siRNA and shRNA Design Guidelines
The design of siRNAs and short hairpin siRNAs (shRNAs) remains an empirical process since the molecular mechanisms underlying RNAi are not yet sufficiently understood to allow for the rational design of siRNAs.
However, based on the research from various laboratories including our own, InvivoGen has been able to develop siRNA Wizard, an online tool accessible from our homepage, that will help you find the best siRNA sequences on your target gene.
The siRNA Wizard tool will also design the pair of oligonucleotides needed to generate shRNAs using InvivoGen's psiRNA plasmids. Below is the list of general rules, used by the siRNA Wizard, that have been revised to better suit the design of shRNAs.
The current guidelines recommend avoiding the first 50-100 nt located downstream of the Start codon and the 100 nt located upstream of the Stop codon, as well as 5' and 3'UTRs. These regions contain binding sequences for regulatory proteins that may affect the accessibility of the RNA target sequence to the RISC complex. However, we and others have successfully silenced the expression of several genes by targeting the 5' or 3'UTRs [1, 2, 3, 4]. Therefore, 5' and 3'UTRs should also be considered when selecting a region on your target gene.
The first nucleotide of the siRNA sequence can either be an A or a G. Although we recommend choosing an A (see Selection criteria for Standard search), a G can also be used since in several examples siRNAs starting with a G and expressed from the human H1 promoter have worked [4, 5].
The pyrimidines C and T should be avoided because expression of RNAs from RNA polymerase III promoters is only efficient when the first transcribed nucleotide is a purine. In cases where your siRNA sequence starts with a C or T, we recommend adding an A as the first nucleotide.
This addition will not affect the activity of your siRNA since it will generate a T at the end of the antisense siRNA strand that will be included in the termination signal maintaining its complementarity with the target sequence. This point is important since according to current knowledge recognition of the specific gene target is achieved by the antisense siRNA strand.
It is usually recommended to choose sequences with low GC content (between 30-55%). There are also many examples of active siRNAs with high GC content [6, 7, 8].
siRNA-mediated RNAi is based on using dsRNA < 30 nt to avoid nonspecific silencing. According to Hannon et al. siRNA of 25-29 nt are generally more effective than shorter ones. However, we and others found that hairpin siRNAs with duplex length of 19-21 nt are as effective as longer hairpin siRNAs [6, 9, 10].
Several teams including ours have tested a variety of sequences for the loop between the two complementary regions of a shRNA, ranging from 3 to 9 nt in length. Similar effectiveness have been obtained for loops of 5, 7 or 9 nt. We use a 7 nt loop sequence (TCAAGAG) for the psiRNA vectors.
Despite the fact that this set of rules is still not well defined, sequences generated by the siRNA Wizard will likely work better than randomly selected sequences.
However, because some candidate siRNAs are more active than others, it is recommended to vary the selection criteria and to compare a panel of three siRNAs to find the most efficient.
REFERENCES
1- Yokota T. et al., 2003. EMBO reports AOP.
2- Yu JY. et al., 2002. PNAS 99(9):6047-6052
3- Rubinson DA. et al., 2003. Nature Genetics 33:401-406
4- Mcmanus MT. et al., 2002. RNA 8:842-850
5- Tiscornia G. et al., 2003. PNAS 100(4):1844-1848
6- Kim MH. et al., 2002. BBRC 296:1372-1377
7- Hasuwa H. et al., 2002. FEBS Letters 532:227-230
8- Bertrand JR. et al., 2002. BBRC 296:1000-1004
9- Yu JY. et al., 2003. Molecular Therapy 7(2):228-236
10- Song E. et al., 2003. Nature Medicine 9(3): 347-351