Background

Cells constantly repair genomic damage caused by UV radiation, chemicals, and metabolic byproducts. The complexly intertwined DNA damage response (DDR) signaling network serves as a key defense line for maintaining genomic stability. Defects in this pathway lead to accumulated genomic instability, resulting in cancer. Paradoxically, these DDR defects in cancer cells can be targeted for synthetic lethality-based anticancer therapies, which selectively kill cancer cells by further blocking specific repair pathways.

Existing functional genomics research has primarily relied on CRISPR-Cas9 screening involving the deletion of single genes. However, human cells have evolved numerous redundant compensatory pathways. Even when a single gene is knocked out, alternative proteins with similar functions perform compensatory actions, making it difficult to fully identify complex genetic interaction networks through single deletions alone. To comprehensively analyze interactions between gene pairs, a large-scale multiplex screening platform capable of precisely deleting two or more genes simultaneously was urgently needed.

Key Findings

Researchers focused on the characteristics of the Cas12a enzyme, which can autonomously process and release multiple guide RNAs under a single promoter. Unlike Cas9, Cas12a possesses intrinsic crRNA processing capability, providing a functional advantage for simultaneous multi-gene targeting. By leveraging this feature, the researchers established a highly efficient multiplex knockout screening system that induces double deletions in tens of thousands of combinations across hundreds of major DDR genes.

By quantitatively tracking changes in cell proliferation and survival rates after knocking out gene pairs in various combinations, they derived genetic interaction scores resulting from the simultaneous loss of two genes. Beyond simple additive effects, synthetic lethal interactions, where cell death is maximized, were clearly distinguished from suppressing interactions, which neutralize the effect of a defect in one gene. The physical and functional connection networks between major DDR sub-pathways, such as homologous recombination repair (HRR), non-homologous end joining (NHEJ), and base excision repair (BER), were precisely visualized on a single map.

In particular, numerous unknown backup gene combinations were identified that entirely sustain cell survival when specific DNA replication stress resolution factors are deleted. It was demonstrated that genes which do not exhibit distinct phenotypes in conventional single-gene screening cause fatal viability defects only when paired.

Significance and Outlook

The completed interaction map provides crucial clues for overcoming drug resistance, a chronic challenge in cancer drug development. It provides a genetic basis for identifying which secondary repair factors should be additionally inhibited in tumors that show resistance to targeted therapies like PARP inhibitors. When combined with large-scale genomic analysis data, it serves as a compass for selecting precision combination therapies tailored to the mutation patterns of individual patient tumors.

Challenges remain before reaching actual clinical application. Synthetic lethal pairs derived from in vitro cell line models must undergo rigorous validation in animal models to ensure they exhibit the same efficacy within the complex tumor microenvironment of living organisms. Systematic optimization is also required regarding the potential for off-target cleavage in Cas12a multiplex editing technology and the variation in cleavage efficiency between target genes.