Repurposing Clinically Safe Drugs for DNA Repair in CRISPR Editing

Study Background and Research Question

Genome integrity is constantly threatened by double-strand DNA breaks (DSBs), which can arise spontaneously from metabolic processes or be induced by external agents such as radiation and chemotherapeutics. The advent of CRISPR-Cas9 genome editing has enabled researchers to introduce targeted DSBs at precise genomic loci, vastly expanding the potential for disease modeling, gene therapy, and functional genomics. However, the outcome of CRISPR-induced DSBs is governed by endogenous cellular repair pathways—primarily non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homology-directed repair (HDR)—each with distinct mutational signatures and implications for editing precision. Modulating the choice of these pathways has remained a key challenge for achieving desired editing outcomes, especially when precise sequence correction is required for therapeutic applications.

The reference study (Macak et al., 2025) addressed the critical question: Can existing, clinically safe drugs be repurposed to influence the balance of DSB repair pathways in human induced pluripotent stem cells (iPSCs) during CRISPR editing, thereby enabling more predictable and controllable genetic modifications?

Key Innovation from the Reference Study

The central innovation of this work is the systematic screening of the majority of FDA-approved drugs—over 7,000 compounds—for their ability to modulate DNA repair pathway choice following CRISPR-induced DSBs in human iPSCs. By directly analyzing the mutational outcomes at targeted loci, the researchers identified both inhibitors and enhancers of NHEJ, MMEJ, and HDR, including compounds that favor precise editing outcomes or induce synthetic lethality when certain repair pathways are disabled. This approach repurposes drugs with known safety profiles to fine-tune genome editing outcomes and potentially enhance therapeutic genome engineering strategies, as detailed in the internal resource summarizing the screen's translational significance.

Methods and Experimental Design Insights

The experimental pipeline combined high-throughput drug screening with next-generation sequencing to quantify the effects of drug treatment on CRISPR-induced DSB repair. Human iPSCs expressing a doxycycline-inducible Cas9 (iCRISPR) system were edited at a defined genomic locus (FRMD7) while exposed to individual drug candidates. Following recovery, cell survival was assessed with a resazurin fluorescence assay, and genomic DNA was extracted for Illumina sequencing to map repair outcomes.

• The analysis distinguished between precise edits (HDR), small indels typical of NHEJ, and larger deletions attributable to MMEJ.
• Relative frequencies of each outcome under drug treatment were compared to DMSO controls, enabling identification of pathway modulators.
• Genetic perturbations, such as silencing of ESR2 and AOX1, were combined with pharmacological inhibition to assess synergistic effects on HDR enhancement.

This robust, quantitative framework allowed the team to catalog drugs that shift the balance of DNA repair, including those capable of enhancing HDR or inducing synthetic lethality in specific repair-deficient contexts.

Core Findings and Why They Matter

The screen identified a diverse set of clinically safe compounds capable of modulating DSB repair pathway choice. Notable findings include:

• Drugs that inhibit NHEJ or MMEJ can bias repair outcomes toward HDR, increasing the efficiency of precise genome edits—an essential requirement for therapeutic correction of pathogenic alleles.
• Silencing of ESR2 (estrogen receptor 2) in combination with NHEJ inhibition resulted in a mean 4.6-fold increase in HDR, suggesting a new node for combinatorial modulation of repair pathways (Macak et al., 2025).
• Several drugs were identified that induce synthetic lethality when NHEJ or HDR pathways are blocked, offering new precision medicine strategies for targeting cancer cells with specific DNA repair deficiencies.
• The ability to pharmacologically steer repair outcomes enables researchers to design more predictable gene knockouts or knock-ins, and to minimize unwanted indels and rearrangements—major concerns in disease modeling and cell therapy development.

These results have far-reaching implications for genome editing precision, synthetic lethality screening, and translational research in hematologic malignancies, neurodegenerative disorders, and inherited disease models.

Comparison with Existing Internal Articles

Several internal reviews and guides contextualize the findings of this drug repurposing screen:

• Dantrolene sodium salt: Precision Ryanodine Receptor Antagonist Workflows highlights the integration of ryanodine receptor antagonists into high-throughput screening and CRISPR workflows, echoing the reference study's focus on optimizing DNA repair via targeted modulation of intracellular signaling pathways.
• The article Dantrolene Sodium Salt: Unraveling RyR Pathways expands on the mechanistic interplay between calcium signaling modulation and DNA repair, supporting the rationale for including calcium channel modulators in synthetic lethality and genome editing screens.
• Additionally, Dantrolene Sodium Salt: Precision Ryanodine Receptor Antagonist Use provides protocol-level insights for employing high-purity ryanodine receptor antagonists in CRISPR and disease modeling assays, complementing the reference workflow with practical guidance.

Together, these resources underscore a growing consensus: that pharmacological modulation of DNA repair and calcium signaling can be strategically leveraged to refine CRISPR genome editing outcomes and synthetic lethality screens.

Limitations and Transferability

Despite its breadth and systematic approach, the drug screening study presents certain limitations:

• The primary screen was conducted in human iPSCs, which may not fully recapitulate DNA repair dynamics in primary cells or disease-specific contexts such as mature neurons or cancer subtypes.
• Many compounds with promising in vitro effects may have pharmacokinetic or off-target liabilities in vivo that were not addressed in this study.
• While single-replicate screening enabled high throughput, deeper validation in multiple cell types and genetic backgrounds is necessary to confirm specific drug-pathway interactions.
• Transferability to clinical gene editing or synthetic lethality protocols requires additional studies assessing compound safety, on-target efficiency, and off-target effects in relevant disease models.

Protocol Parameters

• Drug treatment window: Administer candidate drug during Cas9 induction and editing phase (e.g., 24–48 hours), followed by recovery in drug-free medium.
• Cell type: Human iPSCs with inducible Cas9 recommended for initial screens; validate hits in disease-relevant or primary cell models.
• Readout: Use resazurin-based survival assay for viability and targeted Illumina sequencing for pathway-specific repair quantification.
• Synergy assessment: Combine pharmacological and genetic perturbations (e.g., gene knockdown with pathway inhibitor) to evaluate additive or synergistic effects on HDR or synthetic lethality.

These parameters reflect literature-backed approaches, but adaptation to specific repair pathway targets and experimental systems may be necessary.

Research Support Resources

For researchers seeking to implement similar workflows, validated compounds such as Dantrolene, sodium salt (SKU B6329) are available for precise modulation of ryanodine receptors and calcium signaling in genome editing or disease modeling experiments. Dantrolene sodium salt is recognized for its high purity and well-characterized inhibitory profile, supporting protocols that require ryanodine receptor antagonism or calcium signaling modulation, as described in the product information and related internal articles. Proper storage and solubilization—such as dissolving in DMSO and using solutions promptly—ensure activity and reproducibility in CRISPR and synthetic lethality assays. APExBIO supplies this compound with detailed quality control data to facilitate robust experimental design.