Introduction
Type 2 diabetes (T2D) is a severe metabolic disease caused primarily by the inadequate production or secretion of insulin [
1]. Previous studies have revealed that T2D patients have a significantly increased risk of suffering from peripheral artery disease (PAD) [
2,
3], a chronic circulatory condition characterized by the constriction of arteries that serves to slow the blood flow in the limbs [
4,
5]. Evidence has accumulated for a higher mortality rate among patients with co-morbid T2D and PAD than among patients with PAD alone [
6], illustrating the importance of understanding the pathophysiological link between T2D and PAD.
The observed association between T2D and PAD can be explained in part by a shared genetic basis [
7‐
9]. For example, Strawbridge and van Zuydam identified
CDKN2A/B as candidate genes likely to be involved in regulating both T2D and PAD [
8]. A recent study also replicated
CDKN2A as being associated with PAD in individuals with diabetes [
10]. Vujkovic et al. analyzed ~ 1.4 million individuals of multiple ethnicities (including Europeans, African Americans, Hispanics, South Asians, and East Asians), thereby disclosing significant associations between 318 genetic variants and the risk of both T2D and PAD [
9]. Nevertheless, our knowledge of the nature and extent of the genetic associations between T2D and PAD is still far from clear. Furthermore, whether the associations between T2D and PAD differ among ethnic groups remains uncertain, and whether the genetic associations between the two diseases reflect a causal relationship or pleiotropy is unknown.
The growth of genome-wide association studies (GWAS) over the past two decades has stimulated the development of approaches to analyze cross-trait shared genetic architecture based on GWAS summary statistics. For example, Bulik-Sullivan et al. proposed the GWAS-based linkage disequilibrium score regression (LDSC) statistic as a means to estimate the single nucleotide polymorphism (SNP)-based genetic correlation between traits [
11]. Mendelian randomization (MR) has been proposed as a way to measure the putative causal relationship between traits. MR uses genetic variants as instruments to mimic a random allocation procedure in randomized controlled trials [
12], thereby avoiding issues of confounding and reverse causation [
13]. With the advent of large-scale biobanks involving multiple ethnicities (e.g., UK Biobank [UKB], BioBank Japan [BBJ]), it became possible to compare the cross-trait shared genetic architecture between different ethnicities in order to explore the possibility of trans-ethnic genetic heterogeneity. To date, the applications of these approaches and datasets have achieved considerable success in increasing our understanding of the shared genetic architecture between complex diseases [
14‐
16], thereby superseding traditional experimental models which are much more time-consuming and costlier to implement. Thus, using GWAS-based approaches to analyze GWAS summary data for T2D and PAD derived from different ethnic groups provides an opportunity for us to study the shared genetic basis between the two diseases.
In this study, we applied LDSC and seven MR or MR-equivalent approaches to estimate the genetic correlation and potential causal relationship between T2D and PAD in Europeans and East Asians. Further multivariable Mendelian randomization analysis (MVMR) was used to examine whether the putative causal relationship between T2D and PAD can be affected by traits associated with the increased risk of PAD or T2D. A validation of genetic overlap between T2D and PAD was then performed using independent samples to explore whether the polygenic risk scores (PRS) for T2D could predict the status of PAD, and vice versa. Finally, we performed a multi-trait analysis of GWAS (MTAG) and two gene-based analyses (i.e., multi-marker analysis of genomic annotation [MAGMA] and summary data-based Mendelian randomization [SMR]) to identify the risk SNPs and functional genes that are likely to be responsible for the shared genetic etiology underlying T2D and PAD. We applied the analytical pipeline to Europeans, and also to East Asians independently as a replicated analysis, to investigate the common and distinct mechanisms of co-occurring T2D-PAD between Europeans and East Asians.
Discussion
In this study, we systematically investigated the shared genetic architecture between T2D and PAD in Europeans and East Asians by leveraging their large-scale GWAS summary statistics. Our study disclosed a putative causal effect of T2D upon PAD in both Europeans and East Asians and identified several novel loci/functional genes that might be relevant to the shared genetics underlying T2D and PAD. We additionally used MVMR analysis to exclude an underlying heritable trait as the basis of the causal relationship between T2D and PAD and indicated that the causal effect of T2D on PAD is not influenced by TC, BMI, SBP, or smoking initiation. All these traits are generally considered to be factors that are associated with a high risk of PAD and T2D, and they are known to be related with dysfunction of lipoproteins or hemostasis [
64,
65], etc. Indeed, recent studies have revealed lipoproteins and hemostatic factors to be risk factors for PAD [
66‐
68]. Our study may indirectly indicate that neither the lipoproteins nor hemostatic factors have much influence on the causal relationship between T2D and PAD. Taken together, this study not only improves our understanding of the genetic etiology shared by T2D and PAD but also provides evidence to support the concept of genetically screening T2D patients in order to pre-symptomatically detect/prevent PAD.
To our knowledge, this is the first study to investigate the shared genetic architecture between T2D and PAD, although a significant clinical/phenotypic correlation has been previously reported [
69,
70]. One of the main aims of this study was to investigate the shared genetic relationship between T2D and PAD in Europeans and East Asians. On the basis of an online LDSC power calculation (
https://nealelab.github.io/UKBB_ldsc/viz_sampsize.html), we believe our input GWAS summary statistics have sufficient power (> 95%) to uncover shared genetic associations between T2D and PAD in either Europeans or East Asians. Here, we observed a genetic correlation between the two disease entities in both Europeans and East Asians. The estimated
h2 of T2D and PAD, as well as the
rg between T2D and PAD in Europeans, were largely similar to those in East Asians, suggesting a strong genetic correlation between T2D and PAD in both ethnicities.
In both Europeans and East Asians, we applied multiple MR methods and consistently identified the putative causal effect of T2D in relation to PAD, suggesting a common causal genetic relationship between T2D and PAD in Europeans and East Asians. Among multiple MR methods, CAUSE and LCV were the only models capable of distinguishing both correlated and uncorrelated pleiotropy from causality. LCV can yield conservative results when the genetic correlation is mild and cannot measure the extent of the causal effect directly. Other MR models can only deal with uncorrelated pleiotropy or partially correlated pleiotropy. Therefore, we consider CAUSE to be a prior MR method compared to other models. In our study, CAUSE showed consistent evidence for a putative causal effect of T2D on PAD, indicating that our findings are credible. Furthermore, MVMR analysis indicated that the putative causal effect of T2D on PAD is less likely to be influenced by TC, BMI, SBP, and smoking which have generally been considered to be common factors associated with the increased risk of PAD [
71‐
76]. We further identified a higher PRS for T2D among PAD patients compared to non-PAD individuals using PRS regression, in line with previous studies that have reported T2D as a risk factor for arterial disease (such as coronary artery disease and arterial stiffness [
77,
78]). Taken together, we conclude that T2D is a causal risk factor for PAD, although further studies with larger sample sizes (particularly with reference to additional PAD patients) will be required to establish this causal relationship conclusively. Nevertheless, several MR models (MR-Egger, weighted mode, and LCV) failed to fully distinguish causality from horizontal pleiotropy, after allowing for Bonferroni correction. The underlying reason may be the limitations inherent to these models. For instance, MR-Egger may inflate type I errors when horizontal pleiotropy from instrumental SNPs occurs via the same confounder [
79]. The weighted mode is thought to be less powerful in comparison with other MR methods as it uses a smaller set of instrumental SNPs [
80]. The standard error of GCP statistics produced by LCV analysis may be enhanced when analyzing the trait pair with mild or moderate genetic correlation [
43].
Previous studies have provided support for the conclusion that while East Asians are more prone to T2D than Europeans [
1,
81,
82], the prevalence of PAD in T2D patients is lower in East Asians than in Europeans [
83]. The underlying reason has remained unclear, although genetic factors are thought to make important contributions to the ethnic disparities in relation to T2D and PAD prevalence [
83‐
86]. Herein, we provide the first evidence for the common and distinct mechanisms underlying the shared genetics of T2D and PAD between Europeans and East Asians. We used MAGMA and SMR analyses to identify the
ANKFY1 gene as showing significant associations with the cross-trait shared architecture of T2D and PAD in both Europeans and East Asians.
ANKFY1 is thought to play a role in angiogenesis on the basis of experiments in endothelial cells [
87,
88] and has been reported as a gene conferring T2D risk in a GWAS study [
34]. Another study has identified a homozygous missense mutation in
ANKFY1 to be a potential cause of nephrotic syndrome [
89], a disease commonly comorbid with diabetes and cardiovascular disease [
90,
91]. Moreover, protein-protein interaction analysis provided further evidence for common mechanisms shared between Europeans and East Asians by revealing gene-gene interactions underlying the comorbidity of T2D and PAD, specifically the interactions between
STARD10 (European-specific) and
AP3S2 (East Asian-specific) and between
KCNJ11 (European-specific) and
KCNQ1 (East Asian-specific). In the context of the interaction of
STARD10 (European-specific) and
AP3S2 (East Asian-specific),
STARD10 is a phospholipid transfer protein whose expression in isolated pancreatic islets has been found to influence the production and processing of insulin in the mouse [
92]. Prior studies indicated that
STARD10 harbors SNPs associated with T2D [
93,
94].
AP3S2 encodes a protein involved in protein transport whose variants are associated with the risk of T2D in South Asians and Japanese [
95,
96]. Other studies have implicated
AP3S2 as being important for the pathogenesis of T2D [
97‐
101]. Regarding the interaction between
KCNJ11 and
KCNQ1, both genes contribute to potassium channels whereas genetic variants located near both
KCNJ11 and
KCNQ1 are known to be significantly associated with T2D [
9,
102]. Here, we revealed their potential roles in the occurrence of PAD in T2D patients in both Europeans and East Asians. In addition, numerous genes identified as being associated with the cross-trait shared architecture of T2D and PAD in Europeans have similar functions to those in East Asians. For example,
TP53INP1 (European-specific) and
MOB1B (East Asian-specific) both have functions related to cell migration and death [
103,
104];
CHMP4B (European-specific) and
AP3S2 (East Asian-specific) are involved in protein transport and intracellular trafficking [
105,
106]. These genes were reported as harboring SNPs significantly associated with T2D [
1,
107,
108]. Here, they are suggestive of common genetic mechanisms of comorbid T2D and PAD between the two populations.
The distinct genetic mechanisms of co-occurrence of T2D and PAD between Europeans and East Asians may be explicable in terms of the discrepant prevalence of T2D/PAD between Europeans and East Asians. The discrepant prevalence of T2D between ethnic groups may be attributed to insulin resistance and β-cell dysfunction [
109]. T2D in Europeans tends to be caused by obesity and insulin resistance whereas T2D in East Asians, who are generally characterized by a leaner body mass, is caused by more severe β-cell dysfunction [
82]. Our study potentially supports the view that the distinct genetic mechanisms of co-occurrence of T2D and PAD could be due, at least in part, to the differences in the prevalence of insulin resistance and β-cell dysfunction between the two populations. For instance, we identified the insulin resistance-related gene
TIGAR in Europeans and the β-cell dysfunction-related gene
TLE1 in East Asians as being associated with the cross-trait shared architecture of T2D and PAD (Additional file
1: Table S9) [
110,
111].
TIGAR, activated by p53, promotes insulin resistance in the rat whereas
TLE1 has been found to play a role in inducing pancreatic β-cells and converting α-cells to β-cells [
110,
111]. Further studies are required to explore the different roles that insulin resistance and β-cell dysfunction play in causing T2D-PAD comorbidity in Europeans and East Asians.
Our findings are potentially important for pre-symptomatic screening in order to prevent the occurrence of PAD in those T2D patients with genetic defects associated with PAD. Identifying incipient PAD among T2D patients and carrying out clinical interventions prior to the onset of arterial disease has the potential to dramatically improve the life quality and prognosis of T2D patients [
112]. Our results could lead to the early-stage genetic screening for PAD in T2D patients thereby offering more effective diagnostic and therapeutic approaches for both diseases. Additionally, our study revealed the putative shared/distinct mechanisms underlying the causal relationship between T2D and PAD in Europeans and East Asians, which highlights the need for personalized intervention in designing therapies for PAD in T2D patients with different ethnic backgrounds.
Study limitation
Our study has several limitations. First, the sample size of East Asians in the UKB cohort is insufficient, thereby limiting the power of the study to confirm the putative causal relationship between T2D and PAD in East Asians via PRS analysis. Clearly, the results of this study should be replicated on a much larger group of East Asians. Second, neither European- nor East Asian-based PAD GWAS have enough instrumental SNPs (
p-value < 5 × 10
−8) for testing with some MR models. Alternatively, we relaxed the
p-value threshold and recruited the “proxy” instrumental SNPs (with
p-value < 1 × 10
−5), which may have led to a violation of MR assumptions. Nevertheless, this influence is likely to be negligible as we obtained very similar results when applying multiple MR methods for sensitivity analysis. Third, the SMR results for East Asian-based GWAS were probably underpowered, as the eQTL summary data from eQTLGen were of European descent, which hampered our ability to identify trans-ethnic functional genes underlying T2D and PAD. Fourth, we excluded SNPs in the MHC region from all our analyses because of the complicated LD pattern within the MHC region; this could have led to an underestimation of the shared genetic basis between T2D and PAD. Finally, the classification of ICD-10-based T2D cases in our study may be less precise because we cannot filter out some rare cases of maturity-onset diabetes of the young (MODY). Nevertheless, as the proportion of MODY comprises only 1-5% of total diabetes [
113], we believe such influence is likely to be minimal.
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