June 24, 2023 - Shelly Jones
The field of genetics has greatly advanced our understanding of life, disease, and health. Yet, genetics only forms part of the puzzle. Epigenetics has recently gained recognition for its significant role in various biological processes, including disease onset and progression. By uncovering how our epigenome interacts with our genetic code and the environment, we hope to shed light on how this knowledge could transform the diagnosis, prognosis, and treatment of cancer.
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Epigenetics refers to changes in gene expression that do not involve alterations to the underlying DNA sequence. Epigenetic changes are normal and natural. They are often influenced by age, environment, lifestyle, and disease state. They can lead to specific changes in the body, like cells developing into a liver, brain, or skin cells, even though they all have the same DNA.
Epigenetic changes are modifications that affect gene expression without altering the underlying DNA sequence. These changes, unlike genetic changes (mutations), do not involve a change in the nucleotide sequence of DNA. Instead, they impact how cells read genes.
Think of genetic changes as alterations in the words of a book, altering the fundamental story. Epigenetic changes, on the other hand, can be seen as notes in the margin of the book. They don't change the words but affect how they are interpreted and therefore how the story is understood.
This is the most commonly studied epigenetic modification. It involves the addition of a methyl group (a carbon atom linked to three hydrogen atoms, CH3) to the DNA molecule, typically at a cytosine followed by a guanine, a spot known as a CpG site. When methyl groups attach to a particular spot in a gene, they can prevent the gene from being expressed.
DNA in cells is wrapped around proteins called histones. Together, they form a structure called chromatin. The way DNA is wrapped can affect whether genes can be read by the cell. If the chromatin is open and loose, gene expression can occur. If it's tight, it can prevent gene expression. Histone modifications such as acetylation, methylation, and phosphorylation can change the shape of the chromatin and thus, regulate gene expression.
Although these molecules don't code for proteins, they play significant roles in controlling gene expression. One of the most studied types of ncRNA is microRNA (miRNA), tiny strands of RNA that can bind to messenger RNAs (mRNAs), prevent them from making proteins, and thus influence gene expression. Other types of ncRNAs can influence the way chromatin is structured, thus controlling which genes are accessible for expression.
Methylation typically acts to repress gene transcription. When a gene is silenced by methylation, it's like placing a do not disturb sign on a door. The machinery needed to read the gene gets the signal not to proceed, and thus, the gene's protein isn't made.
Modifications to histones can either encourage or discourage gene expression. For instance, histone acetylation often loosens DNA and makes genes more accessible for transcription, promoting gene expression. In contrast, certain types of histone methylation can lead to condensed, closed chromatin, which can prevent gene expression.
These molecules, particularly microRNAs, can bind to messenger RNAs and prevent them from being translated into proteins, thus acting as powerful repressors of gene expression. Other ncRNAs can recruit proteins that modify chromatin structure, making genes more or less accessible for expression.
It's important to note that these processes often don't act independently. They can interact and influence each other in a complex web of regulation. Understanding these epigenetic modifications and their interplay is essential to appreciate how gene expression is controlled, and how dysregulation of these processes can contribute to diseases such as cancer.
Epigenetic modifications play a crucial role in gene expression. When these modifications become abnormal or inappropriate, they can disrupt the normal functioning of cells and potentially lead to the development of cancer. This disruption can occur in several ways:
Tumor suppressor genes are crucial for keeping cell division in check. When hypermethylation occurs in the promoter region of these genes, it can turn them off, thus eliminating their inhibitory effect on cell proliferation. This can result in uncontrolled cell growth, a characteristic of cancer.
Hypomethylation can activate genes that promote cell growth, known as oncogenes. When these genes are switched on excessively, it can lead to accelerated cell division leading to cancer.
Aberrant histone modifications can alter chromatin structure and lead to either the inappropriate activation or silencing of genes.
Non-coding RNAs, like miRNAs, also play a role in cancer. Some miRNAs, known as onco-miRs, can promote cancer when overexpressed. Others, known as tumor suppressor miRNAs, can prevent cancer. But when their expression is reduced, it can lead to cancer progression.
Almost all types of cancer have been linked to epigenetic changes to some degree. For instance:
DNA hypermethylation often occurs in breast cancer, silencing certain tumor suppressor genes.
Abnormal methylation patterns, including global hypomethylation and hypermethylation of specific genes, are associated with colorectal cancer.
Both DNA methylation and histone modification changes are frequently seen in lung cancer.
Changes in DNA methylation and histone modifications are linked to several types of leukemia. Also, certain types of leukemia are associated with mutations in enzymes that add or remove these epigenetic marks.
Epigenetic changes, especially DNA methylation, are associated with the development and progression of prostate cancer.
Environmental and lifestyle factors are known to influence epigenetic modifications, which can potentially contribute to cancer development.
Exposure to tobacco smoke can lead to DNA methylation changes that silence tumor suppressor genes.
Certain nutrients can affect epigenetic processes. For example, folate, a type of B vitamin, is critical for producing and maintaining cells. It's also necessary for the methylation process. A diet low in folate may lead to DNA hypomethylation, which could contribute to cancer.
Long-term alcohol consumption can lead to abnormal DNA methylation patterns, potentially contributing to cancer.
Exposure to certain environmental toxins, such as asbestos, can result in epigenetic changes linked to lung cancer and mesothelioma.
Aging is associated with changes in the epigenetic landscape, including alterations in DNA methylation, histone modifications, and miRNA expression. These changes can contribute to cancer development.
A biomarker is a measurable indicator of the severity or presence of some disease state. Epigenetic changes, such as DNA methylation patterns and histone modifications, can be specific to certain types of cancer, and hence serve as potential biomarkers.
For example, methylation of certain genes can indicate the presence of specific types of cancer. Similarly, changes in the levels of certain non-coding RNAs (like microRNAs) in blood or other bodily fluids can also act as potential cancer biomarkers. Because these changes often occur early in the disease progression, they can be valuable tools for early detection of cancer, sometimes even before symptoms appear.
Epigenetic therapy involves the use of drugs or other measures to correct the epigenetic modifications contributing to cancer. Unlike traditional cancer treatments that aim to kill cancer cells directly, epigenetic therapies aim to reprogram cancer cells by reversing abnormal epigenetic marks, thus restoring normal control of gene expression.
One of the attractive aspects of targeting epigenetic changes is that, unlike genetic changes, they are potentially reversible. This opens up a new avenue for cancer treatment, one where the faulty switches that contribute to cancer can be reset, offering hope for more effective treatments.
Several epigenetic drugs have already been approved for cancer treatment, and many more are being tested in clinical trials:
Drugs like azacitidine and decitabine inhibit DNA methyltransferases, the enzymes responsible for adding methyl groups to DNA. By doing so, they can reverse hypermethylation, potentially switching on tumor suppressor genes. These drugs are approved for use in myelodysplastic syndrome, a bone marrow disorder that can lead to leukemia.
Drugs such as vorinostat and romidepsin inhibit histone deacetylases, enzymes that remove acetyl groups from histones. This can lead to a more open chromatin structure, potentially allowing for the re-expression of silenced genes. These drugs are used in cutaneous T cell lymphoma.
These drugs, which are still in the early stages of development, inhibit proteins that recognize acetylated histones. They have shown promise in certain types of leukemia and solid tumors.
While still largely in experimental stages, therapies designed to modulate the levels of specific miRNAs or other ncRNAs are being developed and tested.
Several recent advancements have broadened our understanding of the role of epigenetics in cancer:
Developments in sequencing technologies have enabled detailed mapping of the epigenome, leading to the discovery of novel epigenetic modifications and their roles in cancer.
By examining the epigenomes of individual cells, researchers can understand how epigenetic heterogeneity within a tumor might drive cancer progression and affect treatment response.
This innovative technique allows for the detection of circulating tumor DNA (ctDNA) in the bloodstream, which carries the epigenetic alterations found in the tumor. It shows potential for non-invasive cancer detection, monitoring disease progression, and guiding treatment decisions.
Epigenetics is poised to play a key role in the burgeoning field of personalized medicine. By understanding the specific epigenetic changes that characterize an individual's tumor, treatments can be tailored to target those changes, potentially leading to more effective and less toxic therapies. For example:
This could be used to guide the selection of epigenetic drugs. If a tumor has a specific epigenetic alteration, a drug targeting that alteration could be chosen.
Epigenetic markers could help predict which patients are likely to respond to certain treatments. For instance, certain methylation patterns may predict response to DNA methyltransferase inhibitors.
Epigenetic therapies could be used in combination with other treatments, such as traditional chemotherapy or immunotherapy, to increase their effectiveness.
Testosterone exerts its effects by binding to androgen receptors, which are located in various tissues throughout the body. This receptor-hormone complex then enters the cell's nucleus and binds to specific regions of the DNA, called androgen-responsive elements (AREs). This binding regulates the transcription of specific genes, altering the production of proteins within the cell. This process is known as genomic action.
Testosterone's effects can be direct, meaning it binds to androgen receptors itself, or indirect, through its conversion into other hormones. For instance, testosterone can be converted to dihydrotestosterone (DHT) by the enzyme 5-alpha reductase. DHT is a more potent androgen and plays significant roles in certain tissues like the prostate and hair follicles. Alternatively, testosterone can also be converted into estradiol, a form of estrogen, by the enzyme aromatase. This process is particularly important in bone health.
In addition to these genomic actions, testosterone can also exert non-genomic actions, which are faster and do not involve changes in gene transcription. These include modulation of cellular signaling pathways and ion channels, leading to immediate changes in cell function.
This combination of genomic and non-genomic actions allows testosterone to influence a wide array of physiological processes, impacting muscle growth, fat distribution, bone health, cardiovascular health, metabolic health, and more.