Hydroxymethylated cytosine (5hmC) in DNA is an epigenetic modification that regulates gene transcription. Type 2 diabetes (T2D) and obesity (OB) are diseases that are serious global public health challenges. Little is known about the role of 5hmC in the development of T2D and OB. In our exploratory study to investigate hydroxymethylation using a co-twin control design, we included three monozygotic twin pairs (MZ) discordant for incident T2D (iT2D) and five separate MZ pairs discordant for prevalent OB from our previous R21 (1R21HL127368-01) study of cardiovascular disease. Using circulating 5hmC as an aggregate surrogate for cumulative systemic hydroxymethylation, intriguingly, we found statistically significant, whole-genome differentially hydroxymethylated regions (DhMRs) related to iT2D and prevalent OB, respectively, after controlling for genetic variation in germline consisting of 3.2 billion base pairs, age-period-cohort effects, and multiple-testing. Due to the very limited sample size and lack of twin pairs discordant for incident OB (iOB) in our prior exploratory study, it is imperative to perform the prospective research in a relatively large sample to generate strong evidence. The National Heart, Lung, and Blood Institute (NHLBI) Twin Study is a well- characterized, 46-year follow-up study initiated in 1969. Leveraging existing rich resources in the NHLBI Twin Study and our established, multi-mechanism students' research engagement plan, we propose this R15 study. Our overall objectives are to characterize circulating 5hmC in relation to iT2D and iOB and to offer an excellent opportunity for students to be engaged in public health epigenome research. These objectives will be achieved by pursuing three specific aims: 1) measure whole genome hydroxymethylation (5hmC) in blood DNA samples; 2) test if DNA regions are differentially hydroxymethylated in relation to iT2D (aim 2-1) and iOB (aim 2-2) independent of germline and common environment; which, in turn, will identify novel differentially hydroxymethylated regions; and 3) characterize sociobehavioral and other environmental correlates of hydroxymethylation in literature-based candidate and novel DNA regions independent of germline and common environment. We will include all available twin pairs discordant for iT2D [17 MZ and 20 dizygotic twin pairs (DZ)] and separate twin pairs discordant for iOB (11 MZ pairs and 15 DZ pairs) from NHLBI Twin Study. The NHLBI Twin Study twins were born between 1917 and 1927, a period when 2 male MZ pairs and 4 male DZ pairs of twins were born per 1,000 live births. An illustrative example is that inclusion of our dMZ twin sample size is equivalent to that of at least 126,746 singletons. The significance of our study is to provide innovative evidence about iT2D and iOB-discriminating hydroxymethylation induced environmentally, which could impact disease prevention and treatment through the improvement of modifiable environmental factors. Our study will offer a unique opportunity for engaging medical and graduate students at the applicant's institute in the public health epigenome research.
The proposed discordant identical and fraternal twin study of incident type 2 diabetes and incident obesity is pivotal to public health because this study design compares diseased twins with their non-diseased co-twins for a better understanding of environment-induced hydroxymethylation independent of genetic influences as the novel biological mechanism underlying the diseases. By engaging students in the proposed co-twin control study, we will prepare our next generation of public health researchers to sustain our impact on public health across generations. The discovery of new environmentally and epigenetically therapeutic and preventive regimens will pave a way to fight against incident type 2 diabetes and incident obesity.
