Association between IL-1B (-511)/IL-1RN (VNTR) polymorphisms and type 2 diabetes: a systematic review and meta-analysis

Introduction

The latest data from the 9th edition of the IDF Diabetes Atlas show that approximately 463 million adults (20–79 years) have diabetes, and by 2045, the number will rise to 700 million. Type 2 diabetes mellitus (T2DM) is the most prevalent type of diabetes (>90% of diabetes). The proportion of T2DM patients is increasing in most countries, especially in low- and middle-income countries such as China and India (https://www.idf.org/aboutdiabetes/what-is-diabetes/facts-figures.html). Growing evidence suggests that the risk of T2DM is associated with various factors, such as genetics, ethnicity, environment, and lifestyle. In particular, genetic susceptibility seems to play an essential role in the pathogenesis of T2DM (Hivert, Vassy & Meigs, 2014).

T2DM occurs when β-cell function progressively deteriorates and fails to compensate for insulin resistance, partly due to the demise of pancreatic β-cells through apoptosis (Donath & Halban, 2004). The chronic and low-grade inflammation in metabolic organs including the liver, brain, pancreas and adipose tissue is known as metaflammation (metabolism-induced inflammation), which can be widely observed in T2DM (Hotamisligil, 2006; Hotamisligil, 2017). Increasing evidence has shown that metaflammation plays a vital role in the development of T2DM and its cardiovascular complications (Hotamisligil, 2017) and pro-inflammatory cytokines, especially interleukin-1β (IL-1β), play a key role in this process (Dinarello, Donath & Mandrup-Poulsen, 2010; Fève & Bastard, 2009; Sathyapalan & Atkin, 2011). Moreover, the increased level of IL-1β in human pancreatic cells due to the elevated glucose concentrations and decreased level of IL-1 receptor antagonist (IL-1Ra) in islets of T2DM patients result in impaired insulin secretion, decreased cell proliferation, and apoptosis of β-cells (Böni-Schnetzler & Donath, 2013). A previous observational study demonstrated that a combined elevation of IL-1β and IL-6 was associated with a roughly threefold increased risk of T2DM, and IL-1β might induce insulin resistance via activating the IκB kinaseβ (Spranger et al., 2003). Furthermore, CANTOS and TRACK trials have evaluated the role of IL-1 inhibition in the treatment of several inflammatory disorders, such as coronary artery disease, T2DM and rheumatoid arthritis (Qamar & Rader, 2012; Ruscitti et al., 2019). According to the result from CANTOS trials, canakinumab (a fully humanized monoclonal antibody) could selectively block IL-1β, and its efficacy in suppressing both levels of inflammatory markers and clinical symptoms in patients with autoinflammatory disease such as T2DM and impaired glucose tolerance has been observed in this trial (Aday & Ridker, 2018; Rissanen et al., 2012). Therefore, the genetic polymorphisms that regulate the expression levels of IL-1β and IL-1Ra might have an essential impact on the interindividual differences in T2DM.

Consisting of three linked genes and mapping to chromosome 2q13-24, IL-1 cytokine genes encode IL-1α, IL-1β and IL-1Ra. Both IL-1α and IL-1β are pro-inflammatory cytokines, while IL-1Ra can inhibit inflammation by competing for receptor binding (Dinarello, 2000). In the promoter region, IL-1B (-511) (rs16944, Chr 2:112,837,290, C/T Intragenic, Transition Substitution) has a bi-allelic polymorphism at position -511, representing the C/T transition (Kristiansen et al., 2000). IL-1RN (VNTR) (gene for IL-1 receptor antagonist, IL-1RA, 86 bp tandem repeats in intron 2) has five different alleles: 1* (four repeats), 2* (two repeats), 3* (five repeats), 4* (three repeats) and 5* (six repeats) (Tarlow et al., 1993). IL-1RN 1* (four repeats) and IL-1RN 2* (two repeats) are the most common, whereas the others occur much less frequently (<5%) (Santtila, Savinainen & Hurme, 1998). Many genetic association studies have been performed to estimate the relationship of the IL-1B (-511) and IL-1RN (VNTR) polymorphisms with T2DM risk (Achyut et al., 2007; Muktabhant et al., 2013). Several results suggest that there is no relationship between them (Borilova Linhartova et al., 2019; Muktabhant et al., 2013), whereas Achyut et al., (2007) showed that both the IL-1B (-511) and IL-1RN (VNTR) polymorphisms were associated with susceptibility to T2DM as well as complications, and he indicated that T2 (IL-1β −511T/IL-1RN*2) haplotype was associated with a roughly twofold increased risk of T2DM. Considering the small sample sizes (n <  600) and varying population characteristics (ethnic differences) in different studies, the results are conflicting. To illustrate the potential association of the IL-1B (-511) and IL-1RN (VNTR) polymorphisms with T2DM risk, we conducted a meta-analysis including 12 reported publications.

Materials and Methods

This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement guidelines (Moher et al., 2009). A protocol was registered before commencing this review on PROSPERO (CRD42021268494).

Results

Included studies

Figure 1 shows the detailed process of selecting and assessing eligible studies. We identified 450 publications via the initial keyword search. After screening, 12 publications met our inclusion criteria (Achyut et al., 2007; Blakemore et al., 1996; Borilova Linhartova et al., 2019; Cao et al., 2013; Doody et al., 2017; Lin et al., 2016; Liu et al., 2014; Muktabhant et al., 2013; Tayel et al., 2018; Zhang, Xiao & Li, 2004a; Zhang et al., 2004b; Zhou et al., 2010), and 438 studies were excluded for containing duplicate data, missing critical data, and being review papers or studies that were not related to T2DM. Table 1 illustrates the characteristics of all included publications in this meta-analysis. In terms of polymorphisms and diseases, six identified case-control studies, comprising 981 cases and 1,225 controls, regarded IL-1RN (VNTR) to evaluate its association with T2DM risk (Achyut et al., 2007; Blakemore et al., 1996; Borilova Linhartova et al., 2019; Zhang, Xiao & Li, 2004a; Zhang et al., 2004b; Zhou et al., 2010), and seven identified case-control studies, including 1,679 cases and 1,442 controls, tested the association between IL-IB (-511) and T2DM risk (Achyut et al., 2007; Cao et al., 2013; Doody et al., 2017; Lin et al., 2016; Liu et al., 2014; Muktabhant et al., 2013; Tayel et al., 2018). Stratified by ethnicity, six publications were performed in East Asian (EA) populations (Cao et al., 2013; Lin et al., 2016; Liu et al., 2014; Zhang, Xiao & Li, 2004a; Zhang et al., 2004b; Zhou et al., 2010), two studies were undertaken in South Asian (SA) populations (Achyut et al., 2007; Doody et al., 2017), two studies were conducted in Caucasian populations (Blakemore et al., 1996; Borilova Linhartova et al., 2019), and the remaining studies were completed in Southeast Asian (SA) (Muktabhant et al., 2013) and North African (NA₁) populations (Tayel et al., 2018).

Sensitivity and publication bias analysis

We next performed sensitivity analysis on the association between IL-1β (-511) and T2DM. The results showed that Achyut’s study had an influence on the pooled OR in the IL-1β (-511)-dominant model (OR = 0.71, 95% CI [0.53–0.96]) and homozygous model (OR = 0.65, 95% CI [0.44–0.97]) (Figs. 2B, 2D) but did not affect the pooled OR in the recessive model (Fig. 2C). Achyut’s study was performed in a SA population; thus, there was no influence on the pooled OR in the EA population. Due to the lack of heterogeneity in IL-1RN recessive and homozygous models, sensitivity analysis was not performed.

The results of the Harbord test and Peters test showed no publication bias for IL-1B (-511) in the dominant model (P_har = 0.759 and P_pet = 0.881, respectively) (Fig. 4A). Moreover, no evidence of publication bias was found for IL-1RN (VNTR) in the recessive (P_har = 0.498 and P_pet = 0.495, respectively) or homozygous models (P_har = 0.634 and P_pet = 0.481, respectively) (Figs. 4B, 4C).

Discussion

The IL-1B (-511) and IL-1RN (VNTR) polymorphisms have been reported to be related to the pathogenesis of T2DM (Ehses et al., 2009; Masters et al., 2010), and many case-control studies have illustrated the association of IL-1B (-511) and IL-1RN (VNTR) polymorphisms with T2DM risk. However, each individual study may not have been powerful, causing their results to be controversial. Given the above, we conducted the present meta-analysis to derive a more precise evaluation of the association of IL-1B (-511) and IL-1RN (VNTR) polymorphisms with T2DM risk. We found that (i) the T allele of IL-1B (-511) was related with a decreased T2DM risk in the EA subgroup; (ii) significantly elevated risk of developing T2DM was observed to be associated with the 2* allele in all ethnicities and EA subgroup, and its carriers of the IL-1RN (VNTR) polymorphism had an increased risk of developing T2DM among all ethnicities; (iii) compared to the 1*/1* homozygotes, the 2*/2* homozygotes of the IL-1RN (VNTR) polymorphism had an increased risk of T2DM development; and (iv) no significant association was observed between the IL-1RN (VNTR) polymorphism and T2DM risk in the EA subgroup. The results of sensitivity analysis showed that Achyut’s study affected the pooled ORs in the IL-1β (-511) dominant and homozygous models. However, Achyut’s study did not influence the pooled OR in the EA subgroup after stratification by ethnicity. Therefore, the result of the association between IL-1B (-511) and the decreased risk of T2DM in the dominant model in the EA subgroup was convincing.

First, IL-1β was considered a mediator of fever, and rapidly, it was found that IL-1β induced innate immunity to defend against pathogens. However, chronic overexpression of IL-1β has been related to multiple immune diseases, including T2DM (Mandrup-Poulsen, 1996). Eventually, it was suggested that IL-1β can induce the inflammatory microenvironment of islets, leading to impaired insulin secretion, decreased cell proliferation and apoptosis of β-cells, ultimately contributing to the development of T2DM (Rhodes, 2005). Donath, M Y et al. extensively described the development of islet inflammation in GK rats, suggesting that the expression of pro-inflammatory cytokines IL-1β and others (IL-6, TNFα) was increased in islets, while elevated expression levels of many chemokines (CXCL1/KC, MCP-1, MIP-1α) and infiltration of immune cells in islets were observed (Ehses et al., 2009). In all cases, macrophage infiltration is increased in the islet inflammatory process (Ehses et al., 2007). Moreover, two independent studies have now indicated that the numbers of islet-related CD68⁺ cells are increased in T2DM patients (Ehses et al., 2007; Richardson et al., 2009). In addition, data obtained from laser-captured β-cells from T2DM patients have shown that evaluated expression levels of IL-1β and chemokines possibly led to this immune cell infiltration (Böni-Schnetzler et al., 2008). Since it can regulate various inflammatory processes, any change in the level of IL-1β in blood or tissue possibly affects these processes. Larsen et al. suggested that blocking IL-1β is a potential therapy in the treatment of T2DM (Larsen et al., 2007). The IL-1B (-511) C allele is associated with higher expression of IL-1β and with severe inflammation in the liver, while the (-511) T allele is associated with lower levels of IL-1β (Hirankarn et al., 2006; Vishnoi et al., 2008). In this meta-analysis, we found that the IL-1B (-511) T allele had a decreased T2DM risk in the EA subgroup, suggesting an influence of the IL-1B (-511) polymorphism system in this ethnic group.

The IL-1RN 2* allele (two repeats) is associated with increased IL-1Ra levels (Danis et al., 1995), which can compete with an inhibitor of IL-1β by binding to the IL-1 receptor. The 2* allele significantly increases the IL-1β secretion in vitro and balances the expression of IL-1β and IL-1Ra (Santtila, Savinainen & Hurme, 1998), and their ratio (IL-1Ra/IL-1β) determines the severity of inflammation. It has been shown that the IL-1RN 2* allele is associated with a low ratio (IL-1Ra/IL-1β), thereby inducing a longer and more severe pro-inflammation (Witkin, Gerber & Ledger, 2002). The frequency of the 2* allele is increased in inflammatory conditions or autoimmune diseases (Tountas et al., 1999; Van der Paardt et al., 2002). Consistently, this meta-analysis suggested that the IL-1RN 2* allele and 2*2* homozygotes increased T2DM risk, which indicated that the IL-1RN 2* allele and 2*2* homozygote polymorphism play critical roles in the development of T2DM.

Although we have devoted considerable efforts and resources to testing the potential association of the IL-1B (-511) and IL-1RN (VNTR) polymorphisms with T2DM risk, this analysis still has some limitations. First, significant heterogeneity was found in pooled analyses between the IL-1B (-511) polymorphism and T2DM risk. Due to the limited number of included publications, we conducted further ethnic stratification analysis and meta-regression to identify the exact sources of between-study heterogeneity. The meta-regression results showed that ethnicity was not the source of heterogeneity (both values of tau² were not much different (0.34 vs 0.40)). Some possible relevant factors (genotyping method, sex) may lead to heterogeneity. However, subgroup analysis can significantly reduce heterogeneity. Second, the literature articles were primarily published in English or Chinese; thus, some eligible publications may be missing, causing some bias. Third, the publications included have some confounders, such as age, sex, diet, and exercise, whereas we used only the raw data to conduct pooled analysis; therefore, we were unable to control these possible confounders or test the potential gene-environment interactions. Fourth, the number of included publications was limited; thus, additional studies with large sample sizes and a wider variety of ethnicities are needed in the future to evaluate the association.

In conclusion, our meta-analysis first indicated that the IL-1B (-511) T allele polymorphism is associated with decreased T2DM risk in the EA population and that the IL-1RN 2* allele and 2*2 * homozygote polymorphism are strongly associated with increased T2DM risk. Further well-designed studies including different ethnicities with large sample sizes are needed to verify this conclusion.

Supplemental Information