Epigenetics is the study of heritable changes in gene expression that occur without changes to the underlying DNA sequence. These mechanisms are essential for normal development, cell differentiation, and genomic imprinting, and their dysregulation contributes to cancer and other diseases.
DNA Methylation
DNA methylation primarily occurs at cytosine bases within CpG dinucleotides, catalyzed by DNA methyltransferases (DNMTs). DNMT3A and DNMT3B establish methylation patterns during development (de novo methylation), while DNMT1 maintains methylation patterns during DNA replication by copying methylation from the parental to the daughter strand. CpG islands, regions of high CpG density often found in gene promoter regions, are typically unmethylated in active genes. Methylation of promoter CpG islands is associated with transcriptional repression, either by directly blocking transcription factor binding or by recruiting methyl-CpG-binding domain proteins (MeCP2, MBD1–4) that recruit histone deacetylases and chromatin remodeling complexes. DNA methylation patterns are reprogrammed during embryogenesis and gametogenesis, with global demethylation occurring shortly after fertilization followed by lineage-specific re-methylation.
Histone Modifications
Histone proteins undergo a diverse array of post-translational modifications on their N-terminal tails, including acetylation, methylation, phosphorylation, ubiquitination, and sumoylation. Histone acetylation, catalyzed by histone acetyltransferases (HATs), neutralizes the positive charge of lysine residues, reducing the affinity between histones and DNA and creating an open, transcriptionally active chromatin structure. Histone deacetylases (HDACs) reverse this modification, restoring the positive charge and promoting chromatin compaction and gene silencing. Histone methylation can be either activating or repressive depending on the specific residue and degree of methylation. H3 lysine 4 trimethylation (H3K4me3) marks active promoters, H3 lysine 36 trimethylation (H3K36me3) marks transcribed gene bodies, and H3 lysine 27 trimethylation (H3K27me3) deposited by Polycomb repressive complex 2 (PRC2) marks silenced developmental genes. H3 lysine 9 trimethylation (H3K9me3) is associated with constitutive heterochromatin at centromeres and telomeres.
Chromatin Remodeling
ATP-dependent chromatin remodeling complexes use the energy of ATP hydrolysis to slide, eject, or restructure nucleosomes, making DNA accessible to transcription factors and other regulatory proteins. The SWI/SNF family (BAF complexes in mammals) mobilizes nucleosomes to promote transcription factor binding and is frequently mutated in cancer, with mutations in ARID1A, SMARCA4, and SMARCB1 found in ovarian, lung, and pediatric cancers. ISWI family complexes promote nucleosome spacing and chromatin compaction, CHD family complexes (including NuRD) have roles in both activation and repression, and INO80 family complexes are involved in DNA repair and replication. The combinatorial action of these remodeling complexes establishes the landscape of chromatin accessibility that defines cellular identity.
Genomic Imprinting
Genomic imprinting is an epigenetic phenomenon where a subset of genes is expressed in a parent-of-origin-specific manner. Imprinted genes are marked by differential DNA methylation during gametogenesis, with either the maternal or paternal allele being methylated and silenced. The IGF2/H19 locus is a classic example: IGF2 is expressed only from the paternal allele, while H19 is expressed only from the maternal allele, regulated by an imprinting control region (ICR) that binds CTCF on the maternal chromosome to block enhancer access to IGF2. Imprinting disorders include Prader-Willi syndrome (paternal deletion of 15q11-q13, with maternal allele silenced by imprinting) and Angelman syndrome (maternal deletion of the same region, with paternal allele silenced), demonstrating the clinical importance of parent-of-origin effects.
X-Chromosome Inactivation
In female mammals, one of the two X chromosomes is transcriptionally silenced in a process called X-inactivation, equalizing X-linked gene expression between XX females and XY males. The process is initiated by the long non-coding RNA Xist, which is transcribed from the future inactive X chromosome and coats it in cis, recruiting chromatin modifiers that deposit repressive marks including H3K27me3 and H2AK119ub. The inactive X chromosome becomes a compact Barr body, replicates late in S phase, and most of its genes are stably silenced. X-inactivation is random in the embryo proper but imprinted (paternal X silenced) in the extraembryonic tissues. The inactive X is reactivated during oogenesis but remains inactive in sperm.
Epigenetic Reprogramming in Development
After fertilization, the paternal genome undergoes rapid active demethylation before DNA replication, while the maternal genome is demethylated more gradually through passive dilution. This erasure of parent-specific epigenetic marks is followed by de novo methylation at the blastocyst stage, establishing the epigenetic patterns that drive lineage-specific gene expression. Primordial germ cells undergo a more complete epigenetic reprogramming, including erasure of imprints, to restore totipotency. Induced pluripotent stem cells are generated by reprogramming somatic cells through forced expression of transcription factors (Oct4, Sox2, Klf4, c-Myc), accompanied by global epigenetic remodeling that erases somatic cell memory.
Epigenetics in Disease
Aberrant DNA methylation patterns are a hallmark of cancer, with global hypomethylation promoting genomic instability and focal hypermethylation of tumor suppressor gene promoters (such as BRCA1, MLH1, CDKN2A) causing their silencing. Mutations in epigenetic modifiers are themselves oncogenic, including IDH1/IDH2 mutations that produce 2-hydroxyglutarate, inhibiting TET demethylases and causing a CpG island methylator phenotype. Epigenetic therapies include DNA methyltransferase inhibitors (azacitidine, decitabine) for myelodysplastic syndrome and acute myeloid leukemia, and HDAC inhibitors (vorinostat, romidepsin) for cutaneous T cell lymphoma. Environmental factors including diet, stress, and smoking can alter epigenetic marks, linking lifestyle to gene expression changes and disease risk through the emerging field of environmental epigenetics.
