{"gene":"HTT","run_date":"2026-06-10T01:55:22","timeline":{"discoveries":[{"year":1995,"finding":"The IT15/HTT gene product is a ~350 kDa protein widely expressed in neurons throughout the brain (not enriched in striatum), present in the soluble cytoplasmic compartment, loosely associated with membranes or cytoskeleton, concentrated in nerve terminals, and diminished in nuclei and mitochondria, as determined by immunostaining with antibodies against the N-terminus and an internal epitope.","method":"Western blotting, immunohistochemistry, subcellular fractionation with two independent antisera","journal":"Neuron","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiment with fractionation and two independent antibodies, single study but multiple orthogonal methods","pmids":["7748554"],"is_preprint":false},{"year":1998,"finding":"Intranuclear neuronal inclusions containing huntingtin protein (detected by N-terminal huntingtin antibodies and anti-ubiquitin) are present in HD patient brains, and their density correlates with the length of the CAG repeat in IT15, establishing that polyglutamine expansion drives nuclear aggregate formation.","method":"Immunohistochemistry of post-mortem HD brain tissue with anti-ubiquitin and N-terminal huntingtin antibodies; 20 HD cases examined","journal":"Neurobiology of disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct histopathological experiment on human tissue with two orthogonal antibodies; single study, no functional reconstitution","pmids":["9666478"],"is_preprint":false},{"year":2000,"finding":"Mutant huntingtin in STHdh(Q111) striatal knock-in cells causes dominant phenotypes distinct from huntingtin deficiency or excess, including elevated p53, ER stress response, and hypoxia markers, indicating a gain-of-function mechanism separate from normal HTT activity. Full-length huntingtin (both wild-type and mutant) localizes to distinct nuclear and perinuclear organelles involved in RNA biogenesis and membrane trafficking.","method":"Comparative analysis of wild-type vs. Hdh(Q111) knock-in striatal cell lines; Western blotting, immunofluorescence, epitope accessibility assays","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knock-in cell line model with multiple orthogonal readouts, single lab","pmids":["11092756"],"is_preprint":false},{"year":2000,"finding":"FGF-2 increases huntingtin (IT15) protein levels in a dose-dependent manner in striatal neuronal cultures, whereas NGF decreases huntingtin expression, demonstrating that neurotrophic factors regulate HTT protein levels in striatal neurons.","method":"Western blotting of striatal cell cultures treated with FGF-2, NGF, or BDNF","journal":"Cell transplantation","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single method (Western blot), no mechanistic pathway elucidated beyond expression level change","pmids":["11144959"],"is_preprint":false},{"year":2010,"finding":"Full-length huntingtin rescues Golgi apparatus reformation following reversible microtubule disruption when endogenous HTT is silenced. Deletion of the HAP1-interaction domain or the dynein-interaction domain within full-length HTT abolishes this rescue and impairs BDNF vesicular trafficking in neuronal cells, demonstrating that HTT's role in dynein/dynactin-dependent vesicular trafficking requires these specific interaction domains.","method":"RNAi knockdown of endogenous HTT followed by rescue with synthetic full-length HTT constructs (pARIS-htt) containing domain deletions; Golgi reformation assay, BDNF vesicle trafficking assay in neurons","journal":"Molecular brain","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic rescue experiment with domain deletion mutants and two functional readouts (Golgi reformation and BDNF trafficking), single lab","pmids":["20515468"],"is_preprint":false},{"year":2011,"finding":"Knockdown of HTT expression in neuroepithelial cells of the neocortex causes disturbed neuronal migration, reduced proliferation, and increased apoptotic cell death during early neural development. The cell death phenotype in cortex is partially reversed by co-knockdown of Caspase-9, implicating mitochondria-mediated apoptosis. In the cerebellum, HTT knockdown causes cell death but not perturbed migration, demonstrating spatial and temporal requirements for HTT in brain development.","method":"In utero electroporation-mediated knockdown in mouse neocortex and cerebellum; phenotypic analysis of migration, proliferation, and cell death; genetic epistasis with Casp9 co-knockdown","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with defined cellular phenotypes and genetic epistasis in vivo, single lab","pmids":["21994396"],"is_preprint":false},{"year":2013,"finding":"The first 17 amino acids (N17) of huntingtin function as a nuclear export sequence (NES). Mutation of conserved residues within N17 increases nuclear accumulation of Htt exon 1, and nuclear export is sensitive to leptomycin B and requires exportin 1. In HEK293 cells, NES mutations decrease overall Htt aggregation but increase nuclear inclusions; in primary neurons, NES mutations increase both nuclear accumulation and overall aggregation.","method":"Mutational analysis of N17 residues, leptomycin B treatment, exportin 1 knockdown, nuclear/cytoplasmic fractionation, fluorescence microscopy in HEK293 cells and primary neurons","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis with multiple functional readouts and pharmacological validation, single lab","pmids":["23319588"],"is_preprint":false},{"year":2015,"finding":"HTT CAG repeat expansion enhances huntingtin's ability to facilitate polycomb repressive complex 2 (PRC2) activity, leading to altered histone H3K27me3 deposition at bivalent loci and prominent changes in histone H3K4me3 at active loci in embryonic stem cells and neuronal progenitor cells. Wild-type HTT is required for proper H3K27me3 deposition at a subset of bivalent loci.","method":"Genome-wide ChIP-seq for histone modifications across isogenic panel of mouse ESC and NPC lines with Htt null or different-size Htt CAG mutations","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genome-wide ChIP-seq across isogenic panel with multiple genotypes, single lab","pmids":["25574027"],"is_preprint":false},{"year":2015,"finding":"Expanded HTT (expHTT) RNA is retained in the nucleus by the splicing factor MBNL1, which increases nuclear retention and decreases cytosolic expHTT protein expression. The splicing and nuclear export factor U2AF65 has the opposite effect, decreasing expHTT nuclear retention and increasing cytosolic expHTT protein. A fragment of expHTT RNA is cytotoxic in the absence of translation, with cytotoxicity similar to that of an expHTT protein fragment of similar repeat size.","method":"Overexpression of MBNL1 and U2AF65, nuclear/cytoplasmic fractionation, cell toxicity assays with non-translatable expHTT RNA fragments","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — gain-of-function experiments with two opposing regulators and functional cytotoxicity readout, single lab","pmids":["26218986"],"is_preprint":false},{"year":2017,"finding":"The pathogenic exon 1 HTT protein is produced by incomplete splicing of the HTT pre-mRNA (failure to splice exon 1 to exon 2), generating a small polyadenylated HTTexon1 mRNA. This aberrant transcript is detectable in fibroblasts from juvenile HD patients and in post-mortem brain samples (sensorimotor cortex, hippocampus, cerebellum) from adult- and juvenile-onset HD individuals, with higher levels in early-onset disease. The level of this read-through product is proportional to CAG repeat length.","method":"Quantitative RT-PCR protocols specific for HTTexon1 transcript, applied to human fibroblast lines and post-mortem brain samples; comparison with knock-in mouse models","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — quantitative transcript analysis in multiple human tissue types and cell models, findings replicated across mouse models and human tissue in the same study","pmids":["28465506"],"is_preprint":false},{"year":2017,"finding":"A trimeric chaperone complex comprising Hsc70, Hsp110, and a J-protein (DNAJB1) completely suppresses fibrilization of HttExon1Q48 in vitro and can resolubilize preformed Htt fibrils. The J-protein is the concentration-limiting factor in the complex. Single overexpression of DNAJB1 in HEK293T cells is sufficient to profoundly reduce HttExon1Q97 aggregation. These findings were confirmed in HD patient-derived neural cells and in C. elegans.","method":"In vitro fibrilization assay, disaggregation assay, HEK293T overexpression, HD patient-derived neural cells, C. elegans in vivo model","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution combined with cell-based and in vivo validation across multiple systems","pmids":["29212816"],"is_preprint":false},{"year":2018,"finding":"Phosphorylation of HTT exon 1 at Ser-13 and/or Ser-16 inhibits aggregation of mutant Httex1, disrupts the amphipathic α-helix of the N-terminal 17-residue (Nt17) domain, inhibits Nt17 membrane binding, and promotes internalization and nuclear targeting of preformed Httex1 aggregates. A phosphorylation-dependent switch involving cross-talk between phosphorylation at Thr-3 and Ser-13/Ser-16 regulates Httex1 structure.","method":"In vitro phosphorylation with synthetic peptides bearing defined phosphorylation sites, aggregation assays, circular dichroism (helix disruption), membrane binding assays, cell-based subcellular localization experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro phosphorylation with mutagenesis, structural (CD), biochemical (aggregation, membrane binding), and cell-based assays in one study","pmids":["30185623"],"is_preprint":false},{"year":2019,"finding":"Mutant HTT intrinsically dysregulates oligodendrocyte progenitor cell (NG2+) function, causing early myelination deficits and behavioral abnormalities in HD mice independent of neuronal loss. Selective inactivation of mHTT in NG2+ cells prevents myelin abnormalities and rescues behavioral deficits even when mHTT continues to be expressed in neurons, astrocytes, and microglia. mHTT enhances polycomb repressive complex 2 (PRC2) activity in oligodendroglia, identified by RNA-seq and ChIP-seq analyses.","method":"Conditional mHTT inactivation in NG2+ cells (Cre-lox), behavioral testing, myelin staining, RNA-seq, ChIP-seq in HD mouse model","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — cell-type-specific conditional knockout with behavioral, histological, transcriptomic, and epigenomic readouts","pmids":["31015293"],"is_preprint":false},{"year":2019,"finding":"Small molecules (HTT-LC3 linker compounds) that interact with both LC3 (autophagosome protein) and the expanded polyQ stretch of mutant HTT (but not wild-type HTT) target mHTT to autophagosomes, reducing mHTT levels in an allele-selective manner and rescuing disease-relevant phenotypes in cells and in fly and mouse models of HD.","method":"Small-molecule microarray screening, co-immunoprecipitation, autophagic flux assays, cell-based mHTT quantification, Drosophila and mouse model rescue experiments","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic identification by screening followed by biochemical validation and in vivo rescue in multiple model systems","pmids":["31666698"],"is_preprint":false},{"year":2020,"finding":"HTT promotes the physical proximity of protein complexes during the initiation of mitophagy and recruits mitophagy receptors to promote interaction between damaged mitochondria and the nascent autophagosome. Mutant HTT (with expanded polyQ) disrupts formation of these protein complexes, leading to accumulation of damaged mitochondria and increased oxidative stress.","method":"Proximity ligation assay, co-immunoprecipitation, mitophagic flux assays, ROS measurements in cellular HD model","journal":"Autophagy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proximity ligation and co-IP with functional mitophagy readouts, single lab","pmids":["32093570"],"is_preprint":false},{"year":2021,"finding":"FKBP5 interacts and co-localizes with HTT in the striatum and cortex of HD mice. Reducing FKBP5 levels (siRNA or pharmacological inhibition with SAFit2) lowers mHTT levels by increasing LC3-II levels and macroautophagic flux (MTOR-independent mechanism). In vivo treatment with SAFit2 reduces HTT levels in HD mouse models.","method":"Co-immunoprecipitation, co-localization immunofluorescence, siRNA knockdown, pharmacological inhibition (SAFit2), LC3-II Western blotting, autophagic flux assay, in vivo mouse treatment","journal":"Autophagy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with functional autophagy readouts and in vivo validation, single lab","pmids":["34024231"],"is_preprint":false},{"year":2021,"finding":"Orally bioavailable small molecules lower HTT expression by promoting inclusion of a pseudoexon containing a premature termination codon (stop-codon psiExon) in HTT pre-mRNA through selective modulation of pre-mRNA splicing, leading to HTT mRNA degradation and reduction of HTT protein throughout the CNS and periphery.","method":"Splice modulator compound screening, RT-PCR for pseudoexon inclusion, HTT mRNA and protein quantification in cells and animal models","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mechanistic splice modulation demonstrated with molecular readouts and in vivo efficacy, single study","pmids":["34911927"],"is_preprint":false},{"year":2022,"finding":"HTT interacts with type I protein arginine methyltransferases (PRMTs), particularly PRMT4 (CARM1) and PRMT6, via its N-terminal domain. These enzymes methylate HTT at specific arginine residues. Alterations of methylation sites (R200/205) render HTT less soluble, promote phase transition from liquid-like to solid-like assemblies in vitro, and increase HTT toxicity to neuronal cells. Overexpression of PRMT4 and PRMT6 is beneficial for neuronal survival.","method":"Co-immunoprecipitation, in vitro methylation assay, mass spectrometry (data-dependent and targeted quantitative), liquid-liquid phase separation assay, cell toxicity assay, PRMT overexpression","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro methylation, MS identification, and cell-based functional validation with multiple orthogonal methods, single lab","pmids":["34888656"],"is_preprint":false},{"year":2022,"finding":"DNAJB1 binds to HTT exon 1 through a specific HTT-binding motif (HBM) in the hinge region between its C-terminal domains I and II, interacting with the polyQ-adjacent proline-rich domain (PRD) of both soluble and aggregated HTT. Mutation of the conserved H244 residue in the HBM completely abrogates suppression and disaggregation of HTT fibrils by the trimeric chaperone complex without affecting binding or remodeling of other substrates. Overexpression of wild-type DNAJB1 but not DNAJB1(H244A) prevents HTTExon1Q97 aggregation in HEK293 cells.","method":"In vitro aggregation/disaggregation assay with purified proteins, site-directed mutagenesis, pull-down, HEK293 cell aggregation assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with mutagenesis, biochemical binding characterization, and cell-based functional validation","pmids":["35948542"],"is_preprint":false},{"year":2022,"finding":"Full-length HTT and HTT1a (exon 1 only) transcripts are retained together in large nuclear RNA clusters in YAC128 mouse brain, and the HTT1a transcript is translated to produce exon 1 HTT protein. Levels of exon 1 HTT correlate with HTT aggregation, supporting the hypothesis that exon 1 HTT initiates the aggregation process.","method":"RNAscope single-molecule RNA visualization, HTRF protein quantification in YAC128 mouse brain and mouse embryonic fibroblasts","journal":"Brain","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single-molecule RNA visualization combined with protein quantification in vivo, single lab","pmids":["35793238"],"is_preprint":false},{"year":2022,"finding":"HTT (huntingtin) co-migrates retrogradely with RAB7 on a signaling LAMP1-containing late endosome during axonal injury in Drosophila neurons, establishing HTT's association with late endosomal retrograde axonal transport in the context of injury signaling.","method":"Live imaging and co-migration analysis in Drosophila axons with fluorescently tagged HTT, RAB7, and LAMP1; immunoprecipitation","journal":"Autophagy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live imaging with multiple markers and IP in Drosophila model, single lab","pmids":["36048753"],"is_preprint":false},{"year":2016,"finding":"The hnRNP-HTT axis regulates necrotic cell death induced by transcriptional repression (TRIAD). HTT functions as an ER-stabilizing molecule whose transcription and RNA splicing are impaired by transcriptional repression. hnRNP proteins regulate HTT RNA splicing, and fly genetic experiments (suppressor/expresser of Htt and hnRNP) worsened or ameliorated transcriptional repression-induced necrosis, placing HTT in a pathway downstream of hnRNP in protection against TRIAD.","method":"Genetic screen of knockdown fly library, RNA sequencing, fly genetics (genetic epistasis with Htt suppressors/expressers and hnRNP), Drosophila pupa-to-larva survival assay","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in Drosophila with transcriptomic readout, single lab","pmids":["27124581"],"is_preprint":false},{"year":2016,"finding":"SMAD transcription factors (TGF-β pathway) are dysregulated in HD cell models and regulate HTT gene expression itself, identifying the TGF-β/SMAD pathway as a regulator of HTT transcription.","method":"Transcriptional profiling of HD cell models with growth factor stimulation (EGF), SMAD target gene analysis, HTT expression measurement","journal":"Cellular signalling","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, expression-based inference with limited direct mechanistic validation of SMAD-HTT promoter interaction","pmids":["27988204"],"is_preprint":false},{"year":2018,"finding":"Gpr52, an orphan G protein-coupled receptor, regulates mutant HTT protein levels in the striatum. Knockout of Gpr52 reduces mHTT levels and rescues HD-associated behavioral phenotypes in a knock-in HD mouse model. A Gpr52 antagonist (E7) reduces mHTT levels in cellular and mouse models, indicating that Gpr52 signaling positively regulates mHTT levels.","method":"Gpr52 knockout in HD knock-in mice, behavioral testing, mHTT protein quantification, pharmacological antagonism in cellular and mouse models","journal":"Brain","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockout and pharmacological validation with behavioral and molecular readouts in vivo, single lab","pmids":["29608652"],"is_preprint":false},{"year":2018,"finding":"HIPK3 kinase activity negatively regulates autophagy and positively regulates mHTT levels. Knockdown or knockout of HIPK3 reduces mHTT levels by enhancing autophagy in HD cells and in a knock-in mouse model. Mutant HTT positively regulates HIPK3 mRNA levels, forming a positive feedback loop that contributes to autophagy inhibition and mHTT accumulation. HIPK3's effect on mHTT requires its known substrate DAXX.","method":"Genome-scale RNAi screen, HIPK3 knockdown/knockout in HD cells and knock-in mice, autophagic flux assay, mHTT protein quantification, epistasis with DAXX","journal":"Autophagy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genome-scale screen with mechanistic follow-up including in vivo validation and epistasis, single lab","pmids":["29130397"],"is_preprint":false}],"current_model":"HTT encodes a large (~350 kDa) scaffolding protein concentrated in nerve terminals and enriched in the cytoplasm, where it facilitates dynein/dynactin-dependent vesicular trafficking (including BDNF vesicles) via HAP1 and dynein interaction domains, promotes mitophagy by recruiting mitophagy receptors and initiating autophagosome–mitochondria contact, and scaffolds PRC2 to regulate histone H3K27me3 deposition; the N-terminal 17-residue (Nt17) domain acts as a nuclear export sequence whose phosphorylation at Ser-13/Ser-16 disrupts an amphipathic α-helix, inhibits aggregation, and redirects HTT to the nucleus; the expanded polyQ tract confers dominant gain-of-function effects including enhanced PRC2 activity, disrupted protein complex formation during mitophagy, altered phase-separation behavior driven by aberrant arginine methylation by PRMT4/PRMT6, and a disposition for fibril formation that is counteracted by a trimeric chaperone complex (Hsc70–DNAJB1–Hsp110) binding the proline-rich domain adjacent to the polyQ stretch; additionally, incomplete pre-mRNA splicing produces a pathogenic exon 1 HTT protein (HTTexon1) whose levels correlate with aggregation and disease severity."},"narrative":{"mechanistic_narrative":"HTT encodes a large (~350 kDa) cytoplasmic scaffolding protein concentrated in neuronal nerve terminals and largely excluded from nuclei and mitochondria, with roles spanning vesicular trafficking, organelle quality control, chromatin regulation, and neural development [PMID:7748554, PMID:21994396]. As a trafficking scaffold, HTT supports dynein/dynactin-dependent transport: its HAP1-interaction and dynein-interaction domains are required to rescue Golgi reformation and BDNF vesicle trafficking in neurons [PMID:20515468], and it associates with RAB7/LAMP1 late endosomes during retrograde axonal injury signaling [PMID:36048753]. HTT also organizes mitochondrial quality control, recruiting mitophagy receptors and promoting contact between damaged mitochondria and nascent autophagosomes [PMID:32093570]. In the nucleus, wild-type HTT is required for proper PRC2-dependent H3K27me3 deposition at bivalent loci, linking it to chromatin regulation [PMID:25574027]. HTT is developmentally essential: its loss in neuroepithelium disrupts neuronal migration, proliferation, and survival via caspase-9-dependent apoptosis [PMID:21994396]. Subcellular distribution is governed by an N-terminal 17-residue (Nt17) nuclear export sequence that depends on exportin 1; phosphorylation at Ser-13/Ser-16 disrupts its amphipathic α-helix, inhibits membrane binding and aggregation, and redirects HTT to the nucleus [PMID:23319588, PMID:30185623]. Pathogenic CAG/polyQ expansion confers dominant gain-of-function effects, including elevated stress and apoptotic markers [PMID:11092756], enhanced PRC2 activity in neurons and oligodendroglia [PMID:25574027, PMID:31015293], disrupted mitophagy complex assembly with oxidative stress [PMID:32093570], and altered arginine methylation by PRMT4/PRMT6 that drives liquid-to-solid phase transition and toxicity [PMID:34888656]. Aggregation is seeded by an exon 1 HTT species generated by incomplete splicing of HTT pre-mRNA, whose levels scale with CAG length and correlate with disease severity [PMID:28465506, PMID:35793238], and is suppressed by a trimeric Hsc70–Hsp110–DNAJB1 chaperone complex in which DNAJB1 engages the polyQ-adjacent proline-rich domain through a specific HTT-binding motif [PMID:29212816, PMID:35948542]. HTT protein levels are further controlled by FKBP5, GPR52, HIPK3, and pre-mRNA splicing, nodes exploited for allele- and dose-lowering therapeutic strategies [PMID:31666698, PMID:34024231, PMID:34911927, PMID:29608652, PMID:29130397].","teleology":[{"year":1995,"claim":"Establishing where the IT15/HTT product resides was the first step toward function; the work showed HTT is a broadly neuronal ~350 kDa cytoplasmic protein concentrated at nerve terminals rather than enriched in the striatum it most affects.","evidence":"Western blot, immunohistochemistry, and subcellular fractionation with two independent antisera","pmids":["7748554"],"confidence":"Medium","gaps":["No molecular activity assigned","Does not explain striatal selectivity of disease","Membrane/cytoskeleton association only loosely defined"]},{"year":1998,"claim":"Whether polyQ expansion causes aggregation in patients was answered by showing intranuclear neuronal inclusions whose density scales with CAG repeat length, tying expansion directly to nuclear aggregate formation.","evidence":"Immunohistochemistry of 20 post-mortem HD brains with N-terminal HTT and anti-ubiquitin antibodies","pmids":["9666478"],"confidence":"Medium","gaps":["Correlation, not causation, of inclusions with toxicity","Does not identify the aggregating species","No functional reconstitution"]},{"year":2000,"claim":"To distinguish loss- from gain-of-function, knock-in striatal cells showed mutant HTT produces dominant phenotypes (elevated p53, ER stress, hypoxia markers) distinct from HTT deficiency or excess.","evidence":"Comparative analysis of Hdh(Q111) knock-in vs wild-type striatal cell lines with multiple readouts","pmids":["11092756"],"confidence":"Medium","gaps":["Molecular trigger of stress responses not defined","Single cell-line model","Relationship to aggregation unclear"]},{"year":2010,"claim":"The molecular basis of HTT's trafficking role was mapped by domain-deletion rescue, showing the HAP1- and dynein-interaction domains are each required for HTT-dependent Golgi reformation and BDNF vesicle transport.","evidence":"RNAi knockdown with full-length HTT rescue constructs bearing domain deletions; Golgi and BDNF trafficking assays in neurons","pmids":["20515468"],"confidence":"Medium","gaps":["Direct structural interaction interfaces not resolved","How polyQ expansion alters trafficking not tested here","Single lab"]},{"year":2011,"claim":"Whether HTT is developmentally essential was addressed by in vivo knockdown showing region-specific requirements for migration, proliferation, and caspase-9-dependent survival in the developing brain.","evidence":"In utero electroporation knockdown in mouse neocortex and cerebellum with Casp9 epistasis","pmids":["21994396"],"confidence":"Medium","gaps":["Molecular pathway linking HTT to migration unknown","Relevance to adult HD pathology indirect","Single lab"]},{"year":2013,"claim":"How HTT subcellular partitioning is controlled was clarified by identifying the N17 segment as an exportin 1-dependent nuclear export sequence whose disruption shifts HTT to the nucleus and alters aggregation.","evidence":"N17 mutagenesis, leptomycin B treatment, exportin 1 knockdown, and fractionation in HEK293 and primary neurons","pmids":["23319588"],"confidence":"Medium","gaps":["Physiological signals triggering export not defined","Context-dependent aggregation effects (HEK vs neurons) unexplained","Single lab"]},{"year":2015,"claim":"A nuclear chromatin function emerged with the finding that wild-type HTT supports PRC2-mediated H3K27me3 deposition and CAG expansion enhances this activity, genome-wide.","evidence":"ChIP-seq for histone marks across isogenic Htt-null and CAG-mutant mouse ESC/NPC lines","pmids":["25574027"],"confidence":"Medium","gaps":["Direct HTT–PRC2 physical interaction not biochemically resolved","Which target genes drive phenotype unknown","Single lab"]},{"year":2015,"claim":"The contribution of expanded HTT RNA itself was addressed by showing MBNL1 retains expHTT RNA in the nucleus while U2AF65 promotes export, and that non-translated expHTT RNA is intrinsically cytotoxic.","evidence":"MBNL1/U2AF65 overexpression, fractionation, and toxicity assays with non-translatable expHTT RNA fragments","pmids":["26218986"],"confidence":"Medium","gaps":["Mechanism of RNA toxicity not defined","In vivo relevance untested","Overexpression-based"]},{"year":2016,"claim":"Upstream regulation of HTT was probed: the hnRNP–HTT axis was placed in protection against transcriptional-repression-induced necrosis (TRIAD), with HTT acting as an ER-stabilizing factor; separately, TGF-β/SMAD signaling was linked to HTT transcription.","evidence":"Drosophila genetic epistasis with Htt/hnRNP plus RNA-seq; transcriptional profiling of HD cell models with EGF/SMAD analysis","pmids":["27124581","27988204"],"confidence":"Medium","gaps":["Direct SMAD–HTT promoter interaction not demonstrated (Low confidence)","ER-stabilizing mechanism of HTT undefined","Mostly model-organism/expression inference"]},{"year":2017,"claim":"The origin of the aggregation-prone fragment was resolved by showing incomplete HTT pre-mRNA splicing generates a polyadenylated exon 1 (HTTexon1) transcript whose levels scale with CAG length and disease severity in human tissue.","evidence":"Quantitative HTTexon1-specific RT-PCR in HD fibroblasts, post-mortem brain, and knock-in mouse models","pmids":["28465506"],"confidence":"High","gaps":["Direct causal role of HTTexon1 in pathology shown later","Splicing machinery driving read-through not fully defined"]},{"year":2017,"claim":"A protective mechanism against aggregation was established by reconstituting a trimeric Hsc70–Hsp110–DNAJB1 chaperone that fully suppresses and resolubilizes HTT exon 1 fibrils, with DNAJB1 limiting and sufficient.","evidence":"In vitro fibrilization/disaggregation assays, HEK293T overexpression, HD patient neural cells, and C. elegans","pmids":["29212816"],"confidence":"High","gaps":["Binding site on HTT not yet defined here","Endogenous regulation of chaperone levels in neurons unclear"]},{"year":2018,"claim":"A phosphorylation switch controlling HTT conformation was defined: Ser-13/Ser-16 phosphorylation disrupts the Nt17 amphipathic helix, blocks membrane binding and aggregation, and promotes nuclear targeting.","evidence":"In vitro phosphopeptide aggregation/CD/membrane-binding assays plus cell-based localization","pmids":["30185623"],"confidence":"High","gaps":["Kinases/phosphatases acting in vivo not identified","Quantitative occupancy of these sites in patients unknown"]},{"year":2018,"claim":"Druggable regulators of mHTT level were identified: GPR52 signaling positively maintains striatal mHTT, and HIPK3 suppresses autophagic clearance of mHTT through DAXX in a positive feedback loop.","evidence":"Gpr52 knockout and antagonist studies; genome-scale RNAi screen with HIPK3 knockout, autophagy flux, and DAXX epistasis in HD cells and knock-in mice","pmids":["29608652","29130397"],"confidence":"Medium","gaps":["Allele selectivity of GPR52/HIPK3 effects limited","Connection of GPR52 to autophagy vs other clearance unclear","Single lab each"]},{"year":2019,"claim":"The cellular site of pathogenic action was broadened by showing mHTT cell-autonomously disrupts oligodendrocyte progenitors via enhanced PRC2 activity, and that NG2-cell-specific mHTT removal rescues myelin and behavior independent of neurons.","evidence":"Cre-lox conditional mHTT inactivation in NG2+ cells with behavior, myelin staining, RNA-seq, and ChIP-seq","pmids":["31015293"],"confidence":"High","gaps":["Mechanism coupling PRC2 to myelin genes incomplete","Relative contribution of glial vs neuronal mHTT in patients unknown"]},{"year":2019,"claim":"Allele-selective clearance was made tractable by HTT-LC3 linker compounds that bridge expanded polyQ HTT to autophagosomes, lowering mHTT and rescuing phenotypes across cells, flies, and mice.","evidence":"Small-molecule microarray, Co-IP, autophagic flux, and rescue in Drosophila and mouse HD models","pmids":["31666698"],"confidence":"High","gaps":["Therapeutic window and CNS delivery not established","Long-term selectivity for mutant over wild-type HTT untested"]},{"year":2020,"claim":"HTT's role in mitochondrial quality control was demonstrated: it brings mitophagy complexes into proximity and recruits receptors to damaged mitochondria, a function disrupted by polyQ expansion leading to oxidative stress.","evidence":"Proximity ligation, Co-IP, mitophagic flux, and ROS assays in cellular HD model","pmids":["32093570"],"confidence":"Medium","gaps":["Identity of the recruited mitophagy receptors not fully specified","Direct HTT binding partners in the complex undefined","Single lab"]},{"year":2021,"claim":"Additional level-control nodes were validated: FKBP5 interacts with HTT and its inhibition (SAFit2) lowers mHTT via MTOR-independent macroautophagy, and splice modulators promote a premature-stop pseudoexon to degrade HTT mRNA CNS-wide.","evidence":"Reciprocal Co-IP, siRNA/SAFit2 with LC3-II and flux assays in vivo; splice modulator screening with RT-PCR and mRNA/protein quantification","pmids":["34024231","34911927"],"confidence":"Medium","gaps":["FKBP5–HTT direct interaction mode unresolved","Allele selectivity of splice modulators limited","Single studies"]},{"year":2022,"claim":"The mechanistic basis of phase behavior was defined by showing PRMT4/PRMT6 bind HTT's N-terminus and methylate it; altering R200/205 methylation drives liquid-to-solid transition and toxicity, while PRMT overexpression is protective.","evidence":"Co-IP, in vitro methylation, mass spectrometry, LLPS assays, and cell toxicity with PRMT overexpression","pmids":["34888656"],"confidence":"Medium","gaps":["In vivo relevance of arginine methylation untested","Crosstalk with phosphorylation/aggregation not integrated","Single lab"]},{"year":2022,"claim":"The DNAJB1 recognition determinant was pinpointed: a HTT-binding motif in the DNAJB1 hinge (H244) engages the polyQ-adjacent proline-rich domain, and H244A abolishes chaperone suppression of HTT fibrils without affecting other substrates.","evidence":"In vitro disaggregation with purified proteins, site-directed mutagenesis, pull-downs, and HEK293 aggregation assay","pmids":["35948542"],"confidence":"High","gaps":["Structure of the DNAJB1–PRD complex not solved","Whether endogenous DNAJB1 levels limit clearance in neurons unclear"]},{"year":2022,"claim":"Direct visualization confirmed that full-length HTT and HTT1a transcripts co-cluster in nuclear RNA foci, that HTT1a is translated to exon 1 protein, and that exon 1 levels correlate with aggregation, supporting it as the aggregation seed.","evidence":"RNAscope single-molecule imaging and HTRF protein quantification in YAC128 mouse brain and MEFs; plus Drosophila imaging linking HTT to RAB7/LAMP1 retrograde endosomal transport","pmids":["35793238","36048753"],"confidence":"Medium","gaps":["Causal role of nuclear RNA clusters in toxicity untested","How exon 1 protein nucleates full-length aggregation unresolved","Single lab each"]},{"year":null,"claim":"How HTT's diverse activities (trafficking scaffold, mitophagy organizer, PRC2 cofactor) are mechanistically unified, and how polyQ expansion coordinately corrupts them, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No integrated structure of full-length HTT with its partners","Causal hierarchy among RNA toxicity, exon 1 seeding, and protein dysfunction undefined","Tissue selectivity of degeneration unexplained at the molecular level"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[4,14]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[4]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[7,12]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[8,19]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0,2]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1,6]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[4,20]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[4]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[20]}],"pathway":[{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[13,14,15,24]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[4,20]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[7,12]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[5]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[8,9,16,19]}],"complexes":["Hsc70–Hsp110–DNAJB1 trimeric chaperone complex (substrate)","PRC2 (functional partner)"],"partners":["HAP1","DNAJB1","PRMT4","PRMT6","FKBP5","MBNL1","U2AF65","RAB7"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P31645","full_name":"Sodium-dependent serotonin transporter","aliases":["5HT transporter","5HTT","Solute carrier family 6 member 4"],"length_aa":630,"mass_kda":70.3,"function":"Serotonin transporter that cotransports serotonin with one Na(+) ion in exchange for one K(+) ion and possibly one proton in an overall electroneutral transport cycle. Transports serotonin across the plasma membrane from the extracellular compartment to the cytosol thus limiting serotonin intercellular signaling (PubMed:10407194, PubMed:12869649, PubMed:21730057, PubMed:27049939, PubMed:27756841, PubMed:34851672). Essential for serotonin homeostasis in the central nervous system. In the developing somatosensory cortex, acts in glutamatergic neurons to control serotonin uptake and its trophic functions accounting for proper spatial organization of cortical neurons and elaboration of sensory circuits. In the mature cortex, acts primarily in brainstem raphe neurons to mediate serotonin uptake from the synaptic cleft back into the pre-synaptic terminal thus terminating serotonin signaling at the synapse (By similarity). Modulates mucosal serotonin levels in the gastrointestinal tract through uptake and clearance of serotonin in enterocytes. Required for enteric neurogenesis and gastrointestinal reflexes (By similarity). Regulates blood serotonin levels by ensuring rapid high affinity uptake of serotonin from plasma to platelets, where it is further stored in dense granules via vesicular monoamine transporters and then released upon stimulation (PubMed:17506858, PubMed:18317590). Mechanistically, the transport cycle starts with an outward-open conformation having Na1(+) and Cl(-) sites occupied. The binding of a second extracellular Na2(+) ion and serotonin substrate leads to structural changes to outward-occluded to inward-occluded to inward-open, where the Na2(+) ion and serotonin are released into the cytosol. Binding of intracellular K(+) ion induces conformational transitions to inward-occluded to outward-open and completes the cycle by releasing K(+) possibly together with a proton bound to Asp-98 into the extracellular compartment. Na1(+) and Cl(-) ions remain bound throughout the transport cycle (PubMed:10407194, PubMed:12869649, PubMed:21730057, PubMed:27049939, PubMed:27756841, PubMed:34851672). Additionally, displays serotonin-induced channel-like conductance for monovalent cations, mainly Na(+) ions. The channel activity is uncoupled from the transport cycle and may contribute to the membrane resting potential or excitability (By similarity)","subcellular_location":"Cell membrane; Endomembrane system; Endosome membrane; Synapse; Cell junction, focal adhesion; Cell projection, neuron projection","url":"https://www.uniprot.org/uniprotkb/P31645/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/HTT","classification":"Not Classified","n_dependent_lines":108,"n_total_lines":1208,"dependency_fraction":0.08940397350993377},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/HTT","total_profiled":1310},"omim":[{"mim_id":"619200","title":"SORTING NEXIN FAMILY, MEMBER 21; SNX21","url":"https://www.omim.org/entry/619200"},{"mim_id":"617485","title":"WD REPEAT- AND FYVE DOMAIN-CONTAINING PROTEIN 3; WDFY3","url":"https://www.omim.org/entry/617485"},{"mim_id":"617435","title":"LOPES-MACIEL-RODAN SYNDROME; LOMARS","url":"https://www.omim.org/entry/617435"},{"mim_id":"616417","title":"ADHESION G PROTEIN-COUPLED RECEPTOR L3; ADGRL3","url":"https://www.omim.org/entry/616417"},{"mim_id":"615684","title":"HELICASE FOR MEIOSIS 1; HFM1","url":"https://www.omim.org/entry/615684"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/HTT"},"hgnc":{"alias_symbol":["IT15"],"prev_symbol":["HD"]},"alphafold":{"accession":"P31645","domains":[{"cath_id":"1.20.1740,1.20.1730","chopping":"104-214_222-620","consensus_level":"medium","plddt":94.1674,"start":104,"end":620}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P31645","model_url":"https://alphafold.ebi.ac.uk/files/AF-P31645-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P31645-F1-predicted_aligned_error_v6.png","plddt_mean":84.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HTT","jax_strain_url":"https://www.jax.org/strain/search?query=HTT"},"sequence":{"accession":"P31645","fasta_url":"https://rest.uniprot.org/uniprotkb/P31645.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P31645/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P31645"}},"corpus_meta":[{"pmid":"12869766","id":"PMC_12869766","title":"Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene.","date":"2003","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/12869766","citation_count":4900,"is_preprint":false},{"pmid":"11092756","id":"PMC_11092756","title":"Dominant phenotypes produced by the HD mutation in STHdh(Q111) striatal cells.","date":"2000","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/11092756","citation_count":541,"is_preprint":false},{"pmid":"17698401","id":"PMC_17698401","title":"The true story of the HD-Zip family.","date":"2007","source":"Trends in plant science","url":"https://pubmed.ncbi.nlm.nih.gov/17698401","citation_count":451,"is_preprint":false},{"pmid":"7748554","id":"PMC_7748554","title":"Widespread expression of Huntington's disease gene (IT15) protein product.","date":"1995","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/7748554","citation_count":421,"is_preprint":false},{"pmid":"31666698","id":"PMC_31666698","title":"Allele-selective lowering of mutant HTT protein by HTT-LC3 linker compounds.","date":"2019","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/31666698","citation_count":390,"is_preprint":false},{"pmid":"19015103","id":"PMC_19015103","title":"Oxytocin receptor (OXTR) and serotonin transporter (5-HTT) genes associated with observed parenting.","date":"2008","source":"Social cognitive and affective neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/19015103","citation_count":339,"is_preprint":false},{"pmid":"9666478","id":"PMC_9666478","title":"Intranuclear neuronal inclusions in Huntington's disease and dentatorubral and pallidoluysian atrophy: correlation between the density of inclusions and IT15 CAG triplet repeat length.","date":"1998","source":"Neurobiology of disease","url":"https://pubmed.ncbi.nlm.nih.gov/9666478","citation_count":338,"is_preprint":false},{"pmid":"9771716","id":"PMC_9771716","title":"Behavioural abnormalities and selective neuronal loss in HD transgenic mice expressing mutated full-length HD cDNA.","date":"1998","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/9771716","citation_count":313,"is_preprint":false},{"pmid":"8240819","id":"PMC_8240819","title":"Huntington's disease gene (IT15) is widely expressed in human and rat tissues.","date":"1993","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/8240819","citation_count":288,"is_preprint":false},{"pmid":"19548255","id":"PMC_19548255","title":"CAG-repeat length and the age of onset in Huntington disease (HD): a review and validation study of statistical approaches.","date":"2010","source":"American journal of medical genetics. 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membranes or cytoskeleton, concentrated in nerve terminals, and diminished in nuclei and mitochondria, as determined by immunostaining with antibodies against the N-terminus and an internal epitope.\",\n      \"method\": \"Western blotting, immunohistochemistry, subcellular fractionation with two independent antisera\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiment with fractionation and two independent antibodies, single study but multiple orthogonal methods\",\n      \"pmids\": [\"7748554\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Intranuclear neuronal inclusions containing huntingtin protein (detected by N-terminal huntingtin antibodies and anti-ubiquitin) are present in HD patient brains, and their density correlates with the length of the CAG repeat in IT15, establishing that polyglutamine expansion drives nuclear aggregate formation.\",\n      \"method\": \"Immunohistochemistry of post-mortem HD brain tissue with anti-ubiquitin and N-terminal huntingtin antibodies; 20 HD cases examined\",\n      \"journal\": \"Neurobiology of disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct histopathological experiment on human tissue with two orthogonal antibodies; single study, no functional reconstitution\",\n      \"pmids\": [\"9666478\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Mutant huntingtin in STHdh(Q111) striatal knock-in cells causes dominant phenotypes distinct from huntingtin deficiency or excess, including elevated p53, ER stress response, and hypoxia markers, indicating a gain-of-function mechanism separate from normal HTT activity. Full-length huntingtin (both wild-type and mutant) localizes to distinct nuclear and perinuclear organelles involved in RNA biogenesis and membrane trafficking.\",\n      \"method\": \"Comparative analysis of wild-type vs. Hdh(Q111) knock-in striatal cell lines; Western blotting, immunofluorescence, epitope accessibility assays\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knock-in cell line model with multiple orthogonal readouts, single lab\",\n      \"pmids\": [\"11092756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"FGF-2 increases huntingtin (IT15) protein levels in a dose-dependent manner in striatal neuronal cultures, whereas NGF decreases huntingtin expression, demonstrating that neurotrophic factors regulate HTT protein levels in striatal neurons.\",\n      \"method\": \"Western blotting of striatal cell cultures treated with FGF-2, NGF, or BDNF\",\n      \"journal\": \"Cell transplantation\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single method (Western blot), no mechanistic pathway elucidated beyond expression level change\",\n      \"pmids\": [\"11144959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Full-length huntingtin rescues Golgi apparatus reformation following reversible microtubule disruption when endogenous HTT is silenced. Deletion of the HAP1-interaction domain or the dynein-interaction domain within full-length HTT abolishes this rescue and impairs BDNF vesicular trafficking in neuronal cells, demonstrating that HTT's role in dynein/dynactin-dependent vesicular trafficking requires these specific interaction domains.\",\n      \"method\": \"RNAi knockdown of endogenous HTT followed by rescue with synthetic full-length HTT constructs (pARIS-htt) containing domain deletions; Golgi reformation assay, BDNF vesicle trafficking assay in neurons\",\n      \"journal\": \"Molecular brain\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic rescue experiment with domain deletion mutants and two functional readouts (Golgi reformation and BDNF trafficking), single lab\",\n      \"pmids\": [\"20515468\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Knockdown of HTT expression in neuroepithelial cells of the neocortex causes disturbed neuronal migration, reduced proliferation, and increased apoptotic cell death during early neural development. The cell death phenotype in cortex is partially reversed by co-knockdown of Caspase-9, implicating mitochondria-mediated apoptosis. In the cerebellum, HTT knockdown causes cell death but not perturbed migration, demonstrating spatial and temporal requirements for HTT in brain development.\",\n      \"method\": \"In utero electroporation-mediated knockdown in mouse neocortex and cerebellum; phenotypic analysis of migration, proliferation, and cell death; genetic epistasis with Casp9 co-knockdown\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with defined cellular phenotypes and genetic epistasis in vivo, single lab\",\n      \"pmids\": [\"21994396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The first 17 amino acids (N17) of huntingtin function as a nuclear export sequence (NES). Mutation of conserved residues within N17 increases nuclear accumulation of Htt exon 1, and nuclear export is sensitive to leptomycin B and requires exportin 1. In HEK293 cells, NES mutations decrease overall Htt aggregation but increase nuclear inclusions; in primary neurons, NES mutations increase both nuclear accumulation and overall aggregation.\",\n      \"method\": \"Mutational analysis of N17 residues, leptomycin B treatment, exportin 1 knockdown, nuclear/cytoplasmic fractionation, fluorescence microscopy in HEK293 cells and primary neurons\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis with multiple functional readouts and pharmacological validation, single lab\",\n      \"pmids\": [\"23319588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"HTT CAG repeat expansion enhances huntingtin's ability to facilitate polycomb repressive complex 2 (PRC2) activity, leading to altered histone H3K27me3 deposition at bivalent loci and prominent changes in histone H3K4me3 at active loci in embryonic stem cells and neuronal progenitor cells. Wild-type HTT is required for proper H3K27me3 deposition at a subset of bivalent loci.\",\n      \"method\": \"Genome-wide ChIP-seq for histone modifications across isogenic panel of mouse ESC and NPC lines with Htt null or different-size Htt CAG mutations\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genome-wide ChIP-seq across isogenic panel with multiple genotypes, single lab\",\n      \"pmids\": [\"25574027\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Expanded HTT (expHTT) RNA is retained in the nucleus by the splicing factor MBNL1, which increases nuclear retention and decreases cytosolic expHTT protein expression. The splicing and nuclear export factor U2AF65 has the opposite effect, decreasing expHTT nuclear retention and increasing cytosolic expHTT protein. A fragment of expHTT RNA is cytotoxic in the absence of translation, with cytotoxicity similar to that of an expHTT protein fragment of similar repeat size.\",\n      \"method\": \"Overexpression of MBNL1 and U2AF65, nuclear/cytoplasmic fractionation, cell toxicity assays with non-translatable expHTT RNA fragments\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gain-of-function experiments with two opposing regulators and functional cytotoxicity readout, single lab\",\n      \"pmids\": [\"26218986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The pathogenic exon 1 HTT protein is produced by incomplete splicing of the HTT pre-mRNA (failure to splice exon 1 to exon 2), generating a small polyadenylated HTTexon1 mRNA. This aberrant transcript is detectable in fibroblasts from juvenile HD patients and in post-mortem brain samples (sensorimotor cortex, hippocampus, cerebellum) from adult- and juvenile-onset HD individuals, with higher levels in early-onset disease. The level of this read-through product is proportional to CAG repeat length.\",\n      \"method\": \"Quantitative RT-PCR protocols specific for HTTexon1 transcript, applied to human fibroblast lines and post-mortem brain samples; comparison with knock-in mouse models\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — quantitative transcript analysis in multiple human tissue types and cell models, findings replicated across mouse models and human tissue in the same study\",\n      \"pmids\": [\"28465506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A trimeric chaperone complex comprising Hsc70, Hsp110, and a J-protein (DNAJB1) completely suppresses fibrilization of HttExon1Q48 in vitro and can resolubilize preformed Htt fibrils. The J-protein is the concentration-limiting factor in the complex. Single overexpression of DNAJB1 in HEK293T cells is sufficient to profoundly reduce HttExon1Q97 aggregation. These findings were confirmed in HD patient-derived neural cells and in C. elegans.\",\n      \"method\": \"In vitro fibrilization assay, disaggregation assay, HEK293T overexpression, HD patient-derived neural cells, C. elegans in vivo model\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution combined with cell-based and in vivo validation across multiple systems\",\n      \"pmids\": [\"29212816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Phosphorylation of HTT exon 1 at Ser-13 and/or Ser-16 inhibits aggregation of mutant Httex1, disrupts the amphipathic α-helix of the N-terminal 17-residue (Nt17) domain, inhibits Nt17 membrane binding, and promotes internalization and nuclear targeting of preformed Httex1 aggregates. A phosphorylation-dependent switch involving cross-talk between phosphorylation at Thr-3 and Ser-13/Ser-16 regulates Httex1 structure.\",\n      \"method\": \"In vitro phosphorylation with synthetic peptides bearing defined phosphorylation sites, aggregation assays, circular dichroism (helix disruption), membrane binding assays, cell-based subcellular localization experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro phosphorylation with mutagenesis, structural (CD), biochemical (aggregation, membrane binding), and cell-based assays in one study\",\n      \"pmids\": [\"30185623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Mutant HTT intrinsically dysregulates oligodendrocyte progenitor cell (NG2+) function, causing early myelination deficits and behavioral abnormalities in HD mice independent of neuronal loss. Selective inactivation of mHTT in NG2+ cells prevents myelin abnormalities and rescues behavioral deficits even when mHTT continues to be expressed in neurons, astrocytes, and microglia. mHTT enhances polycomb repressive complex 2 (PRC2) activity in oligodendroglia, identified by RNA-seq and ChIP-seq analyses.\",\n      \"method\": \"Conditional mHTT inactivation in NG2+ cells (Cre-lox), behavioral testing, myelin staining, RNA-seq, ChIP-seq in HD mouse model\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — cell-type-specific conditional knockout with behavioral, histological, transcriptomic, and epigenomic readouts\",\n      \"pmids\": [\"31015293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Small molecules (HTT-LC3 linker compounds) that interact with both LC3 (autophagosome protein) and the expanded polyQ stretch of mutant HTT (but not wild-type HTT) target mHTT to autophagosomes, reducing mHTT levels in an allele-selective manner and rescuing disease-relevant phenotypes in cells and in fly and mouse models of HD.\",\n      \"method\": \"Small-molecule microarray screening, co-immunoprecipitation, autophagic flux assays, cell-based mHTT quantification, Drosophila and mouse model rescue experiments\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic identification by screening followed by biochemical validation and in vivo rescue in multiple model systems\",\n      \"pmids\": [\"31666698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"HTT promotes the physical proximity of protein complexes during the initiation of mitophagy and recruits mitophagy receptors to promote interaction between damaged mitochondria and the nascent autophagosome. Mutant HTT (with expanded polyQ) disrupts formation of these protein complexes, leading to accumulation of damaged mitochondria and increased oxidative stress.\",\n      \"method\": \"Proximity ligation assay, co-immunoprecipitation, mitophagic flux assays, ROS measurements in cellular HD model\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proximity ligation and co-IP with functional mitophagy readouts, single lab\",\n      \"pmids\": [\"32093570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FKBP5 interacts and co-localizes with HTT in the striatum and cortex of HD mice. Reducing FKBP5 levels (siRNA or pharmacological inhibition with SAFit2) lowers mHTT levels by increasing LC3-II levels and macroautophagic flux (MTOR-independent mechanism). In vivo treatment with SAFit2 reduces HTT levels in HD mouse models.\",\n      \"method\": \"Co-immunoprecipitation, co-localization immunofluorescence, siRNA knockdown, pharmacological inhibition (SAFit2), LC3-II Western blotting, autophagic flux assay, in vivo mouse treatment\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with functional autophagy readouts and in vivo validation, single lab\",\n      \"pmids\": [\"34024231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Orally bioavailable small molecules lower HTT expression by promoting inclusion of a pseudoexon containing a premature termination codon (stop-codon psiExon) in HTT pre-mRNA through selective modulation of pre-mRNA splicing, leading to HTT mRNA degradation and reduction of HTT protein throughout the CNS and periphery.\",\n      \"method\": \"Splice modulator compound screening, RT-PCR for pseudoexon inclusion, HTT mRNA and protein quantification in cells and animal models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mechanistic splice modulation demonstrated with molecular readouts and in vivo efficacy, single study\",\n      \"pmids\": [\"34911927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"HTT interacts with type I protein arginine methyltransferases (PRMTs), particularly PRMT4 (CARM1) and PRMT6, via its N-terminal domain. These enzymes methylate HTT at specific arginine residues. Alterations of methylation sites (R200/205) render HTT less soluble, promote phase transition from liquid-like to solid-like assemblies in vitro, and increase HTT toxicity to neuronal cells. Overexpression of PRMT4 and PRMT6 is beneficial for neuronal survival.\",\n      \"method\": \"Co-immunoprecipitation, in vitro methylation assay, mass spectrometry (data-dependent and targeted quantitative), liquid-liquid phase separation assay, cell toxicity assay, PRMT overexpression\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro methylation, MS identification, and cell-based functional validation with multiple orthogonal methods, single lab\",\n      \"pmids\": [\"34888656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"DNAJB1 binds to HTT exon 1 through a specific HTT-binding motif (HBM) in the hinge region between its C-terminal domains I and II, interacting with the polyQ-adjacent proline-rich domain (PRD) of both soluble and aggregated HTT. Mutation of the conserved H244 residue in the HBM completely abrogates suppression and disaggregation of HTT fibrils by the trimeric chaperone complex without affecting binding or remodeling of other substrates. Overexpression of wild-type DNAJB1 but not DNAJB1(H244A) prevents HTTExon1Q97 aggregation in HEK293 cells.\",\n      \"method\": \"In vitro aggregation/disaggregation assay with purified proteins, site-directed mutagenesis, pull-down, HEK293 cell aggregation assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with mutagenesis, biochemical binding characterization, and cell-based functional validation\",\n      \"pmids\": [\"35948542\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Full-length HTT and HTT1a (exon 1 only) transcripts are retained together in large nuclear RNA clusters in YAC128 mouse brain, and the HTT1a transcript is translated to produce exon 1 HTT protein. Levels of exon 1 HTT correlate with HTT aggregation, supporting the hypothesis that exon 1 HTT initiates the aggregation process.\",\n      \"method\": \"RNAscope single-molecule RNA visualization, HTRF protein quantification in YAC128 mouse brain and mouse embryonic fibroblasts\",\n      \"journal\": \"Brain\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single-molecule RNA visualization combined with protein quantification in vivo, single lab\",\n      \"pmids\": [\"35793238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"HTT (huntingtin) co-migrates retrogradely with RAB7 on a signaling LAMP1-containing late endosome during axonal injury in Drosophila neurons, establishing HTT's association with late endosomal retrograde axonal transport in the context of injury signaling.\",\n      \"method\": \"Live imaging and co-migration analysis in Drosophila axons with fluorescently tagged HTT, RAB7, and LAMP1; immunoprecipitation\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live imaging with multiple markers and IP in Drosophila model, single lab\",\n      \"pmids\": [\"36048753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The hnRNP-HTT axis regulates necrotic cell death induced by transcriptional repression (TRIAD). HTT functions as an ER-stabilizing molecule whose transcription and RNA splicing are impaired by transcriptional repression. hnRNP proteins regulate HTT RNA splicing, and fly genetic experiments (suppressor/expresser of Htt and hnRNP) worsened or ameliorated transcriptional repression-induced necrosis, placing HTT in a pathway downstream of hnRNP in protection against TRIAD.\",\n      \"method\": \"Genetic screen of knockdown fly library, RNA sequencing, fly genetics (genetic epistasis with Htt suppressors/expressers and hnRNP), Drosophila pupa-to-larva survival assay\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in Drosophila with transcriptomic readout, single lab\",\n      \"pmids\": [\"27124581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SMAD transcription factors (TGF-β pathway) are dysregulated in HD cell models and regulate HTT gene expression itself, identifying the TGF-β/SMAD pathway as a regulator of HTT transcription.\",\n      \"method\": \"Transcriptional profiling of HD cell models with growth factor stimulation (EGF), SMAD target gene analysis, HTT expression measurement\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, expression-based inference with limited direct mechanistic validation of SMAD-HTT promoter interaction\",\n      \"pmids\": [\"27988204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Gpr52, an orphan G protein-coupled receptor, regulates mutant HTT protein levels in the striatum. Knockout of Gpr52 reduces mHTT levels and rescues HD-associated behavioral phenotypes in a knock-in HD mouse model. A Gpr52 antagonist (E7) reduces mHTT levels in cellular and mouse models, indicating that Gpr52 signaling positively regulates mHTT levels.\",\n      \"method\": \"Gpr52 knockout in HD knock-in mice, behavioral testing, mHTT protein quantification, pharmacological antagonism in cellular and mouse models\",\n      \"journal\": \"Brain\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout and pharmacological validation with behavioral and molecular readouts in vivo, single lab\",\n      \"pmids\": [\"29608652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HIPK3 kinase activity negatively regulates autophagy and positively regulates mHTT levels. Knockdown or knockout of HIPK3 reduces mHTT levels by enhancing autophagy in HD cells and in a knock-in mouse model. Mutant HTT positively regulates HIPK3 mRNA levels, forming a positive feedback loop that contributes to autophagy inhibition and mHTT accumulation. HIPK3's effect on mHTT requires its known substrate DAXX.\",\n      \"method\": \"Genome-scale RNAi screen, HIPK3 knockdown/knockout in HD cells and knock-in mice, autophagic flux assay, mHTT protein quantification, epistasis with DAXX\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genome-scale screen with mechanistic follow-up including in vivo validation and epistasis, single lab\",\n      \"pmids\": [\"29130397\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HTT encodes a large (~350 kDa) scaffolding protein concentrated in nerve terminals and enriched in the cytoplasm, where it facilitates dynein/dynactin-dependent vesicular trafficking (including BDNF vesicles) via HAP1 and dynein interaction domains, promotes mitophagy by recruiting mitophagy receptors and initiating autophagosome–mitochondria contact, and scaffolds PRC2 to regulate histone H3K27me3 deposition; the N-terminal 17-residue (Nt17) domain acts as a nuclear export sequence whose phosphorylation at Ser-13/Ser-16 disrupts an amphipathic α-helix, inhibits aggregation, and redirects HTT to the nucleus; the expanded polyQ tract confers dominant gain-of-function effects including enhanced PRC2 activity, disrupted protein complex formation during mitophagy, altered phase-separation behavior driven by aberrant arginine methylation by PRMT4/PRMT6, and a disposition for fibril formation that is counteracted by a trimeric chaperone complex (Hsc70–DNAJB1–Hsp110) binding the proline-rich domain adjacent to the polyQ stretch; additionally, incomplete pre-mRNA splicing produces a pathogenic exon 1 HTT protein (HTTexon1) whose levels correlate with aggregation and disease severity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"HTT encodes a large (~350 kDa) cytoplasmic scaffolding protein concentrated in neuronal nerve terminals and largely excluded from nuclei and mitochondria, with roles spanning vesicular trafficking, organelle quality control, chromatin regulation, and neural development [#0, #5]. As a trafficking scaffold, HTT supports dynein/dynactin-dependent transport: its HAP1-interaction and dynein-interaction domains are required to rescue Golgi reformation and BDNF vesicle trafficking in neurons [#4], and it associates with RAB7/LAMP1 late endosomes during retrograde axonal injury signaling [#20]. HTT also organizes mitochondrial quality control, recruiting mitophagy receptors and promoting contact between damaged mitochondria and nascent autophagosomes [#14]. In the nucleus, wild-type HTT is required for proper PRC2-dependent H3K27me3 deposition at bivalent loci, linking it to chromatin regulation [#7]. HTT is developmentally essential: its loss in neuroepithelium disrupts neuronal migration, proliferation, and survival via caspase-9-dependent apoptosis [#5]. Subcellular distribution is governed by an N-terminal 17-residue (Nt17) nuclear export sequence that depends on exportin 1; phosphorylation at Ser-13/Ser-16 disrupts its amphipathic \\u03b1-helix, inhibits membrane binding and aggregation, and redirects HTT to the nucleus [#6, #11]. Pathogenic CAG/polyQ expansion confers dominant gain-of-function effects, including elevated stress and apoptotic markers [#2], enhanced PRC2 activity in neurons and oligodendroglia [#7, #12], disrupted mitophagy complex assembly with oxidative stress [#14], and altered arginine methylation by PRMT4/PRMT6 that drives liquid-to-solid phase transition and toxicity [#17]. Aggregation is seeded by an exon 1 HTT species generated by incomplete splicing of HTT pre-mRNA, whose levels scale with CAG length and correlate with disease severity [#9, #19], and is suppressed by a trimeric Hsc70\\u2013Hsp110\\u2013DNAJB1 chaperone complex in which DNAJB1 engages the polyQ-adjacent proline-rich domain through a specific HTT-binding motif [#10, #18]. HTT protein levels are further controlled by FKBP5, GPR52, HIPK3, and pre-mRNA splicing, nodes exploited for allele- and dose-lowering therapeutic strategies [#13, #15, #16, #23, #24].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Establishing where the IT15/HTT product resides was the first step toward function; the work showed HTT is a broadly neuronal ~350 kDa cytoplasmic protein concentrated at nerve terminals rather than enriched in the striatum it most affects.\",\n      \"evidence\": \"Western blot, immunohistochemistry, and subcellular fractionation with two independent antisera\",\n      \"pmids\": [\"7748554\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No molecular activity assigned\", \"Does not explain striatal selectivity of disease\", \"Membrane/cytoskeleton association only loosely defined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Whether polyQ expansion causes aggregation in patients was answered by showing intranuclear neuronal inclusions whose density scales with CAG repeat length, tying expansion directly to nuclear aggregate formation.\",\n      \"evidence\": \"Immunohistochemistry of 20 post-mortem HD brains with N-terminal HTT and anti-ubiquitin antibodies\",\n      \"pmids\": [\"9666478\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlation, not causation, of inclusions with toxicity\", \"Does not identify the aggregating species\", \"No functional reconstitution\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"To distinguish loss- from gain-of-function, knock-in striatal cells showed mutant HTT produces dominant phenotypes (elevated p53, ER stress, hypoxia markers) distinct from HTT deficiency or excess.\",\n      \"evidence\": \"Comparative analysis of Hdh(Q111) knock-in vs wild-type striatal cell lines with multiple readouts\",\n      \"pmids\": [\"11092756\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular trigger of stress responses not defined\", \"Single cell-line model\", \"Relationship to aggregation unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The molecular basis of HTT's trafficking role was mapped by domain-deletion rescue, showing the HAP1- and dynein-interaction domains are each required for HTT-dependent Golgi reformation and BDNF vesicle transport.\",\n      \"evidence\": \"RNAi knockdown with full-length HTT rescue constructs bearing domain deletions; Golgi and BDNF trafficking assays in neurons\",\n      \"pmids\": [\"20515468\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct structural interaction interfaces not resolved\", \"How polyQ expansion alters trafficking not tested here\", \"Single lab\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Whether HTT is developmentally essential was addressed by in vivo knockdown showing region-specific requirements for migration, proliferation, and caspase-9-dependent survival in the developing brain.\",\n      \"evidence\": \"In utero electroporation knockdown in mouse neocortex and cerebellum with Casp9 epistasis\",\n      \"pmids\": [\"21994396\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular pathway linking HTT to migration unknown\", \"Relevance to adult HD pathology indirect\", \"Single lab\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"How HTT subcellular partitioning is controlled was clarified by identifying the N17 segment as an exportin 1-dependent nuclear export sequence whose disruption shifts HTT to the nucleus and alters aggregation.\",\n      \"evidence\": \"N17 mutagenesis, leptomycin B treatment, exportin 1 knockdown, and fractionation in HEK293 and primary neurons\",\n      \"pmids\": [\"23319588\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological signals triggering export not defined\", \"Context-dependent aggregation effects (HEK vs neurons) unexplained\", \"Single lab\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"A nuclear chromatin function emerged with the finding that wild-type HTT supports PRC2-mediated H3K27me3 deposition and CAG expansion enhances this activity, genome-wide.\",\n      \"evidence\": \"ChIP-seq for histone marks across isogenic Htt-null and CAG-mutant mouse ESC/NPC lines\",\n      \"pmids\": [\"25574027\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct HTT\\u2013PRC2 physical interaction not biochemically resolved\", \"Which target genes drive phenotype unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"The contribution of expanded HTT RNA itself was addressed by showing MBNL1 retains expHTT RNA in the nucleus while U2AF65 promotes export, and that non-translated expHTT RNA is intrinsically cytotoxic.\",\n      \"evidence\": \"MBNL1/U2AF65 overexpression, fractionation, and toxicity assays with non-translatable expHTT RNA fragments\",\n      \"pmids\": [\"26218986\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of RNA toxicity not defined\", \"In vivo relevance untested\", \"Overexpression-based\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Upstream regulation of HTT was probed: the hnRNP\\u2013HTT axis was placed in protection against transcriptional-repression-induced necrosis (TRIAD), with HTT acting as an ER-stabilizing factor; separately, TGF-\\u03b2/SMAD signaling was linked to HTT transcription.\",\n      \"evidence\": \"Drosophila genetic epistasis with Htt/hnRNP plus RNA-seq; transcriptional profiling of HD cell models with EGF/SMAD analysis\",\n      \"pmids\": [\"27124581\", \"27988204\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct SMAD\\u2013HTT promoter interaction not demonstrated (Low confidence)\", \"ER-stabilizing mechanism of HTT undefined\", \"Mostly model-organism/expression inference\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The origin of the aggregation-prone fragment was resolved by showing incomplete HTT pre-mRNA splicing generates a polyadenylated exon 1 (HTTexon1) transcript whose levels scale with CAG length and disease severity in human tissue.\",\n      \"evidence\": \"Quantitative HTTexon1-specific RT-PCR in HD fibroblasts, post-mortem brain, and knock-in mouse models\",\n      \"pmids\": [\"28465506\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct causal role of HTTexon1 in pathology shown later\", \"Splicing machinery driving read-through not fully defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"A protective mechanism against aggregation was established by reconstituting a trimeric Hsc70\\u2013Hsp110\\u2013DNAJB1 chaperone that fully suppresses and resolubilizes HTT exon 1 fibrils, with DNAJB1 limiting and sufficient.\",\n      \"evidence\": \"In vitro fibrilization/disaggregation assays, HEK293T overexpression, HD patient neural cells, and C. elegans\",\n      \"pmids\": [\"29212816\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding site on HTT not yet defined here\", \"Endogenous regulation of chaperone levels in neurons unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"A phosphorylation switch controlling HTT conformation was defined: Ser-13/Ser-16 phosphorylation disrupts the Nt17 amphipathic helix, blocks membrane binding and aggregation, and promotes nuclear targeting.\",\n      \"evidence\": \"In vitro phosphopeptide aggregation/CD/membrane-binding assays plus cell-based localization\",\n      \"pmids\": [\"30185623\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinases/phosphatases acting in vivo not identified\", \"Quantitative occupancy of these sites in patients unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Druggable regulators of mHTT level were identified: GPR52 signaling positively maintains striatal mHTT, and HIPK3 suppresses autophagic clearance of mHTT through DAXX in a positive feedback loop.\",\n      \"evidence\": \"Gpr52 knockout and antagonist studies; genome-scale RNAi screen with HIPK3 knockout, autophagy flux, and DAXX epistasis in HD cells and knock-in mice\",\n      \"pmids\": [\"29608652\", \"29130397\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Allele selectivity of GPR52/HIPK3 effects limited\", \"Connection of GPR52 to autophagy vs other clearance unclear\", \"Single lab each\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The cellular site of pathogenic action was broadened by showing mHTT cell-autonomously disrupts oligodendrocyte progenitors via enhanced PRC2 activity, and that NG2-cell-specific mHTT removal rescues myelin and behavior independent of neurons.\",\n      \"evidence\": \"Cre-lox conditional mHTT inactivation in NG2+ cells with behavior, myelin staining, RNA-seq, and ChIP-seq\",\n      \"pmids\": [\"31015293\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism coupling PRC2 to myelin genes incomplete\", \"Relative contribution of glial vs neuronal mHTT in patients unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Allele-selective clearance was made tractable by HTT-LC3 linker compounds that bridge expanded polyQ HTT to autophagosomes, lowering mHTT and rescuing phenotypes across cells, flies, and mice.\",\n      \"evidence\": \"Small-molecule microarray, Co-IP, autophagic flux, and rescue in Drosophila and mouse HD models\",\n      \"pmids\": [\"31666698\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Therapeutic window and CNS delivery not established\", \"Long-term selectivity for mutant over wild-type HTT untested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"HTT's role in mitochondrial quality control was demonstrated: it brings mitophagy complexes into proximity and recruits receptors to damaged mitochondria, a function disrupted by polyQ expansion leading to oxidative stress.\",\n      \"evidence\": \"Proximity ligation, Co-IP, mitophagic flux, and ROS assays in cellular HD model\",\n      \"pmids\": [\"32093570\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the recruited mitophagy receptors not fully specified\", \"Direct HTT binding partners in the complex undefined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Additional level-control nodes were validated: FKBP5 interacts with HTT and its inhibition (SAFit2) lowers mHTT via MTOR-independent macroautophagy, and splice modulators promote a premature-stop pseudoexon to degrade HTT mRNA CNS-wide.\",\n      \"evidence\": \"Reciprocal Co-IP, siRNA/SAFit2 with LC3-II and flux assays in vivo; splice modulator screening with RT-PCR and mRNA/protein quantification\",\n      \"pmids\": [\"34024231\", \"34911927\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"FKBP5\\u2013HTT direct interaction mode unresolved\", \"Allele selectivity of splice modulators limited\", \"Single studies\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The mechanistic basis of phase behavior was defined by showing PRMT4/PRMT6 bind HTT's N-terminus and methylate it; altering R200/205 methylation drives liquid-to-solid transition and toxicity, while PRMT overexpression is protective.\",\n      \"evidence\": \"Co-IP, in vitro methylation, mass spectrometry, LLPS assays, and cell toxicity with PRMT overexpression\",\n      \"pmids\": [\"34888656\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance of arginine methylation untested\", \"Crosstalk with phosphorylation/aggregation not integrated\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The DNAJB1 recognition determinant was pinpointed: a HTT-binding motif in the DNAJB1 hinge (H244) engages the polyQ-adjacent proline-rich domain, and H244A abolishes chaperone suppression of HTT fibrils without affecting other substrates.\",\n      \"evidence\": \"In vitro disaggregation with purified proteins, site-directed mutagenesis, pull-downs, and HEK293 aggregation assay\",\n      \"pmids\": [\"35948542\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the DNAJB1\\u2013PRD complex not solved\", \"Whether endogenous DNAJB1 levels limit clearance in neurons unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Direct visualization confirmed that full-length HTT and HTT1a transcripts co-cluster in nuclear RNA foci, that HTT1a is translated to exon 1 protein, and that exon 1 levels correlate with aggregation, supporting it as the aggregation seed.\",\n      \"evidence\": \"RNAscope single-molecule imaging and HTRF protein quantification in YAC128 mouse brain and MEFs; plus Drosophila imaging linking HTT to RAB7/LAMP1 retrograde endosomal transport\",\n      \"pmids\": [\"35793238\", \"36048753\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal role of nuclear RNA clusters in toxicity untested\", \"How exon 1 protein nucleates full-length aggregation unresolved\", \"Single lab each\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How HTT's diverse activities (trafficking scaffold, mitophagy organizer, PRC2 cofactor) are mechanistically unified, and how polyQ expansion coordinately corrupts them, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No integrated structure of full-length HTT with its partners\", \"Causal hierarchy among RNA toxicity, exon 1 seeding, and protein dysfunction undefined\", \"Tissue selectivity of degeneration unexplained at the molecular level\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [4, 14]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [7, 12]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [8, 19]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [4, 20]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [20]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [13, 14, 15, 24]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [4, 20]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [7, 12]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [8, 9, 16, 19]}\n    ],\n    \"complexes\": [\"Hsc70\\u2013Hsp110\\u2013DNAJB1 trimeric chaperone complex (substrate)\", \"PRC2 (functional partner)\"],\n    \"partners\": [\"HAP1\", \"DNAJB1\", \"PRMT4\", \"PRMT6\", \"FKBP5\", \"MBNL1\", \"U2AF65\", \"RAB7\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}