Short Communication

Genes and Immunity (2009) 10, 601–605; doi:10.1038/gene.2009.29; published online 23 April 2009

Association of UCP2 −866 G/A polymorphism with chronic inflammatory diseases

X Yu1, S Wieczorek2, A Franke3, H Yin4, M Pierer5, C Sina6, T H Karlsen7, K M Boberg7, A Bergquist8, M Kunz9, T Witte10, W L Gross11, J T Epplen2, M E Alarcón-Riquelme4, S Schreiber3,6 and S M Ibrahim1

  1. 1Section of Immunogenetics, University of Rostock, Rostock, Germany
  2. 2Department of Human Genetics, Ruhr University, IGSN, Bochum, Germany
  3. 3Institute of Clinical Molecular Biology, Christian-Albrechts-University, Kiel, Germany
  4. 4Department of Genetics and Pathology, Uppsala University, Uppsala, Sweden
  5. 5Medical Department IV, University of Leipzig, Leipzig, Germany
  6. 6Department of General Internal Medicine, University Hospital Schleswig-Holstein, Christian-Albrechts-University, Kiel, Germany
  7. 7Medical Department, Rikshospitalet University Hospital, Oslo, Norway
  8. 8Department of Gastroenterology and Hepatology, Karolinska University Hospital, Huddinge, Stockholm, Sweden
  9. 9Dermatology Clinic, University of Rostock, Rostock, Germany
  10. 10Department of Immunology and Rheumatology, Medical School Hannover, Hannover, Germany
  11. 11Department of Rheumatology, University Hospital of Schleswig-Holstein, Campus Luebeck, Luebeck, Germany

Correspondence: Professor SM Ibrahim, Immunogenetics Group, University of Rostock, Schillingallee 70, Rostock 18055, Germany. E-mail: saleh.ibrahim@med.uni-rostock.de

Received 21 November 2008; Revised 12 February 2009; Accepted 17 February 2009; Published online 23 April 2009.

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Abstract

We reported earlier that two mitochondrial gene polymorphisms, UCP2 –866 G/A (rs659366) and mtDNA nt13708 G/A (rs28359178), are associated with multiple sclerosis (MS). Here we aim to investigate whether these functional polymorphisms contribute to other eight chronic inflammatory diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Wegener' granulomatosis (WG), Churg–Strauss syndrome (CSS), Crohn's disease (CD), ulcerative colitis (UC), primary sclerosing cholangitis (PSC) and psoriasis. Compared with individual control panels, the UCP2 –866 G/A polymorphism was associated with RA and SLE, and the mtDNA nt13708 G/A polymorphism with RA. Compared with combined controls, the UCP2 –866 G/A polymorphism was associated with SLE, WG, CD and UC. When all eight disease panels and the original MS panel were combined in a meta-analysis, the UCP2 was associated with chronic inflammatory diseases in terms of either alleles (odds ratio (OR)=0.91, 95% confidence interval (95% CI): 0.86–0.96), P=0.0003) or genotypes (OR=0.88, (95% CI: 0.82–0.95), P=0.0008), with the –866A allele associated with a decreased risk to diseases. As the −866A allele increases gene expression, our findings suggest a protective role of the UCP2 protein in chronic inflammatory diseases.

Keywords:

mitochondria, chronic inflammatory diseases, uncoupling protein 2

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Introduction

Mitochondria are organelles of all nucleated cells. They are composed of roughly 1500 proteins encoded by the nuclear DNA (nDNA) and the mitochondrial DNA (mtDNA).1 Mitochondria are the main source of the cellular energy and they are critical for a wide spectrum of biological processes.1, 2 In addition, there are many interrelationships between mitochondria and biological processes likely to be involved in autoimmunity, for example, mitochondria play a critical role in apoptosis, a process responsible for negative selection by deletion of self-reactive leukocytes. Mitochondria also provide energy required for the activation and proliferation of peripheral lymphocytes. Several mitochondrial proteins have been suggested to be required for the prevention of autoimmunity.3 Given this influence of mitochondria in autoimmunity, it is conceivable that mitochondrial gene polymorphisms could be associated with autoimmune or chronic inflammatory diseases.

Earlier, we have carried out a case–control study in order to investigate the association of an nDNA-encoded mitochondrial gene, uncoupling protein 2 (UCP2), with multiple sclerosis (MS), an autoimmune disease of the central nervous system. Among the three functional UCP2 variants evaluated, a promoter polymorphism, −866 G/A, was identified to be associated with MS. The less frequent A allele that is associated with an increased expression of the UCP2 gene transcript had a protective effect on the risk for developing MS in our earlier study.4 We have also confirmed and extended the earlier reported associations between mtDNA variants and MS,5 we identified the mtDNA nt13708A variant as a risk allele for MS.6

In this study, we aimed to investigate the two aforementioned MS-associated mitochondrial variants in other chronic inflammatory diseases. That is, we tested UCP2 –866 G/A and mtDNA nt13708 G/A for an association with eight distinct chronic inflammatory diseases, which included rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Wegener' granulomatosis (WG), Churg–Strauss syndrome (CSS), Crohn's disease (CD), ulcerative colitis (UC), primary sclerosing cholangitis (PSC) and psoriasis.

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Results and discussion

Overall 4625 patients and 2841 healthy controls (see in Supplementary Material) were successfully genotyped for the UCP2 –866 G/A and the mtDNA nt13708 G/A polymorphisms. The frequencies of the A allele of the UCP2 –866 G/A polymorphism ranged from 33 to 45% in the different patient panels and from 35 to 45% in the respective control panels. The distribution of the UCP2 –866 genotypes were in the Hardy–Weinberg equilibrium in all individual control panels (Table 1). The frequency of the A allele of the mtDNA nt13708 G/A polymorphism ranged from 9.7 to 13.8% in the patient panels and from 7.4 to 12.5% in the control panels (Supplementary Table 1).


With matched control panels, the UCP2 –866 G/A polymorphism was associated with RA (OR=0.66, 95% CI: 0.47–0.94, P=0.022) and SLE (OR=0.67, 95% CI: 0.45–0.99, P=0.044), in terms of genotype comparison. AG and AA genotypes were associated with decreased risks of both diseases as compared with GG genotype. Those associations of the UCP2 –866 G/A polymorphism with the two diseases were also observed in the allele comparison, −866A allele was associated with decreased risk of RA (OR=0.76, 95% CI: 0.60–0.97, P=0.026) and SLE (OR=0.74, 95% CI: 0.56–0.97, P=0.029) as compared with G allele (Table 2). Yet, when P-values were corrected for multiple testing using the Bonferroni correction, none of the associations remained significant (data not shown).


The PSC panel was recruited from Scandinavia, whereas the other seven disease panels were recruited from Germany. Similar frequencies of the UCP2 −866A allele were observed between Scandinavian and German controls. In order to increase the statistical power, we combined all controls together and included in addition 462 controls from a previous German MS case–control panel (4). Using these combined controls, we observed associations of the UCP2 –866 G/A polymorphism with SLE (OR=0.71, 95% CI: 0.54–0.95, P=0.019), WG (OR=0.82, 95% CI: 0.67–0.99, P=0.036), CD (OR=0.78, 95% CI: 0.66–0.94, P=0.007) and UC (OR=0.81, 95% CI: 0.66–1.00, P=0.048), in terms of genotype comparison. Also, we observed association of the UCP2 –866 G/A polymorphism with SLE (OR=0.81, [95% CI: 0.65–0.99], P=0.044), CD (OR=0.90, 95% CI: 0.83–0.98, P=0.013) and UC (OR=0.90, 95% CI: 0.81–0.99, P=0.033), in terms of allele comparison (Table 2). Yet, associations were not significant after correcting for multiple testing (data not shown).

The odds ratios in all sample panels were <1, with the exception of psoriasis. This indicates that the protective role of the UCP2 –866A allele might be common for chronic inflammatory disease. In order to confirm this hypothesis, we carried out a meta-analysis by combining all eight case–control panels investigated here and an earlier German MS panel (4). The meta-analysis revealed strong association of UCP2 with chronic inflammatory diseases, in terms of both allele association (OR=0.91, 95% CI: 0.86–0.96, P=0.0003) and genotypes association (OR=0.88, [95% CI: 0.82–0.95], P=0.0008), the –866 A allele was associated with decreased risk of chronic inflammatory (Table 2 and Figure 1). This association is weaker than the original MS association. To exclude the MS-specific effect, we carried out the meta-analysis by excluding the MS cohorts. Still, UCP2 was associated with chronic inflammatory diseases, in terms of both allele association (OR=0.93, 95% CI: 0.88–0.98, P=0.0093) and genotypes association (OR=0.90, 95% CI: 0.83–0.98, P=0.0013) (Table 2).

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Forest plots of odds ratios of the association of UCP2 with nine inflammatory diseases. The odds ratios were calculated for comparisons of A vs G alleles (a) and GA+AA vs GG genotypes (b). The forest plots were generated using comprehensive meta-analysis software.

Full figure and legend (15K)

Compared with individual control samples, only RA was associated with the mtDNA nt13708 G/A polymorphism (OR=1.76, 95% CI: 1.00–3.09, P=0.049). This association was not significant after correction for multiple testing. Using combined controls, no significant associations of the polymorphism with any other disease were observed. In addition, meta-analysis showed that there was no association between the polymorphism and the investigated chronic inflammatory diseases overall (Supplementary Table 1).

In this study, we showed that the UCP2 –866 G/A polymorphism was associated with RA, SLE, WG, CD and UC. No associations were significant after correcting for multiple comparisons using the conservative Bonferroni method. Correcting for multiple testing reduces type I errors, while on the other hand increasing the possibility of type II errors. As this study aimed to confirm the association of the two polymorphisms with chronic inflammatory diseases overall, those weak associations could still indicate a role of the UCP2 gene in chronic inflammatory diseases. Moreover, the meta-analysis showed that the UCP2 gene is generally associated with inflammatory diseases. These results suggest a potential role of the UCP2 protein in the development of chronic inflammatory diseases.

The UCP2 gene is a member of a family of genes encoding proteins that uncouple proton entry to the mitochondria from mitochondrial oxidative phosphorylation (OXPHOS). The UCP2 −866 G/A polymorphism is located in the promoter region of the gene. It was reported earlier to alter mRNA expression of UCP2 in vivo and in vitro,with the common G allele being associated with lower mRNA expression levels.4, 7, 8 High expression of UCP2 has been reported in cells of the immune system, and the expression was affected by the –866 G variation in vivo and in vitro.4, 9, 10 The –866 G allele decreasing the UCP2 expression is associated with increased risk to chronic inflammatory diseases. This observation is consistent with findings in mouse studies, where UCP2-deficient mice were more susceptible to experimental autoimmune encephalomyelitis (EAE) and showed stronger immune responses to infections than wild-type controls.9, 11 As the A allele increasing the gene expression is associated with a decreased risk to diseases, it suggests that the UCP2 protein play a protective role in chronic inflammatory diseases.

The protective effect of the UCP2 −866A allele was not demonstrable in psoriasis. A possible reason for this could relate to different onset ages of the diseases. Psoriasis is an early onset chronic inflammatory disease, with most patients presenting initial symptoms in their third decade.12 In contrast, the onset ages of the other eight chronic inflammatory diseases range from 34 to 55 years. The mitochondrial influences on complex diseases/traits are age-related, with increasing influence exerted in the elderly.13

The mechanism underlying the association between UCP2 gene polymorphisms and chronic inflammatory diseases remain unknown. An important mediator in the pathway could be the mitochondria reactive oxygen species (ROS). UCP2 protein uncouples the protons generated from ATP synthesis and thereby negatively regulates the ROS production in the mitochondria. There is growing evidence that ROS generated by the mitochondria could be important for immune responses. The ROS production in macrophages from UCP2-deficient mice is ~80% higher than in wild-type mice, and the deficiency of the UCP2 gene increases the susceptibility to EAE, a mouse model for MS.9, 11 In addition, mouse mtDNA variations affecting mitochondrial ROS production are associated with susceptibility to autoimmune diabetes and EAE.14, 15 This indicates that mitochondrial ROS production could be a regulator of susceptibility to chronic inflammatory diseases. Hence, a possible mechanism underlying the association of the mitochondrial gene with chronic inflammatory diseases could be that gene variations reduce/increase the production of mitochondrial ROS and consequently decrease/increase the susceptibility to chronic inflammatory diseases.

In conclusion, our findings underscore that mitochondrial genes are involved in the regulation of the susceptibility to chronic inflammatory diseases.

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Conflict of interest

The authors declare no conflict of interest.

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References

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Acknowledgements

The authors thank all patients, physicians and volunteer healthy controls for the cooperation. The cooperation of the Deutsche Morbus Crohn und Colitis Vereinigung e.V. and of the contributing gastroenterologists is gratefully acknowledged. The authors thank Rica Waterstadt for assistance in genotyping. This work was supported by grants from EU FP6 (EURO-RA), Hertie Stiftung, German Ministry of Education and Research (BMBF) through the National Genome Research Network (NGFN), Deutsche Forschungsgemeinschaft (KFO170) and the PopGen Biobank. The project received infrastructure support through the DFG excellence cluster ‘Inflammation at Interfaces’. Benedicte A Lie and the Norwegian Bone Marrow Donor Registry at Rikshospitalet University Hospital, Oslo, are acknowledged for contributing the healthy Norwegian control population.

Supplementary Information accompanies the paper on Genes and Immunity website (http://www.nature.com/gene)

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