# Epigenetic Modification: Types, Mechanisms, and Examples

## Introduction to Epigenetic Modification

### What Is Epigenetics?

Epigenetics is the study of heritable changes in gene expression that do not involve alterations to the underlying DNA sequence. The term itself derives from the Greek prefix *epi-* ("above" or "upon"), reflecting that these modifications sit "on top of" the genome, directing when, where, and how strongly genes are transcribed. Unlike mutations, which change the nucleotide sequence itself, epigenetic modifications are reversible and responsive to environmental cues, yet they can be stably passed to daughter cells during mitosis and, in some cases, across generations.

The fundamental unit of epigenetic regulation is the **epigenome**—the collection of chemical marks on DNA and histone proteins that collectively determine chromatin structure and gene accessibility. Every cell in an organism carries the same DNA sequence, yet a neuron, a hepatocyte, and a lymphocyte express vastly different gene sets. This cellular identity is established and maintained by [epigenetic mechanisms](/knowledge/molecular-biology/epigenetic-mechanisms) that lock genes into active or silent states. For a deeper discussion of how these marks are transmitted across cell divisions, see [Epigenetic Inheritance](/knowledge/molecular-biology/epigenetic-inheritance).

### The Epigenome and Gene Regulation

The epigenome operates at the interface between the genome and the environment. It integrates signals from nutrition, stress, toxins, and developmental cues to produce stable changes in gene expression. At its core, epigenetic regulation works by controlling **chromatin structure**—the packaging of DNA around histone proteins. When chromatin is tightly condensed (heterochromatin), [transcription factors](/knowledge/molecular-biology/transcription-factor) cannot access promoter regions, and genes are silenced. When chromatin is relaxed (euchromatin), the transcriptional machinery can bind, and genes are expressed.

Three principal molecular mechanisms achieve this regulation: **DNA methylation**, **histone modification**, and **non-coding RNA-mediated regulation**. These systems do not operate in isolation; they interact extensively, forming a regulatory network that fine-tunes gene expression with remarkable precision. Understanding these mechanisms is essential not only for grasping normal development but also for comprehending how their dysregulation drives diseases such as cancer, neurological disorders, and metabolic syndromes. The conceptual framework of how epigenetic signals differ from genetic ones is explored further in [Difference Between Epigenetic and Genetic](/knowledge/molecular-biology/difference-between-epigenetic-and-genetic).

## Types of Epigenetic Modifications

### DNA Methylation

DNA methylation is the most extensively studied epigenetic modification. It involves the covalent addition of a methyl group (–CH₃) to the fifth carbon of the pyrimidine ring of cytosine, producing 5-methylcytosine (5mC). In mammals, this modification occurs almost exclusively at cytosine residues followed by guanine—the so-called **CpG dinucleotide**. Approximately 70–80% of CpG sites in the human genome are methylated, but the distribution is not uniform. Regions with high CpG density, termed **CpG islands**, are typically found in the promoter regions of about 60–70% of human genes and are usually unmethylated when the associated gene is actively transcribed.

DNA methylation is a stable, covalent modification that is faithfully copied during DNA replication by the enzyme **DNMT1** (DNA methyltransferase 1), which recognizes hemimethylated DNA and methylates the newly synthesized strand. This maintenance methylation ensures that methylation patterns are inherited through cell divisions, providing a molecular memory of gene expression states.

### Histone Modifications

Histones are the protein components of chromatin. Each nucleosome consists of 147 base pairs of DNA wrapped around an octamer of four core histone proteins—H2A, H2B, H3, and H4. The N-terminal tails of these histones protrude from the nucleosome and are subject to a wide array of post-translational modifications, including acetylation, methylation, phosphorylation, ubiquitination, sumoylation, and ADP-ribosylation. These modifications do not change the DNA sequence; instead, they alter chromatin structure and recruit effector proteins that either activate or repress transcription. The combinatorial complexity of these marks has led to the **histone code hypothesis**, which proposes that specific patterns of histone modifications dictate particular chromatin states and functional outcomes. A comprehensive overview of these modifications is provided in [Histone Modification](/knowledge/molecular-biology/histone-modification).

### Non-Coding RNAs

Only about 2% of the human genome encodes proteins; the vast majority is transcribed into non-coding RNAs (ncRNAs). Among these, **microRNAs (miRNAs)** and **long non-coding RNAs (lncRNAs)** play significant roles in epigenetic regulation. MiRNAs are short (~22 nucleotides) RNAs that post-transcriptionally silence genes by base-pairing with messenger RNAs (mRNAs), leading to mRNA degradation or translational repression. LncRNAs, defined as transcripts longer than 200 nucleotides with no protein-coding potential, can regulate gene expression at multiple levels, including recruiting chromatin-modifying complexes to specific genomic loci, acting as scaffolds for protein complexes, and modulating transcription factor activity.

## Mechanisms of DNA Methylation

### Enzymes Involved

The addition of methyl groups to cytosine residues is catalyzed by a family of enzymes known as **DNA methyltransferases (DNMTs)**. Three catalytically active members exist in mammals:

1. **DNMT1** — the maintenance methyltransferase. It has a strong preference for hemimethylated CpG sites (where one strand is methylated and the other is not), which arise during DNA replication. By methylating the daughter strand, DNMT1 ensures that methylation patterns are faithfully propagated.

2. **DNMT3A and DNMT3B** — the de novo methyltransferases. These enzymes establish new methylation patterns during embryonic development and cellular differentiation. They can methylate unmethylated CpG sites, setting up the initial methylation landscape.

3. **DNMT3L** — a catalytically inactive accessory protein that stimulates the activity of DNMT3A and DNMT3B, particularly in germ cells and early embryos.

The reaction uses **S-adenosylmethionine (SAM)** as the methyl donor. SAM donates its methyl group to the cytosine ring, and the byproduct S-adenosylhomocysteine (SAH) is released. The reaction proceeds through a covalent intermediate in which a cysteine residue in the enzyme's active site attacks the C6 position of cytosine, facilitating methyl transfer at C5.

Active DNA demethylation—the removal of methyl groups—is mediated by the **TET (ten-eleven translocation)** family of enzymes (TET1, TET2, TET3). These enzymes oxidize 5mC to 5-hydroxymethylcytosine (5hmC), then to 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). These oxidized derivatives can be converted back to unmethylated cytosine through the [base excision repair](/knowledge/molecular-biology/base-excision-repair) pathway, providing a route for active demethylation.

### CpG Islands and Gene Silencing

**CpG islands** are genomic regions of at least 200 base pairs with a GC content greater than 50% and an observed-to-expected CpG ratio above 0.6. Approximately 70% of human gene promoters are associated with CpG islands. In normal cells, most CpG islands remain unmethylated, regardless of whether the associated gene is expressed or silent. This is because the presence of unmethylated CpG islands is itself a signal that maintains an open chromatin state permissive for transcription.

When CpG islands in promoter regions become methylated, gene expression is silenced through two complementary mechanisms:

1. **Direct interference**: Methylated cytosines physically protrude into the major groove of DNA, blocking the binding of transcription factors that require unmethylated CpG sequences for recognition.

2. **Recruitment of methyl-CpG-binding proteins**: Proteins such as **MeCP2**, **MBD1**, **MBD2**, and **MBD3** specifically recognize and bind methylated CpG sites. These proteins then recruit co-repressor complexes, including histone deacetylases (HDACs) and chromatin remodelers, which condense chromatin and establish a repressive state.

The interplay between DNA methylation and histone modifications is bidirectional. Methylated DNA attracts histone-modifying enzymes that establish repressive marks, while repressive histone marks can, in turn, recruit DNMTs to maintain DNA methylation. This self-reinforcing loop ensures that silenced states are stable and faithfully inherited. The broader regulatory network connecting these marks is discussed in [Epigenetic Mechanisms](/knowledge/molecular-biology/epigenetic-mechanisms).

## Histone Modifications and Chromatin Remodeling

### Histone Acetylation and Deacetylation

**Histone acetylation** is the addition of an acetyl group (–COCH₃) to lysine residues on the N-terminal tails of histones, particularly H3 and H4. The reaction is catalyzed by **histone acetyltransferases (HATs)** , which use acetyl-CoA as the acetyl donor. Acetylation neutralizes the positive charge of lysine, weakening the electrostatic interaction between the histone tail and the negatively charged DNA backbone. This causes the chromatin to relax, exposing promoter and enhancer regions to the transcriptional machinery. Consequently, histone acetylation is strongly associated with active gene expression.

The reverse reaction—removal of acetyl groups—is catalyzed by **histone deacetylases (HDACs)** . HDACs restore the positive charge on lysine, promoting chromatin condensation and gene silencing. The balance between HAT and HDAC activity is tightly regulated; disruption of this balance is implicated in numerous cancers and neurological disorders.

Key acetylation marks include:
- **H3K9ac** (acetylation of lysine 9 on histone H3) — associated with active promoters
- **H3K14ac** — associated with active promoters and enhancers
- **H3K27ac** — distinguishes active enhancers from poised enhancers
- **H4K16ac** — involved in chromatin decondensation and transcriptional activation

### Histone Methylation and Its Effects

**Histone methylation** involves the addition of one, two, or three methyl groups to lysine or arginine residues on histone tails. Unlike acetylation, methylation does not alter the charge of the histone. Instead, it creates binding sites for specific effector proteins that recognize methylated residues and translate the mark into a functional outcome.

Histone methylation is catalyzed by **histone methyltransferases (HMTs)** and removed by **histone demethylases (KDMs)** . The effect of methylation depends on which residue is modified and the degree of methylation:

| Modification | Effect on Transcription | Associated Enzymes |
|--------------|------------------------|-------------------|
| H3K4me1 | Active enhancers | SETD1A/B, MLL3/4 |
| H3K4me3 | Active promoters | SETD1A/B, MLL1/2 |
| H3K9me3 | Heterochromatin, silencing | SUV39H1/2, SETDB1 |
| H3K27me3 | Polycomb silencing | EZH2 (PRC2) |
| H3K36me3 | Active gene bodies, splicing | SETD2 |
| H3K79me2/3 | Active transcription | DOT1L |

The **Polycomb repressive complex 2 (PRC2)** , containing the catalytic subunit EZH2, deposits H3K27me3, a hallmark of developmentally silenced genes. The **Trithorax group (TrxG)** proteins, in contrast, deposit H3K4me3 at active genes. The antagonism between these two systems establishes and maintains cell-type-specific gene expression programs during development.

**Histone phosphorylation** occurs on serine, threonine, and tyrosine residues and is involved in DNA damage response, mitosis, and immediate-early gene activation. **Histone ubiquitination** (e.g., H2AK119ub by PRC1, H2BK123ub) plays roles in both activation and repression, depending on context. These modifications, along with acetylation and methylation, constitute the complex regulatory language of the [Chromatin Modification](/knowledge/molecular-biology/chromatin-modification) system.

## Non-Coding RNAs in Epigenetic Regulation

### MicroRNAs and Gene Silencing

**MicroRNAs (miRNAs)** are endogenous, single-stranded RNAs of approximately 21–23 nucleotides that regulate gene expression post-transcriptionally. They are transcribed by RNA polymerase II as primary transcripts (pri-miRNAs), processed in the nucleus by the RNase III enzyme **Drosha** (with its cofactor DGCR8) into ~70-nucleotide hairpin precursors (pre-miRNAs), exported to the cytoplasm by Exportin-5, and further processed by **Dicer** into mature duplexes. One strand of the duplex is loaded into the **RNA-induced silencing complex (RISC)** , where it guides the complex to complementary sequences in target mRNAs, typically in the 3' untranslated region (3' UTR).

The degree of complementarity determines the silencing mechanism:
- **Perfect complementarity** → mRNA cleavage and degradation (common in plants)
- **Partial complementarity** → translational repression and mRNA deadenylation (common in animals)

A single miRNA can target hundreds of different mRNAs, and over 2,500 mature miRNAs have been identified in humans, collectively regulating more than 60% of protein-coding genes. MiRNAs are themselves subject to epigenetic regulation—many miRNA genes are located within CpG islands and can be silenced by DNA methylation in cancer.

### Long Non-Coding RNAs and Chromatin State

**Long non-coding RNAs (lncRNAs)** are transcripts exceeding 200 nucleotides that lack significant open reading frames. They are expressed in a highly cell-type-specific manner and function through diverse mechanisms:

1. **Chromatin remodeling**: The paradigmatic example is **XIST** (X-inactive specific transcript), which coats the inactive X chromosome and recruits the Polycomb repressive complex PRC2 to deposit H3K27me3, establishing facultative heterochromatin.

2. **Scaffolding**: LncRNAs can bring multiple proteins together into a complex. For example, **HOTAIR** (HOX transcript antisense RNA) serves as a scaffold linking PRC2 (which deposits H3K27me3) and the LSD1/CoREST complex (which demethylates H3K4me2), coordinating the silencing of HOXD genes.

3. **Decoy/sequestration**: Some lncRNAs bind and sequester transcription factors or miRNAs away from their targets. The lncRNA **PTENP1** acts as a decoy for miRNAs that target the tumor suppressor PTEN, thereby increasing PTEN expression.

4. **Enhancer function**: Certain lncRNAs, called enhancer RNAs (eRNAs), are transcribed from enhancer regions and promote enhancer-promoter looping, facilitating transcriptional activation.

The interplay between lncRNAs and chromatin-modifying complexes is a major area of active research, as dysregulation of lncRNAs is increasingly implicated in human diseases. The broader category of [Epigenetic Factors](/knowledge/molecular-biology/epigenetic-factors) encompasses these RNA-based regulators alongside the protein and DNA modifications discussed above.

## Methods to Study Epigenetic Modifications

### Bisulfite Sequencing for DNA Methylation

**Bisulfite sequencing** is the gold standard for detecting DNA methylation at single-nucleotide resolution. The method exploits the differential sensitivity of cytosine and 5-methylcytosine to sodium bisulfite:

1. **Treatment**: Genomic DNA is treated with sodium bisulfite (typically 3–4 M, pH 5.0, at 50–55°C for 4–16 hours). Unmethylated cytosines undergo deamination to uracil, while 5-methylcytosines are resistant and remain as cytosine.

2. **PCR amplification**: The treated DNA is amplified by PCR. Uracils are amplified as thymines, while methylated cytosines are amplified as cytosines.

3. **Sequencing**: The PCR products are sequenced. By comparing the sequence to the reference genome, the methylation status of every CpG site can be determined—unmethylated sites appear as TpG, methylated sites as CpG.

**Bisulfite pyrosequencing** allows quantitative measurement at specific loci, while **whole-genome bisulfite sequencing (WGBS)** provides genome-wide coverage. **Reduced representation bisulfite sequencing (RRBS)** enriches for CpG-rich regions, reducing cost while maintaining high coverage of promoters and CpG islands.

### ChIP-seq for Histone Modifications

**Chromatin immunoprecipitation followed by sequencing (ChIP-seq)** is the standard method for mapping histone modifications and transcription factor binding sites genome-wide. The protocol involves:

1. **Crosslinking**: Cells are treated with formaldehyde (typically 1% for 10 minutes at room temperature) to covalently crosslink proteins to DNA.

2. **Sonication**: Chromatin is sheared by sonication into fragments of 200–600 base pairs.

3. **Immunoprecipitation**: An antibody specific to the histone modification of interest (e.g., anti-H3K4me3) is used to pull down chromatin fragments bearing that modification.

4. **Reverse crosslinking and purification**: The crosslinks are reversed by heating (65°C for 4–6 hours), and DNA is purified.

5. **Sequencing and analysis**: The purified DNA is sequenced, and reads are aligned to the reference genome. Peaks of enrichment indicate genomic regions bearing the modification.

ChIP-seq requires high-quality, modification-specific antibodies; the choice of antibody is the single most important factor determining data quality. **CUT&RUN** (cleavage under targets and release using nuclease) and **CUT&Tag** are newer, lower-background alternatives that use a protein A–micrococcal nuclease fusion to cleave DNA specifically at antibody-bound regions.

### RNA-seq for Non-Coding RNAs

**RNA sequencing (RNA-seq)** is used to quantify and characterize both coding and non-coding transcripts. For miRNA analysis, the protocol involves:

1. **Size selection**: Total RNA is size-fractionated to enrich for small RNAs (18–30 nucleotides).

2. **Adapter ligation**: 3' and 5' adapters are ligated to the small RNAs.

3. **Reverse transcription and PCR amplification**: The ligated RNAs are converted to cDNA and amplified (typically 12–15 cycles to minimize PCR duplicates).

4. **Sequencing**: The cDNA library is sequenced on a high-throughput platform.

For lncRNAs, standard RNA-seq with ribosomal RNA depletion (rather than poly-A selection, since many lncRNAs are not polyadenylated) is used. **Strand-specific RNA-seq** preserves the orientation of transcripts, which is essential for distinguishing sense and antisense lncRNAs.

## Examples of Epigenetic Modifications in Development and Disease

### X-Chromosome Inactivation

**X-chromosome inactivation (XCI)** is a classic example of epigenetic regulation in mammals. Female cells (XX) must silence one X chromosome to achieve dosage compensation with males (XY). This process is initiated early in embryonic development by the lncRNA **XIST**, which is expressed from the future inactive X chromosome (Xi). XIST coats the Xi in *cis* and recruits PRC2, leading to deposition of H3K27me3 and subsequent DNA methylation of CpG islands. The Xi becomes condensed into facultative heterochromatin, visible as the **Barr body**.

XCI is random—each cell independently inactivates either the maternal or paternal X chromosome—and the choice is then stably inherited by all daughter cells. Consequently, female mammals are mosaics for X-linked gene expression. The inactive state is remarkably stable, yet reactivation occurs naturally in the female germline before meiosis, demonstrating that even the most stable epigenetic states are reversible.

### Genomic Imprinting

**Genomic imprinting** is an epigenetic phenomenon in which a subset of genes is expressed exclusively from either the maternal or paternal allele. Imprinted genes are marked during gametogenesis by DNA methylation at **imprinting control regions (ICRs)** , which are differentially methylated between the two parental alleles. These marks are established in the germline and maintained throughout development, despite the genome-wide demethylation that occurs after fertilization.

Approximately 100–200 imprinted genes have been identified in humans. A well-studied example is **IGF2/H19** on chromosome 11p15.5. The IGF2 gene encodes insulin-like growth factor 2 and is expressed only from the paternal allele, while H19, a lncRNA, is expressed only from the maternal allele. The ICR between these genes is methylated on the paternal chromosome, which prevents binding of the insulator protein **CTCF**. On the maternal chromosome, the unmethylated ICR binds CTCF, blocking access of the IGF2 promoter to a downstream enhancer and silencing IGF2 while allowing H19 expression.

Loss of imprinting—either by loss of methylation (leading to biallelic IGF2 expression) or gain of methylation (silencing both alleles)—causes developmental disorders such as **Beckwith-Wiedemann syndrome** (overgrowth and tumor predisposition) and **Silver-Russell syndrome** (growth restriction).

### Epigenetics in Cancer

Cancer is characterized by widespread epigenetic dysregulation alongside genetic mutations. Two major epigenetic hallmarks of cancer are:

1. **Global DNA hypomethylation**: Cancer genomes typically show 20–60% reduction in overall 5mC content, particularly at repetitive elements and gene bodies. Hypomethylation can activate oncogenes and promote genomic instability through reactivation of [transposable elements](/knowledge/molecular-biology/transposable-element).

2. **Focal promoter hypermethylation**: [Tumor suppressor genes](/knowledge/molecular-biology/tumor-suppressor-gene) often acquire dense CpG island methylation at their promoters, leading to heritable silencing. Classic examples include:
   - **CDKN2A** (p16^INK4a^) — cell cycle inhibitor, silenced in many cancers
   - **MLH1** — DNA mismatch repair gene, silenced in colorectal and endometrial cancers
   - **BRCA1** — DNA repair gene, silenced in breast and ovarian cancers

Histone modifications are also altered in cancer. **EZH2**, the catalytic subunit of PRC2, is overexpressed or mutated in lymphomas and solid tumors, leading to excessive H3K27me3 and silencing of tumor suppressors. Conversely, loss of H3K4me3 at promoter regions is associated with gene silencing in several cancer types.

The reversibility of epigenetic modifications makes them attractive therapeutic targets. **DNMT inhibitors** (e.g., 5-azacytidine, decitabine) and **HDAC inhibitors** (e.g., vorinostat, romidepsin) are FDA-approved for the treatment of myelodysplastic syndromes and cutaneous T-cell lymphoma, respectively. These agents reactivate silenced tumor suppressors and are being investigated in combination with immunotherapy. The conceptual basis for targeting these pathways is rooted in [Epigenetic Theory](/knowledge/molecular-biology/epigenetic-theory), which emphasizes the dynamic and reversible nature of these regulatory systems.

## Common Pitfalls and Misconceptions

### Epigenetics vs. Mutations

A frequent error is conflating epigenetic modifications with mutations. Mutations are permanent changes in the DNA sequence—substitutions, insertions, deletions—that alter the genetic code itself. Epigenetic modifications, by contrast, do not change the sequence; they alter the accessibility and expression of genes. A methylated cytosine is still a cytosine; an acetylated histone still has the same amino acid sequence. This distinction has practical implications: mutations are generally irreversible (barring gene editing), whereas epigenetic marks can be reversed by enzymatic activity or pharmacological intervention.

### Not All Methylation Is Silencing

A common oversimplification is that DNA methylation always silences genes. While promoter CpG island methylation is strongly associated with repression, methylation in other contexts can have different effects:

- **Gene body methylation**: Methylation within the transcribed region is positively correlated with gene expression. It may suppress spurious [transcription initiation](/knowledge/molecular-biology/transcription-initiation) from internal promoters and affect splicing.

- **Enhancer methylation**: Methylation of enhancer regions generally reduces enhancer activity, but the relationship is context-dependent.

- **Non-CpG methylation**: In embryonic stem cells and neurons, methylation can occur at CHG and CHH sites (where H is A, C, or T). The function of non-CpG methylation is less well understood but appears to be involved in neuronal gene regulation.

Similarly, not all histone methylation is repressive. H3K4me3 is associated with active promoters, H3K36me3 with active gene bodies, and H3K79me2/3 with active transcription. Only specific marks, such as H3K9me3 and H3K27me3, are reliably associated with silencing.

### Reversibility and Environmental Influence

Another misconception is that epigenetic marks are permanent or that they are entirely determined by genetics. In reality, epigenetic modifications are dynamic and responsive to environmental factors. Diet, stress, toxins, and exercise can all influence the epigenome. For example, the availability of methyl donors (folate, vitamin B12, choline, methionine) affects SAM levels and thus DNA methylation capacity. However, this does not mean that every environmental exposure produces meaningful epigenetic change—many reported associations are correlational, and distinguishing cause from effect requires rigorous experimental design.

A related pitfall is overinterpreting the heritability of epigenetic marks. While mitotic inheritance is well established, transgenerational inheritance (transmission through the germline to offspring) in mammals remains controversial and is the subject of active investigation. The [Epigenetic Inheritance](/knowledge/molecular-biology/epigenetic-inheritance) page provides a more nuanced treatment of this topic.

## Frequently Asked Questions

### What are the main types of epigenetic modifications?

The three main types are: (1) **DNA methylation**—covalent addition of methyl groups to cytosine residues, primarily at CpG dinucleotides; (2) **histone modifications**—post-translational modifications of histone tails including acetylation, methylation, phosphorylation, and ubiquitination; and (3) **non-coding RNA-mediated regulation**—control of gene expression by microRNAs and long non-coding RNAs.

### What is an epigenetic modification?

An epigenetic modification is a chemical change to DNA or chromatin that alters gene expression without changing the DNA sequence. These modifications are heritable through cell division and are reversible. Examples include DNA methylation, histone acetylation, and histone methylation.

### Can you give examples of epigenetic modifications?

Specific examples include: methylation of CpG islands in the promoter of the tumor suppressor CDKN2A in cancer; acetylation of H3K27 at active enhancers; trimethylation of H3K27 by PRC2 during X-chromosome inactivation; and the action of the lncRNA XIST in coating and silencing the inactive X chromosome.

### How do epigenetic modifications affect gene expression?

Epigenetic modifications affect gene expression by altering chromatin structure and recruiting regulatory proteins. DNA methylation at promoters blocks transcription factor binding and recruits methyl-CpG-binding proteins that bring co-repressor complexes. Histone acetylation relaxes chromatin, promoting transcription; histone methylation can either activate or repress depending on the specific residue and degree of methylation. Non-coding RNAs can guide chromatin-modifying complexes to specific loci or silence mRNAs post-transcriptionally.

### Are epigenetic modifications reversible?

Yes. DNA methylation can be removed by TET enzymes through oxidation of 5mC, followed by [base excision repair](/knowledge/molecular-biology/base-excision-repair). Histone acetylation is reversed by HDACs, and histone methylation by histone demethylases. This reversibility is the basis for epigenetic therapies in cancer, such as DNMT and HDAC inhibitors.

### What is the difference between DNA methylation and histone modification?

DNA methylation is a covalent modification of the DNA itself (addition of –CH₃ to cytosine), while histone modifications are post-translational modifications of histone proteins. DNA methylation is generally more stable and is faithfully copied during replication by DNMT1. Histone modifications are more dynamic and diverse, with different marks having opposing effects. The two systems interact: DNA methylation can recruit histone-modifying enzymes, and histone marks can influence DNA methylation patterns.

### Why are epigenetic modifications important in cancer?

Cancer cells exhibit widespread epigenetic dysregulation, including global DNA hypomethylation and focal hypermethylation of [tumor suppressor gene](/knowledge/molecular-biology/tumor-suppressor-gene) promoters. These changes silence tumor suppressors, activate oncogenes, and promote genomic instability. Unlike genetic mutations, epigenetic alterations are reversible, making them attractive therapeutic targets. Drugs that inhibit DNMTs and HDACs can reactivate silenced genes and are used clinically to treat certain hematological malignancies.

## Key Takeaways

- Epigenetic modifications are heritable, reversible changes in gene expression that do not alter the DNA sequence; they include DNA methylation, histone modifications, and non-coding RNA-mediated regulation.
- DNA methylation occurs at CpG dinucleotides, is catalyzed by DNMTs using SAM as the methyl donor, and is associated with gene silencing when present at promoter CpG islands.
- Histone modifications—acetylation, methylation, phosphorylation, ubiquitination—alter chromatin structure and recruit effector proteins; acetylation generally activates transcription, while methylation can activate or repress depending on the residue.
- Non-coding RNAs, including miRNAs and lncRNAs, regulate gene expression at transcriptional and post-transcriptional levels; XIST is a paradigm for lncRNA-mediated chromatin silencing.
- Key techniques for studying epigenetic marks include bisulfite sequencing (DNA methylation), ChIP-seq (histone modifications), and RNA-seq (non-coding RNAs).
- Epigenetic dysregulation is central to cancer, with promoter hypermethylation silencing tumor suppressors and global hypomethylation promoting genomic instability; DNMT and HDAC inhibitors are approved epigenetic therapies.
- Common misconceptions include conflating epigenetic changes with mutations, assuming all methylation is repressive, and overstating the heritability and environmental responsiveness of epigenetic marks.

## Further Reading

- Sun L, Zhang H, Gao P. *Metabolic reprogramming and epigenetic modifications on the path to cancer*. Protein & cell. 2022. [PubMed 34050894](https://doi.org/10.1007/s13238-021-00846-7)
- Mao Z et al. *Glibenclamide targets MDH2 to relieve aging phenotypes through metabolism-regulated epigenetic modification*. Signal transduction and targeted therapy. 2025. [PubMed 39962087](https://doi.org/10.1038/s41392-025-02157-3)
- Rastegar M. *Editorial: Epigenetic modification in neurological diseases*. Frontiers in genetics. 2024. [PubMed 39534079](https://doi.org/10.3389/fgene.2024.1506551)
- Wang Y et al. *Epigenetic modification of m(6)A regulator proteins in cancer*. Molecular cancer. 2023. [PubMed 37391814](https://doi.org/10.1186/s12943-023-01810-1)
- Zhang Y et al. *Epigenetic modification of hepatitis B virus infection and related hepatocellular carcinoma*. Virulence. 2024. [PubMed 39460469](https://doi.org/10.1080/21505594.2024.2421231)
- Liu Z et al. *Epigenetic modification in diabetic kidney disease*. Frontiers in endocrinology. 2023. [PubMed 37455912](https://doi.org/10.3389/fendo.2023.1133970)

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* [Nucleotide Formation: Biosynthesis and Assembly of DNA/RNA Building Blocks](/knowledge/molecular-biology/nucleotide-formation)