Gene Inhibition Therapy

Endogenous RNAi Pathway

The video below will provide an overview of the endogenous or naturally occurring RNA interference (or RNAi) pathway.

In this video, we will be reviewing the endogenous or naturally occurring RNA interference (or RNAi) pathway.
The endogenous RNAi pathway can be triggered by different types of double-stranded RNAs. One of these triggers is microRNA1. Endogenous microRNAs play important roles in the biological processes of cell development, differentiation, apoptosis, and proliferation2–5.

A non-coding microRNA gene is usually transcribed by RNA polymerase II to form primary microRNA or pri-microRNA1,6. Pri-microRNA is a double-stranded, stem-loop hairpin structure that is cleaved to form precursor microRNA or “pre-microRNA” of approximately 70 to 100 nucleotides, which is then transported into the cytoplasm2,6. Once in the cytoplasm, pre-microRNA loses the stem loop and is further cleaved into shorter duplexes of approximately 22 nucleotides and two 3-prime nucleotide overhangs to form microRNA1,2,7.

MicroRNA then associates with the RNA-induced silencing complex, otherwise known as RISC1,6. Within the microRNA–RISC complex, the microRNA is unwound, releasing the passenger strand, while the guide strand — the one that is complementary to the messenger RNA (or mRNA) that is being targeted — remains associated with RISC. This leads RISC to bind to the target mRNA by partial complementarity1,6,8,9.

If the guide strand is only partially complementary to the target mRNA, it can bind to multiple complementary sites or multiple mRNAs and cause translational repression6,9-11.

Less frequently, when the guide strand is fully or almost fully complementary to the target mRNA, cleavage of the target mRNA occurs6,11,12.

RNAi can also be triggered by small pieces of double-stranded RNA called small interfering RNA, or siRNA6. Although not completely understood, endogenous siRNAs may have roles in repression of transposable elements, chromatin organisation, and gene regulation at the transcriptional and post-transcriptional level13,14.

A long double-stranded RNA is first transcribed from cellular genes or infecting pathogens. This RNA is cleaved in the cytoplasm into a smaller double-stranded RNA molecule called siRNA that is 21 to 23 nucleotides long6. siRNA then associates with RISC, and within the siRNA–RISC complex, the siRNA is unwound, releasing and degrading a passenger strand. The guide strand, however, remains associated with RISC and leads RISC to bind to the target mRNA1,6,8,9.

To induce gene inhibition, the guide strand must be fully complementary to its target mRNA, which results in gene inhibition via cleavage of the target mRNA6. This summarises the endogenous RNAi pathway. The next section will discuss key therapeutic triggers of RNAi and how these have been harnessed as therapeutic strategies.

References

  1. Borel F, et al. Mol Ther 2014;22(4):692–701.
  2. Pushparaj PN, et al. J Dent Res 2008;87(11):992–1003.
  3. Catalonotto C, et al. Int J Mol Sci 2016;17(10):1712.
  4. Novák J, et al. Theranostics 2014;4(2):119–133.
  5. Zampetaki A, et al. Circ Res 2010;107(6):810–817.
  6. Lam JK, et al. Mol Ther Nucleic Acids 2015;4(9):e252.
  7. Hydbring P, Badalian-Very G. F1000Res 2013;2:136.
  8. Chery J. Postdoc J 2016;4(7):35–50.
  9. Fellmann C, Lowe SW. Nat Cell Biol 2014;16(1):10–18.
  10. Ramachandran PV, Ignacimuthu S. Appl Biochem Biotechnol 2013;169(6):1774–1789.
  11. Herrera-Carrillo E, et al. Hum Gene Ther Methods 2017;28(4):117–190.
  12. Park JH, Shin C. BMB Rep 2014;47(8):417–423.
  13. Stein P, et al. PLoS Genet 2015;11(2):e1005013.
  14. Piatek MJ, Werner A. Biochem Soc Trans 2014;42(4):1174–1179.