We observed statistically significant associations of the major alleles of rs10010131, rs1801213 and rs734312 in the WFS1 locus with an increased risk of type 2 diabetes in individuals with NFG or IFG in a 9 year prospective study in a French cohort issued from the general population. Only a trend towards an association with one of the variants (rs1801213) was observed when we considered the incidence of all hyperglycaemic events (NFG to IFG, NFG to diabetes or IFG to diabetes change) during follow-up. The variants were also associated with the prevalence of IFG and/or diabetes at baseline. Together, these results suggest that allelic variations of WFS1 increase the risk of diabetes, but have a lesser impact on the risk of IFG in the DESIR cohort. Associations with diabetes were confirmed in cross-sectional studies of three cohorts of type 2 diabetic patients compared with normoglycaemic controls. The most frequent haplotype at the haplotype block containing the WFS1 gene was associated with an increased risk of diabetes in the prospective study and with the prevalence of diabetes in cross-sectional studies.
Our results are in agreement with data from the literature. Sandhu and co-workers identified associations of the major alleles of rs10010131, rs6446482, rs752854 and rs734312 SNPs with WFS1 and diabetes risk in 9,533 cases and 11,389 controls from UK and Ashkenazi populations [11]. Lyssenko and co-workers observed an association between the major allele of rs10010131 and an increased risk of diabetes in two prospective cohorts with a median follow-up of 23.5 years and including 16,061 Swedish and 2,770 Finnish participants [13]. An association between rs752854 and type 2 diabetes was also observed in a Swedish cross-sectional study of 1,296 cases and 1,412 controls, with the minor allele conferring protection against diabetes [12]. Other studies have yielded less conclusive results. Florez and co-workers examined the effects of the rs10010131, rs752854 and rs734312 variants on diabetes incidence and response to interventions in the Diabetes Prevention Program, in which a lifestyle intervention or metformin treatment was compared with placebo [28]. They found no statistically significant interactions between genotypes and Diabetes Prevention Program intervention for any of the SNPs. None showed a statistically significant effect on diabetes incidence in the entire cohort, although in the lifestyle arm homozygosis for the minor alleles was consistent with protection from diabetes. Only a trend towards association with diabetes was observed for the rs10010131 variant in a Chinese cross-sectional study of 1,849 type 2 diabetic patients and 1,785 controls [16]. Although it is clear that WFS1 mutations cause diabetes [7, 8], the genetic basis of the association of these common WFS1 variants with type 2 diabetes is still unclear. A recent fine-mapping study in UK, Swedish and Ashkenazi populations identified six novel, highly correlated SNPs showing strong and comparable associations with type 2 diabetes risk [29]. None had obvious functional properties that could predict deleterious effects on protein function. Given the strong linkage disequilibrium in the WFS1 region, the authors suggested that further refinement of these associations for identification of the variants responsible for increased diabetes risk would require studies in ethnically diverse populations and/or studies with much larger sample sizes (10,000 to 100,000 individuals) [29].
We also observed statistically significant associations of the risk alleles of the three SNPs with higher FPG and HbA1c levels, and lower HOMA%B levels, both at baseline and throughout the study in participants who developed hyperglycaemia or type 2 diabetes. Interestingly, no allelic association with these intermediate phenotypes was observed in individuals who remained normoglycaemic at the end of the follow-up. The design of our study had some intrinsic limitations. Glucose tolerance status of participants was based on FPG only and not on an OGTT. Second, insulin secretion and insulin sensitivity were assessed by HOMA indices, which provide only an estimation of these variables. Nonetheless, we do not believe that these limitations have substantially biased our results, as they are in agreement with other observations. Sparso and co-workers observed a similar interaction between the glycaemic status and the genotype effect of rs734312 on insulin secretion in the Inter99 cohort, which involved 4,568 glucose-tolerant individuals and 1,471 individuals with treatment-naive abnormal glucose regulation (IFG, impaired glucose tolerance or screen-detected diabetes) [14]. The risk allele was associated with a decrease in insulinogenic index and decreased 30 min serum insulin levels after an oral glucose load in individuals with abnormal glucose regulation, but not in participants with normal glucose tolerance. It seems that this interaction between glycaemic status and genotype might explain the heterogeneous results reported by previous studies of associations between WFS1 variants and insulin secretion. Schafer and co-workers studied 1,578 non-diabetic German individuals at increased risk of type 2 diabetes, i.e. persons with a history of gestational diabetes, IFG, impaired glucose tolerance or family history of type 2 diabetes in first-degree relatives [15]. They found that the rs10010131 risk allele was associated with reduced insulin secretion during an OGTT. Interestingly, insulin secretion in response to an IVGTT or a hyperglycaemic clamp was not different between genotypes. However, when the hyperglycaemic clamp was combined with a glucagon-like peptide 1 (GLP-1) infusion, first- and second-phase insulin secretion were significantly decreased in carriers of the risk allele. A study from the Netherlands with similar size and power, but performed in participants with normal glucose tolerance, observed no significant genotype effect on insulin secretion in response to GLP-1 and glucose infusion during a hyperglycaemic clamp [17]. Other studies in large cohorts of non-diabetic Finnish [18] or Chinese [16] participants observed no association between rs10010131 and insulin secretion during an OGTT.
WFS1 protects cells from the damaging effects of hyperactivation of ER stress signalling [3]. ER stress is caused by physiological and pathological stimuli that lead to accumulation of a large load of unfolded and misfolded proteins in the ER. For instance, physiological ER stress occurs in pancreatic beta cells during postprandial stimulation of insulin synthesis [30]. Activation of ER stress signalling is tightly regulated because hyperactivation or chronic activation of this signalling pathway can cause cell death by inducing apoptosis [31]. Recent results have shown that WFS1 has an important function in the tight regulation of ER stress through its interaction with activating transcription factor 6α (ATF6α), a key transcription factor implicated in ER stress signalling [3]. WFS1 recruits ATF6α to the proteasome, where in the absence of ER stress it undergoes degradation. In situations of ER stress, ATF6α is released from WFS1 in the ER membrane and translocates to the nucleus, where it upregulates stress signalling targets. As WFS1 is induced by ER stress, it causes eventual degradation of ATF6α when ER homeostasis is established.
A possible reason why WFS1 variants only associate with insulin secretion in individuals with abnormal glucose regulation is that these polymorphisms have only a minor impact on WFS1 function in physiological situations. Thus, their effect on insulin secretion can be best appreciated in situations of chronic activation of ER stress signalling, such as in participants with decreased insulin sensitivity and chronic stimulation of insulin synthesis. Moreover, participants with abnormal glucose regulation may have other stressors of beta cell function, including unfavourable alleles in other pancreatic beta cell genes, as well as glucotoxicity, lipotoxicity or defects in the potentiation of insulin secretion by incretins (GLP-1, glucose-dependent insulinotropic polypeptide) [32], which could interact with susceptibility variants of WFS1 and aggravate ER stress-mediated apoptosis. WFS1 risk alleles have been shown to be associated with reduced insulin response to oral, but not to intravenous glucose [14, 15], and with reduced response to GLP-1 stimulation during hyperglycaemic clamps [15]. This suggests an incretin-related defect. GLP-1 not only enhances insulin secretion in response to glucose, but also directly modulates the ER stress response, preventing beta cell apoptosis and promoting beta cell adaptation and survival [32, 33]. At the present time, the mechanisms of the putative interaction of GLP-1 signalling and WFS1 on ER function are still unclear.
In conclusion, allelic variations in the WFS1 gene modulated insulin secretion in normoglycaemic French individuals at risk of developing type 2 diabetes and also increased the risk of type 2 diabetes in a 9 year prospective study. In our study, the most frequent haplotype at the haplotype block containing the WFS1 gene was associated with an increased risk of diabetes. The genetic basis of these associations is still unclear. Further studies with much larger sample sizes are required to identify the functional variants responsible for increased diabetes risk.