FAM217A Antibody

What is FAM72A and what is its primary role in cellular function?

FAM72A (Family with sequence similarity 72 member A) is a protein that plays a critical role in the error-prone processing of deoxyuracils in B cells. It acts as an antagonist to uracil DNA glycosylase 2 (UNG2), a key enzyme in the base excision repair (BER) pathway. By antagonizing UNG2, FAM72A enables activation-induced cytidine deaminase (AID) to exert its full effects on antibody maturation through somatic hypermutation (SHM) and class-switch recombination (CSR) . This protein effectively disables DNA repair pathways that would normally efficiently remove deoxyuracils from DNA, thus promoting mutagenesis necessary for antibody diversification .

How does FAM72A contribute to antibody diversification processes?

FAM72A modulates three AID-mediated antibody diversification processes:

• Somatic Hypermutation (SHM): FAM72A-deficient B cells exhibit reduced SHM frequencies at immunoglobulin and Bcl6 genes .

• Class-Switch Recombination (CSR): FAM72A-deficient CH12F3-2 B cells and primary B cells from Fam72a⁻/⁻ mice show reduced CSR rates .

• Immunoglobulin Gene Conversion: Interestingly, Fam72a⁻/⁻ DT40 clones display an approximately twofold increase in gene conversion, consistent with FAM72A's role in antagonizing UNG2 .

These findings demonstrate that FAM72A is essential for normal antibody diversification and adaptive immune responses.

How does FAM72A interact with UNG2 at the molecular level?

FAM72A directly binds to UNG2, which has been confirmed through multiple experimental approaches:

• BioID proximity-labeling assay: This technique demonstrated that human FAM72A is proximal to UNG in HEK293 cells .

• Direct protein interaction assays: Experiments with purified proteins showed that mixing FAM72A and UNG2 at a 1:1 ratio resulted in co-precipitation of both proteins when using either nickel-containing beads or anti-Flag antibodies, confirming their direct interaction .

• Co-immunoprecipitation: N-terminal HA-tagged FAM72A (HA-(G4S)3FAM72A) co-precipitates with both MKLN1 and UNG2, while the W125R mutant of FAM72A fails to bind either UNG2 or MKLN1 .

This interaction results in reduced levels of UNG2 protein specifically during the G1 phase of the cell cycle, which coincides with peak AID activity .

Through what molecular pathway does FAM72A promote UNG2 degradation?

FAM72A recruits the C-terminal to LisH (CTLH) E3 ligase complex to target UNG2 for proteasomal degradation:

• FAM72A acts as an E3 ligase substrate adaptor that directly binds to MKLN1 within the CTLH complex .

• This binding facilitates the recruitment and ubiquitination of UNG2 .

• The ubiquitinated UNG2 is subsequently degraded through the proteasomal pathway .

Cryo-EM structural analysis reveals that CTLH-MKLN1-FAM72A forms a ring-shaped structure with MKLN1 in a dimer configuration. Each of the two MKLN1 dimers directly binds and orients two FAM72A molecules toward the center of the ring .

What experimental models are most effective for studying FAM72A function?

Several experimental models have proven effective for studying FAM72A function:

• Cell Lines:

  • CH12F3-2 B cells: Used for studying class-switch recombination

  • DT40 B cells: Used for analyzing immunoglobulin gene conversion

  • HEK293T cells: Employed for protein interaction studies

• Mouse Models:

  • Fam72a⁻/⁻ knockout mice: Used to study the effects of FAM72A deficiency on antibody diversification in vivo

  • Conditional knockout mice: Such as Maea^fl/fl Mb1^cre mice, which phenocopy certain aspects of FAM72A deficiency

• In vitro Reconstitution:

  • Purified protein interaction assays

  • In vitro ubiquitination assays using purified CTLH-MKLN1-FAM72A components

What controls are essential when using antibodies against FAM72A in flow cytometry experiments?

While the search results don't specifically address FAM72A antibody controls, general high-dimensional flow cytometry control principles should be applied:

• Titration Controls: Critical for validating the optimal antibody concentration.

  • Perform titration experiments by varying antibody amounts while keeping other variables constant

  • Calculate the staining index (SI) for each concentration

  • Select the concentration in the middle of the range providing the highest SI

• Fluorescence Minus One (FMO) Controls: Samples stained with all fluorochromes except one.

  • Essential for revealing data spread in polychromatic flow cytometry

  • One FMO control should be prepared for each fluorescently labeled antibody

• For Mass Cytometry:

  • Careful panel design and sample preparation to avoid mass channel overlap or isotope impurity

  • Mass Minus One (MMO) controls may be used to resolve signal from background

How does FAM72A deficiency impact the somatic hypermutation (SHM) spectrum?

The somatic hypermutation spectrum in B cells from Fam72a⁻/⁻ mice is opposite to that observed in mice deficient in UNG2 . This finding suggests that UNG2 is hyperactive in FAM72A-deficient cells, leading to:

These changes occur because FAM72A normally causes U·G mispairs to persist into S phase, leading to error-prone processing by mismatch repair. Without FAM72A, the elevated UNG2 activity more efficiently removes deoxyuracils, reducing the substrate for mutation .

What is the relationship between FAM72A and cancer development?

The research suggests important implications for FAM72A in cancer:

• FAM72A overexpression is observed in many cancers .

• By promoting mutagenesis through antagonism of UNG2, FAM72A overexpression could contribute to genomic instability in cancer cells .

• The hijacking of the CTLH complex by FAM72A to promote mutagenesis may be a mechanism relevant to cancer development .

This suggests that FAM72A could be a potential therapeutic target or biomarker in certain cancers where mutagenesis driven by impaired DNA repair contributes to disease progression.

What protein interaction techniques are most reliable for studying FAM72A-UNG2 binding?

Based on the search results, several complementary techniques have proven effective:

• BioID proximity-labeling assay: This technique uses a biotin ligase fusion protein to identify proteins in close proximity.

  • HEK 293T cells were transfected with pDEST-NLS3×Flag-miniTURBO or pDEST-NLS-3×Flag-miniTURBO-FAM72A

  • Biotinylation was induced with 50 μM biotin for 1 hour

  • Streptavidin Sepharose High Performance beads were used to capture biotinylated proteins

• Direct protein interaction assays: Using purified proteins to demonstrate direct binding.

  • Purified proteins were mixed at 1:1 ratio

  • Nickel-containing beads or anti-Flag antibodies were used for pull-down experiments

  • Co-precipitation was analyzed by immunoblotting

• Affinity-purification mass spectrometry (AP-MS): To comprehensively identify protein interactors.

  • Performed on immunoprecipitated N-terminal HA-tagged FAM72A

  • Allows for unbiased identification of the entire FAM72A interactome

How can researchers validate FAM72A antibody specificity for experimental applications?

While not explicitly addressed in the search results, standard validation approaches would include:

• Western blot analysis:

  • Compare wild-type cells with Fam72a⁻/⁻ cells to confirm antibody specificity

  • Include appropriate positive and negative controls

• Epitope tagging:

  • Use tagged versions of FAM72A (as shown in the research where N-terminal HA- or FLAG-tagging of FAM72A rescued CSR and reduced UNG2 levels in Fam72a⁻/⁻ CH12 cells)

  • This approach allows validation using antibodies against the tag

• Immunoprecipitation-based validation:

  • Perform IP followed by mass spectrometry to confirm the identity of the precipitated protein

  • Cross-validate with multiple antibodies recognizing different epitopes

How can researchers quantitatively assess the impact of FAM72A on class-switch recombination?

Based on the research methods described in the search results:

• Flow cytometry analysis of B cell populations:

  • Stain cells with antibodies against mouse CD45.2, CD19, IgD, Fas, and GL7

  • Identify germinal center B cells (live CD45+CD4−CD11c−F4/80−CD19+IgD−GL-7+Fas+)

• In vitro CSR assays using CH12F3-2 cells:

  • Compare CSR rates between wild-type and Fam72a⁻/⁻ cells

  • Quantify switching from IgM to IgA following appropriate stimulation

• Immunization-based assays in mouse models:

  • Compare germinal center formation and antibody production in wild-type and Fam72a⁻/⁻ mice

  • Analyze T follicular helper (TFH) cells (live CD4+B220−CXCR5+PD-1+intracellular Bcl-6+)

These approaches provide complementary data on how FAM72A affects CSR at cellular and organismal levels.