{"gene":"IMPDH1","run_date":"2026-04-28T18:06:54","timeline":{"discoveries":[{"year":2002,"finding":"IMPDH1 encodes the rate-limiting enzyme in de novo guanine nucleotide biosynthesis, functioning as a homotetramer; missense mutations (e.g., Asp226Asn, Val268Ile) in the catalytic domain cause the RP10 form of autosomal dominant retinitis pigmentosa without altering enzymatic activity.","method":"DNA sequencing, linkage mapping, identification of mutations in affected individuals; enzymatic activity assay (NADH fluorescence); conservation analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — foundational mutation identification replicated across multiple families and labs, enzymatic assays performed","pmids":["11875050"],"is_preprint":false},{"year":2004,"finding":"IMPDH1 is the primary source of GTP in photoreceptors (versus IMPDH2 and HPRT); Impdh1 knockout mice display slowly progressive retinal degeneration; mutant IMPDH1 proteins expressed in bacterial and mammalian cells show misfolding and aggregation rather than reduced enzyme activity, suggesting protein misfolding/aggregation as the disease mechanism.","method":"Impdh1 null mouse (electroretinography, histology), retinal section expression analysis, recombinant protein expression in bacterial/mammalian cells, computational simulation","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods including in vivo knockout, expression studies, and computational modeling in a single study","pmids":["14981049"],"is_preprint":false},{"year":2006,"finding":"Retinitis pigmentosa-associated IMPDH1 missense mutations (Thr116Met, Asp226Asn, Val268Ile, His372Pro, Arg105Trp, Asn198Lys) do not alter enzymatic activity but alter the affinity and/or specificity of IMPDH1 for single-stranded nucleic acids, implicating nucleic acid binding as a distinct moonlighting function of IMPDH1.","method":"Enzymatic activity assay (NADH fluorescence production), filter-binding assays for single-stranded nucleic acid binding","journal":"Investigative ophthalmology & visual science","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro enzymatic and binding assays with multiple mutants; findings replicated across several RP mutations","pmids":["16384941"],"is_preprint":false},{"year":2006,"finding":"In human retina, IMPDH1 is expressed at higher levels than any other tissue tested and is localized predominantly to inner segments and synaptic terminals of photoreceptors; alternative splicing and alternative translational start sites produce retina-specific IMPDH1 isoforms (distinct proteins) not found in other tissues, and the proportions of transcripts/proteins 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 — multiple orthogonal methods with direct subcellular localization and isoform identification linked to functional implications","pmids":["16936083"],"is_preprint":false},{"year":2008,"finding":"AAV-mediated expression of mutant human IMPDH1 in the mouse retina causes an aggressive retinopathy, while expression of normal IMPDH1 has no pathological effect; AAV-mediated co-expression of shRNA targeting both human and mouse IMPDH1 together with mutant IMPDH1 substantially suppresses retinal degeneration, demonstrating that mutant IMPDH1 is sufficient to cause retinopathy and that suppression of mutant transcript is therapeutic.","method":"Recombinant AAV delivery to mouse retina, in vivo shRNA knockdown, electroretinography, histology","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — in vivo gain-of-function and loss-of-function experiments with functional readout (ERG)","pmids":["18385099"],"is_preprint":false},{"year":2010,"finding":"The IMPDH1 Leu275 variant allozyme has ~10% of wild-type enzymatic activity due to accelerated protein degradation, as supported by the IMPDH1 X-ray crystal structure; decreased activity of certain IMPDH1 allozymes results from reduced protein quantity caused by accelerated degradation, not from altered catalytic mechanism.","method":"Enzymatic activity assay, protein degradation assay, X-ray crystal structure analysis","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 1–2 — single study with enzymatic assay and structural corroboration, but not replicated","pmids":["20718729"],"is_preprint":false},{"year":2011,"finding":"Recombinant IMPDH1 clinical mutants R224P and D226N show impaired folding in vitro; in equimolar mixtures of normal and mutant enzymes, mutant subunits impose their faulty conformation on normal partners in hybrid tetramers (molecular recruitment/dominant-negative mechanism), potentially explaining autosomal dominant inheritance of RP10.","method":"In vitro refolding assay of recombinant IMPDH1, activity assay of mixed normal/mutant tetramers","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstitution assay, single lab, single study","pmids":["21791244"],"is_preprint":false},{"year":2012,"finding":"IMPDH1's function in the retina, apparently independent of enzymatic activity, is mediated by retina-specific variants and may involve posttranscriptional regulation of rhodopsin mRNA; the adRP mutation D226N reduces binding to nucleic acids and reduces association with polyribosomes.","method":"Filter-binding assays, polyribosome association assay","journal":"Advances in experimental medicine and biology","confidence":"Medium","confidence_rationale":"Tier 2–3 — binding and polyribosome assays, mechanistic follow-up, single lab","pmids":["22183375"],"is_preprint":false},{"year":2019,"finding":"Retina-specific IMPDH1 isoforms (mIMPDH1-546 and mIMPDH1-603) with C- and N-terminal extensions show higher Km and Ki values relative to the canonical isoform (mIMPDH1-514), do not exhibit NAD+ substrate inhibition unlike the canonical isoform, and the terminal segments interact with the enzyme's finger domain affecting its pseudo-barrel structure, as shown by molecular dynamics simulation.","method":"Kinetic analysis of recombinant murine IMPDH1 isoforms, molecular dynamics simulation","journal":"Molecular and cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 1–3 — direct kinetic measurements combined with computational modeling, single lab","pmids":["31838626"],"is_preprint":false},{"year":2020,"finding":"In vivo, retinal IMPDH1 undergoes light-dependent phosphorylation at Thr159/Ser160 within the Bateman domain, which desensitizes the enzyme to allosteric inhibition by GDP/GTP; exposure to bright light increases GTP and ATP synthesis rates in mouse retinas concomitantly with IMPDH1 aggregate (filament) formation at the outer segment layer; inhibiting IMPDH activity in living mice delays rod mass recovery after light bleaching.","method":"In vivo phosphorylation mapping, metabolite measurements in mouse retina under light/dark conditions, confocal imaging of filament formation, pharmacological inhibition of IMPDH in living mice with ERG readout","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal in vivo methods (phosphoproteomics, metabolomics, imaging, pharmacology) in a single study with strong mechanistic conclusions","pmids":["32254022"],"is_preprint":false},{"year":2020,"finding":"IMPDH1 forms cytoophidia (intracellular filamentous structures) that physically interact with YB-1 and translocate YB-1 into the cell nucleus; IMPDH1 maintains YB-1 protein stabilization, while YB-1 induces IMPDH1 expression by binding to the IMPDH1 promoter, forming a positive feedback loop associated with renal cell carcinoma metastasis.","method":"Immunofluorescence, co-immunoprecipitation, ChIP/promoter binding assay, YB-1 nuclear translocation assay","journal":"Molecular therapy","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP and nuclear translocation assays from a single lab; mechanistic follow-up with promoter binding","pmids":["32209435"],"is_preprint":false},{"year":2021,"finding":"The major mouse retinal IMPDH1 isoform (mH1603) with terminal extensions shows higher catalytic activity and lower fibrillation capacity in the presence of ATP compared to the canonical isoform; in the presence of GTP and/or MPA, the retinal isoform forms higher mass oligomerization products.","method":"Kinetic analysis of recombinant IMPDH1 isoforms, oligomerization assay, molecular simulation","journal":"Cell biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 1–3 — direct biochemical assays with recombinant proteins combined with computational modeling, single lab","pmids":["33733369"],"is_preprint":false},{"year":2022,"finding":"Cryo-EM structures reveal that human IMPDH1 assembles polymorphic filaments with distinct extended and compressed state assembly interfaces; retina-specific splice variants introduce structural elements that stabilize the extended (high-activity) filament form and reduce sensitivity to GTP inhibition; RP disease mutations fall into two classes—one disrupts GTP regulation and the other has no effect on GTP regulation or filament assembly.","method":"Cryo-electron microscopy, biochemical activity assays, analysis of disease mutations and splice variants","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — multiple cryo-EM structures with functional validation of assembly and allosteric regulation; single rigorous study with orthogonal methods","pmids":["35013599"],"is_preprint":false},{"year":2022,"finding":"IMPDH2 stabilizes IMPDH1 by decreasing its polyubiquitination levels; c-Myc transcriptionally activates IMPDH1/2 to promote de novo GTP biosynthesis and colorectal cancer growth.","method":"Co-immunoprecipitation, protein half-life assay (cycloheximide chase), polyubiquitination assay, ChIP/reporter assay, xenograft model","journal":"Clinical and translational medicine","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP and ubiquitination assay plus transcriptional activation demonstrated; single lab","pmids":["36629054"],"is_preprint":false},{"year":2023,"finding":"The adRP-10 IMPDH1 mutation Asp226Asn (D226N) causes cytoophidium assembly in ~70% of cells and confers resistance to feedback inhibition by GDP/GTP; long-term expression of IMPDH1-D226N decreases cell survival by ~40%; introducing a secondary mutation Y12C that disrupts filament assembly significantly recovers cell survival, establishing that both nucleotide imbalance and toxic cytoophidium contribute to IMPDH1-D226N pathology.","method":"Stable expression in HEp-2 cells, immunofluorescence for cytoophidium, long-term cell survival assay, double-mutant genetic rescue experiment","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis via double mutant rescue, quantitative survival assay, single lab","pmids":["37731818"],"is_preprint":false},{"year":2023,"finding":"MYBL2 transcriptionally activates IMPDH1 by direct binding to its promoter, promoting de novo GTP synthesis and hepatocellular carcinoma cell proliferation; knockout of MYBL2 retards IMPDH1 expression, reduces guanine nucleotide pools (metabolomics), and inhibits tumor growth in xenograft models.","method":"ChIP-seq, ChIP-qPCR, metabolomics, MYBL2 knockout, xenograft tumor model","journal":"BMC cancer","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP-seq with functional metabolomic validation and in vivo xenograft, single lab","pmids":["36494680"],"is_preprint":false},{"year":2023,"finding":"LncRNA UCA1 recruits the transcription factor TWIST1 to the IMPDH1 promoter, increasing IMPDH1 transcription and guanine nucleotide production in bladder cancer cells, thereby stimulating RNA polymerase-dependent pre-ribosomal RNA production and GTPase activity.","method":"ChIP assay for TWIST1 binding at IMPDH1 promoter, reporter assay, guanine nucleotide metabolite measurement, functional cell assays (proliferation, migration)","journal":"International journal of biological sciences","confidence":"Medium","confidence_rationale":"Tier 2–3 — ChIP and reporter assay with metabolite readout, single lab","pmids":["37215997"],"is_preprint":false},{"year":2024,"finding":"Phosphorylation of IMPDH1 at S477 (preferentially phosphorylated in the dark in bovine retinas) acts as a mechanism to downregulate retinal GTP synthesis: phosphomimetic S477D mutation re-sensitizes both IMPDH1(546) and IMPDH1(595) variants to GTP inhibition; cryo-EM structures show S477D specifically blocks the high-activity filament assembly interface of IMPDH1(595) while allowing low-activity IMPDH1(546) filament assembly; S477D exerts a dominant-negative effect in cells, preventing endogenous IMPDH filament assembly.","method":"Phosphomimetic mutagenesis, cryo-EM structure determination, in vitro enzymatic activity assay, cell-based filament assembly assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures plus in vitro enzymatic assays and cell-based validation with mutagenesis; multiple orthogonal methods","pmids":["38323936"],"is_preprint":false},{"year":2025,"finding":"Following radiation therapy, IMPDH1 translocates to the nucleus in glioblastoma cells in a DNA-PK-dependent manner, driving nuclear GTP accumulation that promotes DNA damage repair; pharmacological inhibition of IMPDH with mycophenolate mofetil slows DNA damage repair and extends survival in orthotopic murine GBM models.","method":"Subcellular fractionation/immunofluorescence for nuclear translocation, metabolite measurements (nuclear GTP), DNA-PK inhibition epistasis, orthotopic murine GBM survival assay, phase 0 clinical trial with metabolite measurement in human tumors","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment tied to functional consequence, genetic epistasis (DNA-PK), in vivo survival data, and human phase 0 trial; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2025.11.11.25340023"],"is_preprint":true}],"current_model":"IMPDH1 is the rate-limiting homotetrameric enzyme in de novo guanine nucleotide biosynthesis that assembles into polymorphic filaments to reduce allosteric GTP inhibition and boost nucleotide production; in the retina, unique splice variants with terminal extensions produce isoforms with altered kinetics and filament architectures, and light-dependent phosphorylation at S477 (dephosphorylated in light, phosphorylated in dark) and at Thr159/Ser160 in the Bateman domain dynamically regulate filament assembly and allosteric sensitivity to adjust GTP synthesis to illumination conditions; disease-causing missense mutations either disrupt GTP allosteric regulation or impair nucleic acid binding/cause protein misfolding and dominant-negative aggregation in hybrid tetramers, while the protein also moonlights as a nucleic acid-binding factor and, in response to DNA damage, translocates to the nucleus in a DNA-PK-dependent manner to support nuclear GTP synthesis for DNA repair."},"narrative":{"teleology":[{"year":2002,"claim":"Establishing that IMPDH1 mutations cause autosomal dominant retinitis pigmentosa (RP10) without altering enzymatic activity resolved the genetic basis of RP10 and raised the question of how catalytically normal mutants cause photoreceptor degeneration.","evidence":"DNA sequencing and linkage mapping in RP families; NADH fluorescence enzymatic assay of recombinant mutants","pmids":["11875050"],"confidence":"High","gaps":["Mechanism by which catalytically normal mutants cause disease was unknown","No in vivo disease model yet established","No structural basis for mutations"]},{"year":2004,"claim":"Demonstrating that IMPDH1 is the principal GTP source in photoreceptors and that Impdh1 knockout causes progressive retinal degeneration established that GTP supply is critical for photoreceptor survival, while showing mutant protein misfolding/aggregation pointed to a dominant-negative or toxic gain-of-function mechanism.","evidence":"Impdh1-null mouse with ERG and histology; recombinant mutant protein expression in bacterial and mammalian cells","pmids":["14981049"],"confidence":"High","gaps":["Whether misfolding occurs in hybrid tetramers with wild-type subunits was untested","Retina-specific isoforms not yet identified"]},{"year":2006,"claim":"Discovery that RP mutations alter nucleic acid binding affinity without affecting catalysis, and that retina-specific IMPDH1 splice variants exist, revealed a moonlighting function and tissue-specific molecular context that could explain why mutations selectively damage photoreceptors.","evidence":"Filter-binding assays for single-stranded nucleic acids with multiple RP mutants; Northern blot, immunohistochemistry, and transcript sequencing in human retina","pmids":["16384941","16936083"],"confidence":"High","gaps":["Biological target RNAs in photoreceptors were unidentified","Whether nucleic acid binding is physiologically relevant in vivo was unproven","Functional significance of retina-specific isoforms was unknown"]},{"year":2008,"claim":"Showing that AAV-delivered mutant IMPDH1 is sufficient to cause retinal degeneration in vivo and that shRNA-mediated suppression is therapeutic confirmed a dominant gain-of-function disease mechanism and validated an RNA interference therapeutic strategy.","evidence":"AAV delivery of mutant/WT IMPDH1 and shRNA to mouse retina with ERG and histology","pmids":["18385099"],"confidence":"High","gaps":["Molecular basis of toxicity (aggregation vs. nucleotide imbalance vs. nucleic acid binding) was unresolved","Long-term durability of gene therapy not assessed"]},{"year":2011,"claim":"Reconstitution of hybrid tetramers showed that mutant subunits impose faulty conformation on wild-type partners, providing a molecular explanation for autosomal dominant inheritance through a dominant-negative mechanism.","evidence":"In vitro refolding and activity assays of mixed normal/mutant recombinant IMPDH1 tetramers","pmids":["21791244"],"confidence":"Medium","gaps":["Hybrid tetramer formation not confirmed in cells or in vivo","Contribution of dominant-negative effect relative to gain-of-function aggregation not quantified"]},{"year":2019,"claim":"Kinetic characterization of retina-specific IMPDH1 isoforms with terminal extensions revealed distinct enzymatic properties—higher Km/Ki values and loss of NAD+ substrate inhibition—explaining why the retina requires specialized isoforms for sustained GTP production.","evidence":"Kinetic analysis of recombinant murine IMPDH1 isoforms (mIMPDH1-514, -546, -603); molecular dynamics simulation","pmids":["31838626"],"confidence":"Medium","gaps":["In vivo functional consequence of isoform-specific kinetics was untested","Structural basis for altered kinetics awaited high-resolution structures"]},{"year":2020,"claim":"In vivo demonstration that light-dependent phosphorylation at Thr159/Ser160 desensitizes IMPDH1 to GTP inhibition, with concomitant filament formation and increased nucleotide synthesis in bright light, established a direct mechanism coupling illumination to retinal GTP homeostasis.","evidence":"Phosphoproteomics, retinal metabolite measurements under light/dark, confocal imaging of filaments, pharmacological IMPDH inhibition with ERG in living mice","pmids":["32254022"],"confidence":"High","gaps":["Kinase(s) responsible for Thr159/Ser160 phosphorylation were unidentified","Relationship between filament formation and phosphorylation at other sites was unclear"]},{"year":2020,"claim":"Discovery that IMPDH1 cytoophidia physically interact with YB-1, promote its nuclear translocation, and that YB-1 reciprocally activates IMPDH1 transcription revealed a positive feedback loop linking IMPDH1 filament biology to gene regulation.","evidence":"Co-immunoprecipitation, immunofluorescence, ChIP/promoter binding assay in renal carcinoma cells","pmids":["32209435"],"confidence":"Medium","gaps":["Whether the IMPDH1–YB-1 loop operates in photoreceptors or normal physiology is unknown","Mechanism by which cytoophidia translocate YB-1 is unclear","Single Co-IP study without reciprocal validation in independent labs"]},{"year":2022,"claim":"Cryo-EM structures of IMPDH1 filaments revealed extended and compressed conformational states with distinct assembly interfaces, showed that retina-specific splice elements stabilize the extended (high-activity) form, and classified RP mutations into two mechanistic categories—GTP regulation-disrupting and GTP regulation-neutral.","evidence":"Cryo-electron microscopy of filaments from canonical and retinal splice variants; biochemical activity assays; mapping of disease mutations","pmids":["35013599"],"confidence":"High","gaps":["Structural basis for the GTP-regulation-neutral class of RP mutations was not resolved","In vivo relevance of specific filament conformations in photoreceptors was undemonstrated"]},{"year":2023,"claim":"Genetic epistasis experiments showed that the D226N RP mutation causes both GTP feedback resistance and toxic cytoophidium formation, and that a secondary filament-disrupting mutation partially rescues cell survival, disentangling nucleotide imbalance from filament toxicity.","evidence":"Stable expression in HEp-2 cells, long-term survival assay, double-mutant (Y12C) rescue of D226N filament assembly","pmids":["37731818"],"confidence":"Medium","gaps":["Rescue experiment performed in non-retinal cell line","Relative contribution of nucleotide imbalance versus filament toxicity not fully quantified in photoreceptors"]},{"year":2024,"claim":"Identification of S477 as a dark-phosphorylated regulatory site that re-sensitizes retinal IMPDH1 to GTP inhibition and blocks high-activity filament assembly in an isoform-specific manner completed the model of bidirectional phospho-regulation of retinal GTP synthesis across the light–dark cycle.","evidence":"Phosphomimetic mutagenesis (S477D), cryo-EM structures of filament interfaces, enzymatic assays, cell-based filament assembly assays","pmids":["38323936"],"confidence":"High","gaps":["Identity of the S477 kinase and phosphatase is unknown","How Thr159/Ser160 and S477 phosphorylation are coordinated in vivo is unresolved"]},{"year":null,"claim":"Key unresolved questions include the identity of kinases/phosphatases controlling IMPDH1 phosphorylation in photoreceptors, the physiological RNA targets of IMPDH1's nucleic acid-binding function, and whether nuclear translocation and nuclear GTP synthesis for DNA repair represent a general IMPDH1 function across tissues.","evidence":"","pmids":[],"confidence":"Low","gaps":["Kinases/phosphatases for S477 and T159/S160 unidentified","No validated endogenous RNA targets in photoreceptors","Nuclear translocation for DNA repair shown only in glioblastoma preprint, awaits peer review and replication in other cell types"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016491","term_label":"oxidoreductase activity","supporting_discovery_ids":[0,1,5,8,12]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[2,7]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[12,17]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,10,14]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[10]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,1,8,9,12,15,16]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,4,14]},{"term_id":"R-HSA-9709957","term_label":"Sensory Perception","supporting_discovery_ids":[1,3,9,17]}],"complexes":["IMPDH1 homotetramer","IMPDH1 cytoophidium (filament)"],"partners":["IMPDH2","YBX1","TWIST1","MYBL2","MYC"],"other_free_text":[]},"mechanistic_narrative":"IMPDH1 is the rate-limiting enzyme in de novo guanine nucleotide biosynthesis, functioning as a homotetramer that assembles into polymorphic filaments to modulate allosteric GTP feedback inhibition and tune nucleotide output to cellular demand [PMID:35013599, PMID:32254022]. In the retina, where IMPDH1 is the dominant GTP source, retina-specific splice variants with N- and C-terminal extensions stabilize extended (high-activity) filament conformations, exhibit altered kinetics including loss of NAD+ substrate inhibition, and undergo light-dependent phosphorylation at Thr159/Ser160 and Ser477 that dynamically adjusts allosteric sensitivity and filament architecture to match illumination conditions [PMID:36936083, PMID:31838626, PMID:32254022, PMID:38323936]. Missense mutations in IMPDH1 cause the RP10 form of autosomal dominant retinitis pigmentosa through two distinct mechanisms: one class disrupts GTP allosteric regulation and promotes toxic cytoophidium formation, while another impairs nucleic acid binding and causes dominant-negative misfolding within hybrid tetramers [PMID:11875050, PMID:16384941, PMID:35013599, PMID:21791244, PMID:37731818]. Beyond catalysis, IMPDH1 moonlights as a single-stranded nucleic acid-binding protein associated with polyribosomes, forms cytoophidia that interact with YB-1 to promote its nuclear translocation, and itself translocates to the nucleus in a DNA-PK-dependent manner to support nuclear GTP synthesis for DNA repair [PMID:16384941, PMID:22183375, PMID:32209435]."},"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":85,"is_preprint":false},{"pmid":"14981049","id":"PMC_14981049","title":"On the molecular pathology of neurodegeneration in IMPDH1-based retinitis pigmentosa.","date":"2004","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/14981049","citation_count":84,"is_preprint":false},{"pmid":"16936083","id":"PMC_16936083","title":"Why do mutations in the ubiquitously expressed housekeeping gene IMPDH1 cause retina-specific photoreceptor degeneration?","date":"2006","source":"Investigative ophthalmology & visual science","url":"https://pubmed.ncbi.nlm.nih.gov/16936083","citation_count":50,"is_preprint":false},{"pmid":"18385099","id":"PMC_18385099","title":"Therapeutic benefit derived from RNAi-mediated ablation of IMPDH1 transcripts in a murine model of autosomal dominant retinitis pigmentosa (RP10).","date":"2008","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/18385099","citation_count":49,"is_preprint":false},{"pmid":"17001353","id":"PMC_17001353","title":"IMPDH1 promoter mutations in a patient exhibiting azathioprine resistance.","date":"2006","source":"The pharmacogenomics journal","url":"https://pubmed.ncbi.nlm.nih.gov/17001353","citation_count":47,"is_preprint":false},{"pmid":"32254022","id":"PMC_32254022","title":"Post-translational regulation of retinal IMPDH1 in vivo to adjust GTP synthesis to illumination conditions.","date":"2020","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/32254022","citation_count":42,"is_preprint":false},{"pmid":"15851576","id":"PMC_15851576","title":"Screen of the IMPDH1 gene among patients with dominant retinitis pigmentosa and clinical features associated with the most common mutation, Asp226Asn.","date":"2005","source":"Investigative ophthalmology & visual science","url":"https://pubmed.ncbi.nlm.nih.gov/15851576","citation_count":36,"is_preprint":false},{"pmid":"35013599","id":"PMC_35013599","title":"IMPDH1 retinal variants control filament architecture to tune allosteric regulation.","date":"2022","source":"Nature structural & molecular 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missense mutations (e.g., Asp226Asn, Val268Ile) in the catalytic domain cause the RP10 form of autosomal dominant retinitis pigmentosa without altering enzymatic activity.\",\n      \"method\": \"DNA sequencing, linkage mapping, identification of mutations in affected individuals; enzymatic activity assay (NADH fluorescence); conservation analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — foundational mutation identification replicated across multiple families and labs, enzymatic assays performed\",\n      \"pmids\": [\"11875050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"IMPDH1 is the primary source of GTP in photoreceptors (versus IMPDH2 and HPRT); Impdh1 knockout mice display slowly progressive retinal degeneration; mutant IMPDH1 proteins expressed in bacterial and mammalian cells show misfolding and aggregation rather than reduced enzyme activity, suggesting protein misfolding/aggregation as the disease mechanism.\",\n      \"method\": \"Impdh1 null mouse (electroretinography, histology), retinal section expression analysis, recombinant protein expression in bacterial/mammalian cells, computational simulation\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods including in vivo knockout, expression studies, and computational modeling in a single study\",\n      \"pmids\": [\"14981049\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Retinitis pigmentosa-associated IMPDH1 missense mutations (Thr116Met, Asp226Asn, Val268Ile, His372Pro, Arg105Trp, Asn198Lys) do not alter enzymatic activity but alter the affinity and/or specificity of IMPDH1 for single-stranded nucleic acids, implicating nucleic acid binding as a distinct moonlighting function of IMPDH1.\",\n      \"method\": \"Enzymatic activity assay (NADH fluorescence production), filter-binding assays for single-stranded nucleic acid binding\",\n      \"journal\": \"Investigative ophthalmology & visual science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro enzymatic and binding assays with multiple mutants; findings replicated across several RP mutations\",\n      \"pmids\": [\"16384941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"In human retina, IMPDH1 is expressed at higher levels than any other tissue tested and is localized predominantly to inner segments and synaptic terminals of photoreceptors; alternative splicing and alternative translational start sites produce retina-specific IMPDH1 isoforms (distinct proteins) not found in other tissues, and the proportions of transcripts/proteins 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 — multiple orthogonal methods with direct subcellular localization and isoform identification linked to functional implications\",\n      \"pmids\": [\"16936083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"AAV-mediated expression of mutant human IMPDH1 in the mouse retina causes an aggressive retinopathy, while expression of normal IMPDH1 has no pathological effect; AAV-mediated co-expression of shRNA targeting both human and mouse IMPDH1 together with mutant IMPDH1 substantially suppresses retinal degeneration, demonstrating that mutant IMPDH1 is sufficient to cause retinopathy and that suppression of mutant transcript is therapeutic.\",\n      \"method\": \"Recombinant AAV delivery to mouse retina, in vivo shRNA knockdown, electroretinography, histology\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo gain-of-function and loss-of-function experiments with functional readout (ERG)\",\n      \"pmids\": [\"18385099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The IMPDH1 Leu275 variant allozyme has ~10% of wild-type enzymatic activity due to accelerated protein degradation, as supported by the IMPDH1 X-ray crystal structure; decreased activity of certain IMPDH1 allozymes results from reduced protein quantity caused by accelerated degradation, not from altered catalytic mechanism.\",\n      \"method\": \"Enzymatic activity assay, protein degradation assay, X-ray crystal structure analysis\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — single study with enzymatic assay and structural corroboration, but not replicated\",\n      \"pmids\": [\"20718729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Recombinant IMPDH1 clinical mutants R224P and D226N show impaired folding in vitro; in equimolar mixtures of normal and mutant enzymes, mutant subunits impose their faulty conformation on normal partners in hybrid tetramers (molecular recruitment/dominant-negative mechanism), potentially explaining autosomal dominant inheritance of RP10.\",\n      \"method\": \"In vitro refolding assay of recombinant IMPDH1, activity assay of mixed normal/mutant tetramers\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution assay, single lab, single study\",\n      \"pmids\": [\"21791244\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"IMPDH1's function in the retina, apparently independent of enzymatic activity, is mediated by retina-specific variants and may involve posttranscriptional regulation of rhodopsin mRNA; the adRP mutation D226N reduces binding to nucleic acids and reduces association with polyribosomes.\",\n      \"method\": \"Filter-binding assays, polyribosome association assay\",\n      \"journal\": \"Advances in experimental medicine and biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — binding and polyribosome assays, mechanistic follow-up, single lab\",\n      \"pmids\": [\"22183375\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Retina-specific IMPDH1 isoforms (mIMPDH1-546 and mIMPDH1-603) with C- and N-terminal extensions show higher Km and Ki values relative to the canonical isoform (mIMPDH1-514), do not exhibit NAD+ substrate inhibition unlike the canonical isoform, and the terminal segments interact with the enzyme's finger domain affecting its pseudo-barrel structure, as shown by molecular dynamics simulation.\",\n      \"method\": \"Kinetic analysis of recombinant murine IMPDH1 isoforms, molecular dynamics simulation\",\n      \"journal\": \"Molecular and cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–3 — direct kinetic measurements combined with computational modeling, single lab\",\n      \"pmids\": [\"31838626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In vivo, retinal IMPDH1 undergoes light-dependent phosphorylation at Thr159/Ser160 within the Bateman domain, which desensitizes the enzyme to allosteric inhibition by GDP/GTP; exposure to bright light increases GTP and ATP synthesis rates in mouse retinas concomitantly with IMPDH1 aggregate (filament) formation at the outer segment layer; inhibiting IMPDH activity in living mice delays rod mass recovery after light bleaching.\",\n      \"method\": \"In vivo phosphorylation mapping, metabolite measurements in mouse retina under light/dark conditions, confocal imaging of filament formation, pharmacological inhibition of IMPDH in living mice with ERG readout\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal in vivo methods (phosphoproteomics, metabolomics, imaging, pharmacology) in a single study with strong mechanistic conclusions\",\n      \"pmids\": [\"32254022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"IMPDH1 forms cytoophidia (intracellular filamentous structures) that physically interact with YB-1 and translocate YB-1 into the cell nucleus; IMPDH1 maintains YB-1 protein stabilization, while YB-1 induces IMPDH1 expression by binding to the IMPDH1 promoter, forming a positive feedback loop associated with renal cell carcinoma metastasis.\",\n      \"method\": \"Immunofluorescence, co-immunoprecipitation, ChIP/promoter binding assay, YB-1 nuclear translocation assay\",\n      \"journal\": \"Molecular therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP and nuclear translocation assays from a single lab; mechanistic follow-up with promoter binding\",\n      \"pmids\": [\"32209435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The major mouse retinal IMPDH1 isoform (mH1603) with terminal extensions shows higher catalytic activity and lower fibrillation capacity in the presence of ATP compared to the canonical isoform; in the presence of GTP and/or MPA, the retinal isoform forms higher mass oligomerization products.\",\n      \"method\": \"Kinetic analysis of recombinant IMPDH1 isoforms, oligomerization assay, molecular simulation\",\n      \"journal\": \"Cell biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–3 — direct biochemical assays with recombinant proteins combined with computational modeling, single lab\",\n      \"pmids\": [\"33733369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structures reveal that human IMPDH1 assembles polymorphic filaments with distinct extended and compressed state assembly interfaces; retina-specific splice variants introduce structural elements that stabilize the extended (high-activity) filament form and reduce sensitivity to GTP inhibition; RP disease mutations fall into two classes—one disrupts GTP regulation and the other has no effect on GTP regulation or filament assembly.\",\n      \"method\": \"Cryo-electron microscopy, biochemical activity assays, analysis of disease mutations and splice variants\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple cryo-EM structures with functional validation of assembly and allosteric regulation; single rigorous study with orthogonal methods\",\n      \"pmids\": [\"35013599\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"IMPDH2 stabilizes IMPDH1 by decreasing its polyubiquitination levels; c-Myc transcriptionally activates IMPDH1/2 to promote de novo GTP biosynthesis and colorectal cancer growth.\",\n      \"method\": \"Co-immunoprecipitation, protein half-life assay (cycloheximide chase), polyubiquitination assay, ChIP/reporter assay, xenograft model\",\n      \"journal\": \"Clinical and translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP and ubiquitination assay plus transcriptional activation demonstrated; single lab\",\n      \"pmids\": [\"36629054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The adRP-10 IMPDH1 mutation Asp226Asn (D226N) causes cytoophidium assembly in ~70% of cells and confers resistance to feedback inhibition by GDP/GTP; long-term expression of IMPDH1-D226N decreases cell survival by ~40%; introducing a secondary mutation Y12C that disrupts filament assembly significantly recovers cell survival, establishing that both nucleotide imbalance and toxic cytoophidium contribute to IMPDH1-D226N pathology.\",\n      \"method\": \"Stable expression in HEp-2 cells, immunofluorescence for cytoophidium, long-term cell survival assay, double-mutant genetic rescue experiment\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via double mutant rescue, quantitative survival assay, single lab\",\n      \"pmids\": [\"37731818\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"MYBL2 transcriptionally activates IMPDH1 by direct binding to its promoter, promoting de novo GTP synthesis and hepatocellular carcinoma cell proliferation; knockout of MYBL2 retards IMPDH1 expression, reduces guanine nucleotide pools (metabolomics), and inhibits tumor growth in xenograft models.\",\n      \"method\": \"ChIP-seq, ChIP-qPCR, metabolomics, MYBL2 knockout, xenograft tumor model\",\n      \"journal\": \"BMC cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP-seq with functional metabolomic validation and in vivo xenograft, single lab\",\n      \"pmids\": [\"36494680\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"LncRNA UCA1 recruits the transcription factor TWIST1 to the IMPDH1 promoter, increasing IMPDH1 transcription and guanine nucleotide production in bladder cancer cells, thereby stimulating RNA polymerase-dependent pre-ribosomal RNA production and GTPase activity.\",\n      \"method\": \"ChIP assay for TWIST1 binding at IMPDH1 promoter, reporter assay, guanine nucleotide metabolite measurement, functional cell assays (proliferation, migration)\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — ChIP and reporter assay with metabolite readout, single lab\",\n      \"pmids\": [\"37215997\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Phosphorylation of IMPDH1 at S477 (preferentially phosphorylated in the dark in bovine retinas) acts as a mechanism to downregulate retinal GTP synthesis: phosphomimetic S477D mutation re-sensitizes both IMPDH1(546) and IMPDH1(595) variants to GTP inhibition; cryo-EM structures show S477D specifically blocks the high-activity filament assembly interface of IMPDH1(595) while allowing low-activity IMPDH1(546) filament assembly; S477D exerts a dominant-negative effect in cells, preventing endogenous IMPDH filament assembly.\",\n      \"method\": \"Phosphomimetic mutagenesis, cryo-EM structure determination, in vitro enzymatic activity assay, cell-based filament assembly assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures plus in vitro enzymatic assays and cell-based validation with mutagenesis; multiple orthogonal methods\",\n      \"pmids\": [\"38323936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Following radiation therapy, IMPDH1 translocates to the nucleus in glioblastoma cells in a DNA-PK-dependent manner, driving nuclear GTP accumulation that promotes DNA damage repair; pharmacological inhibition of IMPDH with mycophenolate mofetil slows DNA damage repair and extends survival in orthotopic murine GBM models.\",\n      \"method\": \"Subcellular fractionation/immunofluorescence for nuclear translocation, metabolite measurements (nuclear GTP), DNA-PK inhibition epistasis, orthotopic murine GBM survival assay, phase 0 clinical trial with metabolite measurement in human tumors\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment tied to functional consequence, genetic epistasis (DNA-PK), in vivo survival data, and human phase 0 trial; 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 the rate-limiting homotetrameric enzyme in de novo guanine nucleotide biosynthesis that assembles into polymorphic filaments to reduce allosteric GTP inhibition and boost nucleotide production; in the retina, unique splice variants with terminal extensions produce isoforms with altered kinetics and filament architectures, and light-dependent phosphorylation at S477 (dephosphorylated in light, phosphorylated in dark) and at Thr159/Ser160 in the Bateman domain dynamically regulate filament assembly and allosteric sensitivity to adjust GTP synthesis to illumination conditions; disease-causing missense mutations either disrupt GTP allosteric regulation or impair nucleic acid binding/cause protein misfolding and dominant-negative aggregation in hybrid tetramers, while the protein also moonlights as a nucleic acid-binding factor and, in response to DNA damage, translocates to the nucleus in a DNA-PK-dependent manner to support nuclear GTP synthesis for DNA repair.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"IMPDH1 is the rate-limiting enzyme in de novo guanine nucleotide biosynthesis, functioning as a homotetramer that assembles into polymorphic filaments to modulate allosteric GTP feedback inhibition and tune nucleotide output to cellular demand [PMID:35013599, PMID:32254022]. In the retina, where IMPDH1 is the dominant GTP source, retina-specific splice variants with N- and C-terminal extensions stabilize extended (high-activity) filament conformations, exhibit altered kinetics including loss of NAD+ substrate inhibition, and undergo light-dependent phosphorylation at Thr159/Ser160 and Ser477 that dynamically adjusts allosteric sensitivity and filament architecture to match illumination conditions [PMID:36936083, PMID:31838626, PMID:32254022, PMID:38323936]. Missense mutations in IMPDH1 cause the RP10 form of autosomal dominant retinitis pigmentosa through two distinct mechanisms: one class disrupts GTP allosteric regulation and promotes toxic cytoophidium formation, while another impairs nucleic acid binding and causes dominant-negative misfolding within hybrid tetramers [PMID:11875050, PMID:16384941, PMID:35013599, PMID:21791244, PMID:37731818]. Beyond catalysis, IMPDH1 moonlights as a single-stranded nucleic acid-binding protein associated with polyribosomes, forms cytoophidia that interact with YB-1 to promote its nuclear translocation, and itself translocates to the nucleus in a DNA-PK-dependent manner to support nuclear GTP synthesis for DNA repair [PMID:16384941, PMID:22183375, PMID:32209435].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Establishing that IMPDH1 mutations cause autosomal dominant retinitis pigmentosa (RP10) without altering enzymatic activity resolved the genetic basis of RP10 and raised the question of how catalytically normal mutants cause photoreceptor degeneration.\",\n      \"evidence\": \"DNA sequencing and linkage mapping in RP families; NADH fluorescence enzymatic assay of recombinant mutants\",\n      \"pmids\": [\"11875050\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which catalytically normal mutants cause disease was unknown\", \"No in vivo disease model yet established\", \"No structural basis for mutations\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrating that IMPDH1 is the principal GTP source in photoreceptors and that Impdh1 knockout causes progressive retinal degeneration established that GTP supply is critical for photoreceptor survival, while showing mutant protein misfolding/aggregation pointed to a dominant-negative or toxic gain-of-function mechanism.\",\n      \"evidence\": \"Impdh1-null mouse with ERG and histology; recombinant mutant protein expression in bacterial and mammalian cells\",\n      \"pmids\": [\"14981049\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether misfolding occurs in hybrid tetramers with wild-type subunits was untested\", \"Retina-specific isoforms not yet identified\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Discovery that RP mutations alter nucleic acid binding affinity without affecting catalysis, and that retina-specific IMPDH1 splice variants exist, revealed a moonlighting function and tissue-specific molecular context that could explain why mutations selectively damage photoreceptors.\",\n      \"evidence\": \"Filter-binding assays for single-stranded nucleic acids with multiple RP mutants; Northern blot, immunohistochemistry, and transcript sequencing in human retina\",\n      \"pmids\": [\"16384941\", \"16936083\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Biological target RNAs in photoreceptors were unidentified\", \"Whether nucleic acid binding is physiologically relevant in vivo was unproven\", \"Functional significance of retina-specific isoforms was unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showing that AAV-delivered mutant IMPDH1 is sufficient to cause retinal degeneration in vivo and that shRNA-mediated suppression is therapeutic confirmed a dominant gain-of-function disease mechanism and validated an RNA interference therapeutic strategy.\",\n      \"evidence\": \"AAV delivery of mutant/WT IMPDH1 and shRNA to mouse retina with ERG and histology\",\n      \"pmids\": [\"18385099\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of toxicity (aggregation vs. nucleotide imbalance vs. nucleic acid binding) was unresolved\", \"Long-term durability of gene therapy not assessed\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Reconstitution of hybrid tetramers showed that mutant subunits impose faulty conformation on wild-type partners, providing a molecular explanation for autosomal dominant inheritance through a dominant-negative mechanism.\",\n      \"evidence\": \"In vitro refolding and activity assays of mixed normal/mutant recombinant IMPDH1 tetramers\",\n      \"pmids\": [\"21791244\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Hybrid tetramer formation not confirmed in cells or in vivo\", \"Contribution of dominant-negative effect relative to gain-of-function aggregation not quantified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Kinetic characterization of retina-specific IMPDH1 isoforms with terminal extensions revealed distinct enzymatic properties—higher Km/Ki values and loss of NAD+ substrate inhibition—explaining why the retina requires specialized isoforms for sustained GTP production.\",\n      \"evidence\": \"Kinetic analysis of recombinant murine IMPDH1 isoforms (mIMPDH1-514, -546, -603); molecular dynamics simulation\",\n      \"pmids\": [\"31838626\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo functional consequence of isoform-specific kinetics was untested\", \"Structural basis for altered kinetics awaited high-resolution structures\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"In vivo demonstration that light-dependent phosphorylation at Thr159/Ser160 desensitizes IMPDH1 to GTP inhibition, with concomitant filament formation and increased nucleotide synthesis in bright light, established a direct mechanism coupling illumination to retinal GTP homeostasis.\",\n      \"evidence\": \"Phosphoproteomics, retinal metabolite measurements under light/dark, confocal imaging of filaments, pharmacological IMPDH inhibition with ERG in living mice\",\n      \"pmids\": [\"32254022\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase(s) responsible for Thr159/Ser160 phosphorylation were unidentified\", \"Relationship between filament formation and phosphorylation at other sites was unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery that IMPDH1 cytoophidia physically interact with YB-1, promote its nuclear translocation, and that YB-1 reciprocally activates IMPDH1 transcription revealed a positive feedback loop linking IMPDH1 filament biology to gene regulation.\",\n      \"evidence\": \"Co-immunoprecipitation, immunofluorescence, ChIP/promoter binding assay in renal carcinoma cells\",\n      \"pmids\": [\"32209435\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the IMPDH1–YB-1 loop operates in photoreceptors or normal physiology is unknown\", \"Mechanism by which cytoophidia translocate YB-1 is unclear\", \"Single Co-IP study without reciprocal validation in independent labs\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cryo-EM structures of IMPDH1 filaments revealed extended and compressed conformational states with distinct assembly interfaces, showed that retina-specific splice elements stabilize the extended (high-activity) form, and classified RP mutations into two mechanistic categories—GTP regulation-disrupting and GTP regulation-neutral.\",\n      \"evidence\": \"Cryo-electron microscopy of filaments from canonical and retinal splice variants; biochemical activity assays; mapping of disease mutations\",\n      \"pmids\": [\"35013599\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for the GTP-regulation-neutral class of RP mutations was not resolved\", \"In vivo relevance of specific filament conformations in photoreceptors was undemonstrated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Genetic epistasis experiments showed that the D226N RP mutation causes both GTP feedback resistance and toxic cytoophidium formation, and that a secondary filament-disrupting mutation partially rescues cell survival, disentangling nucleotide imbalance from filament toxicity.\",\n      \"evidence\": \"Stable expression in HEp-2 cells, long-term survival assay, double-mutant (Y12C) rescue of D226N filament assembly\",\n      \"pmids\": [\"37731818\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Rescue experiment performed in non-retinal cell line\", \"Relative contribution of nucleotide imbalance versus filament toxicity not fully quantified in photoreceptors\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identification of S477 as a dark-phosphorylated regulatory site that re-sensitizes retinal IMPDH1 to GTP inhibition and blocks high-activity filament assembly in an isoform-specific manner completed the model of bidirectional phospho-regulation of retinal GTP synthesis across the light–dark cycle.\",\n      \"evidence\": \"Phosphomimetic mutagenesis (S477D), cryo-EM structures of filament interfaces, enzymatic assays, cell-based filament assembly assays\",\n      \"pmids\": [\"38323936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the S477 kinase and phosphatase is unknown\", \"How Thr159/Ser160 and S477 phosphorylation are coordinated in vivo is unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the identity of kinases/phosphatases controlling IMPDH1 phosphorylation in photoreceptors, the physiological RNA targets of IMPDH1's nucleic acid-binding function, and whether nuclear translocation and nuclear GTP synthesis for DNA repair represent a general IMPDH1 function across tissues.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Kinases/phosphatases for S477 and T159/S160 unidentified\", \"No validated endogenous RNA targets in photoreceptors\", \"Nuclear translocation for DNA repair shown only in glioblastoma preprint, awaits peer review and replication in other cell types\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [0, 1, 5, 8, 12]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [2, 7]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [12, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 10, 14]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 1, 8, 9, 12, 15, 16]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 4, 14]},\n      {\"term_id\": \"R-HSA-9709957\", \"supporting_discovery_ids\": [1, 3, 9, 17]}\n    ],\n    \"complexes\": [\n      \"IMPDH1 homotetramer\",\n      \"IMPDH1 cytoophidium (filament)\"\n    ],\n    \"partners\": [\n      \"IMPDH2\",\n      \"YBX1\",\n      \"TWIST1\",\n      \"MYBL2\",\n      \"MYC\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}