Whole genome arrayed shRNA libraries were described starting early in 2004 (1-3) and lentiviral libraries have been commercially available to researchers for over a decade [reviewed in (4)]; and yet only two whole genome screens using these resources have been published (Table 1). Given the enthusiasm for adopting CRISPR-based approaches, let’s take a look at the known limiting factors in using arrayed lentiviral resources, and what we can learn from this work as CRISPR arrayed screening advances as a discovery method.
While lentiviral shRNA reagents have been extremely useful in both small-scale knockdown experiments and large-scale pooled library screens, very few truly high-throughput arrayed screens have been performed and published (Table 1). A review of published arrayed shRNA screens reveals several critical (and potentially limiting) success factors. Clear guidelines for handling lentiviral particles have been established from this body of work, and compliance with these guidelines makes it impractical to work at genome-scale without putting the project at high risk for experimental variability.
Consistency of titer: It is not practical to measure the actual functional titer of every well of a lentiviral particle library, so commercial providers frequently provide the titer as a minimum specification. For example, at least 1 × 106 TU/mL (5, 6). Even when stored at -80 °C, unpurified lentiviral particles will lose ~10x in titer every 6-12 months. A single freeze-thaw of supernatant, low titer lentiviral particles can reduce titer by 2- to 3-fold. Unknown titers will impact transduction efficiency, which is key to success in an arrayed screen. One method for improving transduction efficiency is centrifuging plates after lentiviral particles are added to the cells (6), but this is not feasible for a large screen of hundreds of plates.
Safe handling: Lentiviral particles must be thawed on ice just prior to use, and require appropriate containment. This is impractical for many labs when considering a whole genome consists of hundreds of microtiter plates. If the solution is to divide up the plates and perform the screen across many different days, then variability is inevitably introduced.
Crude supernatant: Because lentiviral particle concentration methods, such as ultracentrifugation and column purification, are not amenable to high-throughput implementation, researchers often use the crude supernatant. Supernatant contains cell debris that can cause cellular toxicity, so filtration may be necessary if you are adding high volumes of lentiviral particles to transduce your cells as was the case for Maier et al. (7).
Extra handling required for antibiotic selection: Experimental protocols for lentiviral transduction require several liquid handling steps. All the publications listed in Table 1 carried out at least one medium change for removing transduction medium (containing cell debris and excess non-transducing lentiviral particles) and adding antibiotic-containing medium for selection (2, 5-10). In addition, antibiotic selection and longer time points also required cell splitting in some cases (5). Splitting cells from multi-well plates without introducing variability and contamination, while complying with safety regulations, is technically challenging, and higher cell density at longer time points prevents the application of high-content analysis of cell phenotypes.
Maier et al. took advantage of the unique quality of a one-gene-per-well format using an shRNA lentiviral library targeting 905 genes. They applied a sophisticated assay where circadian phenotypes were measured over several days using a luciferase reporter (7). This work demonstrated that with smaller libraries and a robust assay, lentiviral arrayed screening is feasible.
The current generation of arrayed lentiviral sgRNA libraries was first described in 2015, but the use of these reagents in gene knockout was demonstrated for only a few genes (11). One arrayed lentiviral sgRNA screen has been published to date; McCleland et al. described using an arrayed lentiviral sgRNA library with 1,030 sgRNA constructs targeting 200 genes (12). From the methods, we deduced that, like arrayed shRNA screens, titer and transduction efficiency are still challenges to consider. These researchers employed centrifugation for 30 minutes to improve transduction, and calculated the ratio of cell death from puromycin-selected vs. unselected cells to determine transduction efficiency. Finally, viability was measured as a phenotype.
Clearly, arrayed lentiviral sgRNA for gene knockout is a valuable resource for experimentation. Almost 2,000 peer-reviewed publications have used smaller numbers of individual lentiviral sgRNA constructs to target genes for knockout. Using these smaller scale experiments, complex phenotypes have been observed. Using lentiviral sgRNA constructs in arrayed format for screening has potential as long as lentiviral handling guidelines and experimental requirements such as medium changes and selection can be applied in a high-throughput setting.
Dharmacon provides the Edit-R Human Lentiviral sgRNA arrayed libraries available for gene families, druggable genome, and whole human genome and also as custom cherry-pick libraries with your favorite genes. These libraries are a renewable resource ideal for:
Authors: Annaleen Vermeulen, Senior Scientist and Melissa Kelley, Senior R&D Leader at Dharmacon
Arrayed collections of lentiviral sgRNA libraries for high-throughput knockout screening across entire human gene families.
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High-titer pooled screening libraries for pre-defined gene sets in human and mouse