Epigenetic Therapies in Cardiovascular Disease
Epigenetic Alterations in Cardiovascular Disease
Sequencing of the human genome at the DNA level identified protein-coding genes. The subsequent ENCODE Project [113–117] revealed that the protein-coding genes account for only a small fraction of all RNA transcripts generated in human cells, and that the non-coding genome is abundantly transcribed into multiple previously unrecognized RNA species with frequently unknown functions.
A further level of complexity was added by studies of the human epigenome[118–142] which is influenced by multiple non-genetic factors and encompasses genome modifications which are—in principle—reversible since they occur above the 'fixed' level of the DNA sequence. Modifications at the epigenetic level can critically influence the transcriptional activity of extended regions of the genome.
Epigenetic mechanisms include DNA methylation, histone modifications, and RNA-based mechanisms. Various studies have identified different DNA methylation patterns in patients with heart failure. A recent study identified epigenetic susceptibility regions linked to myocardial dysfunction. The authors conducted high-density epigenome-wide mapping of DNA methylation in left ventricular myocardial biopsies and whole peripheral blood in parallel with RNA deep sequencing and whole-genome sequencing. They identified distinct epigenetic DNA methylation patterns conserved across tissues and propose that methylation of CpGs sites may represent a novel epigenetic biomarker in heart failure.
Post-translational histone modifications involve, e.g. methylation and acetylation that is regulated by writers (e.g. histone acetyltransferases) and erasers [e.g. histone deacetylases (HDACs)] and is detected by readers such as BET proteins.[143–146] BET proteins are epigenetic readers of lysine acetylation and represent a potential novel therapeutic target.[143–146] The BET protein inhibitor apabetalone[147–155] has been investigated in a phase 3 cardiovascular outcomes trial described below.
Experimental Approaches Towards Epigenetic Therapies
Recent experimental studies suggest epigenetic alterations have pathomechanistic impact upon CVD and may thus constitute several potential novel therapeutic targets (Figure 5). In a murine MI model, intramyocardial transplantation of epigenetically reprogrammed EPCs resulted in improved cardiac function. In a TAC model, pharmacological HDAC inhibition attenuated cardiac hypertrophy. In diabetic mice with cardiac dysfunction, an epigenetic mechanism linking miRs and chromatin-modifying enzymes drove persistent p66Shc transcription and ROS generation. A recent study found that the levels of an N-terminal fragment (HDAC4-NT) of the stress-responsive epigenetic repressor HDAC4 were decreased in failing mouse hearts. Overexpression of HDAC4-NT protected the heart from remodelling and failure. Furthermore, while exercise enhanced HDAC4-NT levels, mice with a cardiomyocyte-specific deletion of Hdac4 showed reduced exercise capacity. This study identified a regulatory axis in which epigenetic regulation of a metabolic pathway affects calcium handling, a well-studied key determinant of myocardial function not only in mice but also in humans.[158–165]
Emerging technologies for human epigenome modulation. The spectrum of tools available for epigenetic therapies ranges from synthetic nucleic acid-based compounds (ASOs, siRNAs) to peptides of different origins. ASOs and siRNAs are in principle capable to inhibit the expression of any epigenome-modulating protein or noncoding RNA, yet to our knowledge, there are so far no preclinical or clinical trials based on this approach. To date, only a limited number of drugs with proven epigenetic mechanism of action have been approved by the FDA of which two are peptides. Concomitant with improved understanding of epigenome-based pathomechanisms, therapeutic strategies employing nucleic acids drugs (ASOs, siRNAs, CRISPR-Cas) and/or peptides are likely to emerge.
A recent study discovered control of LDL uptake into human cells by an LDLR-regulatory lncRNA, thus providing a first link between the prime therapeutic target LDLR and an epigenetic lncRNA-dependent mechanism. GalNAc-coupled siRNAs targeting this lncRNA allowed for direct liver cell targeting and enhanced cholesterol uptake. Another study reported that HDAC9 complex inhibition improves smooth muscle-dependent stenotic vascular disease. Targeting of the HDAC9 complex through either MALAT1 ASOs or inhibition of the methyltransferase EZH2 reduced neointima formation. While this study employed ASOs, the other one employed GalNAc-coupled siRNAs, thus both are building on advanced technologies already evaluated in clinical trials.
Only a limited number of drugs with an epigenetic mechanism of action have been approved and knowledge in the field is still limited. However, the spectrum of tools for epigenetic therapy already ranges from nucleic acid-based compounds to peptides of different origin.[119,143–146,166,168,169] Peptides may influence the expression of lncRNAs and maturation of miRs which then indirectly leads to alterations of the epigenome.
Cardiovascular Clinical Studies Involving Epigenetic Mechanisms
These important in-depth experimental studies shed first light upon the potential of targeting epigenetics in the context of CVD. While epigenetic modification is a highly interesting field of research, it should be emphasized that epigenetic modification will always result in pleiotropic actions which may result in adverse on-target and off-target effects. Furthermore, a given epigenetic modulation may be beneficial in one highly specific pathogenic cardiovascular context only, whereas it may have adverse consequences or no effect in—at first sight—similar situations. Particularly careful patient selection, possibly including novel inclusion criteria referring to directly detected epigenomic changes in the individual patient, may be required to achieve statistical significance for the desired therapeutic effect.
BET Inhibition in CVD. In the clinical arena, the phase III clinical trial (BETonMACE, NCT02586155) has investigated the BET protein inhibitor apabetalone (RVX-208) for its effects on major adverse cardiac events (MACE; i.e. CV death, non-fatal MI, stroke) in high-risk CV patients (i.e. diabetics after an ACS), and publication of the results is expected this year. A prior pooled analysis of short-term non-randomized studies suggested fewer MACE among patients treated with apabetalone compared to placebo. BET proteins interact with acetylated lysines on histones bound to DNA and thus regulate gene transcription in response to epigenetic mechanisms and signalling.[143–146] RVX-208 induced ApoAI expression, increased HDL cholesterol, and decreased C-reactive protein in humans, and acts as anti-thrombotic and anti-inflammatory agent in vitro. In pre-clinical studies it was reported to be beneficial for pulmonary arterial hypertension.
SIRT1—HDAC Modulation. Pharmacological modulation of SIRT-1, that acts as a class III HDAC, by resveratrol restores endothelial functions and cardiometabolic alterations in obese patients.[171,172] A clinical trial is currently investigating the effects of resveratrol on myocardial metabolism and function in patients undergoing cardiovascular surgery (NCT03762096).[147–150,170]
Eur Heart J. 2020;41(40):3884-3899. © 2020 Oxford University Press
Copyright 2007 European Society of Cardiology. Published by Oxford University Press. All rights reserved.