RNA interference and microRNAs

RNAi & miRNA - Silencing the Messengers

• A post-transcriptional gene silencing mechanism using non-coding RNAs to regulate gene expression.
• microRNA (miRNA): Endogenous, derived from hairpin loop precursors (~70 nt). Typically causes translational repression through imperfect pairing with the 3' UTR of target mRNA.
• Small interfering RNA (siRNA): Often exogenous (e.g., viral dsRNA, synthetic). Induces mRNA cleavage via perfect or near-perfect base pairing.

⭐ The degree of complementarity dictates the outcome: perfect match (siRNA-like) leads to cleavage, while imperfect match (miRNA-like) leads to repression.

📌 siRNA → Specific interference (cleavage). 📌 miRNA → Mostly inhibitory (repression).

The Machinery - Molecular Scissors & Co.

• Nuclear Processing (In the Nucleus):

  • pri-miRNA (a long primary transcript) is recognized and cleaved by the microprocessor complex, whose key enzyme is Drosha (an RNase III enzyme).
  • This creates pre-miRNA, a shorter hairpin-loop structure that is then exported to the cytoplasm.

• Cytoplasmic Processing:

  • The Dicer enzyme (another RNase III) dices the pre-miRNA hairpin into a short, 22-nucleotide double-stranded RNA molecule (miRNA/siRNA).

• RISC Loading & Silencing:

  • One strand of this dsRNA is loaded onto the RNA-Induced Silencing Complex (RISC).
  • This single strand guides RISC to its complementary target mRNA.

⭐ The Argonaute protein is the catalytic core of RISC, acting as "molecular scissors" to cleave the target mRNA, leading to gene silencing.

Mechanism of Action - Perfect vs. Imperfect

• Both miRNA and siRNA are loaded into the RNA-Induced Silencing Complex (RISC), which uses one strand as a guide.
• The extent of base pairing between this guide strand and a target mRNA dictates the regulatory outcome.

• Perfect Match (siRNA-like): Leads to direct cleavage and degradation of the target mRNA molecule by Argonaute protein (the catalytic component of RISC).
• Imperfect Match (miRNA-like): Physically hinders translation by ribosomes, leading to translational repression. The mRNA is often sequestered to P-bodies for eventual degradation.

⭐ Most endogenous human miRNAs bind with imperfect complementarity to the 3' untranslated region (3' UTR) of their target mRNAs, causing translational repression rather than cleavage.

Clinical Corner - Therapeutic Silencers

• Therapeutic RNA interference (RNAi) uses synthetic small interfering RNAs (siRNAs) to silence pathogenic genes.
• These custom-designed dsRNA molecules are introduced into cells, where they engage the RISC complex to find and cleave a specific target mRNA.
• This prevents the translation of disease-causing proteins.
• Examples:
  • Patisiran & Inotersen: Target transthyretin (TTR) mRNA for hereditary amyloidosis (hATTR).
  • Inclisiran: Targets PCSK9 mRNA to treat hypercholesterolemia.

⭐ Most siRNA drugs are engineered for targeted delivery to the liver, often by conjugating them to N-acetylgalactosamine (GalNAc), which binds to hepatocyte-specific receptors.

High‑Yield Points - ⚡ Biggest Takeaways

• RNA interference (RNAi) is a potent mechanism of post-transcriptional gene silencing mediated by small non-coding RNAs.
• MicroRNAs (miRNAs) are endogenous regulators, while small interfering RNAs (siRNAs) are often from exogenous sources (e.g., viruses, therapeutics).
• Precursor RNAs are processed by Drosha (nucleus) and Dicer (cytoplasm).
• The mature single-stranded RNA is loaded into the RNA-induced silencing complex (RISC).
• RISC targets mRNA for cleavage (perfect complementarity) or translational repression (imperfect match).