Epigenetics: How Genes Get Switched On and Off

Every cell in your body contains the same DNA — yet a liver cell behaves completely differently from a neuron. The difference is in gene expression: which genes are switched on and which are silenced. Epigenetics studies these heritable changes in gene activity that don't involve changes to the DNA sequence itself.

1. What Is Epigenetics?

Genetics studies the DNA sequence — the letters A, T, G, C that encode genes. Epigenetics (from Greek "epi" = upon/over) studies the layer of chemical marks on top of DNA that control how, when, and in which cells genes are read.

All ~37 trillion cells in the human body have nearly identical DNA (~3 billion base pairs). Yet the proteome (set of expressed proteins) differs massively between cell types. A pancreatic beta cell expresses insulin; a neuron does not. Epigenetic marks are the instruction manual that cell-type-specifically silences or activates parts of the genome.

Key property: epigenetic marks are heritable through cell division — when a liver cell divides, its daughter cells maintain the liver-specific pattern of marks, not a random reset. Some marks are also heritable across generations.

2. DNA Methylation

The most well-studied epigenetic mark. A methyl group (–CH₃) is added to the cytosine (C) base — typically at CpG dinucleotides (cytosine followed by guanine). This is performed by enzymes called DNA methyltransferases (DNMTs).

Methylation of a gene's promoter region generally silences that gene: methyl groups recruit proteins that compact chromatin, making the gene inaccessible to transcription machinery. Conversely, unmethylated promoters are associated with active genes.

5-methylcytosine (5mC) is often called the "fifth base" of DNA. The human genome contains ~28 million CpG dinucleotides. About 70–80% are methylated, with notable unmethylated regions at gene promoters (CpG islands).

Abnormal methylation patterns are a hallmark of cancer: tumour suppressor genes often become hypermethylated (silenced), while oncogenes can become demethylated (activated).

3. Histone Modification

DNA is not naked inside the cell — it's wrapped around proteins called histones. About 147 base pairs of DNA wrap 1.65 times around each histone octamer (2 copies each of H2A, H2B, H3, H4), forming a nucleosome. The nucleosome is the fundamental unit of chromatin.

The histone "tails" (N-terminal extensions) protrude outward and carry numerous chemical modifications:

H3K4me3

Trimethylation of histone H3 at lysine 4. Mark of active promoters. Placed by MLL/SET enzymes.

H3K27me3

Trimethylation at H3K27. Repressive mark. Placed by Polycomb complex (PRC2, EZH2). Silences developmental genes.

H3K9me3

Trimethylation at H3K9. Constitutive heterochromatin — permanently silenced regions.

H3K27ac

Acetylation at H3K27. Active enhancer mark. Neutralises positive charge, loosening DNA-histone contact.

These marks are written (by "writer" enzymes like HDACs, HATs), read (by "reader" proteins with bromodomains/chromodomains), and erased (by "eraser" enzymes). Many cancer drugs target these epigenetic enzymes — e.g., HDAC inhibitors (vorinostat) and EZH2 inhibitors.

4. Chromatin Remodelling

Chromatin can exist in two states: euchromatin (open, loosely packed, transcriptionally active) and heterochromatin (compact, densely packed, silenced). Epigenetic marks define which regions adopt which state.

ATP-dependent chromatin remodelling complexes (SWI/SNF, NuRD, INO80) physically slide or eject nucleosomes, exposing or hiding transcription factor binding sites. Enhancers — distant regulatory elements — can loop to contact promoters and activate genes across millions of base pairs.

5. Role in Development

The clearest demonstration of epigenetics: a fertilised egg contains totipotent cells that can become any cell type. As development proceeds, cells gradually commit to specific lineages by stably silencing genes for other lineages.

Remarkably, differentiation can be reversed. In 2006, Yamanaka showed that adding just 4 transcription factors (Oct4, Sox2, Klf4, c-Myc) to skin cells can reprogram them into induced pluripotent stem cells (iPSCs) — erasing most somatic epigenetic marks. This won the 2012 Nobel Prize in Medicine and opened the field of regenerative medicine.

6. Environment & Lifestyle Effects

Epigenetic marks are dynamic and responsive to environmental inputs — unlike DNA sequence:

Diet: Folate/methionine are methyl group donors. Maternal diet during pregnancy affects offspring methylation patterns. Caloric restriction alters SIRT1-dependent histone deacetylation.
Stress: Glucocorticoid receptor gene (NR3C1) methylation is altered by early-life stress. Abused children show increased methylation → blunted stress response in adulthood.
Exercise: Acute exercise causes rapid genome-wide changes in histone acetylation in muscle cells, upregulating metabolic genes.
Ageing: DNA methylation patterns change predictably with age. "Epigenetic clocks" (Horvath clock) can predict biological age from blood methylation patterns with ±4 year accuracy.

7. Transgenerational Epigenetics

Most epigenetic marks are erased in the germ line (sperm and eggs) during development — a "reset." However, some marks escape erasure and can be transmitted to offspring, providing a non-genetic mechanism for inheritance of acquired traits.

Notable examples: Dutch Hunger Winter (1944–45) — children of mothers who starved during pregnancy showed increased obesity and metabolic disease, and this effect persisted in their grandchildren. Sperm from traumatically stressed male mice carries altered miRNA profiles that transmit anxiety behaviour to offspring.

Epigenetic therapy: DNMT inhibitors (azacitidine, decitabine) and HDAC inhibitors are approved for treating certain cancers. They can restore silenced tumour suppressor genes by demethylating promoters. Highly active areas of research include epigenetic editing with dCas9-fused DNMT/HDAC enzymes for precise, programmable gene silencing.