How to Build a Synthetic Lethality Screen

Build two screens, not one: the same library in a mutant background and in an isogenic wild-type background. The synthetic lethal hits are the guides depleted in the mutant but not in the wild type.

Coverage, MOI, and readout follow the standard rules for a dropout screen — the difference is that the interaction is defined by the difference between screens, not by depletion in either alone.

The first row is what makes the design interpretable and is the most common failure point: two unrelated lines with the same driver mutation are not an isogenic pair.

A guide that depletes in the mutant background may simply target an essential gene. The synthetic lethal signal is the difference in effect between backgrounds.

Practical points:

• • Normalize each screen to its own baseline before comparing.
• • Model the interaction explicitly (for example, a multiplicative model on guide abundance).
• • Aggregate guides to genes and require consistency across guides.
• • Control the false discovery rate across genes, not guides.

Reporting a list of genes depleted in the mutant background, without the wild-type comparison, is not a synthetic lethality result.

• • Non-targeting guides in both backgrounds.
• • Known synthetic lethal pairs as positive controls for the analysis pipeline (for example, a PARP–BRCA-type pair if the backgrounds permit).
• • Known essential genes to confirm the screen detects essentiality at all.
• • Guide-level consistency within each hit gene.

For each candidate, rebuild the three-arm experiment individually:

1. Lesion A only — viable.
2. Target knockout only — viable.
3. Both — not viable.

Arm 3 alone is a combination effect. Only the comparison across arms establishes interaction, and this is where most screen papers stop short.

EDITGENE builds matched backgrounds for differential screens, with documented baseline proliferation, verified isogenicity, and downstream three-arm reconstruction of candidate pairs.

Tell us the lesion and the library scope. Contact EDITGENE to request a project consultation or quote.

Synthetic lethality is defined relative to a specific pair of contexts; results in one background pair are not transferable to another without testing.

Screen hits require three-arm reconstruction; interaction scores depend on the model used and should be reported with it.

• • O'Neil NJ, et al. Synthetic lethality and cancer. Nature Reviews Genetics. 2017;18:613–623.
• • Shen JP, et al. Combinatorial CRISPR-Cas9 screens for de novo mapping of genetic interactions. Nature Methods. 2017;14:573–576.
• • Hart T, et al. Evaluation and design of genome-wide CRISPR/SpCas9 knockout screens. G3. 2017;7:2719–2727.