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Performing target validation well

Performing target validation well

Summary

This blogpost describes issues encountered in target validation and how to safeguard against poor reproducibility in RNAi experiments.

The importance of target validation

More than half of all clinical trials fail from a lack of drug efficacy. One of the major reasons for this is inadequate target validation.

Target validation involves verifying whether a target (protein/nucleic acid) merits the development of a drug (small molecule/biologic) for therapeutic application.

Failing to adequately validate a target can burden a pharma with roughly 800 million to 1.4 billion in drug development costs. Impact is not only monetary as large site closures  often result as companies struggle to save costs and a reduced production effort deprives patients of new medicines.

Performing target validation well

Special attention should therefore be given to performing target validation techniques well.

target validation techniques
Overview of target validation techniques (Lindsay, Nat Review Drug Discovery, 2013)

Many of these techniques involve inhibiting target expression to establish its relevance in a cellular or animal disease model. This can be performed with chemical probes, RNA interference (RNAi), genetic knock-outs, and even targeted protein degradation.

The reproducibility of these techniques however has been an issue of concern for drug developers. Less than half of all findings from peer-reviewed scientific publications was reported to be successfully reproduced.

Dismal rates of reproducibility from several pharma-led cancer-focused studies ranged from 11% (Amgen) to 25% (Bayer). A review by William Kaelin Jr sums up the common pitfalls of preclinical cancer target validation. One of his key points:

Cellular phenotypes caused by a chemical or genetic perturbant should be considered to be off-target until proved otherwise, especially when the phenotypes were detected in a down assay and therefore could reflect a nonspecific loss of cellular fitness. It is only by performing rescue experiments that one can formally address whether the effects of a perturbant are on-target.

The comment highlights the issue of reagent non-specificity as a notable contribution towards poor reproducibility.

Certainly, for RNAi the wide-spread off-target effects of siRNAs has been observed in numerous publications. The mechanism being well-established to be based on microRNA-like seed-based recognition of non-target genes. The effect dominates over on-target effects in many large RNAi screens, illustrating the depth of the problem.

Reagent non-specificity is not restricted to RNAi. There have been multiple reports of non-specificity for gene editing technique, CRISPR, which can be read about in detail here, here and here. Recent publications continue to shed more light on its potential off-targets as we learn more about this relatively new technique.

Even chemical probes may have multiple targets. It is hence imperative that more than one target validation technique be used to avoid confirmation bias.

Target validation – a story from Pharma

Back in 2013, when siTOOLs was just starting out, a pharma approached us with a target validation problem.

They were obtaining different results with 3 different siRNAs in a cellular proliferation assay. Despite all 3 siRNAs potently downregulating the target gene, they produced different effects on cell viability.

Which siRNA tool to trust?

target validation siRNA vs siPOOL pharma story
Three different siRNAs against the same target were tested in a cell proliferation assay. Despite all 3 siRNAs showing potent target gene silencing, effect on cell proliferation differed greatly.

A whole-transcriptome expression analysis performed for the 3 siRNAs and a siPOOL designed against the same target revealed the reason for the large variability.

target validation expression analysis siRNA vs siPOOL pharma story
How many genes can you affect with an siRNA? Whole transcriptome analysis by microarray was performed and number and % of up and down-regulated genes are shown over total number of genes assayed (18567).

Despite all siRNA tools affecting the same target, the difference in extent of gene deregulation was astounding. With the greatest number of off-target effects, it was not surprising that siRNA 3 showed an impact on cell proliferation.

In contrast, siPOOLs had 5 to 25X less differentially expressed genes compared to the 3 commercial siRNAs against the same target. An expression analysis carried out for another gene target showed similar results i.e. siPOOLs having far less off-targets.

The target was dropped from development. A great example where failing early is a good thing, though it was not without costs from validating the multiple siRNAs.

The recommended target validation tool

Functioning like a pack of wolves, siPOOLs increase the chances of capturing large and difficult prey, while making full use of group diversity to compensate for individual weakness.

siPOOLs efficiently counter RNAi off-target effects by high complexity pooling of sequence-defined siRNAs. This enables individual siRNAs to be administered at much lower concentrations, below the threshold for stimulating significant off-target gene deregulation. Due to having multiple siRNAs against the same target gene, target gene knock-down is maintained and in fact becomes more efficient.

siRNA vs siPOOL rtqPCR knock-down efficiency
siPOOLs increase targeting efficiency, avoiding knock-down variability. Figure shows rtqPCR quantification of target RNA levels when two siPOOLs vs two siRNAs against 36 genes were tested.

We still recommend using multiple target validation techniques. As a first evaluation however, siPOOLs are quick, easy and most of all, reliable.

Rescue experiments can also be performed with siPOOL-resistant rescue constructs.

Find out more

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Low complexity pooling does not prevent siRNA off-targets

Low complexity pooling does not prevent siRNA off-targets

Summary: Low-complexity siRNA pooling (e.g. Dharmacon siGENOME SMARTpools) does not prevent siRNA off-targets.  It may in fact exacerbate off-target effects.  Only high-complexity pooling (siPOOLs) can reliably ensure on-target phenotypes.

Low-complexity pooling increases the number of siRNA off-targets

One of the claims often made in favour of low-complexity pooling (e.g Dharmacon siGENOME SMARTpools) is that this pooling reduces the number of seed-based off-target effects compared to single siRNAs.

If this were true, we would expect different low-complexity siRNA pools for the same gene to give similar phenotypes.  But this is not the case.

Published expression data shows that low-complexity pooling actually increases the number of off-targets.

Kittler et al. (2007) looked at the effect of combining differing number of siRNAs in low to medium complexity siRNA pools (siRNA pools sizes were: 1, 3, 5, 9, and 12).

Their work showed that the number of down-regulated genes (50% or greater silencing) actually increases when small numbers of siRNAs are combined.  Only when larger numbers of siRNAs are combined does the number of off-targets start to drop:

 

 

[The figure is based on data from GEO dataset GSE6807.  Down-regulated genes are those whose expression is reduced by 50% or more.  Note that the orange point is taken from our 2014 NAR paper, as we are not aware of other published expression datasets with this many pooled siRNAs.  A few caveats with combining these datasets are that they use different target genes, siRNA concentrations, and the data comes from a different expression platform.]

Low-complexity pooling: a bad solution for siRNA off-targets

Low-complexity pooling does not get rid of the main problem associated with single siRNAs: seed-based off-target effects.   Based the above analysis, it can make it even worse.  It also prevents use of the most effective computational measures against seed effects.

Redundant siRNA Activity (RSA) is a common on-target hit analysis method for single-siRNA screens.  It checks how over-represented the siRNAs for a gene are at the top of a ranked screening list.  If a gene has 2 or more siRNAs near the top of the list, it will score better than a gene that only has a single siRNA near the top of the list.  This is one way to reduce the influence of strong off-target siRNAs.

Correcting single siRNA values by seed medians has also been shown to be an effective way to increase the on-target signal in screens.  This correction is not effective for low-complexity pools, since each pool can contain 3-4 different seeds.

Off-target based hit detection algorithms (e.g. Haystack and GESS) are also only effective for single-siRNA screens.  The advantage of these algorithms is that it permits the detection of hit genes that were not screened with on-target siRNAs.  These algorithms are not effective for low-complexity pool screens.

Our recommendation: do not convert single siRNAs into low-complexity pools, rather use high-complexity siPOOLs to confirm hits

We do not recommend that screeners combine their single siRNA libraries into low-complexity pools (e.g. combining 3 Silencer Select siRNAs for the same target gene).  If possible, it is better to screen the siRNAs individually and then apply seed-based correction, RSA and seed-based hit-detection algorithms.

The time saved by only screening one well per target may prove illusory when the deconvolution experiments show that the individual siRNAs have divergent phenotypes.

It is probably better to deal with off-target effects up front (by screening single siRNAs) than to be surprised by them later in the screen (during pool deconvolution).

Reliable high-complexity siPOOLs, as independent on-target reagents, can then be used to confirm screening hits.

siTOOLs also now has RNAi screening libraries available.  Please contact us for more information.

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What is the probability of an siRNA off-target phenotype?

What is the probability of an siRNA off-target phenotype?

Summary:   Conventional siRNAs have a high probability of giving off-target phenotypes.  siRNA off-target effects can be reduced by using more specific reagents or narrowing the assay focus (to reduce the number of relevant genes).  Even when the assay is relatively focused, more specific reagents significantly increase the probability of observing on-target effects.

Probability of siRNA off-target phenotype depends on reagent specificity and assay biology

The probability of getting an off-target effect from an siRNA depends on several factors, the main ones being reagent specificity and assay biology.  If an siRNA down-regulates a large number of genes, or if an assay phenotype can be induced by a large number of genes, the probability of observing an off-target phenotype increases.

siRNAs can down-regulate many off-target genes

Garcia et al. (2011) compiled 164 different microarray experiments measuring gene expression following transfection with siRNAs.  The mean number of down-regulated genes in these experiments was 132 and the median was 68 (down-regulated genes were silenced by 50% or more).

As noted in earlier studies of gene expression following siRNA treatment (e.g. Jackson et al. 2003), few of the down-regulated genes are shared between siRNAs with the same target gene.  This suggests that the down-regulated genes are not the downstream result of target gene knockdown (i.e. they are mostly off-target).

High-complexity pooling of siRNAs (e.g. with siPOOLs) can reduce the number of down-regulated genes.

The following figure, based on data from Hannus et al. 2014, shows the difference between the gene expression changes caused by a single siRNA (left) and a high-complexity siRNA pool (siPOOL, right), which also includes that same single siRNA:

 

Estimating the probability of siRNA off-target phenotypes

Assuming different numbers of down-regulated genes (off-target) and different numbers of potent genes involved in assay pathways, we can try to estimate the probability of an siRNA giving an off-target effect.

The following plot shows the probability of getting an off-target effect when:

  • assuming RNAi reagents down-regulate varying numbers of off-target genes (5, 25, 50, 100)
    • down-regulated means that gene expression is reduced by 50% or more
    • in the Garcia paper dataset, the mean is 132 and median is 68
  • assuming different numbers of assay-potent genes
    • an assay-potent gene is one whose down-regulation by 50% or more is sufficient to produce a hit phenotype
    • for assays with more general phenotypes (e.g. cell count) we would expect more  assay-potent genes

 

We can see that even if there are only 20 assay-potent genes, there’s a nearly 10% chance of getting an off-target phenotype when siRNAs down-regulate 100 off-target genes (which is close to the average observed in the Garcia dataset).

In a genome-wide screen of 20,000 genes with 3 siRNAs per gene, we would thus expect 2,000 off-target siRNAs.

In contrast, a more specific reagent that only down-regulates 5 off-target genes only has a 0.5% change of producing an off-target phenotype.  For the above-mentioned genome-wide RNAi screen, we would expect only 100 off-target siRNAs (a 20-fold reduction).

The importance of RNAi reagent specificity

The above analysis demonstrates the importance of using specific siRNA reagents.

Changing an assay to make the phenotypic readout narrower (to reduce the number of genes capable of inducing a phenotype) is one way to reduce the risk of off-target phenotypes.  But this may be a lot of work and is not necessarily desirable or even possible.

A more ideal solution is the use of a specific RNAi reagent, like siPOOLs.

postscript

As the number of assay-potent genes increases, the probability of getting an off-target phenotype approaches one.

The following plot (same format as the one above) shows the distribution

 

The p-values were calculated using the hypergeometric distribution, assuming a population size of 20,000 (the approximate number of protein-coding genes in the human genome).

Note that one of the major simplifying assumptions of the above analysis is that all siRNAs have the same number of down-regulated off-target genes.

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5 factors to consider in multi-gene targeting RNAi screens

5 factors to consider in multi-gene targeting RNAi screens

Summary: Effective functional genomic screening depends on a variety of factors that need to be simultaneously addressed to obtain meaningful results. A recent Cell Reports paper demonstrates this by taking a holistic approach to siRNA screening with the use of multi-isoform/multi-gene targeting to address redundant paralogs and pathways in cancer cells.

The case for multi-gene targeting

Many RNAi screens use arrayed single gene knockdowns to find genes that play an important role in a biological process. The idea is that a single bullet is enough to take down its target leaving a gaping hole that one cannot fail to notice. In some cases, this is true, and is certainly relied upon by drug developers seeking to create specific mono-target drugs.  However, in complex diseases like cancer, cells have evolved fail-safe mechanisms to make them more resistant to external assaults. A single bullet is simply not enough.

Take for example oncogenic protein RAF or Rapidly Accelerated Fibrosarcoma, a tyrosine kinase effector that is a component of the MAPK signalling pathway (Ras-Raf-MEK-ERK). RAF has three isoforms – ARAF, BRAF and RAF1 (also called CRAF). Studies in mouse embryonic development show they all share some form of functional redundancy as knocking out two isoforms produces more severe effects than knocking out each isoform alone.

Screens that target single genes/isoforms therefore tends to bias results towards genes that have no paralogs or only have single isoforms. This was indeed the reason why classical Ras effectors were not identified in previous screens.

Factors to consider in a multi-gene targeting RNAi screen

Determining gene combinations that make sense

The authors of the study did a focussed siRNA screen on 41 RAS effector nodes represented by 84 genes. Out of the 41 nodes, 25 of them had 2-4 functional paralogs where combinatorial gene silencing was carried out with multiple siRNAs. 5 nodes knocked down multiple members of a protein complex. 5 nodes had siRNAs targeting multiple steps within a pathway. Only 6 nodes silenced single genes (highlighted).

Multi-gene targeting screen design

The only caveat with designing such a screen is the requirement for prior knowledge to perform meaningful gene silencing combinations. In this instance, many of the Ras effector pathways are characterized sufficiently to do this well however in other less studied fields, this could be a challenge. Useful tools that would help in designing gene knockdown combinations would include pathway or phenotype databases such as KEGG, REACTOME or Wikipathways. The Phenovault which siTOOLs Biotech is developing, is yet another potentially useful tool.. more details to come!

Number and types of phenotypes

The authors also highlight how a screen that reads only one phenotype might miss other important gene functions. Many RNAi screens sadly still stick to measuring cell proliferation as their only read-out which is greatly influenced by siRNA off-target effects. Here, 5 different phenotypes were measured (cell size, proliferation, apoptosis, reactive oxygen species [ROS], and viability). It was noted that silencing of Cdc42 had little effect on cell viability yet a prominent effect on ROS levels.

To take this up a notch, analysis was also performed at the single-cell level in cells expressing uniform levels of GFP and co-transfected with GFP siRNA. This allowed authors to correlate phenotypes with levels of gene knockdown, generating dose-response curves. How clever!

A lot more work, but adds to data robustness especially when using single siRNAs that are known to be rather variable.

Heterogeneity of cell lines

Many reports and our own observations attest to the heterogenous response of different cell lines to the same treatment. In cancer especially, the large heterogeneity necessitates the use of multiple cell lines. Not doing so would be failing to account for the large genetic diversity observed in the clinic. The authors screened 92 cell lines derived from lung, pancreas and colorectal tissue.

Despite seeing heterogenous responses to node knockdowns, phenotypic responses could be distinguished into  several groups based on effector engagement.  A major group dependended on RAF through direct binding with KRAS, a second major group worked via RSK p90 S6 kinases to drive RSK-mTOR signalling. And a third minor group was dependent on RalGDS. They went on to focus on the first two major groups, naming them KRAS-type and RSK-type respectively.

Reagents – choosing siRNAs and siRNA concentrations

The authors used previously characterized siRNAs to select for more potent siRNAs. This involved an RNAi sensor reporter-based assay that required the generation of 20,000 clones. The reporter was also shRNA-based. Due to heterogeneity in Dicer-mediated cleavage of shRNA, its uncertain if knockdown potency is accurately reflected when translated to siRNAs (read about the difference between shRNAs and siRNAs).

siRNA off-target effects are concentration-dependent

In any case, its a lot of work to characterize all siRNAs to be used in a screen. Furthermore, off-target effects are not addressed.

The authors stuck to a maximal concentration of 12 nM where 2 nM of siRNA was applied per gene. At 2 nM per siRNA, one still risks deregulating other genes. One of the first papers by Aimee Jackson et al., demonstrated an siRNA targeting MAPK14 deregulated many other genes even at concentrations of 1-4 nM.

An important consideration is to ensure total siRNA concentrations are kept constant. In which case, a negative control siRNA has to match or follow the maximal siRNA concentration used. Using different levels of siRNAs runs the risk of biasing off-target effects towards sequences present at higher concentrations.

To learn what the causes, extent and consequences of siRNA off-target effects are, read siTOOLs Technote 1)

Validating results

As with all scientific hypothesis, it helps to arrive at the same conclusion with different approaches.

The two different effector response subgroups identified also responded differently to small molecules. The KRAS-type lines being more sensitive to EGFR and ERK inhibition while the RSK-type lines more sensitive to inhibitors of PDK1, RSK, MTOR, S6K1 and DNA repair enzymes. This was attributed to the latter’s higher basal metabolic activity manifested in larger investments towards oxidative phosphorylation and mitochondrial ribosome maintenance.

By also projecting signatures obtained from cell lines into patient samples (in The Cancer Genome Atlas, TCGA), the subtypes were also effective at predicting differential sensitivity to multiple drug treatments. This highlights the importance in designing effective drug combinations in cancer.

Interestingly, the authors also performed CRISPR pooled screens in parallel. However, due to the restraints of being only able to knockout 1 gene at a time, smaller effects were seen due to gene redundancy. However, they did go on to use CRISPR as well to mutate key genes to affirm the pathway relationships established.

siPOOLs have been used successfully for multi-gene targeting for up to 4 genes, and potentially more. They also safely address off-target effects by high complexity pooling, enabling each siRNA to be applied at picomolar concentrations. For more articles on multi-gene targeting, read an older blogpost:

Understanding gene networks with combinatorial gene knockdown

 

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Is it important to avoid microRNA binding sites during siRNA design?

Is it important to avoid microRNA binding sites during siRNA design?

Summary: To address the question of whether one should avoid microRNA binding sites during siRNA design, we examined whether removing siRNAs that share seeds with native microRNAs would reduce the dominance of seed-based off-target effects in RNAi screening.

siRNA design and native microRNA target sites

Recently, we discussed a review of genomics screening strategies.  The authors state:

RNAi screens are powerful and readily implemented discovery tools but suffer from shortcomings arising from their high levels of false negatives and false positives (OTEs) as can be seen when comparing the low concordance among the candidate genes detected in different screens using the same species of virus, e.g., HIV-1, HRV, or IAV (Booker et al., 2011; Bushman et al., 2009; Hao et al., 2013; Perreira et al., 2015; Zhu et al., 2014).

To address these concerns, improvements in the design and synthesis of next-gen RNAi library reagents have been implemented including the elimination of siRNAs with seed sequences that are complementary to microRNA binding sites.

Given that off-target effects via microRNA-like binding are the main source of RNAi screening phenotypes, avoiding native microRNA sites during siRNA design seems like a reasonable strategy.  But does it make much difference in actual RNAi screens?

Hasson et al. 2013 performed a mitophagy screen using the Silencer Select siRNA library.  About 12% of the ~65,000 screened siRNAs have a 7-mer seed shared by a miRBase microRNA.

The screen’s main phenotypic readout, % Parkin translocation (PPT), is strongly affected by seed effects.   The intra-class correlation for siRNAs with the same seed is ~.51 (versus ~.06 for siRNAs with the same target gene).  There appears to be no difference between how siRNAs with or without microRNA seeds behave:

Is it important to avoid microRNA binding sites during siRNA design?

The same thing is found if we look at a less specific phenotype like cell count (which should be more broadly susceptible to off-target effects, as more genes should affect this phenotype):

Is it important to avoid microRNA binding sites during siRNA design?

And if we look at seeds that are enriched at the top of the screening list (sorted by descending PPT), we also don’t see much difference between siRNAs with or without native microRNA seeds.  (Note that the seed p-value is calculated in a similar way to RSA, based on how over-represented a seed is towards the top of a ranked list)

Is it important to avoid microRNA binding sites during siRNA design?

We also examined a general phenotypic readout (cell viability) in a dozen large-scale RNAi screens.

For some screens, we do see a slight shift in the values for siRNAs with or without native microRNA seeds.

For example, a genome-wide screen of Panda et al. 2017 (also using the Silencer Select library) shows a slight decrease in viability for siRNAs with native microRNA seeds:

Is it important to avoid microRNA binding sites during siRNA design?

Removing those siRNAs does not change the dominance of seed-base off-targets.

The intra-class correlation (ICC) for siRNAs with the same 7-mer seed is ~.53, with or without the inclusion of siRNAs with native microRNA seeds, while ICC for siRNAs with the same target gene is only  ~.06.

Coming back to the quote from the review article on genomic screening, next-gen RNAi library reagents that avoid native microRNA seeds are not expected to be much better than siRNAs that include them.

The most effective way to avoid seed-based off-target effects is to use high-complexity siRNA pools (siPOOLs). Learn more about siPOOLs

 

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Disrupting lncRNA function with siPOOLs (RNAi), antisense oligos and CRISPR

Disrupting lncRNA function with siPOOLs (RNAi), antisense oligos and CRISPR

Summary

This blogpost covers methods used in the disruption of lncRNA function. Specifically focusing on RNA interference (with siPOOLs), antisense oligos, and CRISPR approaches. Challenges faced with these approaches are addressed.

Long non-coding RNAs (lncRNAs) make up a major subgroup of RNAs and are defined as over 200 nucleotides long with limited protein-coding potential. There are three times as many genes producing lncRNAs as opposed to proteins. Numerous studies have described functional roles of lncRNAs in development and disease. This has stimulated major global interest and intense efforts to decode lncRNA function.

Disrupting lncRNA function

One way to find out what a lncRNA does is to decrease its expression, thereby disrupting its function. Current methods of downregulating lncRNA expression include knockdown approaches with siRNA and antisense oligos (ASOs), or knockout approaches with CRISPR, TALENs and other techniques involving DNA nucleases.

As we have mentioned before, knockdown and knockout approaches employ different mechanisms and as a result sometimes yield different results. Hence it is highly recommended to employ both techniques when possible to thoroughly validate lncRNA function.

LncRNA functional knockdown – RNAi and antisense approaches

LncRNA knockdown involves the transient downregulation of lncRNAs at the RNA level. This typically involves RNA degradation mediated by the RNA interference (RNAi) machinery for siRNAs, or with RNase H for ASOs.

Disrupting lncRNA function - How ASOs and siRNAs downregulate RNA
How ASOs and siRNAs downregulate RNA

Figure from Watts, J. K. & Corey, D. R. Silencing disease genes in the laboratory and the clinic. J. Pathol. 226, 365–79 (2012).

Some challenges that both technologies face when targeting lncRNAs:

  • low endogenous expression of lncRNA may limit efficiency of knockdown
  • accessibility of siRNA/ASO to lncRNA may be limited by secondary structure (created by folding of the lncRNA and self-base pairing)
  • accessibility to siRNA/ASO to lncRNA may be limited by bound proteins
  • off-target effects

Does cellular localization matter when disrupting lncRNA function?

Cellular localization of lncRNAs was reported to account for differences in knockdown efficiency by ASOs compared with siRNAs. Although there have been observations that RNAi factors are present in the nuclei, siRNAs were reportedley less efficient than ASOs for modulating nuclear-localized lncRNAs (Lennox and Behlke, Nucleic Acids Res, 2016).

This does not appear to apply to all cases as using siPOOLs (high complexity pooled siRNA) or ASOs led to similar downregulation of NEAT1, a nucleus-localized lncRNA:

Disrupting lncRNA function - Downregulation of lncRNA NEAT1 with siPOOLs and ASOs
Downregulation of lncRNA NEAT1 with siPOOLs and ASOs

NEAT1 lncRNA has two isoforms, 3.7kb NEAT1_1 and longer 21.7kb NEAT1_2. MCF7 cells were transfected with either LNA GapmeRs (ASOs) or siPOOLs that target both isoforms (N1) or the long form only (N1_2). RNA levels of both isoforms (NEAT1) or only the long isoform (NEAT1_2) were quantified after 24h. (Adriaens et al., Nat Med, 2016) 

siPOOLs also worked well for XIST and MALAT1 (~80% KD at 1 nM), both nuclear-localized lncRNA. Notably however, cytosolic-localized lncRNAs such as H19 were much more efficiently targeted with the high complexity siRNA pools (> 95% KD at 1 nM).

Disrupting lncRNA function - siTOOLs data lncRNA gene knockdown with siPOOLs, 1-3 nM
siPOOL knockdown efficiency of lncRNAs

siTOOLs Biotech in-house data showing knockdown efficiencies of siPOOLs against 16 lncRNAs tested at 1 or 3 nM in standard cell lines (MCF7, A549, Huh7). Assayed by real-time quantitative PCR after 24h.

Compared to coding genes, the above-mentioned factors do limit efficiencies of knockdown approaches. But with siPOOLs, the greater diversity of siRNA sequences is expected to increase chances of association with the target RNA. In-house data shows 12 of 16 tested lncRNAs showed good knockdown efficiencies of > 70% with siPOOLs.

Importantly, siPOOLs efficiently counter off-target effects commonly associated with siRNA. Off-target effects have also been reported to occur with ASOs, especially since they are also exposed to intronic regions. Hepatotoxicity related to certain sequence motifs on LNA-modified ASOs have also been reported (Burdick et al., 2014)

lncRNA functional knockout with CRISPR

The genomic distribution of lncRNA loci is rather complex. They are typically categorized in relation to their proximity with protein coding genes.

Types of lncrna
Types of lncrna

Figure showing lncRNA loci in green and protein-coding loci in purple. Arrows indicate direction of transcription. Figure and description below from McManus lncRNA presentation: http://mcmanuslab.ucsf.edu/node/251

  • Sense – The lncRNA sequence overlaps with the sense strand of a protein coding gene.
  • Antisense – The lncRNA sequence overlaps with the antisense strand of a protein coding gene.
  • Bidirectional – The lncRNA sequence is located on the opposite strand from a protein coding gene whose transcription is initiated less than 1000 base pairs away.
  • Intronic – The lncRNA sequence is derived entirely from within an intron of another transcript. This may be either a true independent transcript or a product of pre-mRNA processing
  • Intergenic – The lncRNA sequence is not located near any other protein coding loci.

Hence disrupting lncRNAs with DNA nucleases can be a challenging affair that runs the risk of affecting neighbouring genes.

How many lncRNAs can be CRISPRed?

Goyal et al. 2017 performed a genome-wide “CRISPRability” analysis to evaluate the risks and utility of CRISPR for disrupting lncRNA function.

Introducing mutations with CRISPR is generally not applicable for lncRNAs. Mainly due to difficulty predicting active functional domains and the fact that some lncRNAs exert phenotypes through the act of transcription per se.

Deleting the entire lncRNA is an option but not when it overlaps with other genes. Hence, the major approach is to target lncRNA promoters. But then we run into the problem of affecting neighbouring genes that share promoters.

So they came up with three “CRISPRability” rules to avoid potential effects on neighbouring genes:

Rule 1: Sense, antisense and intergenic lncRNAs are considered “non-CRISPRable” when transcribed from bidirectional promoters, defined by presence of another promoter present 2000bp upstream/downstream of lncRNA start.

LncRNA with bidirectional promoter
LncRNA with bidirectional promoter

Rule 2: Sense, antisense and intergenic lncRNAs are considered “non-CRISPRable” when the start of the lncRNA is located closer than 2000p to the start of the neighbouring gene, excluding lncRNAs transcribed from bidirectional promoters – termed “proximal promoters“.

LncRNA with proximal promoter
LncRNA with proximal promoter

Rule 3: Sense and antisense lncRNAs are considered “non-CRISPRable” when transcribed from internal promoters, where the start of the lncRNA falls within the gene body of another coding/non-coding transcript. This would include intronic lncRNAs.

LncRNA with internal promoter
LncRNA with internal promoter

After applying “CRISPRability” rules, only 38% of all lncRNAs were suitable for CRISPR-based functional disruption

CRISPRability of lncRNAs
CRISPRability of lncRNAs

 

Figure from Goyal et al., 2017 showing proportion of lncRNAs that fall within the 3 rules of “CRISPRability”

The study went on to corroborate the relevance of the classification by testing effects of CRISPR/Cas9 compared to ASOs/siRNA on their targets and neighbouring genes.

 

HOTAIR downregulation by CRISPR and siPOOL
HOTAIR downregulation by CRISPR and siPOOL

An example involved lncRNA HOTAIR that arises from the HOXC locus which regulates expression of several genes including HOXC11. They found that dCas9-KRAB , which produces CRISPR-based transient inhibition (CRISPRi) by blocking transcription, caused knockdown of HOXC11 when designed to target HOTAIR. This occurred for all 3 independent sgRNAs. siPOOL-mediated knockdown of HOTAIR, in contrast, did not affect HOXC11.

Similar scenarios were seen with coding genes, in particular for well-known tumour suppressor TP53, where neighbouring gene WRAP53alpha tended to be downregulated by dCas9-KRAB. This effect was absent with siPOOLs targeting TP53.

It therefore pays to carefully note the genomic neighbourhood of lncRNAs when using CRISPR for disruption. A careful scientist would also monitor the expression of neighbouring/overlapping genes in parallel to the target gene. Orthogonal methods such as RNAi (with siPOOLs), or rescue experiments that restore expression of the lncRNA, is recommended to fully evaluate lncRNA function.

Learn more about siPOOLs!

Featured blog image from lncRNA blog, photo credit autism.am

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Correcting seed-based off-target effects in RNAi screens

Correcting seed-based off-target effects in RNAi screens

Summary: Correcting for seed-based off-targets can improve the results from RNAi screening.  However, the correlation between siRNAs for the same gene is still poor and the strongest screening hits remain difficult to interpret.

Seed-based off-target correction has little effect on reagent reproducibility

Given that seed-based off-targets are the main cause of phenotypes in RNAi screening, trying to correct for those effects makes good sense.

The dominance of seed-based off-targets means that independent siRNAs for the same gene usually show poor correlation.

If one could correct for the seed effect, the correlation between siRNAs targeting the same gene may improve.

One straightforward way to do seed correction is to subtract the ‘seed median’ from each siRNA.  (The seed median is the median for all siRNAs having the given seed.)

This was the approach used by Grohar et al. in a recent genome-wide survey of EWS-FLI1 splicing (involved in Ewing sarcoma).  They used the Silencer Select library, which has 3 siRNAs per target gene.

After seed correction, there is only minor improvement in the correlation between siRNAs targeting the same gene.  The intra-class correlation (ICC) improves from 0.031 to 0.037.  The ICC for siRNAs with the same 7-mer seed decreases from 0.576 to 0.261.

Although we have reduced the seed-based signal, it has not resulted in a correspondingly large improvement in the gene-based signal.

More sophisticated seed correction can improve reagent correlation

Grohar et al. used a simple seed-median subtraction method to correct their screening results.

A more sophisticated method (scsR) was developed by Franceshini et al. for seed-based correction of screening data.  It corrects using the mean value for siRNAs with the same seed, and weighs the correction using the standard deviation the values.  This allows seeds with a more consistent effect to contribute more to the data normalisation.

Applying the scsR method to the Grohar data, ICC for siRNAs targeting the same gene increases from 0.031 to 0.041.  It is better than the increase with seed-median subtraction (0.037), but is still only a fairly minor improvement (plot created using random selection of 10,000 pairs of siRNAs that target the same gene):

 

Off-target correction increases double-hit rate in top siRNAs of RNAi screen

The following plot shows the count for single-hit and double-hit genes as we go through the top 1000 siRNAs (of ~60K screened in total).  Double-hit means that the gene is covered by 2 (or more) hit siRNAs.

Despite the small improvement in reagent correlation, the double-hit rate is essentially the same using simple seed-median subtraction or the more advanced scsR method.

Furthermore, the number of double-hits is higher than what we’d expect by chance.

This shows that, despite the noise from off-target effects, there is some on-target signal that can be detected.

siRNAs with the strongest phenotypes remain difficult to interpret

Despite the fact that the double-hit count is higher than expected by change, most of the genes targeted by the strongest siRNAs are single-hits.  siRNAs with the strongest phenotypes remain difficult to interpret.

Seed correction is best suited for single-siRNA libraries.  Low-complexity pools, like siGENOME or ON-TARGETplus, are less amenable to effective seed correction since there are (usually) 4 different seeds per pool.  This reduces the effectiveness of seed-based correction, even though seed-based off-target effects remain the primary determinant of observed phenotypes (as discussed here, here , and here).

The best way to correct for seed-based off-targets is to avoid them in the first place.  Using more specific reagents, like high-complexity siPOOLs, is the key to generating interpretable RNAi screening results.

For help with seed correction or other RNAi screening data analysis with the Phenovault, contact us at info@sitools.de

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“Phenoville” – RNAi & CRISPR Screening Strategies

“Phenoville” – RNAi & CRISPR Screening Strategies

Pleasantville is a movie based on an interesting idea: two teenagers are magically transported through their TV to a town called Pleasantville set in the 1950s where everything is perfect (and also black-and-white).  As they discover the complex, imperfect emotions hidden below the idyllic surface, the black-and-white characters and objects start to gain colour.

In loss-of-function genetic screening, some reagents and screening formats may also give rise to a narrow, black-and-white view of a biological process.  A sort of “Phenoville”.  This was illustrated nicely in a recent review of screening strategies for human-virus interactions by Perreira et al. (2016).

The authors performed screens for human rhinovirus (HRV) infection using arrayed RNAi reagents (siRNAs) and pooled CRISPR reagents (sgRNAs), and then compared the resulting hit lists.

The arrayed RNAi screen produced over 160 high-confidence candidate genes, whereas the CRISPR screen only found 2.  The authors comment:

“The comparison of these two screening approaches side-by-side, using the same cells and virus, raises an interesting point. The number of host factors found for HRV14 was far greater using the MORR/RIGER approach [i.e. RNAi performed with multiple orthologous RNAi reagents and analysed by RNAi gene enrichment ranking method] and is approaching a systems level understanding based on bioinformatic analyses and the near saturation of, or enrichment for, multiple complexes and pathways (Fig. 4) (Perreira et al., 2015). By comparison our matched pooled CRISPR/Cas9 screen for HRV-HFs yielded two high-confidence candidates based on reagent redundancy, ICAM1, the known receptor for HRV14, and EXOC4, a gene involved in exocyst targeting and vesicular transport (He & Guo, 2009). Given the known role of ICAM1 as the host receptor for most HRVs, these results point to entry as the major viral lifecycle stage interrogated by a pooled functional genomic screening approach using a population of randomly biallelic null cells infected by a cytopathic virus.”

In simple terms, RNAi screening produced a richer data set that revealed system level interactions whereas CRISPR screening yielded a small number of specific hits that only affected an early-stage pathway. The ‘systems level understanding’ is nicely shown in the following diagram of the RNAi hits.  The red box at the top left is the only gene (ICAM1) that was common to the RNAi and CRISPR screens.

Perreira et al. conclude that arrayed siRNA screens permit the detection of a larger number of viral dependency factors, albeit with a significant tradeoff in a greater number of false positive hits (mainly due to off-target effects).  In contrast, pooled screens with CRISPR sgRNAs using cell survival as a readout, as also seen with most haploid cell screens, display limited sensitivity but excellent specificity in finding host genes that act early on in viral replication (e.g. ICAM1).

In Perreira et al.‘s words:

“… given the currently available functional genomic strategies if the goal is to find viral entry factors (e.g., host receptors) with high specificity its best to use a pooled survival screen, but alternatively if the aim is to obtain with relative ease a more comprehensive set of host factors, albeit with more prevalent false positives, than an arrayed siRNA screen would be the preferred method.”

Summarizing two options for genetic screeners:

  1. Arrayed RNAi screens
    • provide a richer view of the underlying biology
    • produce more false positives from OTEs
    • produce false negatives from OTEs
  2. Pooled CRISPR screens
    • provide a narrower view of the underlying biology
    • produce fewer false positives
    • produce false negatives because of genetic compensation

Off-target effects (OTEs) are the primary cause of false positives, and the resultant higher assay noise also increases the number of false negatives in arrayed RNAi screens. Reagents like siPOOLs minimize the risk of off-target effects and reduce assay noise.

One key factor not mentioned by Perreira et al. is the presence of genetic compensation in gene knockout approaches.

Putting genetic compensation in terms of human actors, imagine that you are investigating the function of bus drivers in Pleasantville.  To induce loss-of-function, assume that aliens will be abducting the bus drivers.  If the bus drivers are abducted in their sleep (equivalent to a CRISPR knock-out), you may not get a good idea of their function when you film the next day.  People may be compensating by driving, biking or staying home.  Alternatively, the bus company may have found emergency replacement drivers.

Now suppose the bus drivers are abducted in the middle of the day while driving their routes (equivalent to an RNAi knock-down).  The film will show buses crashing (hopefully without any serious injuries, since this is just a TV show!) and the public transportation system will suddenly come to a halt.

RNAi gene knockdown screens with siPOOLs  can provide a significant advantage over CRISPR gene knockout screens in obtaining a system level understanding in biological models.

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CRISPR/Cas9 Screening – The “Copy-Number Effect”

CRISPR/Cas9 Screening – The “Copy-Number Effect”

Several CRISPR/Cas9 screens identifying essential genes in cancer cell lines have been performed to date (Shalem et al., 2014, Hart et al., 2015, Kiessling et al., 2016). These typically take the form of pooled screens where sgRNA libraries targeting all genes or subsets of genes are introduced in parallel into Cas9-expressing cells, at a single sgRNA per cell. The sgRNAs exert a negative or positive selection pressure on cells based on their impact on cell viability and proliferation. The most depleted or enriched sgRNA sequences are determined by next-generation sequencing, revealing relevant gene ‘hits’. Very similar to how pooled shRNA screens are performed.

From these screens, several groups have observed a worrying phenomenon: CRISPR gRNAs targeting genomic regions of high copy number amplification showed a striking reduction in cell proliferation/survival. Dr William Hahn’s group at the Dana Farber Institute was one of the first to characterize this in a publication last year involving a CRISPR/Cas9 screen on 33 cancer cell lines looking for essential genes. In total, 123411 unique sgRNAs were used targeting 19050 genes (6 sgRNAs/gene), 1864 miRNAs and 1000 non-targeting negative control sgRNAs.

What they discovered is a little worrying to say the least.

The figure shows two genomic regions in two different cell lines (SU86.86 and HT29). At genomic coordinates highlighted by the red box, 3 tracks are shown. Top, copy number from the Cancer Cell Line Encyclopaedia (CCLE) SNP arrays, red indicating above average ploidy and blue showing below; middle, CRISPR/Cas9 guide scores with purple trend line indicating the mean CRISPR guide score for each CN segment defined from the above track; bottom, RNAi gene-dependency scores. AKT2 and MYC, known driver oncogenes at these loci, respectively, are highlighted in orange. For RNAi data, shRNAs targeting AKT2 used in Project Achilles were not effective in suppressing AKT2 (hence the negative result).

 

Key findings:

  • A striking enrichment of negative CRISPR guide scores (i.e. sgRNAs that reduced cell proliferation/survival) for genes that reside in genomic regions of high copy-number amplification.

 

  • Genes identified in CRISPR that reduced survival, did not have the same effect when disrupted by RNAi in the same cell lines (this RNAi screen was done by the same group but published 2 years before).

 

  • This enrichment was seen also for unexpressed genes, i.e. genes not transcribed. Meaning the reduced survival was not due to loss-of-function of the targeted gene.

 

  • Even for regions with low absolute copy numbers, a significant reduction in survival was observed compared to non-targeting control sgRNAs. Furthermore, the effect was dose-dependent with greater copy number amplifications producing larger negative CRISPR guide scores.

Notably, the correlation between copy number and genes that were scored high on essentiality was also observed when looking at data from other studies (Hart et al., 2015). The “copy number effect” would therefore produce a high number of false positives in CRISPR screens for essential genes in cancer cell lines. The graph above shows just how big an effect this is. Comparing genes identified as essential in a CRISPR screen vs RNAi screen, increasingly essential CRISPR-identified genes were more likely to reside on copy number amplifications (defined as having average sample ploidy > 2). This effect was notably absent for RNAi-derived essential genes.

Aside from false positives, the increased noise due to “copy number effects” also increases false negatives. MET, a gene identified by shRNA screens, for example, failed to be picked out by CRISPR screens as it is located on a chromosome 7 amplicon (7q31) in MKN45 cells (gastric cancer cell line) where all other gRNAs within that amplicon also scored as essential.

The authors go on to explore mechanisms behind the “copy number effect”. They found it was attributed to a DNA damage response stimulated by excessive cutting by Cas9. This response appeared p53-dependent and induced cell cycle arrest at the G2 phase, explaining the anti-proliferative effect. A similar response was seen for promiscuous sgRNAs that cut at multiple sites, with effects being more pronounced when cuts were spread over several chromosomes as opposed to a single chromosome.

How to manage this?

So far, most simply avoid analysing hits where sgRNAs lie at amplified regions or target multiple sites (Wang et al., 2017). However, these regions of copy number amplifications have been implicated in cancer and may contain relevant hits. Several computational methods have therefore recently been developed to correct for “the copy number effect”. Hahn’s group developed a computational algorithm called CERES based on data obtained from CRISPR sgRNA screens in 342 cancer cell lines representing 27 cell lineages.

Novartis also developed a Local Drop Out (LDO) algorithm that corrects obtained data based on examining gRNAs scores at direct genomic neighbours. When multiple neighbouring genes show similar drop out scores, effects are assumed to be due to “copy number effects”. This method has the advantage of not requiring prior knowledge of copy number, however it does require a sufficient density of gRNAs to accurately capture “copy number effects”.  They also had an alternative method, Generalized Additive Model (GAM) where copy number was taken into account.

 

How the CERES Model Works

The Results – copy number dependency is reduced while preserving essentiality of cancer-specific genes such as KRAS

 

A step towards the right direction but the penetrance of this effect still raises some concerns:

  • Although false positives are reduced with these computational methods, it is difficult to recapture false negatives. This is dependent on the gRNA having a stronger phenotype compared to neighbouring gRNAs on the amplicon which is not always the case. The LDO method for example still failed to recapture MET.

 

  • Guide scores can vary with cell line, sgRNA and experimental conditions, making it difficult to apply the same counter-measures to every experiment.

 

  • Given multiple cut sites trigger the same effect, how do we ensure multiple sgRNAs when introduced into a cell are not inducing a similar response? This is difficult to control in pooled screens, and poses a limitation in multiplex screens. Synthetic lethality screens for example with sgRNAs targeting multiple genes, might be subject to a higher false positive rate.

 

  • With even diploid genes (copy number = 2) having statistically significant growth reduction compared to haploid gene loci, the challenge still remains to delineate a true loss-of-function over a non-specific cellular response.

 

  • Negative sgRNA controls have to be carefully selected. From the study, non-targeting controls had little impact on viability compared to most other sgRNAs. Controls targeting non-expressed genes or non-essential loci have been recommended as better controls.

 

  • Lastly, although this effect seems to apply mostly to cancer cell lines that undergo a high rate of gene amplifications, similar effects may extend to polyploid tissues such as the liver.

Hence as always gene function should be determined by a variety of methods. Using RNAi for example to affirm a CRISPR-knockout phenotype would add greater confidence to a hit. To avoid those RNAi-related false positives however, its probably best to use siPOOLs.

 

Source of figures:

Aguirre, A. J., Meyers, R. M., Weir, B. A., Vazquez, F., Zhang, C.-Z., Ben-David, U., … Hahn, W. C. (2016). Genomic Copy Number Dictates a Gene-Independent Cell Response to CRISPR/Cas9 Targeting. Cancer Discovery, 6(8), 914 LP-929.

Meyers, R. M., Bryan, J. G., McFarland, J. M., Weir, B. A., Sizemore, A. E., Xu, H., … Tsherniak, A. (2017). Computational correction of copy-number effect improves specificity of CRISPR-Cas9 essentiality screens in cancer cells. bioRxiv. Retrieved from http://biorxiv.org/content/early/2017/07/10/160861.abstract

Other relevant sources:

Munoz, D. M., Cassiani, P. J., Li, L., Billy, E., Korn, J. M., Jones, M. D., … Schlabach, M. R. (2016). CRISPR Screens Provide a Comprehensive Assessment of Cancer Vulnerabilities but Generate False-Positive Hits for Highly Amplified Genomic Regions. Cancer Discovery, 6(8), 900 LP-913. Retrieved from http://cancerdiscovery.aacrjournals.org/content/6/8/900.abstract

de Weck, A., Golji, J., Jones, M. D., Korn, J. M., Billy, E., McDonald, E. R., … Kauffmann, A. (2017). Correction of copy number induced false positives in CRISPR screens. bioRxiv. Retrieved from http://biorxiv.org/content/early/2017/06/23/151985.abstract

 

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siRNA vs shRNA – applications and off-targeting

siRNA vs shRNA – applications and off-targeting

Short interfering RNA (siRNA) and short hairpin RNA (shRNA) are both used in RNAi-mediated gene silencing. In this blogpost, we explore the differences in applications of siRNA and shRNA and compare their capacity for off-targeting.

For a summary of their properties, please refer to Table 1 at the end of  the post.

In what situations should we use siRNA or shRNA?

In terms of application, siRNAs are commonly applied for rapid and transient knockdown of gene expression.

It is performed in cell lines amenable to transfection by liposomes/electroporation and effects typically last from 3-7 days though retransfection can be performed to extend the effect.

The amount of siRNA introduced can be highly controlled and efficiency of gene knockdown is dependent on the levels of siRNA in the cell which is influenced by transfection efficiency and siRNA stability. Knockdown is also influenced by characteristics of the gene. A gene that is highly transcribed for example, may experience less siRNA-mediated downregulation compared to a gene where lesser copies of RNA are produced over time. In addition, a gene which expresses a protein with a very long half-life, may require extended periods of siRNA application to see a knockdown effect.

Due to the transient effect of siRNAs, shRNAs were developed to be used for prolonged knockdown of genes.

As they are introduced by viral vectors, cells that are more difficult to transfect are better targeted with shRNA. Furthermore, promoter-driven expression allows for inducible expression of the shRNA. Depending on the viral vector used – refer to Labome’s post that covers siRNA/shRNA delivery in greater detail – the shRNA may be integrated into the host genome, allowing it to be propagated into daughter cells. This maintains a consistent gene knockdown over several generations. However, knockdown efficiency can decline over time. This is mainly due to varying levels of uptake of the shRNA among cells, with a cell population having lower shRNA expression being over-represented with time.

 

What about RNAi screening?

siRNAs and shRNAs are both used in RNAi screening to identify genes of interest in a studied phenotype. These are performed with siRNA/shRNA libraries that target a large variety of genes. There are two RNAi screening formats commonly used – arrayed and pooled.

siRNAs and shRNAs can both be used in an arrayed screening format. This means that the siRNA(s)/shRNA(s) against each gene is tested in distinct cell populations. Arrayed screens have the advantage of being compatible with various phenotypic readouts and do not suffer from possible reagent cross-talk or challenges associated with deconvoluting data. However, they are more energy and resource-intensive to perform. (See Fig. 2)

The pooled screening format in contrast, applies only with shRNAs. Here, all shRNAs (e.g. a whole-genome shRNA library) are introduced to a single cell population. As low titers of viral vectors are used, each cell in the population is expected to take up one shRNA vector.

With pooled screening, only readouts linked to cell number can be assessed. These include measurements for cell viability or altered expression of a cell surface marker assessed by fluorescence activated-cell sorting. shRNAs targeting genes which impact these readouts are expected to skew the cell population, such that only cells affected by the relevant shRNAs can be identified. This is either through negative selection, where lost cell populations are noted, or positive selection, where cells with certain shRNAs become over-represented.

The resulting cell population is then assessed by PCR, microarray hybridization or next generation sequencing to measure which shRNAs are highly or lowly-represented. The shRNAs are identified usually by means of a DNA barcode present in the vector sequence. Of note, pooled screens take up less resources to perform but require longer assay times to allow for significant changes in the overall cell population to occur.

Fig. 2 Simplified workflow for arrayed and pooled RNAi screening formats

 

Off-target effects with shRNAs?

The use of siRNAs are known to produce several off-target effects but what about shRNAs? Given they are processed the same way as siRNAs, shRNAs are also subject to microRNA-like off-target effects. In addition, because they are expressed from DNA and rely on endogenous machinery to be processed into siRNA, several variations may be introduced not found with introducing siRNA directly. Some potential sources of off-target effects for shRNAs include:

1. Promoter-driven expression. shRNAs are typically controlled with a U6 promoter which drives high levels of transcription via RNA polymerase III. The high shRNA expression levels may saturate endogenous RNAi machinery, contributing to off-target effects. To counter this, shRNAs can be expressed in a context mimicking miRNAs, utilizing RNA polymerase II for transcription instead. This has been found by several groups to reduce the incidence of off-target effects (Grimm et al., 2006, Kampman et al., 2015)

2. Dicer-mediated hairpin processing. shRNAs undergo Dicer-mediated cleavage in the cytosol to remove its hairpin loop. Gu et al., 2012 reported that Dicer cleaves with sufficient heterogeneity to generate multiple sequences. This factor was reported to generate the higher noise levels unique to shRNA screens (Bhinder and Djaballah, 2013). As specificity of Dicer cleavage is influenced by neighbouring loop and bulge structures, care should be taken in shRNA design.

3. Multiple shRNA uptake. During viral transduction, the viral titer is minimized to increase the probability that cells take up a single shRNA vector. However, this does not guarantee that multiple shRNA uptake will not occur. In this event, a combinatorial gene knockdown ensues resulting in a mixed phenotype that may generate false hits.

4. Differences in genomic integration between shRNAs. Varying efficiencies in transfection and genome integration between shRNAs may skew results to over-represent certain shRNAs over others, especially in pooled screens. Furthermore, integration into the host genome may disrupt the function of certain genes, producing more off-targets.

Studies comparing results from siRNA and shRNA screens have found extremely poor overlap, both between and within the reagent-specific screens. Bhinder and Djaballah’s (2013) analysis of results from 30 published RNAi screens (16 siRNA, 14 shRNA) searching for genes that impact cell viability saw no common genes identified across the board. Furthermore, different genes were identified depending on whether the screen used siRNA or shRNA. PLK1 for example, was a prominent hit for siRNA screens but was only marginally represented in shRNA screens. In contrast, KRAS was a top hit among shRNA screens.

Fig. 3 Reagent format of RNAi screens analysed in Bhinder and Djaballah, 2013 Screens were performed either with genome-wide (GW) or focused (FD) siRNA/shRNA libraries. For siRNA screens, Pooled refers to pools of 3 siRNAs applied together compared to Singles where a single siRNA duplex was applied. For shRNA screens, Pooled refers to a pooled format screen (Fig. 2) where ~50, 000 shRNAs were applied to a single cell population. Arrayed refers to arrayed format screen where shRNAs were applied individually (Fig. 2).

Fig. 4 Overlap of hits among genome-wide (left) and focused (right) siRNA screens (Bhinder and Djaballah, 2013) Only 4 common hits detected across the 2 lethal gene lists from genome-wide siRNA screens. In focused siRNA screens, a greater overlap was detected but still limited across the 22 lethal gene lists. PLK1 detected in 9 out of 22 gene lists.

Fig. 5 Overlap of hits among genome-wide (left) and focused (right) shRNA screens (Bhinder and Djaballah, 2013) KRAS was a top hit in shRNA GW screens, appearing in 5 out of 9 lists. In focused shRNA screens, KRAS was present in 15 out of 31 lists. 

Worryingly, an enrichment of gene candidates exclusive to pooled shRNA screens was observed as opposed to arrayed shRNA or siRNA screens. Most of the overlap seen in gene lists (80% global overlaps, 60% after stringent filtering) were specific to pooled shRNA screens. Exclusion of data from pooled shRNA screens would have reduced overlap to a mere 27%. This indicates gene targets obtained from shRNA pooled screens is specific to the technique as opposed to specific gene downregulation.

Furthermore, a greater number of hits were obtained from shRNA screens – 6664 candidates from 40 shRNA gene lists – as opposed to 1525 candidates from 24 siRNA gene lists. This indicates a generally noisier dataset associated with shRNA screens.

Bhinder and Djaballah later performed a head-to-head comparison of an arrayed siRNA and shRNA screen and reported similarly dismal results. Despite using a gain-of-function assay, which tends to yield clearer results, only a 29 hit overlap was seen between siRNA and shRNA libraries which shared 15,068 common genes. Based on a known set of positive controls, siRNAs identified 8 known regulators as opposed to shRNA which only identified 3. Furthermore, predicted siRNA sequences obtained after Dicer-processing of shRNA which corresponded to exactly the same siRNA sequence from the siRNA library yielded different phenotypes. The authors highlight that differential intracellular processing of the shRNA contributes significantly to the discrepancies observed.

It is evident that shRNAs are at risk to greater number of off-target effects than siRNAs. Much care should be taken towards the interpretation of pooled shRNA screens in particular. Secondary validation of gene hits plays an increasingly important role. It is recommended to validate gene hits with siPOOLs (high-complexity, defined siRNA pools) which have a lower off-target profile than single siRNAs or low complexity siRNA pools of 3-4. siPOOL-resistant rescue constructs enable further affirmation that the loss-of-function phenotype is attributed to the target gene. Alternative tools such as compounds, antibodies or gene knockout technologies are also highly recommended.

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Table. 1 Comparison of properties between siRNA and shRNA

siRNA shRNA
Structure 20-25 nucleotide long double-stranded RNA (dsRNA) with 2 nucleotide overhangs at the 3’ end

~57-58 nucleotide long RNA sequence with a dsRNA region linked by non-pairing nucleotides to form a stem-loop structure

Delivery RNA itself with liposome/electroporation-mediated delivery into cells Usually delivered to cells via viral vectors. DNA may be incorporated into host genome depending on viral vector used.
Processing In the cytosol, guide or antisense strand* (shown in blue in Fig. 1) is incorporated into RNA induced silencing complex (RISC). RISC is guided towards RNA transcripts with the complementary sequence to mediate cleavage and subsequent degradation of the transcript.

 

*Note that the sense strand may also load into RISC and mediate off-targeting but incidence of this is reduced by designing siRNA with  appropriate thermodynamic properties (refer to previous blogpost on siRNA design)

In the nucleus, shRNA is transcribed from DNA by either RNA polymerase I or III, depending on the promoter.

Drosha, a member of the ribonuclease III family, processes the RNA transcript of its long flanking single-stranded RNA sequences and the resultant shRNA is exported out of the nucleus by Exportin-5.

 

In the cytosol, the enzyme Dicer cuts off the hairpin loop of the shRNA and releases the functional active siRNA which follows the same downstream processing as siRNAs.

 

Length of expression Varies from 3-7 days. Affected by degradation of siRNA within cell and dilution of effect upon cell division. Expression can be reinstated by re-transfecting the siRNA. If the DNA is stably integrated in the host genome, knock-down is theoretically permanent.
Control of knockdown Easily controlled by varying amount of siRNA introduced. Magnitude of knockdown harder to control as determined by promoter-driven efficiency and shRNA vector uptake. Expression however can be made inducible with Tet-on/off systems.

 

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