Epigenetics and Disease

Intro to Epigenetics - Code Above Code

• "Above genetics": Heritable gene expression changes without DNA sequence alteration.
• Dynamic, reversible; influenced by age, environment (diet, toxins, stress), lifestyle.
• Crucial for cellular differentiation, development, and normal cell function.
• Key Mechanisms:
  • DNA Methylation: Addition of methyl group to cytosine (often at CpG islands); generally silences genes.
  • Histone Modification: Acetylation (typically activates genes), methylation (can activate/repress), phosphorylation. Alters chromatin accessibility.
  • Non-coding RNAs (ncRNAs): e.g., microRNAs (miRNAs), long non-coding RNAs (lncRNAs) regulate gene expression post-transcriptionally.

⭐ Aberrant DNA methylation patterns, such as hypermethylation of tumor suppressor gene promoters, are a common feature in many cancers (e.g., colorectal, breast).

Epigenetic Mechanisms - Molecular Makeover

• Dynamic processes modifying gene activity; DNA sequence unchanged.
• 1. DNA Methylation:
  • Methyl ($CH_3$) group added to cytosine, often in CpG islands.
  • Enzymes: DNA Methyltransferases (DNMTs); Demethylation by TETs.
  • Typically causes transcriptional repression (gene silencing). 📌 "Methylation Mutes."
• 2. Histone Modification:
  • Post-translational modifications of N-terminal histone tails (e.g., H3, H4).
  • Acetylation (HATs): Relaxes chromatin, ↑ gene expression.
  • Deacetylation (HDACs): Condenses chromatin, ↓ gene expression.
  • Others: Methylation (HMTs/HDMs), phosphorylation, ubiquitination; context-dependent.
• 3. Non-coding RNAs (ncRNAs):
  • Functional RNAs not translated (e.g., miRNAs, lncRNAs, siRNAs).
  • Regulate gene expression transcriptionally or post-transcriptionally.

⭐ Widespread DNA hypomethylation and focal hypermethylation of tumor suppressor gene promoters are key epigenetic hallmarks in many cancers.

Epigenetics in Disease - Marks of Malady

• Epigenetic dysregulation: Heritable gene expression changes (not DNA sequence) causing disease.
• Key Mechanisms:
  • DNA Methylation: Aberrant patterns.
    • Hypermethylation: Silences tumor suppressor genes (TSGs) (e.g., BRCA1 in cancer).
    • Hypomethylation: Activates oncogenes, genomic instability.
  • Histone Modifications: Altered acetylation, methylation impacting chromatin structure.
  • Non-coding RNAs (ncRNAs): Dysregulated miRNAs, lncRNAs affecting gene expression.
• Associated Diseases:
  • Cancer: Widespread epigenetic alterations drive tumorigenesis.
  • Imprinting Disorders:
    • Prader-Willi Syndrome (PWS): Paternal 15q11-q13 defect/maternal imprinting.
    • Angelman Syndrome (AS): Maternal UBE3A defect/paternal imprinting.
    • Beckwith-Wiedemann Syndrome (BWS): 11p15.5 imprinting abnormalities.
  • Neurological: Rett Syndrome (MECP2 gene mutations).
• Therapeutic Targets:
  • DNMT inhibitors (e.g., Azacitidine).
  • HDAC inhibitors (e.g., Vorinostat).

⭐ Global DNA hypomethylation alongside promoter-specific hypermethylation of tumor suppressor genes is a hallmark of many cancers, offering diagnostic and therapeutic targets.

Epigenetic Therapy & Factors - Editing the Editors

• Epigenetic Drugs: Target enzymes writing/erasing epigenetic marks.
  • DNMT Inhibitors (DNMTi):
    • Azacitidine, Decitabine.
    • Uses: Myelodysplastic Syndromes (MDS), AML.
    • Action: Cause DNA hypomethylation, reactivate silenced genes.
  • HDAC Inhibitors (HDACi):
    • Vorinostat, Romidepsin.
    • Uses: Cutaneous T-cell Lymphoma (CTCL).
    • Action: Increase histone acetylation, open chromatin structure.
• Key Environmental Factors:
  • Diet: Folate, B12 (methyl supply); phytochemicals.
  • Toxins: Smoking, alcohol, pollutants (e.g., arsenic).
  • Stress: Chronic stress impacts DNA methylation.
  • Exercise: Promotes beneficial epigenetic modifications.

⭐ DNMT inhibitors like Azacitidine are crucial in MDS treatment by reversing aberrant DNA methylation and reactivating tumor suppressor genes.

High‑Yield Points - ⚡ Biggest Takeaways

• Epigenetics: Heritable gene function changes without altering DNA sequence.
• Key mechanisms: DNA methylation, histone modifications (acetylation, methylation), and non-coding RNAs.
• DNA methylation at CpG islands typically causes gene silencing.
• Histone acetylation generally activates transcription; deacetylation represses it.
• Genomic imprinting means parent-of-origin specific expression (e.g., Prader-Willi, Angelman syndromes).
• Implicated in cancer (e.g., silencing tumor suppressors), neurodevelopmental, and autoimmune diseases.
• Epigenetic marks are reversible, offering novel therapeutic targets (e.g., DNMT/HDAC inhibitors).