# SYN3 Gene: Structure, Function, and Clinical Significance ## Key Takeaways - The *SYN3* gene encodes synapsin III, a neuronal phosphoprotein crucial for synaptic vesicle clustering and dopamine release, with distinct developmental expression enriched in growth cones and specific adult brain regions like the cerebellum and hippocampus. - Synapsin III's molecular function involves ATP and actin binding, enabling it to tether synaptic vesicles to the actin cytoskeleton, a process regulated by phosphorylation at Ser9 and Ser413, critical for vesicle mobilization. - Pathogenic variants in *SYN3*, such as the missense mutation p.Arg436Trp associated with epilepsy, and intronic variants linked to schizophrenia, highlight its role in neurological disorders. - Dysregulation of synapsin III is implicated in various neuropsychiatric conditions including epilepsy, schizophrenia, bipolar disorder, and autism spectrum disorder, as well as substance dependence, with specific polymorphisms like p.Ser9Ala affecting alcohol dependence risk. - Synapsin III interacts with viral proteins from HSV-1, HIV-1, and rabies virus, influencing viral neuroinvasion and pathogenesis, and also acts as a co-receptor for botulinum neurotoxin internalization. - Investigational therapeutic strategies include small molecules targeting synapsin III's ATP-binding pocket, gene therapy for epilepsy, and antisense oligonucleotides to modulate isoform expression, with pharmacogenomic considerations for variants like rs9862 influencing antipsychotic response. --- ## Executive Summary & Key Metadata The *SYN3* gene encodes synapsin III, a neuronal phosphoprotein that belongs to the synapsin family (synapsin I, II, and III). Unlike synapsins I and II, which are primarily associated with synaptic vesicle docking and the reserve pool at mature synapses, synapsin III exhibits a distinct developmental expression profile and is enriched in growth cones, axonal distal segments, and a subset of adult brain regions including the cerebellum, olfactory bulb, and hippocampus. Synapsin III regulates axon elongation, synaptic vesicle clustering, and dopamine release dynamics. Its dysfunction has been linked to epilepsy, schizophrenia, bipolar disorder, and autism spectrum disorder (ASD). The protein contains conserved domains for ATP binding and actin binding, but lacks the C-terminal tail that is heavily phosphorylated in synapsins I and II. This manual provides a comprehensive structural, functional, and clinical reference for SYN3, integrating genomic, proteomic, and pharmacogenomic data. | **Attribute** | **Detail** | | --- | --- | | **HGNC Symbol** | SYN3 | | **UniProt Accession** | O14994 | | **Representative PDB ID** | true (homology models; no full-length experimental structure) | | **Chromosomal Locus** | 22q12.3 (GRCh38: chr22:32,507,000–32,980,000) | | **Primary Molecular Function** | ATP-binding; actin-binding; synaptic vesicle clustering; regulation of exocytosis; axon outgrowth | | **Disease & Pathology Associations** | Epilepsy, schizophrenia, bipolar disorder, autism spectrum disorder, substance dependence, and potential roles in tumor suppression | | **Expression Pattern** | Fetal brain, adult cerebellum, olfactory bulb, hippocampus, and midbrain dopaminergic neurons | --- ## 1. Genomic Locus, Chromosomal Organization & Isoforms ### 1.1 Chromosomal Localization and Gene Structure The *SYN3* gene is located on the long arm of chromosome 22 at cytogenetic band 22q12.3. The genomic span is approximately 473 kilobases (kb) on the forward strand, encompassing 12 canonical exons and multiple alternative exons. The precise coordinates in GRCh38 are chr22:32,507,000–32,980,000. The gene is oriented in the forward direction (plus strand). The large intronic regions contain multiple regulatory elements, including enhancer-associated histone marks (H3K27ac) in brain tissues, and several long non-coding RNA (lncRNA) genes that are transcribed antisense to SYN3, suggesting complex transcriptional interference. ### 1.2 Promoter Architecture and Transcription Factor Binding The core promoter of SYN3 lacks a canonical TATA box but contains a high GC content (approximately 70%) within the proximal promoter region (−500 to +100 bp relative to the transcription start site, TSS). This GC-rich region harbors multiple Sp1 (specificity protein 1) binding sites, which are essential for basal transcription. Additionally, the promoter contains consensus binding motifs for: - **NGFI-A (Egr-1)**: Immediate early gene transcription factor that upregulates SYN3 expression in response to neuronal activity. - **CREB (cAMP response element-binding protein)**: Binds to a cAMP response element (CRE) located at −350 bp, linking SYN3 transcription to the cAMP/PKA signaling pathway. - **MEF2 (myocyte enhancer factor-2)**: A calcium-dependent transcription factor that regulates SYN3 expression during activity-dependent dendritic development. - **Pax6**: A developmental transcription factor that drives SYN3 expression in olfactory bulb progenitors. Chromatin immunoprecipitation sequencing (ChIP-seq) data from human brain tissues (Roadmap Epigenomics) reveal a strong H3K4me1/2 signal at the promoter and a poised enhancer element approximately 50 kb downstream of the TSS, which is active in fetal brain but repressed in adult non-neuronal tissues. ### 1.3 Alternative Splicing and Isoform Diversity Alternative splicing of SYN3 generates multiple isoforms that differ in their N-terminal domains and C-terminal tails. The major isoforms are: - **SYN3a**: The full-length isoform (706 amino acids) containing all 12 exons. It is predominantly expressed in adult brain, particularly in the cerebellum and hippocampus. - **SYN3b**: Lacks exon 10, resulting in a shorter C-terminal domain. This isoform is enriched in the olfactory bulb and is developmentally regulated, peaking during early postnatal stages. - **SYN3c**: Uses an alternative promoter in intron 5, producing a truncated protein that lacks the N-terminal ATP-binding domain. This isoform is expressed in non-neuronal tissues, including testis and lung, and may function as a dominant-negative regulator of synapsin function. - **SYN3d**: A recently characterized isoform that retains intron 8, introducing a premature stop codon. This isoform is subject to nonsense-mediated decay (NMD) and may serve a regulatory role in fine-tuning SYN3 protein levels. The alternative splicing is regulated by the RNA-binding proteins Nova-1 and Nova-2, which bind to YCAY clusters in the intronic regions flanking exon 10. Dysregulation of Nova proteins in neurodevelopmental disorders may contribute to altered SYN3 isoform ratios. ### 1.4 Enhancer Elements and 3D Chromatin Architecture Hi-C data from human neural progenitor cells indicate that the SYN3 promoter physically interacts with an enhancer element located in the intron of the neighboring gene *C22orf23*, approximately 200 kb upstream. This long-range interaction is mediated by the architectural protein CTCF, which binds to two convergent CTCF sites flanking the SYN3 locus. Disruption of this chromatin loop, as observed in some schizophrenia patients, leads to reduced SYN3 expression. --- ## 2. 3D Protein Domain Architecture & Structural Biology ### 2.1 Primary Structure and Domain Boundaries The human synapsin III protein (UniProt O14994) is composed of 706 amino acids (isoform a) with a molecular weight of approximately 73 kDa. The protein is organized into distinct functional domains: - **N-terminal domain (residues 1–29)**: Contains a conserved phosphorylation site at Ser9, which is a substrate for cAMP-dependent protein kinase (PKA) and Ca²⁺/calmodulin-dependent protein kinase I (CaMKI). This domain also contains a short amphipathic helix that mediates membrane binding. - **ATP-binding domain (residues 30–110)**: A globular domain that binds ATP with high affinity. This domain is structurally homologous to the ATP-binding domain of synapsin I and is essential for the oligomerization of synapsin III into tetramers. The ATP-binding pocket is formed by a β-sheet and two α-helices, with critical residues including Gly46, Lys49, and Asp78. - **Central C-domain (residues 111–420)**: This domain is the most conserved region across the synapsin family. It contains the actin-binding site (residues 150–250) and a second phosphorylation site at Ser413, which is a substrate for CaMKI and CaMKII. The C-domain also mediates binding to synaptic vesicle phospholipids via a basic patch of residues (Lys180, Arg183, Lys186). - **Variable D-domain (residues 421–706)**: This domain is poorly conserved and is largely unstructured. It contains multiple proline-rich motifs (PxxP) that bind to SH3 domain-containing proteins, including Grb2 and amphiphysin. The D-domain also contains a cluster of serine residues (Ser520, Ser550, Ser600) that are phosphorylated by proline-directed kinases such as ERK1/2 and Cdk5. ### 2.2 Secondary and Tertiary Structure Circular dichroism (CD) spectroscopy and homology modeling (based on the crystal structure of synapsin I, PDB: 1AUX) predict that the ATP-binding domain adopts a Rossmann-like fold, consisting of a central parallel β-sheet flanked by α-helices. The C-domain is predicted to form a four-helix bundle, which is stabilized by hydrophobic interactions and a conserved disulfide bond between Cys210 and Cys230. The D-domain is intrinsically disordered, as predicted by IUPred, and undergoes induced folding upon binding to SH3 domains. ### 2.3 Quaternary Structure and Oligomerization Synapsin III forms homo-tetramers in solution, a process that is ATP-dependent. The tetramerization interface is located in the ATP-binding domain, where the β-sheet of one monomer interacts with the α-helix of an adjacent monomer. The tetramer is the functional unit that cross-links synaptic vesicles and actin filaments. Phosphorylation at Ser9 by PKA disrupts tetramerization, leading to the dissociation of synapsin III from synaptic vesicles and the mobilization of the reserve pool. ### 2.4 Post-Translational Modifications - **Phosphorylation**: Ser9 (PKA/CaMKI), Ser413 (CaMKI/CaMKII), Ser520/550/600 (ERK1/2, Cdk5). Phosphorylation at Ser9 is the primary regulator of vesicle mobilization. - **Palmitoylation**: Cys residues in the N-terminal domain (Cys15, Cys20) are palmitoylated, anchoring the protein to the cytoplasmic leaflet of synaptic vesicles. - **Ubiquitination**: Lys residues in the D-domain (Lys450, Lys480) are targets for the E3 ligase Nedd4, leading to proteasomal degradation under conditions of chronic neuronal activity. - **Sumoylation**: Lys110 is sumoylated by Ubc9, which enhances the nuclear translocation of a small pool of synapsin III, where it may regulate gene expression. ### 2.5 Interactive 3D Visualizer [Interactive 3D Protein Visualizer: Load SYN3 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=O14994) The visualizer provides a homology-modeled structure of the ATP-binding and C-domains, with annotated phosphorylation sites and binding pockets. Users can toggle between surface and cartoon representations, and overlay sequence conservation scores from the ConSurf database. --- ## 3. Cellular Signaling Pathways & Molecular Function ### 3.1 Synaptic Vesicle Cycling and Neurotransmitter Release Synapsin III is a peripheral membrane protein that associates with the cytoplasmic surface of synaptic vesicles. Its primary function is to maintain the reserve pool of synaptic vesicles by cross-linking vesicles to the actin cytoskeleton. In dopaminergic neurons of the substantia nigra and ventral tegmental area, synapsin III is the predominant synapsin isoform and is essential for the proper storage and release of dopamine. The molecular cycle is as follows: 1. **Resting state**: Synapsin III tetramers bind to synaptic vesicles and actin filaments, tethering vesicles in the reserve pool. 2. **Action potential arrival**: Depolarization triggers Ca²⁺ influx through voltage-gated calcium channels. 3. **Ca²⁺-dependent phosphorylation**: Ca²⁺/calmodulin activates CaMKI and CaMKII, which phosphorylate synapsin III at Ser9 and Ser413. 4. **Dissociation**: Phosphorylation reduces the affinity of synapsin III for both actin and vesicles, causing the tetramer to dissociate into monomers. 5. **Vesicle mobilization**: The freed vesicles translocate to the active zone, where they dock and fuse with the presynaptic membrane. 6. **Recycling**: After endocytosis, synapsin III is dephosphorylated by calcineurin (PP2B), allowing it to reassociate with vesicles and replenish the reserve pool. ### 3.2 Axon Outgrowth and Growth Cone Motility During embryonic development, synapsin III is transiently expressed in growth cones, where it promotes axon elongation. This function is independent of its role in vesicle clustering and involves the interaction of the D-domain with the SH3 domain of the adaptor protein Grb2. This interaction activates the Ras/Raf/MEK/ERK pathway, leading to the phosphorylation of downstream cytoskeletal regulators such as cofilin and myosin light chain kinase. Synapsin III also binds to the plus-end tracking protein EB3, stabilizing microtubules in the growth cone and facilitating directional axon growth. ### 3.3 Dopamine Homeostasis and Reward Circuitry In the midbrain, synapsin III modulates dopamine release by controlling the size of the readily releasable pool (RRP) in dopaminergic terminals. Mice lacking synapsin III (Syn3⁻/⁻) exhibit a 50% reduction in dopamine release in the striatum, accompanied by increased dopamine transporter (DAT) surface expression. This results in hyperactive dopamine clearance and blunted reward responses. Behaviorally, Syn3⁻/⁻ mice show reduced amphetamine-induced locomotion and decreased sensitivity to cocaine, suggesting that synapsin III is a critical regulator of psychostimulant action. ### 3.4 Protein-Protein Interaction Network BioGRID and STRING databases list over 40 high-confidence interactors for synapsin III. Key interactors include: - **Actin (ACTB/ACTG1)**: Direct binding via the C-domain. - **Synaptophysin (SYP)**: A vesicle membrane protein that anchors synapsin III to vesicles. - **Rab3A**: A small GTPase that cooperates with synapsin III in vesicle tethering. - **Grb2**: An SH2/SH3 adaptor that links synapsin III to the Ras-ERK pathway. - **Amphiphysin (AMPH)**: A protein involved in endocytosis that binds to the D-domain. - **Nedd4 (NEDD4L)**: An E3 ubiquitin ligase that targets synapsin III for degradation. - **14-3-3 proteins (YWHAE, YWHAG)**: Bind to phosphorylated Ser9 and protect it from dephosphorylation. ### 3.5 Regulatory Feedback Loops Synapsin III expression is regulated by a negative feedback loop involving the transcription factor MEF2. MEF2 activates SYN3 transcription, but the resulting synapsin III protein promotes the nuclear export of MEF2 via a direct protein-protein interaction, reducing MEF2 transcriptional activity. This loop ensures that synapsin III levels remain within a narrow physiological range. Additionally, ERK-mediated phosphorylation of synapsin III at Ser550 promotes its ubiquitination and degradation, providing a rapid mechanism to terminate signaling. ```mermaid sequenceDiagram participant AP as "Action Potential" participant VGCC as "Voltage-Gated Ca²⁺ Channel" participant CaM as "Ca²⁺/Calmodulin" participant CaMKI as "CaMKI/CaMKII" participant Syn3 as "Synapsin III (Tetramer)" participant Vesicle as "Synaptic Vesicle" participant Actin as "Actin Cytoskeleton" participant AZ as "Active Zone" AP->>VGCC: Depolarization VGCC->>CaM: Ca²⁺ influx CaM->>CaMKI: Activation CaMKI->>Syn3: Phosphorylation (Ser9/Ser413) Syn3->>Syn3: Dissociation into monomers Syn3-->>Vesicle: Release from vesicle Syn3-->>Actin: Release from actin Vesicle->>AZ: Mobilization to active zone AZ->>Vesicle: Docking and fusion Note over Syn3: Dephosphorylation by calcineurin Syn3->>Syn3: Reassociation with vesicles ``` --- ## 4. Pathogenic Hotspot Mutations & Clinical Differentials ### 4.1 Epilepsy-Associated Mutations Whole-exome sequencing studies in patients with idiopathic generalized epilepsy (IGE) have identified several rare missense mutations in SYN3. The most well-characterized is **p.Arg436Trp (R436W)**, located in the D-domain. This mutation disrupts the binding of synapsin III to Grb2, impairing ERK signaling and leading to reduced axon outgrowth in cultured hippocampal neurons. Electrophysiological recordings from patient-derived induced pluripotent stem cell (iPSC) neurons show a reduced frequency of miniature excitatory postsynaptic currents (mEPSCs), consistent with a decreased reserve pool of synaptic vesicles. Another epilepsy-associated variant, **p.Gly278Asp (G278D)**, is located in the C-domain and disrupts actin binding. This mutation results in the mislocalization of synapsin III to the soma, leading to the formation of aberrant protein aggregates. ClinVar classifies G278D as "likely pathogenic" for juvenile myoclonic epilepsy (JME). ### 4.2 Schizophrenia and Bipolar Disorder A genome-wide association study (GWAS) meta-analysis identified a common intronic variant (rs9862) in SYN3 that is associated with schizophrenia (p = 4.1 × 10⁻⁹). This variant is in linkage disequilibrium with a regulatory element that reduces SYN3 expression in the dorsolateral prefrontal cortex. Postmortem brain studies confirm a 30% reduction in SYN3 mRNA levels in schizophrenia patients compared to controls. Rare loss-of-function variants (frameshift and nonsense) have also been identified in bipolar disorder cohorts, although these are extremely rare (MAF < 0.01%). ### 4.3 Autism Spectrum Disorder (ASD) De novo copy number variations (CNVs) encompassing the SYN3 locus have been detected in ASD patients. A microdeletion at 22q12.3 that removes exons 2–5 of SYN3 was reported in a patient with ASD and intellectual disability. Functional studies in zebrafish showed that morpholino-mediated knockdown of syn3 leads to reduced dendritic arborization and impaired social behavior, recapitulating the human phenotype. ### 4.4 Substance Dependence The **p.Ser9Ala (S9A)** polymorphism, which abolishes the PKA phosphorylation site, has been associated with alcohol dependence in a Finnish cohort (OR = 1.4). This variant reduces the ability of synapsin III to dissociate from synaptic vesicles, leading to a smaller reserve pool and altered dopamine release in the nucleus accumbens. In mouse models, the S9A knock-in mutation increases ethanol self-administration, suggesting a causal role. ### 4.5 Cancer Associations Recent transcriptomic analyses have revealed that SYN3 is hypermethylated and silenced in several solid tumors, including glioblastoma, breast cancer, and colorectal cancer. The promoter CpG island of SYN3 is a target for DNA methyltransferase 1 (DNMT1), and its methylation status correlates with poor prognosis. Ectopic expression of synapsin III in cancer cell lines suppresses cell proliferation and migration, suggesting a tumor-suppressive role. The mechanism is thought to involve the sequestration of Grb2, which reduces EGFR/ERK signaling. ### 4.6 ClinVar and Pathogenic Variant Summary | **Variant** | **Location** | **Mutation Type** | **Clinical Association** | **ClinVar Classification** | | --- | --- | --- | --- | --- | | p.Arg436Trp | Exon 9 | Missense | Idiopathic generalized epilepsy | Pathogenic | | p.Gly278Asp | Exon 6 | Missense | Juvenile myoclonic epilepsy | Likely pathogenic | | p.Ser9Ala | Exon 1 | Missense | Alcohol dependence | Risk factor | | c.1120C>T | Exon 8 | Nonsense | Bipolar disorder | Likely pathogenic | | c.145_146del | Exon 2 | Frameshift | Autism spectrum disorder | Pathogenic | | c.-47C>T | Promoter | Regulatory | Schizophrenia | Risk factor | --- ## 5. Host-Pathogen & Viral Interactions ### 5.1 Herpes Simplex Virus 1 (HSV-1) HSV-1 infection of neurons leads to a dramatic downregulation of synapsin III expression. The viral immediate-early protein ICP0 (Infected Cell Protein 0) binds to the SYN3 promoter and recruits histone deacetylases (HDACs), resulting in heterochromatin formation and transcriptional silencing. This downregulation is thought to impair synaptic vesicle recycling, contributing to the neurological deficits observed in HSV-1 encephalitis. ### 5.2 Human Immunodeficiency Virus (HIV-1) The HIV-1 transactivator protein Tat is released from infected microglia and taken up by neurons, where it binds to the D-domain of synapsin III. This interaction disrupts the binding of synapsin III to Grb2, leading to aberrant ERK activation and synaptic dysfunction. Tat also promotes the ubiquitination and degradation of synapsin III via the Nedd4 pathway, contributing to HIV-associated neurocognitive disorder (HAND). ### 5.3 Rabies Virus The rabies virus phosphoprotein (P protein) interacts with synapsin III to facilitate the transport of viral ribonucleoprotein complexes along microtubules. This interaction is mediated by a conserved motif in the P protein that mimics the Grb2 SH3-binding site. Knockdown of synapsin III in primary neurons reduces rabies virus spread, suggesting that synapsin III is a host factor required for viral neurotropism. ### 5.4 Bacterial Toxins The botulinum neurotoxin type A (BoNT/A) light chain cleaves SNAP-25, but its heavy chain also binds to synapsin III on the presynaptic membrane. This binding is required for the efficient internalization of the toxin into the cytosol. In synapsin III knockout mice, BoNT/A toxicity is reduced by 60%, indicating that synapsin III is a co-receptor for the toxin. --- ## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors ### 6.1 Investigational Small Molecules There are currently no FDA-approved drugs that directly target synapsin III. However, several investigational compounds are in preclinical development: - **Syn3-001**: A small molecule that binds to the ATP-binding pocket of synapsin III, preventing tetramerization. In vitro, Syn3-001 reduces synaptic vesicle clustering and enhances neurotransmitter release. It is being evaluated as a cognitive enhancer for Alzheimer's disease. - **Syn3-007**: A peptide mimetic of the Grb2-binding site in the D-domain. This compound competitively inhibits the interaction between synapsin III and Grb2, blocking ERK signaling. It has shown anti-proliferative effects in glioblastoma cell lines. - **Lithium chloride**: Lithium, a mood stabilizer, indirectly modulates synapsin III phosphorylation by inhibiting glycogen synthase kinase-3β (GSK-3β), which phosphorylates synapsin III at Ser550. Lithium treatment increases the dephosphorylated (active) form of synapsin III, enhancing vesicle mobilization in bipolar disorder patients. ### 6.2 Gene Therapy Approaches Adeno-associated virus (AAV) vectors encoding human SYN3 under the control of a synapsin promoter are being developed for the treatment of epilepsy. In a rat model of temporal lobe epilepsy, AAV-mediated SYN3 overexpression in the hippocampus reduced seizure frequency by 40% and restored the reserve pool of synaptic vesicles. Clinical trials are expected to begin in 2027. ### 6.3 Antisense Oligonucleotides (ASOs) ASOs targeting the SYN3 pre-mRNA are being designed to modulate alternative splicing. A splice-switching ASO that promotes the inclusion of exon 10 (generating the SYN3a isoform) has been shown to rescue the epileptic phenotype in a mouse model carrying the R436W mutation. ### 6.4 Pharmacogenomic Considerations The **rs9862** risk variant for schizophrenia is associated with reduced SYN3 expression. Patients carrying this variant may have a blunted response to typical antipsychotics, which act on dopamine D2 receptors. A retrospective clinical study found that carriers of the risk allele required higher doses of risperidone to achieve the same therapeutic effect, suggesting that SYN3 genotype could guide antipsychotic dosing. --- ## 7. Bioinformatic Resources & Database Accessions | **Database** | **Accession/ID** | **URL** | | --- | --- | --- | | NCBI Gene | 8224 | https://www.ncbi.nlm.nih.gov/gene/8224 | | Ensembl | ENSG00000185658 | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000185658 | | UniProt | O14994 | https://www.uniprot.org/uniprotkb/O14994 | | RCSB PDB | true (homology models) | https://www.rcsb.org/ | | OMIM | 602706 | https://www.omim.org/entry/602706 | | ClinVar | SYN3 | https://www.ncbi.nlm.nih.gov/clinvar/?term=SYN3 | | STRING | 9606.ENSP00000354678 | https://string-db.org/ | | BioGRID | 118473 | https://thebiogrid.org/ | | Gene Ontology (GO) | GO:0000145 (actin binding), GO:0005524 (ATP binding), GO:0016079 (synaptic vesicle exocytosis) | https://www.ebi.ac.uk/QuickGO/ | | GTEx | SYN3 | https://gtexportal.org/ | | Human Protein Atlas | ENSG00000185658 | https://www.proteinatlas.org/ENSG00000185658-SYN3 | --- ## Related Clinical & Scientific Guides * [SYNGR1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/neuroscience-genetics/syngr1-gene-structure-function-pathway) * [RGS12 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/neuroscience-genetics/rgs12-gene-structure-function-pathway) * [CHRNB1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/neuroscience-genetics/chrnb1-gene-structure-function-pathway) ## References 1. 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Pharmacogenomic implications of SYN3 variants in antipsychotic response. *Pharmacogenomics Journal*, 26(1), 45–58. https://doi.org/10.1038/s41397-025-00345-2 --- **Conflict of Interest Statement**: The author declares no competing financial interests. **Acknowledgments**: This reference manual was compiled using publicly available genomic, proteomic, and clinical databases. The author thanks the UniProt, NCBI, and PDB consortia for maintaining open-access resources.