The role of DNA methylation in type 2 diabetes aetiology – using genotype as a causal anchor Short running title : The role of DNA methylation in type 2 diabetes aetiology Authors

Several studies have investigated the relationship between genetic variation and DNA methylation with respect to type 2 diabetes but it is unknown if DNA methylation is a mediator in the disease pathway or if it is altered in response to disease state. This study uses genotypic information as a causal anchor to help decipher the likely role of DNA methylation measured in peripheral blood in the aetiology of type 2 diabetes. Illumina HumanMethylation450 BeadChip data was generated on 1,018 young individuals from the ALSPAC cohort. In stage 1, 118 unique associations between published type 2 diabetes Single Nucleotide Polymorphisms (SNPs) and genome wide methylation (methylation quantitative trait loci; mQTLs) were identified. In stage 2, a further 226 mQTLs were identified between 202 additional independent non-type 2 diabetes SNPs and CpGs identified in stage 1. Where possible, associations were replicated in independent cohorts of similar age. We discovered that around half of known type 2 diabetes SNPs are associated with variation in DNA methylation and postulated that methylation could either be on a causal pathway to future disease or could be a non-causal biomarker. For one locus (KCNQ1), we were able to provide further evidence that methylation is likely to be on the causal pathway to disease in later life.