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Mesothelin
Mesothelin
from Wikipedia
MSLN
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesMSLN, MPF, SMRP, mesothelin
External IDsOMIM: 601051; MGI: 1888992; HomoloGene: 4249; GeneCards: MSLN; OMA:MSLN - orthologs
Orthologs
SpeciesHumanMouse
Entrez
Ensembl
UniProt
RefSeq (mRNA)

NM_001177355
NM_005823
NM_013404

NM_018857
NM_001356286
NM_001374653

RefSeq (protein)

NP_001170826
NP_005814
NP_037536

NP_061345
NP_001343215

Location (UCSC)Chr 16: 0.76 – 0.77 MbChr 17: 25.97 – 25.97 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Mesothelin, also known as MSLN, is a protein that in humans is encoded by the MSLN gene.[5][6]

Function

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Mesothelin is a 40 kDa protein that is expressed in mesothelial cells.[7] The protein was first identified by its reactivity with monoclonal antibody K1.[8] Subsequent cloning studies showed that the mesothelin gene encodes a precursor protein that is processed to yield mesothelin which is attached to the cell membrane by a glycophosphatidylinositol linkage and a 31-kDa shed fragment named megakaryocyte-potentiating factor (MPF). Although it has been proposed that mesothelin may be involved in cell adhesion, its biological function is not known.[9][10] A knockout mouse line that lacks mesothelin reproduces and develops normally.[11]

Mesothelin is over expressed in several human tumors, including mesothelioma, ovarian cancer, pancreatic adenocarcinoma,[7] lung adenocarcinoma,[12] and cholangiocarcinoma.[13] Mesothelin binds MUC16 (also known as CA125), indicating that the interaction of mesothelin and MUC16 may contribute to the implantation and peritoneal spread of tumors by cell adhesion.[14] The region (residues 296-359) consisting of 64 amino acids at the N-terminus of cell surface mesothelin has been identified as the functional binding domain (named IAB) for MUC16/CA125, suggesting the mechanism of mesothelin acting as a MUC16/CA125 functional partner in cancer development.[15]

A protein structure model of human mesothelin and the binding sites of MUC16 (CA125) and antibodies[16]

Medical applications

[edit]

Mesothelin is a tumor differentiation antigen that is normally present on the mesothelial cells lining the pleura,[17] peritoneum and pericardium.[7] Since mesothelin is overexpressed in several cancers and is immunogenic, the protein could be exploited as tumor marker or as the antigenic target of a therapeutic cancer vaccine.[9][18] A 2016 review indicates that some immunotherapeutic strategies have shown encouraging results in early-phase clinical trials.[19] Elevations of serum mesothelin specific to ovarian and other cancer patients may be measured using ELISA assays.[20] Soluble mesothelin is identified as the extracellular domain of membrane-bound mesothelin shed from tumor cells according to the mass spectrometry analysis of soluble mesothelin purified from cell culture supernatant.[21]

Assays for blood-borne mesothelin and MPF for tumor diagnosis, especially applied to asbestos-related mesothelioma have been developed.[22] Elevated serum mesothelin was found in most patients with mesothelioma (71%) and ovarian cancer (67%).[23] Blood MPF and mesothelin levels were correlated, with modest accuracy for malignant pleural mesothelioma and lung cancer (sensitivity 74% and 59%, specificity 90% and 86%, respectively for MPF and mesothelin assays).[24] Circulating mesothelin is reported in nearly all pancreatic cancers,[25] however the levels in healthy persons often exceed 80 ng/mL (using 40 kD molecular weight as the conversion factor) and to widely overlap the values in the pancreatic cancer patients.[26] It was noted that the cutoff levels for normal could differ as much as 10-fold among publications, depending on the assay used[26][24][23] and thus that normal levels must be determined anew when new assays are introduced. Increase of mesothelin-specific antibodies were also detected in the sera of about 40% of patients with mesothelioma and 42% with ovarian cancer, indicating an antibody response to mesothelin was correlated with high expression of mesothelin on tumor cells.[27]

Human monoclonal antibodies HN1 and SD1 targeting mesothelin have been isolated by phage display.[28][29] Mitchell Ho and Ira Pastan at the U.S. National Institutes of Health (NIH) generated rabbit monoclonal antibodies targeting rare and poorly immunogenic epitopes of mesothelin, including the C terminus recognized by the YP218 antibody.[16] The rabbit antibodies have been "humanized" by Ho and Zhang using human immunoglobulin germline framework sequences for CDR grafting based on computational structure modeling.[30] The CAR-T cells derived from the humanized YP218 antibody (hYP218) effectively inhibit the growth of human xenograft tumors in mice.[31][32]

References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia
from Grokipedia
Mesothelin is a (GPI)-anchored that serves as a cell surface , primarily expressed on mesothelial cells lining the pleura, , and . Originally identified in 1992 as a differentiation associated with mesotheliomas and ovarian carcinomas, it is encoded by the MSLN gene on chromosome 16p13.3 and plays a role in and signaling. In normal physiology, its expression is restricted to these mesothelial tissues, where it contributes to maintaining epithelial integrity, though its precise endogenous function remains partially elusive. The protein is synthesized as a 69–71 precursor that undergoes furin-mediated cleavage to yield a 31 soluble fragment known as megakaryocyte potentiating factor (MPF) and a 40 membrane-bound mature mesothelin, which anchors to the cell surface via GPI linkage. Structurally, mature mesothelin features a compact, right-handed superhelical domain composed of 24 short α-helices arranged in nine layered /HEAT-like repeats, stabilized by bonds and four N-linked sites that enhance its stability and shedding. This shedding produces a soluble form detectable in serum, which has diagnostic potential in certain malignancies. In cancer biology, mesothelin is markedly overexpressed in aggressive tumors, including >90% of mesotheliomas, ~85–95% of ovarian and pancreatic adenocarcinomas, and subsets of and gastric cancers, where it promotes tumor progression through enhanced , , and . It facilitates peritoneal dissemination by binding to the MUC16/CA-125 on tumor cells, activating pro-survival pathways such as , PI3K/Akt, and ERK1/2, and conferring resistance to agents like . Genetic polymorphisms in MSLN have been linked to increased cancer risk and poor in some cohorts, underscoring its oncogenic role. Due to its restricted normal expression and high tumor specificity, mesothelin has emerged as a prime target for anticancer therapies, including monoclonal antibodies (e.g., amatuximab/MORAb-009), immunotoxins (e.g., SS1P), and chimeric receptor () T-cell therapies, with multiple agents advancing through clinical trials for and . These approaches leverage (ADCC) and blockade of mesothelin-MUC16 interactions to inhibit tumor growth, though challenges like antigen shedding and heterogeneous expression persist. Ongoing focuses on combination strategies to enhance efficacy and overcome resistance.

Discovery and Molecular Structure

Discovery

Mesothelin was first identified in by researchers at the (NCI) through the development of a , K1, generated by immunizing mice with human ovarian cells. This antibody specifically recognized a 40-kDa glycosylated on the cell surface of ovarian cancers and normal mesothelial cells lining the pleura, , and . Immunohistochemical analysis revealed that the K1 was expressed on normal but absent from most other normal adult tissues, highlighting its potential as a differentiation marker. The of the mesothelin (MSLN) was achieved in 1996 by Kai Chang and Ira Pastan at the NCI, using a cDNA expression derived from the ovarian cell line OVCAR-3. They screened the with the K1 , isolating a full-length cDNA encoding a 69-kDa precursor protein that undergoes proteolytic processing and to produce the mature 40-kDa mesothelin antigen. analysis confirmed MSLN mRNA expression in normal and various cancers, consistent with the antibody's reactivity pattern. These foundational studies by Chang, Pastan, and colleagues at the NCI established mesothelin as a cell surface of interest in mesothelial and , paving the way for subsequent research.

Gene and Protein Structure

The , which encodes mesothelin, is located on the short arm of human at position 16p13.3. It spans approximately 8.1 kb of genomic DNA and consists of 18 exons, producing multiple transcript variants through . The predominant transcript, NM_005823.6, encodes a preproprotein precursor of 622 with a predicted molecular weight of approximately 71 kDa. This precursor undergoes furin-mediated proteolytic processing at residue R295, yielding an N-terminal megakaryocyte-potentiating factor (MPF) of about 31 kDa (residues 1–295) and a C-terminal mesothelin fragment of approximately 40 kDa (residues 296–622). The mature mesothelin protein is a (GPI)-anchored tethered to the cell surface, lacking a transmembrane or intracellular domain. Its structure features a compact, right-handed fold composed of 24 short alpha-helices and connecting loops, as revealed by crystallographic studies. A key functional region is the N-terminal domain spanning residues 296–359, which forms a conformation-sensitive structure essential for protein-protein interactions. Post-translational modifications include N-linked at four potential sites (Asn57, Asn388, Asn488, and Asn515 in the precursor), which contribute to the protein's stability and shedding. The GPI anchor is added post-translationally at the (after residue 598), facilitating attachment, while proteolytic cleavage or activity enables release of soluble mesothelin (sMSLN) into the . Evolutionarily, mesothelin exhibits high conservation across mammals, with to related GPI-anchored proteins such as otoancorin and stereocilin, which are involved in function and share structural motifs in their extracellular domains. This family resemblance underscores mesothelin's role in and signaling, though its precise evolutionary origin traces back to a common of these proteins in vertebrates.

Expression Patterns

In Normal Tissues

Mesothelin, encoded by the MSLN gene, exhibits highly restricted expression in normal human tissues, primarily localized to mesothelial cells lining the pleura, , and . This tissue-specific pattern underscores its role as a differentiation in these serosal membranes, where it is abundantly present on the cell surface. Low-level expression of mesothelin has been detected in select non-mesothelial tissues, including the squamous of tonsils, epithelial cells of the trachea, and the epithelium. In contrast, mesothelin is absent or present at negligible levels in the parenchyma of most vital organs, such as the , liver, and kidney. The expression of mesothelin is tightly regulated at the transcriptional level through its promoter region, which spans approximately 1.85 kb upstream of the transcriptional start site and lacks a but contains tissue-specific enhancer elements that drive mesothelium-restricted activity. In healthy individuals, circulating soluble mesothelin levels are minimal, with median serum concentrations typically below 1 nmol/L, reflecting the limited shedding from normal tissues. Studies in mesothelin knockout mice have demonstrated that the protein is not essential for normal development, , or , as homozygous null animals exhibit no overt phenotypic abnormalities and maintain viable counts and organ function.

In Cancer

Mesothelin is aberrantly overexpressed in various malignant tissues, distinguishing it from its restricted expression in normal mesothelial cells. Expression rates vary by study, tumor subtype, and detection method. In mesotheliomas, overexpression occurs in 70–100% of cases, particularly in the epithelioid subtype where it approaches 95-100%. Similarly, 60–90% of ovarian cancers exhibit significant mesothelin expression, while pancreatic ductal adenocarcinomas show levels around 85-100%. Lung adenocarcinomas display mesothelin in about 69% of tumors, and cholangiocarcinomas have notable expression in approximately 32% of cases, often focally in tumor cells. Elevated mesothelin expression correlates with advanced tumor stages and poorer prognosis in several cancers. For instance, in advanced ovarian cancer, high mesothelin levels are associated with chemoresistance and reduced survival rates. In pancreatic and lung cancers, particularly those with KRAS mutations, increased expression aligns with more aggressive disease progression. Mechanisms driving this upregulation include promoter hypomethylation, which leads to enhanced gene transcription in tumor cells, as observed in gynecological malignancies. Gene amplification of the MSLN locus contributes in select tumor types, such as esophageal adenocarcinomas. Additionally, oncogenic signaling pathways, exemplified by KRAS activation in pancreatic cancer, promote mesothelin overexpression through downstream transcriptional regulation. Shed soluble mesothelin (sMSLN), a fragment released from the cell surface, is elevated in the sera of patients with mesothelin-expressing cancers, including , ovarian, and pancreatic types, reflecting tumor burden. Recent findings from indicate mesothelin expression in rheumatoid arthritis-associated destruction, a non-malignant but tumor-like pathological process involving regulation.

Biological Functions

Physiological Roles

Mesothelin, a glycosylphosphatidylinositol-anchored expressed on the surface of mesothelial cells lining serosal cavities, has been proposed to play a role in protecting these surfaces from mechanical friction and abrasion during organ movement. This protective function is hypothesized based on its abundant localization in normal of the pleura, , and , where it may facilitate adhesion to the , potentially acting as a lectin-like to anchor cells and maintain tissue integrity. However, direct experimental evidence for this role remains limited, as mesothelin's precise contribution to serosal lubrication or repair processes has not been definitively established . In the female reproductive tract, mesothelin interacts with MUC16 (also known as CA125), a expressed on epithelial surfaces, to mediate cell-cell that may contribute to mucosal protection. This high-affinity, N-glycan-dependent binding, with a of approximately 5-10 nM, supports heterotypic between mesothelin-expressing cells and MUC16-positive epithelia, potentially aiding in the barrier function of reproductive tissues. Although primarily studied in pathological contexts, the interaction's presence in normal and endometrial cells suggests a role in maintaining mucosal integrity, though this remains speculative without targeted functional studies. Studies using mesothelin (MSLN) mice demonstrate that the protein is not essential for normal development, , or , as homozygous mutants exhibit no detectable abnormalities in growth, platelet counts, or reproductive capabilities compared to wild-type littermates. These findings indicate that mesothelin provides non-critical supportive functions in healthy tissues rather than indispensable roles. Additionally, the N-terminal fragment of the mesothelin precursor, known as megakaryocyte potentiation factor (MPF), enhances the effects of interleukin-3 (IL-3) and IL-6 on colony formation and maturation , potentially contributing to platelet production under normal physiological conditions. However, MPF lacks intrinsic colony-stimulating activity and its potentiating effects in human megakaryopoiesis have not been confirmed . Data on mesothelin's involvement in signaling pathways within normal cells are sparse, with evidence suggesting minimal activation of pathways such as or PI3K/Akt in non-transformed mesothelial cells, contrasting with robust signaling observed in malignant contexts. This limited signaling profile aligns with mesothelin's non-essential nature, implying subtle modulatory roles in and responsiveness rather than driving proliferation or in healthy tissues.

Role in Tumorigenesis

Mesothelin plays a pivotal role in promoting the , , and of tumor cells, primarily through its high-affinity interaction with MUC16 (also known as CA125), a overexpressed on the surface of many cancer cells. This binding facilitates the attachment of tumor cells to mesothelial surfaces in the , enabling peritoneal commonly observed in ovarian and s. Studies have demonstrated that disrupting this mesothelin-MUC16 interaction significantly reduces cell motility and by inhibiting the expression of matrix metalloproteinase-7 (MMP-7), which degrades the to support tumor spread. In models, coexpression of mesothelin and MUC16 correlates with increased metastatic potential, underscoring the complex's contribution to aggressive disease progression. Mesothelin further enhances tumor cell survival by conferring resistance to chemotherapy, particularly platinum-based agents like in , through activation of pro-survival signaling pathways. High mesothelin expression has been linked to reduced sensitivity to , leading to poorer patient outcomes, as it promotes anti-apoptotic mechanisms that allow cancer cells to evade drug-induced . In high-grade serous , mesothelin mediates this resistance by upregulating survival signals, including those involving the PI3K/AKT pathway, which sustains tumor cell viability under chemotherapeutic stress. Knocking down mesothelin in models restores sensitivity, highlighting its mechanistic role in chemoresistance across mesothelin-expressing malignancies. At the molecular level, mesothelin activates key oncogenic pathways that drive tumorigenesis, including , PI3K/AKT, and MMP-mediated remodeling. In , mesothelin triggers and PI3K/AKT signaling to inhibit TNF-α-induced , upregulating anti-apoptotic proteins like IL-6 and Mcl-1, thereby enhancing cell and proliferation. This pathway also promotes MMP expression, facilitating tumor by breaking down basement membranes and stromal barriers. Mesothelin overexpression in gastrointestinal cancers similarly stimulates PI3K/AKT and MAPK/ERK cascades, contributing to resistance against apoptosis-inducing agents like . Mesothelin contributes to cancer stemness and , particularly in pancreatic ductal and , fostering tumor initiation and vascular support for growth. In these models, mesothelin induces epithelial-mesenchymal transition (EMT) and stem cell-like traits, such as increased self-renewal and invasiveness, which blockage with antibodies like amatuximab reverses, reducing stemness markers and enhancing sensitivity. For , secreted mesothelin from cells recruits macrophages to secrete VEGFA and S100A9, promoting vascularization in metastatic lesions and supporting tumor colonization.

Clinical Significance

As a Diagnostic Biomarker

Mesothelin, particularly in its soluble form known as soluble mesothelin-related (sMRP or SMRP), has been established as a serum for detecting mesothelin-expressing cancers, with elevated levels exceeding 2-3 nM commonly observed in patients with malignant pleural and epithelial . In , sMRP demonstrates a pooled sensitivity of 61% (95% CI: 58-63%) and specificity of 87% (95% CI: 86-88%) for , while in , sensitivities range from 40% to 82% at high specificity thresholds, often outperforming standalone use in early-stage detection when combined with . The primary assay for measuring sMRP is the enzyme-linked immunosorbent assay (ELISA), such as the FDA-approved MESOMARK kit, which quantifies levels in serum with cutoff values typically ranging from 0.93 to 2.0 nM depending on laboratory protocols and population studied. Alternative assays, like those for megakaryocyte potentiating factor (MPF)—a related mesothelin fragment—are less commonly employed due to comparable but not superior performance. These assays facilitate non-invasive monitoring, though inter-laboratory variability in cutoffs can affect reproducibility. Elevated sMRP levels also hold prognostic significance, correlating with advanced disease stage, metastatic spread, and poorer survival outcomes in both ovarian and pancreatic cancers. In ovarian cancer, high preoperative sMRP is independently associated with chemoresistance and reduced progression-free survival. Similarly, in pancreatic cancer, increased mesothelin expression links to tumor aggressiveness and diminished overall survival. Combining sMRP with other markers enhances diagnostic accuracy; for instance, pairing it with CA125 in improves sensitivity at 98% specificity, reaching up to 67% for high-risk populations. However, limitations include variability across platforms and false-positive results in conditions like renal impairment, where reduced clearance elevates sMRP independently of . As of 2025, sMRP shows significant differential expression in patient cohorts and has been evaluated in multi-biomarker screening panels for early detection using machine learning-based models alongside markers like , though it was not included in the optimal panel identified in recent studies.

Therapeutic Applications

Mesothelin-targeted therapies have emerged as a promising approach for treating cancers with high mesothelin expression, such as , ovarian, and s, primarily through antibody-based, cellular, and modalities. These strategies leverage mesothelin's restricted expression in normal tissues to minimize off-target effects while exploiting its overexpression in tumors. Monoclonal antibodies against mesothelin, such as amatuximab (MORAb-009), a chimeric anti-mesothelin antibody, have been evaluated in phase II trials for advanced unresectable pleural , often in combination with and . Amatuximab demonstrated tolerability at doses up to 200 mg/m², with 90% of patients achieving objective responses or stable disease, though it did not significantly extend overall survival compared to alone. Antibody-drug conjugates like anetumab ravtansine, which links an anti-mesothelin antibody to a maytansinoid , have shown robust antitumor activity in preclinical models of and . In phase II trials, anetumab ravtansine showed similar to vinorelbine (4.3 months vs. 4.5 months) with manageable toxicity in relapsed , but failed to outperform in high-grade serous as of 2025. Chimeric antigen receptor (CAR) T-cell therapies targeting mesothelin, such as anti-MSLN CAR-T cells (e.g., CART-meso), have been tested in phase I trials for solid tumors including and . These therapies demonstrate a favorable safety profile with regional administration reducing systemic toxicity, but efficacy remains limited due to on-target effects on normal , leading to pulmonary complications in some cases. For instance, mesothelin affinity-tuned CAR-T cells have shown enhanced persistence and reduced toxicity in preclinical models by mitigating high-affinity binding issues observed in earlier trials. Ongoing trials, such as those with FC004 for platinum-resistant , report preliminary antitumor activity but highlight challenges with T-cell exhaustion. Immunotoxins like SS1P, comprising an anti-mesothelin disulfide-stabilized Fv fragment fused to A (PE38), were developed to deliver cytotoxic payloads directly to mesothelin-expressing cells. Phase I trials established a maximum tolerated dose with of tumor regression in and patients, but development was halted due to and vascular leak . Insights from SS1P have informed next-generation immunotoxins, such as LMB-100, which exhibit reduced antigenicity while retaining efficacy in preclinical studies. Vaccine approaches include CRS-207, a live-attenuated engineered to express mesothelin, which stimulates both innate and adaptive immunity. In a phase II trial for metastatic , CRS-207 combined with GVAX vaccine and improved median overall survival to 6.1 months versus 3.9 months with chemotherapy alone, though a subsequent phase IIb study did not meet its primary endpoint. As of 2025, emerging therapies include bispecific antibodies and engineered scaffold proteins. Bispecific T-cell engagers like CT-95, a humanized anti-mesothelin construct, are in phase I trials for mesothelin-expressing advanced cancers, showing promising safety and T-cell activation in preclinical data. Similarly, JNJ-79032421, a mesothelin-targeted bispecific, is advancing in phase I/II studies for solid tumors. Engineered Fn3 scaffold proteins, non-antibody variants of type III domains, bind mesothelin with high affinity for targeted delivery of therapeutics, demonstrating internalization and enhanced cytotoxicity in models. These scaffolds offer advantages in stability and reduced over traditional antibodies. Ongoing challenges in mesothelin-targeted therapies include tumor heterogeneity leading to variable expression and loss, development of resistance mechanisms such as immune evasion, and on-target affecting mesothelial cells. Strategies to address these, including combination therapies and patient selection via assays, are under investigation to improve clinical outcomes.

References

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