Epigenetic mechanisms in cancer

Epigenetics Overview - The Cancer Conductor

Heritable, reversible changes in gene expression without altering the DNA sequence. These modifications can silence tumor suppressor genes or activate oncogenes, acting as a conductor for carcinogenesis.

• Key Mechanisms:
  • DNA Methylation: Hypermethylation of CpG islands in promoter regions → silences tumor suppressor genes (e.g., CDKN2A).
  • Histone Modification: Altered acetylation/deacetylation patterns change chromatin accessibility and gene expression.

⭐ Epigenetic alterations are reversible, forming a basis for novel cancer therapies (e.g., DNMT and HDAC inhibitors).

DNA Methylation - The Master Switch

• Mechanism: Covalent addition of a methyl group ($CH_3$) to cytosine, primarily at CpG dinucleotides.
• Enzyme: Catalyzed by DNA Methyltransferases (DNMTs).
• Function: Acts as a master switch for gene expression.
  • ↑ Methylation (Hypermethylation) → condensed chromatin → gene silencing.
  • ↓ Methylation (Hypomethylation) → relaxed chromatin → gene activation.

• Role in Cancer:
  • Hypermethylation: Silences tumor suppressor genes (e.g., p16, BRCA1).
  • Hypomethylation: Activates oncogenes and promotes genomic instability.

⭐ High-Yield: Aberrant hypermethylation of CpG islands in the promoter regions of tumor suppressor genes is a hallmark of many cancers, leading to their transcriptional inactivation.

Histone Modification - Unspooling Trouble

• Core Concept: Chemical changes to histone proteins alter chromatin structure & gene accessibility. Tightly wound heterochromatin silences genes; loosely packed euchromatin allows transcription.

• Key Modifications:

  • Acetylation: Histone Acetyltransferases (HATs) add acetyl groups → relaxed chromatin (euchromatin) → ↑ gene expression. Histone Deacetylases (HDACs) reverse this.
  • Methylation: Histone Methyltransferases (HMTs) add methyl groups. Effect varies: can activate or repress genes depending on the site.

• Role in Cancer:

  • Mutations in genes for HATs/HDACs or HMTs can silence tumor suppressor genes (e.g., p53) or activate oncogenes.

⭐ Exam Favorite: Histone Deacetylase (HDAC) inhibitors (e.g., Vorinostat, Romidepsin) are a class of anti-cancer drugs. They work by promoting histone acetylation, leading to re-expression of silenced tumor suppressor genes.

Epigenetic Drugs - Hacking the Code

Goal: Reactivate tumor suppressor genes silenced by epigenetic modifications.

• DNMT Inhibitors: Azacitidine, Decitabine. Hypomethylating agents that incorporate into DNA, trapping DNMTs and leading to ↓ methylation.
• HDAC Inhibitors: Vorinostat, Romidepsin. Prevent removal of acetyl groups from histones, leading to ↑ acetylation, open chromatin, and gene expression.

⭐ Azacitidine and Decitabine are workhorses for myelodysplastic syndromes (MDS), often improving blood counts and delaying progression to acute myeloid leukemia (AML).

• DNA hypermethylation at CpG islands in promoter regions silences tumor suppressor genes (e.g., MLH1, BRCA1).
• Widespread genomic hypomethylation can lead to chromosomal instability and activation of proto-oncogenes.
• Histone modifications, like deacetylation, can condense chromatin and inhibit transcription of key regulatory genes.
• MicroRNAs (miRNAs) can function as oncogenes or tumor suppressors by regulating mRNA translation.
• Unlike genetic mutations, epigenetic alterations are potentially reversible, offering novel therapeutic targets (e.g., DNMT and HDAC inhibitors).