Pleural mesothelioma (PMe): The evolving molecular knowledge of a rare and aggressive cancer

Mesothelioma is a type of late-onset cancer that develops in cells covering the outer surface of organs. Although it can affect the peritoneum, heart, or testicles, it mainly targets the lining of the lungs, making pleural mesothelioma (PMe) the most common and widely studied mesothelioma type. PMe is caused by exposure to fibres of asbestos, which when inhaled leads to inflammation and scarring of the pleura. Despite the ban on asbestos by most Western countries, the incidence of PMe is on the rise, also facilitated by a lack of specific symptomatology and diagnostic methods. Therapeutic options are also limited to mainly palliative care, making this disease untreatable. Here we present an overview of biological aspects underlying PMe by listing genetic and molecular mechanisms behind its onset, aggressive nature, and fast-paced progression. To this end, we report on the role of deubiquitinase BRCA1-associated protein-1 (BAP1), a tumour suppressor gene with a widely acknowledged role in the corrupted signalling and metabolism of PMe. This review aims to enhance our understanding of this devastating malignancy and propel efforts for its investigation.

Preclinical evaluation of CDK4 phosphorylation predicts high sensitivity of pleural mesotheliomas to CDK4/6 inhibition

Malignant pleural mesothelioma (MPM) is an aggressive cancer with limited therapeutic options. We evaluated the impact of CDK4/6 inhibition by palbociclib in 28 MPM cell lines including 19 patient-derived ones, using various approaches including RNA-sequencing. Palbociclib strongly and durably inhibited the proliferation of 23 cell lines, indicating a unique sensitivity of MPM to CDK4/6 inhibition. When observed, insensitivity to palbociclib was mostly explained by the lack of active T172-phosphorylated CDK4. This was associated with high p16INK4A (CDKN2A) levels that accompany RB1 defects or inactivation, or (unexpectedly) CCNE1 overexpression in the presence of wild-type RB1. Prolonged palbociclib treatment irreversibly inhibited proliferation despite re-induction of cell cycle genes upon drug washout. A senescence-associated secretory phenotype including various potentially immunogenic components was irreversibly induced. Phosphorylated CDK4 was detected in 80% of 47 MPMs indicating their sensitivity to CDK4/6 inhibitors. Its absence in some highly proliferative MPMs was linked to very high p16 (CDKN2A) expression, which was also observed in public datasets in tumours from short-survival patients. Our study supports the evaluation of CDK4/6 inhibitors for MPM treatment, in monotherapy or combination therapy.

YB-1 regulates mesothelioma cell migration via snail but not EGFR, MMP1, EPHA5 or PARK2

Pleural mesothelioma (PM) is characterized by rapid growth, local invasion, and limited therapeutic options. The multifunctional oncoprotein Y-box-binding protein-1 (YB-1) is frequently overexpressed in cancer and its inhibition reduces aggressive behavior in multiple tumor types. Here, we investigated the effects of YB-1 on target gene regulation and PM cell behavior. Whereas siRNA-mediated YB-1 knockdown reduced cell motility, YB-1 overexpression resulted in scattering, increased migration, and intravasation in vitro. Furthermore, YB-1 stimulated PM cell spreading in zebrafish. Combined knockdown and inducible overexpression of YB-1 allowed bidirectional control and rescue of cell migration, the pattern of which was closely followed by the mRNA and protein levels of EGFR and the protein level of snail, whereas the mRNA levels of MMP1, EPHA5, and PARK2 showed partial regulation by YB-1. Finally, we identified snail as a critical regulator of YB-1-mediated cell motility in PM. This study provides insights into the mechanism underlying the aggressive nature of PM and highlights the important role of YB-1 in this cancer. In this context, we found that YB-1 closely regulates EGFR and snail, and, moreover, that YB-1-induced cell migration depends on snail.

Comparison of Immunohistochemistry, Next-generation Sequencing and Fluorescence In Situ Hybridization for Detection of MTAP Loss in Pleural Mesothelioma

9p21 deletions involving MTAP/CDKN2A genes are detected in diffuse pleural mesotheliomas (DPM) but are absent in benign mesothelial proliferations. Loss of MTAP expression by immunohistochemistry (IHC) is well accepted as a surrogate for 9p21 deletion to support a diagnosis of DPM. Accurate interpretation can be critical in the diagnosis of DPM, but variations in antibody performance may impact interpretation. The objectives of this study were to compare the performance of MTAP monoclonal antibodies (mAbs) EPR6893 and 1813 and to compare MTAP expression by IHC with 9p21 copy number status in DPM. Cytoplasmic expression of MTAP IHC with mAbs EPR6893 (ab126770; Abcam) and 1813 (NBP2-75730, Novus Biologicals) was evaluated in 56 DPM (47 epithelioid, 7 biphasic, and 2 sarcomatoid) profiled by targeted next-generation sequencing. 9p21 Copy number status was assessed by Fraction and Allele-Specific Copy Number Estimates from Tumor Sequencing (FACETS) analysis and also by CDKN2A fluorescence in situ hybridization in discrepant cases when material was available. MTAP mAb 1813 showed stronger immunoreactivity, more specific staining, and no equivocal interpretations compared to mAb EPR6893 which showed equivocal staining in 19 (34%) of cases due to weak or heterogenous immunoreactivity, lack of definitive internal positive control, and/or nonspecific background staining. MTAP expression with mAb 1813 showed near perfect agreement with 9p21 copy number by combined FACETS/fluorescence in situ hybridization calls (κ = 0.85; 95% CI, 0.71-0.99; P < .001). MTAP IHC with mAb 1813 was 96% sensitive, 86% specific, and 93% accurate for 9p21 homozygous deletion. The findings of this study suggest that interpretation of MTAP IHC is improved with mAb 1813 because mAb EPR6893 was often limited by equivocal interpretations. We show that MTAP IHC and molecular assays are complementary in detecting 9p21 homozygous deletion. MTAP IHC may be particularly useful for low tumor purity samples and in low-resource settings.

p53 dry gene powder enhances anti-cancer effects of chemotherapy against malignant pleural mesothelioma

Dry gene powder is a novel non-viral gene-delivery system, which is inhalable with high gene expression. Previously, we showed that the transfection of p16INK4a or TP53 by dry gene powder resulted in growth inhibitions of lung cancer and malignant pleural mesothelioma (MPM) in vitro and in vivo. Here, we report that dry gene powder containing p53- expression-plasmid DNA enhanced the therapeutic effects of cisplatin (CDDP) against MPM even in the presence of endogenous p53. Furthermore, our results indicated that the safe transfection with a higher plasmid DNA (pDNA) concentration suppressed MPM growth independently of chemotherapeutic agents. To develop a new therapeutic alternative for MPM patients without safety concerns over "vector doses", our in vitro data provide basic understandings for dry gene powder.

POLD1 Is Required for Cell Cycle Progression by Overcoming DNA Damage in Malignant Pleural Mesothelioma

BACKGROUND/AIM

The prognosis of patients with malignant pleural mesothelioma (MPM) remains poor due to lack of effective therapeutic targets. DNA damage caused by long-time exposure to asbestos fibers has been associated with the development of MPM, with mutations at genes encoding DNA damage repair (DDR)-related molecules frequently expressed in patients with MPM. The present study was designed to identify novel therapeutic targets in MPM using large public databases, such as The Cancer Genome Atlas (TCGA) and Genotype Tissue Expression project (GTEx) focused on DDR pathways.

MATERIALS AND METHODS

The correlations between mRNA expression levels of DDR-related genes and overall survival (OS) were analyzed in mesothelioma patients in TCGA mesothelioma (TCGA-MESO) datasets. The anti-tumor effects of small interfering RNAs (siRNA) against DDR-related genes associated with OS were subsequently tested in MPM cell lines.

RESULTS

High levels of mRNA encoding DNA polymerase delta 1, catalytic subunit (POLD1) were significantly associated with reduced OS in patients with MPM (p<0.001, Log-rank test). In addition, siRNA targeting POLD1 (siPOLD1) caused cell cycle arrest at the G1/S checkpoint and induced apoptosis involving accumulation of DNA damage in MPM cell lines.

CONCLUSION

POLD1 plays essential roles in overcoming DNA damage and cell cycle progression at the G1/S checkpoint in MPM cells. These findings suggest that POLD1 may be a novel therapeutic target in MPM.

Genes and Pathways Involved in the Progression of Malignant Pleural Mesothelioma: A Meta-analysis of Genome-Wide Expression Studies

Malignant pleural mesothelioma (MPM) is a rare and aggressive neoplasm of the pleural tissue that lines the lungs and is mainly associated with long latency from asbestos exposure. This tumor has no effective therapeutic opportunities nowadays and has a very low five-year survival rate. In this sense, identifying molecular events that trigger the development and progression of this tumor is highly important to establish new and potentially effective treatments. We conducted a meta-analysis of genome-wide expression studies publicly available at the Gene Expression Omnibus (GEO) and ArrayExpress databases. The differentially expressed genes (DEGs) were identified, and we performed functional enrichment analysis and protein-protein interaction networks (PPINs) to gain insight into the biological mechanisms underlying these genes. Additionally, we constructed survival prediction models for selected DEGs and predicted the minimum drug inhibition concentration of anticancer drugs for MPM. In total, 115 MPM tumor transcriptomes and 26 pleural tissue controls were analyzed. We identified 1046 upregulated DEGs in the MPM samples. Cellular signaling categories in tumor samples were associated with the TNF, PI3K-Akt, and AMPK pathways. The inflammatory response, regulation of cell migration, and regulation of angiogenesis were overrepresented biological processes. Expression of SOX17 and TACC1 were associated with reduced survival rates. This meta-analysis identified a list of DEGs in MPM tumors, cancer-related signaling pathways, and biological processes that were overrepresented in MPM samples. Some therapeutic targets to treat MPM are suggested, and the prognostic potential of key genes is shown.

Single-cell transcriptomic analysis of human pleura reveals stromal heterogeneity and informs in vitro models of mesothelioma

The pleural lining of the thorax regulates local immunity, inflammation and repair. A variety of conditions, both benign and malignant, including pleural mesothelioma, can affect this tissue. A lack of knowledge concerning the mesothelial and stromal cells comprising the pleura has hampered the development of targeted therapies. Here, we present the first comprehensive single-cell transcriptomic atlas of the human parietal pleura and demonstrate its utility in elucidating pleural biology. We confirm the presence of known universal fibroblasts and describe novel, potentially pleural-specific, fibroblast subtypes. We also present transcriptomic characterisation of multiple in vitro models of benign and malignant mesothelial cells, and characterise these through comparison with in vivo transcriptomic data. While bulk pleural transcriptomes have been reported previously, this is the first study to provide resolution at the single-cell level. We expect our pleural cell atlas will prove invaluable to those studying pleural biology and disease. It has already enabled us to shed light on the transdifferentiation of mesothelial cells, allowing us to develop a simple method for prolonging mesothelial cell differentiation in vitro.

Genomic profiling of tissue and blood predicts survival outcomes in patients with resected pleural mesothelioma

PURPOSE

Pleural mesothelioma (PM) is an aggressive tumor still considered incurable, in part due to the lack of predictive biomarkers. Little is known about the clinical implications of molecular alterations in resectable PM tissues and blood. Here, we characterized genetic alterations to identify prognostic and predictive biomarkers in patients with resected PM.

EXPERIMENTAL DESIGN

Targeted next-generation sequencing was performed in retrospective pleural tumor tissue and paired plasma samples from stage IB-IIIB resected PM. Association between prognosis and presence of specific mutations was validated in silico.

RESULTS

Thirty PM tissues and paired blood samples from 12 patients were analyzed. High tissue tumor mutational burden (TMB) (>10 mutations/Mb), tissue median minor allele frequency (MAF) (>9 mutations/Mb), and blood TMB (>6 mutations/Mb), tissue KMT2C, PBRM1, PKHD1,EPHB1 and blood LIFR mutations correlated with longer disease-free survival and/or overall survival. High concordance (>80%) between tissue and blood was found for some mutations.

CONCLUSIONS

Tissue TMB and MAF, blood TMB, and specific mutations correlated with outcomes in patients with resected PM and should be further studied to validate their role as prognostic biomarkers and potentially predictive factors for combinations with immune-checkpoint inhibitors. This suggest that molecular profiling could identify longer survivors in patients with resected PM.

LAG3 is an independent prognostic biomarker and potential target for immune checkpoint inhibitors in malignant pleural mesothelioma: a retrospective study

BACKGROUND

Lymphocyte-activation gene 3 (LAG3) is an immune checkpoint receptor; novel LAG3 immune checkpoint inhibitors (ICIs) exhibit therapeutic activity in melanoma. The role of LAG3and ICIs of LAG3 are unknown in malignant pleural mesothelioma (MPM). This study aimed to uncover the prognostic landscape of LAG3 in multiple cancers and investigate the potential of using LAG3 as an ICIs target in patients with MPM.

METHODS

We used The Cancer Genome Atlas (TCGA) cohort for assessing mRNA expression and our cohort for immunohistochemical expression. TCGA cohort were analyzed using the Wilcoxon rank-sum test to compare mRNA expression between normal and tumor tissues in multiple cancers. We used 86 MPM cases from TCGA and 38 MPM cases from our cohort to analyze the expression of LAG3 in tumor-infiltrating lymphocytes. The mean LAG3 mRNA expression was set as the cut-off and samples were classified as positive/negative for immunohistochemical expression. Overall survival (OS) of patients with MPM was determined using the Kaplan-Meier method based on LAG3 mRNA and immunohistochemical expression. OS analysis was performed using the multivariate Cox proportional hazards model. The correlation of LAG3 expression and mRNA expression of tumor immune infiltration cells (TIICs) gene markers were estimated using Spearman correlation. To identify factors affecting the correlation of LAG3 mRNA expression, a multivariate linear regression model was performed.

RESULTS

LAG3 mRNA was associated with prognosis in multiple cancers. Elevated LAG3 mRNA expression was correlated with a better prognosis in MPM. LAG3 expression was detected immunohistochemically in the membrane of infiltrating lymphocytes in MPM. LAG3 immunohistochemical expression was correlated with a better prognosis in MPM. The multivariate Cox proportional hazards model revealed that elevated LAG3 immunohistochemical expression indicated a better prognosis. In addition, LAG3 mRNA expression was correlated with the expression of various gene markers of TIICs, the most relevant to programmed cell death 1 (PD-1) with the multivariate linear regression model in MPM.

CONCLUSIONS

LAG3 expression was correlated with prognosis in multiple cancers, particularly MPM; LAG3 is an independent prognostic biomarker of MPM. LAG3 regulates cancer immunity and is a potential target for ICIs therapy. PD-1 and LAG3 inhibitors may contribute to a better prognosis in MPM.

TRIAL REGISTRATION

This study was registered with UMIN000049240 (registration day: August 19, 2022) and approved by the Institutional Review Board (approval date: August 22, 2022; approval number: 2022-0048) at Tokyo Women's Medical University.

Genomic Landscape of Pleural Mesothelioma and Therapeutic Aftermaths

PURPOSE OF REVIEW

In this article, we provide a comprehensive analysis of recent progress in the genetic characterisation of pleural mesothelioma, and the translation of these findings to clinical practice.

RECENT FINDINGS

Advancements in sequencing technology have allowed the identification of driver mutations and improved our understanding of how these mutations may shape the mesothelioma tumour microenvironment. However, the identification of frequently mutated regions including CDKN2A, BAP1 and NF2 have, to date, not yet yielded targeted therapy options that outperform standard chemo- and immunotherapies. Similarly, the association between mutational profile and the immune microenvironment or immunotherapy response is not well characterised. Further research into the link between tumour mutational profile and response to therapy is critical for identifying targetable vulnerabilities and stratifying patients for therapy.

Sarcomatoid mesothelioma diagnosed in a patient with mesothelioma in situ: a case report on morphologic differences after 9-month interval with details analysis of cytology in early-stage mesothelioma

BACKGROUND

Overlapping morphological features of mesothelial cells have been rendered it difficult to distinguish between reactive and malignant conditions. The development of methods based on detecting genomic abnormalities using immunohistochemistry and fluorescence in situ hybridization have contributed markedly to solving this problem. It is important to identify bland mesothelioma cells on cytological screening, perform efficient genomic-based testing, and diagnose mesothelioma, because the first clinical manifestation of pleural mesothelioma is pleural effusion, which is the first sample available for pathological diagnosis. However, certain diagnostic aspects remain challenging even for experts.

CASE PRESENTATION

This report describes a case of a 72-year-old man with a history of asbestos exposure who presented with pleural effusion as the first symptom and was eventually diagnosed as mesothelioma. Mesothelioma was suspected owing to prominent cell-in-cell engulfment in mesothelial cells on the first cytological sample, and the diagnosis of mesothelioma in situ was confirmed by histology. Unexpectedly, sarcomatoid morphology of mesothelioma was found in the second pathology samples 9 months after the first pathological examination. Both the mesothelioma in situ and invasive lesion showed immunohistochemical loss of methylthioadenosine phosphorylase (MTAP) and homozygous deletion of cyclin dependent kinase inhibitor 2A (CDKN2A) on fluorescence in situ hybridization. The patient received medication therapy but died of disease progression 12 months after the diagnosis of the sarcomatoid morphology of mesothelioma.

CONCLUSION

Our case suggests that cell-in-cell engulfment can be conspicuous in early-stage mesothelioma with inconspicuous nuclear atypia and few multinucleated cells. In addition, the presence of MTAP loss and CDKN2A homozygous deletion are suspected to be involved in early formation to invasive lesions and/or sarcomatoid morphology. We believe that it is important to consider genetic abnormalities when deciding on individual patient management. Furthermore, cases of mesothelioma, even those of an in situ lesion, with MTAP loss and/or CDKN2A deletion should be carefully followed up or subjected to early treatment.

[Effects of microRNA-106b on migration and invasion of human malignant pleural mesothelioma cell NCI-H2452]

Objective: To investigate the inhibitory effect of microRNA-106b in the process of migration and invasion of human malignant pleural mesothelioma cell NCI-H2452. Methods: In April 2017, the expression level of miRNA-106b in malignant pleural mesothelioma cells (NCI-H2452, MSTO-211H, NCI-H2052) and normal mesothelial cells MeT-5A was detected and analyzed. Using NCI-H2452 cells as a model, the NCI-H2452 cell model with miRNA-106b overexpression was established by transfecting miRNA-106b mimics. The expression level of miRNA-106b in the cells was detected by real-time fluorescent quantitative PCR. The effect of miRNA-106b on the migration and invasion ability of NCI-H2452 cells was analyzed. The gene expression data of malignant mesothelioma and the downstream target gene data of miRNA-106b in public databases were analyzed to screen the downstream target genes of miRNA-106b in mesothelioma cells that affect cell migration and invasion ability, and to verify the expression of this gene in NCI-H2452 cells with miRNA-106b overexpression. Results: The expression of miRNA-106b in three MPM cells was decreased compared with MeT-5A cells (P<0.001) . The expression level of miRNA-106b was significantly increased after transfection of miRNA-106b mimics (P<0.001) . The scratch migration levels of the experimental group were 28.45%±4.37%, 38.12%±4.82% and 50.06%±8.92% at 24h, 31h and 48h, respectively. Compared with the control group, the migration level decreased by 37.48%±2.65%, 49.21%±3.45% and 68.14%±3.81% (P<0.01) . The number of cell migration and invasion decreased in the experimental group compared with the control group (P<0.001) . Public databases were used to screen and analyze the possibility that TCF21 gene, as a downstream target gene, could affect the migration and invasion ability of MPM cells. The expression level of TCF21 gene was increased after transfection of miRNA-106b mimics in NCI-H2452 cells (P=0.009) . Conclusion: MiRNA-106b can inhibit the migration and invasion of NCI-H2452 cells and increase the expression of TCF21 gene.

Characterizing the distribution of alterations in mesothelioma and their correlation to morphology

OBJECTIVES

Mesothelioma is a lethal disease that arises from the serosal lining of organ cavities. Several recurrent alterations have been observed in pleural and peritoneal -mesotheliomas, including in BAP1, NF2, and CDKN2A. Although specific histopathologic parameters have been correlated with prognosis, it is not as well known whether genetic alterations correlate with histologic findings.

METHODS

We reviewed 131 mesotheliomas that had undergone next-generation sequencing (NGS) at our institutions after pathologic diagnosis. There were 109 epithelioid mesotheliomas, 18 biphasic mesotheliomas, and 4 sarcomatoid mesotheliomas. All our biphasic and sarcomatoid cases arose in the pleura. Of the epithelioid mesotheliomas, 73 were from the pleura and 36 were from the peritoneum. On average, patients were 66 years of age (range, 26-90 years) and predominantly male (92 men, 39 women).

RESULTS

The most common alterations identified were in BAP1, CDKN2A, NF2, and TP53. Twelve mesotheliomas did not show a pathogenic alteration on NGS. For epithelioid mesotheliomas in the pleura, the presence of an alteration in BAP1 correlated with low nuclear grade (P = .04), but no correlation was found in the peritoneum (P = .62). Similarly, there was no correlation between the amount of solid architecture in epithelioid mesotheliomas and any alterations in the pleura (P = .55) or peritoneum (P = .13). For biphasic mesotheliomas, cases with either no alteration detected or with an alteration in BAP1 were more likely to be epithelioid predominant (>50% of the tumor, P = .0001), and biphasic mesotheliomas with other alterations detected and no alteration in BAP1 were more likely to be sarcomatoid predominant (>50% of the tumor, P = .0001).

CONCLUSIONS

This study demonstrates a significant association between morphologic features associated with a better prognosis and an alteration in BAP1.

Cullin 4B Ubiquitin Ligase Is Important for Cell Survival and Regulates TGF-β1 Expression in Pleural Mesothelioma

We previously demonstrated that cullin 4B (CUL4B) upregulation was associated with worse outcomes of pleural mesothelioma (PM) patients, while the overexpression of its paralog CUL4A was not associated with clinical outcomes. Here, we aimed to identify the distinct roles of CUL4B and CUL4A in PM using an siRNA approach in PM cell lines (ACC Meso-1 and Mero82) and primary culture. The knockdown of CUL4B and CUL4A resulted in significantly reduced colony formation, increased cell death, and delayed cell proliferation. Furthermore, similar to the effect of CUL4A knockdown, downregulation of CUL4B led to reduced expression of Hippo pathway genes including YAP1, CTGF, and survivin. Interestingly, CUL4B and not CUL4A knockdown reduced TGF-β1 and MMP2 expression, suggesting a unique association of CUL4B with this pathway. However, the treatment of PM cells with exogenous TGF-β1 following CUL4B knockdown did not rescue PM cell growth. We further analyzed ACC Meso-1 xenograft tumor tissues treated with the cullin inhibitor, pevonedistat, which targets protein neddylation, and observed the downregulation of human TGF-β1 and MMP2. In summary, our data suggest that CUL4B overexpression is important for tumor cell growth and survival and may drive PM aggressiveness via the regulation of TGF-β1 expression and, furthermore, reveal a new mechanism of action of pevonedistat.

Primary and hTERT-Transduced Mesothelioma-Associated Fibroblasts but Not Primary or hTERT-Transduced Mesothelial Cells Stimulate Growth of Human Mesothelioma Cells

Pleural mesothelioma (PM) is an aggressive malignancy that develops in a unique tumor microenvironment (TME). However, cell models for studying the TME in PM are still limited. Here, we have generated and characterized novel human telomerase reverse transcriptase (hTERT)-transduced mesothelial cell and mesothelioma-associated fibroblast (Meso-CAF) models and investigated their impact on PM cell growth. Pleural mesothelial cells and Meso-CAFs were isolated from tissue of pneumothorax and PM patients, respectively. Stable expression of hTERT was induced by retroviral transduction. Primary and hTERT-transduced cells were compared with respect to doubling times, hTERT expression and activity levels, telomere lengths, proteomes, and the impact of conditioned media (CM) on PM cell growth. All transduced derivatives exhibited elevated hTERT expression and activity, and increased mean telomere lengths. Cell morphology remained unchanged, and the proteomes were similar to the corresponding primary cells. Of note, the CM of primary and hTERT-transduced Meso-CAFs stimulated PM cell growth to the same extent, while CM derived from mesothelial cells had no stimulating effect, irrespective of hTERT expression. In conclusion, all new hTERT-transduced cell models closely resemble their primary counterparts and, hence, represent valuable tools to investigate cellular interactions within the TME of PM.

Circulating FGF18 is decreased in pleural mesothelioma but not correlated with disease prognosis

BACKGROUND

Pleural mesothelioma (PM) is a relatively rare malignancy with limited treatment options and dismal prognosis. We have previously found elevated FGF18 expression in PM tissue specimens compared with normal mesothelium. The objective of the current study was to further explore the role of FGF18 in PM and evaluate its suitability as a circulating biomarker.

METHODS

FGF18 mRNA expression was analyzed by real-time PCR in cell lines and in silico in datasets from the Cancer Genome Atlas (TCGA). Cell lines overexpressing FGF18 were generated by retroviral transduction and cell behavior was investigated by clonogenic growth and transwell assays. Plasma was collected from 40 PM patients, six patients with pleural fibrosis, and 40 healthy controls. Circulating FGF18 was measured by ELISA and correlated to clinicopathological parameters.

RESULTS

FGF18 showed high mRNA expression in PM and PM-derived cell lines. PM patients with high FGF18 mRNA expression showed a trend toward longer overall survival (OS) in the TCGA dataset. In PM cells with low endogenous FGF18 expression, forced overexpression of FGF18 resulted in reduced growth but increased migration. Surprisingly, despite the high FGF18 mRNA levels observed in PM, circulating FGF18 protein was significantly lower in PM patients and patients with pleural fibrosis than in healthy controls. No significant association of circulating FGF18 with OS or other disease parameters of PM patients was observed.

CONCLUSIONS

FGF18 is not a prognostic biomarker in PM. Its role in PM tumor biology and the clinical significance of decreased plasma FGF18 in PM patients warrant further investigation.

Genetic analysis of familial predisposition in the pathogenesis of malignant pleural mesothelioma

PURPOSE

Mesothelioma is the primary tumor of the mesothelial cell membrane. The most important etiology is asbestos exposure. The development of malignant mesothelioma in very few of the population exposed to asbestos and its frequent occurrence in some families may be significant in terms of genetic predisposition. Again, the presence of relatives with mesothelioma who did not have asbestos contact strengthens this argument. This disease, which has limited treatment options and has a poor prognosis, revealing a genetic predisposition, if any, may prolong survival with early diagnosis and effective treatment.

METHODS

Based on the genetic predisposition idea, we diagnosed and followed a total of ten individuals of relatives with mesothelioma. DNA was isolated from peripheral blood and whole genome sequencing analysis was done. Common gene mutations in ten individuals were filtered using bioinformatics. After this filter, from the remaining variants, very rare in the population and damaging mutations are selected.

RESULTS

Eight thousand six hundred and twenty-two common variants have been identified in ten individuals with this analysis. In total, 120 variants were found on 37 genes in 15 chromosomes. These genes are PIK3R4, SLC25A5, ITGB6, PLK2, RAD17, HLA-B, HLA-DRB1, HLA-DQB1, GRM, IL20RA, MAP3K7, RIPK2, and MUC16.

CONCLUSION

Our finding, PIK3R4 gene, is directly associated with mesothelioma development. Twelve genes, which are associated with cancer, were detected in literature. Additional studies, which scan first-degree relatives of individual, are needed to find the specific gene region.

Spatially resolved, high-dimensional transcriptomics sorts out the evolution of biphasic malignant pleural mesothelioma: new paradigms for immunotherapy

BACKGROUND

Malignant Pleural Mesothelioma (MPM) is a dreadful disease escaping the classical genetic model of cancer evolution and characterized by wide heterogeneity and transcriptional plasticity. Clinical evolution of MPM is marked by a progressive transdifferentiation that converts well differentiated epithelioid (E) cells into undifferentiated and pleomorphic sarcomatoid (S) phenotypes. Catching the way this transition takes place is necessary to understand how MPM develops and progresses and it is mandatory to improve patients' management and life expectancy. Bulk transcriptomic approaches, while providing a significant overview, failed to resolve the timing of this evolution and to identify the hierarchy of molecular events through which this transition takes place.

METHODS

We applied a spatially resolved, high-dimensional transcriptomic approach to study MPM morphological evolution. 139 regions across 8 biphasic MPMs (B-MPMs) were profiled using the GeoMx™Digital Spatial Profiler to reconstruct the positional context of transcriptional activities and the spatial topology of MPM cells interactions. Validation was conducted on an independent large cohort of 84 MPMs by targeted digital barcoding analysis.

RESULTS

Our results demonstrated the existence of a complex circular ecosystem in which, within a strong asbestos-driven inflammatory environment, MPM and immune cells affect each other to support S-transdifferentiation. We also showed that TGFB1 polarized M2-Tumor Associated Macrophages foster immune evasion and that TGFB1 expression correlates with reduced survival probability.

CONCLUSIONS

Besides providing crucial insights into the multidimensional interactions governing MPM clinical evolution, these results open new perspectives to improve the use of immunotherapy in this disease.

Multiomic analysis of malignant pleural mesothelioma identifies molecular axes and specialized tumor profiles driving intertumor heterogeneity

Malignant pleural mesothelioma (MPM) is an aggressive cancer with rising incidence and challenging clinical management. Through a large series of whole-genome sequencing data, integrated with transcriptomic and epigenomic data using multiomics factor analysis, we demonstrate that the current World Health Organization classification only accounts for up to 10% of interpatient molecular differences. Instead, the MESOMICS project paves the way for a morphomolecular classification of MPM based on four dimensions: ploidy, tumor cell morphology, adaptive immune response and CpG island methylator profile. We show that these four dimensions are complementary, capture major interpatient molecular differences and are delimited by extreme phenotypes that-in the case of the interdependent tumor cell morphology and adapted immune response-reflect tumor specialization. These findings unearth the interplay between MPM functional biology and its genomic history, and provide insights into the variations observed in the clinical behavior of patients with MPM.

TROP2 expression and SN38 antitumor activity in malignant pleural mesothelioma cells provide a rationale for antibody-drug conjugate therapy

OBJECTIVES

Malignant pleural mesothelioma (MPM) is an aggressive cancer which at large is not amenable to curative surgery. Despite the recent approval of immune checkpoint inhibitor therapy, the response rates and survival following systemic therapy is still limited. Sacituzumab govitecan is an antibody-drug conjugate targeting the topoisomerase I inhibitor SN38 to trophoblast cell-surface antigen 2 (TROP-2)-positive cells. Here we have explored the therapeutic potential of sacituzumab govitecan in MPM models.

MATERIALS AND METHODS

TROP2 expression was analyzed in a panel of two well established and 15 pleural effusion derived novel lines by RT-QPCR and immunoblotting, TROP2 membrane-localization was studied by flow cytometry and immunohistochemistry. Cultured mesothelial cells and pneumothorax pleura served as controls. The sensitivity of MPM cell lines to irinotecan and SN38 was studied using cell viability, cell cycle, apoptosis and DNA damage assays. Drug sensitivity of cell lines was correlated with RNA expression of DNA repair genes. Drug sensitivity was defined as an IC50 below 5 nM in the cell viability assay.

RESULTS

TROP2 expression was detected at RNA and protein level in 6 of the 17 MPM cell lines, but not in in cultured mesothelial control cells or in the mesothelial layer of the pleura. TROP2 was detectable on the cell membrane in 5 MPM lines and was present in the nucleus in 6 cell models. Ten of 17 MPM cell lines showed sensitivity to SN38 treatment, among those 4 expressed TROP2. High AURKA RNA expression and high proliferation rate correlated with sensitivity to SN38-induced cell death, DNA damage response, cell cycle arrest and cell death. Sacituzumab govitecan treatment effectively induced cell cycle arrest and cell death in TROP2-positive MPM cells.

CONCLUSION

TROP2 expression and sensitivity to SN38 in MPM cell lines support biomarker-selected clinical exploration of sacituzumab govitecan in patients with MPM.

Blood cell DNA methylation biomarkers in preclinical malignant pleural mesothelioma: The EPIC prospective cohort

Malignant pleural mesothelioma (MPM) is a rare and aggressive cancer mainly caused by asbestos exposure. Specific and sensitive noninvasive biomarkers may facilitate and enhance screening programs for the early detection of cancer. We investigated DNA methylation (DNAm) profiles in MPM prediagnostic blood samples in a case-control study nested in the European Prospective Investigation into Cancer and nutrition (EPIC) cohort, aiming to characterise DNAm biomarkers associated with MPM. From the EPIC cohort, we included samples from 135 participants who developed MPM during 20 years of follow-up and from 135 matched, cancer-free, controls. For the discovery phase we selected EPIC participants who developed MPM within 5 years from enrolment (n = 36) with matched controls. We identified nine differentially methylated CpGs, selected by 10-fold cross-validation and correlation analyses: cg25755428 (MRI1), cg20389709 (KLF11), cg23870316, cg13862711 (LHX6), cg06417478 (HOOK2), cg00667948, cg01879420 (AMD1), cg25317025 (RPL17) and cg06205333 (RAP1A). Receiver operating characteristic (ROC) analysis showed that the model including baseline characteristics (age, sex and PC1wbc) along with the nine MPM-related CpGs has a better predictive value for MPM occurrence than the baseline model alone, maintaining some performance also at more than 5 years before diagnosis (area under the curve [AUC] < 5 years = 0.89; AUC 5-10 years = 0.80; AUC >10 years = 0.75; baseline AUC range = 0.63-0.67). DNAm changes as noninvasive biomarkers in prediagnostic blood samples of MPM cases were investigated for the first time. Their application can improve the identification of asbestos-exposed individuals at higher MPM risk to possibly adopt more intensive monitoring for early disease identification.

A molecular phenotypic map of malignant pleural mesothelioma

BACKGROUND

Malignant pleural mesothelioma (MPM) is a rare understudied cancer associated with exposure to asbestos. So far, MPM patients have benefited marginally from the genomics medicine revolution due to the limited size or breadth of existing molecular studies. In the context of the MESOMICS project, we have performed the most comprehensive molecular characterization of MPM to date, with the underlying dataset made of the largest whole-genome sequencing series yet reported, together with transcriptome sequencing and methylation arrays for 120 MPM patients.

RESULTS

We first provide comprehensive quality controls for all samples, of both raw and processed data. Due to the difficulty in collecting specimens from such rare tumors, a part of the cohort does not include matched normal material. We provide a detailed analysis of data processing of these tumor-only samples, showing that all somatic alteration calls match very stringent criteria of precision and recall. Finally, integrating our data with previously published multiomic MPM datasets (n = 374 in total), we provide an extensive molecular phenotype map of MPM based on the multitask theory. The generated map can be interactively explored and interrogated on the UCSC TumorMap portal (https://tumormap.ucsc.edu/?p=RCG_MESOMICS/MPM_Archetypes ).

CONCLUSIONS

This new high-quality MPM multiomics dataset, together with the state-of-art bioinformatics and interactive visualization tools we provide, will support the development of precision medicine in MPM that is particularly challenging to implement in rare cancers due to limited molecular studies.

The Rocky Road from Preclinical Findings to Successful Targeted Therapy in Pleural Mesothelioma

Pleural mesothelioma (PM) is a rare and aggressive disease that arises from the mesothelial cells lining the pleural cavity. Approximately 80% of PM patients have a history of asbestos exposure. The long latency period of 20-40 years from the time of asbestos exposure to diagnosis, suggests that multiple somatic genetic alterations are required for the tumorigenesis of PM. The genomic landscape of PM has been characterized by inter- and intratumor heterogeneity associated with the impairment of tumor suppressor genes such as CDKN2A, NF2, and BAP1. Current systemic therapies have shown only limited efficacy, and none is approved for patients with relapsed PM. Advances in understanding of the molecular landscape of PM has facilitated several biomarker-driven clinical trials but so far, no predictive biomarkers for targeted therapies are in clinical use. Recent advances in the PM genetics have provided optimism for successful molecular strategies in the future. Here, we summarize the molecular mechanism underlying PM pathogenesis and review potential therapeutic targets.

Inhibition of Cyclin Dependent Kinase 4/6 Overcomes Primary Resistance to Programmed Cell Death 1 Blockade in Malignant Mesothelioma

BACKGROUND

Despite the profound number of malignant pleural mesothelioma (MPM) patients now treated with programmed cell death 1 (PD-1) blockade, insight into the underpinnings of rational therapeutic strategies to treat resistance to checkpoint immunotherapy remains unrealized. Our objective was to develop a novel therapeutic approach to overcome primary resistance to PD-1 blockade in MPM.

METHODS

We generated a transcriptome signature of resistance to PD-1 blockade in MPM patients treated with nivolumab (4 responders and 4 nonresponders). We used The Cancer Genome Atlas MPM cohort (n = 73) to determine what genomic alterations were associated with the resistance signature. We tested whether regulation of identified molecules could overcome resistance to PD-1 blockade in an immunocompetent mouse malignant mesothelioma model.

RESULTS

Immunogenomic analysis by applying our anti-PD-1 resistance signature to The Cancer Genome Atlas cohort revealed that deletion of cyclin dependent kinase inhibitor 2A (CDKN2A) was highly associated with primary resistance to PD-1 blockade. Under the hypothesis that resistance to PD-1 blockade can be overcome by cyclin dependent kinase 4/6 (CDK4/6) inhibition, we tested whether CDK4/6 inhibitors could overcome resistance to PD-1 blockade in subcutaneous tumors derived from Cdkn2a-/- AB1 malignant mesothelioma cells, which were resistant to PD-1 blockade. The combination of daily oral administration of CDK4/6 inhibitors (abemaciclib or palbociclib) and intraperitoneal anti-PD-1 treatment markedly suppressed tumor growth compared with anti-PD-1 or CDK4/6 inhibitor alone.

CONCLUSIONS

We identified a therapeutic target, CDK4/6, to overcome primary resistance to PD-1 blockade through comprehensive immunogenomic approaches. These data provide a rationale for undertaking clinical trials of CDK4/6 inhibitors in more than 40% of patients with MPM who demonstrate loss of CDKN2A.

Clinical and molecular validation of BAP1, MTAP, P53, and Merlin immunohistochemistry in diagnosis of pleural mesothelioma

BAP1 and MTAP immunostains play an important role in diagnosis of mesothelioma, but additional markers are needed to increase sensitivity. We analyzed 84 pleural mesotheliomas (51 epithelioid, 27 biphasic, 6 sarcomatoid) by a hybrid-capture next-generation sequencing (NGS) panel including complete coverage of coding and splicing regions for BAP1, CDKN2A/MTAP, NF2, and TP53 and correlated molecular findings with diagnostic immunostains for BAP1, MTAP, Merlin, and p53, respectively. Fifty-seven reactive mesothelial proliferations served as benign comparators. Loss of BAP1, MTAP, and Merlin protein expression were, respectively, 54%, 46%, and 52% sensitive and 100% specific for mesothelioma. Two-marker immunopanels of BAP1 + MTAP, BAP1 + Merlin, and MTAP + Merlin were 79%, 85%, and 71% sensitive for mesothelioma, while a three-marker immunopanel of BAP1 + MTAP + Merlin was 90% sensitive. Diffuse (mutant-pattern) p53 immunostaining was seen in only 6 (7%) tumors but represented the only immunohistochemical abnormality in 2 cases. Null-pattern p53 was not specific for malignancy. An immunopanel of BAP1 + MTAP + Merlin + p53 was 93% sensitive for mesothelioma, and panel NGS detected a pathogenic alteration in BAP1, MTAP, NF2, and/or TP53 in 95%. Together, 83 (99%) of 84 tumors showed a diagnostic alteration by either immunohistochemistry or panel NGS. Adding Merlin to the standard BAP1 + MTAP immunopanel increases sensitivity for mesothelioma without sacrificing specificity. p53 immunohistochemistry and panel NGS with complete coverage of BAP1, CDKN2A/MTAP, TP53, and NF2 may be useful in diagnostically challenging cases.

Define the Two Molecular Subtypes of Epithelioid Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is a fatal disease of respiratory system. Despite the availability of invasive biomarkers with promising results, there are still significant diagnostic and therapeutic challenges in the treatment of MPM. One of three main mesothelioma cell types, epithelioid mesothelioma makes up approximately 70% of all mesothelioma cases. Different observational findings are under process, but the molecular heterogeneity and pathogenesis of epithelioid malignant pleural mesothelioma (eMPM) are still not well understood. Through molecular analysis, expression profiling data were used to determine the possibility and optimal number of eMPM molecular subtypes. Next, clinicopathological characteristics and different molecular pathways of each subtype were analyzed to prospect the clinical applications and advanced mechanisms of eMPM. In this study, we identified two distinct epithelioid malignant pleural mesothelioma subtypes with distinct gene expression patterns. Subtype I eMPMs were involved in steroid hormone biosynthesis, porphyrin and chlorophyll metabolism, and drug metabolism, while subtype II eMPMs were involved in rational metabolism, tyrosine metabolism, and chemical carcinogenesis pathways. Additionally, we identified potential subtype-specific therapeutic targets, including CCNE1, EPHA3, RNF43, ROS1, and RSPO2 for subtype I and CDKN2A and RET for subtype II. Considering the need for potent diagnostic and therapeutic biomarkers for eMPM, we are anticipating that our findings will help both in exploring underlying mechanisms in the development of eMPM and in designing targeted therapy for eMPM.

A comprehensive study of alternative splicing in malignant pleural mesothelioma identifies potential therapeutic targets in a new cluster with poor survival

Background

Malignant pleural mesothelioma (MPM) is one of the most aggressive tumors with few effective treatments worldwide. It has been suggested that alternative splicing at the transcriptome level plays an indispensable role in MPM.

Methods

We analyzed the splicing profile of 84 MPM patients from the TCGA cohort by using seven typical splicing types. We classified MPM patients based on their splicing status and conducted a comprehensive analysis of the correlation between the splicing classification and clinical characteristics, genetic variation, pathway changes, immune heterogeneity, and potential therapeutic targets.

Results

The expression of the alternative splicing regulator SRPK1 is significantly higher in MPM tissues than in normal tissues, and correlates with poor survival. SRPK1 deficiency promotes MPM cell apoptosis and inhibits cell migration in vitro. We divided the MPM patients into four clusters based on their splicing profile and identified two clusters associated with the shortest (cluster 3) and longest (cluster 4) survival time. We present the different gene signatures of each cluster that are related to survival and splicing. Comprehensive analysis of data from the GDSC and TCGA databases revealed that cluster 3 MPM patients could respond well to the small‐molecule inhibitor CHIR‐99021, a small‐molecule inhibitor of GSK‐3.

Conclusion

We performed unsupervised clustering of alternative splicing data from 84 MPM patients from the TCGA database and identified a cluster associated with the worst prognosis that was sensitive to a GSK‐3 inhibitor.

[Expression and clinical significance of COL1A1 and COL1A2 genes in malignant pleural mesothelioma tissues]

Objective: To investigate the expression levels and clinical significance of collagen typeⅠ α1 chain (COL1A1) and collagen type Ⅰ α2 chain (COL1A2) in malignant pleural mesothelioma (MPM) tissues. Methods: In January 2020, MPM tissues and adjacent normal pleural tissues were collected from 26 MPM patients, and the expression levels of COL1A1 and COL1A2 genes in the tissues were determined by quantitative reverse transcription PCR, and the efficacy of both levels in diagnosing MPM was assessed using receiver operating characteristic (ROC) curves. The relationship between COL1A1 and COL1A2 gene expression and clinicopathological features was analyzed by the Cancer Genome Atlas (TCGA) database, and the relationship between the expression levels of both and overall survival (OS) and disease-free progression survival (DFS) of MPM patients was dynamically analyzed by gene expression profiling, and the factors affecting the prognosis of MPM patients were explored by Cox proportional risk regression model. The TIMER 2.0 platform was used to explore the relationship between COL1A1 and COL1A2 gene expression in MPM and tumor immune infiltrative cells. Results: Compared with normal pleural tissues, the expression of COL1A1 and COL1A2 genes was significantly increased in MPM tissues (P<0.01) , and their expression was positively correlated (P<0.001) . The ROC curves showed that the area under the curve for COL1A1 and COL1A2 expression levels diagnostic of MPM was 0.900 and 0.897, respectively. The expression of COL1A1 gene was correlated with tumor type in MPM patients (P<0.05) , and COL1A2 gene expression was correlated with T stage in MPM patients (P<0.05) . Both COL1A1 and COL1A2 gene expression were associated with OS in MPM patients (Logrank P<0.05) , but there was no significant correlation with DFS (Logrank P>0.05) . Cox multivariate analysis showed that patients with high COL1A1 and COL1A2 gene expression and biphasic mixed MPM had a higher risk of death (P<0.05) . TIMER 2.0 platform analysis showed that COL1A1 and COL1A2 gene expression in MPM patients was positively correlated with macrophages, COL1A2 gene expression in MPM was negatively correlated with neutrophils (P<0.05) . Conclusion: High expression of COL1A1 and COL1A2 genes in MPM tissues is valuable for diagnosis, disease prediction and prognostic assessment of MPM, and both may jointly contribute to the development of MPM.

Epithelioid Pleural Mesothelioma Is Characterized by Tertiary Lymphoid Structures in Long Survivors: Results from the MATCH Study

Pleural mesothelioma (PM) is an aggressive tumor with few therapeutic options. Although patients with epithelioid PM (ePM) survive longer than non-epithelioid PM (non-ePM), heterogeneity of tumor response in ePM is observed. The role of the tumor immune microenvironment (TIME) in the development and progression of PM is currently considered a promising biomarker. A few studies have used high-throughput technologies correlated with TIME evaluation and morphologic and clinical data. This study aimed to identify different morphological, immunohistochemical, and transcriptional profiles that could potentially predict the outcome. A retrospective multicenter cohort of 129 chemonaive PM patients was recruited. Tissue slides were reviewed by dedicated pathologists for histotype classification and immunophenotype of tumor-infiltrating lymphocytes (TILs) and lymphoid aggregates or tertiary lymphoid structures (TLS). ePM (n = 99) survivors were further classified into long (>36 months) or short (<12 months) survivors. RNAseq was performed on a subset of 69 samples. Distinct transcriptional profiling in long and short ePM survivors was found. An inflammatory background with a higher number of B lymphocytes and a prevalence of TLS formations were detected in long compared to short ePM survivors. These results suggest that B cell infiltration could be important in modulating disease aggressiveness, opening a pathway for novel immunotherapeutic approaches.

Integrated genomics point to immune vulnerabilities in pleural mesothelioma

Pleural mesothelioma is an aggressive malignancy with limited effective therapies. In order to identify therapeutic targets, we integrated SNP genotyping, sequencing and transcriptomics from tumours and low-passage patient-derived cells. Previously unrecognised deletions of SUFU locus (10q24.32), observed in 21% of 118 tumours, resulted in disordered expression of transcripts from Hedgehog pathways and the T-cell synapse including VISTA. Co-deletion of Interferon Type I genes and CDKN2A was present in half of tumours and was a predictor of poor survival. We also found previously unrecognised deletions in RB1 in 26% of cases and show sub-micromolar responses to downstream PLK1, CHEK1 and Aurora Kinase inhibitors in primary mesothelioma cells. Defects in Hippo pathways that included RASSF7 amplification and NF2 or LATS1/2 mutations were present in 50% of tumours and were accompanied by micromolar responses to the YAP1 inhibitor Verteporfin. Our results suggest new therapeutic avenues in mesothelioma and indicate targets and biomarkers for immunotherapy.

Clonal architecture in mesothelioma is prognostic and shapes the tumour microenvironment

Malignant Pleural Mesothelioma (MPM) is typically diagnosed 20-50 years after exposure to asbestos and evolves along an unknown evolutionary trajectory. To elucidate this path, we conducted multi-regional exome sequencing of 90 tumour samples from 22 MPMs acquired at surgery. Here we show that exomic intratumour heterogeneity varies widely across the cohort. Phylogenetic tree topology ranges from linear to highly branched, reflecting a steep gradient of genomic instability. Using transfer learning, we detect repeated evolution, resolving 5 clusters that are prognostic, with temporally ordered clonal drivers. BAP1/-3p21 and FBXW7/-chr4 events are always early clonal. In contrast, NF2/-22q events, leading to Hippo pathway inactivation are predominantly late clonal, positively selected, and when subclonal, exhibit parallel evolution indicating an evolutionary constraint. Very late somatic alteration of NF2/22q occurred in one patient 12 years after surgery. Clonal architecture and evolutionary clusters dictate MPM inflammation and immune evasion. These results reveal potentially drugable evolutionary bottlenecking in MPM, and an impact of clonal architecture on shaping the immune landscape, with potential to dictate the clinical response to immune checkpoint inhibition.

Identification of Overexpressed Genes in Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is a fatal tumor lacking effective therapies. The characterization of overexpressed genes could constitute a strategy for identifying drivers of tumor progression as targets for novel therapies. Thus, we performed an integrated gene-expression analysis on RNAseq data of 85 MPM patients from TCGA dataset and reference samples from the GEO. The gene list was further refined by using published studies, a functional enrichment analysis, and the correlation between expression and patients' overall survival. Three molecular signatures defined by 15 genes were detected. Seven genes were involved in cell adhesion and extracellular matrix organization, with the others in control of the mitotic cell division or apoptosis inhibition. Using Western blot analyses, we found that ADAMTS1, PODXL, CIT, KIF23, MAD2L1, TNNT1, and TRAF2 were overexpressed in a limited number of cell lines. On the other hand, interestingly, CTHRC1, E-selectin, SPARC, UHRF1, PRSS23, BAG2, and MDK were abundantly expressed in over 50% of the six MPM cell lines analyzed. Thus, these proteins are candidates as drivers for sustaining the tumorigenic process. More studies with small-molecule inhibitors or silencing RNAs are fully justified and need to be undertaken to better evaluate the cancer-driving role of the targets herewith identified.

Challenges in lung and thoracic pathology: molecular advances in the classification of pleural mesotheliomas

The diagnosis and classification of malignant pleural mesothelioma (MPM) is extremely challenging; obtaining an accurate histopathological diagnosis of the different types and subtypes requires expert assessment and suitable biopsies that are not always available, which can leave doctors uncertain about the patient's diagnosis, sometimes resulting in a delay in the start of treatment. In this review, we discuss recent major advances in the molecular characterisation of MPM and their implications for histological classification. We detail what is known of the molecular landscape of MPM at the genomic, transcriptomic, and epigenomic levels, describe the similarities and dissimilarities of the multiple molecular classifications that have been proposed, and provide an overview of the current state of knowledge regarding inter- and intra-tumour heterogeneity. We also highlight the current gaps in knowledge and how addressing them would benefit classification, as well as the patients in general.

UHRF1 Is a Novel Druggable Epigenetic Target in Malignant Pleural Mesothelioma

INTRODUCTION

Ubiquitin-like with plant homeodomain and ring finger domains 1 (UHRF1) encodes a master regulator of DNA methylation that has emerged as an epigenetic driver in human cancers. To date, no studies have evaluated UHRF1 expression in malignant pleural mesothelioma (MPM). This study was undertaken to explore the therapeutic potential of targeting UHRF1 in MPM.

METHODS

Microarray, real-time quantitative reverse transcription-polymerase chain reaction, immunoblot, and immunohistochemistry techniques were used to evaluate UHRF1 expression in normal mesothelial cells (NMCs) cultured with or without asbestos, MPM lines, normal pleura, and primary MPM specimens. The impact of UHRF1 expression on MPM patient survival was evaluated using two independent databases. RNA-sequencing, proliferation, invasion, and colony formation assays, and murine xenograft experiments were performed to evaluate gene expression and growth of MPM cells after biochemical or pharmacologic inhibition of UHRF1 expression.

RESULTS

UHRF1 expression was significantly higher in MPM lines and specimens relative to NMC and normal pleura. Asbestos induced UHRF1 expression in NMC. The overexpression of UHRF1 was associated with decreased overall survival in patients with MPM. UHRF1 knockdown reversed genomewide DNA hypomethylation, and inhibited proliferation, invasion, and clonogenicity of MPM cells, and growth of MPM xenografts. These effects were phenocopied by the repurposed chemotherapeutic agent, mithramycin. Biochemical or pharmacologic up-regulation of p53 significantly reduced UHRF1 expression in MPM cells. RNA-sequencing experiments exhibited the pleiotropic effects of UHRF1 down-regulation and identified novel, clinically relevant biomarkers of UHRF1 expression in MPM.

CONCLUSIONS

UHRF1 is an epigenetic driver in MPM. These findings support the efforts to target UHRF1 expression or activity for mesothelioma therapy.

Genomic characterization of malignant pleural mesothelioma and associated clinical outcomes

INTRODUCTION

Malignant pleural mesothelioma (MPM) is a rare malignancy with a poor prognosis. While treatment with a platinum-based chemotherapy is the standard of care, many patients have rapid progression of disease with a median overall survival of ~12 months. Limited data exist about the genomic alterations associated with MPM and their clinical implications.

METHODS

We report genomic alterations and clinical data for 17 patients with MPM who had next generation sequencing performed. Overall survival (OS) and progression-free survival (PFS) were analyzed with Kaplan-Meier method.

RESULTS

Median age at diagnosis was 70 years (range 55-85), and 47% of the patients were male. The most common genomic alterations in the 17 patients were NF2 (53%), BAP1 (41%), CDKN2A (41%) and TP53 (29%). The median OS was 10.8 months. When stratified by mutational status, patients had better median OS if they had a BAP1 alteration compared to TP53 alteration (median OS 14.5 vs 7.2 months). Median PFS with first-line chemotherapy was 7 months (SD ± 3.3). However, patients with TP53 mutations had worse PFS with chemotherapy with median of only 3.9 months. Tumor mutation burden (TMB) was available for 12 patients and all had low TMB (range 1 to 8.1 mutation/Mb). Median PFS with immunotherapy was poor with at 1.5 months (SD ±0.4) and there was no significant difference in PFS with immunotherapy based on molecular profile.

CONCLUSION

Our study has identified that TP53 confers worse survival and response to platinum chemotherapy compared to BAP1. Overall PDL1 expression and TMB is low in patients with MPM resulting in limited benefit from single agent PD-1/PD-L1 agent.

Large-scale analysis of BAP1 expression reveals novel associations with clinical and molecular features of malignant pleural mesothelioma

BRCA1‐associated protein‐1 (BAP1) expression is commonly lost in several tumors including malignant pleural mesothelioma (MPM). Presence or absence of immunohistochemical BAP1 nuclear staining in tumor cells is currently used for differential diagnosis of MPM. In this study, a large cohort of 596 MPM tumors with available clinical data was analyzed to examine associations of BAP1 staining pattern with clinical and molecular features that may reflect the impact of BAP1 mutation on MPM biology. Cases were classified according to the BAP1 staining pattern of tumor cells. Exome and RNA‐sequencing data were available for subsets of cases. Levels of mRNA encoding claudin 15 (CLDN15) and vimentin (VIM) were determined using RT‐qPCR on 483 cases to estimate the relative proportions of epithelial‐like and mesenchymal‐like components in each tumor. Four BAP1 staining patterns were observed: single‐pattern nuclear staining (36%), single‐pattern cytoplasmic staining (25%), single‐pattern absent staining (12%), and combinations of these staining patterns (27%). This study confirmed prior reports that nuclear BAP1 is more frequently associated with wild‐type BAP1 and sarcomatoid histology. However, no associations between BAP1 staining pattern(s) and mutations in specific protein domains and/or mutation type were observed. BAP1 staining patterns were significantly associated (p < 0.001) with BAP1 gene expression, MPM histologic subtypes, molecular clusters, and markers of epithelial‐to‐mesenchymal transition. Frequent observation of combinations of BAP1 staining patterns in MPM tumors indicated intra‐tumoral heterogeneity of BAP1 status. Cytoplasmic BAP1 staining was identified as a putative indicator of favorable prognosis in non‐epithelioid MPM. In conclusion, novel significant associations among different BAP1 staining patterns and subgroups of MPM tumors were observed, suggesting that the role of BAP1 in tumor progression may be more complex than its presumed tumor suppressor function. Cytoplasmic staining was identified as a putative indicator of favorable prognosis in non‐epithelioid MPM, potentially addressing a critical need in clinical decision‐making in this disease. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Verification of a Blood-Based Targeted Proteomics Signature for Malignant Pleural Mesothelioma

BACKGROUND

We have verified a mass spectrometry (MS)-based targeted proteomics signature for the detection of malignant pleural mesothelioma (MPM) from the blood.

METHODS

A seven-peptide biomarker MPM signature by targeted proteomics in serum was identified in a previous independent study. Here, we have verified the predictive accuracy of a reduced version of that signature, now composed of six-peptide biomarkers. We have applied liquid chromatography-selected reaction monitoring (LC-SRM), also known as multiple-reaction monitoring (MRM), for the investigation of 402 serum samples from 213 patients with MPM and 189 cancer-free asbestos-exposed donors from the United States, Australia, and Europe.

RESULTS

Each of the biomarkers composing the signature was independently informative, with no apparent functional or physical relation to each other. The multiplexing possibility offered by MS proteomics allowed their integration into a single signature with a higher discriminating capacity than that of the single biomarkers alone. The strategy allowed in this way to increase their potential utility for clinical decisions. The signature discriminated patients with MPM and asbestos-exposed donors with AUC of 0.738. For early-stage MPM, AUC was 0.765. This signature was also prognostic, and Kaplan-Meier analysis showed a significant difference between high- and low-risk groups with an HR of 1.659 (95% CI, 1.075-2.562; P = 0.021).

CONCLUSIONS

Targeted proteomics allowed the development of a multianalyte signature with diagnostic and prognostic potential for MPM from the blood.

IMPACT

The proteomic signature represents an additional diagnostic approach for informing clinical decisions for patients at risk for MPM.

Genomics and Functional Genomics of Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is a rare, aggressive cancer of the mesothelial cells lining the pleural surface of the chest wall and lung. The etiology of MPM is strongly associated with prior exposure to asbestos fibers, and the median survival rate of the diagnosed patients is approximately one year. Despite the latest advancements in surgical techniques and systemic therapies, currently available treatment modalities of MPM fail to provide long-term survival. The increasing incidence of MPM highlights the need for finding effective treatments. Targeted therapies offer personalized treatments in many cancers. However, targeted therapy in MPM is not recommended by clinical guidelines mainly because of poor target definition. A better understanding of the molecular and cellular mechanisms and the predictors of poor clinical outcomes of MPM is required to identify novel targets and develop precise and effective treatments. Recent advances in the genomics and functional genomics fields have provided groundbreaking insights into the genomic and molecular profiles of MPM and enabled the functional characterization of the genetic alterations. This review provides a comprehensive overview of the relevant literature and highlights the potential of state-of-the-art genomics and functional genomics research to facilitate the development of novel diagnostics and therapeutic modalities in MPM.

Nucleolin and nucleophosmin expression patterns in pulmonary adenocarcinoma invading the pleura and in pleural malignant mesothelioma

BACKGROUND

Visceral pleural invasion (VPI) in adenocarcinoma of the lung is considered a poor prognostic factor. The purpose of this study was to analyze nucleolin and nucleophosmin expression in pulmonary adenocarcinoma (PA) with VPI and in pleural malignant mesothelioma.

METHODS

The study was conducted on the basis of 19 pathologically-confirmed cases of adenocarcinoma of the lung and 29 cases of epithelioid malignant mesothelioma. The nucleolin and nucleophosmin expression was assessed immunohistochemically and analyzed with image analysis software.

RESULTS

Nucleolin expression was lower while nucleophosmin was higher in pleural invasion of pulmonary adenocarcinoma than in the central part of the tumor. Differences in subpopulations of cells with different expression of proteins studied were also found. Malignant mesothelioma showed lower nucleolin expression than adenocarcinoma of the lung but no differences in nucleophosmin expression were found.

CONCLUSIONS

The results of our study suggested that lower nucleolin and higher nucleophosmin expression may be related to higher invasiveness of adenocarcinoma of the lung. Differences in nucleolin expression between pulmonary adenocarcinoma and malignant mesothelioma indicate another aspect of biology of these pleura-invading cancers that requires further study.

KEY POINTS

SIGNIFICANT FINDINGS OF THE STUDY: Differences in nucleolin and nucleophosmin expression in pleura invading pulmonary adenocarcinoma indicate the involvement of these proteins in its locoregional spread while differences in nucleolin expression between pulmonary adenocarcinoma and malignant mesothelioma suggest another aspect of biology of these cancers.

WHAT THIS STUDY ADDS

This is the first study on nucleolin and nucleophosmin expression in pleural malignant mesothelioma and pleura-invading pulmonary adenocarcinoma. Our findings may assist in understanding the mechanisms of locoregional spread of adenocarcinoma and differences between these two pleura-invading cancers.

Telomerase Reverse Transcriptase Promoter Mutations Identify a Genomically Defined and Highly Aggressive Human Pleural Mesothelioma Subgroup

PURPOSE

Human malignant pleural mesothelioma (MPM) is characterized by dismal prognosis. Consequently, dissection of molecular mechanisms driving malignancy is of key importance. Here we investigate whether activating mutations in the telomerase reverse transcriptase (TERT) gene promoter are present in MPM and associated with disease progression, cell immortalization, and genomic alteration patterns.

EXPERIMENTAL DESIGN

TERT promoters were sequenced in 182 MPM samples and compared with clinicopathologic characteristics. Surgical specimens from 45 patients with MPM were tested for in vitro immortalization. The respective MPM cell models (N = 22) were analyzed by array comparative genomic hybridization, gene expression profiling, exome sequencing as well as TRAP, telomere length, and luciferase promoter assays.

RESULTS

TERT promoter mutations were detected in 19 of 182 (10.4%) MPM cases and significantly associated with advanced disease and nonepithelioid histology. Mutations independently predicted shorter overall survival in both histologic MPM subtypes. Moreover, 9 of 9 (100%) mutated but only 13 of 36 (36.1%) wild-type samples formed immortalized cell lines. TERT promoter mutations were associated with enforced promoter activity and TERT mRNA expression, while neither telomerase activity nor telomere lengths were significantly altered. TERT promoter-mutated MPM cases exhibited distinctly reduced chromosomal alterations and specific mutation patterns. While BAP1 mutations/deletions were exclusive with TERT promoter mutations, homozygous deletions at the RBFOX1 and the GSTT1 loci were clearly enriched in mutated cases.

CONCLUSIONS

TERT promoter mutations independently predict a dismal course of disease in human MPM. The altered genomic aberration pattern indicates that TERT promoter mutations identify a novel, highly aggressive MPM subtype presumably based on a specific malignant transformation process.

EIF4G1 and RAN as Possible Drivers for Malignant Pleural Mesothelioma

For malignant pleural mesothelioma (MPM) novel therapeutic strategies are urgently needed. In a previous study, we identified 51 putative cancer genes over-expressed in MPM tissues and cell lines. Here, we deepened the study on nine of them (ASS1, EIF4G1, GALNT7, GLUT1, IGF2BP3 (IMP3), ITGA4, RAN, SOD1, and THBS2) to ascertain whether they are truly mesothelial cancer driver genes (CDGs) or genes overexpressed in an adaptive response to the tumoral progression (“passenger genes”). Through a fast siRNA-based screening, we evaluated the consequences of gene depletion on migration, proliferation, colony formation capabilities, and caspase activities of four MPM (Mero-14, Mero-25, IST-Mes2, and NCI-H28) and one SV40-immortalized mesothelial cell line (MeT-5A) as a non-malignant model. The depletion of EIF4G1 and RAN significantly reduced cell proliferation and colony formation and increased caspase activity. In particular, the findings for RAN resemble those observed for other types of cancer. Thus, we evaluated the in vitro effects of importazole (IPZ), a small molecule inhibitor of the interaction between RAN and importin-β. We showed that IPZ could have effects similar to those observed following RAN gene silencing. We also found that primary cell lines from one out of three MPM patients were sensitive to IPZ. As EIF4G1 and RAN deserve further investigation with additional in vitro and in vivo studies, they emerged as promising CDGs, suggesting that their upregulation could play a role in mesothelial tumorigenesis and aggressiveness. Furthermore, present data propose the molecular pathways dependent on RAN as a putative pharmacological target for MPM patients in the view of a future personalized medicine.

Analysis of mismatch repair (MMR) proteins expression in a series of malignant pleural mesothelioma (MPM) patients

BACKGROUND

Promising results have been reported with immune checkpoint inhibitors (ICI) in a small proportion of MPM patients. MMR deficiency (dMMR) has been well described in several malignancies and was approved as a biomarker for anti-PD-1 inhibitors. Next generation sequencing (NGS) data demonstrated that 2% of MPM harbor microsatellite instability. The aim of this study is to characterize MMR by immunohistochemistry (IHC) in a series of MPM including a subset of patients treated with immunotherapy.

METHODS

Tumors of 159 MPM p diagnosed between 2002 and 2017 were reviewed. Formalin-fixed, paraffin-embedded tissue was stained for MLH1, MSH2, MSH6 and PMS2 and tumors were classified as dMMR (MMR protein expression negative) and MMR intact (all MMR proteins positively expressed). We retrospectively collected clinical outcomes under standard chemotherapy and experimental immunotherapy in the entire cohort.

RESULTS

MMR protein expression was analyzed in 158 samples with enough tissue and was positive in all of the cases. Twenty two patients received ICI with anti-CTLA4 or anti-PD-1 blockade in clinical trials, 58% had a response or stable disease for more than 6 m, with median progression-free survival (PFS) of 5.7 m (2.1-26.1 m). The median overall survival (mOS) in all population was 15 months (m) (13.5-18.8 m). In a multivariable model factors associated to improved mOS were PS 0, neutrophil-lymphocyte ratio (NLR) < 5 and epithelioid histology (p < 0.001).

CONCLUSIONS

In our series we were unable to identify any MPM patient with dMMR by IHC. Further studies are needed to elucidate potential predictive biomarkers of ICI benefit in MPM.

Inhibition of cyclin-dependent kinase 7 down-regulates yes-associated protein expression in mesothelioma cells

Cyclin-dependent kinase 7 (CDK7) is a protein kinase that plays a major role in transcription initiation. Yes-associated protein (YAP) is a main effector of the Hippo/YAP signalling pathway. Here, we investigated the role of CDK7 on YAP regulation in human malignant pleural mesothelioma (MPM). We found that in microarray samples of human MPM tissue, immunohistochemistry staining showed correlation between the expression level of CDK7 and YAP (n = 70, r = .513). In MPM cells, CDK7 expression level was significantly correlated with GTIIC reporter activity (r = .886, P = .019). Inhibition of CDK7 by siRNA decreased the YAP protein level and the GTIIC reporter activity in the MPM cell lines 211H, H290 and H2052. Degradation of the YAP protein was accelerated after CDK7 knockdown in 211H, H290 and H2052 cells. Inhibition of CDK7 reduced tumour cell migration and invasion, as well as tumorsphere formation ability. Restoration of the CDK7 gene rescued the YAP protein level and GTIIC reporter activity after siRNA knockdown in 211H and H2052 cells. Finally, we performed a co-immunoprecipitation analysis using an anti-YAP antibody and captured the CDK7 protein in 211H cells. Our results suggest that CDK7 inhibition reduces the YAP protein level by promoting its degradation and suppresses the migration and invasion of MPM cells. Cyclin-dependent kinase 7 may be a promising therapeutic target for MPM.

Metabolic rewiring and redox alterations in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is a rare malignancy of mesothelial cells with increasing incidence, and in many cases, dismal prognosis due to its aggressiveness and lack of effective therapies. Environmental and occupational exposure to asbestos is considered the main aetiological factor for MPM. Inhaled asbestos fibres accumulate in the lungs and induce the generation of reactive oxygen species (ROS) due to the presence of iron associated with the fibrous silicates and to the activation of macrophages and inflammation. Chronic inflammation and a ROS-enriched microenvironment can foster the malignant transformation of mesothelial cells. In addition, MPM cells have a highly glycolytic metabolic profile and are positive in 18F-FDG PET analysis. Loss-of-function mutations of BRCA-associated protein 1 (BAP1) are a major contributor to the metabolic rewiring of MPM cells. A subset of MPM tumours show loss of the methyladenosine phosphorylase (MTAP) locus, resulting in profound alterations in polyamine metabolism, ATP and methionine salvage pathways, as well as changes in epigenetic control of gene expression. This review provides an overview of the perturbations in metabolism and ROS homoeostasis of MPM cells and the role of these alterations in malignant transformation and tumour progression.

Dissecting heterogeneity in malignant pleural mesothelioma through histo-molecular gradients for clinical applications

Malignant pleural mesothelioma (MPM) is recognized as heterogeneous based both on histology and molecular profiling. Histology addresses inter-tumor and intra-tumor heterogeneity in MPM and describes three major types: epithelioid, sarcomatoid and biphasic, a combination of the former two types. Molecular profiling studies have not addressed intra-tumor heterogeneity in MPM to date. Here, we use a deconvolution approach and show that molecular gradients shed new light on the intra-tumor heterogeneity of MPM, leading to a reconsideration of MPM molecular classifications. We show that each tumor can be decomposed as a combination of epithelioid-like and sarcomatoid-like components whose proportions are highly associated with the prognosis. Moreover, we show that this more subtle way of characterizing MPM heterogeneity provides a better understanding of the underlying oncogenic pathways and the related epigenetic regulation and immune and stromal contexts. We discuss the implications of these findings for guiding therapeutic strategies, particularly immunotherapies and targeted therapies.

Expression of Numb and Gli1 in malignant pleural mesothelioma and their clinical significance

AIM OF STUDY

Malignant pleural mesothelioma (MPM) is a highly lethal and refractory to multimodal treatment tumor. Numb is considered as a tumor suppressor playing critical roles in determining cell fate and has been shown to target the oncogenic transcription factor Gli1 for Itch-dependent ubiquitination, resulting in suppression of the oncogenic sonic hedgehog signaling in medulloblastoma. This study was designed to analysis the role of Numb and Gli1 in MPM.

MATERIALS AND METHODS

Tissues of 61 MPM patients and 22 normal pleura as control were investigated. Numb and Gli1 expression were evaluated by immunohistochemistry. The associations with clinical and pathological parameters of the two markers were statistically analyzed, and the correlation between them was also demonstrated.

RESULTS

The expression levels of Numb with nuclear Gli1 exhibited a significant inverse correlation (r = -0.361 P < 0.05). In addition, Numb has an inverse correlation with ki-67 labeling index (P < 0.05), and nuclear Gli1 was found in associated with the tumor International Mesothelioma Interest Group-stage (P < 0.05). The overall survival was influenced by the expression of Numb (P < 0.05) and histological subtype (P < 0.05), further regression analysis showed that only histological subtype has a prognostic influence on survival (P < 0.05).

CONCLUSION

The results provide new evidence of Numb and Gli1 on the clinical characteristics of MPM, which may be helpful in clinical diagnosis and targeted therapy. Further research with larger sample size is needed.

Secreted and Tissue miRNAs as Diagnosis Biomarkers of Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is a rare but aggressive tumor that originates in the pleura, is diagnosed in advanced stages and has a poor prognosis. Accurate diagnosis of MPM is often difficult and complex, and the gold standard diagnosis test is based on qualitative analysis of markers in pleural tissue by immunohistochemical staining. Therefore, it is necessary to develop quantitative and non-subjective alternative diagnostic tools. MicroRNAs are non-coding RNAs that regulate essential cellular mechanisms at the post-transcriptional level. Recent evidence indicates that miRNA expression in tissue and body fluids is aberrant in various tumors, revealing miRNAs as promising diagnostic biomarkers. This review summarizes evidence regarding secreted and tissue miRNAs as biomarkers of MPM and the biological characteristics associated with their potential diagnostic value. In addition to studies regarding miRNAs with potential diagnostic value for MPM, studies that aimed to identify the miRNAs involved in molecular mechanisms associated with MPM development are described with an emphasis on relevant aspects of the experimental designs that may influence the accuracy, consistency and real diagnostic value of currently reported data.

Dysregulated Expression of the MicroRNA miR-137 and Its Target YBX1 Contribute to the Invasive Characteristics of Malignant Pleural Mesothelioma

INTRODUCTION

Malignant pleural mesothelioma (MPM) is an aggressive malignancy linked to asbestos exposure. On a genomic level, MPM is characterized by frequent chromosomal deletions of tumor suppressors, including microRNAs. MiR-137 plays a tumor suppressor role in other cancers, so the aim of this study was to characterize it and its target Y-box binding protein 1 (YBX1) in MPM.

METHODS

Expression, methylation, and copy number status of miR-137 and its host gene MIR137HG were assessed by polymerase chain reaction. Luciferase reporter assays confirmed a direct interaction between miR-137 and Y-box binding protein 1 gene (YBX1). Cells were transfected with a miR-137 inhibitor, miR-137 mimic, and/or YBX1 small interfering RNA, and growth, colony formation, migration and invasion assays were conducted.

RESULTS

MiR-137 expression varied among MPM cell lines and tissue specimens, which was associated with copy number variation and promoter hypermethylation. High miR-137 expression was linked to poor patient survival. The miR-137 inhibitor did not affect target levels or growth, but interestingly, it increased miR-137 levels by means of mimic transfection suppressed growth, migration, and invasion, which was linked to direct YBX1 downregulation. YBX1 was overexpressed in MPM cell lines and inversely correlated with miR-137. RNA interference-mediated YBX1 knockdown significantly reduced cell growth, migration, and invasion.

CONCLUSIONS

MiR-137 can exhibit a tumor-suppressive function in MPM by targeting YBX1. YBX1 knockdown significantly reduces tumor growth, migration, and invasion of MPM cells. Therefore, YBX1 represents a potential target for novel MPM treatment strategies.

Synthetic lethality in malignant pleural mesothelioma with PARP1 inhibition

Malignant pleural mesotheliomas (MPM) are most often surgically unresectable, and they respond poorly to current chemotherapy and radiation therapy. Between 23 and 64% of malignant pleural mesothelioma have somatic inactivating mutations in the BAP1 gene. BAP1 is a homologous recombination (HR) DNA repair component found in the BRCA1/BARD1 complex. Similar to BRCA1/2 deficient cancers, mutation in the BAP1 gene leads to a deficient HR pathway and increases the reliance on other DNA repair pathways. We hypothesized that BAP1-mutant MPM would require PARP1 for survival, similar to the BRCA1/2 mutant breast and ovarian cancers. Therefore, we used the clinical PARP1 inhibitors niraparib and olaparib to assess whether they could induce synthetic lethality in MPM. Surprisingly, we found that all MPM cell lines examined, regardless of BAP1 status, were addicted to PARP1-mediated DNA repair for survival. We found that niraparib and olaparib exposure markedly decreased clonal survival in multiple MPM cell lines, with and without BAP1 mutations. This clonal cell death may be due to the extensive replication fork collapse and genomic instability that PARP1 inhibition induces in MPM cells. The requirement of MPM cells for PARP1 suggests that they may generally arise from defects in HR DNA repair. More importantly, these data demonstrate that the PARP1 inhibitors could be effective in the treatment of MPM, for which little effective therapy exists.