Common genetic variants associated with disease from genome-wide association studies are mutually exclusive in prostate cancer and rheumatoid arthritis

What's known on the subject? and What does the study add? The link between inflammation and cancer has long been reported and inflammation is thought to play a role in the pathogenesis of many cancers, including prostate cancer (PrCa). Over the last 5 years, genome-wide association studies (GWAS) have reported numerous susceptibility loci that predispose individuals to many different traits. The present study aims to ascertain if there are common genetic risk profiles that might predispose individuals to both PrCa and the autoimmune inflammatory condition, rheumatoid arthritis. These results could have potential public heath impact in terms of screening and chemoprevention. Objectives To investigate if potential common pathways exist for the pathogenesis of autoimmune disease and prostate cancer (PrCa). To ascertain if the single nucleotide polymorphisms (SNPs) reported by genome-wide association studies (GWAS) as being associated with susceptibility to PrCa are also associated with susceptibility to the autoimmune disease rheumatoid arthritis (RA). Materials and Methods The original Wellcome Trust Case Control Consortium (WTCCC) UK RA GWAS study was expanded to include a total of 3221 cases and 5272 controls. In all, 37 germline autosomal SNPs at genome-wide significance associated with PrCa risk were identified from a UK/Australian PrCa GWAS. Allele frequencies were compared for these 37 SNPs between RA cases and controls using a chi-squared trend test and corrected for multiple testing (Bonferroni). Results In all, 33 SNPs were able to be analysed in the RA dataset. Proxies could not be located for the SNPs in 3q26, 5p15 and for two SNPs in 17q12. After applying a Bonferroni correction for the number of SNPs tested, the SNP mapping to CCHCR1 (rs130067) retained statistically significant evidence for association (P = 6 × 10–4; odds ratio [OR] = 1.15, 95% CI: 1.06–1.24); this has also been associated with psoriasis. However, further analyses showed that the association of this allele was due to confounding by RA-associated HLA-DRB1 alleles. Conclusions There is currently no evidence that SNPs associated with PrCa at genome-wide significance are associated with the development of RA. Studies like this are important in determining if common genetic risk profiles might predispose individuals to many diseases, which could have implications for public health in terms of screening and chemoprevention.


Objectives
• To investigate if potential common pathways exist for the pathogenesis of autoimmune disease and prostate cancer (PrCa). • To ascertain if the single nucleotide polymorphisms (SNPs) reported by genome-wide association studies (GWAS) as being associated with susceptibility to PrCa are also associated with susceptibility to the autoimmune disease rheumatoid arthritis (RA).

Materials and Methods
• The original Wellcome Trust Case Control Consortium (WTCCC) UK RA GWAS study was expanded to include a total of 3221 cases and 5272 controls.
• In all, 37 germline autosomal SNPs at genome-wide significance associated with PrCa risk were identified from a UK/Australian PrCa GWAS. • Allele frequencies were compared for these 37 SNPs between RA cases and controls using a chi-squared trend test and corrected for multiple testing (Bonferroni).

Results
• In all, 33 SNPs were able to be analysed in the RA dataset. Proxies could not be located for the SNPs in 3q26, 5p15 and for two SNPs in 17q12. • After applying a Bonferroni correction for the number of SNPs tested, the SNP mapping to CCHCR1 (rs130067) retained statistically significant evidence for association (P = 6 ¥ 10 -4 ; odds ratio [OR] = 1.15, 95% CI: 1.06-1.24); this has also been associated with psoriasis. • However, further analyses showed that the association of this allele was due to confounding by RA-associated HLA-DRB1 alleles.

Introduction
Although prostate cancer (PrCa) is the most frequent non-cutaneous cancer among men in America and the UK, very little is known regarding the underlying aetiology [ Nevertheless, these variants alone cannot explain fully the variation of PrCa incidence seen amongst populations. Environmental factors are also thought to play a key role in PrCa aetiology. These factors include the immune system and inflammation [25].
The link between inflammation and cancer has long been established [26]. Chronic inflammation is thought to influence carcinogenesis of many tumour sites, including PrCa [27][28][29]. The role of the immune system in the aetiology of PrCa has been further enhanced by the results from the IMPACT study, which for the first time showed an improved survival for patients with PrCa using sipuleucel-T immunotherapy [30].
There have also been attempts to investigate the relationship between autoimmune diseases and PrCa. Reports from longitudinal studies looking at the incidence of cancers in cohorts with autoimmune diseases have been conflicting. Cohorts with Crohn's or ulcerative colitis have been reported to have a slightly increased incidence of PrCa, but this was not statistically significant [31,32]. A group from the National Cancer Institute in the USA reported no difference in the incidence of PrCa in a scleroderma cohort [33]. Conversely, cohorts with rheumatoid arthritis (RA), psoriasis and systemic lupus erythematosus seemed to have a decreased risk of PrCa [34][35][36]. This seems to go against the previously reported studies showing inflammation as a cause of carcinogenesis. Furthermore there is an increased incidence of non-Hodgkin's lymphoma in relatives of patients with PrCa [37,38]. There are, however, potential confounding factors that might account for these findings, such as the medication usage in these cohorts. The chemopreventive effects of aspirin and NSAIDs have been reported in many longitudinal studies [39]. The increased intake of NSAIDs in the cohorts with RA could have skewed the results, leading to the perceived protective effect.
The interest in the effects of autoimmunity and inflammation on PrCa has also led to groups exploring the association of PrCa risk with genes in these pathways. A candidate gene approach was initially adopted, with conflicting results [25,27]. Zheng et al. [40] then conducted a pathway analysis approach looking at sets of inflammatory pathways genes and their association with PrCa. There were some positive associations with PrCa risk, but further validation is still awaited. More recently, GWAS identified a coding SNP associated with PrCa risk at CCHCR1 (coding for coiled-coil alpha-helical rod protein 1), which is also associated with the autoimmune disease psoriasis [17]. This is encouraging as it highlights a potential autoimmune aetiology for PrCa. The question that arises from this is: could there still be other susceptibility loci that are common to both PrCa and autoimmune diseases? It is known that the functions for most PrCa GWAS SNPs have not been established, as they are non-coding, lying in intronic or intergenic regions [41]. It would be important to determine if there are any associations between these PrCa-risk SNPs and autoimmune diseases, as it could improve our understanding of the biology of these SNPs, as well as potentially offering a chemopreventive and/or therapeutic target. The present study aims to evaluate this further by investigating if the GWAS SNPs associated with Associations between pathogenesis of RA and prostate cancer susceptibility to PrCa are also associated with susceptibility to the autoimmune disease RA.
Rheumatoid arthritis is a chronic autoimmune disease affecting 0.5-1% of the population worldwide. The inherited link has been established in twin studies, where the genetic contribution to risk is estimated to be between 50% and 60% [42]. As with PrCa, significant progress has been made recently with the advent of GWAS to identify the genetic factors that contribute to this disease, with over 33 SNPs reported [42,43]. However, it is still estimated that more than 50% of the genetic risks remain unaccounted for [43]. The present study could also potentially highlight new susceptibility loci for RA. RA was chosen as the autoimmune disease to compare with PrCa as the former also occurs in males, whereas many other autoimmune diseases are predominant in females.

Materials and Methods
We

Results
Of the 37 autosomal PrCa loci identified to date, we could not find proxies for rs10936632 in 3q26, rs2242652 in 5p15, and rs11649743 and rs4430796 in 17q12. Association with RA was tested for the remaining 33 PrCa-associated markers ( mapping to CCHCR1 retained statistically significant evidence for association (P = 6 ¥ 10 -4 ; OR = 1.15, 95% CI: 1.06-1.24).
CCHCR1 maps to the human leucocyte antigen (HLA) region, around 100 kb from the HLA-C gene. The major RA susceptibility locus, HLA-DRB1, and a number of additional RA loci also map to the HLA. The region is characterized by the presence of strong linkage disequilibrium, and therefore the association of rs130067 with RA might be due to linkage disequilibrium with previously known RA loci. We carried out a study aimed at identifying HLA-DRB1-independent RA susceptibility loci by pairwise matching WTCCC cases and controls on DRB1 genotypes [47]. A perfect proxy (r 2 = 1) of rs130067, rs1265074, was included in the above-mentioned analysis.
When we compared non-matched cases and controls, this SNP was associated with RA with a P value of 0.03. However, when we repeated the analysis using case-control pairs with identical DRB1 genotypes, the SNP was no longer significant (P = 0.91). This suggests that the association of rs130067 was due to confounding by RA-associated HLA-DRB1 alleles.

Discussion
This is the first study exploring the association between the common genetic variants associated with PrCa and RA. The inflammation pathways remain an important factor in PrCa biology, especially with the improved survival reported with the use of the immunotherapy drug sipuleucel-T in castration-resistant PrCa [30]. The main aim of the present study was to ascertain if PrCa is related to autoimmunity with shared genetic variants that could predispose individuals to both diseases.

Associations between pathogenesis of RA and prostate cancer
For the present study, a large sample size was used, including 3221 cases and 5272 controls. However, no PrCa risk SNPs were found to be significantly associated in the RA cohorts. This suggests that PrCa is not genetically linked to autoimmune diseases, at least for those SNPs at highest significance on GWAS.
There could be several possible reasons for the lack of association found. Firstly, we have still yet to discover the full extent of the genetic inheritability of PrCa and GWAS continue to report new SNPs. So far 18 GWAS have been reported in PrCa [48], with new studies due to be published, such as the Collaborative Oncological Gene-environment Study (COGS) analysis [49]. These undiscovered genes might yet uncover potential links. Different papers have reported a chronic inflammatory pathogenesis for many cancers, which include PrCa [26][27][28][29]. Epidemiological studies have also shown a correlation between PrCa incidence and autoimmune diseases, which lead to chronic inflammation and hence cancer [31,32]. It is therefore still possible that we might be missing genetic variants common to both diseases. Future analyses should be done to include a newer risk SNP profile. Conversely, analyses of the published RA risk loci in PrCa case-control cohorts should also be performed to determine any associations.
The second possible reason for the lack of association is that the present study design only allows us to explore common germline genetic variants identified from the PrCa GWAS. Rarer variants, which are not evaluated here, could be important in RA. The rare variants associated with PrCa risk have so far only been identified in DNA repair genes and the HOX gene, HOXB13 [50][51][52]. More recently, PrCa has also been shown to be a feature of Lynch syndrome, which is an autosomal condition caused by germline mutations in the DNA mismatch repair (MMR) genes [53]. Rare variants like these could also be important in autoimmune diseases such as RA. Previous groups have shown the association of defects in DNA repair genes with diseases involving the immune system, for example in severe common immunodeficiency (SCID). There have also been reports of polymorphisms in the DNA repair gene XRCC1 associated with the risk of developing RA [54,55]. GWAS have not been designed to study the contribution of rare or structural variants that the current next generation sequencing studies are targeting, and where it is likely that more genetic variants relating to PrCa risk will be discovered [56]. Future analyses should include these other forms of heritability. Furthermore, as only the SNP identified in PrCa GWAS was tested in RA, it could be that a different variant in the same gene/region is responsible for risk in the second disease.
In addition, although this is a relatively large study, it still has limited power to detect all the associations (averaged power across SNPs with MAF > 5% is 47% for OR = 1.1 and >90% for OR > 1.2), so failure to detect a signal could just be the result of stochastic variations.
Lastly, it could transpire that the autoimmune inflammatory pathogenesis pathways for both diseases are actually mutually exclusive. Most of the SNPs reported in RA are unsurprisingly associated with immune-related pathways [42], which relate to its pathogenesis. Unlike RA, the molecular basis for the aetiology of SNPs associated with PrCa risk remains unknown and it is possible that they exert their effect on other pathways via promoter or enhancer elements, leading to control of gene expression in genes located elsewhere [41]. Evaluating how the genetic variants initiate disease would allow a better understanding of the pathogenesis of PrCa. Currently, the only GWAS-risk SNP found to be potentially linked with autoimmunity is on chromosome 6p21, coding for CCHCR1 [17], which was not significantly associated with the RA cohorts in the present study after correction of confounding variables. The discovery of the functional elements of the other risk SNPs might feature common pathways in the future, which could be evaluated further to ascertain any true associations between PrCa and RA.
Studies like this are important to determine if common genetic risk profiles might predispose individuals to many diseases. The results could have implications for public health in terms of screening and chemoprevention. Future research is therefore warranted to investigate this link further.
In conclusion, there is currently no evidence that SNPs associated with PrCa at genome-wide significance are associated with the development of RA. Further work should be done using an updated profile to potentially include rare or structural variants. Associations between pathogenesis of RA and prostate cancer