Glucokinase Gene May Be a More Suitable Target Than the Insulin Gene for Detection of β Cell Death

Detection and quantification of unmethylated circulating insulin (INS) DNA presumably released from β cells has been previously used for assessing their destruction. As the targets within the INS gene suffer from suboptimal specificity, we sought to improve the assay parameters by using the glucokinase gene (GCK) tissue-specific pancreatic promoter. The amount of methylated and unmethylated GCK DNA was measured using a droplet polymerase chain reaction assay and compared with the previously published INS-targeted assay. The method was tested using synthetic target sequences and DNA from pancreatic islets, blood, brain, kidney, large intestine, liver, lung, small intestine, and stomach. Circulating serum DNA was obtained from children with recent-onset type 1 diabetes (T1D) (n = 25), autoantibody-positive first-degree relatives of T1D patients (n = 14), and healthy controls (n = 20). The unmethylated GCK DNA was found to be more islet specific than unmethylated INS DNA. The proportion of the unmethylated GCK DNA was lower than INS in all tested extrapancreatic tissues, except kidney. Although the amounts of methylated DNA measured by the two assays were similar, the INS assay detected considerably more unmethylated DNA. Whereas none of the assays showed significant increase in the amount of unmethylated DNA, the ratio of unmethylated/methylated GCK DNA was borderline significantly increased in autoantibody-positive relatives compared with T1D patients (P = 0.04) and controls (P = 0.06). Targeting the assay into the GCK gene improved analytical parameters of the assay. As the amount of unmethylated target DNA in properly treated samples is very low, the clinical utility of this method remains to be evaluated.