Epigenetics and DNA Methylation

Epigenetics and DNA Methylation - Genes Beyond Sequence

• Epigenetics: Heritable gene expression changes; no DNA sequence alteration. Regulates gene activity.
• Key Mechanisms:
  • DNA Methylation: Methyl group ($CH_3$) addition to DNA, primarily at CpG sites.
  • Histone Modification: Acetylation, methylation, phosphorylation of histone tails.
  • Non-coding RNAs (ncRNAs): e.g., microRNAs (miRNAs), long non-coding RNAs (lncRNAs).
• DNA Methylation Focus:
  • Enzymes: DNA Methyltransferases (DNMTs).
  • Effect: Usually gene silencing (transcriptional repression).
  • CpG islands (promoters): methylation often leads to ↓ gene expression.
  • Reversible: Demethylation (e.g., by TET enzymes) can reactivate genes.
• Significance: Vital for normal development, cellular differentiation, genomic imprinting, X-chromosome inactivation, and diseases (e.g., cancer).

⭐ Epigenetic changes, unlike genetic mutations, do not alter the DNA sequence itself but can be stably inherited through cell divisions.

Epigenetics and DNA Methylation - Silencing Secrets

• Epigenetics: Heritable changes in gene expression without altering DNA sequence. Environmentally influenced.
• DNA Methylation: Key epigenetic modification.
  • Mechanism: Addition of a methyl group ($-CH_3$) to DNA, primarily at cytosine bases.
    • Enzymes: DNA Methyltransferases (DNMTs).
    • Methyl donor: S-adenosylmethionine (SAM).
  • Primary Site: CpG dinucleotides (Cytosine-phosphate-Guanine).
    • Clusters of CpGs = CpG islands, often in promoter regions.
  • Effect: Typically leads to gene silencing.
    • Blocks transcription factor binding.
    • Recruits methyl-CpG-binding domain proteins (MBDs) $\rightarrow$ chromatin condensation.
  • Demethylation: Active process involving TET (Ten-Eleven Translocation) enzymes.
• Clinical Relevance: Implicated in cancer (tumor suppressor gene silencing), imprinting disorders (Prader-Willi, Angelman), developmental processes.

⭐ DNA methylation predominantly occurs at CpG dinucleotides, and methylation of CpG islands in promoter regions is typically associated with gene silencing.

Epigenetics and DNA Methylation - Chromatin's Chat

• Histone Modifications: Reversible chemical changes to histone proteins, altering chromatin structure & gene activity.
• Key Modifications:
  • Acetylation:
    • By HATs (Histone Acetyltransferases).
    • Loosens chromatin (euchromatin) → ↑ transcription. 📌 "A"cetylation = "A"ctive.
  • Deacetylation:
    • By HDACs (Histone Deacetylases).
    • Compacts chromatin (heterochromatin) → ↓ transcription.
  • Methylation:
    • By HMTs (Histone Methyltransferases).
    • Context-dependent: can activate (e.g., H3K4me3) or repress (e.g., H3K9me3, H3K27me3).
    • Reversed by HDMs (Histone Demethylases).
  • Phosphorylation & Ubiquitination: Also influence chromatin.
• "Histone Code": Patterns of modifications recruit specific proteins, dictating gene expression outcomes.

⭐ Histone acetylation, mediated by HATs, generally opens up chromatin and activates transcription, while deacetylation by HDACs has the opposite effect.

Epigenetics and DNA Methylation - Disease & Drugs

• Cancer: Aberrant DNA methylation (e.g., ↑methylation of tumor suppressors, ↓methylation of oncogenes).
• Imprinting Disorders:
  • Prader-Willi (PWS), Angelman (AS) syndromes.
  • Beckwith-Wiedemann, Russell-Silver syndromes.
• Other Genetic Conditions: Fragile X, Rett, ICF syndromes.
• Therapeutic Agents (Epi-drugs):
  • DNMT Inhibitors: Azacitidine, Decitabine (for Myelodysplastic Syndromes (MDS), Acute Myeloid Leukemia (AML)).
  • HDAC Inhibitors: Vorinostat, Romidepsin (for Cutaneous T-cell Lymphoma (CTCL), Peripheral T-cell Lymphoma (PTCL)).

⭐ Genomic imprinting, an epigenetic phenomenon, results in monoallelic gene expression depending on the parent of origin, exemplified by Prader-Willi and Angelman syndromes.

High‑Yield Points - ⚡ Biggest Takeaways

• DNA methylation at CpG islands predominantly leads to heritable gene silencing.
• SAM (S-adenosylmethionine) is the crucial methyl donor for DNA methyltransferases (DNMTs).
• Histone acetylation by HATs activates transcription; HDACs cause repression.
• Histone methylation effects (activation/repression) are highly context-dependent.
• Genomic imprinting dictates parent-of-origin specific gene expression.
• Epigenetic dysregulation drives cancer and syndromes like Prader-Willi/Angelman.
• X-chromosome inactivation (Barr body formation) is a key epigenetic process.