{"gene":"IMPDH1","run_date":"2026-06-10T01:55:23","timeline":{"discoveries":[{"year":2002,"finding":"IMPDH1 encodes a homotetramer that catalyzes the rate-limiting step in de novo guanine nucleotide synthesis (IMP → XMP); the Asp226Asn missense mutation at a universally conserved residue causes the RP10 form of autosomal dominant retinitis pigmentosa.","method":"DNA sequencing of affected families, linkage mapping, evolutionary conservation analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — mutation identified in multiple independent families across three countries, conserved-residue analysis, replicated by multiple subsequent studies","pmids":["11875050"],"is_preprint":false},{"year":2006,"finding":"IMPDH1 disease-causing missense mutations (Thr116Met, Asp226Asn, Val268Ile, Gly324Asp, His372Pro, Arg105Trp, Asn198Lys) do not alter enzymatic activity but alter the affinity and/or specificity of single-stranded nucleic acid binding, as measured by filter-binding assays; the Gly324Asp variant is an exception and does not alter nucleic acid binding.","method":"NADH fluorescence enzymatic assay; filter-binding assays for single-stranded nucleic acids","journal":"Investigative ophthalmology & visual science","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro enzymatic assay and nucleic acid binding assay on multiple mutants, single lab with two orthogonal methods","pmids":["16384941"],"is_preprint":false},{"year":2004,"finding":"IMPDH1 is the primary source of GTP in photoreceptors (vs. IMPDH2 and HPRT-mediated salvage), shown by transcript analysis in retinal sections. Impdh1−/− null mice develop slowly progressive retinal degeneration with compromised electroretinographic responses. Expression of mutant IMPDH1 in bacterial and mammalian cells, combined with computational simulations, indicates protein misfolding and aggregation rather than reduced enzymatic activity is the likely disease mechanism.","method":"Transcript analysis in mouse retinal sections; Impdh1 knockout mouse ERG; recombinant protein expression in bacterial and mammalian cells; computational folding simulations","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (knockout mouse phenotype, expression studies, computational analysis) in a single focused study, replicated by subsequent work","pmids":["14981049"],"is_preprint":false},{"year":2006,"finding":"IMPDH1 is expressed at higher levels in the retina than any other tested tissue, predominantly in the inner segments and synaptic terminals of photoreceptors. Human retina contains unique alternatively spliced and alternate-start-site IMPDH1 isoforms that encode distinct proteins not found in other tissues, and the proportions of these isoforms differ between human and mouse retina.","method":"Northern blot, SAGE, immunohistochemistry, transcript sequencing, Western blot","journal":"Investigative ophthalmology & visual science","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Northern, IHC, Western, sequencing) in a single study establishing retina-specific isoforms and localization","pmids":["16936083"],"is_preprint":false},{"year":2008,"finding":"AAV-mediated expression of mutant human IMPDH1 in mouse retina causes aggressive retinopathy modelling the human disease, whereas expression of normal human IMPDH1 has no pathological effect. AAV co-expression of shRNAs targeting both human and mouse IMPDH1 together with mutant IMPDH1 substantially suppresses photoreceptor loss caused by the mutant protein.","method":"Recombinant AAV subretinal injection in mice; electroretinography; histology; shRNA knockdown validated in vitro and in vivo","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — rigorous in vivo model with multiple controls (WT vs mutant overexpression, shRNA rescue), ERG and histological readouts","pmids":["18385099"],"is_preprint":false},{"year":2020,"finding":"Retinal IMPDH1 undergoes light-dependent phosphorylation at Thr159/Ser160 in the Bateman domain in vivo; this phosphorylation desensitizes the enzyme to allosteric inhibition by GDP/GTP. Bright-light exposure in mice increases the rate of GTP and ATP synthesis in the retina, coincident with IMPDH1 aggregate (filament) formation at the outer segment layer. Pharmacological inhibition of IMPDH activity in living mice delays rod recovery after light exposure.","method":"Mass spectrometry phosphoproteomics of mouse retina; metabolic flux assays; immunofluorescence of IMPDH1 aggregates; pharmacological inhibition (IMPDH inhibitor) in vivo with ERG","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo phosphoproteomics, metabolic flux, imaging, and pharmacological intervention with functional ERG readout in a single study","pmids":["32254022"],"is_preprint":false},{"year":2022,"finding":"Human IMPDH1 assembles polymorphic filaments with distinct assembly interfaces in extended (high-activity) and compressed (low-activity) states, as resolved by cryo-EM. Retina-specific splice variants introduce structural elements that reduce sensitivity to GTP inhibition, including stabilization of the extended filament form. Disease mutations fall into two classes: one class disrupts GTP regulation (e.g., Asp226Asn confers resistance to GDP/GTP feedback inhibition), while the second class has no effect on GTP regulation or filament assembly.","method":"Cryo-electron microscopy structures of IMPDH1 filaments; enzymatic assays with GTP inhibition; comparison of retinal splice variants vs. canonical isoform","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple cryo-EM structures with functional validation by enzymatic assays; rigorous structure-function analysis of retinal variants and disease mutants","pmids":["35013599"],"is_preprint":false},{"year":2024,"finding":"Phosphomimetic S477D mutation in retinal IMPDH1 splice variants re-sensitizes both IMPDH1(546) and IMPDH1(595) to GTP inhibition and specifically blocks assembly of the high-activity IMPDH1(595) filament interface (but not the low-activity IMPDH1(546) interface), as shown by cryo-EM. S477D exerts a dominant-negative effect in cells, preventing endogenous IMPDH filament assembly. In bovine retinas, S477 is preferentially phosphorylated in the dark, establishing phosphorylation at S477 as a mechanism for downregulating retinal GTP synthesis in the dark.","method":"Phosphomimetic mutagenesis; cryo-EM structures; enzymatic GTP inhibition assays; cell-based dominant-negative filament assembly assay; bovine retina phosphoproteomics","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures combined with mutagenesis, enzymatic assays, and cell-based assays; corroborated by bovine retina phosphoproteomics","pmids":["38323936"],"is_preprint":false},{"year":2011,"finding":"IMPDH1 clinical mutants R224P and D226N show impaired folding in vitro. In equimolar mixtures of normal and mutant IMPDH1, activity regain after refolding is close to the mutant level rather than the midpoint, indicating that mutant subunits impose their misfolded conformation on wild-type partners in hybrid tetramers (molecular recruitment), providing a mechanism for dominant-negative inheritance.","method":"Recombinant protein expression; in vitro refolding assay; enzymatic activity measurement of mixed wild-type/mutant tetramers","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution with functional readout, but single lab, single method, and the canonical retinal isoform was not used","pmids":["21791244"],"is_preprint":false},{"year":2012,"finding":"The adRP mutation D226N reduces IMPDH1 binding to nucleic acids and reduces association with polyribosomes, suggesting a role for IMPDH1 in posttranscriptional regulation of rhodopsin mRNA in the retina.","method":"Nucleic acid binding assays; polyribosome fractionation (referenced from prior experimental work)","journal":"Advances in experimental medicine and biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — experimental data cited but detailed methods not fully described in the abstract; supported by prior filter-binding data","pmids":["22183375"],"is_preprint":false},{"year":2010,"finding":"A novel IMPDH1 allozyme (Leu275) has approximately 10% of wild-type enzymatic activity, due to accelerated protein degradation rather than intrinsic catalytic impairment, as supported by X-ray crystal structure analysis.","method":"Enzymatic activity assay; protein stability/degradation assay; structural analysis using IMPDH1 X-ray crystal structure","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro enzymatic assay and structural rationalization, single lab","pmids":["20718729"],"is_preprint":false},{"year":2021,"finding":"Retinal IMPDH1 isoforms bearing C/N-terminal extensions (mIMPDH1(546) and mIMPDH1(603)) have higher Km and Ki values relative to the canonical isoform (mIMPDH1(514)), do not exhibit NAD+ substrate inhibition unlike the canonical isoform, and the retinal mIMPDH1(603) isoform shows lower fibrillation capacity under ATP but forms higher-mass oligomers in the presence of GTP and/or MPA. Molecular dynamic simulations indicate terminal peptides interact with the enzyme's finger domain affecting its pseudo barrel structure.","method":"Recombinant protein kinetic assays; native PAGE oligomerization analysis; thioflavin T fibrillation assay; molecular dynamics simulation","journal":"Cell biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro enzymatic kinetics and oligomerization assays with computational support, single lab","pmids":["33733369"],"is_preprint":false},{"year":2019,"finding":"Mouse retinal IMPDH1 isoforms (mIMPDH1(546) and mIMPDH1(603)) show higher Km and Ki values than the canonical mIMPDH1(514), and molecular dynamics simulations indicate that terminal peptide extensions interact with the enzyme's finger domain affecting the critical pseudo-barrel structure.","method":"Recombinant protein kinetic assays; enzymatic inhibition assays; molecular modeling and molecular dynamics simulation","journal":"Molecular and cellular biochemistry","confidence":"Low","confidence_rationale":"Tier 1 for in vitro kinetics / Weak — single lab, computational rationalization without mutagenesis validation","pmids":["31838626"],"is_preprint":false},{"year":2020,"finding":"IMPDH1 and YB-1 form an autoregulatory positive feedback loop in clear cell renal cell carcinoma: IMPDH1 maintains YB-1 protein stabilization, and YB-1 induces IMPDH1 expression by binding the IMPDH1 promoter. IMPDH1-assembled cytoophidia (filaments) physically interact with YB-1 and translocate YB-1 into the cell nucleus.","method":"Co-immunoprecipitation; immunofluorescence; promoter binding assay; ChIP; YB-1 protein stability assay; nuclear fractionation","journal":"Molecular therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP and ChIP with functional nuclear translocation readout, single lab with multiple orthogonal methods","pmids":["32209435"],"is_preprint":false},{"year":2023,"finding":"IMPDH2 stabilizes IMPDH1 protein by decreasing its polyubiquitination levels. c-Myc transcriptionally activates both IMPDH1 and IMPDH2 to promote de novo GTP biosynthesis.","method":"Co-immunoprecipitation; protein half-life assay; polyubiquitination assay; ChIP/transcriptional activation assay; colorectal cancer cell lines and xenograft models","journal":"Clinical and translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP for protein-protein interaction, ubiquitination assay, and transcriptional assay, single lab","pmids":["36629054"],"is_preprint":false},{"year":2022,"finding":"MYBL2 directly binds the IMPDH1 promoter and transcriptionally activates IMPDH1 expression in hepatocellular carcinoma cells, as demonstrated by ChIP-seq and ChIP-qPCR. MYBL2 knockout reduces IMPDH1 expression and inhibits guanine nucleotide synthesis.","method":"ChIP-seq; ChIP-qPCR; MYBL2 knockout; metabolomics; xenograft tumor models","journal":"BMC cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP-seq and ChIP-qPCR with functional metabolomic validation, single lab","pmids":["36494680"],"is_preprint":false},{"year":2023,"finding":"The IMPDH1 Asp226Asn (D226N) mutation causes cytoophidium (filament) assembly in ~70% of HEp-2 cells and reduces long-term cell survival by ~40% compared to wild-type. Introduction of a secondary Y12C mutation disrupts filament assembly and significantly rescues cell survival (only ~10% decrease vs. WT), indicating that cytoophidium formation contributes to D226N cytotoxicity. D226N also confers resistance to feedback inhibition by GDP/GTP.","method":"Stable cell line expression; immunofluorescence; long-term survival assay; double-mutant (Y12C/D226N) functional analysis","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — double-mutant genetic dissection of filament assembly vs. GTP regulation with cell survival readout, single lab","pmids":["37731818"],"is_preprint":false},{"year":2006,"finding":"A 9-bp insertion in the IMPDH1 P3 promoter abolishes a cAMP-response element (CRE) and significantly reduces IMPDH1 P3 promoter activity in a luciferase reporter assay, identifying a transcriptional regulatory element controlling IMPDH1 expression.","method":"Luciferase reporter gene assay; dHPLC and DNA sequencing","journal":"The pharmacogenomics journal","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct luciferase reporter assay establishing promoter element function, single patient/single lab","pmids":["17001353"],"is_preprint":false},{"year":1994,"finding":"The human IMPDH1 gene is located on chromosome 7q31.3-q32, established by PCR analysis of somatic cell hybrid panels and fluorescence in situ hybridization.","method":"PCR of human-mouse and human-hamster somatic cell hybrids; FISH with metaphase chromosomes","journal":"Genomics","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent methods (somatic cell hybrid PCR and FISH) concordantly assign chromosomal location; replicated by linkage studies","pmids":["7896275"],"is_preprint":false},{"year":2025,"finding":"Following radiation therapy, IMPDH1 translocates from the cytoplasm to the nucleus in glioblastoma cells in a DNA-PK-dependent manner, leading to nuclear GTP accumulation that promotes DNA damage repair and radiation resistance. Pharmacological inhibition of IMPDH with mycophenolate mofetil slows DNA damage repair, extends survival in orthotopic murine models with combined RT/TMZ, and achieves active intracranial drug concentrations in a human phase 0 clinical trial with reversal of IMPDH upstream/downstream metabolites.","method":"Subcellular fractionation/imaging of IMPDH1 localization; DNA-PK inhibitor epistasis; metabolic flux assays; orthotopic murine GBM models with ERG/survival endpoints; phase 0 clinical trial with tumor metabolomics","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal in vitro and in vivo methods plus human clinical data; preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.11.11.25340023"],"is_preprint":true}],"current_model":"IMPDH1 is a homotetrameric enzyme that catalyzes the rate-limiting step in de novo guanine nucleotide synthesis; in the retina it exists as tissue-specific splice variants that assemble polymorphic filaments regulated by allosteric GDP/GTP feedback and light-dependent phosphorylation at S477 (dark) and Thr159/Ser160 (Bateman domain, light), with filament formation desensitizing the enzyme to feedback inhibition to boost GTP synthesis during light exposure; disease-causing missense mutations cluster in the Bateman/CBS domain and predominantly disrupt GTP allosteric regulation and/or nucleic acid binding rather than catalytic activity, with dominant-negative effects propagated through hybrid tetramer misfolding and toxic cytoophidium formation."},"narrative":{"mechanistic_narrative":"IMPDH1 is a homotetrameric enzyme that catalyzes the rate-limiting step of de novo guanine nucleotide synthesis (IMP → XMP) and is the principal source of GTP in photoreceptors, where its loss in knockout mice produces slowly progressive retinal degeneration [PMID:11875050, PMID:14981049]. The enzyme assembles into polymorphic filaments resolved by cryo-EM in extended high-activity and compressed low-activity states, and the retina expresses tissue-specific alternatively spliced and alternate-start isoforms bearing N/C-terminal extensions that raise Km/Ki, eliminate canonical NAD+ substrate inhibition, and reduce sensitivity to GTP feedback by stabilizing the extended filament form [PMID:16936083, PMID:35013599, PMID:33733369]. Retinal activity is set by light-dependent phosphorylation: phosphorylation of Thr159/Ser160 in the Bateman domain during light desensitizes the enzyme to GDP/GTP feedback and coincides with filament formation and increased GTP/ATP synthesis, whereas phosphorylation at S477 in the dark re-sensitizes the enzyme to GTP inhibition and blocks high-activity filament assembly, the S477D phosphomimetic acting dominant-negatively on endogenous filaments [PMID:32254022, PMID:38323936]. Missense mutations in the Bateman/CBS domain cause RP10 autosomal dominant retinitis pigmentosa; rather than abolishing catalysis, these mutations impair single-stranded nucleic acid binding and/or confer resistance to GDP/GTP feedback inhibition (e.g., Asp226Asn), and the dominant disease mechanism reflects protein misfolding, recruitment of wild-type subunits into hybrid tetramers, and toxic cytoophidium formation that reduces cell survival—a toxicity rescued by disrupting filament assembly [PMID:11875050, PMID:16384941, PMID:35013599, PMID:21791244, PMID:37731818]. AAV-delivered mutant IMPDH1 reproduces the retinopathy in mice and is suppressed by shRNA knockdown, supporting a gain-of-function/dominant-negative basis [PMID:18385099]. Beyond the retina, IMPDH1 expression is driven by c-Myc, MYBL2, and a P3-promoter CRE, its protein stabilized by IMPDH2, and it participates in cancer-associated circuits including a YB-1 positive-feedback loop and DNA-PK-dependent nuclear translocation supporting DNA-damage repair [PMID:32209435, PMID:36629054, PMID:36494680, PMID:17001353, PMID:bio_10.1101_2025.11.11.25340023].","teleology":[{"year":1994,"claim":"Establishing the chromosomal location of human IMPDH1 provided the genetic anchor later needed to link the gene to an inherited retinal disease locus.","evidence":"PCR of somatic cell hybrids and FISH","pmids":["7896275"],"confidence":"High","gaps":["Does not address gene function or expression","No tissue or isoform information"]},{"year":2002,"claim":"Identifying the Asp226Asn mutation in RP10 families connected this metabolic enzyme to autosomal dominant retinitis pigmentosa, raising the question of why a housekeeping enzyme causes a tissue-specific dominant disease.","evidence":"Linkage mapping and sequencing across multiple families, conserved-residue analysis","pmids":["11875050"],"confidence":"High","gaps":["Mechanism of dominance unresolved","Does not explain retina specificity","No functional consequence of the mutation tested"]},{"year":2004,"claim":"Showing IMPDH1 is the dominant GTP source in photoreceptors and that mutants misfold rather than lose activity reframed the disease as a protein-conformation problem, not simple enzyme deficiency.","evidence":"Retinal transcript analysis, Impdh1 knockout mouse ERG, recombinant expression, folding simulations","pmids":["14981049"],"confidence":"High","gaps":["Knockout phenotype milder than dominant disease","Aggregation mechanism in photoreceptors not directly imaged","Does not identify the misfolding propagation route"]},{"year":2006,"claim":"Demonstrating retina-enriched, retina-unique splice/start-site isoforms and showing disease mutants alter nucleic-acid binding rather than catalysis explained tissue specificity and pointed to a non-catalytic role for IMPDH1.","evidence":"Northern/SAGE/IHC/Western isoform mapping; NADH enzymatic assays and filter-binding assays on seven mutants; P3 promoter CRE luciferase reporter","pmids":["16936083","16384941","17001353"],"confidence":"High","gaps":["Physiological nucleic-acid targets not identified","Function of retina-specific extensions unknown at this stage","Gly324Asp exception unexplained"]},{"year":2008,"claim":"An AAV mouse model recapitulating retinopathy with mutant but not wild-type IMPDH1, and rescue by shRNA, confirmed a gain-of-function/dominant-negative mechanism amenable to knockdown therapy.","evidence":"Subretinal AAV expression and shRNA rescue with ERG and histology","pmids":["18385099"],"confidence":"High","gaps":["Molecular nature of the toxic species not defined","Does not distinguish aggregation from regulatory dysfunction"]},{"year":2011,"claim":"In vitro refolding of mixed wild-type/mutant tetramers showed mutant subunits impose their misfolded state on wild-type partners, providing a concrete dominant-negative recruitment mechanism.","evidence":"Recombinant refolding and activity assays of equimolar WT/mutant mixtures","pmids":["21791244"],"confidence":"Medium","gaps":["Single lab, single method","Canonical retinal isoform not used","Relevance in vivo not demonstrated"]},{"year":2012,"claim":"Linking D226N nucleic-acid binding loss to reduced polyribosome association suggested IMPDH1 contributes to posttranscriptional regulation of rhodopsin mRNA, a candidate non-catalytic retinal function.","evidence":"Nucleic-acid binding and polyribosome fractionation (cited prior work)","pmids":["22183375"],"confidence":"Medium","gaps":["Direct mRNA targets not identified","Methods incompletely described","Causal link to disease unproven"]},{"year":2020,"claim":"Discovery of light-dependent Bateman-domain phosphorylation (Thr159/Ser160) that desensitizes feedback inhibition, coupled to filament formation and increased GTP flux, established a physiological logic for retinal IMPDH1 regulation tied to light exposure.","evidence":"In vivo phosphoproteomics, metabolic flux, filament imaging, pharmacological IMPDH inhibition with ERG","pmids":["32254022"],"confidence":"High","gaps":["Kinase responsible not identified","How phosphorylation couples to filament assembly structurally unresolved"]},{"year":2021,"claim":"Kinetic characterization of retinal isoforms showed terminal extensions raise Km/Ki, abolish NAD+ substrate inhibition, and alter oligomerization, explaining how retina-specific isoforms tune activity and assembly.","evidence":"Recombinant kinetics, native PAGE, thioflavin T fibrillation, molecular dynamics","pmids":["33733369","31838626"],"confidence":"Medium","gaps":["Computational finger-domain interaction not validated by mutagenesis","In vivo relevance of altered kinetics untested"]},{"year":2022,"claim":"Cryo-EM of extended high-activity and compressed low-activity filaments, with retinal variants stabilizing the extended form and disease mutants partitioning into GTP-regulation-disrupting vs. neutral classes, provided the structural framework unifying regulation and disease.","evidence":"Multiple cryo-EM structures with enzymatic GTP-inhibition validation and variant comparison","pmids":["35013599"],"confidence":"High","gaps":["Mechanism of the second mutation class (no GTP/filament effect) unexplained","Filament function in living photoreceptors not directly resolved"]},{"year":2023,"claim":"Genetic dissection of D226N showed cytoophidium assembly itself drives cytotoxicity—a Y12C filament-blocking mutation rescued survival—separating toxic filament formation from the mutation's GTP-regulation defect.","evidence":"Stable cell lines, immunofluorescence, long-term survival assay, double-mutant analysis","pmids":["37731818"],"confidence":"Medium","gaps":["HEp-2 cell model not photoreceptor-specific","Does not weigh filament toxicity against feedback-resistance in vivo"]},{"year":2023,"claim":"Identification of c-Myc/MYBL2 transcriptional activation and IMPDH2-mediated stabilization of IMPDH1 protein revealed how IMPDH1 levels and GTP biosynthesis are amplified in cancer.","evidence":"ChIP/transcriptional assays, Co-IP, ubiquitination and half-life assays in colorectal and hepatocellular models with xenografts","pmids":["36629054","36494680"],"confidence":"Medium","gaps":["Direct contribution of IMPDH1 vs IMPDH2 to tumor GTP pools not separated","Single-lab interaction data"]},{"year":2024,"claim":"Showing S477 is phosphorylated in the dark to re-sensitize the enzyme to GTP inhibition and to selectively block the high-activity filament interface defined the antagonistic dark/light phosphorylation switch governing retinal GTP synthesis.","evidence":"Phosphomimetic cryo-EM, GTP-inhibition assays, cell-based dominant-negative filament assays, bovine retina phosphoproteomics","pmids":["38323936"],"confidence":"High","gaps":["Kinase/phosphatase that toggles S477 not identified","Integration with Thr159/Ser160 signaling unresolved"]},{"year":2025,"claim":"DNA-PK-dependent nuclear translocation of IMPDH1 supplying GTP for DNA-damage repair extended IMPDH1's role beyond the retina to a druggable mediator of radiation resistance in glioblastoma.","evidence":"Subcellular fractionation, DNA-PK epistasis, flux assays, orthotopic GBM models, phase 0 trial metabolomics (preprint)","pmids":["bio_10.1101_2025.11.11.25340023"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed","Mechanism of nuclear import signal undefined","Relation to filament assembly in this context unknown"]},{"year":null,"claim":"The identity of the kinases/phosphatases driving the light-dependent Thr159/Ser160 and dark S477 phosphorylation switch, and the physiological nucleic-acid/mRNA targets of IMPDH1's non-catalytic binding activity, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No kinase/phosphatase identified for either phosphosite","Endogenous nucleic-acid ligands not defined","Second class of disease mutations (no GTP/filament effect) lacks a mechanism"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016491","term_label":"oxidoreductase activity","supporting_discovery_ids":[0,2]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,6,7]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1,9]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[13,19]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[13,19]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[6,16]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,2,5]},{"term_id":"R-HSA-9709957","term_label":"Sensory Perception","supporting_discovery_ids":[2,3,5]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,4,16]}],"complexes":["IMPDH1 homotetramer/filament (cytoophidium)"],"partners":["IMPDH2","YB-1","MYBL2","C-MYC","DNA-PK"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P20839","full_name":"Inosine-5'-monophosphate dehydrogenase 1","aliases":["IMPDH-I"],"length_aa":514,"mass_kda":55.4,"function":"Catalyzes the conversion of inosine 5'-phosphate (IMP) to xanthosine 5'-phosphate (XMP), the first committed and rate-limiting step in the de novo synthesis of guanine nucleotides, and therefore plays an important role in the regulation of cell growth. Could also have a single-stranded nucleic acid-binding activity and could play a role in RNA and/or DNA metabolism. It may also have a role in the development of malignancy and the growth progression of some tumors","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/P20839/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/IMPDH1","classification":"Not Classified","n_dependent_lines":123,"n_total_lines":1208,"dependency_fraction":0.10182119205298014},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"PARP1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/IMPDH1","total_profiled":1310},"omim":[{"mim_id":"618605","title":"ANKYRIN REPEAT DOMAIN-CONTAINING PROTEIN 9; ANKRD9","url":"https://www.omim.org/entry/618605"},{"mim_id":"613837","title":"LEBER CONGENITAL AMAUROSIS 11; LCA11","url":"https://www.omim.org/entry/613837"},{"mim_id":"268000","title":"RETINITIS PIGMENTOSA; RP","url":"https://www.omim.org/entry/268000"},{"mim_id":"204000","title":"LEBER CONGENITAL AMAUROSIS 1; LCA1","url":"https://www.omim.org/entry/204000"},{"mim_id":"180105","title":"RETINITIS PIGMENTOSA 10; RP10","url":"https://www.omim.org/entry/180105"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Rods & Rings","reliability":"Supported"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"retina","ntpm":98.6}],"url":"https://www.proteinatlas.org/search/IMPDH1"},"hgnc":{"alias_symbol":["sWSS2608","LCA11"],"prev_symbol":["RP10"]},"alphafold":{"accession":"P20839","domains":[{"cath_id":"3.20.20.70","chopping":"20-111_237-499","consensus_level":"high","plddt":94.9512,"start":20,"end":499},{"cath_id":"3.10.580.10","chopping":"128-227","consensus_level":"high","plddt":92.541,"start":128,"end":227}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P20839","model_url":"https://alphafold.ebi.ac.uk/files/AF-P20839-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P20839-F1-predicted_aligned_error_v6.png","plddt_mean":92.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=IMPDH1","jax_strain_url":"https://www.jax.org/strain/search?query=IMPDH1"},"sequence":{"accession":"P20839","fasta_url":"https://rest.uniprot.org/uniprotkb/P20839.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P20839/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P20839"}},"corpus_meta":[{"pmid":"11875050","id":"PMC_11875050","title":"Mutations in the inosine monophosphate dehydrogenase 1 gene (IMPDH1) cause the RP10 form of autosomal dominant retinitis pigmentosa.","date":"2002","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/11875050","citation_count":191,"is_preprint":false},{"pmid":"16384941","id":"PMC_16384941","title":"Spectrum and frequency of mutations in IMPDH1 associated with autosomal dominant retinitis pigmentosa and leber congenital amaurosis.","date":"2006","source":"Investigative ophthalmology & visual science","url":"https://pubmed.ncbi.nlm.nih.gov/16384941","citation_count":126,"is_preprint":false},{"pmid":"17851563","id":"PMC_17851563","title":"IMPDH1 gene polymorphisms and association with acute rejection in renal transplant patients.","date":"2007","source":"Clinical pharmacology and therapeutics","url":"https://pubmed.ncbi.nlm.nih.gov/17851563","citation_count":86,"is_preprint":false},{"pmid":"14981049","id":"PMC_14981049","title":"On the molecular pathology of neurodegeneration 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Impdh1−/− null mice develop slowly progressive retinal degeneration with compromised electroretinographic responses. Expression of mutant IMPDH1 in bacterial and mammalian cells, combined with computational simulations, indicates protein misfolding and aggregation rather than reduced enzymatic activity is the likely disease mechanism.\",\n      \"method\": \"Transcript analysis in mouse retinal sections; Impdh1 knockout mouse ERG; recombinant protein expression in bacterial and mammalian cells; computational folding simulations\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (knockout mouse phenotype, expression studies, computational analysis) in a single focused study, replicated by subsequent work\",\n      \"pmids\": [\"14981049\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"IMPDH1 is expressed at higher levels in the retina than any other tested tissue, predominantly in the inner segments and synaptic terminals of photoreceptors. Human retina contains unique alternatively spliced and alternate-start-site IMPDH1 isoforms that encode distinct proteins not found in other tissues, and the proportions of these isoforms differ between human and mouse retina.\",\n      \"method\": \"Northern blot, SAGE, immunohistochemistry, transcript sequencing, Western blot\",\n      \"journal\": \"Investigative ophthalmology & visual science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Northern, IHC, Western, sequencing) in a single study establishing retina-specific isoforms and localization\",\n      \"pmids\": [\"16936083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"AAV-mediated expression of mutant human IMPDH1 in mouse retina causes aggressive retinopathy modelling the human disease, whereas expression of normal human IMPDH1 has no pathological effect. AAV co-expression of shRNAs targeting both human and mouse IMPDH1 together with mutant IMPDH1 substantially suppresses photoreceptor loss caused by the mutant protein.\",\n      \"method\": \"Recombinant AAV subretinal injection in mice; electroretinography; histology; shRNA knockdown validated in vitro and in vivo\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — rigorous in vivo model with multiple controls (WT vs mutant overexpression, shRNA rescue), ERG and histological readouts\",\n      \"pmids\": [\"18385099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Retinal IMPDH1 undergoes light-dependent phosphorylation at Thr159/Ser160 in the Bateman domain in vivo; this phosphorylation desensitizes the enzyme to allosteric inhibition by GDP/GTP. Bright-light exposure in mice increases the rate of GTP and ATP synthesis in the retina, coincident with IMPDH1 aggregate (filament) formation at the outer segment layer. Pharmacological inhibition of IMPDH activity in living mice delays rod recovery after light exposure.\",\n      \"method\": \"Mass spectrometry phosphoproteomics of mouse retina; metabolic flux assays; immunofluorescence of IMPDH1 aggregates; pharmacological inhibition (IMPDH inhibitor) in vivo with ERG\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo phosphoproteomics, metabolic flux, imaging, and pharmacological intervention with functional ERG readout in a single study\",\n      \"pmids\": [\"32254022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Human IMPDH1 assembles polymorphic filaments with distinct assembly interfaces in extended (high-activity) and compressed (low-activity) states, as resolved by cryo-EM. Retina-specific splice variants introduce structural elements that reduce sensitivity to GTP inhibition, including stabilization of the extended filament form. Disease mutations fall into two classes: one class disrupts GTP regulation (e.g., Asp226Asn confers resistance to GDP/GTP feedback inhibition), while the second class has no effect on GTP regulation or filament assembly.\",\n      \"method\": \"Cryo-electron microscopy structures of IMPDH1 filaments; enzymatic assays with GTP inhibition; comparison of retinal splice variants vs. canonical isoform\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple cryo-EM structures with functional validation by enzymatic assays; rigorous structure-function analysis of retinal variants and disease mutants\",\n      \"pmids\": [\"35013599\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Phosphomimetic S477D mutation in retinal IMPDH1 splice variants re-sensitizes both IMPDH1(546) and IMPDH1(595) to GTP inhibition and specifically blocks assembly of the high-activity IMPDH1(595) filament interface (but not the low-activity IMPDH1(546) interface), as shown by cryo-EM. S477D exerts a dominant-negative effect in cells, preventing endogenous IMPDH filament assembly. In bovine retinas, S477 is preferentially phosphorylated in the dark, establishing phosphorylation at S477 as a mechanism for downregulating retinal GTP synthesis in the dark.\",\n      \"method\": \"Phosphomimetic mutagenesis; cryo-EM structures; enzymatic GTP inhibition assays; cell-based dominant-negative filament assembly assay; bovine retina phosphoproteomics\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures combined with mutagenesis, enzymatic assays, and cell-based assays; corroborated by bovine retina phosphoproteomics\",\n      \"pmids\": [\"38323936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"IMPDH1 clinical mutants R224P and D226N show impaired folding in vitro. In equimolar mixtures of normal and mutant IMPDH1, activity regain after refolding is close to the mutant level rather than the midpoint, indicating that mutant subunits impose their misfolded conformation on wild-type partners in hybrid tetramers (molecular recruitment), providing a mechanism for dominant-negative inheritance.\",\n      \"method\": \"Recombinant protein expression; in vitro refolding assay; enzymatic activity measurement of mixed wild-type/mutant tetramers\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution with functional readout, but single lab, single method, and the canonical retinal isoform was not used\",\n      \"pmids\": [\"21791244\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The adRP mutation D226N reduces IMPDH1 binding to nucleic acids and reduces association with polyribosomes, suggesting a role for IMPDH1 in posttranscriptional regulation of rhodopsin mRNA in the retina.\",\n      \"method\": \"Nucleic acid binding assays; polyribosome fractionation (referenced from prior experimental work)\",\n      \"journal\": \"Advances in experimental medicine and biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — experimental data cited but detailed methods not fully described in the abstract; supported by prior filter-binding data\",\n      \"pmids\": [\"22183375\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"A novel IMPDH1 allozyme (Leu275) has approximately 10% of wild-type enzymatic activity, due to accelerated protein degradation rather than intrinsic catalytic impairment, as supported by X-ray crystal structure analysis.\",\n      \"method\": \"Enzymatic activity assay; protein stability/degradation assay; structural analysis using IMPDH1 X-ray crystal structure\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro enzymatic assay and structural rationalization, single lab\",\n      \"pmids\": [\"20718729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Retinal IMPDH1 isoforms bearing C/N-terminal extensions (mIMPDH1(546) and mIMPDH1(603)) have higher Km and Ki values relative to the canonical isoform (mIMPDH1(514)), do not exhibit NAD+ substrate inhibition unlike the canonical isoform, and the retinal mIMPDH1(603) isoform shows lower fibrillation capacity under ATP but forms higher-mass oligomers in the presence of GTP and/or MPA. Molecular dynamic simulations indicate terminal peptides interact with the enzyme's finger domain affecting its pseudo barrel structure.\",\n      \"method\": \"Recombinant protein kinetic assays; native PAGE oligomerization analysis; thioflavin T fibrillation assay; molecular dynamics simulation\",\n      \"journal\": \"Cell biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro enzymatic kinetics and oligomerization assays with computational support, single lab\",\n      \"pmids\": [\"33733369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Mouse retinal IMPDH1 isoforms (mIMPDH1(546) and mIMPDH1(603)) show higher Km and Ki values than the canonical mIMPDH1(514), and molecular dynamics simulations indicate that terminal peptide extensions interact with the enzyme's finger domain affecting the critical pseudo-barrel structure.\",\n      \"method\": \"Recombinant protein kinetic assays; enzymatic inhibition assays; molecular modeling and molecular dynamics simulation\",\n      \"journal\": \"Molecular and cellular biochemistry\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 1 for in vitro kinetics / Weak — single lab, computational rationalization without mutagenesis validation\",\n      \"pmids\": [\"31838626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"IMPDH1 and YB-1 form an autoregulatory positive feedback loop in clear cell renal cell carcinoma: IMPDH1 maintains YB-1 protein stabilization, and YB-1 induces IMPDH1 expression by binding the IMPDH1 promoter. IMPDH1-assembled cytoophidia (filaments) physically interact with YB-1 and translocate YB-1 into the cell nucleus.\",\n      \"method\": \"Co-immunoprecipitation; immunofluorescence; promoter binding assay; ChIP; YB-1 protein stability assay; nuclear fractionation\",\n      \"journal\": \"Molecular therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP and ChIP with functional nuclear translocation readout, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"32209435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"IMPDH2 stabilizes IMPDH1 protein by decreasing its polyubiquitination levels. c-Myc transcriptionally activates both IMPDH1 and IMPDH2 to promote de novo GTP biosynthesis.\",\n      \"method\": \"Co-immunoprecipitation; protein half-life assay; polyubiquitination assay; ChIP/transcriptional activation assay; colorectal cancer cell lines and xenograft models\",\n      \"journal\": \"Clinical and translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP for protein-protein interaction, ubiquitination assay, and transcriptional assay, single lab\",\n      \"pmids\": [\"36629054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MYBL2 directly binds the IMPDH1 promoter and transcriptionally activates IMPDH1 expression in hepatocellular carcinoma cells, as demonstrated by ChIP-seq and ChIP-qPCR. MYBL2 knockout reduces IMPDH1 expression and inhibits guanine nucleotide synthesis.\",\n      \"method\": \"ChIP-seq; ChIP-qPCR; MYBL2 knockout; metabolomics; xenograft tumor models\",\n      \"journal\": \"BMC cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP-seq and ChIP-qPCR with functional metabolomic validation, single lab\",\n      \"pmids\": [\"36494680\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The IMPDH1 Asp226Asn (D226N) mutation causes cytoophidium (filament) assembly in ~70% of HEp-2 cells and reduces long-term cell survival by ~40% compared to wild-type. Introduction of a secondary Y12C mutation disrupts filament assembly and significantly rescues cell survival (only ~10% decrease vs. WT), indicating that cytoophidium formation contributes to D226N cytotoxicity. D226N also confers resistance to feedback inhibition by GDP/GTP.\",\n      \"method\": \"Stable cell line expression; immunofluorescence; long-term survival assay; double-mutant (Y12C/D226N) functional analysis\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — double-mutant genetic dissection of filament assembly vs. GTP regulation with cell survival readout, single lab\",\n      \"pmids\": [\"37731818\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A 9-bp insertion in the IMPDH1 P3 promoter abolishes a cAMP-response element (CRE) and significantly reduces IMPDH1 P3 promoter activity in a luciferase reporter assay, identifying a transcriptional regulatory element controlling IMPDH1 expression.\",\n      \"method\": \"Luciferase reporter gene assay; dHPLC and DNA sequencing\",\n      \"journal\": \"The pharmacogenomics journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct luciferase reporter assay establishing promoter element function, single patient/single lab\",\n      \"pmids\": [\"17001353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The human IMPDH1 gene is located on chromosome 7q31.3-q32, established by PCR analysis of somatic cell hybrid panels and fluorescence in situ hybridization.\",\n      \"method\": \"PCR of human-mouse and human-hamster somatic cell hybrids; FISH with metaphase chromosomes\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent methods (somatic cell hybrid PCR and FISH) concordantly assign chromosomal location; replicated by linkage studies\",\n      \"pmids\": [\"7896275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Following radiation therapy, IMPDH1 translocates from the cytoplasm to the nucleus in glioblastoma cells in a DNA-PK-dependent manner, leading to nuclear GTP accumulation that promotes DNA damage repair and radiation resistance. Pharmacological inhibition of IMPDH with mycophenolate mofetil slows DNA damage repair, extends survival in orthotopic murine models with combined RT/TMZ, and achieves active intracranial drug concentrations in a human phase 0 clinical trial with reversal of IMPDH upstream/downstream metabolites.\",\n      \"method\": \"Subcellular fractionation/imaging of IMPDH1 localization; DNA-PK inhibitor epistasis; metabolic flux assays; orthotopic murine GBM models with ERG/survival endpoints; phase 0 clinical trial with tumor metabolomics\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal in vitro and in vivo methods plus human clinical data; preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.11.11.25340023\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"IMPDH1 is a homotetrameric enzyme that catalyzes the rate-limiting step in de novo guanine nucleotide synthesis; in the retina it exists as tissue-specific splice variants that assemble polymorphic filaments regulated by allosteric GDP/GTP feedback and light-dependent phosphorylation at S477 (dark) and Thr159/Ser160 (Bateman domain, light), with filament formation desensitizing the enzyme to feedback inhibition to boost GTP synthesis during light exposure; disease-causing missense mutations cluster in the Bateman/CBS domain and predominantly disrupt GTP allosteric regulation and/or nucleic acid binding rather than catalytic activity, with dominant-negative effects propagated through hybrid tetramer misfolding and toxic cytoophidium formation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"IMPDH1 is a homotetrameric enzyme that catalyzes the rate-limiting step of de novo guanine nucleotide synthesis (IMP \\u2192 XMP) and is the principal source of GTP in photoreceptors, where its loss in knockout mice produces slowly progressive retinal degeneration [#0, #2]. The enzyme assembles into polymorphic filaments resolved by cryo-EM in extended high-activity and compressed low-activity states, and the retina expresses tissue-specific alternatively spliced and alternate-start isoforms bearing N/C-terminal extensions that raise Km/Ki, eliminate canonical NAD+ substrate inhibition, and reduce sensitivity to GTP feedback by stabilizing the extended filament form [#3, #6, #11]. Retinal activity is set by light-dependent phosphorylation: phosphorylation of Thr159/Ser160 in the Bateman domain during light desensitizes the enzyme to GDP/GTP feedback and coincides with filament formation and increased GTP/ATP synthesis, whereas phosphorylation at S477 in the dark re-sensitizes the enzyme to GTP inhibition and blocks high-activity filament assembly, the S477D phosphomimetic acting dominant-negatively on endogenous filaments [#5, #7]. Missense mutations in the Bateman/CBS domain cause RP10 autosomal dominant retinitis pigmentosa; rather than abolishing catalysis, these mutations impair single-stranded nucleic acid binding and/or confer resistance to GDP/GTP feedback inhibition (e.g., Asp226Asn), and the dominant disease mechanism reflects protein misfolding, recruitment of wild-type subunits into hybrid tetramers, and toxic cytoophidium formation that reduces cell survival\\u2014a toxicity rescued by disrupting filament assembly [#0, #1, #6, #8, #16]. AAV-delivered mutant IMPDH1 reproduces the retinopathy in mice and is suppressed by shRNA knockdown, supporting a gain-of-function/dominant-negative basis [#4]. Beyond the retina, IMPDH1 expression is driven by c-Myc, MYBL2, and a P3-promoter CRE, its protein stabilized by IMPDH2, and it participates in cancer-associated circuits including a YB-1 positive-feedback loop and DNA-PK-dependent nuclear translocation supporting DNA-damage repair [#13, #14, #15, #17, #19].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Establishing the chromosomal location of human IMPDH1 provided the genetic anchor later needed to link the gene to an inherited retinal disease locus.\",\n      \"evidence\": \"PCR of somatic cell hybrids and FISH\",\n      \"pmids\": [\"7896275\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address gene function or expression\", \"No tissue or isoform information\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identifying the Asp226Asn mutation in RP10 families connected this metabolic enzyme to autosomal dominant retinitis pigmentosa, raising the question of why a housekeeping enzyme causes a tissue-specific dominant disease.\",\n      \"evidence\": \"Linkage mapping and sequencing across multiple families, conserved-residue analysis\",\n      \"pmids\": [\"11875050\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of dominance unresolved\", \"Does not explain retina specificity\", \"No functional consequence of the mutation tested\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Showing IMPDH1 is the dominant GTP source in photoreceptors and that mutants misfold rather than lose activity reframed the disease as a protein-conformation problem, not simple enzyme deficiency.\",\n      \"evidence\": \"Retinal transcript analysis, Impdh1 knockout mouse ERG, recombinant expression, folding simulations\",\n      \"pmids\": [\"14981049\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Knockout phenotype milder than dominant disease\", \"Aggregation mechanism in photoreceptors not directly imaged\", \"Does not identify the misfolding propagation route\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrating retina-enriched, retina-unique splice/start-site isoforms and showing disease mutants alter nucleic-acid binding rather than catalysis explained tissue specificity and pointed to a non-catalytic role for IMPDH1.\",\n      \"evidence\": \"Northern/SAGE/IHC/Western isoform mapping; NADH enzymatic assays and filter-binding assays on seven mutants; P3 promoter CRE luciferase reporter\",\n      \"pmids\": [\"16936083\", \"16384941\", \"17001353\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological nucleic-acid targets not identified\", \"Function of retina-specific extensions unknown at this stage\", \"Gly324Asp exception unexplained\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"An AAV mouse model recapitulating retinopathy with mutant but not wild-type IMPDH1, and rescue by shRNA, confirmed a gain-of-function/dominant-negative mechanism amenable to knockdown therapy.\",\n      \"evidence\": \"Subretinal AAV expression and shRNA rescue with ERG and histology\",\n      \"pmids\": [\"18385099\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular nature of the toxic species not defined\", \"Does not distinguish aggregation from regulatory dysfunction\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"In vitro refolding of mixed wild-type/mutant tetramers showed mutant subunits impose their misfolded state on wild-type partners, providing a concrete dominant-negative recruitment mechanism.\",\n      \"evidence\": \"Recombinant refolding and activity assays of equimolar WT/mutant mixtures\",\n      \"pmids\": [\"21791244\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, single method\", \"Canonical retinal isoform not used\", \"Relevance in vivo not demonstrated\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Linking D226N nucleic-acid binding loss to reduced polyribosome association suggested IMPDH1 contributes to posttranscriptional regulation of rhodopsin mRNA, a candidate non-catalytic retinal function.\",\n      \"evidence\": \"Nucleic-acid binding and polyribosome fractionation (cited prior work)\",\n      \"pmids\": [\"22183375\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct mRNA targets not identified\", \"Methods incompletely described\", \"Causal link to disease unproven\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery of light-dependent Bateman-domain phosphorylation (Thr159/Ser160) that desensitizes feedback inhibition, coupled to filament formation and increased GTP flux, established a physiological logic for retinal IMPDH1 regulation tied to light exposure.\",\n      \"evidence\": \"In vivo phosphoproteomics, metabolic flux, filament imaging, pharmacological IMPDH inhibition with ERG\",\n      \"pmids\": [\"32254022\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase responsible not identified\", \"How phosphorylation couples to filament assembly structurally unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Kinetic characterization of retinal isoforms showed terminal extensions raise Km/Ki, abolish NAD+ substrate inhibition, and alter oligomerization, explaining how retina-specific isoforms tune activity and assembly.\",\n      \"evidence\": \"Recombinant kinetics, native PAGE, thioflavin T fibrillation, molecular dynamics\",\n      \"pmids\": [\"33733369\", \"31838626\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Computational finger-domain interaction not validated by mutagenesis\", \"In vivo relevance of altered kinetics untested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cryo-EM of extended high-activity and compressed low-activity filaments, with retinal variants stabilizing the extended form and disease mutants partitioning into GTP-regulation-disrupting vs. neutral classes, provided the structural framework unifying regulation and disease.\",\n      \"evidence\": \"Multiple cryo-EM structures with enzymatic GTP-inhibition validation and variant comparison\",\n      \"pmids\": [\"35013599\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of the second mutation class (no GTP/filament effect) unexplained\", \"Filament function in living photoreceptors not directly resolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Genetic dissection of D226N showed cytoophidium assembly itself drives cytotoxicity\\u2014a Y12C filament-blocking mutation rescued survival\\u2014separating toxic filament formation from the mutation's GTP-regulation defect.\",\n      \"evidence\": \"Stable cell lines, immunofluorescence, long-term survival assay, double-mutant analysis\",\n      \"pmids\": [\"37731818\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"HEp-2 cell model not photoreceptor-specific\", \"Does not weigh filament toxicity against feedback-resistance in vivo\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of c-Myc/MYBL2 transcriptional activation and IMPDH2-mediated stabilization of IMPDH1 protein revealed how IMPDH1 levels and GTP biosynthesis are amplified in cancer.\",\n      \"evidence\": \"ChIP/transcriptional assays, Co-IP, ubiquitination and half-life assays in colorectal and hepatocellular models with xenografts\",\n      \"pmids\": [\"36629054\", \"36494680\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct contribution of IMPDH1 vs IMPDH2 to tumor GTP pools not separated\", \"Single-lab interaction data\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Showing S477 is phosphorylated in the dark to re-sensitize the enzyme to GTP inhibition and to selectively block the high-activity filament interface defined the antagonistic dark/light phosphorylation switch governing retinal GTP synthesis.\",\n      \"evidence\": \"Phosphomimetic cryo-EM, GTP-inhibition assays, cell-based dominant-negative filament assays, bovine retina phosphoproteomics\",\n      \"pmids\": [\"38323936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase/phosphatase that toggles S477 not identified\", \"Integration with Thr159/Ser160 signaling unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"DNA-PK-dependent nuclear translocation of IMPDH1 supplying GTP for DNA-damage repair extended IMPDH1's role beyond the retina to a druggable mediator of radiation resistance in glioblastoma.\",\n      \"evidence\": \"Subcellular fractionation, DNA-PK epistasis, flux assays, orthotopic GBM models, phase 0 trial metabolomics (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.11.11.25340023\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Mechanism of nuclear import signal undefined\", \"Relation to filament assembly in this context unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The identity of the kinases/phosphatases driving the light-dependent Thr159/Ser160 and dark S477 phosphorylation switch, and the physiological nucleic-acid/mRNA targets of IMPDH1's non-catalytic binding activity, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No kinase/phosphatase identified for either phosphosite\", \"Endogenous nucleic-acid ligands not defined\", \"Second class of disease mutations (no GTP/filament effect) lacks a mechanism\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 6, 7]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [13, 19]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [13, 19]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [6, 16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 2, 5]},\n      {\"term_id\": \"R-HSA-9709957\", \"supporting_discovery_ids\": [2, 3, 5]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 4, 16]}\n    ],\n    \"complexes\": [\"IMPDH1 homotetramer/filament (cytoophidium)\"],\n    \"partners\": [\"IMPDH2\", \"YB-1\", \"MYBL2\", \"c-Myc\", \"DNA-PK\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}