# AXL Gene: Structure, Function, and Clinical Significance ## Key Takeaways - AXL is a receptor tyrosine kinase (RTK) belonging to the TAM family, encoded by a gene located on chromosome 19q13.2, and its primary ligand is Growth Arrest-Specific 6 (GAS6). - AXL signaling is initiated by GAS6 binding, leading to receptor dimerization and activation of downstream pathways including PI3K/AKT, RAS/MAPK, and JAK/STAT, which promote cell survival, proliferation, migration, and immune evasion. - Aberrant AXL activation, often due to overexpression, is a significant driver of oncogenesis and is strongly correlated with poor prognosis and resistance to targeted therapies (e.g., EGFR inhibitors like osimertinib) across various solid and hematological malignancies. - AXL functions as a viral entry receptor for enveloped viruses like Zika and Ebola, utilizing phosphatidylserine on the viral envelope to mediate entry and subsequently suppressing host innate immune responses. - AXL is a critical therapeutic target, with multiple small-molecule inhibitors (e.g., cabozantinib, bemcentinib) and antibody-drug conjugates (e.g., enapotamab vedotin) in clinical development or approved for cancer treatment. - Soluble AXL (sAXL), generated by alternative splicing, acts as a decoy receptor and is being investigated as a prognostic and pharmacodynamic biomarker in cancer patients. --- ## Executive Summary & Key Metadata The AXL gene encodes a receptor tyrosine kinase (RTK) that belongs to the TAM (TYRO3, AXL, MERTK) family. AXL was originally identified as a transforming gene from chronic myelogenous leukemia (CML) patients, where it was found fused to a ubiquitously expressed protein. The gene product is a single-pass type I transmembrane protein that transduces signals from the extracellular matrix to the intracellular compartment upon binding its primary ligand, growth arrest-specific 6 (GAS6). AXL is a master regulator of multiple oncogenic processes, including epithelial-to-mesenchymal transition (EMT), immune evasion, drug resistance, and metastasis. Its overexpression and aberrant activation are correlated with poor prognosis across a broad spectrum of solid and hematological malignancies. Consequently, AXL has emerged as a high-priority therapeutic target, with multiple small-molecule inhibitors and monoclonal antibodies in clinical development. | **Attribute** | **Detail** | | :--- | :--- | | **HGNC Symbol** | AXL | | **UniProt Accession** | P30530 | | **Representative PDB ID** | 2C5D (Intracellular kinase domain) | | **Chromosomal Locus** | 19q13.2 (GRCh38: chr19:41,219,223-41,261,766) | | **Primary Molecular Function** | Receptor tyrosine kinase; signal transduction; cell survival, proliferation, migration, and immune regulation | | **Ligand** | Growth arrest-specific 6 (GAS6) | | **Disease & Pathology Associations** | Non-small cell lung cancer (NSCLC), breast cancer, pancreatic ductal adenocarcinoma (PDAC), acute myeloid leukemia (AML), glioblastoma, melanoma, and resistance to targeted therapies (e.g., osimertinib, BRAF inhibitors) | --- ## 1. Genomic Locus, Chromosomal Organization & Isoforms ### 1.1 Chromosomal Localization and Gene Structure The human AXL gene is located on the long arm of chromosome 19 at cytogenetic band 19q13.2. In the GRCh38 assembly, the gene spans approximately 42.5 kilobases (kb) of genomic DNA, from base pair 41,219,223 to 41,261,766 on the forward strand. The gene is oriented in a head-to-tail fashion relative to its neighboring genes, which include *MEGF8* (multiple EGF-like domains 8) upstream and *SEPT3* (septin 3) downstream. The genomic locus is characterized by a high density of Alu repetitive elements, which have been implicated in the generation of genomic rearrangements and fusion events involving AXL in certain malignancies. The coding sequence is organized into 20 exons, with the translation initiation codon (ATG) located in exon 1 and the stop codon in exon 20. The exon-intron boundaries conform to the canonical GT-AG splice donor-acceptor rule. The intronic regions vary considerably in size, with intron 1 being the largest at approximately 8.5 kb. The 5' untranslated region (UTR) is encoded within exon 1 and a portion of intron 1, while the 3' UTR is exceptionally long (~2.5 kb) and contains multiple AU-rich elements (AREs) that regulate mRNA stability. ### 1.2 Promoter Architecture and Transcriptional Regulation The core promoter of AXL lacks a canonical TATA box but contains a high GC content (~70%) and multiple Sp1 transcription factor binding sites. This GC-rich region is characteristic of housekeeping and growth-related genes. The minimal promoter region required for basal transcription has been mapped to a 300-base pair (bp) region upstream of the transcription start site (TSS). Within this region, functional binding sites for the following transcription factors have been experimentally validated: - **Sp1**: Binds to GC boxes and is essential for basal promoter activity. - **AP-1 (Fos/Jun)**: Mediates transcriptional activation in response to growth factor stimulation and cellular stress. - **NF-κB**: Drives AXL expression in response to inflammatory cytokines (e.g., TNF-α) and in the context of the tumor microenvironment. - **HIF-1α**: Directly binds to a hypoxia-responsive element (HRE) in the promoter, leading to AXL upregulation under hypoxic conditions, a common feature of solid tumors. - **Ets-1**: Cooperates with AP-1 to enhance transcription during EMT. Beyond the core promoter, several enhancer elements have been identified in the first intron. These enhancers are marked by histone H3 lysine 27 acetylation (H3K27ac) and are bound by the transcriptional co-activator BRD4. Pharmacological inhibition of BRD4 with BET inhibitors (e.g., JQ1) leads to a significant reduction in AXL mRNA levels, indicating that these intronic enhancers are functionally relevant for high-level AXL expression in cancer cells. ### 1.3 Alternative Splicing and Isoforms Alternative splicing of the AXL pre-mRNA generates multiple transcript variants, although the functional significance of many of these isoforms remains under investigation. The major transcript (NM_001699.6) encodes the full-length, 894-amino acid (aa) receptor tyrosine kinase. The following isoforms have been cataloged: - **Isoform 1 (Canonical)**: 894 aa. Contains the full complement of extracellular, transmembrane, and intracellular domains. - **Isoform 2 (Soluble AXL, sAXL)**: Generated by alternative splicing that introduces a premature stop codon in the region encoding the juxtamembrane domain. This results in a truncated protein that is secreted into the extracellular space. sAXL acts as a decoy receptor, sequestering GAS6 and thereby negatively regulating AXL signaling. Elevated levels of sAXL in plasma are being investigated as a prognostic and pharmacodynamic biomarker. - **Isoform 3 (AXL-ΔEC)**: A splice variant lacking exons 1-3, which removes a portion of the extracellular domain. This isoform retains the transmembrane and kinase domains and exhibits ligand-independent, constitutive activation. It has been detected in a subset of glioblastoma and breast cancer cell lines. - **Isoform 4 (AXL-TV)**: A variant with a truncated C-terminal tail, resulting in altered downstream signaling kinetics. The regulation of alternative splicing is mediated by serine/arginine-rich (SR) proteins and heterogeneous nuclear ribonucleoproteins (hnRNPs). Notably, the splicing factor ESRP1 (epithelial splicing regulatory protein 1) has been shown to suppress the generation of the soluble isoform, thereby promoting full-length AXL expression in epithelial cells. --- ## 2. 3D Protein Domain Architecture & Structural Biology ### 2.1 Primary Structure and Domain Organization The AXL protein is a 894-amino acid type I transmembrane glycoprotein with a theoretical molecular weight of ~98 kDa (unmodified). Due to extensive N-linked glycosylation, the mature protein migrates at approximately 140 kDa on SDS-PAGE. The protein is organized into distinct functional domains from the N-terminus to the C-terminus: 1. **Signal Peptide (aa 1-32)**: Directs the nascent polypeptide to the endoplasmic reticulum for co-translational translocation into the secretory pathway. This peptide is cleaved off in the mature protein. 2. **Extracellular Domain (aa 33-472)**: - **Two Immunoglobulin (Ig)-like Domains (aa 33-129 and aa 130-220)**: These domains are responsible for high-affinity binding to the ligand GAS6. The N-terminal Ig-like domain (Ig1) is the primary binding site, while the second Ig domain (Ig2) stabilizes the interaction. The Ig domains adopt a classic β-sandwich fold composed of two anti-parallel β-sheets. - **Two Fibronectin Type III (FNIII) Domains (aa 221-330 and aa 331-440)**: These domains are structurally similar to those found in adhesion molecules and are thought to mediate protein-protein interactions that facilitate receptor dimerization and cell-cell adhesion. They also contribute to the overall rigidity and extension of the extracellular region. 3. **Transmembrane Domain (aa 473-495)**: A single hydrophobic α-helix that anchors the receptor in the plasma membrane. This domain is critical for correct receptor dimerization and signal propagation. 4. **Intracellular Domain (aa 496-894)**: - **Juxtamembrane Domain (aa 496-550)**: This region is subject to regulatory phosphorylation and interacts with intracellular signaling molecules. It also contains a conserved YXXΦ motif that mediates internalization and trafficking. - **Tyrosine Kinase Domain (aa 551-860)**: This is the catalytic core of the receptor. It adopts the canonical bilobed protein kinase fold, with an N-terminal lobe (rich in β-sheets) and a C-terminal lobe (rich in α-helices). The ATP-binding pocket is located in the cleft between the two lobes. The activation loop (A-loop) contains three critical tyrosine residues (Y779, Y821, and Y866) whose phosphorylation is required for full catalytic activity. - **C-terminal Tail (aa 861-894)**: Contains additional tyrosine residues (Y888, Y893) that serve as docking sites for downstream signaling proteins containing Src homology 2 (SH2) domains. ### 2.2 Quaternary Structure and Ligand Binding AXL functions as a homodimer. Ligand-induced dimerization is the primary mechanism of activation. GAS6 is a vitamin K-dependent protein that binds to AXL with high affinity (Kd ~ 0.4 nM). GAS6 itself is a multi-domain protein, and its interaction with AXL is complex. The GAS6-AXL complex is formed with a 2:2 stoichiometry, where a single GAS6 dimer bridges two AXL receptors. The interaction is primarily mediated by the Ig1 domains of AXL and the laminin G-like (LG) domains of GAS6. The binding of GAS6 induces a conformational change in the extracellular domain, bringing the two transmembrane domains into close proximity. This proximity allows the intracellular kinase domains to trans-phosphorylate each other on key tyrosine residues within the activation loop, leading to a fully activated state. ### 2.3 Structural Basis of Kinase Activation and Inhibition The crystal structure of the AXL kinase domain (PDB: 2C5D) has been solved in its inactive conformation. In this state, the activation loop is folded in a manner that blocks the substrate-binding site. The DFG motif (Asp-Phe-Gly) at the N-terminus of the activation loop adopts a "DFG-out" conformation, which is incompatible with ATP binding. Upon receptor dimerization and trans-phosphorylation of the activation loop tyrosines, the loop undergoes a conformational switch to a "DFG-in" state, opening the catalytic cleft and allowing ATP and protein substrates to bind. This structural plasticity has been exploited for drug design. Type II kinase inhibitors (e.g., cabozantinib, foretinib) bind to the ATP-binding pocket and extend into an adjacent allosteric hydrophobic pocket that is only accessible in the "DFG-out" conformation. In contrast, Type I inhibitors (e.g., gilteritinib) bind to the active "DFG-in" conformation. The structural differences between these inhibitor classes have significant implications for selectivity and the development of resistance mutations. > **[Interactive 3D Protein Visualizer: Load AXL (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=P30530)** > > Use the interactive visualizer to explore the atomic coordinates of the AXL kinase domain (PDB: 2C5D). The tool allows you to rotate the molecule, highlight specific residues (e.g., the DFG motif, activation loop tyrosines), and measure atomic distances within the ATP-binding pocket. This is an essential resource for understanding the structural basis of inhibitor binding and resistance. --- ## 3. Cellular Signaling Pathways & Molecular Function ### 3.1 Canonical GAS6-AXL Signaling The primary ligand for AXL is GAS6, a vitamin K-dependent protein that shares homology with the anticoagulation factor Protein S. GAS6 binding to AXL triggers receptor dimerization and autophosphorylation on multiple tyrosine residues. These phosphotyrosine residues serve as docking sites for a variety of SH2-domain and phosphotyrosine-binding (PTB) domain-containing proteins, initiating a complex signaling network. The major downstream signaling cascades activated by AXL include: - **PI3K/AKT/mTOR Pathway**: The p85 regulatory subunit of PI3K binds directly to phospho-Y821 on AXL. This recruits PI3K to the membrane, where it converts PIP2 to PIP3. PIP3 then recruits AKT and PDK1 to the membrane, leading to AKT phosphorylation at T308 and S473. Activated AKT promotes cell survival by phosphorylating and inactivating pro-apoptotic proteins such as BAD and FOXO3a, and by activating the mTORC1 complex, which drives protein synthesis and cell growth. - **RAS/RAF/MEK/ERK Pathway**: The adaptor protein GRB2 binds to AXL, either directly or via the scaffolding protein SHC. GRB2 recruits SOS, a guanine nucleotide exchange factor (GEF) for RAS. This leads to RAS activation and the subsequent activation of the RAF/MEK/ERK kinase cascade. ERK translocates to the nucleus and phosphorylates transcription factors such as ELK1 and c-FOS, driving cell proliferation and differentiation. - **JAK/STAT Pathway**: AXL can activate JAK kinases, which in turn phosphorylate STAT transcription factors (primarily STAT3). Phosphorylated STAT3 dimerizes and translocates to the nucleus, where it drives the expression of genes involved in cell survival, angiogenesis (e.g., VEGF), and immune suppression (e.g., PD-L1). - **NF-κB Pathway**: AXL activates IKK (IκB kinase), leading to the phosphorylation and proteasomal degradation of IκBα. This releases NF-κB, allowing it to translocate to the nucleus and transactivate genes involved in inflammation, EMT, and anti-apoptosis. - **SRC Family Kinases**: AXL can also activate SRC, which phosphorylates a variety of substrates, including focal adhesion kinase (FAK), to promote cell migration and invasion. ### 3.2 Non-Canonical and Ligand-Independent Signaling In addition to GAS6-dependent signaling, AXL can be activated through ligand-independent mechanisms. These include: - **Homophilic Interactions**: AXL can bind to AXL molecules on adjacent cells (trans-homophilic binding), leading to juxtacrine signaling. This is particularly relevant in dense tumor microenvironments. - **Heterodimerization with Other RTKs**: AXL can form heterodimers with other receptor tyrosine kinases, such as EGFR, HER2, and MET. This cross-talk can lead to the trans-phosphorylation and activation of AXL in the absence of GAS6, and conversely, AXL can sustain the activation of these receptors. This is a major mechanism of resistance to targeted therapies against EGFR and HER2. - **Integrin-Mediated Signaling**: AXL can associate with β3 and β5 integrins. This interaction enhances integrin-mediated adhesion and migration and can activate downstream signaling pathways such as FAK and SRC. ### 3.3 Regulation of AXL Signaling AXL signaling is tightly regulated at multiple levels to prevent uncontrolled cell growth: - **Dephosphorylation**: Protein tyrosine phosphatases (PTPs), such as PTPN1 (PTP1B) and PTPN11 (SHP2), can dephosphorylate AXL and its downstream substrates, attenuating signaling. - **Receptor Internalization and Degradation**: Upon activation, AXL is internalized via clathrin-mediated endocytosis. It can then be either recycled back to the cell surface or targeted for lysosomal degradation. The E3 ubiquitin ligase CBL binds to phospho-AXL and ubiquitinates it, tagging it for proteasomal degradation. - **Soluble AXL (sAXL)**: The alternatively spliced soluble form of AXL acts as a decoy receptor, binding to GAS6 and preventing it from activating membrane-bound AXL. This is a natural negative feedback mechanism. - **Transcriptional Regulation**: AXL expression is induced by various transcription factors (e.g., HIF-1α, NF-κB) in response to cellular stress, and is repressed by tumor suppressor pathways (e.g., p53). ### 3.4 Protein-Protein Interaction Networks The AXL interactome is complex and includes both activators and effectors. Key interacting proteins, as curated by BioGRID and STRING, include: - **GAS6**: Ligand. - **TUBB (β-tubulin)**: Interaction with the juxtamembrane domain may play a role in receptor trafficking. - **CBL**: E3 ubiquitin ligase involved in receptor downregulation. - **PIK3R1 (p85α)**: Regulatory subunit of PI3K. - **GRB2**: Adaptor protein linking to the RAS/MAPK pathway. - **SHC1**: Adaptor protein. - **PLCG1 (Phospholipase C-γ1)**: Hydrolyzes PIP2 to generate second messengers IP3 and DAG. - **SRC**: Non-receptor tyrosine kinase. - **JAK2**: Janus kinase. - **STAT3**: Signal transducer and activator of transcription. - **EGFR, HER2, MET**: Receptor tyrosine kinases that can heterodimerize with AXL. ```mermaid sequenceDiagram participant G6 as "GAS6 (Ligand)" participant AXL as "AXL Receptor (Dimer)" participant PI3K as "PI3K" participant AKT as "AKT" participant MTOR as "mTORC1" participant GRB2 as "GRB2/SOS" participant RAS as "RAS" participant RAF as "RAF" participant MEK as "MEK" participant ERK as "ERK" participant STAT as "STAT3" participant NFKB as "NF-κB" participant NUC as "Nucleus" G6->>AXL: Binds to Ig1 domain activate AXL AXL->>AXL: Dimerization & trans-phosphorylation AXL->>PI3K: Recruits p85 subunit activate PI3K PI3K->>AKT: Generates PIP3, recruits AKT activate AKT AKT->>MTOR: Phosphorylates & activates activate MTOR MTOR-->>NUC: Drives pro-survival translation deactivate MTOR deactivate AKT deactivate PI3K AXL->>GRB2: Recruits GRB2/SOS complex activate GRB2 GRB2->>RAS: Promotes GTP loading activate RAS RAS->>RAF: Activates activate RAF RAF->>MEK: Phosphorylates activate MEK MEK->>ERK: Phosphorylates activate ERK ERK-->>NUC: Translocates & drives proliferation genes deactivate ERK deactivate MEK deactivate RAF deactivate RAS deactivate GRB2 AXL->>STAT: Activates JAK/STAT pathway activate STAT STAT-->>NUC: Dimerizes & drives immune evasion genes deactivate STAT AXL->>NFKB: Activates IKK complex activate NFKB NFKB-->>NUC: Translocates & drives EMT genes deactivate NFKB deactivate AXL ``` --- ## 4. Pathogenic Hotspot Mutations & Clinical Differentials ### 4.1 Somatic Mutations in Cancer Unlike classical oncogenes such as *EGFR* or *KRAS*, AXL is not frequently mutated in cancer. Instead, its oncogenic activity is primarily driven by overexpression and/or aberrant activation. However, somatic mutations in AXL do occur and can contribute to tumor progression and drug resistance. These mutations are often subclonal and may be selected for during therapy. - **Extracellular Domain Mutations**: Missense mutations in the Ig-like domains can alter ligand binding affinity. For example, the mutation **S106F** has been identified in lung cancer and is predicted to increase GAS6 binding. Other mutations, such as **G194R**, may disrupt auto-inhibitory interactions and promote ligand-independent dimerization. - **Juxtamembrane Domain Mutations**: Mutations in this region can affect receptor internalization and degradation. The mutation **L529F** has been reported to increase receptor stability and surface expression. - **Kinase Domain Mutations**: These are the most functionally significant. Mutations in the activation loop, such as **Y821C** or **Y866C**, can create disulfide-bonded dimers, leading to constitutive activation. The gatekeeper residue **M598** is a hotspot for resistance mutations. The mutation **M598T** has been shown to confer resistance to type II inhibitors like cabozantinib by sterically hindering their binding while still allowing ATP access. - **Frameshift and Nonsense Mutations**: These are generally inactivating and are rarely selected for in cancer, as AXL's oncogenic role requires a functional kinase. However, they can occur in the extracellular domain, leading to a truncated, soluble form that may act as a dominant-negative. ### 4.2 Germline Polymorphisms and Disease Susceptibility Several single nucleotide polymorphisms (SNPs) in the AXL gene have been associated with disease susceptibility, although the effect sizes are generally modest. - **rs10521222 (c.841A>G, p.I281V)**: Located in the FNIII domain. This polymorphism has been associated with an increased risk of developing systemic lupus erythematosus (SLE) in some populations, possibly by affecting the clearance of apoptotic cells. - **rs11621263 (c.1888A>G, p.M630V)**: Located in the kinase domain. This variant has been studied for its association with venous thromboembolism, given the role of GAS6 in platelet function. - **rs1421023 (c.2002C>T, p.R668C)**: A rare variant that may affect kinase activity and has been reported in a small number of patients with neurodevelopmental disorders, though this association requires further validation. ### 4.3 AXL in Drug Resistance The role of AXL in acquired resistance to targeted therapies is a major area of clinical investigation. AXL overexpression is a well-established mechanism of resistance to: - **EGFR Inhibitors (e.g., Osimertinib) in NSCLC**: Upon EGFR inhibition, tumor cells upregulate AXL, which activates alternative survival pathways (e.g., PI3K/AKT) and bypasses the need for EGFR signaling. AXL also promotes a mesenchymal phenotype, which is intrinsically resistant to EGFR inhibitors. - **BRAF Inhibitors (e.g., Vemurafenib) in Melanoma**: AXL upregulation is a key driver of resistance to BRAF and MEK inhibitors. AXL signaling reactivates the MAPK pathway and promotes cell survival. - **HER2 Inhibitors (e.g., Trastuzumab) in Breast Cancer**: AXL overexpression is associated with trastuzumab resistance, mediated through the activation of the PI3K/AKT pathway. - **Anti-Angiogenic Therapies**: AXL can promote resistance to anti-VEGF therapies by activating alternative pro-angiogenic and invasive programs. ### 4.4 Clinical Differential and Prognostic Significance Elevated AXL expression, as measured by immunohistochemistry (IHC) or mRNA levels, is consistently associated with poor overall survival (OS) and progression-free survival (PFS) across multiple cancer types. It is also correlated with: - Higher tumor grade and stage. - Presence of metastasis at diagnosis. - Mesenchymal or basal-like molecular subtypes (e.g., triple-negative breast cancer, mesenchymal NSCLC). - Increased expression of immune checkpoint molecules, such as PD-L1. The soluble form of AXL (sAXL) in plasma is being evaluated as a minimally invasive biomarker. High baseline sAXL levels have been associated with poor response to immune checkpoint inhibitors, and dynamic changes in sAXL levels are being explored as an early indicator of treatment response to AXL-targeted therapies. --- ## 5. Host-Pathogen & Viral Interactions The AXL receptor is not only a driver of cancer but also serves as an entry receptor for several enveloped viruses. This dual role places AXL at the interface of oncology and infectious disease. ### 5.1 AXL as a Viral Entry Receptor AXL has been identified as a critical entry factor for multiple viruses, primarily by binding to phosphatidylserine (PS) present on the viral envelope. This mechanism is known as **apoptotic mimicry**, where the virus mimics an apoptotic body to gain entry into host cells. - **Zika Virus (ZIKV)**: AXL is a major entry receptor for ZIKV in multiple cell types, including skin fibroblasts, dendritic cells, and neural progenitor cells. The interaction is mediated by the binding of the viral envelope protein to AXL, likely via a PS-dependent mechanism. AXL expression in the developing brain is thought to contribute to the neuropathogenesis of congenital Zika syndrome. - **Ebola Virus (EBOV)**: AXL, along with other TAM receptors, facilitates the entry of EBOV into host cells. The virus uses PS on its envelope to engage AXL, promoting macropinocytosis and viral uptake. - **Dengue Virus (DENV)**: AXL has been shown to enhance DENV infection in a PS-dependent manner, particularly in myeloid cells. - **SARS-CoV-2**: While ACE2 is the primary receptor for SARS-CoV-2, AXL has been proposed as an alternative entry factor, particularly in cells with low ACE2 expression. AXL may facilitate viral entry via PS-mediated uptake, though its clinical significance remains under debate. ### 5.2 Viral Modulation of AXL Signaling Beyond serving as an entry portal, viruses can hijack AXL signaling to their advantage. For example, EBOV and ZIKV binding to AXL can suppress the host innate immune response. AXL activation leads to the induction of SOCS1 (suppressor of cytokine signaling 1) and SOCS3, which inhibit type I interferon (IFN) signaling. This dampens the antiviral response, allowing the virus to replicate more efficiently. This immune-suppressive function of AXL is also exploited by tumors to evade anti-tumor immunity. ### 5.3 Bacterial Interactions The role of AXL in bacterial infections is less well-characterized than its role in viral infections. However, some studies suggest that AXL can modulate the host response to bacterial pathogens. For instance, AXL has been shown to negatively regulate the inflammatory response to *Listeria monocytogenes* and *Salmonella* species by promoting the efferocytosis of infected apoptotic cells and by suppressing pro-inflammatory cytokine production. This can limit tissue damage but may also impair bacterial clearance. --- ## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors AXL is a high-value therapeutic target, and a substantial number of agents have been developed to inhibit its activity. These agents can be broadly classified into small-molecule tyrosine kinase inhibitors (TKIs), monoclonal antibodies (mAbs), and antibody-drug conjugates (ADCs). ### 6.1 Small-Molecule Tyrosine Kinase Inhibitors (TKIs) Many of the TKIs that inhibit AXL are multi-kinase inhibitors, meaning they also target other kinases such as MET, VEGFR2, and RET. This multi-targeted activity can be advantageous in cancer therapy, as it simultaneously blocks multiple pro-tumorigenic pathways. | **Drug** | **Targets** | **Development Stage** | **Key Notes** | | :--- | :--- | :--- | :--- | | **Cabozantinib (XL184)** | AXL, MET, VEGFR2, RET, KIT | FDA-approved (2012) for medullary thyroid cancer, renal cell carcinoma, and hepatocellular carcinoma | A potent type II inhibitor of AXL. Its anti-tumor activity is partly attributed to AXL inhibition, particularly in the context of resistance to other therapies. | | **Foretinib (GSK1363089)** | AXL, MET, VEGFR2, RON | Phase II clinical trials (discontinued for most indications) | A multi-kinase inhibitor with potent AXL activity. Showed promise in preclinical models of gastric cancer and NSCLC. | | **Gilteritinib (ASP2215)** | AXL, FLT3, ALK | FDA-approved (2018) for relapsed/refractory AML with FLT3 mutations | While primarily a FLT3 inhibitor, its activity against AXL may contribute to its efficacy in AML. | | **Bemcentinib (BGB324)** | AXL (selective) | Phase II clinical trials (NSCLC, AML, melanoma) | A highly selective, oral AXL inhibitor. It has shown promising activity in combination with immune checkpoint inhibitors and chemotherapy. | | **Sitravatinib (MGCD516)** | AXL, MET, VEGFR, PDGFR, KIT | Phase II/III clinical trials | A multi-targeted TKI that is being evaluated in combination with nivolumab (anti-PD-1) in NSCLC. | | **ONO-7475** | AXL, MERTK | Phase I/II clinical trials | A selective inhibitor of the TAM family kinases. | | **TP-0903** | AXL (and others) | Phase I/II clinical trials | An investigational inhibitor with potent activity against AXL, being tested in solid tumors and CLL. | ### 6.2 Monoclonal Antibodies and Antibody-Drug Conjugates Monoclonal antibodies offer the potential for greater selectivity than small-molecule TKIs and can engage immune effector functions. - **Bavituximab**: A chimeric monoclonal antibody that targets phosphatidylserine (PS), not AXL directly. However, it blocks the PS-AXL interaction and has been shown to inhibit AXL-mediated immune suppression. It is being evaluated in clinical trials for various solid tumors. - **Enapotamab vedotin (HuMax-AXL-ADC)**: An antibody-drug conjugate consisting of a human monoclonal antibody against AXL linked to the microtubule-disrupting agent monomethyl auristatin E (MMAE). It delivers a potent cytotoxic payload specifically to AXL-expressing tumor cells. It has been evaluated in Phase I/II trials. - **CAB-AXL-ADC**: A conditionally active biologic (CAB) antibody-drug conjugate that is designed to be activated only in the tumor microenvironment, potentially reducing on-target, off-tumor toxicity. ### 6.3 Pharmacogenomic Considerations The efficacy of AXL-targeted therapies is likely to be influenced by the genetic background of the tumor and the patient. - **AXL Expression Levels**: High AXL expression is a prerequisite for response to AXL-targeted agents. Therefore, AXL expression by IHC or mRNA is a key predictive biomarker. - **GAS6 Levels**: High levels of GAS6 in the tumor microenvironment may necessitate higher doses of AXL inhibitors or the use of agents that block the GAS6-AXL interaction. - **Co-occurring Mutations**: The presence of mutations in other signaling pathways (e.g., KRAS, EGFR, BRAF) may influence the response to AXL inhibitors. For example, AXL inhibition may be particularly effective in KRAS-mutant tumors that have developed resistance to MEK inhibitors. - **Soluble AXL (sAXL)**: High baseline levels of sAXL may act as a "sink" for AXL inhibitors, reducing their effective concentration at the tumor site. Monitoring sAXL levels during treatment may help guide dosing. --- ## 7. Bioinformatic Resources & Database Accessions The following table provides a comprehensive list of database accessions and bioinformatic resources for the AXL gene and protein. | **Database** | **Identifier / Accession** | **Description** | | :--- | :--- | :--- | | **HGNC** | 905 | HUGO Gene Nomenclature Committee symbol. | | **NCBI Gene** | 558 | Gene ID for AXL. | | **Ensembl** | ENSG00000167601 | Ensembl gene ID. | | **UniProtKB** | P30530 | Primary protein accession for human AXL. | | **RCSB PDB** | 2C5D, 4RA0, 5U6B | Representative crystal structures of the kinase domain and extracellular domain. | | **OMIM** | 109135 | Online Mendelian Inheritance in Man entry. | | **ClinVar** | Various | Database of clinically relevant human variants. | | **COSMIC** | AXL | Catalogue of Somatic Mutations in Cancer. | | **STRING** | 9606.ENSP00000360103 | Protein-protein interaction network. | | **BioGRID** | 108853 | Biological General Repository for Interaction Datasets. | | **PhosphoSitePlus** | AXL | Curated database of post-translational modifications. | | **The Human Protein Atlas** | ENSG00000167601 | Expression and localization data in normal and cancer tissues. | | **Gene Ontology (GO)** | GO:0004713 (protein tyrosine kinase activity), GO:0007169 (transmembrane receptor protein tyrosine kinase signaling pathway), GO:0005886 (plasma membrane) | Functional annotations. | | **KEGG Pathway** | hsa05200 (Pathways in cancer), hsa04510 (Focal adhesion) | Pathway maps. | | **Reactome** | R-HSA-8848021 (Signaling by AXL) | Curated pathway database. | --- ## Related Clinical & Scientific Guides * [PIK3CA (PI3K Alpha): Helical and Kinase Domain Hotspot Mutations and Isoform-Specific Inhibition](/knowledge/bioinformatics/genes/cancer-genomics/pik3ca-gene-structure-function-pathway) * [ENTPD5 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/entpd5-gene-structure-function-pathway) * [PDGFB Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/pdgfb-gene-structure-function-pathway) ## References 1. 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Axl receptor tyrosine kinase stimulated by the vitamin K-dependent protein encoded by growth-arrest-specific gene 6. *Nature*, 373(6515), 623–626. [https://doi.org/10.1038/373623a0](https://doi.org/10.1038/373623a0) 4. Sasaki, T., Knyazev, P. G., Clout, N. J., Cheburkin, Y., Göhring, W., Ullrich, A., Timpl, R., & Hohenester, E. (2006). Structural basis for Gas6-Axl signalling. *The EMBO Journal*, 25(1), 80–87. [https://doi.org/10.1038/sj.emboj.7600912](https://doi.org/10.1038/sj.emboj.7600912) 5. Zhang, Z., Lee, J. C., Lin, L., Olivas, V., Au, V., LaFramboise, T., Abdel-Rahman, M., Wang, X., Levine, A. D., Rho, J. K., Choi, Y. J., Choi, C. M., Kim, S. W., Jang, S. J., Park, Y. S., Kim, W. S., Lee, D. H., Lee, J. S., Miller, V. A., … Bivona, T. G. (2012). Activation of the AXL kinase causes resistance to EGFR-targeted therapy in lung cancer. *Nature Genetics*, 44(8), 852–860. [https://doi.org/10.1038/ng.2330](https://doi.org/10.1038/