{"gene":"PAICS","run_date":"2026-06-10T05:19:53","timeline":{"discoveries":[{"year":2006,"finding":"Human PAICS forms a homo-octamer with an octameric carboxylase core and four peripheral dimers formed by the synthetase domains. Each AIRc active site is formed by structural elements from three AIRc domains, demonstrating that the octamer is essential for carboxylation activity. Four tunnel systems connecting the AIRc and SAICARs active sites were identified, suggesting intermediate (CAIR) channeling between the two active sites.","method":"X-ray crystallography (crystal structure at 2.8 Å resolution); functional complementation analyses to identify active sites","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with functional complementation, replicated and extended by subsequent structural and biochemical studies","pmids":["17224163"],"is_preprint":false},{"year":2009,"finding":"Zebrafish paics encodes a bifunctional enzyme catalyzing steps 6 and 7 of IMP synthesis; loss-of-function causes pigmentation defects (absent xanthophore/iridophore pigment, reduced melanin) and microphthalmia due to defects in cell cycle exit of retinoblasts. Genetic epistasis showed pigmentation defects depend on GTP synthesis pathway deficiency and microphthalmia on ATP synthesis pathway deficiency, placing paics at a bifurcation point in purine biosynthesis affecting two independent downstream pathways.","method":"Zebrafish recessive mutant analysis; maternal-zygotic and maternal-effect mutant analysis; genetic epistasis with separable ATP/GTP pathway outputs; purine supplementation rescue","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple mutant classes and pathway rescue, replicated across maternal and zygotic contexts","pmids":["19570845"],"is_preprint":false},{"year":1995,"finding":"The chicken GPAT (PPAT) and AIRC (PAICS) genes are divergently transcribed from a ~230 bp intergenic bidirectional promoter. An initiator-like element overlapping the AIRC transcription start site plays a central role in coordinating expression of both genes; removal of GC/CCAAT boxes from the AIRC proximal half disrupts bidirectional transcription.","method":"Deletion mutagenesis in a bireporter vector transfected into HepG2 and chicken LMH cells; gel retardation, DNase I, and methylation interference assays with HeLa nuclear proteins","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — deletion mutagenesis with multiple readouts in transfected cells, single lab, no in vivo validation","pmids":["7836476"],"is_preprint":false},{"year":1997,"finding":"NRF-1 and Sp1 bind to a cluster of sites (nt 215–260) in the human GPAT-AIRC bidirectional promoter; NRF-1 is required for stable Sp1 binding at this locus, and point mutations in the NRF-1 site or flanking Sp1 sites decrease expression of both GPAT and AIRC (PAICS) in transfected HepG2 cells.","method":"Promoter deletion and point-mutation analysis in transfected HepG2 cells; gel retardation assays identifying NRF-1 and Sp1 binding","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — point mutagenesis and binding assays, single lab, functional readout in transfected cells","pmids":["9108165"],"is_preprint":false},{"year":1988,"finding":"The bovine PAIS (PAICS) gene is syntenic with PRGS (PPAT) and SOD1, mapping to cattle syntenic group U10, which is conserved with human chromosome 21; this synteny was established by somatic cell hybrid complementation requiring bovine PAIS for growth on selective media.","method":"Somatic cell genetics with cow-hamster hybrid cell lines; enzyme electrophoresis for biochemical marker concordance","journal":"Biochemical genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — somatic cell hybrid complementation establishes functional gene identity and chromosomal synteny, single lab","pmids":["3377762"],"is_preprint":false},{"year":2019,"finding":"A homozygous missense mutation p.Lys53Arg in PAICS reduces the catalytic activity of the enzyme to ~10% in patient fibroblasts and ~25% in recombinant purified protein compared to wild-type, establishing a direct catalytic deficiency. The mutation also prevents purinosome formation in patient fibroblasts, which was rescued by transfection with wild-type but not mutant PAICS.","method":"Enzyme activity assay in patient skin fibroblasts and recombinant E. coli-expressed protein; purinosome formation assessed by transfection rescue","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1 / Strong — recombinant enzyme activity assay combined with cellular rescue experiment establishing catalytic site importance and purinosome assembly role","pmids":["31600779"],"is_preprint":false},{"year":2020,"finding":"Crystal structures of human PAICS in complex with native ligands revealed CAIR bound in both AIRc and SAICARs active sites and SAICAR bound in the SAICARs domain, as well as a structure with SAICAR and an ATP analog in the SAICARs active site. These structures define the architecture of both active sites and substrate/product binding modes.","method":"X-ray crystallography of PAICS complexes with native substrates and ATP analog","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple crystal structures with native ligands, extends prior structural work with substrate-bound complexes","pmids":["32571877"],"is_preprint":false},{"year":2020,"finding":"PAICS knockdown is required for growth and survival of prostate cancer cells, as demonstrated by clonogenic survival and cell viability assays.","method":"Gene knockdown with clonogenic survival and cell viability assays in prostate cancer cell lines","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean knockdown with two orthogonal cell-based phenotypic assays, single lab","pmids":["32571877"],"is_preprint":false},{"year":2022,"finding":"PAICS physically interacts with all other known de novo purine biosynthesis (DNPB) enzymes (except amidophosphoribosyltransferase/PPAT) and with MTHFD1, as demonstrated by bimolecular fluorescence complementation in live cells and co-immunoprecipitation of StrepTag-labeled PAICS reintegrated into PAICS-knockout HeLa cells. These interactions occur in both purine-depleted and purine-rich conditions. C-terminal tagging of PAICS disrupts these interactions and correlates with impaired DNPB activity and perturbed IMP partitioning into AMP and GMP.","method":"Bimolecular fluorescence complementation (BiFC) in live cells; Co-IP with StrepTag-labeled PAICS in CRISPR knockout HeLa cells; metabolic flux analysis of DNPB intermediates","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — two orthogonal PPI methods (BiFC + reciprocal Co-IP in KO background), correlated with metabolic functional readout, single lab but rigorous controls","pmids":["35331738"],"is_preprint":false},{"year":2022,"finding":"Time-course mass spectrometric analysis of 13C-bicarbonate incorporation demonstrated that the SAICAR synthetase domain of PAICS selectively uses enzyme-generated CAIR over exogenously added CAIR, providing biochemical evidence for substrate channeling of CAIR between the two active sites of PAICS.","method":"Time-course mass spectrometry with 13C-bicarbonate isotope tracing using recombinant human PAICS","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro isotope-tracing experiment with recombinant enzyme directly demonstrating channeling, consistent with structural data","pmids":["35285625"],"is_preprint":false},{"year":2022,"finding":"Density functional theory calculations based on PAICS crystal structures elucidated the reaction mechanism: AIRc carboxylation proceeds in two steps (C-C bond formation forming isoCAIR, then deprotonation assisted by an active-site histidine); SAICARs phosphorylation of CAIR precedes condensation with aspartate; three active-site magnesium ions bind substrates and stabilize transition states.","method":"Quantum chemical (DFT) calculations using large active-site models built from crystal structures; barriers compared to experimental kinetic data","journal":"Journal of the American Chemical Society","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — computational mechanistic study with experimental validation of calculated barriers, but no mutagenesis or direct biochemical confirmation of each step","pmids":["35914774"],"is_preprint":false},{"year":2023,"finding":"K6-polyubiquitination of PAICS by the Cul5/ASB11-based ubiquitin E3 ligase recruits UBAP2 (a ubiquitin-binding protein with intrinsically disordered regions) to induce phase separation and drive purinosome assembly. ASB11 upregulation upon purinosome-inducing stress (via relief of H3K9me3/HP1α transcriptional silencing) triggers this cascade. In human melanoma, constitutive ASB11 overexpression drives purinosome formation supporting proliferation and tumorigenesis.","method":"Co-immunoprecipitation; ubiquitination assays; phase separation/condensate imaging; CRISPR/genetic manipulation; xenograft tumor model","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal biochemical and cell biological methods identifying the E3 ligase, the ubiquitin linkage type, the reader protein, and the functional consequence in vivo","pmids":["37848033"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of invertebrate (Trichoplusia ni) PAICS at 2.8 Å resolution confirmed the bifunctional domain architecture is highly conserved across divergent species and provided insights into substrate binding; comparison with human and prokaryotic homologs revealed a conserved enzymatic framework.","method":"X-ray crystallography (SAD phasing) of insect PAICS; comparative structural analysis","journal":"Proteins","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — independent crystal structure confirming domain architecture, no functional mutagenesis reported in this paper alone","pmids":["23553965"],"is_preprint":false},{"year":2015,"finding":"PAICS knockdown modulates pyruvate kinase activity, and PAICS expression is induced by L-glutamine. A glutamine antagonist (DON) blocked glutamine-mediated induction of PAICS and reduced pyruvate kinase activity, placing PAICS expression under metabolic regulation by glutamine in lung cancer cells.","method":"Gene knockdown and overexpression studies; pyruvate kinase activity assay; DON treatment; cell proliferation and invasion assays","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knockdown and chemical inhibition with enzymatic activity readout, single lab, two orthogonal approaches","pmids":["26140362"],"is_preprint":false},{"year":2016,"finding":"PAICS expression in prostate cancer is transcriptionally regulated by MYC; the BET bromodomain inhibitor JQ1 reduces PAICS expression and causes loss of MYC occupancy at the PAICS promoter. PAICS knockdown inhibits proliferation and invasion in prostate cancer cells in vitro and in vivo (CAM and xenograft models).","method":"BET inhibitor (JQ1) treatment; ChIP showing loss of MYC occupancy on PAICS promoter; siRNA/shRNA knockdown; CAM and murine xenograft models","journal":"The Prostate","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP plus functional knockdown studies linking MYC to PAICS transcription, single lab","pmids":["27550065"],"is_preprint":false},{"year":2018,"finding":"PAICS expression is negatively regulated post-transcriptionally by miR-128, which binds the PAICS 3'-UTR. In bladder cancer cells, PAICS induces EMT by positively regulating SNAI1 and reducing E-cadherin expression.","method":"3'-UTR luciferase reporter assay; miR-128 overexpression; qRT-PCR and immunoblot for SNAI1 and E-cadherin; in vitro proliferation/invasion and in vivo CAM assays","journal":"Neoplasia","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — luciferase reporter validates miRNA-target interaction, EMT markers measured by Western blot, single lab","pmids":["30121007"],"is_preprint":false},{"year":2018,"finding":"PAICS knockdown in breast cancer cells blocks G1-S cell cycle transition, suppresses Cyclin E, upregulates Cyclin D1, P21, and CDK4, and activates PARP and caspase-3 while downregulating Bcl-2 and Bcl-xl, inducing apoptosis.","method":"Lentiviral shRNA knockdown; flow cytometry for cell cycle analysis; Western blot for cell cycle and apoptosis markers","journal":"Biological research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean knockdown with multiple molecular readouts for cell cycle and apoptosis, single lab","pmids":["30097015"],"is_preprint":false},{"year":2020,"finding":"In colorectal cancer cells, PAICS expression is transcriptionally activated by MYC (BET inhibitor JQ1 reduces PAICS expression) and negatively regulated by miR-128. PAICS knockdown upregulates E-cadherin (EMT marker) and reduces tumor growth and metastatic dissemination to liver, lungs, and bone in mice.","method":"JQ1 treatment; miR-128 overexpression; stable PAICS knockdown; murine xenograft and metastasis model with PET imaging","journal":"Cancers","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo validation with multiple metastatic readouts, mechanistic links to MYC and miR-128 from prior studies, single lab","pmids":["32218208"],"is_preprint":false},{"year":2020,"finding":"PAICS interacts with HDAC1 and HDAC2 in gastric cancer cells; PAICS deficiency decreases RAD51 expression, impairs RAD51 recruitment to DNA damage sites by reducing HDAC1/2 deacetylase activity, and thereby prevents DNA damage repair, sensitizing cells to cisplatin.","method":"Co-immunoprecipitation (PAICS–HDAC1/HDAC2 interaction); RAD51 ChIP at damage sites; HDAC activity assay; cisplatin sensitivity assay in vitro and in vivo","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP and functional HDAC activity assay with cisplatin sensitization readout, single lab","pmids":["32632107"],"is_preprint":false},{"year":2022,"finding":"PAICS promotes FAK phosphorylation in breast cancer cells; miR-4731-5p inhibits this by targeting PAICS mRNA. PAICS-dependent FAK phosphorylation drives glycolysis, EMT, migration, and invasion, defining a miR-4731-5p/PAICS/FAK signaling axis.","method":"Luciferase 3'-UTR reporter confirming miR-4731-5p targets PAICS; Western blot for FAK phosphorylation; functional assays for glycolysis and EMT; xenograft model","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reporter assay validates miRNA-target, phosphorylation readout links PAICS to FAK pathway, in vivo validation, single lab","pmids":["35379785"],"is_preprint":false},{"year":2023,"finding":"PAICS interacts with DYRK3 kinase and co-regulates purinosome formation in oral squamous cell carcinoma; disrupting PAICS inhibits purinosome formation and affects survival of radiation-resistant OSCC cells. The DYRK3/PAICS axis contributes to radiotherapy resistance.","method":"In vitro cell models of radiation-resistant OSCC; protein interaction assays; GSK-626616 (DYRK3 inhibitor); purinosome formation imaging; in vivo tumor model","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — interaction and functional data from single lab with limited mechanistic detail in abstract; note: a correction was issued for this paper","pmids":["38139175"],"is_preprint":false},{"year":2025,"finding":"ACSS2 directly interacts with PAICS and promotes its acetylation; acetylated PAICS undergoes autophagy-mediated degradation, limiting purine biosynthesis and reducing dNTP pools, which exacerbates cytoplasmic chromatin fragment accumulation and drives the senescence-associated secretory phenotype (SASP).","method":"Co-immunoprecipitation (ACSS2–PAICS interaction); acetylation assays; autophagy inhibition experiments; Acss2 pharmacological inhibition and deletion in mice; dNTP pool measurement; SASP marker analysis","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP identifying interaction plus acetylation/degradation assays and in vivo mouse model, single lab but multiple orthogonal methods","pmids":["40021646"],"is_preprint":false},{"year":2025,"finding":"In lung cancer cells, GART deletion inhibits the PAICS-Akt-β-catenin signaling pathway, suggesting GART acts upstream of PAICS to activate Akt-β-catenin and promote proliferation and migration.","method":"GART knockdown; qRT-PCR and Western blot for PAICS, p-Akt, and β-catenin; xenograft tumor model","journal":"Frontiers in oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, Western blot only for pathway placement, indirect evidence for PAICS in Akt signaling","pmids":["40201340"],"is_preprint":false},{"year":2026,"finding":"PAICS loss-of-function in zebrafish (paics knockout) recapitulates cerebellar neuronal loss, neuromuscular junction disruption, motor impairment, and widespread DNA damage repair defects including suppression of key DNA repair pathways. Restoring paics expression in C9orf72-deficient zebrafish resolves DNA damage and preserves Purkinje and Granule cells, identifying PAICS as a critical mediator of cerebellar neuronal survival downstream of C9orf72.","method":"Zebrafish paics knockout; single-cell transcriptomics; rescue by paics re-expression in C9orf72 zebrafish; DNA damage assays; neuromuscular junction and motor behavioral analysis","journal":"Brain : a journal of neurology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic rescue in vivo establishes PAICS as epistatic downstream mediator of C9orf72 cerebellar degeneration, single lab","pmids":["41810938"],"is_preprint":false},{"year":2026,"finding":"IRF4 transcriptionally activates PAICS in DLBCL cells; PAICS physically interacts with LDHA and augments its activity, skewing the NAD+/NADH balance toward metabolic immunosuppression (elevated TGF-β and IL-10, reduced IFN-γ, enhanced CD8+ T cell exhaustion). This defines an IRF4-PAICS-LDHA axis.","method":"Co-immunoprecipitation (PAICS–LDHA interaction); LDHA activity assay; ChIP or transcriptional assay for IRF4-PAICS; cytokine profiling; PAICS/LDHA knockdown with T cell co-culture assays; tumor model","journal":"NPJ precision oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — Co-IP and activity assay support PAICS–LDHA interaction, but abstract does not detail rigor of IRF4 transcriptional assay; single lab, single paper","pmids":["41991742"],"is_preprint":false}],"current_model":"PAICS is a bifunctional homo-octameric enzyme that catalyzes steps 6 (AIRc) and 7 (SAICARs) of de novo purine biosynthesis by converting AIR sequentially to CAIR and then to SAICAR via a two-step carboxylation mechanism and an ATP-dependent phosphorylation-condensation reaction, with CAIR channeled between the two active sites through internal tunnels; its octameric architecture is essential for carboxylase activity. PAICS assembles into the purinosome metabolon by interacting with all other DNPB enzymes and MTHFD1, a process driven by K6-polyubiquitination of PAICS by the Cul5/ASB11 E3 ligase, which recruits the disordered protein UBAP2 to trigger phase separation. PAICS expression is transcriptionally activated by MYC (and IRF4 in lymphoma) and repressed post-transcriptionally by miR-128; its protein stability is regulated by ACSS2-mediated acetylation leading to autophagic degradation. Beyond its biosynthetic role, PAICS participates in DNA damage response through interaction with HDAC1/2 (supporting RAD51 recruitment) and promotes EMT, proliferation, and invasion partly through PAICS-dependent FAK phosphorylation and activation of the Akt-β-catenin pathway."},"narrative":{"mechanistic_narrative":"PAICS is a bifunctional enzyme of de novo purine biosynthesis that catalyzes steps 6 and 7 of IMP synthesis, converting AIR to CAIR (AIR carboxylase) and then CAIR to SAICAR (SAICAR synthetase) [PMID:19570845, PMID:32571877]. The enzyme assembles into a homo-octamer in which an octameric carboxylase core is decorated by four peripheral synthetase dimers; this quaternary arrangement is itself essential, because each carboxylase active site is built from elements contributed by three subunits, and tunnel systems connecting the AIRc and SAICARs sites permit internal channeling of the CAIR intermediate [PMID:17224163]. Isotope-tracing confirms that the synthetase domain selectively uses enzyme-generated CAIR over exogenous CAIR, providing direct biochemical evidence for substrate channeling [PMID:35285625], and structural and computational work define a two-step carboxylation mechanism, an ATP-dependent phosphorylation-condensation with aspartate, and the magnesium ions that stabilize the transition states [PMID:32571877, PMID:35914774]. Beyond catalysis, PAICS nucleates the purinosome metabolon by physically interacting with the other DNPB enzymes and MTHFD1, with C-terminal tagging disrupting both these contacts and metabolic flux through the pathway [PMID:35331738]; purinosome assembly is driven by K6-polyubiquitination of PAICS by the Cul5/ASB11 E3 ligase, which recruits the disordered protein UBAP2 to trigger phase separation [PMID:37848033]. Enzymatic and assembly functions are clinically validated: a homozygous p.Lys53Arg mutation that reduces catalytic activity and abolishes purinosome formation causes a developmental disorder, with rescue by wild-type but not mutant protein [PMID:31600779]. PAICS is transcriptionally activated by MYC and post-transcriptionally repressed by miR-128, and across multiple cancers its knockdown suppresses proliferation, invasion, EMT, and metastasis [PMID:27550065, PMID:30121007, PMID:32218208]; reported effectors include HDAC1/2-dependent RAD51 recruitment for DNA repair [PMID:32632107] and PAICS-dependent FAK phosphorylation [PMID:35379785]. Protein abundance is further controlled by ACSS2-mediated acetylation that targets PAICS for autophagic degradation, linking purine and dNTP supply to senescence [PMID:40021646].","teleology":[{"year":1988,"claim":"Before molecular characterization, establishing that the mammalian PAICS gene was a functional, mappable locus was needed; somatic cell hybrid complementation identified bovine PAICS and placed it in a synteny conserved with human chromosome 21 alongside PPAT and SOD1.","evidence":"Cow-hamster somatic cell hybrids with selective growth complementation and enzyme electrophoresis","pmids":["3377762"],"confidence":"Medium","gaps":["No protein-level or catalytic characterization","Human gene structure not yet defined"]},{"year":1995,"claim":"How PAICS expression is coordinated with another purine-pathway gene was unknown; the chicken PPAT and PAICS genes were shown to share a compact bidirectional promoter, with an initiator-like element at the PAICS start site coordinating both genes.","evidence":"Deletion mutagenesis in a bireporter vector with DNase I/methylation interference footprinting","pmids":["7836476"],"confidence":"Medium","gaps":["Trans-acting factors not yet identified","Avian promoter, not validated in human in vivo"]},{"year":1997,"claim":"The trans-acting factors driving the bidirectional promoter were undefined; NRF-1 and Sp1 were shown to bind a site cluster, with NRF-1 required for stable Sp1 binding and both needed for full expression of both genes.","evidence":"Promoter point-mutation analysis and gel retardation in transfected HepG2 cells","pmids":["9108165"],"confidence":"Medium","gaps":["No in vivo occupancy data","Regulation under metabolic demand not addressed"]},{"year":2006,"claim":"The structural basis of PAICS bifunctionality was unknown; the human crystal structure revealed a homo-octamer whose carboxylase core requires three subunits per active site and contains tunnels linking the two catalytic sites, implying intermediate channeling.","evidence":"X-ray crystallography at 2.8 Å with functional complementation to map active sites","pmids":["17224163"],"confidence":"High","gaps":["Substrate-bound states not captured","Channeling inferred from tunnels, not directly demonstrated"]},{"year":2009,"claim":"The physiological consequences of PAICS loss in a vertebrate were untested; zebrafish mutants placed paics at a bifurcation point where pigmentation defects depend on GTP-pathway output and microphthalmia on ATP-pathway output.","evidence":"Zebrafish recessive mutant analysis with genetic epistasis and purine supplementation rescue","pmids":["19570845"],"confidence":"High","gaps":["Tissue-specific mechanisms not resolved","Relevance to human disease not yet established"]},{"year":2013,"claim":"Whether the bifunctional architecture was conserved was addressed by an invertebrate PAICS structure, confirming a deeply conserved domain organization and enzymatic framework.","evidence":"X-ray crystallography (SAD phasing) of Trichoplusia ni PAICS with comparative analysis","pmids":["23553965"],"confidence":"Medium","gaps":["No functional mutagenesis in this study","Catalytic mechanism still not defined at atomic detail"]},{"year":2016,"claim":"The drivers of elevated PAICS in cancer were unclear; in prostate cancer MYC was shown to occupy the PAICS promoter and JQ1 reduced this occupancy and expression, with knockdown suppressing proliferation, invasion, and tumor growth.","evidence":"ChIP, BET inhibition, knockdown, and CAM/xenograft models in prostate cancer","pmids":["27550065"],"confidence":"Medium","gaps":["Whether catalytic vs non-catalytic activity drives phenotype not separated","Single cancer context"]},{"year":2018,"claim":"Post-transcriptional control and downstream oncogenic effects were addressed: miR-128 targets the PAICS 3'-UTR, and PAICS promotes EMT (SNAI1 up, E-cadherin down) and the G1-S transition while restraining apoptosis.","evidence":"3'-UTR luciferase reporters, knockdown with cell-cycle/EMT/apoptosis marker profiling in bladder and breast cancer","pmids":["30121007","30097015"],"confidence":"Medium","gaps":["Mechanism linking purine synthesis to EMT/cell-cycle markers not defined","Effectors downstream of PAICS unclear"]},{"year":2019,"claim":"Whether PAICS catalytic activity and purinosome assembly are physiologically essential in humans was tested by a homozygous p.Lys53Arg mutation that lowers activity to ~10–25% and abolishes purinosome formation, rescued only by wild-type protein.","evidence":"Enzyme assays in patient fibroblasts and recombinant protein with transfection rescue of purinosome formation","pmids":["31600779"],"confidence":"High","gaps":["Full clinical spectrum from a single residue not generalizable","Link between catalysis and assembly defect mechanistically unresolved"]},{"year":2020,"claim":"Ligand-bound structures and cancer dependency were established together: structures captured CAIR in both sites, SAICAR with an ATP analog, and PAICS knockdown impaired prostate cancer growth and survival.","evidence":"X-ray crystallography of native-ligand complexes plus clonogenic and viability assays","pmids":["32571877"],"confidence":"High","gaps":["Reaction trajectory between sites not directly observed","Dependency mechanism (metabolic vs other) not dissected"]},{"year":2020,"claim":"A non-canonical nuclear role was proposed: PAICS interacts with HDAC1/2 to support RAD51 recruitment and DNA damage repair, with loss sensitizing gastric cancer cells to cisplatin.","evidence":"Reciprocal Co-IP, RAD51 ChIP at damage sites, HDAC activity assay, and cisplatin sensitivity in vitro and in vivo","pmids":["32632107"],"confidence":"Medium","gaps":["How a metabolic enzyme localizes to chromatin to modulate HDAC activity unexplained","Single cancer type"]},{"year":2020,"claim":"Metastatic relevance and converging regulation were confirmed in colorectal cancer, where MYC activates and miR-128 represses PAICS, and knockdown reduced metastasis to liver, lung, and bone.","evidence":"JQ1, miR-128 overexpression, stable knockdown, and PET-imaged xenograft/metastasis models","pmids":["32218208"],"confidence":"Medium","gaps":["Causal step between PAICS and metastatic colonization not defined"]},{"year":2022,"claim":"Direct evidence for substrate channeling and metabolon assembly was obtained: isotope tracing showed the synthetase site preferentially uses enzyme-generated CAIR, while PAICS was shown to physically organize the purinosome by binding all other DNPB enzymes and MTHFD1.","evidence":"13C-bicarbonate time-course MS on recombinant PAICS; BiFC and reciprocal Co-IP in CRISPR-KO HeLa with metabolic flux analysis","pmids":["35285625","35331738"],"confidence":"High","gaps":["Stoichiometry and architecture of the assembled metabolon not resolved","PPAT exclusion from PAICS interactome unexplained"]},{"year":2022,"claim":"The atomic reaction mechanism was elucidated computationally: two-step carboxylation via isoCAIR with histidine-assisted deprotonation, phosphorylation-before-condensation in the synthetase site, and three catalytic magnesium ions; a parallel oncogenic axis linked PAICS to FAK phosphorylation.","evidence":"DFT calculations from crystal structures benchmarked against kinetics; Western blot/functional assays for the miR-4731-5p/PAICS/FAK axis","pmids":["35914774","35379785"],"confidence":"Medium","gaps":["Mechanistic steps not confirmed by mutagenesis","Direct vs indirect basis of PAICS-driven FAK phosphorylation unknown"]},{"year":2023,"claim":"The molecular trigger for purinosome assembly was defined: K6-polyubiquitination of PAICS by Cul5/ASB11 recruits the disordered reader UBAP2 to induce phase separation, with ASB11 overexpression driving melanoma tumorigenesis.","evidence":"Co-IP, ubiquitination assays, condensate imaging, CRISPR manipulation, and xenografts","pmids":["37848033"],"confidence":"High","gaps":["How ubiquitinated PAICS templates inclusion of other enzymes not detailed","Reversibility/turnover of the condensate not addressed"]},{"year":2025,"claim":"A protein-stability control axis was established: ACSS2 directly interacts with and acetylates PAICS, targeting it for autophagic degradation, thereby limiting dNTP pools and promoting the senescence-associated secretory phenotype.","evidence":"Co-IP, acetylation and autophagy assays, ACSS2 inhibition/deletion in mice, dNTP and SASP measurements","pmids":["40021646"],"confidence":"Medium","gaps":["Acetylated residues and the degradation receptor not defined","Interplay with ubiquitination-driven assembly not addressed"]},{"year":2026,"claim":"A neurodevelopmental/neurodegenerative role was placed downstream of C9orf72: paics-knockout zebrafish show cerebellar neuronal loss and DNA repair defects, and restoring paics in C9orf72-deficient fish preserves Purkinje and Granule cells.","evidence":"Zebrafish knockout with single-cell transcriptomics, DNA damage assays, and cross-genotype rescue","pmids":["41810938"],"confidence":"Medium","gaps":["Mechanism connecting purine supply to C9orf72-dependent neuronal survival unresolved","Mammalian validation absent"]},{"year":2026,"claim":"An immunometabolic moonlighting function was proposed: IRF4 activates PAICS in DLBCL, and PAICS binds and augments LDHA to skew NAD+/NADH toward immunosuppression and CD8+ T cell exhaustion.","evidence":"Co-IP, LDHA activity assay, transcriptional assay for IRF4, cytokine profiling, and T cell co-culture/tumor models","pmids":["41991742"],"confidence":"Low","gaps":["Single lab, single paper; IRF4 transcriptional regulation rigor not detailed","PAICS-LDHA interaction not reciprocally validated across systems"]},{"year":null,"claim":"It remains unresolved how PAICS's catalytic, metabolon-scaffolding, and reported non-canonical (chromatin, FAK, LDHA) functions are mechanistically separated and which are direct versus secondary to altered purine/dNTP supply.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure-function separation of catalytic vs scaffolding mutants in disease/cancer models","Direct biochemical basis of nuclear and signaling roles undefined","Regulatory cross-talk between ubiquitination, acetylation, and phase separation unmapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,1,6,9,10]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[6,9,10]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[6,10]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[8,11]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[8,11]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,1,6,9]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[9,10]}],"complexes":["purinosome"],"partners":["MTHFD1","UBAP2","ASB11","CUL5","HDAC1","HDAC2","ACSS2","LDHA"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P22234","full_name":"Bifunctional phosphoribosylaminoimidazole carboxylase/phosphoribosylaminoimidazole succinocarboxamide synthetase","aliases":[],"length_aa":425,"mass_kda":47.1,"function":"Bifunctional phosphoribosylaminoimidazole carboxylase and phosphoribosylaminoimidazole succinocarboxamide synthetase catalyzing two reactions of the de novo purine biosynthetic pathway","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/P22234/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/PAICS","classification":"Common Essential","n_dependent_lines":598,"n_total_lines":1208,"dependency_fraction":0.49503311258278143},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"ACTR2","stoichiometry":0.2},{"gene":"AP2S1","stoichiometry":0.2},{"gene":"CNBP","stoichiometry":0.2},{"gene":"SAR1B","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PAICS","total_profiled":1310},"omim":[{"mim_id":"619859","title":"PHOSPHORIBOSYLAMINOIMIDAZOLE CARBOXYLASE DEFICIENCY; PAICSD","url":"https://www.omim.org/entry/619859"},{"mim_id":"608222","title":"ADENYLOSUCCINATE LYASE; ADSL","url":"https://www.omim.org/entry/608222"},{"mim_id":"605594","title":"DEAFNESS, AUTOSOMAL DOMINANT 39, WITH 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Each AIRc active site is formed by structural elements from three AIRc domains, demonstrating that the octamer is essential for carboxylation activity. Four tunnel systems connecting the AIRc and SAICARs active sites were identified, suggesting intermediate (CAIR) channeling between the two active sites.\",\n      \"method\": \"X-ray crystallography (crystal structure at 2.8 Å resolution); functional complementation analyses to identify active sites\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with functional complementation, replicated and extended by subsequent structural and biochemical studies\",\n      \"pmids\": [\"17224163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Zebrafish paics encodes a bifunctional enzyme catalyzing steps 6 and 7 of IMP synthesis; loss-of-function causes pigmentation defects (absent xanthophore/iridophore pigment, reduced melanin) and microphthalmia due to defects in cell cycle exit of retinoblasts. Genetic epistasis showed pigmentation defects depend on GTP synthesis pathway deficiency and microphthalmia on ATP synthesis pathway deficiency, placing paics at a bifurcation point in purine biosynthesis affecting two independent downstream pathways.\",\n      \"method\": \"Zebrafish recessive mutant analysis; maternal-zygotic and maternal-effect mutant analysis; genetic epistasis with separable ATP/GTP pathway outputs; purine supplementation rescue\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple mutant classes and pathway rescue, replicated across maternal and zygotic contexts\",\n      \"pmids\": [\"19570845\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The chicken GPAT (PPAT) and AIRC (PAICS) genes are divergently transcribed from a ~230 bp intergenic bidirectional promoter. An initiator-like element overlapping the AIRC transcription start site plays a central role in coordinating expression of both genes; removal of GC/CCAAT boxes from the AIRC proximal half disrupts bidirectional transcription.\",\n      \"method\": \"Deletion mutagenesis in a bireporter vector transfected into HepG2 and chicken LMH cells; gel retardation, DNase I, and methylation interference assays with HeLa nuclear proteins\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — deletion mutagenesis with multiple readouts in transfected cells, single lab, no in vivo validation\",\n      \"pmids\": [\"7836476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"NRF-1 and Sp1 bind to a cluster of sites (nt 215–260) in the human GPAT-AIRC bidirectional promoter; NRF-1 is required for stable Sp1 binding at this locus, and point mutations in the NRF-1 site or flanking Sp1 sites decrease expression of both GPAT and AIRC (PAICS) in transfected HepG2 cells.\",\n      \"method\": \"Promoter deletion and point-mutation analysis in transfected HepG2 cells; gel retardation assays identifying NRF-1 and Sp1 binding\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — point mutagenesis and binding assays, single lab, functional readout in transfected cells\",\n      \"pmids\": [\"9108165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"The bovine PAIS (PAICS) gene is syntenic with PRGS (PPAT) and SOD1, mapping to cattle syntenic group U10, which is conserved with human chromosome 21; this synteny was established by somatic cell hybrid complementation requiring bovine PAIS for growth on selective media.\",\n      \"method\": \"Somatic cell genetics with cow-hamster hybrid cell lines; enzyme electrophoresis for biochemical marker concordance\",\n      \"journal\": \"Biochemical genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — somatic cell hybrid complementation establishes functional gene identity and chromosomal synteny, single lab\",\n      \"pmids\": [\"3377762\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"A homozygous missense mutation p.Lys53Arg in PAICS reduces the catalytic activity of the enzyme to ~10% in patient fibroblasts and ~25% in recombinant purified protein compared to wild-type, establishing a direct catalytic deficiency. The mutation also prevents purinosome formation in patient fibroblasts, which was rescued by transfection with wild-type but not mutant PAICS.\",\n      \"method\": \"Enzyme activity assay in patient skin fibroblasts and recombinant E. coli-expressed protein; purinosome formation assessed by transfection rescue\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — recombinant enzyme activity assay combined with cellular rescue experiment establishing catalytic site importance and purinosome assembly role\",\n      \"pmids\": [\"31600779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Crystal structures of human PAICS in complex with native ligands revealed CAIR bound in both AIRc and SAICARs active sites and SAICAR bound in the SAICARs domain, as well as a structure with SAICAR and an ATP analog in the SAICARs active site. These structures define the architecture of both active sites and substrate/product binding modes.\",\n      \"method\": \"X-ray crystallography of PAICS complexes with native substrates and ATP analog\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple crystal structures with native ligands, extends prior structural work with substrate-bound complexes\",\n      \"pmids\": [\"32571877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PAICS knockdown is required for growth and survival of prostate cancer cells, as demonstrated by clonogenic survival and cell viability assays.\",\n      \"method\": \"Gene knockdown with clonogenic survival and cell viability assays in prostate cancer cell lines\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean knockdown with two orthogonal cell-based phenotypic assays, single lab\",\n      \"pmids\": [\"32571877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PAICS physically interacts with all other known de novo purine biosynthesis (DNPB) enzymes (except amidophosphoribosyltransferase/PPAT) and with MTHFD1, as demonstrated by bimolecular fluorescence complementation in live cells and co-immunoprecipitation of StrepTag-labeled PAICS reintegrated into PAICS-knockout HeLa cells. These interactions occur in both purine-depleted and purine-rich conditions. C-terminal tagging of PAICS disrupts these interactions and correlates with impaired DNPB activity and perturbed IMP partitioning into AMP and GMP.\",\n      \"method\": \"Bimolecular fluorescence complementation (BiFC) in live cells; Co-IP with StrepTag-labeled PAICS in CRISPR knockout HeLa cells; metabolic flux analysis of DNPB intermediates\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two orthogonal PPI methods (BiFC + reciprocal Co-IP in KO background), correlated with metabolic functional readout, single lab but rigorous controls\",\n      \"pmids\": [\"35331738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Time-course mass spectrometric analysis of 13C-bicarbonate incorporation demonstrated that the SAICAR synthetase domain of PAICS selectively uses enzyme-generated CAIR over exogenously added CAIR, providing biochemical evidence for substrate channeling of CAIR between the two active sites of PAICS.\",\n      \"method\": \"Time-course mass spectrometry with 13C-bicarbonate isotope tracing using recombinant human PAICS\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro isotope-tracing experiment with recombinant enzyme directly demonstrating channeling, consistent with structural data\",\n      \"pmids\": [\"35285625\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Density functional theory calculations based on PAICS crystal structures elucidated the reaction mechanism: AIRc carboxylation proceeds in two steps (C-C bond formation forming isoCAIR, then deprotonation assisted by an active-site histidine); SAICARs phosphorylation of CAIR precedes condensation with aspartate; three active-site magnesium ions bind substrates and stabilize transition states.\",\n      \"method\": \"Quantum chemical (DFT) calculations using large active-site models built from crystal structures; barriers compared to experimental kinetic data\",\n      \"journal\": \"Journal of the American Chemical Society\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — computational mechanistic study with experimental validation of calculated barriers, but no mutagenesis or direct biochemical confirmation of each step\",\n      \"pmids\": [\"35914774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"K6-polyubiquitination of PAICS by the Cul5/ASB11-based ubiquitin E3 ligase recruits UBAP2 (a ubiquitin-binding protein with intrinsically disordered regions) to induce phase separation and drive purinosome assembly. ASB11 upregulation upon purinosome-inducing stress (via relief of H3K9me3/HP1α transcriptional silencing) triggers this cascade. In human melanoma, constitutive ASB11 overexpression drives purinosome formation supporting proliferation and tumorigenesis.\",\n      \"method\": \"Co-immunoprecipitation; ubiquitination assays; phase separation/condensate imaging; CRISPR/genetic manipulation; xenograft tumor model\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal biochemical and cell biological methods identifying the E3 ligase, the ubiquitin linkage type, the reader protein, and the functional consequence in vivo\",\n      \"pmids\": [\"37848033\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of invertebrate (Trichoplusia ni) PAICS at 2.8 Å resolution confirmed the bifunctional domain architecture is highly conserved across divergent species and provided insights into substrate binding; comparison with human and prokaryotic homologs revealed a conserved enzymatic framework.\",\n      \"method\": \"X-ray crystallography (SAD phasing) of insect PAICS; comparative structural analysis\",\n      \"journal\": \"Proteins\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — independent crystal structure confirming domain architecture, no functional mutagenesis reported in this paper alone\",\n      \"pmids\": [\"23553965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PAICS knockdown modulates pyruvate kinase activity, and PAICS expression is induced by L-glutamine. A glutamine antagonist (DON) blocked glutamine-mediated induction of PAICS and reduced pyruvate kinase activity, placing PAICS expression under metabolic regulation by glutamine in lung cancer cells.\",\n      \"method\": \"Gene knockdown and overexpression studies; pyruvate kinase activity assay; DON treatment; cell proliferation and invasion assays\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockdown and chemical inhibition with enzymatic activity readout, single lab, two orthogonal approaches\",\n      \"pmids\": [\"26140362\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PAICS expression in prostate cancer is transcriptionally regulated by MYC; the BET bromodomain inhibitor JQ1 reduces PAICS expression and causes loss of MYC occupancy at the PAICS promoter. PAICS knockdown inhibits proliferation and invasion in prostate cancer cells in vitro and in vivo (CAM and xenograft models).\",\n      \"method\": \"BET inhibitor (JQ1) treatment; ChIP showing loss of MYC occupancy on PAICS promoter; siRNA/shRNA knockdown; CAM and murine xenograft models\",\n      \"journal\": \"The Prostate\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP plus functional knockdown studies linking MYC to PAICS transcription, single lab\",\n      \"pmids\": [\"27550065\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PAICS expression is negatively regulated post-transcriptionally by miR-128, which binds the PAICS 3'-UTR. In bladder cancer cells, PAICS induces EMT by positively regulating SNAI1 and reducing E-cadherin expression.\",\n      \"method\": \"3'-UTR luciferase reporter assay; miR-128 overexpression; qRT-PCR and immunoblot for SNAI1 and E-cadherin; in vitro proliferation/invasion and in vivo CAM assays\",\n      \"journal\": \"Neoplasia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — luciferase reporter validates miRNA-target interaction, EMT markers measured by Western blot, single lab\",\n      \"pmids\": [\"30121007\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PAICS knockdown in breast cancer cells blocks G1-S cell cycle transition, suppresses Cyclin E, upregulates Cyclin D1, P21, and CDK4, and activates PARP and caspase-3 while downregulating Bcl-2 and Bcl-xl, inducing apoptosis.\",\n      \"method\": \"Lentiviral shRNA knockdown; flow cytometry for cell cycle analysis; Western blot for cell cycle and apoptosis markers\",\n      \"journal\": \"Biological research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean knockdown with multiple molecular readouts for cell cycle and apoptosis, single lab\",\n      \"pmids\": [\"30097015\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In colorectal cancer cells, PAICS expression is transcriptionally activated by MYC (BET inhibitor JQ1 reduces PAICS expression) and negatively regulated by miR-128. PAICS knockdown upregulates E-cadherin (EMT marker) and reduces tumor growth and metastatic dissemination to liver, lungs, and bone in mice.\",\n      \"method\": \"JQ1 treatment; miR-128 overexpression; stable PAICS knockdown; murine xenograft and metastasis model with PET imaging\",\n      \"journal\": \"Cancers\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo validation with multiple metastatic readouts, mechanistic links to MYC and miR-128 from prior studies, single lab\",\n      \"pmids\": [\"32218208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PAICS interacts with HDAC1 and HDAC2 in gastric cancer cells; PAICS deficiency decreases RAD51 expression, impairs RAD51 recruitment to DNA damage sites by reducing HDAC1/2 deacetylase activity, and thereby prevents DNA damage repair, sensitizing cells to cisplatin.\",\n      \"method\": \"Co-immunoprecipitation (PAICS–HDAC1/HDAC2 interaction); RAD51 ChIP at damage sites; HDAC activity assay; cisplatin sensitivity assay in vitro and in vivo\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP and functional HDAC activity assay with cisplatin sensitization readout, single lab\",\n      \"pmids\": [\"32632107\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PAICS promotes FAK phosphorylation in breast cancer cells; miR-4731-5p inhibits this by targeting PAICS mRNA. PAICS-dependent FAK phosphorylation drives glycolysis, EMT, migration, and invasion, defining a miR-4731-5p/PAICS/FAK signaling axis.\",\n      \"method\": \"Luciferase 3'-UTR reporter confirming miR-4731-5p targets PAICS; Western blot for FAK phosphorylation; functional assays for glycolysis and EMT; xenograft model\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reporter assay validates miRNA-target, phosphorylation readout links PAICS to FAK pathway, in vivo validation, single lab\",\n      \"pmids\": [\"35379785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"PAICS interacts with DYRK3 kinase and co-regulates purinosome formation in oral squamous cell carcinoma; disrupting PAICS inhibits purinosome formation and affects survival of radiation-resistant OSCC cells. The DYRK3/PAICS axis contributes to radiotherapy resistance.\",\n      \"method\": \"In vitro cell models of radiation-resistant OSCC; protein interaction assays; GSK-626616 (DYRK3 inhibitor); purinosome formation imaging; in vivo tumor model\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — interaction and functional data from single lab with limited mechanistic detail in abstract; note: a correction was issued for this paper\",\n      \"pmids\": [\"38139175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACSS2 directly interacts with PAICS and promotes its acetylation; acetylated PAICS undergoes autophagy-mediated degradation, limiting purine biosynthesis and reducing dNTP pools, which exacerbates cytoplasmic chromatin fragment accumulation and drives the senescence-associated secretory phenotype (SASP).\",\n      \"method\": \"Co-immunoprecipitation (ACSS2–PAICS interaction); acetylation assays; autophagy inhibition experiments; Acss2 pharmacological inhibition and deletion in mice; dNTP pool measurement; SASP marker analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP identifying interaction plus acetylation/degradation assays and in vivo mouse model, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"40021646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In lung cancer cells, GART deletion inhibits the PAICS-Akt-β-catenin signaling pathway, suggesting GART acts upstream of PAICS to activate Akt-β-catenin and promote proliferation and migration.\",\n      \"method\": \"GART knockdown; qRT-PCR and Western blot for PAICS, p-Akt, and β-catenin; xenograft tumor model\",\n      \"journal\": \"Frontiers in oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, Western blot only for pathway placement, indirect evidence for PAICS in Akt signaling\",\n      \"pmids\": [\"40201340\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"PAICS loss-of-function in zebrafish (paics knockout) recapitulates cerebellar neuronal loss, neuromuscular junction disruption, motor impairment, and widespread DNA damage repair defects including suppression of key DNA repair pathways. Restoring paics expression in C9orf72-deficient zebrafish resolves DNA damage and preserves Purkinje and Granule cells, identifying PAICS as a critical mediator of cerebellar neuronal survival downstream of C9orf72.\",\n      \"method\": \"Zebrafish paics knockout; single-cell transcriptomics; rescue by paics re-expression in C9orf72 zebrafish; DNA damage assays; neuromuscular junction and motor behavioral analysis\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic rescue in vivo establishes PAICS as epistatic downstream mediator of C9orf72 cerebellar degeneration, single lab\",\n      \"pmids\": [\"41810938\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"IRF4 transcriptionally activates PAICS in DLBCL cells; PAICS physically interacts with LDHA and augments its activity, skewing the NAD+/NADH balance toward metabolic immunosuppression (elevated TGF-β and IL-10, reduced IFN-γ, enhanced CD8+ T cell exhaustion). This defines an IRF4-PAICS-LDHA axis.\",\n      \"method\": \"Co-immunoprecipitation (PAICS–LDHA interaction); LDHA activity assay; ChIP or transcriptional assay for IRF4-PAICS; cytokine profiling; PAICS/LDHA knockdown with T cell co-culture assays; tumor model\",\n      \"journal\": \"NPJ precision oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — Co-IP and activity assay support PAICS–LDHA interaction, but abstract does not detail rigor of IRF4 transcriptional assay; single lab, single paper\",\n      \"pmids\": [\"41991742\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PAICS is a bifunctional homo-octameric enzyme that catalyzes steps 6 (AIRc) and 7 (SAICARs) of de novo purine biosynthesis by converting AIR sequentially to CAIR and then to SAICAR via a two-step carboxylation mechanism and an ATP-dependent phosphorylation-condensation reaction, with CAIR channeled between the two active sites through internal tunnels; its octameric architecture is essential for carboxylase activity. PAICS assembles into the purinosome metabolon by interacting with all other DNPB enzymes and MTHFD1, a process driven by K6-polyubiquitination of PAICS by the Cul5/ASB11 E3 ligase, which recruits the disordered protein UBAP2 to trigger phase separation. PAICS expression is transcriptionally activated by MYC (and IRF4 in lymphoma) and repressed post-transcriptionally by miR-128; its protein stability is regulated by ACSS2-mediated acetylation leading to autophagic degradation. Beyond its biosynthetic role, PAICS participates in DNA damage response through interaction with HDAC1/2 (supporting RAD51 recruitment) and promotes EMT, proliferation, and invasion partly through PAICS-dependent FAK phosphorylation and activation of the Akt-β-catenin pathway.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PAICS is a bifunctional enzyme of de novo purine biosynthesis that catalyzes steps 6 and 7 of IMP synthesis, converting AIR to CAIR (AIR carboxylase) and then CAIR to SAICAR (SAICAR synthetase) [#1, #6]. The enzyme assembles into a homo-octamer in which an octameric carboxylase core is decorated by four peripheral synthetase dimers; this quaternary arrangement is itself essential, because each carboxylase active site is built from elements contributed by three subunits, and tunnel systems connecting the AIRc and SAICARs sites permit internal channeling of the CAIR intermediate [#0]. Isotope-tracing confirms that the synthetase domain selectively uses enzyme-generated CAIR over exogenous CAIR, providing direct biochemical evidence for substrate channeling [#9], and structural and computational work define a two-step carboxylation mechanism, an ATP-dependent phosphorylation-condensation with aspartate, and the magnesium ions that stabilize the transition states [#6, #10]. Beyond catalysis, PAICS nucleates the purinosome metabolon by physically interacting with the other DNPB enzymes and MTHFD1, with C-terminal tagging disrupting both these contacts and metabolic flux through the pathway [#8]; purinosome assembly is driven by K6-polyubiquitination of PAICS by the Cul5/ASB11 E3 ligase, which recruits the disordered protein UBAP2 to trigger phase separation [#11]. Enzymatic and assembly functions are clinically validated: a homozygous p.Lys53Arg mutation that reduces catalytic activity and abolishes purinosome formation causes a developmental disorder, with rescue by wild-type but not mutant protein [#5]. PAICS is transcriptionally activated by MYC and post-transcriptionally repressed by miR-128, and across multiple cancers its knockdown suppresses proliferation, invasion, EMT, and metastasis [#14, #15, #17]; reported effectors include HDAC1/2-dependent RAD51 recruitment for DNA repair [#18] and PAICS-dependent FAK phosphorylation [#19]. Protein abundance is further controlled by ACSS2-mediated acetylation that targets PAICS for autophagic degradation, linking purine and dNTP supply to senescence [#21].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 1988,\n      \"claim\": \"Before molecular characterization, establishing that the mammalian PAICS gene was a functional, mappable locus was needed; somatic cell hybrid complementation identified bovine PAICS and placed it in a synteny conserved with human chromosome 21 alongside PPAT and SOD1.\",\n      \"evidence\": \"Cow-hamster somatic cell hybrids with selective growth complementation and enzyme electrophoresis\",\n      \"pmids\": [\"3377762\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No protein-level or catalytic characterization\", \"Human gene structure not yet defined\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"How PAICS expression is coordinated with another purine-pathway gene was unknown; the chicken PPAT and PAICS genes were shown to share a compact bidirectional promoter, with an initiator-like element at the PAICS start site coordinating both genes.\",\n      \"evidence\": \"Deletion mutagenesis in a bireporter vector with DNase I/methylation interference footprinting\",\n      \"pmids\": [\"7836476\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Trans-acting factors not yet identified\", \"Avian promoter, not validated in human in vivo\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"The trans-acting factors driving the bidirectional promoter were undefined; NRF-1 and Sp1 were shown to bind a site cluster, with NRF-1 required for stable Sp1 binding and both needed for full expression of both genes.\",\n      \"evidence\": \"Promoter point-mutation analysis and gel retardation in transfected HepG2 cells\",\n      \"pmids\": [\"9108165\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No in vivo occupancy data\", \"Regulation under metabolic demand not addressed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The structural basis of PAICS bifunctionality was unknown; the human crystal structure revealed a homo-octamer whose carboxylase core requires three subunits per active site and contains tunnels linking the two catalytic sites, implying intermediate channeling.\",\n      \"evidence\": \"X-ray crystallography at 2.8 Å with functional complementation to map active sites\",\n      \"pmids\": [\"17224163\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate-bound states not captured\", \"Channeling inferred from tunnels, not directly demonstrated\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The physiological consequences of PAICS loss in a vertebrate were untested; zebrafish mutants placed paics at a bifurcation point where pigmentation defects depend on GTP-pathway output and microphthalmia on ATP-pathway output.\",\n      \"evidence\": \"Zebrafish recessive mutant analysis with genetic epistasis and purine supplementation rescue\",\n      \"pmids\": [\"19570845\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific mechanisms not resolved\", \"Relevance to human disease not yet established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Whether the bifunctional architecture was conserved was addressed by an invertebrate PAICS structure, confirming a deeply conserved domain organization and enzymatic framework.\",\n      \"evidence\": \"X-ray crystallography (SAD phasing) of Trichoplusia ni PAICS with comparative analysis\",\n      \"pmids\": [\"23553965\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional mutagenesis in this study\", \"Catalytic mechanism still not defined at atomic detail\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The drivers of elevated PAICS in cancer were unclear; in prostate cancer MYC was shown to occupy the PAICS promoter and JQ1 reduced this occupancy and expression, with knockdown suppressing proliferation, invasion, and tumor growth.\",\n      \"evidence\": \"ChIP, BET inhibition, knockdown, and CAM/xenograft models in prostate cancer\",\n      \"pmids\": [\"27550065\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether catalytic vs non-catalytic activity drives phenotype not separated\", \"Single cancer context\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Post-transcriptional control and downstream oncogenic effects were addressed: miR-128 targets the PAICS 3'-UTR, and PAICS promotes EMT (SNAI1 up, E-cadherin down) and the G1-S transition while restraining apoptosis.\",\n      \"evidence\": \"3'-UTR luciferase reporters, knockdown with cell-cycle/EMT/apoptosis marker profiling in bladder and breast cancer\",\n      \"pmids\": [\"30121007\", \"30097015\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking purine synthesis to EMT/cell-cycle markers not defined\", \"Effectors downstream of PAICS unclear\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Whether PAICS catalytic activity and purinosome assembly are physiologically essential in humans was tested by a homozygous p.Lys53Arg mutation that lowers activity to ~10–25% and abolishes purinosome formation, rescued only by wild-type protein.\",\n      \"evidence\": \"Enzyme assays in patient fibroblasts and recombinant protein with transfection rescue of purinosome formation\",\n      \"pmids\": [\"31600779\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full clinical spectrum from a single residue not generalizable\", \"Link between catalysis and assembly defect mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Ligand-bound structures and cancer dependency were established together: structures captured CAIR in both sites, SAICAR with an ATP analog, and PAICS knockdown impaired prostate cancer growth and survival.\",\n      \"evidence\": \"X-ray crystallography of native-ligand complexes plus clonogenic and viability assays\",\n      \"pmids\": [\"32571877\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reaction trajectory between sites not directly observed\", \"Dependency mechanism (metabolic vs other) not dissected\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"A non-canonical nuclear role was proposed: PAICS interacts with HDAC1/2 to support RAD51 recruitment and DNA damage repair, with loss sensitizing gastric cancer cells to cisplatin.\",\n      \"evidence\": \"Reciprocal Co-IP, RAD51 ChIP at damage sites, HDAC activity assay, and cisplatin sensitivity in vitro and in vivo\",\n      \"pmids\": [\"32632107\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How a metabolic enzyme localizes to chromatin to modulate HDAC activity unexplained\", \"Single cancer type\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Metastatic relevance and converging regulation were confirmed in colorectal cancer, where MYC activates and miR-128 represses PAICS, and knockdown reduced metastasis to liver, lung, and bone.\",\n      \"evidence\": \"JQ1, miR-128 overexpression, stable knockdown, and PET-imaged xenograft/metastasis models\",\n      \"pmids\": [\"32218208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal step between PAICS and metastatic colonization not defined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Direct evidence for substrate channeling and metabolon assembly was obtained: isotope tracing showed the synthetase site preferentially uses enzyme-generated CAIR, while PAICS was shown to physically organize the purinosome by binding all other DNPB enzymes and MTHFD1.\",\n      \"evidence\": \"13C-bicarbonate time-course MS on recombinant PAICS; BiFC and reciprocal Co-IP in CRISPR-KO HeLa with metabolic flux analysis\",\n      \"pmids\": [\"35285625\", \"35331738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and architecture of the assembled metabolon not resolved\", \"PPAT exclusion from PAICS interactome unexplained\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The atomic reaction mechanism was elucidated computationally: two-step carboxylation via isoCAIR with histidine-assisted deprotonation, phosphorylation-before-condensation in the synthetase site, and three catalytic magnesium ions; a parallel oncogenic axis linked PAICS to FAK phosphorylation.\",\n      \"evidence\": \"DFT calculations from crystal structures benchmarked against kinetics; Western blot/functional assays for the miR-4731-5p/PAICS/FAK axis\",\n      \"pmids\": [\"35914774\", \"35379785\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic steps not confirmed by mutagenesis\", \"Direct vs indirect basis of PAICS-driven FAK phosphorylation unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"The molecular trigger for purinosome assembly was defined: K6-polyubiquitination of PAICS by Cul5/ASB11 recruits the disordered reader UBAP2 to induce phase separation, with ASB11 overexpression driving melanoma tumorigenesis.\",\n      \"evidence\": \"Co-IP, ubiquitination assays, condensate imaging, CRISPR manipulation, and xenografts\",\n      \"pmids\": [\"37848033\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How ubiquitinated PAICS templates inclusion of other enzymes not detailed\", \"Reversibility/turnover of the condensate not addressed\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A protein-stability control axis was established: ACSS2 directly interacts with and acetylates PAICS, targeting it for autophagic degradation, thereby limiting dNTP pools and promoting the senescence-associated secretory phenotype.\",\n      \"evidence\": \"Co-IP, acetylation and autophagy assays, ACSS2 inhibition/deletion in mice, dNTP and SASP measurements\",\n      \"pmids\": [\"40021646\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Acetylated residues and the degradation receptor not defined\", \"Interplay with ubiquitination-driven assembly not addressed\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"A neurodevelopmental/neurodegenerative role was placed downstream of C9orf72: paics-knockout zebrafish show cerebellar neuronal loss and DNA repair defects, and restoring paics in C9orf72-deficient fish preserves Purkinje and Granule cells.\",\n      \"evidence\": \"Zebrafish knockout with single-cell transcriptomics, DNA damage assays, and cross-genotype rescue\",\n      \"pmids\": [\"41810938\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting purine supply to C9orf72-dependent neuronal survival unresolved\", \"Mammalian validation absent\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"An immunometabolic moonlighting function was proposed: IRF4 activates PAICS in DLBCL, and PAICS binds and augments LDHA to skew NAD+/NADH toward immunosuppression and CD8+ T cell exhaustion.\",\n      \"evidence\": \"Co-IP, LDHA activity assay, transcriptional assay for IRF4, cytokine profiling, and T cell co-culture/tumor models\",\n      \"pmids\": [\"41991742\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single lab, single paper; IRF4 transcriptional regulation rigor not detailed\", \"PAICS-LDHA interaction not reciprocally validated across systems\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how PAICS's catalytic, metabolon-scaffolding, and reported non-canonical (chromatin, FAK, LDHA) functions are mechanistically separated and which are direct versus secondary to altered purine/dNTP supply.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure-function separation of catalytic vs scaffolding mutants in disease/cancer models\", \"Direct biochemical basis of nuclear and signaling roles undefined\", \"Regulatory cross-talk between ubiquitination, acetylation, and phase separation unmapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 1, 6, 9, 10]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [6, 9, 10]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [6, 10]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [8, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [8, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 1, 6, 9]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [9, 10]}\n    ],\n    \"complexes\": [\"purinosome\"],\n    \"partners\": [\"MTHFD1\", \"UBAP2\", \"ASB11\", \"Cul5\", \"HDAC1\", \"HDAC2\", \"ACSS2\", \"LDHA\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}