The DCCT/EDIC study: epigenetic clues after three decades.

The advent of insulin therapy in the 1920s prolonged life expectancy in patients with type 1 diabetes (T1D). However, along with extended lifespan came increased morbidity and mortality associated with the microand macrovascular complications of diabetes. Because early insulin treatment regimens did not anticipate the need for multiple daily injections for tight glycemic control, hyperglycemia was proposed as the culprit for these vascular complications. In the 1980s, this hypothesis was tested in the Diabetes Control and Complications Trial (DCCT). The DCCT was a randomized trial of a cohort of recently diagnosed T1D patients that compared conventional insulin therapy at the time, which consisted of one or two daily injections, to an intensive multiple daily injection regimen along with strict monitoring of glycemia (1). In DCCT, patients in the intensive control arm achieved and maintained HbA1c levels of approximately 7%, two percentage points lower than those in the conventional therapy group. At study end, improved glycemic control was associated with a lower incidence and progression of retinopathy, nephropathy, and neuropathy, thus providing evidence of a link between glycemia (defined by HbA1c levels) and the incidence of diabetic microvascular complications (2). The clinical benefits of intensive control were so clear that DCCT was terminated early and intensive insulin therapy was proposed as the new standard of care (3). The DCCT’s conventional therapy group was switched to the more beneficial intensive regimen, and DCCT cohort began an observational follow-up phase—the Epidemiology of Diabetic Interventions and Complications (EDIC) study— which began in 1993 (4). EDIC aimed to gather information on the longer-term development of microvascular complications and more advanced diabetes morbidities that were not observed in DCCT due to its relatively short duration. During EDIC, the two former DCCT arms (intensive insulin therapy and conventional treatment) quickly converged with equalized HbA1c levels (4). Despite the loss of separation in HbA1c that was observed as early as the first year of EDIC, DCCT/EDIC, now entering its fourth decade, still shows persistent beneficial effects of earlier allocation to the DCCT’s intensive therapy arm, as demonstrated by reductions in severe diabetes complications (5). The persistence of these benefits in the EDIC cohort implies “memory” of the DCCT’s intensive versus conventional treatment regimen. Among the proposed mechanisms mediating this “glycemic” or “metabolic memory” is the occurrence of persistent epigenetic modifications induced by transitory hyperglycemia and subsequent oxidative stress (6) (reviewed in 7–9). Epigenetic modifications involve transcriptionally suppressive cytosine DNA methylation (10) and histone posttranslational modifications (PTMs). Histone PTMs can be transcriptionally permissive, such as histone H3 acetylation and lysine-4 methylation, or transcriptionally repressive, such as H3 lysine-9 methylation (11). DNA methylation and histone PTMs have been investigated as epigenetic modifications that are potentially associated with glycemic memory in experimental models ranging from primary endothelial cells (12,13) to zebrafish (14). Prolonged responses to transient exposure to high glucose also resulted in sustained expression of protein markers of high-glucose stress, including the NADPH oxidase subunit p47 and fibronectin (15), indicating that epigenetic changes modulate both gene transcription and associated protein expression. An early, clinically oriented study comparing histone PTMs in lymphocytes from patients with T1D and control subjects provided evidence for an association between T1D and altered histone methylation on genes implicated in the pathophysiology of T1D (16). These findings suggested an association between epigenetic modifications and metabolic memory. However, whether epigenetic events play a clinically relevant role in