Fig. 1

The scheme of study design. This figure describes how the study sample generated (below part) and the statistical analysis plan (up part). The study sample is a sub-sample of SATSA population who participated in in-person testing (IPT) from the first IPT (IPT1, 1986–1988) to the last one (IPT10, 2012–2014). Participants in IPT1 were those who responded to questionnaire 1 and were above 50 years old at the start of IPT1. From IPT2 to IPT5, participants included those who had taken part in the previous IPT and new twins who had recently turned 50 years old. From IPT6 and onwards, there were no new twins added, only longitudinal followed-up (Arrows from IPT5 to IPT10 represent only following-up and no new twins added). DNA methylation was measured in participants attending at least one of these IPTs (IPT3, IPT5, IPT6, IPT8, IPT9, IPT10). Based on the samples with DNA measurements in SATSA, we conducted a CpG-based epigenetic association study in a longitudinal Swedish twin sample to test the effect of statin treatment on the longitudinal trend of blood lipids and on the longitudinal changes of DNA methylation in selected CpGs, and to explore the long-term co-varying pattern of blood lipids and DNA methylation levels of these CpGs in response to statin therapy. SATSA: the Swedish Adoption/Twin Study of Aging; IPT: in-person testing