Findings from the present study provide some evidence for a causal role of pancreatic β-cell function in the development of type 2 diabetes. First, our systematic analysis of up to 55 436 diabetes cases and 106 020 controls demonstrated robust associations of the genetic variants of TCF7L2 gene with both measures of β-cell function and the risk of type 2 diabetes. Moreover, our MR causal estimates by leveraging available genetic association evidence showed that lifelong averaged levels of various β-cell function measures were significantly and inversely associated with risk of type 2 diabetes. Such significant MR estimates indicate a causal role of β-cell function alone in the etiology of type 2 diabetes free of reverse causation and confounding from many factors after birth, especially during adulthood.
Our MR approach using TCF7L2
genotypes as an instrument represents an important and unique quantitative approach to assess the causal impact of genetically determined β-cell function on risk of type 2 diabetes. In agreement with recent evidence suggesting the role of TCF7L2
polymorphisms in the insulin function pathway (14
), our meta-analysis showed that the TCF7L2
genotypes were robustly associated with measures of β-cell function, supporting the hypothesis that the genetic effects on type 2 diabetes are likely mediated through β-cell function as the intermediate phenotypic product. The strength of the instrument is determined by the absolute magnitude of its association with the intermediate phenotype. Although we found evidence of an association between TCF7L2
variants and surrogates of insulin resistance such as fasting glucose, fasting insulin and 2 h glucose responses, such associations were expected because these biomarkers are not very specific and may to some extent reflect the concomitant β-cell function. Functional and genetic data supported that rs7903146 is the causal variant at this locus (17
). Although the precise molecular mechanisms remain to be determined, there is no clear evidence that the TCF7L2
alleles associated with diabetes risk have pleiotropic effects on metabolic or physiological systems other than pancreatic β-cell function (15
The robustness of associations between TCF7L2 variants and type 2 diabetes in genetically heterogeneous populations based on our systematic analyses of up to 55 436 diabetes cases and 106 020 controls further strengthens the precision and certainty of our inferences. In previous studies, the heterogeneous nature of observational studies and limited sample sizes make the results from individual studies less reliable. Furthermore, ethnic/racial differences in the frequencies of these known TCF7L2 variants are pronounced, with the allele frequency of the minor T allele at rs7903146 being low in East Asians but common in other populations. Such genetic heterogeneity of TCF7L2 in populations of various ethnic origins not only affects the power of individual studies but also indicates different patterns and magnitudes of linkage disequilibrium (LD) between variants in this gene among different ethnic groups (i.e. population-specific LD). Despite substantial ethnic difference in the frequency of TCF7L2 risk allele, we observed a robust association between TCF7L2 genotypes and type 2 diabetes risk in a strong genotype–dose manner with similar magnitude across subgroups. In addition, the assumption of no LD with causal variants or population stratification required for genetic instruments seemed not to be violated in our MR approach. Our meta-analysis synthesizing the final result from each ethnically homogenous population minimized the potential of population stratification.
Meeting all three basic assumptions (8
), in the present study, the MR analysis based on the evidence from the genotype–intermediate phenotype and the genotype–disease relations provided strong evidence supporting and quantifying the causal role of β-cell function in the etiology of type 2 diabetes. It has been hypothesized that impairments in early β-cell development can lead to fetal malnutrition and predisposes individuals to the development of type 2 diabetes later in life (24
). Beyond obesity and insulin resistance, β-cell function has been increasingly recognized as a central cause of type 2 diabetes. Because of measurement error, however, a single measure of β-cell function is not sufficient in capturing lifelong β-cell function levels. Dynamic and longitudinal measures of β-cell function are usually not practically feasible in large population studies. In addition, clinical assessment of insulin function is usually conducted during adulthood and has a hyperbolic relation with insulin action (22
). As such, studies based on these assessments cannot separate the impact of β-cell function from that of insulin/glucose action in peripheral tissues. As genetic variants in the TCF7L2
gene exert their effects across a lifetime, our estimates of genotype–phenotype relation may reflect the vast majority of variation of β-cell function measures attributable to TCF7L2
polymorphisms. Our MR estimates of the impact of β-cell function on type 2 diabetes risk may therefore largely represent a lifelong effect —a measure of average lifelong exposure, which incorporates various mitigating, compensating and attenuating patterns and lifestyle-related changes over a lifetime. In other words, our MR analysis provided precise estimates of the causal link from genetically determined lifetime levels of β-cell function to adult-onset type 2 diabetes, which were free of reverse causation or confounding by insulin resistance or impaired glucose metabolisms or other diabetes risk factors in adulthood.
Like all studies adopting MR approach, some limitations inherent in MR analysis merit consideration (8
). First, hypothetically, MR analysis is analogous to a randomized controlled trial of a drug treatment or intervention—where there is always a degree of non-adherence to the treatment drug, non-compliance to protocol, drop-out, drop-in and necessary compensatory/rescue drugs used in placebo/control groups that mimic canalization and incomplete penetrance. While MR analysis does require several important assumptions that need to be carefully evaluated prior to making causal inference, some of these assumptions cannot be empirically verified and requires subject-matter knowledge (e.g. absence of pleiotropy/heterogeneity of effects and canalization/the buffering effects of germline variants by developmental compensation via other biologic pathways) (8
). For example, it remains difficult to evaluate the assumptions of no canalization and no gene–environment interactions, which would require further experimental and intervention work. In the present study, however, the robustness and the strength of associations of multiple measures of β-cell function with type 2 diabetes risk using both conventional multivariable method and the MR analysis provide further some assurance of precision and certainty of our inferences. Secondly, β-cell function measures derived from IVGTT are usually regarded as the gold standard measures. Due to the limited number of available studies with IVGTT measures and the heterogeneity in IVGTT test protocols, confidence intervals (CI) of odds ratio (OR) or relative risk (RR) estimates of the genotype–phenotype association and the MR estimates of causal effect based on the IVGTT measures were somewhat wider and may be relatively less precise.
In conclusion, findings from the present study based on MR approach and systematic analyses of available population data lend support to the causal role of pancreatic β-cell function in the pathogenesis of type 2 diabetes. These findings not only further highlight the compelling need for the development and application of validated β-cell function measures for population studies, but also further support using preservation of pancreatic β-cell function as a target for the prevention or treatment of type 2 diabetes in general population.