GO:0045003 double-strand break repair via synthesis-dependent strand annealing: Mechanism, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0045003 (synthesis-dependent strand annealing, SDSA) is a mitotic double-strand break (DSB) repair pathway that achieves error-free repair without exchange of adjacent sequences.
• SDSA is mechanistically distinct from double Holliday junction (dHJ) resolution and is the predominant mitotic gene conversion pathway in many organisms.
• Key proteins include RAD51, RECQ5, MCM10, and RAD-51 paralogs, which mediate strand invasion, helicase-driven displacement, and annealing.
• SDSA is conserved from yeast to plants and humans, with organism-specific variations in pathway usage.
• Defects in SDSA are linked to genome instability, cancer predisposition, and impaired homologous recombination repair.
• CRISPR-based models (knockout, point mutation, knock-in, overexpression) enable functional dissection of SDSA genes in disease and basic research.
Description
Double-strand breaks (DSBs) are among the most cytotoxic DNA lesions, and cells repair them through multiple pathways. One major error-free mechanism is synthesis-dependent strand annealing (SDSA), annotated as GO:0045003. SDSA is a mitotic gene conversion pathway that repairs DSBs without exchanging flanking sequences, making it critical for genome stability. Unlike repair via double Holliday junctions, SDSA avoids crossovers and is therefore preferred in somatic cells to prevent loss of heterozygosity. The process begins with resection of the broken DNA ends, followed by strand invasion into a homologous template, DNA synthesis, and displacement of the newly synthesized strand, which then anneals with the other side of the break. This mechanism is conserved across eukaryotes, from Drosophila to plants and humans, though pathway usage varies by organism and cell cycle stage. Researchers study SDSA to understand genome maintenance, cancer biology, and the mechanisms of homologous recombination. Defects in SDSA proteins such as RECQ5 and RAD51 lead to impaired DSB repair and increased genomic instability. Moreover, SDSA is essential for accurate repair in mitotic cells, and its dysregulation is implicated in cancer and developmental disorders. This article provides a comprehensive overview of GO:0045003, covering its definition, molecular mechanism, key genes, disease relevance, and experimental approaches including CRISPR-based models.
double-strand break repair via synthesis-dependent strand annealing At A Glance
| GO ID | GO:0045003 |
|---|---|
| GO term | double-strand break repair via synthesis-dependent strand annealing |
| Ontology | biological_process |
| Synonym | mitotic gene conversion, SDSA |
| Major function | Error-free repair of DNA double-strand breaks without exchange of adjacent sequences |
| Organism conservation | Conserved from yeast to humans; studied in Drosophila, plants, and mammals |
| Key proteins | RAD51, RECQ5, MCM10, RAD-51 paralogs |
| Pathway distinction | Distinct from double Holliday junction resolution; avoids crossovers |
| Disease relevance | Genome instability, cancer predisposition |
What Is GO:0045003?
GO:0045003, double-strand break repair via synthesis-dependent strand annealing (SDSA), is a biological process that repairs DNA double-strand breaks in an error-free manner without exchanging adjacent sequences. The broken DNA ends are processed to expose 3' single-stranded tails, one of which invades a homologous intact chromosome. DNA synthesis then extends the invading strand using the intact chromosome as a template. The newly synthesized strand is displaced and anneals with complementary sequences on the other side of the break, followed by fill-in synthesis and ligation. This process is a major mechanism of mitotic gene conversion and is also known as mitotic gene conversion.
Why Is double-strand break repair via synthesis-dependent strand annealing Important in Cell Biology?
SDSA is a central mechanism for maintaining genome integrity during mitosis. By repairing DSBs without crossovers, it prevents loss of heterozygosity and chromosomal rearrangements, which are hallmarks of cancer and genetic disorders. Understanding SDSA is therefore crucial for cancer research, gene editing, and the development of therapeutic strategies targeting DNA repair.
• SDSA is the predominant error-free DSB repair pathway in mitotic cells, preventing mutations and chromosomal instability.
• It avoids crossover formation, thus preserving genome organization and preventing loss of heterozygosity.
• Defects in SDSA proteins such as RECQ5 lead to impaired DSB repair and increased sensitivity to DNA-damaging agents.
• SDSA is conserved across eukaryotes, making model organisms valuable for mechanistic studies.
• Dysregulation of SDSA is implicated in cancer predisposition and neurodegenerative disorders.
• SDSA is critical for homologous recombination-based gene editing outcomes.
• Studying SDSA helps understand pathway choice between error-free and error-prone repair.
• SDSA proteins are potential targets for cancer therapy and chemosensitization.
• SDSA facilitates accurate repair during DNA replication stress.
• SDSA mechanisms inform the design of CRISPR-based knock-in strategies.
What Happens During double-strand break repair via synthesis-dependent strand annealing?
Initiation and End Resection
In simple terms: The broken DNA ends are chewed back to create single-stranded tails.
Upon DSB formation, the MRN complex and CtIP initiate short-range resection, followed by extensive resection by EXO1 and BLM to generate 3' single-stranded DNA (ssDNA) tails. These tails are coated by RPA, which is subsequently replaced by RAD51 to form a nucleoprotein filament. This step is essential for homology search and strand invasion.
Strand Invasion and Homology Search
In simple terms: The single-stranded tail invades a matching sequence on an intact chromosome.
The RAD51-ssDNA filament searches for a homologous sequence and invades the intact duplex, forming a displacement loop (D-loop). This process is facilitated by RAD51 paralogs and accessory factors. In Drosophila, RAD-51 separation-of-function alleles reveal distinct roles in SDSA versus double Holliday junction pathways. The invading 3' end is then extended by DNA polymerase using the intact strand as a template.
DNA Synthesis and Displacement
In simple terms: New DNA is made using the intact chromosome as a template, then the new strand is released.
DNA synthesis extends the invading strand, copying the homologous template. The newly synthesized strand is then displaced from the template by helicases such as RECQ5, which promotes SDSA by unwinding the D-loop and preventing double Holliday junction formation. In Arabidopsis, AtMCM10 facilitates SDSA-mediated intermolecular homologous recombination repair via liquid-liquid phase separation.
Annealing and Ligation
In simple terms: The displaced new strand pairs with the other side of the break and is sealed.
The displaced strand anneals with complementary sequences on the other side of the DSB, followed by fill-in synthesis and ligation. This step completes repair without crossovers. In Drosophila, SDSA is a major pathway for gene conversion, and its usage is differentially regulated compared to other DSB repair pathways. In plants, SDSA operates in somatic cells and has evolutionary consequences.
Key Genes Involved in GO:0045003 double-strand break repair via synthesis-dependent strand annealing
The following genes and proteins are central to the execution and regulation of SDSA, based on experimental evidence from model organisms and human cells.
| Gene | Major Role | Research Relevance |
|---|---|---|
| RAD51 | Forms nucleoprotein filament for strand invasion | Core recombinase; knockout is lethal in vertebrates |
| RECQ5 | Helicase that promotes SDSA by disrupting D-loops | Knockout leads to impaired DSB repair and genomic instability |
| MCM10 | Facilitates SDSA via liquid-liquid phase separation | Plant homolog AtMCM10; involved in intermolecular HR |
| RAD-51 | Drosophila RAD51 homolog; separation-of-function alleles | Distinguishes SDSA from dHJ pathways |
| RAD54 | Stimulates RAD51-mediated D-loop formation | Conserved in yeast and humans; supports SDSA |
| RPA | Binds ssDNA and removes secondary structures | Essential for resection and strand invasion |
| BLM | Helicase involved in resection and D-loop disruption | Mutations cause Bloom syndrome; affects SDSA |
| EXO1 | Exonuclease for long-range end resection | Required for SDSA initiation |
| CtIP | Initiates end resection | Regulates pathway choice |
| BRCA2 | Mediates RAD51 loading onto ssDNA | Defects cause cancer predisposition |
| PALB2 | Links BRCA2 to RAD51 | Fanconi anemia and cancer |
| RAD52 | Mediates single-strand annealing and SDSA | Backup pathway in BRCA-deficient cells |
| RAD51C | RAD51 paralog; involved in filament stability | Mutations cause Fanconi anemia |
| RAD51D | RAD51 paralog; involved in SDSA | Cancer susceptibility |
| XRCC2 | RAD51 paralog; promotes SDSA | Defects lead to HR deficiency |
| XRCC3 | RAD51 paralog; promotes SDSA | Defects lead to HR deficiency |
| SRS2 | Yeast helicase that disrupts D-loops | Regulates SDSA vs. dHJ |
| MUS81 | Structure-specific nuclease | Resolves recombination intermediates |
How Is double-strand break repair via synthesis-dependent strand annealing Regulated?
SDSA is regulated at multiple levels. Cell cycle stage controls pathway choice, with SDSA predominant in S/G2 phases when a sister chromatid is available. Post-translational modifications of RAD51 and its paralogs modulate filament stability and invasion. Helicases such as RECQ5 and SRS2 actively disrupt D-loops to favor SDSA over double Holliday junction formation. In plants, AtMCM10 undergoes liquid-liquid phase separation to facilitate SDSA. Additionally, the DNA damage response kinases ATM and ATR coordinate resection and repair factor recruitment.
double-strand break repair via synthesis-dependent strand annealing and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| RECQ5 | Cancer, genomic instability | Knockout cell lines, xenografts |
| RAD51C | Fanconi anemia, breast/ovarian cancer | Patient-derived iPSCs, knock-in mutations |
| BRCA2 | Hereditary breast/ovarian cancer | Conditional knockout mouse models |
| RAD51D | Ovarian cancer | CRISPR knockout organoids |
| MCM10 | Plant development, genome stability | Arabidopsis knockout and overexpression |
Cancer Predisposition and Genome Instability
Defects in SDSA proteins lead to impaired DSB repair and genomic instability, a hallmark of cancer. Mutations in RAD51 paralogs (RAD51C, RAD51D, XRCC2, XRCC3) are associated with breast and ovarian cancer susceptibility. RECQ5 deficiency impairs SDSA and increases sensitivity to DNA-damaging agents, suggesting a tumor-suppressive role. BRCA2 and PALB2, which load RAD51, are well-known cancer predisposition genes.
Fanconi Anemia and Bone Marrow Failure
Biallelic mutations in RAD51C and other SDSA-related genes cause Fanconi anemia, a disorder characterized by bone marrow failure, developmental abnormalities, and cancer predisposition. These defects highlight the importance of SDSA in maintaining hematopoietic stem cell genome integrity.
Neurodegeneration and Aging
Impaired DSB repair, including SDSA, contributes to neuronal loss in neurodegenerative diseases such as ataxia telangiectasia and Alzheimer's disease. Oxidative DNA damage in post-mitotic neurons requires efficient repair, and SDSA defects may accelerate aging.
From double-strand break repair via synthesis-dependent strand annealing-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does gene X promote SDSA? | Knockout cell line + I-SceI DSB repair assay |
| Does a point mutation in RAD51 affect SDSA? | Point-mutation knock-in via CRISPR |
| How does RECQ5 helicase activity regulate SDSA? | Overexpression of wild-type vs. helicase-dead RECQ5 |
| What is the role of MCM10 phase separation in SDSA? | Knock-in of phase-separation-deficient MCM10 |
| How does SDSA pathway choice change in cancer? | Patient-derived organoids with defined mutations |
| Does SDSA prevent crossovers in mitosis? | Drosophila SDSA reporter assays |
How to Study the double-strand break repair via synthesis-dependent strand annealing Process
| Method | What It Measures | Typical Application |
|---|---|---|
| I-SceI reporter assay | SDSA frequency and gene conversion | Pathway choice in knockout cells |
| Fluorescence microscopy | Repair foci formation and dynamics | Live-cell imaging of RAD51 |
| In vitro D-loop assay | Strand invasion and displacement | Biochemical reconstitution |
| CRISPR screen | Identification of SDSA genes | Genome-wide knockout libraries |
| RNA-seq | Transcriptional changes after DSB | Differential gene expression |
| Proteomics | Protein interactions and modifications | RAD51 interactome |
| ChIP-seq | Binding sites of repair proteins | RAD51 and RECQ5 localization |
| Phase separation assays | Liquid-liquid phase separation | MCM10 condensate formation |
DSB Repair Reporter Assays
The I-SceI endonuclease-based reporter system is widely used to measure SDSA frequency and gene conversion tract length. In Drosophila, direct repeat GFP reporters allow quantification of SDSA versus other pathways. These assays are essential for determining pathway choice and the impact of gene knockouts.
Live-Cell Imaging of Repair Factors
Fluorescent tagging of RAD51, RECQ5, and MCM10 enables real-time visualization of repair foci and D-loop dynamics. Super-resolution microscopy can reveal phase-separated condensates of MCM10 during SDSA. This method provides spatial and temporal insights into SDSA.
Biochemical Reconstitution
In vitro assays with purified proteins (RAD51, RPA, RECQ5, DNA polymerase) reconstitute key SDSA steps, including D-loop formation, strand displacement, and annealing. These experiments define minimal requirements and kinetic parameters.
Genomic and Proteomic Profiling
CRISPR screens and proteomics identify novel SDSA factors and their interaction networks. RNA-seq after DSB induction reveals transcriptional responses, while ChIP-seq maps repair factor binding.
How CRISPR Can Be Used to Study GO:0045003 double-strand break repair via synthesis-dependent strand annealing
Knockout
CRISPR knockout of SDSA genes (e.g., RECQ5, RAD51 paralogs) in cell lines enables loss-of-function studies. These models reveal essential roles in DSB repair and sensitivity to DNA-damaging agents. Knockout of RAD51 is lethal, so conditional or inducible systems are used.
Point Mutation
Point mutations in SDSA genes can dissect specific domains, such as the helicase activity of RECQ5 or the separation-of-function alleles of RAD-51. CRISPR-mediated knock-in of these mutations allows precise functional analysis without confounding effects of complete loss.
Knock-in
Knock-in of tagged versions (e.g., GFP-RAD51, mCherry-RECQ5) enables live-cell imaging and proteomic pull-downs. Knock-in of patient-derived mutations (e.g., RAD51C) creates disease models for drug testing.
Overexpression
Overexpression of wild-type or mutant SDSA proteins (e.g., RECQ5, MCM10) can test gain-of-function effects and dominant-negative activities. This approach is useful for studying phase separation and D-loop disruption.
How EDITGENE Supports double-strand break repair via synthesis-dependent strand annealing Research
Researchers studying double-strand break repair via synthesis-dependent strand annealing-related genes often need to determine whether a candidate gene is causally involved in SDSA, and to dissect its molecular function. EDITGENE provides comprehensive CRISPR-based services to accelerate this research.
Contact EDITGENE today to design your custom CRISPR model for double-strand break repair via synthesis-dependent strand annealing research.
Frequently Asked Questions About double-strand break repair via synthesis-dependent strand annealing
What is GO:0045003?
GO:0045003 is the Gene Ontology term for double-strand break repair via synthesis-dependent strand annealing (SDSA), a mitotic error-free DNA repair pathway that avoids crossovers.
What genes are involved in double-strand break repair via synthesis-dependent strand annealing?
Key genes include RAD51, RECQ5, MCM10, RAD54, RPA, BLM, and RAD51 paralogs such as RAD51C and RAD51D.
How does SDSA differ from double Holliday junction repair?
SDSA displaces the newly synthesized strand and anneals it without forming a double Holliday junction, thus avoiding crossovers, whereas dHJ resolution can produce crossovers.
Why is SDSA important for genome stability?
SDSA repairs DSBs accurately without exchanging flanking sequences, preventing loss of heterozygosity and chromosomal rearrangements.
What diseases are associated with defective SDSA?
Defects in SDSA genes are linked to cancer predisposition, Fanconi anemia, and neurodegenerative disorders.
How can I study SDSA in the lab?
Common methods include I-SceI reporter assays, live-cell imaging of RAD51 foci, and CRISPR knockout of SDSA genes.
What is the role of RECQ5 in SDSA?
RECQ5 is a helicase that promotes SDSA by disrupting D-loops, preventing double Holliday junction formation.
How does MCM10 facilitate SDSA?
MCM10 undergoes liquid-liquid phase separation to facilitate SDSA-mediated intermolecular homologous recombination repair.
Can CRISPR be used to model SDSA defects?
Yes, CRISPR knockout, point mutation, and knock-in models are widely used to dissect SDSA gene functions.
What model organisms are used to study SDSA?
Drosophila, Arabidopsis, yeast, and human cell lines are commonly used, each offering unique genetic tools.
Conclusion
GO:0045003, double-strand break repair via synthesis-dependent strand annealing, is a fundamental error-free DNA repair pathway that safeguards genome integrity in mitotic cells. Its mechanism, conserved across eukaryotes, involves strand invasion, DNA synthesis, and annealing without crossovers. Defects in SDSA proteins are linked to cancer and other diseases, making it a critical area of research. EDITGENE provides comprehensive CRISPR services to study SDSA genes, from knockout to knock-in and screening, empowering researchers to uncover new therapeutic targets.
References
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