⚡ Core Concept: Heritable Phenotypes Without Altering DNA Sequence
Epigenetics encompasses stable, heritable modifications in gene expression and cellular phenotype that do not involve changes to the underlying genomic DNA sequence. Major epigenetic mechanisms include DNA cytosine methylation, post-translational histone modifications, and non-coding RNA-mediated chromatin remodeling.
1. Major Epigenetic Regulatory Mechanisms
- DNA Methylation: Covalent transfer of a methyl group (−CH3) from S-adenosylmethionine (SAM) to the 5-position of cytosine within CpG dinucleotides by DNA methyltransferases (DNMTs):
- DNMT1: Maintenance methyltransferase preserving methylation during DNA replication.
- DNMT3A & DNMT3B: De novo methyltransferases establishing new methylation patterns during embryonic development.
- CpG Islands: Dense CpG clusters located in gene promoters; when hypermethylated, they recruit methyl-CpG-binding domain proteins (MBDs) and histone deacetylases, causing stable transcriptional silencing.
- Histone Post-Translational Modifications (Histone Code):
- Histone Acetylation: Histone acetyltransferases (HATs) transfer acetyl groups to lysine residues on histone tails, neutralizing positive charges, weakening DNA-histone electrostatic interaction → relaxes chromatin into euchromatin (active transcription). Counteracted by Histone Deacetylases (HDACs).
- Histone Methylation: Histone methyltransferases (HMTs) add methyl groups.
H3K4me3marks active promoters;H3K9me3andH3K27me3mark repressive heterochromatin.
Figure 1: Epigenetic Transition: Euchromatin vs Heterochromatin
Chromatin Biology
Dynamic equilibrium between relaxed euchromatin (acetylated histones, hypomethylated DNA) allowing transcription factor access, and condensed heterochromatin (deacetylated histones, hypermethylated DNA) inducing gene silencing.
2. Physiological Epigenetic Phenomena
In female mammalian cells, dosage compensation occurs via transcriptional silencing of one X chromosome into a dense Barr body, driven by the non-coding RNA Xist coating the inactive X.
Monoallelic gene expression determined by parental origin. E.g., Igf2 is expressed exclusively from the paternal allele, while H19 is expressed exclusively from the maternal allele.
Global DNA hypomethylation accompanied by focal promoter hypermethylation of tumor suppressor genes (e.g., p16INK4a, BRCA1), contributing to oncogenesis.
🎓 High-Yield Exam Checkpoint
- Prader-Willi vs. Angelman Syndrome: Microdeletion on chromosome 15q11-q13:
- Loss of paternal contribution → Prader-Willi Syndrome (hyperphagia, obesity, hypogonadism).
- Loss of maternal contribution (UBE3A gene) → Angelman Syndrome (“happy puppet”, ataxia, seizures, severe cognitive impairment).
- Epigenetic Therapeutics: Azacitidine / Decitabine (DNMT inhibitors) and Vorinostat / Trichostatin A (HDAC inhibitors) are approved for myelodysplastic syndromes and cutaneous T-cell lymphomas.
