This web page was produced as an assignment for Genetics 564, an undergraduate course at UW‐Madison.
What is RNA interference?RNA interference (RNAi) is an experimental method which allows for the study of genes by determining what happens when their RNA transcripts are inhibited. The mechanism of RNAi is shown in Figure 1. Double stranded RNA is introduced into a cell and gets chopped up by the enzyme dicer to form siRNA. siRNA then binds to the RNA-induced silencing complex (RISC) and is unwound. The antisense RNA complexed with RISC binds to its corresponding mRNA which is then cleaved by the enzyme slicer and makes it inactive. [1]
How does RNAi illustrate gene expression?By silencing the RNA products of the gene of interest, we can observe the phenotypic changes to determine the functions of the gene. By utilizing RNAi, scientists can silence selected genes quickly and reversibly.
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Figure 1. Mechanism of RNAi. (Source)
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Limitations of RNAi
There are a few limitations of using RNAi method to study gene expression:
- RNAi does not silence the gene completely, as there will be some mRNA that remain unbound by antisense RNA or undegraded by enzyme slicer. [2]
- Off target effects might arise if the antisense RNA binds to a sequence that is similar to the target sequence and affect the expression of an undesired gene. [2]
- Effectiveness of RNAi relies on the delivery and stability of siRNA. Delivery of siRNA depends on the sequence and the structure of the chosen siRNA, as well as the receptiveness of the cell type to siRNA uptake. The chosen siRNA also require a certain level of chemical and thermodynamic stability when circulated in vivo. [3]
- RNAi may induce an undesired immune response. RNAi may not work in mammalian cells, because RNAi may activate the interferon response, a nonspecific viral defense mechanism.[4]
RNAi and MAOA
As MAOA is not present in non-vertebrates, RNAi for MAOA cannot be found from databases of worms, flies and zebrafish. However, I managed to obtain phenotypic results of MAOA RNAi in mouse and human via Mouse Genome Informatics (MGI) and Cellular Phenotypes Databases respectively.
Given the association of MAOA and aggression, the resulted phenotypic changes such as increased aggression and hypoactivity, altered response to stress, inhibition of protein secretion and abnormal nuclear shape of the MAOA protein are not surprising. Reduced LV systolic pressure and resistance to induced cardiac stress indicate that MAOA expression has something to do with heart rate. Increased heart rate usually accompanies aggression, reduced LV systolic pressure might be why heart has to pump at a faster rate to supply oxygenated blood. Resistance to induced cardiac stress is the opposite of what I expected and would be interesting to look further into.
Reference:
- RNA Interference Fact Sheet. National Institute of General Medical Sciences. Retrieved May 9, 2014.
- Sugimoto A. (2004). High-throughput RNAi in Caenorhabditis elegans: genome-wide screens and functional genomics. Differentiation, 72(2-3):81-91.
- Sledz, C.A., & Williams, B.G.R. (2005). RNA interference in biology and disease. Blood, 106(3): 787–794.
- Tuschl T. (2001). RNA interference and small interfering RNAs. Chem Bio Chem, 2: 239-145.