miRNA and RNA Interference

RNAi Fundamentals - Silencing Secrets

⭐ RNAi is a highly conserved gene silencing mechanism found in most eukaryotic organisms, from yeast to mammals.

• RNA Interference (RNAi): A natural cellular process for post-transcriptional gene silencing (PTGS).
• Mechanism: Double-stranded RNA (dsRNA) triggers sequence-specific degradation or translational repression of homologous messenger RNA (mRNA).
• Key Components:
  • Dicer: Endoribonuclease that cleaves dsRNA into short interfering RNAs (siRNAs) or microRNAs (miRNAs) (typically 20-25 base pairs).
  • RISC (RNA-Induced Silencing Complex): Incorporates one strand of siRNA/miRNA to guide target recognition.
  • Argonaute (Ago) proteins: Core components of RISC, mediate mRNA cleavage or translational repression.
• Outcome: Sequence-specific reduction in gene expression.
• Discovery: Andrew Fire and Craig Mello (Nobel Prize 2006).

miRNAs - Tiny Gene Tamers

• Tiny (~22 nt) endogenous non-coding RNAs; key post-transcriptional gene regulators.

• Biogenesis: 📌 Pri-Mi-Di-Ri

  • Pri-miRNA (nucleus) processed by Drosha-DGCR8 to pre-miRNA.
  • Pre-miRNA exported (Exportin-5) to cytoplasm.
  • Dicer cleaves pre-miRNA → mature miRNA duplex.
  • One strand loaded into Argonaut (Ago) → RiSC (RNA-induced silencing complex).

• Mechanism:

  • RISC guides miRNA to target mRNA (often 3' UTR).
  • Imperfect pairing → translational repression.
  • Near-perfect pairing → mRNA cleavage.

• Functions: Regulate development, differentiation, proliferation, apoptosis.

• Clinical Links: Dysregulation in cancer, cardiovascular disease, viral infections. Potential biomarkers/therapeutics.

⭐ The 'seed region' (nucleotides 2-8) of a miRNA is crucial for target mRNA recognition and binding, often leading to translational repression through imperfect pairing.

siRNAs & Key Players - Interference Engines

• Small interfering RNAs (siRNAs):
  • Source: Exogenous (e.g., viral) or endogenous long dsRNA.
  • Structure: ~21-23 bp dsRNA, 2-nt 3' overhangs.
  • Function: Gene silencing via RNAi (PTGS).
• Core RNAi Machinery:
  • Dicer:
    • Type: RNase III enzyme.
    • Action: Processes long dsRNA → siRNAs.
    • Produces characteristic 2-nt 3' overhangs.
  • RISC (RNA-Induced Silencing Complex):
    • Role: Main effector complex in RNAi.
    • Composition: siRNA guide strand + Argonaut (Ago) proteins.
    • Action: Unwinds siRNA, incorporates guide strand.
  • Argonaute (Ago) proteins:
    • Function: Catalytic core of RISC.
    • Ago2 (human): 'Slicer' activity, cleaves target mRNA.
    • Mechanism: Uses guide siRNA for mRNA target recognition & cleavage.

⭐ Dicer, an RNase III enzyme, is essential for processing both pre-miRNAs into mature miRNAs and long dsRNAs into siRNAs.

RNAi Applications - Therapeutic Targets

• Oncology: Targeting oncogenes (e.g., KRAS, MYC), anti-apoptotic proteins (e.g., BCL2), drug resistance genes.
• Viral Infections: Targeting viral genome/transcripts.
  • Hepatitis B & C (HBV, HCV)
  • HIV
  • Respiratory viruses (Influenza, RSV)
• Genetic Disorders:
  • Huntington's disease (targeting mutant huntingtin mRNA)
  • Dominant dystrophic epidermolysis bullosa
  • Transthyretin-mediated amyloidosis (hATTR)
• Metabolic Diseases:
  • Hypercholesterolemia (targeting PCSK9)
• Ocular Diseases:
  • Age-related macular degeneration (AMD) (targeting VEGF)
• Inflammatory Diseases: Targeting cytokines, chemokines.

⭐ Patisiran (Onpattro) is the first FDA-approved siRNA therapeutic, targeting transthyretin mRNA for the treatment of hereditary transthyretin-mediated amyloidosis (hATTR).

High‑Yield Points - ⚡ Biggest Takeaways

• miRNAs are ~22 nucleotide non-coding RNAs regulating gene expression.
• RNA Interference (RNAi) is a gene silencing mechanism by miRNAs/siRNAs targeting mRNA.
• Drosha (nuclear) and Dicer (cytoplasmic) are key RNase III enzymes in miRNA biogenesis.
• The RISC complex, with Argonaute protein, mediates target mRNA recognition and silencing.
• Imperfect miRNA-mRNA pairing typically leads to translational repression; perfect siRNA-mRNA pairing usually causes mRNA cleavage.
• RNAi pathways are vital for cellular defense, developmental control, and genome stability.