Repurposing Approved Drugs to Guide DNA Repair in Genome Editing

Study Background and Research Question

DNA double-strand breaks (DSBs) are critical lesions that threaten genomic stability. In human cells, DSBs can arise spontaneously through metabolic byproducts or be induced deliberately, as in CRISPR-Cas9 genome editing. DSB repair is primarily managed by non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homologous recombination (HR), with each pathway affecting the precision and outcome of DNA repair. While NHEJ and MMEJ are error-prone, leading to insertions or deletions, HR and its related homology-directed repair (HDR) enable precise genetic changes—crucial for disease modeling, gene therapy, and engineered cell therapies. Despite the centrality of these pathways, pharmacological tools to modulate repair choice in a targeted fashion are limited, hindering efforts to improve efficiency and precision in genome engineering. This study addresses whether clinically safe, FDA-approved drugs can be repurposed to selectively influence DSB repair outcomes in human cells, thereby expanding the toolkit for precision genome editing and synthetic lethality in cancer treatment according to the reference study.

Key Innovation from the Reference Study

The principal innovation of this work lies in its systematic, high-throughput screening of over 7,000 FDA-approved drugs to identify compounds that can either inhibit or enhance specific DSB repair pathways in human induced pluripotent stem cells (hiPSCs). Previous efforts often focused on single inhibitors or small panels targeting key proteins (e.g., DNA-PKcs, RAD51), whereas this study widens the scope to drugs with established clinical safety profiles. The approach enables rapid identification of pharmacological modulators for NHEJ, MMEJ, and HDR, including potential candidates for precision medicine and synthetic lethality in oncology.

Methods and Experimental Design Insights

The authors employed a robust screening workflow using 409B2 hiPSCs engineered to express a doxycycline-inducible Cas9 (iCRISPR). During the treatment phase, cells underwent CRISPR-mediated editing at a defined genomic locus (FRMD7) in the presence of a test drug. After recovery, cell survival was quantified by a resazurin fluorescence assay, and mutational outcomes were characterized by Illumina sequencing. The sequencing data were analyzed to assign each repair event to a specific pathway—NHEJ, MMEJ, or HDR—based on the type and size of indels or the presence of precise edits. This design provided quantitative measures of both cell viability and pathway-specific editing outcomes for each compound.

• Drug screening was conducted with a single replicate for each of the >7,000 compounds, maximizing throughput and coverage.
• Mock treatment with DMSO established the baseline distribution of repair outcomes.
• Sequencing enabled precise discrimination between small indels (NHEJ), microhomology-dependent deletions (MMEJ), and template-driven edits (HDR/SSTR).

Core Findings and Why They Matter

The screen identified numerous clinically approved drugs capable of modulating DSB repair pathway choice. Notable findings include:

• Identification of both inhibitors and enhancers of NHEJ, MMEJ, and HDR. Some drugs shifted the balance toward precise HDR, while others promoted error-prone end joining.
• Demonstration that silencing of ESR2 (estrogen receptor 2) synergistically increases HDR, especially when combined with NHEJ inhibition—achieving a mean 4.6-fold increase in HDR frequency.
• Discovery of drug-induced synthetic lethality in contexts where NHEJ or HDR is genetically or pharmacologically suppressed, highlighting potential avenues for targeted cancer therapy.
• Mapping of the influence of certain metabolic enzymes (e.g., AOX1) and DNA repair proteins (e.g., ATM, 53BP1) in repair pathway selection, suggesting novel intervention points.

These findings are significant because they demonstrate that DNA repair outcomes after CRISPR editing can be pharmacologically tuned using compounds with established safety profiles. This expands the potential for precise genome engineering in therapeutic, disease modeling, and immuno-oncology contexts. The use of synthetic lethality via drug combinations offers additional strategies for targeting cancer cells with specific DNA repair defects.

Comparison with Existing Internal Articles

Several internal resources provide complementary perspectives on pharmacological modulation of DNA repair and its translational implications:

• The article "Dantrolene Sodium Salt: Unraveling RyR Pathways for Precision DNA Repair" explores the unique mechanism of dantrolene as a ryanodine receptor antagonist, focusing on its role in calcium signaling modulation and potential impact on DNA repair—suggesting future cross-talk between calcium signaling and DNA repair pathway choice.
• "Dantrolene, sodium salt (SKU B6329): Scenario-Driven Solutions" emphasizes workflow optimization for calcium signaling and DNA repair studies, highlighting best practices in experimental design for researchers using compounds like dantrolene sodium salt.
• The internal summary "Repurposing Approved Drugs to Modulate DNA Repair in CRISPR Editing" provides an accessible overview of the reference study’s methodology and implications for genome editing precision and synthetic lethality strategies.

While the reference study did not specifically investigate ryanodine receptor antagonists or calcium signaling modifiers such as dantrolene sodium salt in its primary screening, the workflow and approach are readily adaptable to evaluating Dantrolene, sodium salt and similar compounds for their potential effects on DNA repair pathway modulation, as discussed in the internal literature.

Limitations and Transferability

There are several limitations to consider in translating these findings to other cell types or clinical applications:

• The study was conducted in a single human iPSC line, and it is not yet clear whether the identified drug effects are broadly generalizable to other cell types, especially primary cells or differentiated derivatives.
• Screening was performed at a single concentration and time point per drug; dose-response relationships and optimal treatment windows remain to be established for most compounds.
• Genomic context, chromatin state, and sequence composition at the edited site can influence repair outcomes, potentially modulating the drug effects observed.
• Off-target effects and cytotoxicity profiles of some drugs in specific cell types were not comprehensively assessed in this initial screen.

Nevertheless, the workflow provides a generalizable platform for future studies, including the evaluation of additional pathways, cell types, and drug classes.

Protocol Parameters

• CRISPR editing with drug treatment: Add the test compound at the desired concentration during transfection or induction of Cas9 activity in hiPSCs. For initial screens, use concentrations supported by safety data or established pharmacological ranges in cell culture.
• Cell survival measurement: Employ a resazurin-based fluorescence assay 48-72 hours post-editing to quantify viability effects of drug treatment.
• Genomic outcome assessment: Extract genomic DNA after recovery and perform targeted Illumina sequencing around the edited site to assign repair events to NHEJ, MMEJ, or HDR categories.
• Replicates and controls: Include DMSO-only controls and, where possible, positive controls for pathway inhibition (e.g., DNA-PKcs inhibitors for NHEJ blockade).
• Data analysis: Use established bioinformatics pipelines for indel and HDR quantification, adjusting for sequencing depth and baseline editing rates.

Why this cross-domain matters, maturity, and limitations

The intersection of pharmacological DNA repair modulation and precision genome editing is of high translational value. Compounds able to shift pathway choice can improve the fidelity of gene correction or inactivation in disease models and therapeutics. However, the functional crosstalk between calcium signaling, ryanodine receptor activity, and DNA repair remains underexplored, as highlighted by the limited direct investigation of such mechanisms in the current reference screen. Maturity in this area will require additional mechanistic studies and cross-validation in diverse biological contexts.

Research Support Resources

Researchers interested in modulating DNA repair pathways in genome editing or synthetic lethality studies can leverage a range of compounds identified in this and related studies. For those exploring the role of calcium signaling in DNA repair, Dantrolene, sodium salt (SKU B6329) is a well-characterized ryanodine receptor antagonist suitable for mechanistic studies of intracellular calcium modulation and its impact on genome stability. High-purity formulations and comprehensive quality control facilitate reproducibility in advanced editing and pathophysiology workflows.