FAM35AFAM35/B Antibody

What is FAM35A and what are its known biological functions?

FAM35A (also known as SHLD2, RINN2, or Shieldin complex subunit 2) is a protein that plays a critical role in DNA double-strand break repair pathways. It belongs to the FAM35 family, with two isoforms of the human protein produced by alternative splicing . FAM35A functions as a novel effector of REV7 in the Non-Homologous End Joining (NHEJ) pathway and is involved in modulating DNA damage sensitivity in both normal and BRCA1-defective cells . The C-terminal half of FAM35A contains three OB-fold domains similar to those in the single-stranded DNA-binding protein RPA large subunit, while its N-terminal portion is disordered and contains sites for DNA damage-dependent post-translational modification . FAM35A acts downstream of 53BP1, RIF1, and REV7 to antagonize homologous recombination (HR) by limiting DNA end resection, thereby promoting NHEJ-mediated repair .

How does FAM35A relate to cancer research?

FAM35A has significant implications for cancer research based on several key findings. The gene is altered at a notably high frequency in prostate cancers (up to 13%) and is significantly less expressed in metastatic cases, positioning FAM35A as a potentially valuable cancer marker for therapeutic considerations . Interestingly, FAM35A has been found to be absent in HCC1937, a widely used BRCA1-mutant cancer cell line with anomalous resistance to PARP inhibitors . This suggests that FAM35A status may influence response to certain cancer therapies, particularly those targeting DNA repair pathways. The role of FAM35A in DNA repair pathway choice also makes it relevant to understanding cancer development and treatment resistance mechanisms.

What protein interactions have been identified for FAM35A?

FAM35A has been shown to interact with several key proteins involved in DNA damage response pathways:

These interactions place FAM35A in a network of proteins that coordinate double-strand break repair pathway choice, particularly in the context of antagonizing homologous recombination and promoting NHEJ.

What are the key specifications of commercially available FAM35AFAM35/B antibodies?

Commercial FAM35AFAM35/B antibodies have specific characteristics researchers should be aware of when planning experiments:

When selecting an antibody, researchers should consider the specific application needs and validation status for their experimental systems.

What methodological considerations should be taken when using FAM35AFAM35/B antibodies for Western blotting?

When using FAM35AFAM35/B antibodies for Western blotting, several methodological considerations should be addressed:

• Sample preparation: Given FAM35A's role in DNA damage response, consider treating cell cultures with DNA-damaging agents (such as mitomycin C at 100 ng/ml for 18-24h or etoposide) to enhance detection of damage-induced modifications .

• Controls: Include both positive controls (cell lines known to express FAM35A) and negative controls. HCC1937 cells have been documented to lack FAM35A expression and could serve as a useful negative control .

• Detection of isoforms: Be aware that FAM35A has two isoforms produced by alternative splicing . The antibody may detect one or both forms depending on the epitope location.

• Cross-reactivity considerations: The human genome contains three pseudogenes with high sequence identity (>98%) to FAM35A, two of them on chromosome 10q22 . Verify the antibody's specificity and potential cross-reactivity with these highly similar sequences.

• Signal validation: Validate signal specificity through siRNA knockdown of FAM35A. Complete knockout using nuclease-based methods is challenging due to the presence of pseudogenes with high sequence similarity .

How can FAM35AFAM35/B antibodies be utilized for investigating DNA damage response pathways?

FAM35AFAM35/B antibodies can be powerful tools for investigating DNA damage response pathways through several experimental approaches:

• Co-immunoprecipitation studies: FAM35A antibodies can be used to immunoprecipitate the protein and its interacting partners to study complex formation with REV7, RIF1, 53BP1, and other DNA repair factors. This approach has successfully identified novel interactions following DNA damage induction with agents like mitomycin C .

• Immunofluorescence for localization studies: Use immunofluorescence to track FAM35A recruitment to DNA damage sites. This can be performed in conjunction with markers for double-strand breaks (γH2AX), Fanconi anemia pathway (FANCD2), or homologous recombination (RAD51) .

• Chromatin immunoprecipitation: FAM35A antibodies can potentially be used for ChIP experiments to investigate its association with chromatin at sites of DNA damage.

• Analysis of post-translational modifications: Given that FAM35A has DNA damage-dependent post-translational modification sites in its N-terminal region , antibodies can be used to study these modifications in response to various DNA-damaging agents.

• Functional studies with domain-specific antibodies: Antibodies targeting specific domains (like the N-terminal NUMOD3 motif essential for recruitment to damage sites) can provide insights into domain-specific functions .

How does the absence or presence of FAM35A affect cellular responses to PARP inhibitors?

The relationship between FAM35A expression and PARP inhibitor sensitivity represents an important research area with therapeutic implications. FAM35A has been found to be absent in HCC1937, a BRCA1-mutant cancer cell line that displays unusual resistance to PARP inhibitors . This suggests that FAM35A status may influence PARP inhibitor sensitivity through several potential mechanisms:

• DNA repair pathway balance: FAM35A promotes NHEJ while limiting HR by restricting DNA end resection . Loss of FAM35A may shift repair pathway choice toward HR, potentially affecting cellular responses to PARP inhibitors that exploit synthetic lethality with HR defects.

• Relationship with BRCA1 deficiency: In BRCA1-deficient contexts, the absence of FAM35A may restore certain repair capabilities, potentially explaining the anomalous resistance to PARP inhibitors observed in HCC1937 cells .

• Potential biomarker value: The correlation between FAM35A status and PARP inhibitor response suggests FAM35A could serve as a biomarker for predicting therapy effectiveness in BRCA1-mutant cancers.

Researchers investigating PARP inhibitor resistance mechanisms should consider analyzing FAM35A expression and function as a potentially significant factor influencing treatment outcomes.

What are the methodological approaches for studying FAM35A's role in double-strand break repair pathway choice?

To investigate FAM35A's role in regulating double-strand break repair pathway choice, researchers can employ several methodological approaches:

• Plasmid integration assays: Use plasmid integration assays to measure NHEJ efficiency following FAM35A depletion or overexpression. Previous studies have demonstrated decreased plasmid integration ratios after FAM35A depletion, which can be restored by expressing FAM35A isoform 1 .

• EJ5-GFP reporter assays: These assays provide a quantitative measurement of NHEJ and have revealed that FAM35A depletion impairs NHEJ, with effects similar to REV7 depletion .

• Class switch recombination (CSR) analysis: In B-cell models like CH12F3-2 cells, CSR from IgM to IgA following cytokine stimulation (IL-4/TGF-β/anti-CD40) can be used to assess FAM35A's function in physiologically relevant NHEJ processes .

• Domain-specific mutant analysis: Generate domain-specific mutants (particularly targeting the N-terminal NUMOD3 motif and C-terminal OB-fold domains) to dissect the structural requirements for FAM35A's function in repair pathway choice .

• Resection marker analysis: Quantify resection markers to determine how FAM35A affects the extent of DNA end processing, which influences repair pathway choice.

• FokI system for localized damage: The FokI system can be employed to study the recruitment of FAM35A to DNA damage sites and the hierarchical dependencies with other factors like 53BP1, RIF1, and REV7 .

What is known about the complex formation between FAM35A, REV7, and C20orf196/SHDL1?

FAM35A forms part of a larger complex with REV7 and C20orf196/SHDL1 that collectively promotes NHEJ and limits HR . Understanding this complex is important for deciphering DNA repair regulation:

• Complex composition: Mass spectrometry studies have identified FAM35A as a top specific hit in REV7 immunoprecipitation experiments, along with previously identified REV7-associated proteins . Reciprocally, REV7 was identified in FAM35A immunoprecipitation experiments .

• Functional relationship: FAM35A and REV7 appear to function epistatically in the NHEJ pathway, as co-depletion of REV7 with FAM35A does not further reduce NHEJ efficiency compared to individual depletions .

• Recruitment hierarchy: FAM35A accumulates at double-strand breaks in a manner dependent on 53BP1, RIF1, and REV7, suggesting that these factors function upstream of FAM35A in the recruitment cascade .

• DNA damage dependency: The association between FAM35A and DNA repair proteins may be enhanced following DNA damage, as evidenced by the increased detection of RIF1 peptides and 53BP1 in FAM35A immunoprecipitation experiments following mitomycin C exposure .

• Complex function: The FAM35A-REV7-C20orf196/SHDL1 complex appears to collectively function in antagonizing homologous recombination by limiting DNA end resection, thereby promoting NHEJ-mediated repair .

What experimental strategies can effectively validate the specificity of FAM35AFAM35/B antibodies?

Validating antibody specificity is crucial for reliable research results. For FAM35AFAM35/B antibodies, consider these validation strategies:

• siRNA/shRNA knockdown controls: Use siRNA or shRNA targeting FAM35A to deplete the protein and confirm the specificity of antibody signal in Western blot or immunofluorescence. This approach has been successfully employed in previous studies .

• Overexpression systems: Complement knockdown studies with overexpression of tagged FAM35A constructs to verify antibody recognition of the target protein.

• Multiple antibodies targeting different epitopes: Use different antibodies recognizing distinct regions of FAM35A to confirm consistent detection patterns.

• Analysis in cell lines with known FAM35A status: Test antibodies in cell lines with confirmed FAM35A expression (positive controls) and in the HCC1937 cell line, which has been documented to lack FAM35A expression (negative control) .

• Peptide competition assays: Pre-incubate the antibody with the immunizing peptide prior to application in Western blot or immunostaining to demonstrate signal specificity.

• Cross-species reactivity testing: Though current commercial antibodies are specific to human FAM35A , testing cross-reactivity with orthologous proteins from model organisms can provide additional validation information.

How can researchers effectively study the recruitment dynamics of FAM35A to DNA damage sites?

Studying FAM35A recruitment to DNA damage sites requires specialized approaches:

• Laser microirradiation: Use laser microirradiation to induce localized DNA damage within cell nuclei, followed by fixed-time point or live-cell imaging to track FAM35A recruitment kinetics.

• FokI nuclease system: The FokI system has been successfully employed to study FAM35A recruitment to DNA damage sites and dependency on factors like 53BP1, RIF1, and REV7 .

• Domain truncation analysis: Generate truncated versions of FAM35A to determine domain requirements for recruitment. Previous studies have shown that the N-terminal NUMOD3 motif is essential, while the C-terminal OB3 domain and S/Q motif (S339) are dispensable for recruitment to damage sites .

• Co-localization with established DNA damage markers: Perform co-immunostaining with established markers like γH2AX, 53BP1, or RIF1 to confirm localization to genuine DNA damage sites.

• Recruitment kinetics under various damage conditions: Compare recruitment dynamics following different DNA damaging agents (e.g., mitomycin C, etoposide, ionizing radiation) to identify potential damage-specific responses.

• Dependency analysis through protein depletion: Systematically deplete upstream factors (53BP1, RIF1, REV7) to confirm the recruitment hierarchy and potentially identify additional regulators of FAM35A localization .

What approaches can be used to study FAM35A's role in DNA repair in the context of cancer?

To investigate FAM35A's role in DNA repair within cancer contexts, researchers can employ several methodological approaches:

• Expression analysis in cancer cohorts: Analyze FAM35A expression across cancer types and stages. Previous studies have identified reduced expression in metastatic prostate cancers, suggesting prognostic relevance .

• Correlation with therapy response: Investigate correlations between FAM35A expression/mutation status and response to DNA-damaging therapies or PARP inhibitors in patient cohorts or cell line panels.

• Genetic manipulation in cancer models: Use CRISPR/Cas9 (with careful design considering pseudogenes) or siRNA approaches to modulate FAM35A expression in cancer models to assess impacts on therapy sensitivity, particularly to agents targeting DNA repair pathways.

• Analysis of FAM35A in BRCA1-deficient contexts: Study how FAM35A status influences DNA repair and therapy responses in BRCA1-deficient settings, given the observation that FAM35A is absent in the BRCA1-mutant HCC1937 cell line with anomalous PARP inhibitor resistance .

• Functional assays in patient-derived models: Implement DNA repair assays (NHEJ, HR) in patient-derived cell lines or xenografts with varying FAM35A status to assess functional consequences of FAM35A alterations.

• Combination therapy testing: Evaluate whether FAM35A status influences responses to combination therapies involving DNA-damaging agents and repair pathway inhibitors.

How should researchers interpret differences in FAM35A-mediated DNA damage response between cell types?

Interpreting cell type-specific differences in FAM35A function requires careful consideration of several factors:

• Baseline expression levels: Quantify baseline FAM35A expression across cell types using techniques like qPCR and Western blotting to determine whether observed functional differences correlate with expression levels .

• Expression of interacting partners: Assess the expression of key FAM35A interacting partners (REV7, RIF1, 53BP1) as variations in these factors may influence FAM35A-dependent phenotypes.

• Genetic background considerations: Consider how the broader genetic background, particularly the status of DNA repair genes, might influence FAM35A function. For example, BRCA1 status appears to interact with FAM35A function in determining repair outcomes and therapy responses .

• Cell cycle profile analysis: Since DNA repair pathway choice is cell cycle-dependent, analyze cell cycle profiles to determine whether cell type-specific differences might be explained by variations in cell cycle distribution.

• Post-translational modification patterns: Investigate potential differences in FAM35A post-translational modifications across cell types, particularly within the N-terminal region containing DNA damage-dependent modification sites .

• Subcellular localization: Examine whether FAM35A shows differential subcellular localization patterns across cell types, which might contribute to functional variations.

What are the key considerations when analyzing FAM35A's impact on cellular sensitivity to DNA-damaging agents?

When analyzing how FAM35A affects cellular sensitivity to DNA-damaging agents, researchers should consider:

• Agent-specific effects: Different DNA-damaging agents induce distinct types of damage. FAM35A-depleted HEK293 cultures have shown hypersensitivity to both mitomycin C (MMC) and etoposide , but responses may vary with other agents.

• Pathway-specific markers: Analyze pathway-specific markers to determine mechanistic effects. For instance, FAM35A depletion does not affect γH2AX formation following MMC exposure, indicating intact damage signaling, but does influence RAD51 foci formation, suggesting altered HR dynamics .

• Repair pathway balance: FAM35A influences the balance between NHEJ and HR. Its depletion leads to increased resection, making NHEJ less effective . Consider how this pathway shift might differently impact sensitivity to various agents.

• Genetic context dependencies: The effect of FAM35A on damage sensitivity may depend on genetic context, particularly the status of other repair factors. For example, FAM35A absence in BRCA1-deficient HCC1937 cells correlates with PARP inhibitor resistance .

• Temporal dynamics: Consider the timing of sensitivity assessments, as early and late responses to damage may differ and provide complementary insights into FAM35A function.

• Dose-response relationships: Perform detailed dose-response analyses rather than single-dose experiments to fully characterize how FAM35A impacts the therapeutic window of DNA-damaging agents.

How can contradictory findings about FAM35A function be reconciled in research interpretation?

Researchers may encounter seemingly contradictory findings regarding FAM35A function. To reconcile such discrepancies:

• Context-dependent effects: Consider whether contradictions reflect genuine context-dependent functions. For example, FAM35A's role may differ between normal and cancer cells or between different genetic backgrounds.

• Isoform-specific functions: The two FAM35A isoforms produced by alternative splicing may have distinct or even opposing functions in certain contexts. Verify which isoform(s) were studied in conflicting reports.

• Domain-specific activities: FAM35A contains multiple functional domains, including OB-fold domains and the N-terminal NUMOD3 motif . Contradictory findings might reflect domain-specific activities that manifest differently depending on experimental conditions.

• Methodological differences: Carefully compare methodological approaches between studies, including cell types, knockdown efficiency, detection methods, and experimental timelines, as these factors can significantly influence outcomes.

• Integration with broader pathway knowledge: Interpret FAM35A findings within the broader context of DNA repair pathway knowledge. For instance, its position downstream of 53BP1, RIF1, and REV7 in the recruitment cascade provides a framework for reconciling certain functional observations .

• Consideration of compensatory mechanisms: Assess whether acute versus chronic depletion of FAM35A triggers different compensatory responses that could explain contradictory phenotypes.