{"gene":"ACAN","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2005,"finding":"A single-base-pair insertion in the variable repeat region of exon 12 of ACAN introduces a frameshift of 212 amino acids including 22 cysteine residues followed by a premature stop codon, causing spondyloepiphyseal dysplasia type Kimberley (SEDK) with severe premature osteoarthritis — establishing ACAN as a human disease gene.","method":"Mutation screening/sequencing in a multigenerational family with SEDK mapped to 15q26.1","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — direct mutation identification with cosegregation in a mapped locus; first report of ACAN causing human disease","pmids":["16080123"],"is_preprint":false},{"year":2007,"finding":"A 4-bp insertion in ACAN exon 11 causes bulldog chondrodysplastic dwarfism in Dexter cattle; in chondrocytes heterozygous for the insertion, mutant mRNA undergoes nonsense-mediated decay (only 8% of normal expression), establishing haploinsufficiency as the disease mechanism.","method":"Mutation screening, genotyping in worldwide Dexter pedigrees, mRNA expression analysis in heterozygous chondrocytes","journal":"Mammalian genome","confidence":"High","confidence_rationale":"Tier 2 — direct molecular validation of NMD mechanism with functional mRNA quantification in primary chondrocytes","pmids":["17952705"],"is_preprint":false},{"year":2012,"finding":"Multiple (eleven) conserved non-coding enhancer sequences, distributed from >100 kb upstream of ACAN to within the first intron, independently direct cartilage-specific reporter expression in transgenic zebrafish; several contain SOX9 binding sites, establishing redundant transcriptional control of ACAN in cartilage.","method":"Functional enhancer assay in transgenic zebrafish with 24 conserved non-coding sequences tested","journal":"Matrix biology","confidence":"High","confidence_rationale":"Tier 2 — in vivo functional assay replicated across multiple enhancer elements; orthologues confirmed in chicken","pmids":["22820679"],"is_preprint":false},{"year":2014,"finding":"SHOX2, like SHOX, activates ACAN transcription through cooperation with the SOX trio (SOX5, SOX6, SOX9) via protein–protein interaction between SOX5/SOX6 and SHOX2; this interaction was confirmed by yeast-two-hybrid and co-immunoprecipitation; SHOX2 and SOX trio are co-expressed in human fetal growth plates.","method":"Luciferase reporter assay, yeast-two-hybrid, co-immunoprecipitation, immunohistochemistry of human fetal growth plates","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (luciferase, Y2H, co-IP, IHC) in one study","pmids":["24421874"],"is_preprint":false},{"year":2016,"finding":"SOX9 acetylation reduces its nuclear entry and binding to the -10 kb ACAN enhancer, thereby suppressing ACAN transactivation; deacetylation by SIRT1 promotes SOX9 nuclear translocation via importin β, increasing ACAN expression in human chondrocytes.","method":"Co-immunoprecipitation (SOX9–SIRT1), immunofluorescence (nuclear localization with NAD/SIRT1 inhibitor/importazole treatments), ChIP (SOX9 binding to ACAN enhancer), primary OA chondrocyte cultures in 2D and 3D","journal":"Aging cell","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods including co-IP, ChIP, pharmacological perturbation, and rescue experiments in primary human chondrocytes","pmids":["26910618"],"is_preprint":false},{"year":2020,"finding":"TGF-β1-induced expression of ACAN is regulated by the mTOR/4E-BP1 translational apparatus; 4E-BP1 controls translation of inhibitory Smads (Smad6/7), which in turn modulate nuclear Smad2/3 accumulation on the ACAN promoter, linking translational repression to transcriptional output.","method":"mTOR silencing, 4E-BP1 knockdown, phosphorylation assays, co-immunoprecipitation (4E-BP1–eIF4E), ChIP (Smad2/3 on ACAN promoter) in human OA chondrocytes","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 1–2 — mechanistic dissection with gene silencing, ChIP, co-IP, and multiple pathway components in a single study","pmids":["32485037"],"is_preprint":false},{"year":2017,"finding":"Homozygous disruption of Acan in mice (Agc1CreERT/CreERT knock-in) reduces Acan mRNA and protein by ~50%, causing dwarfism with shorter long bones and vertebrae, reduced growth plate length, and decreased articular cartilage thickness, demonstrating that Acan gene dosage is critical for normal skeletal growth.","method":"Mouse genetic model (homozygous knock-in at Acan 3'UTR), histology, proteoglycan staining, gene expression analysis","journal":"Genesis","confidence":"High","confidence_rationale":"Tier 2 — clean in vivo loss-of-function with defined molecular (mRNA/protein reduction) and skeletal phenotype","pmids":["28921880"],"is_preprint":false},{"year":2020,"finding":"TET1-mediated deposition of 5-hydroxymethylcytosine (5hmC) at SOX9 class II binding sites in the Col2a1 and Acan loci is required for SOX9 occupancy and subsequent chondrogenic gene activation; Tet1 knockdown blocks SOX9 binding to Acan despite unchanged SOX9 levels.","method":"Tet1 shRNA knockdown in ATDC5 chondroprogenitors, genome-wide 5hmC mapping (CATCH-seq), RNA-seq, ChIP for SOX9 binding","journal":"JBMR plus","confidence":"High","confidence_rationale":"Tier 1–2 — genome-wide epigenomic mapping combined with functional ChIP and loss-of-function in chondrogenic cell model","pmids":["33134768"],"is_preprint":false},{"year":2020,"finding":"miR-140 (miR-140-5p and miR-140-3p) promotes aggrecan (ACAN) protein expression at the translational level in differentiating mesenchymal stem cells without changing ACAN mRNA; inhibition of miR-140 reduces ACAN protein; RALA (a miR-140 target) regulates SOX9 at the protein level and thereby indirectly controls ACAN.","method":"Transient and stable miR-140 inhibition/overexpression in MSCs and articular chondrocytes; RALA knockdown; protein and mRNA measurements","journal":"Stem cells and development","confidence":"Medium","confidence_rationale":"Tier 2 — clean loss/gain-of-function with mRNA vs. protein dissociation, single lab","pmids":["24063364"],"is_preprint":false},{"year":2022,"finding":"Missense ACAN variants in the G3 domain (C-type lectin repeat) linked to familial osteochondritis dissecans cause reduced secretion of variant aggrecan proteins and decreased binding of variant aggrecan to known cartilage extracellular matrix ligands, demonstrating that G3 domain integrity is required for proper aggrecan secretion and matrix interaction.","method":"Recombinant variant protein production, secretion assays, binding assays to ECM ligands, analysis of full-length aggrecan from heterozygous patient cartilage","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1–2 — functional studies with recombinant proteins and patient cartilage using multiple assays","pmids":["35338222"],"is_preprint":false},{"year":2023,"finding":"HDAC2 in parvalbumin-positive (PV+) interneurons represses Acan expression; PV+-cell specific Hdac2 deletion reduces Acan expression in prefrontal cortex and basolateral amygdala, decreases perineuronal net aggregation around PV+ cells, and reduces spontaneous fear memory recovery. Brief siRNA-mediated Acan knockdown alone (before extinction training) recapitulates the reduced fear recovery, establishing Acan as a downstream effector of HDAC2 in PV+ cell maturation and fear memory.","method":"Conditional Hdac2 KO in PV+ cells (mouse genetic model), Hdac2 pharmacological inhibition, intravenous siRNA-mediated Acan knockdown, immunofluorescence for PNN, behavioral fear conditioning/extinction assays","journal":"Molecular psychiatry","confidence":"High","confidence_rationale":"Tier 2 — genetic KO, rescue by Hdac2 re-expression, pharmacological confirmation, and direct Acan knockdown with behavioral readout; multiple orthogonal methods","pmids":["37131076"],"is_preprint":false},{"year":2020,"finding":"Aggrecan (ACAN) in perineuronal nets provides neuroprotection by forming an external shield that prevents internalization of pathological tau; reduced aggrecan levels in a bigenic TauP301L-Acan mouse model are accompanied by increased total tau protein and reduced Tau-1-positive neurons (indicative of increased tau phosphorylation), demonstrating that aggrecan modulates tau expression and phosphorylation.","method":"Bigenic mouse model (TauP301L × heterozygous Acan mice), immunohistochemistry, Western blotting, ELISA","journal":"European journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo genetic model with protein quantification, single lab, no direct mechanistic rescue","pmids":["32737917"],"is_preprint":false},{"year":2022,"finding":"Co-immunoprecipitation in a TauP301L-Acan mouse model reveals a physical interaction between perineuronal net components (including aggrecan) and tau protein; tau modulates levels of PN components such as brevican, and protein phosphatase 2A expression differs between conditions, indicating a bidirectional relationship.","method":"Co-immunoprecipitation, immunohistochemistry, Western blotting in bigenic TauP301L-acan mouse model","journal":"Biomolecules","confidence":"Medium","confidence_rationale":"Tier 3 — co-IP establishes physical association; mechanistic follow-up partial","pmids":["35454094"],"is_preprint":false},{"year":2024,"finding":"The G1 region of aggrecan (ACAN) forms a single structural unit comprising one immunoglobulin domain and two Link modules; hyaluronan (HA) is clamped inside a groove spanning the tandem Link domains. Point mutations eliminating HA-binding activity reduce but do not abolish ACAN integration into perineuronal nets, indicating HA binding is important but not essential for PNN assembly.","method":"Co-crystal structure of ACAN G1 with HA decasaccharide, site-directed mutagenesis of glycosaminoglycan-binding site, PNN integration assay","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with functional mutagenesis validation","pmids":[],"is_preprint":true},{"year":2024,"finding":"Conditional knockout of aggrecan (Acan) specifically from CA2 pyramidal neurons (Amigo2-Cre) impairs social memory and reversal learning, and reduces supramammillary nucleus input to CA2; conditional knockout from PV+ interneurons (PV-Cre) impairs contextual fear memory — demonstrating cell-type-specific roles of Acan/PNNs in hippocampal-dependent memory.","method":"Conditional Acan knockout mouse strains (Amigo2-Cre and PV-Cre), behavioral testing (social memory, reversal learning, contextual fear), electrophysiology (LFP), circuit tracing","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 2 — two independent conditional KO lines with multiple behavioral and circuit-level readouts","pmids":[],"is_preprint":true},{"year":2024,"finding":"Exosomal U2AF2 from bone marrow mesenchymal stem cells promotes maturation of circ_0036763 in nucleus pulposus cells; circ_0036763 acts as a sponge for miR-583, relieving miR-583-mediated suppression of ACAN, thereby increasing ACAN expression and collagen II and reducing IDD-associated collagen I.","method":"miRNA pull-down, RNA immunoprecipitation (RIP), co-culture of bMSCs/exosomes with HNPCs, qRT-PCR, Western blot","journal":"Regenerative therapy","confidence":"Medium","confidence_rationale":"Tier 2–3 — RIP and pull-down establish direct interactions; single lab","pmids":["38362337"],"is_preprint":false},{"year":2025,"finding":"The E3 ubiquitin ligase MKRN1 interacts with AGC1 (aspartate/glutamate carrier 1, SLC25A12) and facilitates its degradation via K11- and K29-linked ubiquitination; MKRN1-mediated AGC1 degradation reprograms mitochondrial energy metabolism and antioxidant responses, enhancing expression of HSP90AA1 and HSPD1 and reducing oxidative stress, thereby promoting oxaliplatin resistance in colorectal cancer.","method":"Co-immunoprecipitation mass spectrometry (CO-IP MS) from DCM heart tissue, MKRN1 gain/loss-of-function, AGC1 knockdown rescue in xenograft model, ubiquitination assays","journal":"Molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 — CO-IP MS identification with functional rescue; single lab but multiple methods including in vivo xenograft","pmids":["40722058"],"is_preprint":false},{"year":2024,"finding":"A non-canonical splicing variant (c.630-13G>A) in intron 4 of ACAN creates a cryptic splice site, incorporating an 11 bp intronic sequence into the final transcript and causing a frameshift with a premature termination codon, impairing aggrecan protein structure and causing familial short stature.","method":"Sanger sequencing for pedigree verification; minigene splicing assay to functionally validate the aberrant splice site","journal":"Gene","confidence":"High","confidence_rationale":"Tier 1 — minigene functional assay directly demonstrates aberrant splicing mechanism","pmids":["38782218"],"is_preprint":false}],"current_model":"ACAN encodes aggrecan, a large chondroitin sulfate proteoglycan that is the principal structural proteoglycan of cartilage extracellular matrix; its transcription is activated by SOX9 (binding to redundant cartilage enhancers) and co-activated by SHOX/SHOX2 via SOX5/SOX6 protein interactions, with SOX9 activity gated by SIRT1-mediated deacetylation that promotes nuclear entry, and with TET1-dependent 5hmC deposition required for SOX9 occupancy at the ACAN locus; ACAN protein is secreted and integrates into perineuronal nets via its G1 domain HA-binding groove, where it provides neuroprotection and regulates synaptic plasticity and memory in a cell-type-specific manner; loss-of-function mutations cause a spectrum of aggrecanopathies (short stature, spondyloepiphyseal dysplasias, osteochondritis dissecans) through haploinsufficiency, while G3 domain missense variants reduce secretion and ECM ligand binding, and MKRN1-mediated ubiquitination targets AGC1/SLC25A12 (a distinct protein sharing the abbreviation in some contexts) for proteasomal degradation."},"narrative":{"teleology":[{"year":2005,"claim":"Identifying a frameshift mutation in ACAN cosegregating with spondyloepiphyseal dysplasia type Kimberley established ACAN as a human skeletal disease gene, resolving the question of whether aggrecan deficiency could cause Mendelian bone disease.","evidence":"Mutation screening and cosegregation analysis in a multigenerational SEDK family mapped to 15q26.1","pmids":["16080123"],"confidence":"High","gaps":["Mechanism of disease limited to genetic linkage; no in vitro functional assay of the truncated protein","Whether haploinsufficiency or dominant-negative effects drive pathology was unresolved"]},{"year":2007,"claim":"Demonstrating that a frameshift allele in Dexter cattle undergoes nonsense-mediated decay (retaining only 8% mutant mRNA) established haploinsufficiency as the primary disease mechanism for ACAN loss-of-function alleles.","evidence":"mRNA quantification in heterozygous chondrocytes from Dexter cattle with a 4-bp ACAN insertion; genotyping in worldwide pedigrees","pmids":["17952705"],"confidence":"High","gaps":["Whether human SEDK mutations similarly undergo NMD was not tested","Protein-level consequences of heterozygous loss not quantified"]},{"year":2012,"claim":"Functional testing of conserved non-coding sequences revealed that ACAN transcription in cartilage is controlled by at least eleven redundant enhancers spanning >100 kb, many harboring SOX9 sites, explaining the robustness of cartilage-specific expression.","evidence":"Transgenic zebrafish enhancer reporter assays for 24 conserved non-coding sequences around the ACAN locus","pmids":["22820679"],"confidence":"High","gaps":["Individual enhancer necessity (deletion studies) not performed","Combinatorial or hierarchical relationships among enhancers unknown"]},{"year":2014,"claim":"Showing that SHOX2 activates ACAN transcription through physical interaction with SOX5/SOX6 established a cooperative SOX trio–SHOX axis in growth plate chondrocytes, explaining how short stature transcription factors converge on the ACAN promoter.","evidence":"Luciferase reporter, yeast-two-hybrid, co-immunoprecipitation, and immunohistochemistry of human fetal growth plates","pmids":["24421874"],"confidence":"High","gaps":["Whether SHOX2 contacts specific ACAN enhancers (ChIP) was not tested","Relative contribution of SHOX vs SHOX2 to endogenous ACAN expression unclear"]},{"year":2016,"claim":"Revealing that SIRT1-mediated deacetylation of SOX9 promotes its nuclear import via importin-β and increases SOX9 binding to the ACAN -10 kb enhancer explained how metabolic (NAD+) status gates chondrogenic gene expression.","evidence":"Co-IP of SOX9–SIRT1, ChIP at ACAN enhancer, pharmacological perturbation with NAD/SIRT1 inhibitors, and importazole in primary human OA chondrocytes","pmids":["26910618"],"confidence":"High","gaps":["Whether SIRT1 deacetylation acts at acetylation sites relevant in vivo remains to be mapped","Contribution of other HDACs to SOX9 activity at ACAN not addressed"]},{"year":2017,"claim":"A mouse knock-in model with ~50% reduction in Acan expression recapitulated dwarfism with shorter bones, reduced growth plates, and thinner articular cartilage, formally demonstrating that Acan gene dosage is critical for skeletal growth.","evidence":"Homozygous Agc1CreERT/CreERT knock-in mice with histology, proteoglycan staining, and gene expression analysis","pmids":["28921880"],"confidence":"High","gaps":["Heterozygous phenotype not fully characterized","Molecular compensation by other proteoglycans not examined"]},{"year":2020,"claim":"Three studies collectively deepened understanding of ACAN regulation and neural function: TET1-dependent 5hmC deposition was shown to be required for SOX9 occupancy at the Acan locus; the mTOR/4E-BP1 pathway was found to control ACAN transcription via translational regulation of inhibitory Smads; and in a tauopathy mouse model, aggrecan in perineuronal nets was shown to shield neurons from pathological tau internalization.","evidence":"Tet1 knockdown with ChIP and 5hmC mapping in ATDC5 cells; mTOR/4E-BP1 silencing with Smad ChIP in OA chondrocytes; bigenic TauP301L×Acan mouse with IHC, Western blot, and ELISA","pmids":["33134768","32485037","32737917"],"confidence":"High","gaps":["Whether TET1 acts directly at the ACAN locus or through broader chromatin remodeling is unclear","The mTOR–Smad–ACAN axis lacks in vivo validation","The neuroprotective mechanism (physical shielding vs. signaling) is not resolved"]},{"year":2022,"claim":"Functional studies of G3-domain missense variants linked to familial osteochondritis dissecans demonstrated that G3 integrity is required for aggrecan secretion and ECM ligand binding, establishing a distinct pathogenic mechanism from haploinsufficiency.","evidence":"Recombinant variant protein secretion assays, ECM ligand binding assays, and analysis of full-length aggrecan from heterozygous patient cartilage","pmids":["35338222"],"confidence":"High","gaps":["Structural basis for secretion defect not determined","Whether retained protein triggers ER stress was not examined"]},{"year":2023,"claim":"Conditional Hdac2 deletion from PV+ interneurons reduced Acan expression and perineuronal net density, and direct siRNA knockdown of Acan recapitulated impaired fear memory recovery, establishing Acan as a functional effector of HDAC2-regulated PV+ cell maturation and fear extinction.","evidence":"PV+-specific Hdac2 conditional KO, pharmacological HDAC inhibition, intravenous Acan siRNA, immunofluorescence for PNNs, and behavioral fear conditioning in mice","pmids":["37131076"],"confidence":"High","gaps":["Which HDAC2-regulated transcription factor directly controls Acan in PV+ cells is unknown","Whether HDAC2–Acan axis operates outside amygdala/PFC circuits not tested"]},{"year":2024,"claim":"A non-canonical intronic ACAN splice variant was functionally validated by minigene assay, expanding the mutational spectrum of aggrecanopathies to include deep-intronic variants causing familial short stature.","evidence":"Sanger sequencing in a pedigree with short stature; minigene splicing assay demonstrating cryptic splice site activation and frameshift","pmids":["38782218"],"confidence":"High","gaps":["Endogenous mRNA from patient cells not characterized","Whether NMD degrades the aberrant transcript was not confirmed"]},{"year":null,"claim":"Key unresolved questions include the precise structural basis for how G3-domain variants impair aggrecan secretion, the identity of the transcription factor directly linking HDAC2 to Acan in PV+ neurons, and whether HA-binding-independent mechanisms contribute substantially to aggrecan integration into perineuronal nets in vivo.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No full-length aggrecan structure available","Cell-type-specific transcriptional regulation in neural contexts poorly defined","Relative contribution of HA-dependent vs HA-independent PNN integration not quantified in vivo"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,6,9,13]}],"localization":[{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[0,9,11,13]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[9,11,12]}],"pathway":[{"term_id":"R-HSA-1474244","term_label":"Extracellular matrix organization","supporting_discovery_ids":[0,1,6,9]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[3,6,17]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[10,11,14]}],"complexes":["perineuronal net"],"partners":["SOX9","SOX5","SOX6","SHOX2","SIRT1","HDAC2","MAPT"],"other_free_text":[]},"mechanistic_narrative":"ACAN encodes aggrecan, the principal chondroitin sulfate proteoglycan of cartilage extracellular matrix and a core structural component of perineuronal nets in the brain. In cartilage, ACAN transcription is driven by SOX9 binding to multiple redundant enhancers distributed across >100 kb, with SOX9 occupancy gated by SIRT1-mediated deacetylation that promotes nuclear import and by TET1-dependent 5-hydroxymethylcytosine deposition at SOX9 binding sites; additional transcriptional input comes from SHOX/SHOX2 cooperating with SOX5/SOX6 and from TGF-β1 signaling through the mTOR/4E-BP1–Smad axis [PMID:22820679, PMID:26910618, PMID:33134768, PMID:24421874, PMID:32485037]. Loss-of-function mutations—including frameshifts causing nonsense-mediated decay and G3-domain missense variants that impair secretion and ECM ligand binding—cause a spectrum of aggrecanopathies encompassing spondyloepiphyseal dysplasia, familial short stature, and osteochondritis dissecans through haploinsufficiency [PMID:16080123, PMID:17952705, PMID:35338222, PMID:38782218]. In the brain, aggrecan integrates into perineuronal nets via its G1 domain hyaluronan-binding groove and exerts cell-type-specific functions: it provides neuroprotection against pathological tau in neurons, and conditional deletion from PV+ interneurons or CA2 pyramidal neurons selectively impairs fear memory, social memory, and reversal learning [PMID:32737917, PMID:37131076]."},"prefetch_data":{"uniprot":{"accession":"P16112","full_name":"Aggrecan core protein","aliases":["Cartilage-specific proteoglycan core protein","CSPCP","Chondroitin sulfate proteoglycan core protein 1","Chondroitin sulfate proteoglycan 1"],"length_aa":2530,"mass_kda":261.3,"function":"This proteoglycan is a major component of extracellular matrix of cartilagenous tissues. A major function of this protein is to resist compression in cartilage. It binds avidly to hyaluronic acid via an N-terminal globular region","subcellular_location":"Secreted, extracellular space, extracellular matrix","url":"https://www.uniprot.org/uniprotkb/P16112/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ACAN","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ACAN","total_profiled":1310},"omim":[{"mim_id":"620882","title":"SECONDARY OSSIFICATION CENTER-ASSOCIATED REGULATOR OF CHONDROCYTE MATURATION; SNORC","url":"https://www.omim.org/entry/620882"},{"mim_id":"619710","title":"HYALURONAN AND PROTEOGLYCAN LINK PROTEIN 4; HAPLN4","url":"https://www.omim.org/entry/619710"},{"mim_id":"614923","title":"BRANCHED-CHAIN KETO ACID DEHYDROGENASE KINASE DEFICIENCY; BCKDKD","url":"https://www.omim.org/entry/614923"},{"mim_id":"614901","title":"BRANCHED-CHAIN ALPHA-KETO ACID DEHYDROGENASE KINASE; BCKDK","url":"https://www.omim.org/entry/614901"},{"mim_id":"612813","title":"SPONDYLOEPIMETAPHYSEAL DYSPLASIA, AGGRECAN TYPE; SEMDAG","url":"https://www.omim.org/entry/612813"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"blood vessel","ntpm":35.7}],"url":"https://www.proteinatlas.org/search/ACAN"},"hgnc":{"alias_symbol":["CSPGCP"],"prev_symbol":["MSK16","CSPG1","AGC1"]},"alphafold":{"accession":"P16112","domains":[{"cath_id":"2.60.40.10","chopping":"31-60_68-152","consensus_level":"medium","plddt":86.1067,"start":31,"end":152},{"cath_id":"3.10.100.10","chopping":"153-250","consensus_level":"medium","plddt":89.8593,"start":153,"end":250},{"cath_id":"3.10.100.10","chopping":"254-354","consensus_level":"medium","plddt":89.1487,"start":254,"end":354},{"cath_id":"3.10.100.10","chopping":"477-571","consensus_level":"medium","plddt":88.2367,"start":477,"end":571},{"cath_id":"3.10.100.10","chopping":"576-677","consensus_level":"medium","plddt":86.167,"start":576,"end":677},{"cath_id":"2.10.25.10","chopping":"2284-2315","consensus_level":"medium","plddt":69.0897,"start":2284,"end":2315},{"cath_id":"3.10.100.10","chopping":"2319-2445","consensus_level":"medium","plddt":88.4828,"start":2319,"end":2445},{"cath_id":"2.10.70.10","chopping":"2452-2520","consensus_level":"medium","plddt":74.4691,"start":2452,"end":2520}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P16112","model_url":"https://alphafold.ebi.ac.uk/files/AF-P16112-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P16112-F1-predicted_aligned_error_v6.png","plddt_mean":51.91},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ACAN","jax_strain_url":"https://www.jax.org/strain/search?query=ACAN"},"sequence":{"accession":"P16112","fasta_url":"https://rest.uniprot.org/uniprotkb/P16112.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P16112/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P16112"}},"corpus_meta":[{"pmid":"18607376","id":"PMC_18607376","title":"Altered 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region of exon 12 of ACAN introduces a frameshift of 212 amino acids including 22 cysteine residues followed by a premature stop codon, causing spondyloepiphyseal dysplasia type Kimberley (SEDK) with severe premature osteoarthritis — establishing ACAN as a human disease gene.\",\n      \"method\": \"Mutation screening/sequencing in a multigenerational family with SEDK mapped to 15q26.1\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct mutation identification with cosegregation in a mapped locus; first report of ACAN causing human disease\",\n      \"pmids\": [\"16080123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"A 4-bp insertion in ACAN exon 11 causes bulldog chondrodysplastic dwarfism in Dexter cattle; in chondrocytes heterozygous for the insertion, mutant mRNA undergoes nonsense-mediated decay (only 8% of normal expression), establishing haploinsufficiency as the disease mechanism.\",\n      \"method\": \"Mutation screening, genotyping in worldwide Dexter pedigrees, mRNA expression analysis in heterozygous chondrocytes\",\n      \"journal\": \"Mammalian genome\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct molecular validation of NMD mechanism with functional mRNA quantification in primary chondrocytes\",\n      \"pmids\": [\"17952705\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Multiple (eleven) conserved non-coding enhancer sequences, distributed from >100 kb upstream of ACAN to within the first intron, independently direct cartilage-specific reporter expression in transgenic zebrafish; several contain SOX9 binding sites, establishing redundant transcriptional control of ACAN in cartilage.\",\n      \"method\": \"Functional enhancer assay in transgenic zebrafish with 24 conserved non-coding sequences tested\",\n      \"journal\": \"Matrix biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo functional assay replicated across multiple enhancer elements; orthologues confirmed in chicken\",\n      \"pmids\": [\"22820679\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"SHOX2, like SHOX, activates ACAN transcription through cooperation with the SOX trio (SOX5, SOX6, SOX9) via protein–protein interaction between SOX5/SOX6 and SHOX2; this interaction was confirmed by yeast-two-hybrid and co-immunoprecipitation; SHOX2 and SOX trio are co-expressed in human fetal growth plates.\",\n      \"method\": \"Luciferase reporter assay, yeast-two-hybrid, co-immunoprecipitation, immunohistochemistry of human fetal growth plates\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (luciferase, Y2H, co-IP, IHC) in one study\",\n      \"pmids\": [\"24421874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SOX9 acetylation reduces its nuclear entry and binding to the -10 kb ACAN enhancer, thereby suppressing ACAN transactivation; deacetylation by SIRT1 promotes SOX9 nuclear translocation via importin β, increasing ACAN expression in human chondrocytes.\",\n      \"method\": \"Co-immunoprecipitation (SOX9–SIRT1), immunofluorescence (nuclear localization with NAD/SIRT1 inhibitor/importazole treatments), ChIP (SOX9 binding to ACAN enhancer), primary OA chondrocyte cultures in 2D and 3D\",\n      \"journal\": \"Aging cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods including co-IP, ChIP, pharmacological perturbation, and rescue experiments in primary human chondrocytes\",\n      \"pmids\": [\"26910618\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"TGF-β1-induced expression of ACAN is regulated by the mTOR/4E-BP1 translational apparatus; 4E-BP1 controls translation of inhibitory Smads (Smad6/7), which in turn modulate nuclear Smad2/3 accumulation on the ACAN promoter, linking translational repression to transcriptional output.\",\n      \"method\": \"mTOR silencing, 4E-BP1 knockdown, phosphorylation assays, co-immunoprecipitation (4E-BP1–eIF4E), ChIP (Smad2/3 on ACAN promoter) in human OA chondrocytes\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mechanistic dissection with gene silencing, ChIP, co-IP, and multiple pathway components in a single study\",\n      \"pmids\": [\"32485037\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Homozygous disruption of Acan in mice (Agc1CreERT/CreERT knock-in) reduces Acan mRNA and protein by ~50%, causing dwarfism with shorter long bones and vertebrae, reduced growth plate length, and decreased articular cartilage thickness, demonstrating that Acan gene dosage is critical for normal skeletal growth.\",\n      \"method\": \"Mouse genetic model (homozygous knock-in at Acan 3'UTR), histology, proteoglycan staining, gene expression analysis\",\n      \"journal\": \"Genesis\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean in vivo loss-of-function with defined molecular (mRNA/protein reduction) and skeletal phenotype\",\n      \"pmids\": [\"28921880\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"TET1-mediated deposition of 5-hydroxymethylcytosine (5hmC) at SOX9 class II binding sites in the Col2a1 and Acan loci is required for SOX9 occupancy and subsequent chondrogenic gene activation; Tet1 knockdown blocks SOX9 binding to Acan despite unchanged SOX9 levels.\",\n      \"method\": \"Tet1 shRNA knockdown in ATDC5 chondroprogenitors, genome-wide 5hmC mapping (CATCH-seq), RNA-seq, ChIP for SOX9 binding\",\n      \"journal\": \"JBMR plus\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genome-wide epigenomic mapping combined with functional ChIP and loss-of-function in chondrogenic cell model\",\n      \"pmids\": [\"33134768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"miR-140 (miR-140-5p and miR-140-3p) promotes aggrecan (ACAN) protein expression at the translational level in differentiating mesenchymal stem cells without changing ACAN mRNA; inhibition of miR-140 reduces ACAN protein; RALA (a miR-140 target) regulates SOX9 at the protein level and thereby indirectly controls ACAN.\",\n      \"method\": \"Transient and stable miR-140 inhibition/overexpression in MSCs and articular chondrocytes; RALA knockdown; protein and mRNA measurements\",\n      \"journal\": \"Stem cells and development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean loss/gain-of-function with mRNA vs. protein dissociation, single lab\",\n      \"pmids\": [\"24063364\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Missense ACAN variants in the G3 domain (C-type lectin repeat) linked to familial osteochondritis dissecans cause reduced secretion of variant aggrecan proteins and decreased binding of variant aggrecan to known cartilage extracellular matrix ligands, demonstrating that G3 domain integrity is required for proper aggrecan secretion and matrix interaction.\",\n      \"method\": \"Recombinant variant protein production, secretion assays, binding assays to ECM ligands, analysis of full-length aggrecan from heterozygous patient cartilage\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — functional studies with recombinant proteins and patient cartilage using multiple assays\",\n      \"pmids\": [\"35338222\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"HDAC2 in parvalbumin-positive (PV+) interneurons represses Acan expression; PV+-cell specific Hdac2 deletion reduces Acan expression in prefrontal cortex and basolateral amygdala, decreases perineuronal net aggregation around PV+ cells, and reduces spontaneous fear memory recovery. Brief siRNA-mediated Acan knockdown alone (before extinction training) recapitulates the reduced fear recovery, establishing Acan as a downstream effector of HDAC2 in PV+ cell maturation and fear memory.\",\n      \"method\": \"Conditional Hdac2 KO in PV+ cells (mouse genetic model), Hdac2 pharmacological inhibition, intravenous siRNA-mediated Acan knockdown, immunofluorescence for PNN, behavioral fear conditioning/extinction assays\",\n      \"journal\": \"Molecular psychiatry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO, rescue by Hdac2 re-expression, pharmacological confirmation, and direct Acan knockdown with behavioral readout; multiple orthogonal methods\",\n      \"pmids\": [\"37131076\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Aggrecan (ACAN) in perineuronal nets provides neuroprotection by forming an external shield that prevents internalization of pathological tau; reduced aggrecan levels in a bigenic TauP301L-Acan mouse model are accompanied by increased total tau protein and reduced Tau-1-positive neurons (indicative of increased tau phosphorylation), demonstrating that aggrecan modulates tau expression and phosphorylation.\",\n      \"method\": \"Bigenic mouse model (TauP301L × heterozygous Acan mice), immunohistochemistry, Western blotting, ELISA\",\n      \"journal\": \"European journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic model with protein quantification, single lab, no direct mechanistic rescue\",\n      \"pmids\": [\"32737917\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Co-immunoprecipitation in a TauP301L-Acan mouse model reveals a physical interaction between perineuronal net components (including aggrecan) and tau protein; tau modulates levels of PN components such as brevican, and protein phosphatase 2A expression differs between conditions, indicating a bidirectional relationship.\",\n      \"method\": \"Co-immunoprecipitation, immunohistochemistry, Western blotting in bigenic TauP301L-acan mouse model\",\n      \"journal\": \"Biomolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — co-IP establishes physical association; mechanistic follow-up partial\",\n      \"pmids\": [\"35454094\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The G1 region of aggrecan (ACAN) forms a single structural unit comprising one immunoglobulin domain and two Link modules; hyaluronan (HA) is clamped inside a groove spanning the tandem Link domains. Point mutations eliminating HA-binding activity reduce but do not abolish ACAN integration into perineuronal nets, indicating HA binding is important but not essential for PNN assembly.\",\n      \"method\": \"Co-crystal structure of ACAN G1 with HA decasaccharide, site-directed mutagenesis of glycosaminoglycan-binding site, PNN integration assay\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional mutagenesis validation\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Conditional knockout of aggrecan (Acan) specifically from CA2 pyramidal neurons (Amigo2-Cre) impairs social memory and reversal learning, and reduces supramammillary nucleus input to CA2; conditional knockout from PV+ interneurons (PV-Cre) impairs contextual fear memory — demonstrating cell-type-specific roles of Acan/PNNs in hippocampal-dependent memory.\",\n      \"method\": \"Conditional Acan knockout mouse strains (Amigo2-Cre and PV-Cre), behavioral testing (social memory, reversal learning, contextual fear), electrophysiology (LFP), circuit tracing\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two independent conditional KO lines with multiple behavioral and circuit-level readouts\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Exosomal U2AF2 from bone marrow mesenchymal stem cells promotes maturation of circ_0036763 in nucleus pulposus cells; circ_0036763 acts as a sponge for miR-583, relieving miR-583-mediated suppression of ACAN, thereby increasing ACAN expression and collagen II and reducing IDD-associated collagen I.\",\n      \"method\": \"miRNA pull-down, RNA immunoprecipitation (RIP), co-culture of bMSCs/exosomes with HNPCs, qRT-PCR, Western blot\",\n      \"journal\": \"Regenerative therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — RIP and pull-down establish direct interactions; single lab\",\n      \"pmids\": [\"38362337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The E3 ubiquitin ligase MKRN1 interacts with AGC1 (aspartate/glutamate carrier 1, SLC25A12) and facilitates its degradation via K11- and K29-linked ubiquitination; MKRN1-mediated AGC1 degradation reprograms mitochondrial energy metabolism and antioxidant responses, enhancing expression of HSP90AA1 and HSPD1 and reducing oxidative stress, thereby promoting oxaliplatin resistance in colorectal cancer.\",\n      \"method\": \"Co-immunoprecipitation mass spectrometry (CO-IP MS) from DCM heart tissue, MKRN1 gain/loss-of-function, AGC1 knockdown rescue in xenograft model, ubiquitination assays\",\n      \"journal\": \"Molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CO-IP MS identification with functional rescue; single lab but multiple methods including in vivo xenograft\",\n      \"pmids\": [\"40722058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"A non-canonical splicing variant (c.630-13G>A) in intron 4 of ACAN creates a cryptic splice site, incorporating an 11 bp intronic sequence into the final transcript and causing a frameshift with a premature termination codon, impairing aggrecan protein structure and causing familial short stature.\",\n      \"method\": \"Sanger sequencing for pedigree verification; minigene splicing assay to functionally validate the aberrant splice site\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — minigene functional assay directly demonstrates aberrant splicing mechanism\",\n      \"pmids\": [\"38782218\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACAN encodes aggrecan, a large chondroitin sulfate proteoglycan that is the principal structural proteoglycan of cartilage extracellular matrix; its transcription is activated by SOX9 (binding to redundant cartilage enhancers) and co-activated by SHOX/SHOX2 via SOX5/SOX6 protein interactions, with SOX9 activity gated by SIRT1-mediated deacetylation that promotes nuclear entry, and with TET1-dependent 5hmC deposition required for SOX9 occupancy at the ACAN locus; ACAN protein is secreted and integrates into perineuronal nets via its G1 domain HA-binding groove, where it provides neuroprotection and regulates synaptic plasticity and memory in a cell-type-specific manner; loss-of-function mutations cause a spectrum of aggrecanopathies (short stature, spondyloepiphyseal dysplasias, osteochondritis dissecans) through haploinsufficiency, while G3 domain missense variants reduce secretion and ECM ligand binding, and MKRN1-mediated ubiquitination targets AGC1/SLC25A12 (a distinct protein sharing the abbreviation in some contexts) for proteasomal degradation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ACAN encodes aggrecan, the principal chondroitin sulfate proteoglycan of cartilage extracellular matrix and a core structural component of perineuronal nets in the brain. In cartilage, ACAN transcription is driven by SOX9 binding to multiple redundant enhancers distributed across >100 kb, with SOX9 occupancy gated by SIRT1-mediated deacetylation that promotes nuclear import and by TET1-dependent 5-hydroxymethylcytosine deposition at SOX9 binding sites; additional transcriptional input comes from SHOX/SHOX2 cooperating with SOX5/SOX6 and from TGF-β1 signaling through the mTOR/4E-BP1–Smad axis [PMID:22820679, PMID:26910618, PMID:33134768, PMID:24421874, PMID:32485037]. Loss-of-function mutations—including frameshifts causing nonsense-mediated decay and G3-domain missense variants that impair secretion and ECM ligand binding—cause a spectrum of aggrecanopathies encompassing spondyloepiphyseal dysplasia, familial short stature, and osteochondritis dissecans through haploinsufficiency [PMID:16080123, PMID:17952705, PMID:35338222, PMID:38782218]. In the brain, aggrecan integrates into perineuronal nets via its G1 domain hyaluronan-binding groove and exerts cell-type-specific functions: it provides neuroprotection against pathological tau in neurons, and conditional deletion from PV+ interneurons or CA2 pyramidal neurons selectively impairs fear memory, social memory, and reversal learning [PMID:32737917, PMID:37131076].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Identifying a frameshift mutation in ACAN cosegregating with spondyloepiphyseal dysplasia type Kimberley established ACAN as a human skeletal disease gene, resolving the question of whether aggrecan deficiency could cause Mendelian bone disease.\",\n      \"evidence\": \"Mutation screening and cosegregation analysis in a multigenerational SEDK family mapped to 15q26.1\",\n      \"pmids\": [\"16080123\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of disease limited to genetic linkage; no in vitro functional assay of the truncated protein\", \"Whether haploinsufficiency or dominant-negative effects drive pathology was unresolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstrating that a frameshift allele in Dexter cattle undergoes nonsense-mediated decay (retaining only 8% mutant mRNA) established haploinsufficiency as the primary disease mechanism for ACAN loss-of-function alleles.\",\n      \"evidence\": \"mRNA quantification in heterozygous chondrocytes from Dexter cattle with a 4-bp ACAN insertion; genotyping in worldwide pedigrees\",\n      \"pmids\": [\"17952705\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether human SEDK mutations similarly undergo NMD was not tested\", \"Protein-level consequences of heterozygous loss not quantified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Functional testing of conserved non-coding sequences revealed that ACAN transcription in cartilage is controlled by at least eleven redundant enhancers spanning >100 kb, many harboring SOX9 sites, explaining the robustness of cartilage-specific expression.\",\n      \"evidence\": \"Transgenic zebrafish enhancer reporter assays for 24 conserved non-coding sequences around the ACAN locus\",\n      \"pmids\": [\"22820679\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Individual enhancer necessity (deletion studies) not performed\", \"Combinatorial or hierarchical relationships among enhancers unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showing that SHOX2 activates ACAN transcription through physical interaction with SOX5/SOX6 established a cooperative SOX trio–SHOX axis in growth plate chondrocytes, explaining how short stature transcription factors converge on the ACAN promoter.\",\n      \"evidence\": \"Luciferase reporter, yeast-two-hybrid, co-immunoprecipitation, and immunohistochemistry of human fetal growth plates\",\n      \"pmids\": [\"24421874\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SHOX2 contacts specific ACAN enhancers (ChIP) was not tested\", \"Relative contribution of SHOX vs SHOX2 to endogenous ACAN expression unclear\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Revealing that SIRT1-mediated deacetylation of SOX9 promotes its nuclear import via importin-β and increases SOX9 binding to the ACAN -10 kb enhancer explained how metabolic (NAD+) status gates chondrogenic gene expression.\",\n      \"evidence\": \"Co-IP of SOX9–SIRT1, ChIP at ACAN enhancer, pharmacological perturbation with NAD/SIRT1 inhibitors, and importazole in primary human OA chondrocytes\",\n      \"pmids\": [\"26910618\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SIRT1 deacetylation acts at acetylation sites relevant in vivo remains to be mapped\", \"Contribution of other HDACs to SOX9 activity at ACAN not addressed\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"A mouse knock-in model with ~50% reduction in Acan expression recapitulated dwarfism with shorter bones, reduced growth plates, and thinner articular cartilage, formally demonstrating that Acan gene dosage is critical for skeletal growth.\",\n      \"evidence\": \"Homozygous Agc1CreERT/CreERT knock-in mice with histology, proteoglycan staining, and gene expression analysis\",\n      \"pmids\": [\"28921880\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Heterozygous phenotype not fully characterized\", \"Molecular compensation by other proteoglycans not examined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Three studies collectively deepened understanding of ACAN regulation and neural function: TET1-dependent 5hmC deposition was shown to be required for SOX9 occupancy at the Acan locus; the mTOR/4E-BP1 pathway was found to control ACAN transcription via translational regulation of inhibitory Smads; and in a tauopathy mouse model, aggrecan in perineuronal nets was shown to shield neurons from pathological tau internalization.\",\n      \"evidence\": \"Tet1 knockdown with ChIP and 5hmC mapping in ATDC5 cells; mTOR/4E-BP1 silencing with Smad ChIP in OA chondrocytes; bigenic TauP301L×Acan mouse with IHC, Western blot, and ELISA\",\n      \"pmids\": [\"33134768\", \"32485037\", \"32737917\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TET1 acts directly at the ACAN locus or through broader chromatin remodeling is unclear\", \"The mTOR–Smad–ACAN axis lacks in vivo validation\", \"The neuroprotective mechanism (physical shielding vs. signaling) is not resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Functional studies of G3-domain missense variants linked to familial osteochondritis dissecans demonstrated that G3 integrity is required for aggrecan secretion and ECM ligand binding, establishing a distinct pathogenic mechanism from haploinsufficiency.\",\n      \"evidence\": \"Recombinant variant protein secretion assays, ECM ligand binding assays, and analysis of full-length aggrecan from heterozygous patient cartilage\",\n      \"pmids\": [\"35338222\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for secretion defect not determined\", \"Whether retained protein triggers ER stress was not examined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Conditional Hdac2 deletion from PV+ interneurons reduced Acan expression and perineuronal net density, and direct siRNA knockdown of Acan recapitulated impaired fear memory recovery, establishing Acan as a functional effector of HDAC2-regulated PV+ cell maturation and fear extinction.\",\n      \"evidence\": \"PV+-specific Hdac2 conditional KO, pharmacological HDAC inhibition, intravenous Acan siRNA, immunofluorescence for PNNs, and behavioral fear conditioning in mice\",\n      \"pmids\": [\"37131076\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which HDAC2-regulated transcription factor directly controls Acan in PV+ cells is unknown\", \"Whether HDAC2–Acan axis operates outside amygdala/PFC circuits not tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"A non-canonical intronic ACAN splice variant was functionally validated by minigene assay, expanding the mutational spectrum of aggrecanopathies to include deep-intronic variants causing familial short stature.\",\n      \"evidence\": \"Sanger sequencing in a pedigree with short stature; minigene splicing assay demonstrating cryptic splice site activation and frameshift\",\n      \"pmids\": [\"38782218\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous mRNA from patient cells not characterized\", \"Whether NMD degrades the aberrant transcript was not confirmed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the precise structural basis for how G3-domain variants impair aggrecan secretion, the identity of the transcription factor directly linking HDAC2 to Acan in PV+ neurons, and whether HA-binding-independent mechanisms contribute substantially to aggrecan integration into perineuronal nets in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No full-length aggrecan structure available\", \"Cell-type-specific transcriptional regulation in neural contexts poorly defined\", \"Relative contribution of HA-dependent vs HA-independent PNN integration not quantified in vivo\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 6, 9, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [0, 9, 11, 13]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [9, 11, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [0, 1, 6, 9]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [3, 6, 17]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [10, 11, 14]}\n    ],\n    \"complexes\": [\n      \"perineuronal net\"\n    ],\n    \"partners\": [\n      \"SOX9\",\n      \"SOX5\",\n      \"SOX6\",\n      \"SHOX2\",\n      \"SIRT1\",\n      \"HDAC2\",\n      \"MAPT\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}