We have here performed an EWAS for plasma resistin level in participants of the KING study with the use of the Infinium HumanMethylation450 array. Our analysis of 191 elderly Japanese men detected a genome-wide significant association of plasma resistin level with one DNAm site (cg02346997) located in the promoter region of RETN. We then focused our association analysis on the flanking region of RETN and found nominally significant associations of plasma resistin concentration with two additional DNAm sites (cg21777015, cg22322184) in the promoter region and 5′ untranslated region of RETN. A replication study of the relation between plasma resistin level and DNAm sites revealed genome-wide significant associations of cg02346997 and cg22322184 as well as a nominally significant association of cg21777015 in the Fukuoka study. A study of the relation between RETN mRNA abundance and DNAm sites in the flanking region of RETN with a publicly available data set for monocyte samples from the MESA study revealed genome-wide significant associations of cg02346997, cg22322184 and cg21777015 with RETN mRNA level in monocytes. Furthermore, we found that DNAm status for these three DNAm sites was associated with SNPs located in the promoter region of RETN in two Japanese cohorts, those of the KING and Fukuoka studies. In particular, the SNPs rs34861192 and rs3219175 and DNAm status at cg02346997 showed the most significant associations. The DNAm level at cg02346997 was thus negatively associated with plasma resistin level and RETN mRNA abundance in monocytes, and it decreased as the number of minor alleles for rs34861192 and rs3219175 increased. The circulating resistin level was previously found to increase as the number of minor alleles for these SNPs increased [14, 20]. Our results thus suggest that a causal relation between the plasma resistin level and SNPs in the promoter region of RETN is mediated via changes in DNAm in this region and in RETN transcription in monocytes.
The circulating concentration of resistin has previously been associated with SNPs in the promoter region of RETN including rs34681192, rs3219175 and rs1862513. These SNPs are located in the same LD block in the Japanese population [20], with rs34861192 and rs3219175 being in nearly complete LD [14, 20]. Among these three SNPs, rs1862513 was previously found to affect RETN promoter activity [31] and rs3219175 was suggested to influence the binding of proteins such as nuclear factor (NF)-κB [20]. However, the detailed mechanism by which rs34861192 and rs3219175 might affect plasma resistin level has remained unclear. The DNAm site cg02346997, found to be significantly associated with plasma resistin level in our study, is located only 28 bp downstream of the SNP rs3219175 (Fig. 1a), consistent with the notion that DNAm level at cg02346997 might be affected by rs3219175. Our results thus suggest that rs3219175 and rs34861192 might regulate plasma resistin level by influencing DNAm at cg02346997 and RETN transcription in monocytes.
We found that DNAm sites in a CpG island shore such as cg02346997, cg22322184 and cg21777015 were more significantly associated with plasma resistin level than were DNAm sites within the CpG island itself such as cg06379095, cg06411096 and cg13380624 (Fig. 1). A CpG island shore is defined as a 2000 bp region upstream (N shore) or downstream (S shore) of a CpG island [32]. Most differences in DNAm among normal tissues, or between normal and cancer tissues, have been found to occur in CpG island shores rather than in the islands themselves [32]. Our results are thus consistent with this finding, and they indicate that DNAm sites in the N shore of the CpG island of RETN, rather than those within the island itself, are functional with regard to regulation of circulating resistin level.
To identify potential monocyte-specific functional regions of RETN and its flanking region, we examined CTCF binding sites, histone modifications and DNase I peaks previously described for a human monocyte (CD14+) sample [33] (ESM Fig. 3). The region including DNAm sites cg02346997, cg21777015 and cg22322184 as well as SNPs rs34861192 and rs3219175 overlaps with a region characterised by DNase I hypersensitivity [34] and a high level of histone modification (H3K4m1/2/3, H3K27ac), which are indicators of a potential regulatory region. Binding sites for the transcription factors c-Rel, CCAAT/enhancer binding protein α (C/EBPα), activating transcription factor 2 (ATF2) and activator protein (AP1) have been identified within a 619 bp region upstream of the translation start site of RETN [35]. The DNAm site cg02346997 is located within this region. The transcription factor Sp1 was also found to play an important role in RETN transcription, with a predicted Sp1 binding site also being located near cg02346997 [35]. It is thus possible that DNAm at cg02346997 regulates RETN transcription by preventing the binding of these transcription factors to DNA.
Variation in DNAm level at cg02346997 might be affected by SNPs within ±1 bp of this site. Inspection of dbSNP build 142 revealed the SNP rs575656735 at 7,733,883 bp on chromosome 19, which is the same position as cg02346997. This SNP has major and minor alleles of C and T, respectively, and was reported by the Genome of the Netherlands (GoNL) project [36, 37], which aims to characterise DNA sequence variation in the Dutch population. The website of this project (www.nlgenome.nl) states that the frequency of the minor T allele of rs575656735 is low (0.1%) in the Dutch population. Furthermore, this SNP was not reported in the 1000 Genomes Project, which includes Japanese samples. It is therefore likely that this SNP does not contribute to the variation in DNAm at cg02346997 in Japanese individuals. Both rs545198984 and rs143039347 are located within ±1 bp of cg22322184, but these SNPs were not reported in Asian samples of the 1000 Genomes Project, suggesting that they also do not contribute to the variation in DNAm at cg22322184 in Japanese people.
The frequency of the minor A allele of rs3219175 was found to be high in Africans (12.9%) and East Asians (17.9%), but relatively low in Americans (1.2%), Europeans (1.0%) and South Asians (2.4%), in phase III of the 1000 Genomes Project. The SNPs rs3219175 and rs34861192 might thus be expected to contribute to the circulating resistin level in other East Asian and African populations in addition to Japanese. Indeed, rs34861192 and rs3219175 were found to be associated with the plasma resistin concentration, accounting for 13.3% and 12.8% of the variance in this variable, respectively in a homogeneous Malay population in Malaysia [38]. Our results suggest that these associations might also be mediated via changes in DNAm at cg02346997 and cg22322184. Furthermore, we detected a similar pattern of association between DNAm and either plasma resistin level or RETN expression in the KING and MESA data sets, which are derived from Japanese and multiple ethnic groups (whites, African-Americans, Hispanics) [28], respectively. This similarity in the association pattern might thus be due to the presence of African-American samples with a higher frequency of the A allele of rs3219175 in the MESA cohort.
Resistin has been implicated as a key player in obesity-related insulin resistance and type 2 diabetes [1]. We previously investigated the relations of the plasma resistin concentration and the SNP rs34861192 to type 2 diabetes and type 2 diabetes-related traits in 3133 Japanese individuals from the KING study, and we found that the plasma resistin level was positively associated with the fasting serum immunoreactive insulin concentration (IRI) and HOMA-IR [14]. The contribution rates of IRI and HOMA-IR to the plasma resistin level were low (R
2 = 0.005 and 0.003, respectively), however [14]. In addition, there was no significant association of rs34861192 with type 2 diabetes-related traits. In the present study, we examined the association of plasma resistin level, the SNP rs34861192, and the DNAm sites cg02346997 and cg22322184 with type 2 diabetes-related traits for the KING study participants. However, we detected no significant associations, possibly as a result of insufficient power due to the small sample size (the sample sizes required to detect contribution rates of 0.005 and 0.003 for the correlation analysis are 1566 and 2613, respectively, assuming α = 0.05 and power = 0.8). Further population-based association studies with larger sample sizes are thus required to elucidate the relations of circulating resistin level and cg02346997 and cg22322184 to type 2 diabetes and related traits.
With regard to limitations of the present study, the EWAS was performed with the Infinium HumanMethylation450 array, with the consequence that not all DNAm sites in the vicinity of RETN were inspected. Further insight into regulation of RETN expression by DNAm will require fine-mapping analysis with bisulfite sequencing. In addition, the present study is cross-sectional in nature and therefore does not establish a cause-and-effect relation between DNAm level at DNAm sites and plasma resistin concentration. Future studies are thus necessary to evaluate such relations in prospective cohorts. Finally, plasma resistin level might be influenced by post-transcriptional mechanisms, with the association between RETN mRNA abundance and plasma resistin level thus still requiring confirmation in the KING or Fukuoka studies.
In conclusion, we have demonstrated an association of DNAm level at cg02346997 with plasma resistin level and RETN mRNA abundance in monocytes. Furthermore, we found that DNAm level at this site was associated with the SNPs rs34861192 and rs3219175 located in the promoter region of RETN. Our results thus suggest the possibility that plasma resistin level might be influenced by SNPs in the promoter region of RETN via changes in DNAm in this region and in RETN mRNA abundance in monocytes.