GO:0035825 homologous recombination: DNA Repair Pathway, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0035825 homologous recombination is a biological process that exchanges equal amounts of genetic material between highly homologous DNA molecules.
• It is essential for accurate repair of DNA double-strand breaks and for faithful chromosome segregation during meiosis.
• Defects in homologous recombination cause homologous recombination deficiency (HRD), a hallmark of many cancers including breast, ovarian, pancreatic, and biliary tract cancers.
• HRD creates therapeutic vulnerabilities to platinum salts and PARP inhibitors, making HR status a key biomarker.
• Core HR proteins include BRCA1, BRCA2, RAD51, PALB2, and the MRN complex, which are frequently mutated in hereditary cancers.
• CRISPR-based models (knockout, point mutation, knock-in, overexpression) enable functional dissection of HR genes and drug response.
Description
Homologous recombination (GO:0035825) is a fundamental DNA recombination process that results in the exchange of an equal amount of genetic material between highly homologous DNA molecules. This process is critical for the error-free repair of DNA double-strand breaks (DSBs) and for the proper segregation of chromosomes during meiosis. In somatic cells, homologous recombination (HR) uses a homologous sister chromatid or homologous chromosome as a template to accurately restore the DNA sequence at the break site, thereby maintaining genomic integrity. The importance of HR is underscored by the fact that its dysfunction leads to homologous recombination deficiency (HRD), a condition associated with increased mutational burden, genomic instability, and predisposition to various cancers. Understanding the molecular mechanisms, regulation, and clinical implications of HR is therefore essential for researchers in cancer biology, genetics, and therapeutic development.
homologous recombination At A Glance
| GO ID | GO:0035825 |
|---|---|
| GO term | homologous recombination |
| Ontology | biological_process |
| Synonym | chromosomal crossover, interchromosomal DNA recombination, interstrand DNA recombination, reciprocal DNA recombination |
| Major function | Exchange of equal amounts of genetic material between highly homologous DNA molecules |
| Related processes | DNA double-strand break repair, meiosis, chromosome segregation |
| Clinical relevance | Homologous recombination deficiency (HRD) in hereditary cancers and therapeutic response |
| Key proteins | BRCA1, BRCA2, RAD51, PALB2, MRN complex |
What Is GO:0035825?
According to the Gene Ontology, GO:0035825 homologous recombination is defined as a DNA recombination process that results in the exchange of an equal amount of genetic material between highly homologous DNA molecules. This definition encompasses the reciprocal exchange of genetic information between homologous sequences, which can occur during meiosis (chromosomal crossover) or during the repair of DNA double-strand breaks in somatic cells. The process is characterized by the use of a homologous DNA template to guide the accurate repair or exchange of genetic material, distinguishing it from non-homologous end joining and other error-prone repair pathways.
Why Is homologous recombination Important in Cell Biology?
Homologous recombination is a cornerstone of genome maintenance and heredity. It ensures the accurate repair of DNA double-strand breaks, which are among the most lethal forms of DNA damage, and it is required for the proper segregation of homologous chromosomes during meiosis. Defects in HR lead to genomic instability, a hallmark of cancer, and are causally linked to hereditary breast and ovarian cancer syndromes, as well as other malignancies. Moreover, HR status predicts response to specific cancer therapies, such as PARP inhibitors and platinum-based chemotherapy, making it a critical biomarker in clinical oncology. Research into HR mechanisms continues to reveal new therapeutic targets and strategies for overcoming resistance.
• Maintains genomic integrity by error-free repair of DNA double-strand breaks.
• Essential for meiotic recombination and proper chromosome segregation.
• Deficiency causes homologous recombination deficiency (HRD), a common feature in breast, ovarian, pancreatic, and biliary tract cancers.
• HRD is a predictive biomarker for platinum salts and PARP inhibitor sensitivity.
• Germline mutations in HR genes (e.g., BRCA1, BRCA2, PALB2) confer hereditary cancer predisposition.
• HR status influences tumor mutational signatures and neoantigen load.
• Understanding HR mechanisms informs development of targeted therapies and resistance strategies.
• Single-molecule imaging reveals dynamic HR intermediate steps, advancing mechanistic knowledge.
• HR defects are being explored as targets for synthetic lethality approaches.
• HR research spans cancer biology, genetics, and molecular imaging.
What Happens During homologous recombination?
Initiation and resection of DNA double-strand breaks
In simple terms: The process starts when a DNA double-strand break is detected and the broken ends are chewed back to create single-stranded DNA tails.
Homologous recombination is initiated by the detection of a DNA double-strand break (DSB), which can be caused by ionizing radiation, replication stress, or chemotherapeutic agents. The MRN complex (MRE11-RAD50-NBS1) recognizes the break and recruits ATM kinase, leading to the activation of the DNA damage response. Subsequently, the broken DNA ends undergo 5' to 3' resection, generating 3' single-stranded DNA (ssDNA) overhangs. This resection is mediated by the MRN complex and CtIP, followed by extensive resection by EXO1 and BLM helicase. The resulting ssDNA is rapidly coated by replication protein A (RPA), which removes secondary structures and protects the ssDNA.
Formation of the RAD51 nucleoprotein filament
In simple terms: The single-stranded DNA tails are coated with RAD51, forming a filament that can search for a matching DNA sequence.
The RPA-coated ssDNA is then bound by BRCA2, which facilitates the replacement of RPA with RAD51, forming a helical nucleoprotein filament. This RAD51 filament is the active species that performs homology search and strand invasion. BRCA1, in complex with BARD1, promotes the resection and the formation of the RAD51 filament by counteracting the anti-recombinogenic factors 53BP1 and RIF1. PALB2 acts as a bridge between BRCA1 and BRCA2, stabilizing the complex at the break site. The RAD51 filament then searches for a homologous DNA sequence, typically on a sister chromatid or homologous chromosome, and catalyzes strand invasion, forming a D-loop structure.
Strand invasion, DNA synthesis, and resolution
In simple terms: The RAD51 filament invades the matching DNA, uses it as a template to copy the missing information, and then the DNA strands are cut and rejoined.
Once the RAD51 filament invades the homologous duplex, it forms a D-loop, and DNA synthesis extends the invading strand using the homologous template. This synthesis can proceed via several subpathways, including synthesis-dependent strand annealing (SDSA), which leads to non-crossover gene conversion, or the double Holliday junction (dHJ) pathway, which can result in crossovers. The resolution of Holliday junctions involves structure-specific endonucleases such as GEN1 and SLX4-SLX1, or the BLM-TOPIIIα-RMI1-RMI2 complex, which dissolves the dHJ to produce non-crossover products. The final steps restore the original DNA sequence, ensuring error-free repair.
Meiotic recombination and crossover formation
In simple terms: During meiosis, homologous recombination creates crossovers between maternal and paternal chromosomes, ensuring genetic diversity and proper chromosome segregation.
In meiosis, homologous recombination is programmed to generate crossovers between homologous chromosomes, which are essential for their proper segregation and for generating genetic diversity. Meiotic recombination is initiated by SPO11-induced DSBs, followed by resection and strand invasion mediated by the meiosis-specific RAD51 paralogs DMC1 and RAD51. A subset of these events is resolved as crossovers, which are marked by MLH1 foci and are subject to crossover interference and homeostasis. Defects in meiotic HR lead to aneuploidy and infertility.
Regulation of homologous recombination choice
In simple terms: Cells decide whether to use homologous recombination or other repair pathways based on the cell cycle stage and the presence of specific proteins.
The choice between HR and non-homologous end joining (NHEJ) is tightly regulated. HR is favored in the S/G2 phases of the cell cycle when a sister chromatid is available, while NHEJ predominates in G1. This cell-cycle regulation is mediated by cyclin-dependent kinases (CDKs), which phosphorylate key HR factors such as CtIP and BRCA1. The antagonistic relationship between BRCA1 and 53BP1 is central to this decision: BRCA1 promotes resection and HR, whereas 53BP1 inhibits resection and promotes NHEJ. Additionally, the availability of BRCA2 and RAD51, and the presence of anti-recombinogenic factors like HELB and RECQL5, further modulate HR activity.
Key Genes Involved in GO:0035825 homologous recombination
The following genes encode core components of the homologous recombination machinery, and their mutations or dysregulation are frequently implicated in cancer and hereditary diseases.
| Gene | Major Role | Research Relevance |
|---|---|---|
| BRCA1 | Promotes end resection and RAD51 filament formation; antagonizes 53BP1 | Germline mutations cause hereditary breast/ovarian cancer; key HRD biomarker |
| BRCA2 | Mediates RAD51 loading onto ssDNA; stabilizes RAD51 filament | Mutations cause Fanconi anemia and hereditary breast/ovarian cancer; PARP inhibitor target |
| RAD51 | Forms nucleoprotein filament; catalyzes strand invasion | Overexpression linked to chemoresistance; target for HRD therapies |
| PALB2 | Bridges BRCA1 and BRCA2; facilitates RAD51 loading | Mutations confer hereditary breast/pancreatic cancer risk |
| MRE11 | Part of MRN complex; initiates DSB resection | Mutations cause ataxia-telangiectasia-like disorder; HRD biomarker |
| RAD50 | Part of MRN complex; DNA binding and nuclease stimulation | Mutations linked to Nijmegen breakage syndrome-like disorder |
| NBS1 (NBN) | Part of MRN complex; recruits ATM and regulates resection | Mutations cause Nijmegen breakage syndrome; HRD |
| CtIP (RBBP8) | Promotes end resection; regulated by CDKs | Overexpression associated with HR proficiency; target for sensitization |
| EXO1 | Executes long-range resection of DSB ends | Loss impairs HR; potential biomarker for PARP inhibitor response |
| BLM | Helicase involved in resection and dHJ dissolution | Mutations cause Bloom syndrome; HRD and cancer predisposition |
| ATM | Kinase that activates DNA damage response and HR | Mutations cause ataxia-telangiectasia; predicts radiotherapy response |
| BARD1 | Partners with BRCA1 to promote HR | Mutations linked to breast cancer susceptibility |
| RAD51C | RAD51 paralog; facilitates filament formation | Mutations cause Fanconi anemia and hereditary breast/ovarian cancer |
| RAD51D | RAD51 paralog; involved in strand invasion | Mutations associated with ovarian cancer risk |
| BRIP1 (FANCJ) | Helicase that interacts with BRCA1; promotes HR | Mutations cause Fanconi anemia and ovarian cancer predisposition |
| GEN1 | Holliday junction resolvase | Loss leads to HR defects and genome instability |
| SLX4 (FANCP) | Scaffold for structure-specific endonucleases | Mutations cause Fanconi anemia; HRD |
| DMC1 | Meiosis-specific recombinase; forms filaments with RAD51 | Essential for meiotic recombination; mutations cause infertility |
How Is homologous recombination Regulated?
Homologous recombination is regulated at multiple levels, including cell-cycle-dependent phosphorylation, ubiquitination, and transcriptional control. CDK-mediated phosphorylation of CtIP and BRCA1 is required for timely HR activation in S/G2 phases. The antagonistic interplay between BRCA1 and 53BP1 determines the choice between HR and NHEJ. Additionally, the ubiquitin ligases RNF8 and RNF168, and the deubiquitinase USP48, modulate the recruitment of HR factors to damage sites. Post-translational modifications of RAD51, such as phosphorylation by CHK1, regulate its activity and filament stability. In cancer, HR proficiency can be restored by secondary mutations in BRCA1/2 or by overexpression of RAD51, leading to therapy resistance. Understanding these regulatory mechanisms is crucial for developing strategies to overcome HRD and resistance.
homologous recombination and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| BRCA1 | Hereditary breast and ovarian cancer; HRD | BRCA1 knockout cell lines (e.g., U2OS, MCF10A) for HR assays |
| BRCA2 | Hereditary breast and ovarian cancer; Fanconi anemia | BRCA2 knockout or point-mutant models to study RAD51 loading |
| PALB2 | Hereditary breast and pancreatic cancer | PALB2 knockout cells to assess HR efficiency and drug sensitivity |
| RAD51C | Hereditary ovarian cancer; Fanconi anemia | RAD51C knockout models for synthetic lethality screens |
| MRE11 | Ataxia-telangiectasia-like disorder; HRD | MRE11 hypomorphic knock-in models to study resection |
Homologous recombination deficiency in hereditary cancers
Germline mutations in HR genes such as BRCA1, BRCA2, PALB2, RAD51C, and RAD51D cause hereditary breast and ovarian cancer syndromes, as well as increased risk for pancreatic and prostate cancers. These mutations lead to homologous recombination deficiency (HRD), characterized by genomic instability and a distinct mutational signature. HRD tumors are sensitive to platinum-based chemotherapy and PARP inhibitors, which exploit the inability to repair DNA damage. However, resistance can emerge through secondary mutations that restore HR function.
HRD in triple-negative breast cancer and ovarian cancer
Triple-negative breast cancer (TNBC) and high-grade serous ovarian cancer frequently exhibit HRD, even in the absence of BRCA1/2 mutations, through epigenetic silencing or mutations in other HR genes. HRD status is a predictive biomarker for PARP inhibitor response in these cancers. Clinical assays for HRD include genomic scar scores and functional assays, but standardization remains a challenge. Research into HRD mechanisms in TNBC aims to identify new therapeutic targets and overcome resistance.
HRD in biliary tract cancer and other malignancies
Homologous recombination deficiency has been observed in biliary tract cancer, where germline and somatic mutations in HR genes are associated with improved response to platinum-based therapy. Similarly, HRD is being investigated in pancreatic, prostate, and gastric cancers. The prevalence of HRD varies by tumor type, and its detection can guide treatment decisions. Ongoing studies aim to define the optimal HRD biomarkers and therapeutic strategies for these cancers.
Therapeutic implications and synthetic lethality
The concept of synthetic lethality exploits HRD by targeting backup DNA repair pathways, such as poly(ADP-ribose) polymerase (PARP). PARP inhibitors have shown clinical benefit in HRD-positive breast, ovarian, pancreatic, and prostate cancers. However, resistance mechanisms, including restoration of HR and drug efflux, limit long-term efficacy. Combination therapies and novel agents targeting HR-associated proteins are under active investigation.
From homologous recombination-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of a candidate gene cause HRD? | Knockout cell lines (e.g., CRISPR-Cas9) followed by HR reporter assays |
| Does a specific point mutation affect HR function? | Point-mutation knock-in models (e.g., BRCA1 missense variants) |
| Can a gene fusion or isoform restore HR? | Knock-in of tagged or mutant alleles (e.g., RAD51 variants) |
| Does overexpression of a gene induce chemoresistance? | Overexpression models (e.g., RAD51 overexpression) |
| What is the role of a gene in meiotic recombination? | Germline knockout or conditional knock-in mouse models |
| Can we identify synthetic lethal partners of HR genes? | CRISPR library screening in HRD backgrounds |
How to Study the homologous recombination Process
| Method | What It Measures | Typical Application |
|---|---|---|
| DR-GFP reporter assay | HR repair efficiency of a specific DSB | Assessing HR capacity in knockout or mutant cells |
| RAD51 foci immunofluorescence | Formation of RAD51 nucleoprotein filaments at damage sites | Diagnosing HRD in tumor samples and cell lines |
| Genomic scar scores (e.g., myChoice) | Genomic instability patterns associated with HRD | Clinical stratification for PARP inhibitor therapy |
| Single-molecule TIRF microscopy | Real-time dynamics of RAD51 filament and strand invasion | Mechanistic studies of HR intermediates |
| CRISPR library screening | Identification of genes that modulate HR or synthetic lethality | Discovery of new HR regulators and drug targets |
| Whole-exome sequencing | Mutations in HR genes and mutational signatures | Hereditary cancer predisposition testing |
| Phosphoproteomics | DNA damage-induced phosphorylation events | Mapping signaling pathways that regulate HR |
| Immunoprecipitation-mass spectrometry | Protein interactions of HR complexes | Identifying novel HR components and regulators |
Functional assays for homologous recombination
The most direct way to measure HR activity is using reporter assays, such as the DR-GFP or EJ5-GFP systems, which quantify HR or NHEJ repair of a specific DSB induced by I-SceI. These assays can be performed in cell lines with knockout or knockdown of candidate genes to assess their role in HR. Additionally, the formation of RAD51 foci after DNA damage (e.g., ionizing radiation or olaparib) is a widely used marker of HR proficiency. Immunofluorescence for RAD51 and other HR proteins allows visualization of their recruitment to damage sites.
Genomic and transcriptomic profiling of HRD
HRD can be assessed through genomic scar scores, such as the myChoice HRD test or the COSMIC mutational signature 3, which reflect the characteristic patterns of genomic instability caused by HR deficiency. Whole-exome or targeted sequencing can identify mutations in HR genes, while RNA sequencing can reveal aberrant splicing or expression of HR factors. These methods are used in clinical trials to stratify patients for PARP inhibitor therapy.
Single-molecule imaging of HR intermediates
Single-molecule fluorescence microscopy techniques, such as total internal reflection fluorescence (TIRF) microscopy, allow real-time visualization of RAD51 filament formation, strand invasion, and D-loop dynamics. These methods provide mechanistic insights into HR at the molecular level and can reveal how mutations affect the kinetics of HR. They are particularly useful for studying the action of anti-recombinogenic factors and drug effects on HR.
Proteomic and interactomic approaches
Mass spectrometry-based proteomics can identify protein-protein interactions and post-translational modifications of HR factors. For example, affinity purification of BRCA1 or RAD51 complexes followed by mass spectrometry has revealed new components and regulators. Phosphoproteomics can map DNA damage-induced signaling events that control HR. These approaches complement genetic and cell biology studies to build a comprehensive understanding of HR regulation.
How CRISPR Can Be Used to Study GO:0035825 homologous recombination
Knockout
CRISPR-Cas9 knockout of HR genes (e.g., BRCA1, BRCA2, RAD51) is widely used to create isogenic cell models of HRD. These models are invaluable for studying the cellular consequences of HR loss, including sensitivity to PARP inhibitors and platinum drugs. Knockout of HR genes in cancer cell lines can also reveal synthetic lethal interactions and resistance mechanisms.
Point Mutation
Point mutations in HR genes, such as BRCA1 missense variants, can be introduced using CRISPR base editing or homology-directed repair (HDR) with donor templates. These models help determine whether specific variants are pathogenic or benign, which is critical for clinical interpretation. Point-mutant knock-in models also allow structure-function studies of HR proteins.
Knock-in
Knock-in of tagged alleles (e.g., GFP-RAD51 or AID-BRCA2) enables live-cell imaging and rapid degradation of HR proteins to study their dynamics. Knock-in of patient-derived mutations or fusion genes can recapitulate disease phenotypes and test drug responses. CRISPR-mediated knock-in is also used to create reporter cell lines for HR assays.
Overexpression
Overexpression of HR genes, such as RAD51, is associated with chemoresistance and poor prognosis in several cancers. CRISPR activation (CRISPRa) or lentiviral overexpression can model these conditions to study resistance mechanisms and test sensitizing agents. Overexpression models are also useful for biochemical purification of HR complexes.
How EDITGENE Supports homologous recombination Research
Researchers studying homologous recombination-related genes often need to determine whether a candidate gene is causally involved in HR, how specific mutations affect protein function, and whether targeting the gene can sensitize cancer cells to therapy. Generating precise, isogenic cell models is essential for such functional studies. EDITGENE provides a comprehensive suite of CRISPR-based services to accelerate HR research.
Contact EDITGENE today to design your custom CRISPR model for homologous recombination research.
Frequently Asked Questions About homologous recombination
What is GO:0035825 homologous recombination?
GO:0035825 homologous recombination is a biological process defined by the Gene Ontology as a DNA recombination process that results in the exchange of an equal amount of genetic material between highly homologous DNA molecules. It is essential for DNA double-strand break repair and meiosis.
What genes are involved in homologous recombination?
Key genes include BRCA1, BRCA2, RAD51, PALB2, RAD51C, RAD51D, MRE11, RAD50, NBS1, and others that encode proteins functioning in the HR pathway.
How is homologous recombination deficiency (HRD) detected?
HRD can be detected through genomic scar scores, mutational signatures (e.g., COSMIC signature 3), RAD51 foci assays, and sequencing of HR genes.
What cancers are associated with homologous recombination deficiency?
HRD is common in breast, ovarian, pancreatic, prostate, and biliary tract cancers, among others.
Why is homologous recombination important for cancer treatment?
HRD makes cancer cells sensitive to PARP inhibitors and platinum-based chemotherapy, and HR status is used to guide treatment decisions.
What is the role of BRCA1 and BRCA2 in homologous recombination?
BRCA1 promotes end resection and antagonizes 53BP1, while BRCA2 mediates RAD51 loading onto single-stranded DNA, both being critical for HR.
How can CRISPR be used to study homologous recombination?
CRISPR can create knockout, point mutation, knock-in, and overexpression models of HR genes to study their function, drug response, and synthetic lethality.
What are the main steps of homologous recombination?
The main steps are initiation and resection of DNA double-strand breaks, formation of the RAD51 nucleoprotein filament, strand invasion and DNA synthesis, and resolution of recombination intermediates.
What is the difference between homologous recombination and non-homologous end joining?
Homologous recombination uses a homologous template for error-free repair, while non-homologous end joining directly ligates broken ends and is error-prone.
Can homologous recombination deficiency be inherited?
Yes, germline mutations in HR genes such as BRCA1 and BRCA2 cause hereditary cancer predisposition syndromes.
Conclusion
Homologous recombination (GO:0035825) is a vital biological process that safeguards genome stability and drives genetic diversity. Its dysfunction, termed homologous recombination deficiency, is a key driver of cancer predisposition and a predictive biomarker for targeted therapies. Continued research into the molecular mechanisms, regulation, and clinical implications of HR is essential for developing new treatments and overcoming resistance. EDITGENE's CRISPR-based services provide powerful tools to dissect HR gene function and accelerate translational discoveries.
References
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- 3. Yamamoto H et al.. 2021. Homologous Recombination Deficiencies and Hereditary Tumors.. Int J Mol Sci 23(1) PMID: 35008774
- 4. Gibbs DR et al.. 2019. Homologous Recombination under the Single-Molecule Fluorescence Microscope.. Int J Mol Sci 20(23) PMID: 31816946
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- 6. Belli C et al.. 2019. Homologous recombination deficiency in triple negative breast cancer.. Breast 45:15-21 PMID: 30818144
- 7. Piazza A et al.. 2019. Homologous Recombination and the Formation of Complex Genomic Rearrangements.. Trends Cell Biol 29(2):135-149 PMID: 30497856
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