Gene silencing
[5] Oligonucleotides, which are short nucleic acid fragments, bind to complementary target mRNA molecules when added to the cell.
Sidney Altman and Thomas Cech first discovered catalytic RNA molecules, RNase P and group II intron ribozymes, in 1989 and won the Nobel Prize for their discovery.
[12] The process to silence genes first begins with the entrance of a double-stranded RNA (dsRNA) molecule into the cell, which triggers the RNAi pathway.
[12][13] The guide or antisense strand of the fragment that remains bound to RISC directs the sequence-specific silencing of the target mRNA molecule.
[13] With the cleavage or translational repression of the mRNA molecules, the genes that form them are rendered essentially inactive.
[12] RNAi is thought to have evolved as a cellular defense mechanism against invaders, such as RNA viruses, or to combat the proliferation of transposons within a cell's DNA.
The three prime untranslated regions (3'UTRs) of messenger RNAs (mRNAs) often contain regulatory sequences that post-transcriptionally cause gene silencing.
[citation needed] As of 2014, the miRBase web site,[17] an archive of miRNA sequences and annotations, listed 28,645 entries in 233 biologic species.
[citation needed] Direct experiments show that a single miRNA can reduce the stability of hundreds of unique mRNAs.
[22] For instance, in gastrointestinal cancers, nine miRNAs have been identified as epigenetically altered and effective in down regulating DNA repair enzymes.
In in vitro studies of chronic myelogenous leukemia (CML), siRNA was used to cleave the fusion protein, BCR-ABL, which prevents the drug Gleevec (imatinib) from binding to the cancer cells.
[28] Receptors involved in mitogenic pathways that lead to the increased production of cancer cells there have also been targeted by siRNA molecules.
[30][31] Clusterin and survivin-targeting siRNAs were used to reduce the number of antiapoptotic proteins and, thus, increase the sensitivity of the cancer cells to chemotherapy treatments.
RNAi has been used to target genes in several viral diseases, such as the human immunodeficiency virus (HIV) and hepatitis.
[35] This prevented the virus from entering the human peripheral blood lymphocytes and the primary hematopoietic stem cells.
[45] These papayas also remain in use at present, although not without significant public protest,[46][47] which is notably less evident in medical uses of gene silencing.
The E6 gene in tumor samples retrieved from patients with the human papilloma virus was targeted and found to cause apoptosis in the infected cells.
[52][53] For instance, siRNA was used to reduce the amount of pro-inflammatory cytokines expressed in the cells of mice treated with lipopolysaccharide (LPS).
[52][54] The reduced expression of the inflammatory cytokine, tumor necrosis factor α (TNFα), in turn, caused a reduction in the septic shock felt by the LPS-treated mice.
[54] In addition, siRNA was used to prevent the bacteria, Psueomonas aeruginosa, from invading murine lung epithelial cells by knocking down the caveolin-2 (CAV2) gene.
[citation needed] Ribozymes, antisense oligonucleotides, and more recently RNAi have been used to target mRNA molecules involved in asthma.
[57] Mucus secretion was found to be reduced when the transforming growth factor (TGF)-α was targeted by siRNA in NCI-H292 human airway epithelial cells.
[58] In addition to mucus hypersecretion, chronic inflammation and damaged lung tissue are characteristic of COPD and asthma.
[61][62] Huntington's disease (HD) results from a mutation in the huntingtin gene that causes an excess of CAG repeats.
[citation needed] Gene silencing can be used to treat HD by targeting the mutant huntingtin protein.
In addition, when antisense oligonucleotides were used to target an HD-associated SNP in mice, there was a 50% decrease in the mutant huntingtin protein.
For instance, for treatment of neurodegenerative disorders, molecules for a prospective gene silencing therapy must be delivered to the brain.
[63] Thus, researchers are searching for more efficient methods to deliver and develop specific gene silencing therapeutics that are still safe and effective.
