Epigenetic therapy refers to the use of drugs or other interventions to modify gene expression patterns, potentially treating diseases by targeting epigenetic mechanisms such as DNA methylation and histone modifications.
Epigenetics is the study of changes in gene expression that do not arise from alterations in the DNA sequence, resulting in the heritable silencing of genes without changing the coding sequence. Epigenetic therapy involves using drugs or other techniques to influence these epigenetic mechanisms in addressing specific medical conditions. Various diseases, such as diabetes, cancer, heart disease, and mental illnesses, are influenced by epigenetic mechanisms. Emerging areas of epigenetic therapy include its application in heart disease, primarily focusing on tissue regeneration, and in schizophrenia, where the focus lies on alleviating symptoms. Overall, epigenetic therapies aim to target the underlying epigenetic molecular pathways responsible for disease manifestation.
Epigenetics
Epigenetics refers to the study of changes in gene expressions that do not result from alterations in the DNA sequence'. Altered gene expression patterns can result from chemical modifications in DNA and chromatin, to changes in several regulatory mechanisms. Epigenetic markings can be inherited in some cases, and can change in response to environmental stimuli over the course of an organism's life.
Many diseases are known to have a genetic component, but the epigenetic mechanisms underlying many conditions are still being discovered. A significant number of diseases are known to change the expression of genes within the body, and epigenetic involvement is a plausible hypothesis for how they do this. These changes can be the cause of symptoms to the disease. Several diseases, especially cancer, have been suspected of selectively turning genes on or off, thereby resulting in a capability for the tumorous tissues to escape the host's immune reaction.
Known epigenetic mechanisms typically cluster into three categories. The first is DNA methylation, where a cytosine residue that is followed by a guanine residue (CpG) is methylated. In general, DNA methylation attracts proteins which fold that section of the chromatin and repress the related genes. The second category is histone modifications. Histones are proteins which are involved in the folding and compaction of the chromatin. There are several different types of histones, and they can be chemically modified in a number of ways. Acetylation of histone tails typically leads to weaker interactions between the histones and the DNA, which is associated with gene expression. Histones can be modified in many positions, with many different types of chemical modifications, but the precise details of the histone code are currently unknown. The final category of epigenetic mechanism is regulatory RNA. MicroRNAs are small, noncoding sequences that are involved in gene expression. Thousands of miRNAs are known, and the extent of their involvement in epigenetic regulation is an area of ongoing research. Epigenetic therapies are reversible, unlike gene therapy. This means that they are druggable for targeted therapies.
Potential applications
Cancer
Epigenetic alterations can play a major role in the development and progression of cancer. This can be seen as a result of the changes made by various epigenetic mechanisms (i.e. DNA methylation, histone modification, non-coding RNAs) which may work to silence or activate cancer-related genes (i.e. tumor-suppressor genes or oncogenes), therefore invoking changes in gene expression patterns that are responsible for the initiation, progression, and metastasis of cancer. One example includes DNA hypermethylation of tumor-suppressor genes which can cause transcriptional inactivation of genes (i.e. CDKN2A) encoding for other regulatory proteins (i.e. p14 and p16) that play a vital role in the cell cycle. However, there are numerous avenues in which cancer can arise epigenetically given the various hallmarks of cancer. Because of this, epigenetic drugs are utilized to target and combat the internal dysregulation caused by these epigenetic alterations.
DNA methyltransferase inhibitors (DNMTis) are used as a treatment option against various forms of cancer. Some of these drugs, including 5-azacytidine, Decitabine, and Zebularine, have been shown to reactivate the cellular anti-tumor systems repressed by cancer, enabling the body to weaken and kill off the tumor. This can specifically result due to DNMTis ability to block overly active DNMTs, resulting in the hypomethylation of CpG regions which can allow for normal cell growth. Other treatment options including histone deacetylase inhibitors (HDACis), such as vorinostat, have also shown promising epigenetic therapeutic effects through the reactivation of apoptosis, cell death of undesirable cells, and the inhibition of angiogenesis, the formation of new blood vessels that can be utilized by cancerous cells as an extra source of nutrients. HDACis are an example of a chromatin-modifying drug, which specifically targets the acetylation portion of cancerous cells, and like DMNTis, is able to reactivate the cellular anti-tumor systems within a cell. Now because of their wide-ranging effects throughout the entire body, many of these drugs may have major side effects, making dosage regulation important; however, the survival rates are increased significantly when they are used for treatment, whether independently or in congruence with chemotherapy or other cancer-related forms of medication.
Phytochemical interventions have also gained traction in serving as an epigenetic treatment option against cancer. Phytochemicals, which are compounds produced by plants that act as a protectant to aid in their survival, can have an impact on the function of DNMTs. Polyphenols, a type of phytochemical that can be found in everyday foods and ingredients like tea, apples, and various forms of berries, have been shown to have anticancer effects. For example, epigallocatechin gallate (EGCG), the most prevalent polyphenol found in green tea, can work to inhibit cell proliferation by inducing apoptosis, preventing angiogenesis, and disrupting regulatory pathways, such as MAPK pathways.
Although various mechanisms have been utilized to mitigate the negative effects of cancer development and progression, drug resistance does still occur. Drug resistance can emerge as a result of continued and repetitive usage of a drug, giving pathogens time to develop defenses against drugs designed to kill them. In cancer, overexpression, for instance in DNA methylation and histone deactylase, can cause deleterious effects which can enhance drug resistance behaviors such as drug efflux. Drug efflux is regulatory process in which toxic compounds (i.e. drugs for cancer) get removed from a cell to reduce its cytotoxic effects. However, research suggests there are ways to overcome this type of drug resistance, with one being through the use of epigenetic factor inhibitors.
Cardiovascular disease Cardiovascular disease is one of the leading cause of death. Factors such as age, smoking, obesity, diabetes, and hypertension increase the likelihood of developing cardiovascular diseases. As of now, initial investigations have established links between DNA methylation, histone modifications, RNA-based mechanisms, and the onset of cardiovascular diseases such as atherosclerosis, cardiac hypertrophy, myocardial infarction, and heart failure. Epigenetic alterations like DNA methylation enzymes can reverse abnormal DNA methylation, histone modifications, and non-coding RNA regulation play critical roles in cardiovascular disease. A number of cardiac dysfunctions have been linked to cytosine methylation patterns. DNA methylation is an important epigenetic mechanism, it is able to transfer to offspring DNA through the regulation of DNMT. DNA demethylation can happen through either an active or passive process. Passive demethylation refers to the failure of maintenance DNA methyltransferases (DNMTs) to methylate the newly synthesized DNA strand during mitosis. DNMT deficient mice show upregulation of inflammatory mediators, which cause increased atherosclerosis and inflammation. Atherosclerotic tissue has increased methylation in the promoter region for the estrogen gene, although any connection between the two is unknown. Hypermethylation of the HSD11B2 gene, which catalyzes conversions between cortisone and cortisol, and is therefore influential in the stress response in mammals, has been correlated with hypertension. Decreased LINE-1 methylation is a strong predictive indicator of ischemic heart disease and stroke, although the mechanism is unknown. Various impairments in lipid metabolism, leading to clogging of arteries, has been associated with the hypermethylation of GNASAS, IL-10, MEG3, ABCA1, and the hypomethylation of INSIGF and IGF2. Additionally, upregulation of a number of miRNAs has been shown to be associated with acute myocardial infarction, coronary artery disease, and heart failure. Strong research efforts into this area are very recent, with all of the aforementioned discoveries being made since 2009. Mechanisms are entirely speculative at this point, and an area of future research. It has been demonstrated that miRNAs regulate gene expression by suppressing messenger RNA (mRNA) translation and increasing mRNA degradation. Studies have shown that miRNAs can influence myocardial angiogenesis and the survival and proliferation of cardiomyocytes by regulating target gene expression.
Epigenetic treatment methods for cardiac dysfunction are still highly speculative. SiRNA therapy targeting the miRNAs mentioned above is being investigated. The primary area of research in this field is on using epigenetic methods to increase the regeneration of cardiac tissues damaged by various diseases. By comprehending the roles these mechanisms play in disease pathology, researchers can devise precise treatments to regulate gene expression, potentially halting or alleviating the progression of cardiovascular diseases.
Diabetic retinopathy Diabetes is a disease where an affected individual is unable to convert food into energy. When left untreated, the condition can lead to other, more severe complications. A common sign of diabetes is the degradation of blood vessels in various tissues throughout the body. Retinopathy refers to damage from this process in the retina, the part of the eye that senses light. Diabetic retinopathy is known to be associated with a number of epigenetic markers, including methylation of the Sod2 and MMP-9 genes, an increase in transcription of LSD1, a H3K4 and H3K9 demethylase, and various DNA Methyl-Transferases (DNMTs), and increased presence of miRNAs for transcription factors and VEGF.
It is believed that much of the retinal vascular degeneration characteristic of diabetic retinopathy is due to impaired mitochondrial activity in the retina. Sod2 codes for a superoxide disputes enzyme, which scavenges free radicals and prevents oxidative damage to cells. LSD1 may play a major role in diabetic retinopathy through the downregulation of Sod2 in retinal vascular tissue, leading to oxidative damage in those cells. MMP-9 is believed to be involved in cellular apoptosis, and is similarly downregulated, which may help to propagate the effects of diabetic retinopathy.