;454:470–477. doi: 10.1038/nature07202. [DOI] [PubMed] [Google Scholar]
- 15.Zhou C, Tabb MM, Nelson EL, Grün F, Verma S, Sadatrafiei A, Lin M, Mallick S, Forman BM, Thummel KE, et al. Mutual repression between steroid and xenobiotic receptor and NF-kappaB signaling pathways links xenobiotic metabolism and inflammation. J Clin Invest. 2006;116:2280–2289. doi: 10.1172/JCI26283. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.di Masi A, De Marinis E, Ascenzi P, Marino M. Nuclear receptors CAR and PXR: Molecular, functional, and biomedical aspects. Mol Aspects Med. 2009;30:297–343. doi: 10.1016/j.mam.2009.04.002. [DOI] [PubMed] [Google Scholar]
- 17.Ahn KS, Aggarwal BB. Transcription factor NF-kappaB: a sensor for smoke and stress signals. Ann N Y Acad Sci. 2005;1056:218–233. doi: 10.1196/annals.1352.026. [DOI] [PubMed] [Google Scholar]
- 18.Greten FR, Arkan MC, Bollrath J, Hsu LC, Goode J, Miething C, Göktuna SI, Neuenhahn M, Fierer J, Paxian S, et al. NF-kappaB is a negative regulator of IL-1beta secretion as revealed by genetic and pharmacological inhibition of IKKbeta. Cell. 2007;130:918–931. doi: 10.1016/j.cell.2007.07.009. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Steinbrecher KA, Harmel-Laws E, Sitcheran R, Baldwin AS. Loss of epithelial RelA results in deregulated intestinal proliferative/apoptotic homeostasis and susceptibility to inflammation. J Immunol. 2008;180:2588–2599. doi: 10.4049/jimmunol.180.4.2588. [DOI] [PubMed] [Google Scholar]
- 20.Nenci A, Becker C, Wullaert A, Gareus R, van Loo G, Danese S, Huth M, Nikolaev A, Neufert C, Madison B, et al. Epithelial NEMO links innate immunity to chronic intestinal inflammation. Nature. 2007;446:557–561. doi: 10.1038/nature05698. [DOI] [PubMed] [Google Scholar]
- 21.De Vry CG, Prasad S, Komuves L, Lorenzana C, Parham C, Le T, Adda S, Hoffman J, Kahoud N, Garlapati R, et al. Non-viral delivery of nuclear factor-kappaB decoy ameliorates murine inflammatory bowel disease and restores tissue homeostasis. Gut. 2007;56:524–533. doi: 10.1136/gut.2006.096487. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Neurath MF, Pettersson S, Meyer zum Büschenfelde KH, Strober W. Local administration of antisense phosphorothioate oligonucleotides to the p65 subunit of NF-kappa B abrogates established experimental colitis in mice. Nat Med. 1996;2:998–1004. doi: 10.1038/nm0996-998. [DOI] [PubMed] [Google Scholar]
- 23.Rogler G, Brand K, Vogl D, Page S, Hofmeister R, Andus T, Knuechel R, Baeuerle PA, Schölmerich J, Gross V. Nuclear factor kappaB is activated in macrophages and epithelial cells of inflamed intestinal mucosa. Gastroenterology. 1998;115:357–369. doi: 10.1016/s0016-5085(98)70202-1. [DOI] [PubMed] [Google Scholar]
- 24.Takedatsu H, Taylor KD, Mei L, McGovern DP, Landers CJ, Gonsky R, Cong Y, Vasiliauskas EA, Ippoliti A, Elson CO, et al. Linkage of Crohn's disease-related serological phenotypes: NFKB1 haplotypes are associated with anti-CBir1 and ASCA, and show reduced NF-kappaB activation. Gut. 2009;58:60–67. doi: 10.1136/gut.2008.156422. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25.Karban AS, Okazaki T, Panhuysen CI, Gallegos T, Potter JJ, Bailey-Wilson JE, Silverberg MS, Duerr RH, Cho JH, Gregersen PK, et al. Functional annotation of a novel NFKB1 promoter polymorphism that increases risk for ulcerative colitis. Hum Mol Genet. 2004;13:35–45. doi: 10.1093/hmg/ddh008. [DOI] [PubMed] [Google Scholar]
- 26.Borm ME, van Bodegraven AA, Mulder CJ, Kraal G, Bouma G. A NFKB1 promoter polymorphism is involved in susceptibility to ulcerative colitis. Int J Immunogenet. 2005;32:401–405. doi: 10.1111/j.1744-313X.2005.00546.x. [DOI] [PubMed] [Google Scholar]
- 27.Glas J, Török HP, Tonenchi L, Müller-Myhsok B, Mussack T, Wetzke M, Klein W, Epplen JT, Griga T, Schiemann U, et al. Role of the NFKB1 -94ins/delATTG promoter polymorphism in IBD and potential interactions with polymorphisms in the CARD15/NOD2, IKBL, and IL-1RN genes. Inflamm Bowel Dis. 2006;12:606–611. doi: 10.1097/01.ibd.0000225346.23765.6b. [DOI] [PubMed] [Google Scholar]
- 28.De Jager PL, Franchimont D, Waliszewska A, Bitton A, Cohen A, Langelier D, Belaiche J, Vermeire S, Farwell L, Goris A, et al. The role of the Toll receptor pathway in susceptibility to inflammatory bowel diseases. Genes Immun. 2007;8:387–397. doi: 10.1038/sj.gene.6364398. [DOI] [PubMed] [Google Scholar]
- 29.Latiano A, Palmieri O, Valvano MR, Bossa F, Latiano T, Corritore G, DeSanto E, Andriulli A, Annese V. Evaluating the role of the genetic variations of PTPN22, NFKB1, and FcGRIIIA genes in inflammatory bowel disease: a meta-analysis. Inflamm Bowel Dis. 2007;13:1212–1219. doi: 10.1002/ibd.20185. [DOI] [PubMed] [Google Scholar]
- 30.Oliver J, Gómez-García M, Paco L, López-Nevot MA, Piñero A, Correro F, Martín L, Brieva JA, Nieto A, Martín J. A functional polymorphism of the NFKB1 promoter is not associated with ulcerative colitis in a Spanish population. Inflamm Bowel Dis. 2005;11:576–579. doi: 10.1097/01.mib.0000161916.20007.76. [DOI] [PubMed] [Google Scholar]
- 31.Mirza MM, Fisher SA, Onnie C, Lewis CM, Mathew CG, Sanderson J, Forbes A. No association of the NFKB1 promoter polymorphism with ulcerative colitis in a British case control cohort. Gut. 2005;54:1205–1206. doi: 10.1136/gut.2005.070029. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 32.Zelcer N, Tontonoz P. Liver X receptors as integrators of metabolic and inflammatory signaling. J Clin Invest. 2006;116:607–614. doi: 10.1172/JCI27883. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 33.Hong C, Tontonoz P. Coordination of inflammation and metabolism by PPAR and LXR nuclear receptors. Curr Opin Genet Dev. 2008;18:461–467. doi: 10.1016/j.gde.2008.07.016. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 34.Ghisletti S, Huang W, Ogawa S, Pascual G, Lin ME, Willson TM, Rosenfeld MG, Glass CK. Parallel SUMOylation-dependent pathways mediate gene- and signal-specific transrepression by LXRs and PPARgamma. Mol Cell. 2007;25:57–70. doi: 10.1016/j.molcel.2006.11.022. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 35.Pascual G, Fong AL, Ogawa S, Gamliel A, Li AC, Perissi V, Rose DW, Willson TM, Rosenfeld MG, Glass CK. A SUMOylation-dependent pathway mediates transrepression of inflammatory response genes by PPAR-gamma. Nature. 2005;437:759–763. doi: 10.1038/nature03988. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 36.Yamamoto K, Ninomiya Y, Iseki M, Nakachi Y, Kanesaki-Yatsuka Y, Yamanoue Y, Itoh T, Nishii Y, Petrovsky N, Okazaki Y. 4-Hydroxydocosahexaenoic acid, a potent peroxisome proliferator-activated receptor gamma agonist alleviates the symptoms of DSS-induced colitis. Biochem Biophys Res Commun. 2008;367:566–572. doi: 10.1016/j.bbrc.2007.12.188. [DOI] [PubMed] [Google Scholar]
- 37.Sugawara K, Olson TS, Moskaluk CA, Stevens BK, Hoang S, Kozaiwa K, Cominelli F, Ley KF, McDuffie M. Linkage to peroxisome proliferator-activated receptor-gamma in SAMP1/YitFc mice and in human Crohn's disease. Gastroenterology. 2005;128:351–360. doi: 10.1053/j.gastro.2004.11.001. [DOI] [PubMed] [Google Scholar]
- 38.Ramakers JD, Verstege MI, Thuijls G, Te Velde AA, Mensink RP, Plat J. The PPARgamma agonist rosiglitazone impairs colonic inflammation in mice with experimental colitis. J Clin Immunol. 2007;27:275–283. doi: 10.1007/s10875-007-9074-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 39.Dubuquoy L, Jansson EA, Deeb S, Rakotobe S, Karoui M, Colombel JF, Auwerx J, Pettersson S, Desreumaux P. Impaired expression of peroxisome proliferator-activated receptor gamma in ulcerative colitis. Gastroenterology. 2003;124:1265–1276. doi: 10.1016/s0016-5085(03)00271-3. [DOI] [PubMed] [Google Scholar]
- 40.Doney A, Fischer B, Frew D, Cumming A, Flavell DM, World M, Montgomery HE, Boyle D, Morris A, Palmer CN. Haplotype analysis of the PPARgamma Pro12Ala and C1431T variants reveals opposing associations with body weight. BMC Genet. 2002;3:21. doi: 10.1186/1471-2156-3-21. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 41.Masugi J, Tamori Y, Mori H, Koike T, Kasuga M. Inhibitory effect of a proline-to-alanine substitution at codon 12 of peroxisome proliferator-activated receptor-gamma 2 on thiazolidinedione-induced adipogenesis. Biochem Biophys Res Commun. 2000;268:178–182. doi: 10.1006/bbrc.2000.2096. [DOI] [PubMed] [Google Scholar]
- 42.Shrestha UK, Karimi O, Crusius JB, Zhou F, Wang Z, Chen Z, van Bodegraven AA, Xiao J, Morré SA, Wang H, et al. Distribution of peroxisome proliferator-activated receptor-gamma polymorphisms in Chinese and Dutch patients with inflammatory bowel disease. Inflamm Bowel Dis. 2010;16:312–319. doi: 10.1002/ibd.21059. [DOI] [PubMed] [Google Scholar]
- 43.Wang F, Tahara T, Arisawa T, Sakata M, Takahama K, Watanabe M, Hirata I, Nakano H. Polymorphism of peroxisome proliferator-activated receptor gamma is not associated to Japanese ulcerative colitis. Hepatogastroenterology. 2008;55:73–75. [PubMed] [Google Scholar]
- 44.Leung E, Hong J, Fraser AG, Merriman TR, Vishnu P, Krissansen GW. Peroxisome proliferator-activated receptor-gamma gene polymorphisms and Crohn's disease. Int J Colorectal Dis. 2007;22:453–454. doi: 10.1007/s00384-005-0085-2. [DOI] [PubMed] [Google Scholar]
- 45.Langmann T, Moehle C, Mauerer R, Scharl M, Liebisch G, Zahn A, Stremmel W, Schmitz G. Loss of detoxification in inflammatory bowel disease: dysregulation of pregnane X receptor target genes. Gastroenterology. 2004;127:26–40. doi: 10.1053/j.gastro.2004.04.019. [DOI] [PubMed] [Google Scholar]
- 46.Zhang J, Kuehl P, Green ED, Touchman JW, Watkins PB, Daly A, Hall SD, Maurel P, Relling M, Brimer C, et al. The human pregnane X receptor: genomic structure and identification and functional characterization of natural allelic variants. Pharmacogenetics. 2001;11:555–572. doi: 10.1097/00008571-200110000-00003. [DOI] [PubMed] [Google Scholar]
- 47.Dring MM, Goulding CA, Trimble VI, Keegan D, Ryan AW, Brophy KM, Smyth CM, Keeling PW, O'Donoghue D, O'Sullivan M, et al. The pregnane X receptor locus is associated with susceptibility to inflammatory bowel disease. Gastroenterology. 2006;130:341–348; quiz 592. doi: 10.1053/j.gastro.2005.12.008. [DOI] [PubMed] [Google Scholar]
- 48.Amre DK, Mack DR, Israel D, Morgan K, Krupoves A, Costea I, Lambrette P, Grimard G, Deslandres C, Levy E, et al. Investigation of associations between the pregnane-X receptor gene (NR1I2) and Crohn's disease in Canadian children using a gene-wide haplotype-based approach. Inflamm Bowel Dis. 2008;14:1214–1218. doi: 10.1002/ibd.20461. [DOI] [PubMed] [Google Scholar]
- 49.Ho GT, Soranzo N, Tate SK, Drummond H, Nimmo ER, Tenesa A, Arnott ID, Satsangi J. Lack of association of the pregnane X receptor (PXR/NR1I2) gene with inflammatory bowel disease: parallel allelic association study and gene wide haplotype analysis. Gut. 2006;55:1676–1677. doi: 10.1136/gut.2006.105106. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 50.Martínez A, Márquez A, Mendoza J, Taxonera C, Fernández-Arquero M, Díaz-Rubio M, de la Concha EG, Urcelay E. Role of the PXR gene locus in inflammatory bowel diseases. Inflamm Bowel Dis. 2007;13:1484–1487. doi: 10.1002/ibd.20252. [DOI] [PubMed] [Google Scholar]
- 51.Adighibe O, Arepalli S, Duckworth J, Hardy J, Wavrant-De Vrièze F. Genetic variability at the LXR gene (NR1H2) may contribute to the risk of Alzheimer's disease. Neurobiol Aging. 2006;27:1431–1434. doi: 10.1016/j.neurobiolaging.2005.08.010. [DOI] [PubMed] [Google Scholar]
- 52.Rodríguez-Rodríguez E, Llorca J, Mateo I, Infante J, Sánchez-Quintana C, García-Gorostiaga I, Fernández-Viadero C, Peña N, Berciano J, Combarros O. No association of genetic variants of liver X receptor-beta with Alzheimer's disease risk. Am J Med Genet B Neuropsychiatr Genet. 2008;147B:650–653. doi: 10.1002/ajmg.b.30652. [DOI] [PubMed] [Google Scholar]
- 53.Infante J, Rodríguez-Rodríguez E, Mateo I, Llorca J, Vázquez-Higuera JL, Berciano J, Combarros O. Gene-gene interaction between heme oxygenase-1 and liver X receptor-beta and Alzheimer's disease risk. Neurobiol Aging. 2010;31:710–714. doi: 10.1016/j.neurobiolaging.2008.05.025. [DOI] [PubMed] [Google Scholar]
- 54.Hermann M, Krum H, Ruschitzka F. To the heart of the matter: coxibs, smoking, and cardiovascular risk. Circulation. 2005;112:941–945. doi: 10.1161/CIRCULATIONAHA.105.567842. [DOI] [PubMed] [Google Scholar]
- 55.Mahid SS, Minor KS, Soto RE, Hornung CA, Galandiuk S. Smoking and inflammatory bowel disease: a meta-analysis. Mayo Clin Proc. 2006;81:1462–1471. doi: 10.4065/81.11.1462. [DOI] [PubMed] [Google Scholar]
- 56.Aldhous MC, Prescott RJ, Roberts S, Samuel K, Waterfall M, Satsangi J. Does nicotine influence cytokine profile and subsequent cell cycling/apoptotic responses in inflammatory bowel disease? Inflamm Bowel Dis. 2008;14:1469–1482. doi: 10.1002/ibd.20523. [DOI] [PubMed] [Google Scholar]
- 57.Mishra NC, Rir-Sima-Ah J, Langley RJ, Singh SP, Peña-Philippides JC, Koga T, Razani-Boroujerdi S, Hutt J, Campen M, Kim KC, et al. Nicotine primarily suppresses lung Th2 but not goblet cell and muscle cell responses to allergens. J Immunol. 2008;180:7655–7663. doi: 10.4049/jimmunol.180.11.7655. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 58.Mian MF, Stämpfli MR, Mossman KL, Ashkar AA. Cigarette smoke attenuation of poly I:C-induced innate antiviral responses in human PBMC is mainly due to inhibition of IFN-beta production. Mol Immunol. 2009;46:821–829. doi: 10.1016/j.molimm.2008.09.007. [DOI] [PubMed] [Google Scholar]
- 59.Vangsted AJ, Klausen TW, Ruminski W, Gimsing P, Andersen NF, Gang AO, Abildgaard N, Knudsen LM, Nielsen JL, Gregersen H, et al. The polymorphism IL-1beta T-31C is associated with a longer overall survival in patients with multiple myeloma undergoing auto-SCT. Bone Marrow Transplant. 2009;43:539–545. doi: 10.1038/bmt.2008.351. [DOI] [PubMed] [Google Scholar]
- 60.Vogel U, Christensen J, Nexø BA, Wallin H, Friis S, Tjønneland A. Peroxisome proliferator-activated [corrected] receptor-gamma2 [corrected] Pro12Ala, interaction with alcohol intake and NSAID use, in relation to risk of breast cancer in a prospective study of Danes. Carcinogenesis. 2007;28:427–434. doi: 10.1093/carcin/bgl170. [DOI] [PubMed] [Google Scholar]
- 61.Vogel U, Christensen J, Dybdahl M, Friis S, Hansen RD, Wallin H, Nexø BA, Raaschou-Nielsen O, Andersen PS, Overvad K, et al. Prospective study of interaction between alcohol, NSAID use and polymorphisms in genes involved in the inflammatory response in relation to risk of colorectal cancer. Mutat Res. 2007;624:88–100. doi: 10.1016/j.mrfmmm.2007.04.006. [DOI] [PubMed] [Google Scholar]
- 62.Greenland S. Dose-response and trend analysis in epidemiology: alternatives to categorical analysis. Epidemiology. 1995;6:356–365. doi: 10.1097/00001648-199507000-00005. [DOI] [PubMed] [Google Scholar]
- 63.Vogel U, Christensen J, Wallin H, Friis S, Nexø BA, Tjønneland A. Polymorphisms in COX-2, NSAID use and risk of basal cell carcinoma in a prospective study of Danes. Mutat Res. 2007;617:138–146. doi: 10.1016/j.mrfmmm.2007.01.005. [DOI] [PubMed] [Google Scholar]
- 64.Genetic Power Calculator. 2009. Available from: http://pngu.mgh.harvard.edu/~purcell/gpc/cc2.html.
- 65.Daly AK. Candidate gene case-control studies. Pharmacogenomics. 2003;4:127–139. doi: 10.1517/phgs.4.2.127.22629. [DOI] [PubMed] [Google Scholar]
- 66.Hirschhorn JN, Lohmueller K, Byrne E, Hirschhorn K. A comprehensive review of genetic association studies. Genet Med. 2002;4:45–61. doi: 10.1097/00125817-200203000-00002. [DOI] [PubMed] [Google Scholar]
- 67.Statistics Denmark. 2009. Available from: http://www.dst.dk/HomeUK/Statistics/Key_indicators/Population/pop_quarterly.aspx.
- 68.Fowler EV, Doecke J, Simms LA, Zhao ZZ, Webb PM, Hayward NK, Whiteman DC, Florin TH, Montgomery GW, Cavanaugh JA, et al. ATG16L1 T300A shows strong associations with disease subgroups in a large Australian IBD population: further support for significant disease heterogeneity. Am J Gastroenterol. 2008;103:2519–2526. doi: 10.1111/j.1572-0241.2008.02023.x. [DOI] [PubMed] [Google Scholar]
- 69.Achkar JP, Dassopoulos T, Silverberg MS, Tuvlin JA, Duerr RH, Brant SR, Siminovitch K, Reddy D, Datta LW, Bayless TM, et al. Phenotype-stratified genetic linkage study demonstrates that IBD2 is an extensive ulcerative colitis locus. Am J Gastroenterol. 2006;101:572–580. doi: 10.1111/j.1572-0241.2006.00451.x. [DOI] [PubMed] [Google Scholar]
- 70.Shah YM, Ma X, Morimura K, Kim I, Gonzalez FJ. Pregnane X receptor activation ameliorates DSS-induced inflammatory bowel disease via inhibition of NF-kappaB target gene expression. Am J Physiol Gastrointest Liver Physiol. 2007;292:G1114–G1122. doi: 10.1152/ajpgi.00528.2006. [DOI] [PubMed] [Google Scholar]
- 71.Wahli W. A gut feeling of the PXR, PPAR and NF-kappaB connection. J Intern Med. 2008;263:613–619. doi: 10.1111/j.1365-2796.2008.01951.x. [DOI] [PubMed] [Google Scholar]
- 72.Panwala CM, Jones JC, Viney JL. A novel model of inflammatory bowel disease: mice deficient for the multiple drug resistance gene, mdr1a, spontaneously develop colitis. J Immunol. 1998;161:5733–5744. [PubMed] [Google Scholar]
- 73.Langmann T, Schmitz G. Loss of detoxification in inflammatory bowel disease. Nat Clin Pract Gastroenterol Hepatol. 2006;3:358–359. doi: 10.1038/ncpgasthep0545. [DOI] [PubMed] [Google Scholar]
- 74.Annese V, Valvano MR, Palmieri O, Latiano A, Bossa F, Andriulli A. Multidrug resistance 1 gene in inflammatory bowel disease: a meta-analysis. World J Gastroenterol. 2006;12:3636–3644. doi: 10.3748/wjg.v12.i23.3636. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 75.Thornton SJ, Wong E, Lee SD, Wasan KM. Effect of dietary fat on hepatic liver X receptor expression in P-glycoprotein deficient mice: implications for cholesterol metabolism. Lipids Health Dis. 2008;7:21. doi: 10.1186/1476-511X-7-21. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 76.Chisaki I, Kobayashi M, Itagaki S, Hirano T, Iseki K. Liver X receptor regulates expression of MRP2 but not that of MDR1 and BCRP in the liver. Biochim Biophys Acta. 2009;1788:2396–2403. doi: 10.1016/j.bbamem.2009.08.014. [DOI] [PubMed] [Google Scholar]
- 77.Jones DT, Osterman MT, Bewtra M, Lewis JD. Passive smoking and inflammatory bowel disease: a meta-analysis. Am J Gastroenterol. 2008;103:2382–2393. doi: 10.1111/j.1572-0241.2008.01999.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 78.Lashner BA, Shaheen NJ, Hanauer SB, Kirschner BS. Passive smoking is associated with an increased risk of developing inflammatory bowel disease in children. Am J Gastroenterol. 1993;88:356–359. [PubMed] [Google Scholar]