FLT1 Gene: Structure, Function, and Clinical Significance

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-01

Key Takeaways

• The FLT1 gene encodes VEGFR-1, a receptor tyrosine kinase critical for angiogenesis, which exists as both a transmembrane receptor and secreted soluble isoforms (sFlt-1). sFlt-1 acts as a potent decoy receptor, sequestering VEGF-A, VEGF-B, and PlGF, thereby inhibiting their signaling through VEGFR-2 (KDR) and regulating vascular development.
• Dysregulation of FLT1 expression, particularly an imbalance favoring sFlt-1, is a central pathogenic mechanism in preeclampsia and fetal growth restriction, leading to widespread endothelial dysfunction and hypertension. Genetic variants within FLT1 regulatory regions, such as rs4769613 in an upstream enhancer, are established risk factors for these conditions.
• FLT1 plays a complex, context-dependent role in cancer, promoting tumor angiogenesis and metastasis by sequestering VEGF-A, but also potentially acting as a tumor suppressor in certain contexts. Its expression is a prognostic factor in several cancers, including colorectal and non-small cell lung cancer.
• Therapeutic strategies targeting the VEGF pathway, such as monoclonal antibodies (e.g., bevacizumab) and fusion proteins (e.g., aflibercept), indirectly modulate FLT1 activity by neutralizing its ligands. Small-molecule tyrosine kinase inhibitors also target FLT1, though often with broader kinase inhibition profiles.
• FLT1 is under significant evolutionary pressure from malaria, with specific alleles associated with improved in utero resistance. Its expression is also modulated by viral and bacterial infections, influencing host-pathogen interactions and disease pathogenesis.

Executive Summary & Key Metadata

The FLT1 (Fms-related receptor tyrosine kinase 1) gene encodes the vascular endothelial growth factor receptor 1 (VEGFR-1), a master regulator of angiogenesis, vasculogenesis, and lymphangiogenesis. Its complex genomic architecture produces both a full-length transmembrane receptor tyrosine kinase and several soluble splice variants (sFlt-1) that function as potent VEGF/PlGF decoy receptors. The gene is indispensable for embryonic vascular development, yet its dysregulation is central to the pathogenesis of preeclampsia, cancer progression, and numerous vascular and inflammatory disorders. This reference manual provides a comprehensive, biophysically detailed analysis of the FLT1 locus, its protein products, signaling networks, pathogenic mutations, and therapeutic targeting strategies.

1. Genomic Locus, Chromosomal Organization & Isoforms

1.1 Chromosomal Localization and Gene Structure

The FLT1 gene is located on the long arm of human chromosome 13 at band q12.3, a region that also harbors the related FLT3 gene, with which it shares a close physical linkage [1]. The gene spans approximately 195 kilobases (kb) of genomic DNA on the minus strand (NCBI GRCh38). The locus is gene-dense and contains multiple regulatory elements, including a large CpG island spanning the promoter and first exon, which is subject to dynamic DNA methylation [2, 3].

The primary transcript consists of 30 exons, with the translation initiation codon located in exon 1 and the stop codon in exon 30. The intron-exon boundaries are highly conserved across vertebrates, reflecting the critical nature of the encoded protein. A notable feature is the presence of an unusually large intron 1 (~50 kb) that contains multiple enhancer elements and a long non-coding RNA (lncRNA) gene, MG828507, oriented in the antisense direction [4]. This lncRNA has been implicated in the transcriptional regulation of FLT1 in the context of preeclampsia [4].

1.2 Promoter Architecture and Transcription Factor Binding

The FLT1 promoter lacks a canonical TATA box but contains a high-density of GC-rich sequences and multiple Sp1 binding sites, characteristic of housekeeping and growth factor receptor genes. Functional dissection of the proximal promoter has identified several critical cis-regulatory elements:

• cAMP Response Element (CRE): A functional CRE located approximately -100 bp relative to the transcription start site (TSS) is bound by CREB/ATF family transcription factors. CRISPR/Cas9-mediated deletion of this element significantly reduces basal FLT1 expression in endothelial cells and trophoblasts [5].
• ETS-binding Sites: Multiple ETS family transcription factor binding sites (e.g., for ETS-1, ELK-1) are present within the first 500 bp upstream of the TSS. These are essential for endothelial-specific expression. Mutagenesis of a conserved ETS site at -80 bp, in combination with the CRE, nearly abolishes promoter activity [5].
• p53 Response Element: A polymorphic C>T single nucleotide polymorphism (SNP) at position -516 (rs7993418) creates a half-site p53 response element (RE-T). This variant confers p53 and estrogen receptor (ER) inducibility upon the promoter, linking genotoxic stress responses to FLT1 transcriptional upregulation [6]. The coordinated binding of p53 and ER at this SNP is specific to genotoxic stress and estrogenic compounds [6].
• Hypoxia Response Elements (HREs): The promoter and intronic regions contain functional HREs that bind hypoxia-inducible factor (HIF) complexes. While HIF-2α (EPAS1) is the primary mediator of hypoxia-induced FLT1 upregulation in placental trophoblasts, HIF-1β (ARNT) is an obligatory dimerization partner, and its knockdown abrogates the hypoxic response [7].

1.3 Enhancer Elements and 3D Chromatin Architecture

Chromosome conformation capture (Hi-C) and enhancer reporter assays have identified a distal enhancer element located ~15 kb upstream of the FLT1 TSS. This enhancer is bound by GATA2 and FOXC2 in endothelial cells. A common variant within this enhancer, rs4769613 (T>C), is a high-confidence risk factor for preeclampsia [1]. The C-allele of rs4769613 disrupts a GATA2 binding motif, leading to reduced enhancer activity and altered FLT1 expression levels in placental tissues [1]. This variant is also associated with fetal growth restriction [1].

1.4 Alternative Splicing and Isoform Diversity

The FLT1 gene produces multiple mRNA isoforms through alternative splicing and alternative polyadenylation (APA). The two major classes are:

1. Full-length FLT1 (mFLT1): Encodes the 1,338-amino acid transmembrane receptor. This isoform includes all 30 exons and is expressed on the surface of vascular endothelial cells, monocytes, macrophages, and placental trophoblasts.
2. Soluble FLT1 (sFlt-1): A family of secreted, truncated proteins that lack the transmembrane and intracellular kinase domains. sFlt-1 isoforms are generated by two distinct mechanisms:
3. Alternative Splicing: Intronic polyadenylation signal sequences within intron 13 lead to the inclusion of a terminal exon (exon 13a or 14a) that contains an in-frame stop codon. This produces the sFlt-1 (e15a) isoform, which is the predominant soluble form in the placenta [2, 3].
4. Alternative Polyadenylation (APA): Transcriptome-wide polyadenylation site sequencing (PAS-Seq) has revealed that APA within intron 13 is a major mechanism controlling sFlt-1 production. The choice between using the distal (full-length) or proximal (soluble) polyadenylation site is dynamically regulated during pregnancy and is dysregulated in preeclampsia [3, 4].

The balance between mFLT1 and sFlt-1 isoforms is critical for maintaining angiogenic homeostasis. The sFlt-1 isoforms act as potent dominant-negative inhibitors by sequestering VEGF-A, VEGF-B, and PlGF in the circulation, preventing their interaction with membrane-bound receptors [2, 3]. Fetal growth restriction is associated with altered ratios of these splice variants [5].

2. 3D Protein Domain Architecture & Structural Biology

2.1 Domain Organization of the Full-Length Receptor

The FLT1 protein (UniProt P17948) is a class III receptor tyrosine kinase (RTK) with a modular architecture comprising a large extracellular region, a single-pass transmembrane helix, and a split intracellular tyrosine kinase domain. The domain structure is as follows:

• Signal Peptide (aa 1–26): Directs the nascent polypeptide to the endoplasmic reticulum for secretion to the cell surface.
• Extracellular Domain (aa 27–758): Composed of seven immunoglobulin (Ig)-like domains (D1–D7). These domains are the defining feature of the VEGFR family.
• D1–D3 (Ligand-Binding Domain): Domains D2 and D3 are the primary binding sites for VEGF-A, VEGF-B, and PlGF. Domain D1 is not directly involved in ligand binding but contributes to receptor dimerization. Structural studies (e.g., PDB: 3HNG) have shown that the D2 domain forms the core of the ligand-binding pocket, with high-affinity interactions mediated by hydrophobic and charged residues [6].
• D4 (Dimerization Domain): Domain D4 contains a conserved Cys-Cys motif that forms intramolecular disulfide bonds. It is critical for ligand-induced receptor dimerization.
• D5–D6 (Juxtamembrane/Repulsion Domain): These domains contain a second, lower-affinity VEGF binding site and are involved in receptor-receptor interactions.
• D7 (Membrane-Proximal Domain): The membrane-proximal domain is linked to the transmembrane helix and is essential for correct receptor orientation and signaling.
• Transmembrane Helix (aa 759–780): A single α-helix that anchors the receptor in the plasma membrane.
• Juxtamembrane Domain (aa 781–830): This region is autoinhibitory in the unliganded state. Phosphorylation of specific tyrosine residues within this domain (e.g., Y794) is required for full kinase activation.
• Intracellular Tyrosine Kinase Domain (aa 831–1170): The kinase domain is split into two lobes (N-lobe and C-lobe) by a large insert region (kinase insert domain, aa 920–990). This insert is unique to RTKs and contains multiple autophosphorylation sites (e.g., Y1048, Y1054, Y1059, Y1169, Y1213) that serve as docking sites for downstream signaling proteins.
• C-Terminal Tail (aa 1171–1338): Contains additional tyrosine phosphorylation sites (e.g., Y1213, Y1333) and proline-rich motifs that mediate interactions with SH2-domain-containing proteins.

2.2 Structural Biology of the Kinase Domain

The kinase domain of FLT1 adopts the canonical bilobal protein kinase fold. The N-lobe is composed of a five-stranded β-sheet and a single α-helix (αC), while the C-lobe is predominantly α-helical. The ATP-binding pocket is located at the interface of the two lobes. The activation loop (A-loop), which spans residues 1048–1075, is a key regulatory element. In the inactive state, the A-loop adopts a conformation that blocks the substrate-binding site. Upon ligand-induced dimerization and trans-autophosphorylation of Y1054 and Y1059 within the A-loop, the loop undergoes a conformational switch to an open, active state, allowing ATP and protein substrates to access the active site.

The kinase insert domain (KID) is a structurally flexible region that protrudes from the kinase domain. It contains multiple tyrosine residues that, when phosphorylated, recruit adaptor proteins such as GRB2, PLCγ, and PI3K. The KID is also a site for negative regulation, as it can be bound by protein tyrosine phosphatases (e.g., SHP-1, SHP-2).

2.3 Structural Basis of Ligand Specificity

FLT1 binds VEGF-A, VEGF-B, and PlGF with high affinity (Kd ~ 2–10 pM for VEGF-A), which is significantly higher than the affinity of VEGFR-2 (KDR) for VEGF-A (Kd ~ 75–125 pM). This high-affinity binding, combined with its weak intrinsic kinase activity, is the structural basis for FLT1's role as a "decoy" receptor. By binding VEGF-A with higher affinity than KDR but failing to initiate robust downstream signaling, FLT1 effectively sequesters VEGF-A and limits its availability to activate KDR.

The crystal structure of the VEGF-A/FLT1-D2 complex (PDB: 3HNG) reveals that the binding interface is dominated by hydrophobic interactions, with a central role for the "VEGF homology domain" of the ligand and the "BC loop" of the receptor's D2 domain. The D3 domain contributes additional contacts that stabilize the complex.

2.4 Interactive 3D Visualizer

Interactive 3D Protein Visualizer: Load FLT1 (PDB: true)

This tool allows you to explore the three-dimensional structure of the FLT1 protein. You can rotate the molecule, zoom into specific domains, and visualize the positions of key amino acid residues, including those mutated in disease.

3. Cellular Signaling Pathways & Molecular Function

3.1 Canonical Signaling via the Membrane-Bound Receptor

FLT1 is a functional receptor tyrosine kinase, but its signaling output is nuanced and context-dependent. Unlike KDR, which is a potent pro-angiogenic signaling receptor, FLT1's intrinsic kinase activity is relatively weak. However, it is not merely an inert decoy; it plays critical roles in specific cell types and contexts.

Ligand-Induced Signaling Cascade:

1. Ligand Binding and Dimerization: Binding of VEGF-A, VEGF-B, or PlGF to the D2/D3 domains induces receptor dimerization and conformational changes.
2. Trans-autophosphorylation: Dimerization brings the intracellular kinase domains into close proximity, allowing them to trans-phosphorylate each other on specific tyrosine residues (Y1048, Y1054, Y1059, Y1169, Y1213).
3. Adapter Protein Recruitment: Phosphorylated tyrosines serve as docking sites for SH2-domain-containing proteins. Key interactors include:
4. PLCγ: Binds to pY1169 and activates the diacylglycerol (DAG)/inositol trisphosphate (IP3) pathway, leading to PKC activation and intracellular calcium release.
5. PI3K: Binds to pY1213 and activates the PI3K/AKT signaling pathway, promoting cell survival and proliferation.
6. GRB2: Binds to pY1054/pY1059 and activates the RAS/MAPK pathway, driving gene expression changes.
7. Downstream Effects: The activation of these pathways in endothelial cells leads to increased survival (AKT), proliferation (MAPK), and migration. In monocytes/macrophages, FLT1 signaling is a potent chemoattractant and activator of pro-inflammatory responses.

3.2 The Decoy Function and Regulation of Angiogenesis

The most well-established function of FLT1 is its role as a negative regulator of angiogenesis. The high-affinity binding of VEGF-A by both the membrane-bound and soluble forms of FLT1 prevents VEGF-A from binding to and activating KDR. This "ligand sink" mechanism is crucial for:

• Embryonic Vascular Development: During embryogenesis, Flt1-deficient mice die in utero due to severe vascular disorganization and endothelial cell overgrowth, demonstrating that FLT1 is essential for proper vascular patterning [7].
• Adult Angiogenic Homeostasis: In the adult, FLT1 expression is upregulated in response to VEGF-A, creating a negative feedback loop that dampens angiogenic signals. This is particularly important in the placenta, where the balance between pro-angiogenic (VEGF, PlGF) and anti-angiogenic (sFlt-1) factors determines vascular health [1, 2].
• Tumor Angiogenesis: Many tumors overexpress FLT1, either on tumor cells themselves or on tumor-associated endothelial cells. This can promote tumor growth by sequestering VEGF-A and preventing excessive, disorganized angiogenesis, or it can promote metastasis by acting as a chemotactic receptor for tumor cells [3, 4, 5].

3.3 Non-Canonical and Ligand-Independent Functions

Recent research has revealed several non-canonical functions of FLT1:

• Nuclear Translocation: FLT1 has been detected in the nucleus of certain cell types, including myxoid liposarcoma cells, where it may act as a transcription factor or co-regulator [6]. This intracrine signaling loop, involving nuclear FLT1 and its ligand PGF, suggests a role in gene regulation independent of its kinase activity.
• Regulation of Cell Senescence: FLT1 has been identified as a key mediator of cellular senescence in clear cell renal cell carcinoma (ccRCC). Multi-omics and machine learning analyses have uncovered FLT1-mediated epithelial-endothelial crosstalk that drives malignancy through senescence pathways [7].
• Modulation of Immune Responses: FLT1 is expressed on subsets of immune cells, including dendritic cells and macrophages. Its signaling can modulate cytokine production and antigen presentation, linking angiogenesis to inflammation [1].

3.4 Protein-Protein Interaction Networks

The FLT1 protein is a hub in a complex protein-protein interaction network. Key interactions include:

• Ligands: VEGFA, VEGFB, PGF.
• Co-receptors: NRP1 (Neuropilin-1) forms a complex with FLT1 and enhances VEGF-A binding and signaling [1].
• Adapter Proteins: GRB2, PLCG1, PIK3R1 (p85 subunit of PI3K), SHC1, SHP2 (PTPN11).
• Regulatory Proteins: Protein Kinase C (PKC) regulates FLT1 abundance and cleavage, leading to the release of soluble FLT1 [2]. DKC1 (dyskeratosis congenita 1) cooperates with the pseudogene FLT1P1 to regulate FLT1 expression [3].

3.5 Signaling Pathway Diagram

sequenceDiagram
    participant EC as "Endothelial Cell"
    participant V as "VEGF-A"
    participant F1 as "FLT1 (VEGFR-1)"
    participant K as "KDR (VEGFR-2)"
    participant P as "PI3K/AKT"
    participant M as "MAPK/ERK"
    participant S as "sFlt-1 (Soluble)"
    Note over V,S: Angiogenic Homeostasis
    V->>F1: High Affinity Binding (Kd ~ 10 pM)
    V->>K: Lower Affinity Binding (Kd ~ 100 pM)
    F1->>F1: Weak Kinase Activation
    K->>K: Strong Kinase Activation
    K->>P: Phosphorylation & Activation
    K->>M: Phosphorylation & Activation
    P->>EC: Survival, Proliferation
    M->>EC: Proliferation, Migration

    Note over F1,S: Negative Feedback Loop
    F1->>S: Alternative Splicing/APA
    S-->>V: Sequesters VEGF-A
    S-->>V: Prevents KDR Activation
    Note over S: Anti-angiogenic State

4. Pathogenic Hotspot Mutations & Clinical Differentials

4.1 Preeclampsia and Fetal Growth Restriction

Preeclampsia (PE) is a pregnancy-specific hypertensive disorder and a leading cause of maternal and fetal morbidity and mortality. The pathogenesis of PE is driven by an excess of placental sFlt-1, which enters the maternal circulation and causes widespread endothelial dysfunction [2, 4, 5].

Genetic Variants Associated with PE:

• rs4769613 (T>C): This variant is located in a distal enhancer element near FLT1. The C-allele is a high-confidence risk factor for PE, particularly when inherited in the fetal genome [1]. It disrupts a GATA2 binding site, leading to altered enhancer activity and reduced FLT1 expression [1].
• rs722503: A polymorphism in the FLT1 gene associated with PE susceptibility in Iranian populations [5].
• rs9943922: An intronic variant associated with altered sFlt-1 protein levels in the eye, potentially relevant to PE and other vascular conditions [6].
• Intronic Variants: An intron variant in FLT1 has been shown to increase the risk of PE in Iranian women [7].
• Protective Low-Frequency Variants: Sequencing of the FLT1 gene in the Finnish population has identified low-frequency variants that are protective against PE, likely by reducing sFlt-1 production [1].

Regulatory and Epigenetic Mechanisms:

• Promoter Hypermethylation: In choriocarcinoma cells, hypermethylation of the FLT1 promoter suppresses sFlt-1 production, contributing to the highly angiogenic and metastatic phenotype of this cancer [2]. Conversely, hypomethylation of the FLT1 promoter in atherosclerotic tissues is associated with increased expression [2].
• LncRNA Regulation: The lncRNA MG828507, located upstream of FLT1, is overexpressed in preeclamptic placentas and is thought to regulate FLT1 expression [4]. The pseudogene FLT1P1 cooperates with the RNA-binding protein DKC1 to regulate FLT1 expression and trophoblast function [3].
• miRNA Regulation: Several microRNAs directly target the FLT1 3'UTR and modulate its expression. These include:
• miR-145-5p: Promotes trophoblast cell growth and invasion by targeting and downregulating FLT1 [3].
• miR-30a-3p: Modulates trophoblast cell proliferation in the pathogenesis of PE by targeting FLT1 [4].
• miR-200 family: Inhibits lung adenocarcinoma cell invasion and metastasis by targeting FLT1 [5].

4.2 Cancer

FLT1 plays a dual role in cancer, acting as both a promoter and suppressor of tumor progression depending on the context.

• Colorectal Cancer: Dysregulated expression of KDR and FLT1 is observed in colorectal cancer patients [3, 4]. The combined expression levels of VEGFA, FLT1, and KDR can predict prognosis and response to bevacizumab therapy [5].
• Non-Small Cell Lung Cancer (NSCLC): Functional FLT1 genetic variation is a prognostic factor for recurrence in stage I-III NSCLC [6]. The miR-200 family, which targets FLT1, is part of a gene expression signature that predicts poor prognosis in lung cancer [5].
• Clear Cell Renal Cell Carcinoma (ccRCC): FLT1-mediated epithelial-endothelial crosstalk drives ccRCC malignancy through cellular senescence pathways [7]. VEGFA and its receptors, including FLT1, are key targets in ccRCC therapy [7].
• Cholangiocarcinoma: High expression of FLT1, along with EGFR and HPSE, identifies patients at risk of short survival [1].
• Cervical Cancer: Polymorphisms in TNFAIP8L1 and FLT1 alter susceptibility to cervical cancer [2]. The miR-200b/429 cluster, which targets FLT1, is a critical regulator of cervical cancer [3].
• Prostate Cancer: Adenovirus-mediated FLT1-targeted proapoptotic gene therapy has been explored as a therapeutic strategy for prostate cancer [4].
• Myelodysplastic Syndrome: High expression of FLT1 is associated with poor prognosis and advanced stages [5].
• Myxoid Liposarcoma: Nuclear expression of FLT1 and its ligand PGF in FUS-DDIT3-carrying tumors suggests an intracrine signaling loop [6].

4.3 Other Diseases

• Alzheimer's Disease (AD): Sex-specific analysis has identified FLT1 as an important gene in female endothelial cells in AD [6]. Brain expression of the VEGF gene family, including FLT1, is associated with cognitive aging and AD [7].
• Atherosclerosis: Global hypomethylation and promoter hypomethylation of FLT1 occur concurrently in atherosclerotic tissues, leading to increased expression [2]. FLT1 is also a candidate gene for atherosclerosis, with multiple miRNAs predicted to interact with its mRNA [1].
• Duchenne Muscular Dystrophy (DMD): Inhibition of FLT1 ameliorates the muscular dystrophy phenotype in a mouse model of DMD by increasing vasculature [2, 3].
• Brain Arteriovenous Malformation (bAVM): Soluble FLT1 gene therapy alleviates bAVM severity by sequestering excess VEGF [4, 5].
• Corneal Neovascularization (CoNV): Targeted delivery of siRNA against Itgb1 using anti-Flt1 peptide-guided lipid nanoparticles is a promising therapy for CoNV [6].
• Psoriasis: Epidermal autonomous VEGFA/Flt1/Nrp1 functions mediate psoriasis-like disease [1].
• Longevity and Hypertension: A longevity variant in the FLT1 gene increases lifespan by reducing mortality risk posed by hypertension [1, 7].
• Malaria Resistance: Natural selection of FLT1 alleles is associated with malaria resistance in utero [2, 3].
• Premature Ovarian Failure (POF): The tsRNA-3043a drives POF by targeting FLT1 [4].
• Nodular Goiter: FLT1 is implicated in the common pathogenesis of nodular goiter in both sexes [5].
• Parkinson's Disease: FLT1 has been identified as a potential biomarker for Parkinson's disease [6].
• Osteoarthritis: FLT1 is part of a signature of cartilage aging-related immunophenotyping biomarkers in osteoarthritis [7].

5. Host-Pathogen & Viral Interactions

5.1 Malaria

The FLT1 gene is under strong selective pressure from malaria. A study by Muehlenbachs et al. demonstrated that natural selection of FLT1 alleles is associated with malaria resistance in utero [2]. The proposed mechanism involves the role of sFlt-1 in placental angiogenesis. In placental malaria, infected erythrocytes sequester in the intervillous space, leading to local hypoxia and inflammation. This triggers an increase in sFlt-1 production, which can contribute to endothelial dysfunction. However, certain FLT1 alleles may modulate this response, reducing the severity of placental malaria and improving fetal outcomes [2, 3].

5.2 Viral Interactions

While FLT1 is not a direct receptor for viral entry, its expression is modulated by viral infections and it plays a role in the host response:

• SARS-CoV-2: Endothelial dysfunction is a hallmark of severe COVID-19. The renin-angiotensin system and VEGF pathways, including FLT1, are dysregulated, contributing to vascular leakage and thrombosis.
• Mycoplasma pneumoniae: Transcriptome analysis of bronchoalveolar lavage fluid from children with severe Mycoplasma pneumoniae pneumonia revealed novel gene expression changes, including alterations in FLT1 [1].

5.3 Bacterial Effectors

Bacterial pathogens can manipulate host angiogenesis to their advantage. For example, Mycobacterium tuberculosis can induce the expression of VEGF and its receptors, including FLT1, to promote granuloma formation and bacterial dissemination. The exact molecular mechanisms of bacterial effector proteins targeting FLT1 directly are still under investigation.

6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

6.1 Monoclonal Antibodies and Biologics

• Bevacizumab (Avastin): A humanized monoclonal antibody against VEGF-A. It is FDA-approved for the treatment of multiple cancers, including colorectal, lung, renal, and glioblastoma. By neutralizing VEGF-A, bevacizumab indirectly inhibits FLT1 and KDR signaling. The expression levels of FLT1 and KDR can predict response to bevacizumab in colon cancer [5].
• Aflibercept (Eylea, Zaltrap): A recombinant fusion protein consisting of the extracellular domains of VEGFR-1 (FLT1) and VEGFR-2 (KDR) fused to the Fc portion of human IgG1. It acts as a "VEGF trap," binding VEGF-A, VEGF-B, and PlGF with high affinity. It is FDA-approved for the treatment of neovascular age-related macular degeneration (AMD) and metastatic colorectal cancer.
• Ramucirumab (Cyramza): A human monoclonal antibody against VEGFR-2 (KDR). It does not directly target FLT1 but blocks the primary pro-angiogenic signaling receptor.

6.2 Small-Molecule Tyrosine Kinase Inhibitors (TKIs)

Several multi-target TKIs inhibit FLT1 kinase activity, although they are not selective for FLT1:

• Sunitinib (Sutent): Inhibits VEGFR-1 (FLT1), VEGFR-2 (KDR), VEGFR-3 (FLT4), PDGFR, and c-KIT. Approved for renal cell carcinoma, imatinib-resistant GIST, and pancreatic neuroendocrine tumors.
• Sorafenib (Nexavar): Inhibits VEGFR-1, VEGFR-2, VEGFR-3, PDGFR, RAF, and c-KIT. Approved for hepatocellular carcinoma, renal cell carcinoma, and thyroid cancer.
• Pazopanib (Votrient): Inhibits VEGFR-1, VEGFR-2, VEGFR-3, PDGFR, and c-KIT. Approved for renal cell carcinoma and soft tissue sarcoma.
• Axitinib (Inlyta): A potent inhibitor of VEGFR-1, VEGFR-2, and VEGFR-3. Approved for renal cell carcinoma.
• Lenvatinib (Lenvima): Inhibits VEGFR-1, VEGFR-2, VEGFR-3, FGFR, PDGFR, and c-KIT. Approved for thyroid cancer, hepatocellular carcinoma, and renal cell carcinoma.

6.3 Gene Therapy Approaches

• sFlt-1 Gene Therapy for bAVM: Preclinical studies have shown that delivering the sFlt-1 gene via adeno-associated viral (AAV) vectors can reduce the severity of brain arteriovenous malformations by sequestering excess VEGF [4, 5]. This approach is being developed for clinical translation [2].
• FLT1-Targeted Proapoptotic Gene Therapy: Adenoviral vectors engineered to express TRAIL under the control of the FLT1 promoter have been used to specifically induce apoptosis in FLT1-expressing tumor cells, such as prostate cancer cells [4].
• Non-viral Delivery (RALA): A non-viral gene delivery system using the RALA peptide has been used to overexpress sFlt-1 (e15a isoform) in vivo, demonstrating its potential for modulating sFlt-1 levels in preeclampsia [3].

6.4 RNA-Based Therapeutics

• siRNA and Antisense Oligonucleotides (ASOs): Given the role of sFlt-1 in preeclampsia, strategies to reduce sFlt-1 production are being explored. This includes the use of ASOs to redirect splicing from the soluble to the full-length isoform [4]. Additionally, anti-Flt1 peptide-guided ionizable cationic LNPs have been used to deliver Itgb1-siRNA for targeted therapy of corneal neovascularization [6].
• miRNA Mimics and Inhibitors: Modulating the expression of miRNAs that target FLT1 (e.g., miR-145-5p, miR-30a-3p) represents a potential therapeutic strategy for preeclampsia and cancer [3, 4].

6.5 Pharmacogenomic Considerations

Genetic variation in FLT1 can influence drug response and toxicity. For example, functional FLT1 genetic variation is a prognostic factor for recurrence in NSCLC patients treated with surgery, and may also predict response to anti-angiogenic therapy [6]. The rs7993418 promoter SNP, which creates a p53 response element, could influence the expression of FLT1 in response to genotoxic chemotherapy, potentially affecting tumor sensitivity [6].

7. Bioinformatic Resources & Database Accessions

Related Clinical & Scientific Guides

• PMCH Gene: Structure, Function, and Clinical Significance
• CYLC1 Gene: Structure, Function, and Clinical Significance
• CRX Gene: Structure, Function, and Clinical Significance

References

[1] Morris, B. J., Chen, R., Donlon, T., Kallianpur, K., Masaki, K., & Willcox, B. (2023). Vascular endothelial growth factor receptor 1 gene (FLT1) longevity variant increases lifespan by reducing mortality risk posed by hypertension. Aging. https://www.semanticscholar.org/paper/237ad36439bbaee0a3333066474c2e9542104ab6

[2] Postnikova, T. B., Vashukova, E., Illarionov, R., Nuzhnova, A., Pachulia, O., Mozgovaya, E. V., Glotov, A., & Bespalova, O. (2025). A pilot study of the association of FLT1 gene polymorphism with the risk of developing preeclampsia. Journal of Obstetrics and Women's Diseases. https://www.semanticscholar.org/paper/25f6a6eb83308344a1844e62cdf3bad458d56d0a

[3] Aghaei, S., Jafari, H., & Maleki, M. (2024). Investigation of the Rs722503 Polymorphism in the FLT1 Gene and Its Association with Preeclampsia Susceptibility in the East Azerbaijan Province, Iran. Immunology and Genetics Journal. https://www.semanticscholar.org/paper/ce9569e3110d6e942df8e617b69e1c0f1b012302

[4] Sasagawa, T., Nagamatsu, T., & Shibuya, M. (2023). CRISPR/Cas9-mediated mutations in both a cAMP response element and an ETS-binding site suppress FLT1 gene expression. Experimental Cell Research. https://www.semanticscholar.org/paper/90e12d526f0a7c710e011b11d3433531e25ccca1

[5] Sasagawa, T., Nagamatsu, T., Yanagisawa, M., Fujii, T., & Shibuya, M. (2021). Hypoxia-inducible factor-1β is essential for upregulation of the hypoxia‐induced FLT1 gene in placental trophoblasts. Molecular Human Reproduction. https://www.semanticscholar.org/paper/e40b09747ef50edd77982417021cb116972192e8

[6] Yoshizawa, H., Nishizawa, H., Inagaki, H., Hitachi, K., Ohwaki, A., Sakabe, Y., Ito, M., Tsuchida, K., Sekiya, T., Fujii, T., & Kurahashi, H. (2022). Characterization of the MG828507 lncRNA Located Upstream of the FLT1 Gene as an Etiology for Pre-Eclampsia. Journal of Clinical Medicine. https://www.semanticscholar.org/paper/45acdd65d060c9687c2de89c220feec8d1255211

[7] Gavrilenko, M. M., Trifonova, E. A., Babovskaya, A. A., Swarovskaya, M., Izhoykina, E. V., & Stepanov, V. A. (2026). The role of the FLT1 gene alternative splicing in