Arthritis Res Ther. 2013 Jun 2;15(4):R71.

Epistasis with HLA DR3 implicates the P2X7 receptor in the pathogenesis of primary Sjögren’s syndrome.

Susan Lester1, Leanne Stokes2,3, Kristen K Skarratt2, Ben J Gu2,4, Kathy L Sivils5, Christopher J Lessard5, James S Wiley2,4† and Maureen Rischmueller1,6*†

Author Affiliations

1 Department of Rheumatology, The Queen Elizabeth Hospital, Woodville South, South Australia, Australia

2 Sydney Medical School Nepean, University of Sydney, Nepean Hospital, Penrith, NSW, Australia

3 Health Innovations Research Institute, School of Medical Sciences, RMIT University, Bundoora, Victoria, Australia

4 Florey Neuroscience Institutes, University of Melbourne, Parkville, Victoria, Australia

5 Arthritis and Clinical Immunology Research Program, Oklahoma Medical Research Foundation, Oklahoma City, OK, USA

6 Discipline of Medicine, University of Adelaide, South Australia, Australia

† Equal contributors

*Corresponding author: Maureen Rischmueller (maureen.rischmueller@health.sa.gov.au)

 

Abstract

Introduction: The aim of this study was to examine the association between functional polymorphisms in the pro-inflammatory P2X7 receptor and the Ro/La autoantibody response in primary Sjögren’s syndrome (pSS).

Methods: Twelve functional P2RX7 polymorphisms were genotyped in 114 pSS patients fulfilling the Revised American-European Consensus Criteria for pSS, and 136 controls. Genotyping of the A1405G (rs2230912) polymorphism was performed on a replication cohort consisting of 281 pSS patients and 534 controls. P2X7 receptor function in lymphocytes and monocytes was assessed by measurement of ATP-induced ethidium+ uptake. Serum IL-18 levels were determined by ELISA.

Results: The minor allele of P2RX7 A1405G is a tag for a common haplotype associated with gain in receptor function, as assessed by ATP-induced ethidium+ uptake. A positive association between 1405G and anti-Ro±La seropositive pSS patients was observed in Cohort 1. Although not replicated in Cohort 2, there was a consistent, significant, negative epistatic interaction effect with HLA-DR3 in seropositive pSS patients from both cohorts, thereby implicating this gain of function variant in the pathogenesis of pSS. Serum IL-18 was elevated in seropositive pSS patients, but was not influenced by P2RX7 A1405G.

Conclusions: The P2RX7 1405G gain-of-function haplotype may be a risk factor for seropositive pSS in a subset of subjects who do not carry HLA risk alleles, but has no effect in subjects who do (epistasis). Potential mechanisms relate to autoantigen exposure and inflammatory cytokine expression. The observed elevation of IL-18 levels is consistent with P2X7 receptor activation in seropositive pSS patients. Collectively these findings implicate P2X7 receptor function in the pathogenesis of pSS.

PMID 23819992

 

Supplement:

Primary Sjögren’s syndrome is an autoimmune disease that primarily manifests as inflammation and destruction of the salivary and lacrimal glands resulting in reduced secretion of saliva and tears and extreme dryness at mucosal surfaces (sicca symptoms). In addition to these sicca symptoms, the disease may also affect organs such as the lungs, kidney and skin, and in addition, pSS patients have a greatly increased risk of developing lymphoma. pSS is primarily a disease of women (9:1 female:male ratio), and patients may have a wide array of other symptoms which, as we have previously shown, include irritable bladder, sleep apnoea and autonomic symptoms, and depression and anxiety.

We don’t know what causes pSS, and consequently there is no treatment due to our poor understanding of disease pathogenesis. What we do know is that (1) approximately two thirds of pSS patients have autoantibodies to the nuclear components Ro (SSA) and La (SSB), and that these autoantibodies are associated with more severe inflammatory disease and may be directly involved in the underlying inflammatory processes and (2) there is a strong genetic component to pSS, and specifically to autoantibody positive pSS. The strongest genetic association identified so far is between autoantibody positive pSS and the HLA-DR3 allele in the HLA class II region of the Major Histocompatibility Complex (MHC) on chromosome 6, although the molecular mechanisms underpinning this association have not yet been elucidated.

Genetic studies are a primary research tool to aid in unravelling the pathogenesis of human disease. In this study we evaluated the relationship between the P2RX7 candidate gene (chromosome 12) which encodes the P2X7 receptor, HLA-DR3 and autoantibody positive pSS. We observed an epistatic interaction between a P2RX7 genetic variant associated with increased receptor function, and HLA-DR3 in autoantibody positive pSS. Moreover, this epistatic interaction was observed in two separate pSS-control cohorts indicating that this is unlikely to be a false result.

What is epistasis? Epistasis is a form of genetic interaction, similar to dominance/recessive relationships between alleles within a locus, except that the interaction occurs between alleles at different loci on different chromosomes. It is usually defined as suppression of the effect of one gene by another gene. In our study we observed that the P2RX7 gain of function allele was only important in autoantibody positive pSS patients who did not carry HLA-DR3. In other words HLA-DR3 suppressed/masked/was dominant to the effect of the P2RX7 gain of function allele.

What is the significance of this finding? We interpret this result as implicating the P2X7 receptor in the pathogenesis of pSS. The P2X7 receptor is found on many cells and tissues throughout the body and is known to be important in inflammatory processes, fluid secretion, bladder irritability, the autonomic system, the central nervous system and pain response, all of which are directly relevant to pSS. There are several recently published papers which add support to our original conclusion. The P2X7 receptor, together with down-stream inflammatory components, has recently been demonstrated to be up-regulated in the salivary glands of pSS patients [1]. Moreover this up-regulation was more pronounced in pSS patients with autoantibodies and in patients with more salivary gland destruction. Another study demonstrated that the P2X7 receptor expression was increased in the peripheral blood mononuclear cells (PBMC) of pSS patients and correlated with anxiety and depression symptoms [2]. A third study demonstrated that P2X7 blockade attenuated lupus nephritis in a murine model [3], a mechanism that may be relevant in kidney disease associated with pSS.

Conclusion: Collectively, these results suggest that the P2X7 receptor may be a potential therapeutic target for pSS. Importantly, P2X7 receptor antagonists are available for pre-clinical and clinical studies [4] and warrant evaluation for the treatment of pSS.

References

  1. Baldini C, Rossi C, Ferro F, Santini E, Seccia V, Donati V, et al. The P2X7 receptor-inflammasome complex has a role in modulating the inflammatory response in primary Sjogren’s syndrome. J Intern Med. 2013 Nov; 274(5):480-489.
  2. Xie B, Chen Y, Zhang S, Wu X, Zhang Z, Peng Y, et al. The expression of P2X7 receptors on peripheral blood mononuclear cells in patients with primary Sjogren’s syndrome and its correlation with anxiety and depression. Clin Exp Rheumatol. 2014 May-Jun; 32(3):354-360.
  3. Zhao J, Wang H, Dai C, Zhang H, Huang Y, Wang S, et al. P2X7 blockade attenuates murine lupus nephritis by inhibiting activation of the NLRP3/ASC/caspase 1 pathway. Arthritis Rheum. 2013 Dec; 65(12):3176-3185.
  4. Broom DC, Matson DJ, Bradshaw E, Buck ME, Meade R, Coombs S, et al. Characterization of N-(adamantan-1-ylmethyl)-5-[(3R-amino-pyrrolidin-1-yl)methyl]-2-chloro-benzamide, a P2X7 antagonist in animal models of pain and inflammation. J Pharmacol Exp Ther. 2008 Dec; 327(3):620-633.

 

 

 

 

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