What is RNAi (RNA interference) ?

Question: in Civil Services (Prelims) 2019 GS Paper  

‘RNA interference (RNAi)’ technology has gained popularity in the last few Why?

1. It is used in developing gene silencing therapies.
2. It can be used in developing therapies for the treatment of cancer.
3. It can be used to develop hormone replacement therapies.
4. It can be used to produce crop plants that are resistant to viral pathogens.

Select the correct answer using the code given below.

(a) 1, 2 and 4

(b) 2 and 3

(c) 1 and 3

(d) 1 and 4 only

Ans: a 

RNA interference (RNAi) or Post- Transcriptional Gene Silencing (PTGS) is a conserved biological response to double- stranded RNA that mediates resistance to both endogenous parasitic and exogenous pathogenic nucleic acids, and regulates the expression of protein-coding genes.

What is RNAi (RNA interference) ?

The New Technology:

 RNA interference (RNAi) technology is a type of gene silencing that turns off the expression of a target gene in the cell. Gene silencing is currently used to combat various human diseases, including cancers, neurodegenerative and infectious diseases. A gene can be silenced (prevented from the expression of its gene product) either during the formation of its complementary mRNA (post-transcriptionally) or during the synthesis of proteins from the mRNA (posttranslationally). The technology is emerging as a potential alternative to combat stubborn pests infecting commercial crops.

RNA interference (RNAi)

The technology used to silence a gene post-translationally was discovered by Andrew Fire of the Carnegie Institution for Science and Craig Mellow of Chan Medical School, University of Massachusetts, in 1998. The technology received remarkable attention for its potential to shut off disease-related genes. Fire and Mellow were awarded the Nobel Prize in Physiology and Medicine in 2006 for their work. RNAi is now on the cusp of ushering in a new era in pesticide control. RNAi is, in fact, a natural phenomenon that evolved from various animals and plants to combat infections caused by several RNA viruses. A simplified description of the mechanism of action of RNAi is as follows:

1)  Several RNA viruses have double-stranded RNAs (dsRNA) as their genetic material, and the RNAi process is initiated soon after dsRNA enters the host cells. The enzyme Dicer (RNAase III), present in the cells, recognises and cuts the dsRNA into double-stranded fragments designated as short interfering RNAs (siRNA) and micro RNAs (miRNAs)

2)The siRNA has 21-23 nucleotides with two nucleotide overhangs at the 3’end. The siRNA is picked up by a protein complex called RNA-induced Silencing Complex (RISC) that contains the Argonaute proteins. Binding is followed by the degradation of one of the strands of siRNA called the “passenger” strand by an Argonaute enzyme (Ago2). The “guide” (antisense) strand is not degraded but incorporated into the RISC complex. The single-stranded RNA fragment (the guide strand) in the complex hybridises with the RNA target (RNA-bearing complementary sequence), which is degraded by the enzyme RNAse H (the slicer)

3)A gene is silenced either by degradation of RNA bearing complementary sequence to the antisense strand or by miRNAs that bind to the complementary RNA and suppress their expression. The major difference between siRNAs and miRNAs is that the former inhibits one specific target mRNA, while the latter can regulate the expression of multiple RNAs

4)It is theoretically possible to silence any given gene in a cell provided its nucleotide sequence, or that of the mRNA synthesised by it is known. The first essential step is designing siRNA with a nucleotide sequence specific for the target mRNA. Experimentally, RNAi can be induced with the help of synthetic siRNA duplexes or constructs that express short hairpin RNAs (shRNAs) that contain a short double-stranded, hairpin-like turn formed due to the folding back of part of the RNA. siRNA and shRNA are synthesised to contain a sequence of bases in the guide strand complementary to the target mRNA to be silenced

5)  shRNA is an advantageous mediator of RNAi since it is quite resistant to nonspecific degradation by the nucleases in the cell. It, however, requires an expression vector like a plasmid or viral vector for entry into the cell. The shRNA is cleaved by the endogenous DICER to generate desired siRNA duplexes, which in turn associate with RISC and processed as described above.