DNA Repair Mechanisms and Their Critical Role in Cancer Development

The stability of the genome is fundamental to the survival and proper functioning of all living organisms. Genomic integrity ensures the faithful transmission of genetic information during cell division and is tightly regulated by a complex network of cellular surveillance mechanisms. When this integrity is compromised, the risk of mutagenesis and disease, particularly cancer, increases substantially.

DNA is constantly exposed to a variety of damaging agents. Endogenous sources include reactive oxygen species (ROS) generated during normal cellular metabolism, spontaneous hydrolytic reactions, and replication errors. Exogenous sources, on the other hand, encompass ultraviolet (UV) radiation, ionizing radiation, environmental mutagens, and certain chemotherapeutic agents. These insults can lead to a range of DNA lesions, including single- and double-strand breaks, base modifications, crosslinks, and bulky adducts.

To counteract this continuous threat, cells have evolved a sophisticated array of DNA repair mechanisms that detect, signal, and resolve damage to preserve genomic fidelity. However, when these repair pathways are defective—due to inherited mutations or acquired dysfunctions—cells accumulate mutations at an accelerated rate, leading to genomic instability. This instability is a driving force in oncogenesis and is recognized as one of the enabling characteristics of cancer.

In this article, we will explore the major DNA repair mechanisms, their molecular components, and their relevance to cancer biology. We will also examine how deficiencies in these pathways contribute to tumor development and how their dysfunctions are being exploited therapeutically in modern oncology.

II. Types of DNA Damage and Genomic Instability

Cells are continually exposed to a wide range of DNA-damaging agents that threaten genomic stability. The resulting DNA lesions, if not promptly and accurately repaired, can lead to mutations, chromosomal aberrations, and ultimately carcinogenesis. Understanding the types of DNA damage is essential for grasping the biological significance of the corresponding repair mechanisms.

2.1 Single-Strand Breaks (SSBs)

Single-strand breaks occur when the phosphodiester backbone of only one DNA strand is disrupted. These lesions are common and typically result from oxidative stress, ionizing radiation, or spontaneous base loss. While often less lethal than double-strand breaks, unrepaired SSBs can interfere with transcription and replication, and may convert into double-strand breaks (DSBs) if encountered by the replication machinery.

2.2 Double-Strand Breaks (DSBs)

DSBs represent one of the most cytotoxic types of DNA damage. They involve the cleavage of both DNA strands and can result from ionizing radiation, replication fork collapse, or mechanical stress. If left unrepaired or misrepaired, DSBs can trigger chromosomal translocations, deletions, or aneuploidy—hallmarks of many cancers.

2.3 Oxidative DNA Damage

Reactive oxygen species (ROS), generated as byproducts of aerobic metabolism or inflammation, can modify DNA bases, particularly guanine, resulting in 8-oxoguanine and other mutagenic lesions. These base modifications can cause mispairing during replication and lead to point mutations.

2.4 Alkylation and Hydrolytic Damage

Endogenous alkylating agents or environmental chemicals can add alkyl groups to DNA bases, distorting DNA structure and blocking replication. Hydrolytic reactions can lead to base loss (abasic sites) or deamination (e.g., cytosine to uracil), further contributing to mutagenesis if not corrected.

2.5 Bulky DNA Adducts and UV-Induced Lesions

Bulky DNA adducts are typically caused by exposure to carcinogens like benzo[a]pyrene or platinum-based drugs. UV radiation induces cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts, distorting the DNA helix and obstructing transcription and replication.

2.6 Genomic Instability in Cancer

Persistent DNA damage, when left unrepaired or misrepaired, accumulates over time and leads to genomic instability—a key enabling feature of cancer. This instability manifests as chromosomal rearrangements, gene amplifications, deletions, and microsatellite instability. Defects in DNA repair genes such as BRCA1/2, MLH1, or ATM are strongly associated with familial and sporadic cancers. The accumulation of such mutations accelerates cancer progression and contributes to intratumoral heterogeneity and therapeutic resistance.

III. Overview of Major DNA Repair Mechanisms

To preserve genomic integrity in the face of continuous DNA damage, eukaryotic cells have evolved multiple, highly specialized DNA repair pathways. Each pathway is tailored to detect and resolve specific types of lesions. Dysregulation or loss of function in these pathways contributes to cancer initiation, progression, and therapy resistance. This section provides an overview of the five major DNA repair mechanisms and their relevance to oncogenesis.

3.1 Base Excision Repair (BER)

Function: Corrects small, non-helix-distorting base lesions caused by oxidative stress, alkylation, or deamination.

Key Steps and Proteins:

• Initiation by DNA glycosylases that recognize and excise damaged bases.
• Formation of an abasic (AP) site.
• AP endonuclease (APE1) cleaves the phosphodiester bond.
• DNA polymerase β fills the gap; DNA ligase seals the nick.

Clinical Relevance:
Defective BER is associated with increased mutational burden and may sensitize tumors to alkylating agents and ROS-generating therapies. XRCC1 polymorphisms have been linked to cancer susceptibility in some populations.

3.2 Nucleotide Excision Repair (NER)

Function: Removes bulky DNA adducts and UV-induced lesions, such as cyclobutane pyrimidine dimers (CPDs).

Pathways:

• Global Genomic NER (GG-NER): Scans the entire genome.
• Transcription-Coupled NER (TC-NER): Targets lesions that stall RNA polymerase during transcription.

Key Proteins:
XPC (damage recognition), TFIIH (helicase), XPA/XPD (verification), ERCC1/XPF and XPG (incision), DNA polymerase δ/ε (resynthesis), and DNA ligase.

Clinical Relevance:
Inherited defects in NER genes cause xeroderma pigmentosum (XP), characterized by extreme UV sensitivity and early-onset skin cancers. NER status can influence tumor response to cisplatin.

3.3 Mismatch Repair (MMR)

Function: Repairs base-base mismatches and insertion/deletion loops generated during DNA replication.

Key Proteins:
MSH2, MSH6 (MutSα complex); MLH1, PMS2 (MutLα complex). These proteins identify and excise mismatches, followed by resynthesis.

Clinical Relevance:
MMR deficiency leads to microsatellite instability (MSI), a hallmark of Lynch syndrome (hereditary nonpolyposis colorectal cancer). MSI-high tumors respond well to immune checkpoint blockade due to increased neoantigen load.

3.4 Homologous Recombination Repair (HRR)

Function: High-fidelity repair of DNA double-strand breaks (DSBs) using the sister chromatid as a template.

Key Proteins:
BRCA1, BRCA2, RAD51, PALB2, ATM.

Clinical Relevance:
Loss of HRR function results in homologous recombination deficiency (HRD), which is common in breast, ovarian, and prostate cancers. BRCA mutations sensitize tumors to PARP inhibitors via synthetic lethality.

3.5 Non-Homologous End Joining (NHEJ)

Function: Error-prone repair of DSBs by directly ligating broken ends without a homologous template.

Key Proteins:
Ku70/Ku80, DNA-PKcs, XRCC4, Ligase IV.

Clinical Relevance:
NHEJ contributes to chromosomal translocations and mutagenesis in lymphoid malignancies. Hyperactivation or dysregulation can promote oncogenic rearrangements, particularly in leukemia and lymphoma.

IV. The DNA Damage Response (DDR) Network

The DNA Damage Response (DDR) is a complex signaling network that detects DNA lesions, halts cell cycle progression, and initiates repair processes. When DNA damage is beyond repair, DDR can also induce senescence or apoptosis to prevent the propagation of genetically unstable cells. In cancer biology, DDR plays a dual role—initially acting as a barrier to transformation, but later often subverted to support tumor cell survival and therapy resistance.

4.1 Sensing DNA Damage

Damage sensing is initiated by specialized protein complexes that recognize different types of lesions:

• ATM (ataxia-telangiectasia mutated) is activated primarily by double-strand breaks (DSBs).
• ATR (ATM and Rad3-related) responds to replication stress and single-stranded DNA regions.
• DNA-PKcs (DNA-dependent protein kinase catalytic subunit) is also involved in DSB detection, mainly via the NHEJ pathway.

These kinases form the core of the initial DDR signaling cascade and are rapidly recruited to sites of damage.

4.2 Signal Transduction and Amplification

Upon activation, ATM and ATR phosphorylate numerous downstream targets, amplifying the damage signal and coordinating the repair response. Key substrates include:

• CHK1 and CHK2 (Checkpoint Kinases 1 and 2): Mediate cell cycle arrest, especially at G1/S and G2/M checkpoints.
• p53 (TP53): A critical tumor suppressor that induces cell cycle arrest, DNA repair genes, or apoptosis depending on damage severity.
• H2AX: Phosphorylated by ATM to form γH2AX, a well-known marker of DSBs and a platform for repair protein recruitment.

4.3 Cell Cycle Checkpoints

The DDR tightly regulates cell cycle transitions to prevent replication or division of damaged DNA:

• G1/S checkpoint: Prevents damaged DNA from entering S phase.
• Intra-S phase checkpoint: Slows replication in response to stalled forks or lesions.
• G2/M checkpoint: Prevents mitotic entry until damage is resolved.

Checkpoint dysfunction is a frequent feature of cancers, often due to p53 or ATM mutations, allowing damaged cells to continue proliferating.

4.4 Apoptosis and Senescence

If DNA damage is extensive or irreparable, the DDR activates programmed cell death or permanent growth arrest to eliminate damaged cells:

• p53-mediated apoptosis involves transcription of pro-apoptotic genes (e.g., BAX, PUMA, NOXA).
• Senescence is enforced via p21 and RB pathways, serving as a durable tumor-suppressive mechanism.

4.5 DDR in Tumorigenesis and Therapy

While the DDR suppresses tumor initiation by maintaining genome stability, many tumors retain partial DDR functions to survive under genotoxic stress, including chemotherapy and radiation. This selective retention creates therapeutic vulnerabilities—such as reliance on specific repair pathways—that can be exploited using targeted DDR inhibitors.

V. Consequences of Defective DNA Repair in Cancer

Defects in DNA repair mechanisms are among the most significant contributors to cancer development and progression. When the cellular ability to detect and repair DNA damage is impaired, the genome becomes unstable, leading to the accumulation of mutations, chromosomal aberrations, and oncogenic transformations. This section highlights how dysfunctional DNA repair pathways contribute to tumorigenesis and their broader implications in cancer biology.

5.1 Mutation Accumulation and Oncogene Activation

Impaired DNA repair allows for the survival and replication of cells harboring DNA lesions, resulting in a high mutational load. These mutations may activate proto-oncogenes (e.g., RAS, MYC) or inactivate tumor suppressor genes (e.g., TP53, RB1), leading to unchecked cell proliferation. Cancers arising from mismatch repair (MMR) deficiencies, such as colorectal tumors in Lynch syndrome, exhibit particularly high mutation burdens, often manifesting as microsatellite instability (MSI).

5.2 Chromosomal Instability (CIN)

Chromosomal instability involves frequent changes in chromosome number and structure due to defective double-strand break (DSB) repair pathways like HRR and NHEJ. This instability contributes to:

• Aneuploidy (abnormal chromosome numbers)
• Gene amplifications and deletions
• Structural rearrangements, including translocations and inversions

Such alterations are common in aggressive cancers and are associated with poor prognosis and resistance to treatment.

5.3 Hallmarks of Cancer and DNA Repair Defects

DNA repair deficiencies intersect with several key hallmarks of cancer, including:

• Evasion of apoptosis: Mutations in TP53 allow damaged cells to survive.
• Replicative immortality: Telomere dysfunction can be bypassed through alternative lengthening mechanisms.
• Sustained proliferative signaling: DNA repair defects accelerate oncogenic pathway activation.
• Genome instability and mutation: A direct hallmark enabled by defective repair.

5.4 Hereditary Cancer Predisposition Syndromes

Germline mutations in DNA repair genes are strongly associated with familial cancer syndromes:

• BRCA1/2 mutations: Increase risk for breast, ovarian, pancreatic, and prostate cancers due to impaired homologous recombination repair.
• MLH1, MSH2, MSH6 mutations: Cause Lynch syndrome (MMR deficiency), predisposing to colorectal, endometrial, and other cancers.
• ATM and TP53 mutations: Linked to ataxia-telangiectasia and Li-Fraumeni syndrome, respectively.

These inherited conditions highlight the critical role of DNA repair in tumor suppression and the value of genetic testing for risk assessment.

5.5 Tumor Evolution and Heterogeneity

In the context of chronic repair defects, tumors evolve under selective pressure, generating intratumoral heterogeneity. This diversity enables subclonal populations to adapt to environmental stressors, including immune surveillance and therapy. As a result, tumors with defective DNA repair often exhibit:

• High clonal diversity
• Greater adaptability to treatment
• Increased risk of relapse

VI. Therapeutic Exploitation of DNA Repair Defects

The concept of exploiting DNA repair deficiencies in cancer therapy has transformed the landscape of precision oncology. Tumors harboring specific repair defects are more reliant on backup pathways to survive. Targeting these compensatory mechanisms creates a therapeutic window that selectively kills cancer cells while sparing normal tissue. This strategy is best exemplified by synthetic lethality, a principle now central to the development of targeted cancer therapies.

6.1 Synthetic Lethality and PARP Inhibitors

Synthetic lethality occurs when the simultaneous loss of two genes or pathways is lethal to cells, whereas the loss of only one is tolerated. This concept is exploited in cancers with homologous recombination deficiency (HRD), particularly BRCA1/2-mutated tumors.

• PARP inhibitors (e.g., Olaparib, Rucaparib, Niraparib) block the base excision repair (BER) pathway.
• In HR-deficient cells, PARP inhibition leads to accumulation of single-strand breaks that convert into double-strand breaks during replication.
• Without a functional HRR pathway, these lesions become lethal.

Clinical Applications: PARP inhibitors are approved for BRCA-mutant breast, ovarian, prostate, and pancreatic cancers and are under investigation for broader indications.

6.2 Inhibitors Targeting DDR Pathways

Beyond PARP inhibitors, other molecules are being developed to target key components of the DDR, especially in tumors exhibiting high replicative stress or checkpoint dependence.

• ATR inhibitors (e.g., Berzosertib): Target replication stress in ATM-deficient tumors.
• CHK1/CHK2 inhibitors: Block cell cycle checkpoints, forcing mitotic entry with damaged DNA.
• WEE1 inhibitors: Disrupt G2/M checkpoint control, synergizing with DNA-damaging agents.

These agents enhance the cytotoxicity of chemotherapies and radiation by overwhelming the DNA repair capacity of tumor cells.

6.3 Overcoming Resistance to DNA-Damaging Therapies

Many traditional anticancer therapies, such as radiation and alkylating agents (e.g., temozolomide, cisplatin), exert their effects by inducing DNA damage. However, resistance often arises due to:

• Upregulation of repair proteins (e.g., MGMT in glioblastoma)
• Restoration of homologous recombination via secondary mutations in BRCA genes
• Activation of compensatory pathways (e.g., increased NHEJ activity)

Understanding these resistance mechanisms allows for rational combination therapies and timing strategies to improve treatment outcomes.

6.4 Combination Strategies

To overcome monotherapy limitations, DDR-targeted agents are being combined with:

• Immunotherapy: Tumors with high mutational burden (e.g., MMR-deficient/MSI-high) respond well to immune checkpoint inhibitors.
• Chemotherapy: DDR inhibitors sensitize cancer cells to DNA-damaging agents.
• Radiotherapy: DDR inhibitors impair DNA repair during radiation-induced damage.

These combination approaches are under active investigation in numerous clinical trials, with promising early results.

6.5 Biomarker-Guided Therapy

Effective use of DDR-targeting drugs depends on identifying tumors with specific repair deficiencies:

• BRCA1/2 mutation status
• HRD scores (e.g., genomic scarring assays)
• Microsatellite instability (MSI)
• Loss of ATM, RAD51, or TP53

These biomarkers guide therapeutic decisions and help stratify patients for precision treatment strategies.

VII. DNA Repair Pathways as Biomarkers and Targets

As our understanding of DNA repair biology deepens, it has become clear that DNA repair status is not only a determinant of cancer behavior but also a valuable source of predictive and prognostic biomarkers. Identifying specific DNA repair defects allows for personalized treatment approaches and helps select patients who are most likely to benefit from targeted therapies such as PARP inhibitors or immune checkpoint inhibitors. This section highlights key biomarkers and therapeutic targets derived from DNA repair pathways.

7.1 Homologous Recombination Deficiency (HRD) and BRCA Mutations

The most clinically advanced DNA repair biomarker is the presence of BRCA1/2 mutations, which indicate homologous recombination deficiency (HRD).

• Germline or somatic mutations in BRCA1/2 sensitize tumors to PARP inhibitors.
• HRD scores, based on genomic scars (loss of heterozygosity, telomeric allelic imbalance), are being used to broaden eligibility for these therapies.
• Other HRR-related genes, such as RAD51C, PALB2, and ATM, are also under investigation as predictive markers.

7.2 Mismatch Repair Deficiency (MMRd) and Microsatellite Instability (MSI)

Mismatch repair-deficient tumors exhibit microsatellite instability (MSI-high), which results in a high mutational load and increased neoantigen formation.

• MSI-high status serves as a predictive biomarker for immune checkpoint blockade (e.g., anti-PD-1 therapies).
• Approved indications include colorectal, endometrial, gastric, and other solid tumors with MSI-high profiles.

7.3 γH2AX and DNA Damage Signaling

Phosphorylated H2AX (γH2AX) is a marker of DNA double-strand breaks and an early indicator of DDR activation.

• It is used in both preclinical and clinical research as a pharmacodynamic biomarker to assess the activity of DNA-damaging agents or DDR inhibitors.
• Although not yet a standard clinical biomarker, it is instrumental in drug development.

7.4 RAD51 Foci Formation

RAD51 forms foci at sites of DNA double-strand breaks during homologous recombination.

• RAD51 foci assays help assess HR functionality in tumors.
• The absence of RAD51 foci, despite DNA damage, indicates HR deficiency and potential sensitivity to PARP inhibitors.

7.5 ATM and ATR Loss

Loss or inactivation of ATM and ATR, key DDR kinases, is observed in various cancers.

• ATM loss (common in prostate cancer and lymphoid malignancies) confers sensitivity to ATR inhibitors.
• Co-inhibition strategies (e.g., ATR + PARP inhibitors) are under clinical evaluation in ATM-deficient tumors.

7.6 Emerging Biomarkers and Liquid Biopsy Potential

New approaches, including circulating tumor DNA (ctDNA) profiling, aim to monitor DDR defects in real time and guide therapy:

• Detection of BRCA reversion mutations associated with therapy resistance
• Monitoring HRD or MSI status from blood samples
• Development of non-invasive functional assays for DNA repair capacity

7.7 Targeting DNA Repair for Precision Oncology

Beyond their use as biomarkers, DNA repair pathways themselves are therapeutic targets:

• PARP, ATR, CHK1, and WEE1 inhibitors are advancing in trials.
• Rational combinations based on repair pathway interdependence are enhancing response rates.
• Tumor profiling for repair gene mutations is becoming routine in many cancer centers.

VIII. Current Research Trends and Future Directions

The study of DNA repair mechanisms and their role in cancer has entered a transformative era, driven by technological advances and a growing emphasis on precision medicine. Recent developments in molecular biology, genomics, and functional assays are deepening our understanding of DNA repair dynamics in tumors and opening new avenues for diagnostics and therapy.

8.1 CRISPR/Cas9 for DNA Repair Studies

Genome editing technologies, particularly CRISPR/Cas9, have revolutionized functional genomics and cancer modeling:

• Gene knockouts of DNA repair factors (e.g., BRCA1, ATM, MLH1) allow mechanistic studies of repair pathways.
• CRISPR screens identify synthetic lethal interactions and novel drug targets.
• Engineered cell lines and organoids mimic tumor-specific DNA repair deficiencies for preclinical testing.

8.2 Single-Cell and Spatial Profiling Technologies

High-resolution approaches are shedding light on intratumoral heterogeneity and microenvironmental influences on DNA repair:

• Single-cell RNA sequencing reveals variability in DNA repair gene expression across cell populations.
• Spatial transcriptomics allows visualization of DDR activity within tissue architecture.
• These tools are helping identify resistant clones and mechanisms of therapeutic escape.

8.3 Functional Assays for Personalized Therapy

Emerging functional assays are shifting the paradigm from mutation-based to function-based diagnostics:

• RAD51 foci assays, DNA fiber assays, and reporter constructs are being developed to measure DNA repair capacity directly.
• These approaches may better predict PARP inhibitor response than static genomic tests.

8.4 Integration of Multi-Omics Data

Combining genomics, transcriptomics, proteomics, and metabolomics is enhancing our ability to characterize DNA repair networks and vulnerabilities:

• Multi-omics integration improves biomarker discovery and subtype classification.
• DDR-based signatures are being incorporated into predictive models for therapy response and prognosis.

8.5 Tumor Microenvironment and DDR Interactions

Recent findings indicate that DNA repair is influenced by the tumor microenvironment (TME):

• Hypoxia impairs homologous recombination, creating transient HRD.
• Immune responses are modulated by DNA damage–induced inflammation and neoantigen production.
• DDR inhibitors may synergize with immunotherapy by enhancing antigenicity and reducing immune suppression.

8.6 Overcoming Resistance and Adaptive Responses

Tumors often develop adaptive resistance to DDR-targeting agents through:

• Restoration of HR via secondary BRCA mutations
• Upregulation of drug efflux pumps
• Activation of alternative repair pathways (e.g., NHEJ, alt-EJ)

New strategies aim to prevent or delay resistance using combination therapies, sequential treatment protocols, or alternating pathway inhibitors.

8.7 Future Directions in Clinical Application

Looking ahead, several promising trends are expected to shape the field:

• Pan-cancer trials based on DDR biomarkers rather than tumor type
• Expanded use of liquid biopsy for non-invasive monitoring of DNA repair status
• Development of next-generation DDR inhibitors with greater specificity and reduced toxicity
• Integration of AI and machine learning to predict repair defects and optimize treatment plans

Conclusion

DNA repair mechanisms are essential for maintaining genomic stability and preventing malignant transformation. Defects in these pathways not only drive cancer development but also shape tumor behavior, prognosis, and response to therapy. Advances in our understanding of DNA repair have led to transformative clinical applications, including biomarker-driven treatments and targeted DDR inhibitors. Continued research in this area holds great promise for refining cancer diagnostics, overcoming therapeutic resistance, and advancing precision oncology.

Frequently Asked Questions (FAQ)

1. What are the DNA repair mechanisms in cancer?

DNA repair mechanisms are cellular pathways that detect and correct damage to the DNA molecule. In the context of cancer, these pathways are often compromised, leading to genomic instability and tumor progression. The main DNA repair mechanisms include:

• Base Excision Repair (BER) – fixes small, non-bulky lesions like oxidative damage and alkylation.
• Nucleotide Excision Repair (NER) – removes bulky adducts and UV-induced photoproducts.
• Mismatch Repair (MMR) – corrects replication errors such as base mismatches and insertion-deletion loops.
• Homologous Recombination Repair (HRR) – a high-fidelity pathway for repairing double-strand breaks using a sister chromatid.
• Non-Homologous End Joining (NHEJ) – an error-prone mechanism for joining DNA double-strand breaks without a homologous template.

When these pathways are disrupted by genetic mutations or epigenetic silencing, the risk of tumorigenesis increases significantly.

2. Why are people who have poor DNA repair mechanisms at greater risk for cancer development?

Individuals with defective DNA repair systems are at greater risk for cancer because unrepaired or misrepaired DNA damage leads to mutation accumulation, chromosomal instability, and oncogene activation. Over time, this results in the loss of genomic integrity and promotes malignant transformation.

Hereditary cancer syndromes are strong evidence of this link:

• BRCA1/2 mutations impair HRR and increase the risk of breast, ovarian, and prostate cancers.
• MMR gene mutations (e.g., MLH1, MSH2) cause Lynch syndrome and are associated with colorectal and endometrial cancers.

In such individuals, cells fail to properly correct DNA lesions, allowing oncogenic mutations to accumulate unchecked.

3. What is the relationship between DNA and cancer?

Cancer is fundamentally a genetic disease driven by changes in the DNA. These changes can be:

• Point mutations affecting oncogenes or tumor suppressor genes
• Chromosomal rearrangements, deletions, or amplifications
• Epigenetic alterations that silence repair genes or activate survival pathways

DNA damage can result from both endogenous processes (e.g., oxidative stress, replication errors) and external exposures (e.g., UV light, carcinogens). If these lesions are not repaired, they can initiate or promote carcinogenesis by disrupting key regulatory pathways involved in cell proliferation, apoptosis, and genome maintenance.

4. How can DNA repair pathways be used as targets for cancer therapy?

DNA repair pathways can be therapeutically exploited through a concept known as synthetic lethality—whereby cancer cells with existing repair defects are selectively killed by inhibiting compensatory pathways. Examples include:

• PARP inhibitors in BRCA1/2-deficient tumors (e.g., Olaparib)
• ATR and CHK1 inhibitors in tumors with replication stress or ATM loss
• WEE1 inhibitors to abrogate the G2/M checkpoint in DNA-damaged cells

Additionally, DNA repair defects serve as biomarkers to guide treatment decisions (e.g., MSI-high status for immunotherapy). Targeting the DNA damage response (DDR) not only enhances the efficacy of traditional therapies like chemotherapy and radiation but also provides a precision approach to treating genetically defined tumors.

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