We report the association testing of six recently discovered type 2 diabetes risk variants (6
) with intermediary diabetes-related phenotypes. Our results, if replicated in independent and statistically well-powered studies, suggest an impairment of pancreatic β-cell function for diabetes risk alleles in or near JAZF1
, and TSPAN8
, since these variants were associated with various surrogate measures of insulin release during an OGTT. Further support of the role of the CDC123
variants in altered pancreatic β-cell function was provided when analyzing an OGTT-based disposition index and for JAZF1
variants when doing multivariate analysis of estimates of insulin sensitivity and insulin release. The observed associations for all three variants are concordant with an impaired oral glucose-stimulated insulin release in subjects carrying the reported type 2 diabetes risk alleles (6
In the analyses, we primarily focused on glucose-tolerant subjects to avoid the confounding influence of disturbances in glucose homeostasis and to circumvent the risk that associations with especially impaired insulin response were driven by the known association with type 2 diabetes. We did, however, observe similar results when including subjects with impaired fasting glycemia, impaired glucose tolerance, or screen-detected type 2 diabetes.
rs864745 resides in intron 1 of the JAZF1
(juxtaposed with another zinc finger gene 1) gene, which encodes a transcriptional repressor of the nuclear receptor subfamily 2, group C, member 2 (NR2C2
) gene (17
). NR2C2 (also known as TR4) is a member of the nuclear hormone receptor family and acts as a ligand-activated transcription factor (18
is widely expressed and Nr2c2−/−
knockout mice display a phenotype of growth retardation, hypoglycemia, and reduced gluconeogenesis by decreased activation of PEPCK
); however, no obvious involvement in pancreatic β-cell function has been demonstrated. Yet, since JAZF1
is expressed in the pancreas (17
), one might speculate that a gain-of-function variant in JAZF1
may lead to postnatal growth restriction also affecting pancreatic β-cell mass and function.
rs12779790 is located ~90 kb from CDC123
and ~63.5 kb from CAMK1D. CDC123
(cell division cycle 123 homolog [S. cerevisiae
]) encodes a protein involved in cell cycle regulation and nutritional control of gene transcription with no known relation to type 2 diabetes pathogenesis (21
). Because CAMK1D
(calcium/calmodulin-dependent protein kinase I delta) regulates granulocyte function (22
), it is also possible that a causative variant in this region is related to CAMK1D
and affects pancreatic β-cell function through increased apoptosis.
Lastly, rs7961581 resides ~110 kb upstream of TSPAN8
(tetraspanin 8), which encodes a widely expressed cell surface glycoprotein known to form complexes with integrins to regulate cell motility in cancer cell lines (23
). Because α6-integrin binding to laminin has been shown to negatively affect pancreatic β-cell mass maintenance (24
), it is possible that variation in TSPAN8
biologically influences pancreatic β-cell function.
In this article, we have performed a thorough evaluation of a range of OGTT-based surrogate estimates of insulin release and insulin sensitivity. The associations of examined gene variants to various measures of pancreatic β-cell function highlight the need for cautious interpretation of outcomes. Variants in the CDC123
regions associate with the insulinogenic index, the corrected insulin response, and the ratio of AUC-insulin to AUC-glucose, which are widely used and well-documented estimates of insulin release (25
), yet not with the recently described BIGTT-AIR index (16
), and the opposite is true for the JAZF1
variant. These discrepancies may be caused by different accuracy and/or sensitivity of the applied surrogate indexes or the possibility that the different indexes capture particular and diverse roles of the encoded proteins in specific steps of insulin biosynthesis, insulin secretion, or insulin elimination. However, we cannot exclude that the associations to various measures are caused by statistical type I or II errors. Although we analyzed a range of OGTT-based surrogate indexes of insulin release, we acknowledge that application of more precise measures of insulin release, such as estimates based on an intravenous glucose tolerance test, may have modified the outcome of our analyses.
Type 2 diabetes–associated variants in the THADA, ADAMTS9, and NOTCH2 loci did not associate with metabolic traits in the Inter99 cohort. Lack of statistical power is a possible explanation, since these variants confer a modestly increased risk of type 2 diabetes. Based on 95% CIs of effect size estimates, we can with confidence exclude an allele-dependent effect in the current study on BMI, insulinogenic index, BIGTT-AIR, and ISI above 4.5% for THADA rs7578597, 3% for ADAMTS9 rs4607103, and 4% for NOTCH2 rs10923931. However, we are unable to estimate potential associations below these effect sizes.
We recognize that since no correction for multiple hypothesis testing was applied, the present results are of an explorative nature and call for validation in statistically powered and well-characterized cohorts. If, however, stringent Bonferroni correction for multiple testing (252 tests) was performed, only the associations of the CDC123/CAMK1D rs12779790 variant with measures of insulin response (insulinogenic index and serum insulin at 30 min during the OGTT) would remain statistically significant, underlining the need for replication. Based on the effect sizes of the current study, we estimate that ~3,300, 6,100, and 3,900 subjects are needed for future studies to achieve 80% statistical power to replicate associations of JAZF1 rs864745 with BIGTT-AIR (additive model), CDC123/CAMK1D rs12779790 with insulinogenic index (recessive model), and TSPAN8 rs7961581 with insulinogenic index (dominant model), respectively.
In conclusion, we report data suggesting an impaired pancreatic β-cell function in glucose-tolerant carriers of novel type 2 diabetes risk alleles in the JAZF1, CDC123/CAMK1D, and TSPAN8 regions. No associations of common variants in THADA, ADAMTS9, and NOTCH2 with quantitative measures of insulin release or insulin sensitivity could be shown in the cohort of middle-aged people.