Repurposing Clinically Approved Drugs to Control DNA Repair Outcomes

Study Background and Research Question

DNA double-strand breaks (DSBs) represent a central event in genome editing, cancer therapy, and the natural maintenance of genomic stability. While spontaneous DSBs arise from endogenous metabolic processes and exogenous agents such as radiation or genotoxic chemicals, programmable nucleases like CRISPR/Cas9 enable researchers to induce DSBs at specific genomic loci. The repair of these lesions is primarily executed by two error-prone pathways—non-homologous end joining (NHEJ) and microhomology-mediated end joining (MMEJ)—or by the more precise homology-directed repair (HDR) pathway. The choice of repair pathway is a central determinant of genome editing outcomes, influencing the efficiency, specificity, and types of mutations introduced.

Despite broad applications in gene therapy, disease modeling, and cancer research, the ability to pharmacologically steer DNA repair pathway choice has remained limited. The reference study (Macak et al., 2025) addresses this gap by systematically evaluating whether clinically safe, FDA-approved drugs can be repurposed to modulate DSB repair pathway usage and outcomes during CRISPR genome editing in human stem cells.

Key Innovation from the Reference Study

The central innovation lies in the large-scale, unbiased screening of over 7,000 FDA-approved drugs to determine their capacity to shift DSB repair pathway usage. By leveraging high-throughput sequencing and mutational outcome profiling in human induced pluripotent stem cells (hiPSCs) undergoing CRISPR editing, the study identifies candidate compounds that either inhibit or enhance specific repair pathways (NHEJ, MMEJ, HDR). This work enables programmable editing outcomes without the need for genetic manipulation of endogenous repair factors, thus accelerating translational applications in disease modeling, gene therapy, and precision oncology.

Methods and Experimental Design Insights

The study employs a robust workflow wherein hiPSCs expressing a doxycycline-inducible Cas9 (iCRISPR) system are treated with individual drug candidates during CRISPR-mediated targeting of a genomic locus (FRMD7). After a recovery period, cell survival is quantified via resazurin fluorescence assays, and DNA repair outcomes are determined using targeted Illumina sequencing. Each drug condition is tested in a single replicate, encompassing over 7,200 total experimental conditions. Mutational outcomes are parsed and assigned to canonical repair pathways: precise edits (HDR), small insertions (NHEJ), and deletions with microhomology (MMEJ). This design enables a high-resolution, quantitative view of how each compound alters the distribution of repair outcomes (Macak et al., 2025).

In addition to phenotypic screening, the study integrates siRNA-based gene perturbation to explore the impact of silencing key regulatory proteins (e.g., ESR2, AOX1) on repair pathway selection. Synthetic lethality is examined by combining pharmacological inhibition of one pathway with genetic or pharmacological disruption of another, revealing drugs that selectively induce cell death in repair-deficient contexts.

Core Findings and Why They Matter

The screen reveals a diverse set of clinically approved drugs that can be repurposed as modulators of DSB repair pathway choice. Key findings include:

• Pathway-Specific Modulation: Several compounds were shown to either enhance HDR or suppress NHEJ/MMEJ, thereby increasing the frequency of precise edits or altering the spectrum of indels introduced by CRISPR. This has immediate implications for improving the fidelity of genome editing in both research and therapeutic settings.
• Synergistic Effects: Silencing of estrogen receptor 2 (ESR2) combined with NHEJ inhibition led to a marked (mean 4.6-fold) increase in HDR rates, demonstrating that combined pharmacological and genetic interventions can achieve greater control over repair outcomes.
• Synthetic Lethality Candidates: The study identifies drugs that are selectively toxic when a major repair pathway is blocked, providing a basis for precision oncology strategies that exploit tumor-specific DNA repair deficiencies.
• Programmable Control for Disease Modeling: By using drugs to tune repair pathway choice, researchers can generate desired mutational signatures for modeling genetic diseases, optimizing CAR-T cell engineering, or performing genome-wide knockout screens.

These findings significantly expand the toolkit for modulating genome editing outcomes and open the door to rapid, non-genetic pathway manipulation using well-characterized pharmaceuticals (internal review).

Comparison with Existing Internal Articles

Recent internal reviews have highlighted parallel advances in the use of ryanodine receptor antagonists to modulate calcium signaling during genome editing. For instance, one article discusses the nanomolar potency and calmodulin-dependent specificity of Dantrolene sodium salt as a ryanodine receptor antagonist. While the reference study focuses on DNA repair pathway modulation via FDA-approved drugs, internal resources provide complementary protocol insights for integrating calcium signaling modulators into CRISPR workflows.

Additionally, mechanistic analyses have detailed Dantrolene's impact on intracellular calcium flux, which is implicated in the regulation of DNA repair machinery. These resources collectively underscore the importance of cross-talk between calcium signaling and DNA repair pathway choice, particularly relevant when designing high-fidelity experimental protocols for disease modeling or neurodegenerative disease models.

Limitations and Transferability

Despite the breadth of the drug screen, several limitations should be considered. The study relies on single-replicate screening per drug, which, while enabling scale, may not capture subtle or context-dependent effects. The experimental system employs hiPSCs and a single genomic locus, so transferability to other cell types, genomic contexts, or primary patient samples may require additional validation. Furthermore, the functional consequences of modulating repair pathway usage must be rigorously assessed in the context of therapeutic gene correction, especially to avoid unintended large deletions or chromosomal rearrangements.

Not all DNA repair modulators identified in the study may be equally effective or specific in different biological systems or disease models. Optimization of drug dosing, timing, and combinatorial regimens remains necessary for each new application.

Why this cross-domain matters, maturity, and limitations

The intersection of pharmacological DNA repair modulation and calcium signaling is especially relevant in translational research domains such as neurodegeneration, ischemia and hypoxia research, and pancreatitis models. Modulators like Dantrolene sodium salt have been widely used as ryanodine receptor antagonists in these contexts (see further discussion), and their potential to influence DNA repair pathways through calcium signaling offers a promising, yet still maturing, avenue for cross-domain protocol development. However, direct evidence connecting these mechanisms within the context of genome editing remains limited, emphasizing the need for further integrative studies.

Protocol Parameters

• Drug screening concentration: As per the reference study, drugs were typically tested at concentrations reflecting clinical plasma maxima or established in vitro efficacies.
• CRISPR editing induction: Doxycycline-inducible Cas9 expression in hiPSCs, with drug treatment initiated during editing window.
• Cell survival assay: Resazurin-based fluorescence quantification performed after drug and editing treatments.
• Mutation quantification: DNA extraction followed by targeted Illumina sequencing to classify HDR, NHEJ, and MMEJ outcomes.
• Genetic perturbation: Optional siRNA knockdown (e.g., ESR2) can be performed in parallel with drug treatment to probe synergistic effects on repair outcomes.
• Calcium signaling modulation: For workflows targeting calcium homeostasis, Dantrolene sodium salt can be dissolved in DMSO at concentrations ≥12.2 mg/mL and used acutely during editing or stress induction, as supported by product guidelines.

Research Support Resources

To extend findings from the reference study or to incorporate calcium signaling modulation into genome editing protocols, researchers may consider using Dantrolene, sodium salt (SKU B6329). This compound, available from APExBIO, is a potent ryanodine receptor antagonist with robust documentation for use in calcium signaling and DNA repair modulation workflows. Its high purity and validated performance characteristics facilitate reproducible results in pancreatitis research, neurodegenerative disease models, and ischemia and hypoxia research. For additional protocol insights and troubleshooting strategies, consult recent internal reviews and product information.