Molecular pathways and diagnosis in malignant mesothelioma: A review of the 14th International Conference of the International Mesothelioma Interest Group

Integrative eQTL and Mendelian randomization analysis reveals key genetic markers in mesothelioma

BACKGROUND

Mesothelioma is a rare cancer that originates from the pleura and peritoneum, with its incidence increasing due to asbestos exposure. Patients are frequently diagnosed at advanced stages, resulting in poor survival rates. Therefore, the identification of molecular markers for early detection and diagnosis is essential.

METHODS

Three mesothelioma datasets were downloaded from the GEO database for differential gene expression analysis. Instrumental variables (IVs) were identified based on expression quantitative trait locus (eQTL) data for Mendelian randomization (MR) analysis using mesothelioma Genome-Wide Association Study (GWAS) data from the FINNGEN database. The intersecting genes from MR-identified risk genes and differentially expressed genes were identified as key co-expressed genes for mesothelioma. Functional enrichment analyses, including Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Gene Set Enrichment Analysis (GSEA), as well as immune cell correlation analysis, were performed to elucidate the roles of key genes in mesothelioma. Additionally, the differential expression of key genes in mesothelioma was validated in independent GEO datasets and TCGA datasets. This integrative research combining multiple databases and analytical methods established a robust model for identifying mesothelioma risk genes.

RESULTS

The research conducted in our study identified 1608 genes that were expressed differentially in mesothelioma GEO datasets. By combining these genes with 192 genes from MR analysis, we identified 14 key genes. Notably, MPZL1, SOAT1, TACC3, and CYBRD1 are linked to a high risk of mesothelioma, while TGFBR3, NDRG2, EPAS1, CPA3, MNDA, PRKCD, MTUS1, ALOX15, LRRN3, and ITGAM are associated with a lower risk. These genes were found to be enriched in pathways associated with superoxide metabolism, cell cycle regulation, and proteasome function, all of which are linked to the development of mesothelioma. Noteworthy observations included a significant infiltration of M1 macrophages and CD4 + T cells in mesothelioma, with genes SOAT1, MNDA, and ITGAM showing a positive correlation with the level of M1 macrophage infiltration. Furthermore, the differential expression analyses conducted on the GEO validation set and TCGA data confirmed the significance of the identified key genes.

CONCLUSION

This integrative eQTL and Mendelian randomization analysis provides evidence of a positive causal association between 14 key co-expressed genes and mesothelioma genetically. These disease critical genes are implicated in correlations with biological processes and infiltrated immune cells related to mesothelioma. Moreover, our study lays a theoretical foundation for further research into the mechanisms of mesothelioma and potential clinical applications.

Diagnostic performance of immunohistochemistry markers for malignant pleural mesothelioma diagnosis and subtypes. A systematic review and meta-analysis

BACKGROUND

Malignant pleural mesothelioma (MPM) poses diagnostic challenges due to its resemblance to benign pleural pathologies and different histological subtypes. Several immunohistochemistry markers have been employed to aid in accurate diagnosis.

METHODS

The present systematic review and meta-analysis aimed to assess the diagnostic performance of various immunohistochemistry markers in malignant pleural mesothelioma diagnosis and its histological subtypes. Following the PRISMA guidelines, we systematically searched the literature for articles on using different immunohistochemical markers in MPM and its histological subtypes. EMBASE, LILACS, MEDLINE, and Virtual Health Library were searched for studies published up to August 2023. We used the QUADAS-2 (Quality Assessment of Diagnostic Accuracy Studies) criteria to assess the quality of the included articles. Meta-analyses were performed to determine prevalence using a random-effects model.

RESULTS

103 studies met the inclusion criteria, comprising a diverse range of immunohistochemistry markers. EMA and desmin-loss exhibited high sensitivity (96% and 92%, respectively) in distinguishing malignant pleural mesothelioma from benign pleural pathologies. Specificity was notably high for both BAP1-loss and survivin expression at 100%. Subtype-specific analyses demonstrated that EMA and HEG1 were sensitive markers for epithelioid mesothelioma, while GLUT1 showed high sensitivity for sarcomatoid mesothelioma. In cases comparing epithelioid mesothelioma and lung adenocarcinoma, CAM5.2 and calretinin displayed high sensitivity, while WT1 and BAP1-loss demonstrated exceptional specificity for malignant epithelioid mesothelioma. In the case of sarcomatoid mesothelioma and sarcomatoid lung carcinoma, GATA3 exhibited the most heightened sensitivity, while GATA3 and D2-40 displayed the best specificity for sarcomatoid malignant mesothelioma diagnosis.

CONCLUSION

Immunohistochemistry markers are essential in accurately diagnosing malignant pleural mesothelioma and its histological subtypes. This systematic review and meta-analysis provide a comprehensive insight into the diagnostic performance of these markers, facilitating their potential clinical utility in the discrimination of malignant pleural mesothelioma from other pleural pathologies and the differentiation of malignant pleural mesothelioma subtypes.

Membranous Expression of Heart Development Protein with EGF-like Domain 1 Is Associated with a Good Prognosis in Patients with Bladder Cancer

OBJECTIVE

To investigate the correlation between total protein expression of heart development protein with EGF-like domain 1 (HEG1) and clinicopathological characteristics in patients with bladder cancer (BC) after radical cystectomy (RC).

PATIENTS AND METHODS

We retrospectively analyzed data from 110 patients who underwent RC at Kitasato University Hospital. And we prepared an anti-HEG1 monoclonal antibody W10B9, which can detect total HEG1 protein. HEG1 protein expression in tumor cells was evaluated separately for membrane and cytoplasmic staining using immunohistochemistry.

RESULTS

Membranous HEG1 expression was associated with absent lymphovascular invasion (p < 0.01) and low pT stage (p < 0.01). Kaplan-Meier analysis revealed that the membranous HEG1-positive group had significantly long recurrence-free survival (RFS) (p < 0.01) and cancer-specific survival (p = 0.01). Expression of membranous HEG1 was identified as an independent prognostic factor for RFS (p = 0.04). There were no significant differences between cytoplasmic HEG1 expression and clinicopathologic factors including prognosis.

CONCLUSION

The expression of membranous HEG1 could serve as a favorable prognostic indicator in patients with BC treated with RC.

Peritoneal Mesothelioma-An Update

A remarkable amount of new information has been generated on peritoneal mesothelioma (PeM), ranging from nomenclature changes, including the removal of "malignant" when referring to this neoplasm and the use of the term "tumor" rather than "mesothelioma" to designate the neoplasm formerly known as "well-differentiated papillary mesothelioma", to the acknowledgment that PeMs can be associated with tumor predisposition syndromes or germline mutations. Although the disease is still more frequently seen in caucasian males, PeM is not uncommon in women. In addition, it can represent a diagnostic challenge when it has an uncommon presentation (ie, paraneoplastic syndrome or incidental finding) or when it has confounding histologic features. Ancillary testing, including immunohistochemical stains, in situ hybridization for CDKN2A or NF2 , and molecular studies, in selected cases, allows its correct diagnosis. The molecular landscape of PeM is still a work in progress; however, some findings, such as ALK gene rearrangements and EWSR1/FUS-ATF1 fusions, are specifically seen in PeM of young patients. The biological behavior of PeM is variable; however, cytoreductive surgery and hyperthermic intraperitoneal chemotherapy have markedly improved the survival of patients affected by this disease.

Cytological Assessment of Desmoplastic Malignant Pleural Mesothelioma in an Autopsy Case

INTRODUCTION

Desmoplastic malignant pleural mesothelioma (DMPM) is a sarcoma-type mesothelioma, comprising approximately 5% of malignant pleural mesotheliomas. Although effusion cytology is commonly used as the primary diagnostic approach for mesothelioma, it may not be useful for DMPM because of the presence of desmoplasia and bland cellular atypia. We report a case, and previously undescribed cytological features, of DMPM that was diagnosed during autopsy.

CASE PRESENTATION

A man in his 60s with a history of occupational asbestos exposure was referred to our hospital with right chest pain. A chest CT scan showed right pleural effusion. Thirteen months later, the patient died of respiratory failure. During autopsy, scrape-imprint smears were prepared and cytology of pleural effusions was performed. The scrape-imprint smear samples showed spindle cells with mild nuclear atypia and grooves with fibrous stroma. Pleural effusion cytology revealed spindle cells with mild nuclear atypia, as well as grooves with loose epithelial connections. Histological examination of the right pleura showed spindle cells proliferating with dense collagen fibers, as seen in the cytological samples, thus indicating a diagnosis of DMPM, which was confirmed by fluorescence in situ hybridization.

CONCLUSION

Cytological procedures such as pleural effusion cytology and scrape-imprinting cytology may help in diagnosing rare tumors such as DMPM.

Prognostic Bone Metastasis-Associated Immune-Related Genes Regulated by Transcription Factors in Mesothelioma

Mesothelioma (MESO) is a mesothelial originate neoplasm with high morbidity and mortality. Despite advancement in technology, early diagnosis still lacks effectivity and is full of pitfalls. Approaches of cancer diagnosis and therapy utilizing immune biomarkers and transcription factors (TFs) have attracted more and more attention. But the molecular mechanism of these features in MESO bone metastasis has not been thoroughly studied. Utilizing high-throughput genome sequencing data and lists of specific gene subsets, we performed several data mining algorithm. Single-sample Gene Set Enrichment Analysis (ssGSEA) was applied to identify downstream immune cells. Potential pathways involved in MESO bone metastasis were identified using Gene Oncology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, Gene Set Variation Analysis (GSVA), Gene Set Enrichment Analysis (GSEA), and Cox regression analysis. Ultimately, a model to help early diagnosis and to predict prognosis was constructed based on differentially expressed immune-related genes between bone metastatic and nonmetastatic MESO groups. In conclusion, immune-related gene SDC2, regulated by TFs TCF7L1 and POLR3D, had an important role on immune cell function and infiltration, providing novel biomarkers and therapeutic targets for metastatic MESO.

Correlation of MET-Receptor Overexpression with MET Gene Amplification and Patient Outcome in Malignant Mesothelioma

Thanks to clinically newly introduced inhibitors of the mesenchymal–epithelial transition (MET) receptor tyrosine-kinase, MET-gene copy number gain/amplification (MET-GCNG/GA) and increased expression of the MET protein are considered very promising therapeutic targets in lung cancer and other malignancies. However, to which extent these MET alterations occur in malignant mesothelioma (MM) remains unclear. Thus, we investigated by well-established immunohistochemistry and fluorescence in situ hybridization methods, the frequency of these alterations in specimens from 155 consecutive MMs of different subtypes obtained from pleural or peritoneal biopsies and pleurectomies. Thirty-three benign reactive mesothelial proliferations (RMPs) were used as controls. MET-protein upregulation was observed in 35% of all MM-cases, though restricted to predominantly epithelioid MMs. We detected low-/intermediate-level MET-GCNG/GA in 22.2% of MET-overexpressing MMs (7.8% of whole MM-cohort) and no MET-GCNG/GA in the other 77.8%, suggesting other upregulating mechanisms. In contrast, 100% of RMPs exhibited no MET-upregulation or MET-GCNG/-GA. Neither MET exon 14 skipping mutations nor MET-fusions were detected as mechanisms of MET overexpression in MM using RNA next-generation sequencing. Finally, in two cohorts of 30 MM patients with or without MET overexpression (MET-positive/-negative) that were matched for several variables and received the same standard chemotherapy, the MET-positive cases showed a significantly lower response rate, but no significant difference in progression-free or overall survival. Our results imply that MET overexpression occurs in a substantial fraction of predominantly epithelioid MMs, but correlates poorly with MET-amplification status, and may impact the likelihood of response to mesothelioma standard chemotherapy. The predictive significance of MET-IHC and -FISH for possible MET-targeted therapy of MM remains to be elucidated.

Membranous HEG1 expression is a useful marker in the differential diagnosis of epithelioid and biphasic malignant mesothelioma versus carcinomas

Sialylated HEG1 has been reported as a highly specific and sensitive mesothelioma marker but a comprehensive evaluation of its expression in carcinomas in different organs, various sarcomas and reactive mesothelial proliferations has not been reported. The aim of this study was to evaluate the clinical applicability of HEG1 as a marker in the diagnosis of mesothelioma. HEG1 immunoreactivity was evaluated in whole sections of 122 mesotheliomas, 75 pulmonary carcinomas, 55 other carcinomas, 16 mesenchymal tumors, and 24 reactive mesothelial proliferations and in tissue microarrays containing 70 epithelioid (EM), 36 biphasic (BM), and 2 sarcomatoid mesotheliomas (SM). In whole sections and tissue microarrays, respectively, membranous HEG1 was expressed in 93.0% and 85.5% of EM, 81.3% and 69.4% of BM, 0% and 0% of SM. HEG1 was not expressed in pulmonary adenocarcinomas. HEG1 was expressed as cytoplasmic immunoreactivity in pulmonary squamous cell carcinomas (21.7%). Membranous HEG1 staining was seen in ovarian carcinomas (66.7%), thyroid carcinomas (100%), reactive conditions (16.7%), and mesenchymal tumors (18.8%). The sensitivity of membranous HEG1 expression to distinguish EM/BM from all carcinomas was 88.8%. The specificity for the differential diagnosis between EM/BM and all carcinomas and pulmonary carcinomas was 92.3% and 98.7%, respectively.

Cell-blocks and immunohistochemistry

The interpretation of results on immunostained cell-block sections has to be compared with the cumulative published data derived predominantly from formalin-fixed paraffin-embedded (FFPE) tissue sections. Because of this, it is important to recognize that the fixation and processing protocol should not be different from the routinely processed FFPE surgical pathology tissue. Exposure to non-formalin fixatives or reagents may interfere with the diagnostic immunoreactivity pattern. The immunoprofile observed on such cell-blocks, which are not processed in a manner similar to the surgical pathology specimens, may not be representative resulting in aberrant results. The field of immunohistochemistry (IHC) is advancing continuously with the standardization of many immunomarkers. A variety of technical advances such as multiplex IHC with refined methodologies and automation is increasing its role in clinical applications. The recent addition of rabbit monoclonal antibodies has further improved sensitivity. As compared to the mouse monoclonal antibodies, the rabbit monoclonal antibodies have 10 to 100 fold higher antigen affinity. Most of the scenarios involve the evaluation of coordinate immunostaining patterns in cell-blocks with relatively scant diagnostic material without proper orientation which is usually retained in most of the surgical pathology specimens. These challenges are addressed if cell-blocks are prepared with some dedicated methodologies such as NextGen CelBloking™ (NGCB) kits. Cell-blocks prepared by NGCB kits also facilitate the easy application of the SCIP (subtractive coordinate immunoreactivity pattern) approach for proper evaluation of coordinate immunoreactivity. Various cell-block and IHC-related issues are discussed in detail.

Adenomatoid Tumor: A Review of Pathology With Focus on Unusual Presentations and Sites, Histogenesis, Differential Diagnosis, and Molecular and Clinical Aspects With a Historic Overview of Its Description

Adenomatoid tumors have been described almost a century ago, and their nature has been the subject of debate for decades. They are tumors of mesothelial origin usually involving the uterus, the Fallopian tubes, and the paratesticular region. Adenomatoid tumors of the adrenal gland, the liver, the extragenital peritoneum, the pleura, and the mediastinum have been rarely reported. They are usually small incidental findings, but large, multicystic and papillary tumors, as well as multiple tumors have been described. Their pathogenesis is related to immunosuppression and to TRAF7 mutations. Despite being benign tumors, there are several macroscopic or clinical aspects that could raise diagnostic difficulties. The aim of this review was to describe the microscopic and macroscopic aspects of adenomatoid tumor with a special focus on its differential diagnosis and pathogenesis and the possible link of adenomatoid tumor with other mesothelial lesions, such as the well-differentiated papillary mesothelioma and the benign multicystic mesothelioma, also known as multilocular peritoneal cysts.

HEG1 Is a Highly Specific and Sensitive Marker of Epithelioid Malignant Mesothelioma

Malignant mesothelioma can be difficult to distinguish from other malignancies, particularly non-small cell lung carcinomas (NSCLCs), without immunohistochemistry. However, conventional markers of mesothelial lineage all have variable degrees of cross-reactivity with other neoplasms, including NSCLCs, necessitating the use of multiple mesothelioma and carcinoma markers in every case for accurate diagnosis. A recently described monoclonal HEG homolog 1 (HEG1) antibody was proposed to be a specific marker for mesothelioma. Here we performed a large scale assessment of the SKM9-2 HEG1 antibody using tissue microarrays containing 69 epithelioid mesotheliomas, 32 sarcomatoid mesotheliomas, 167 NSCLCs, and 17 ovarian high-grade serous carcinomas. Strong membrane staining, usually diffuse, for HEG1 was seen in 65/69 (94%) epithelioid mesotheliomas, 0/60 pulmonary squamous cell carcinomas, 0/73 pulmonary adenocarcinomas, and 0/13 pulmonary large cell carcinomas. HEG1 showed staining in 14/32 (44%) sarcomatoid mesotheliomas compared with 0/21 sarcomatoid pulmonary carcinomas. Three of 17 (18%) high-grade serous carcinomas demonstrated membrane staining. Ten B3 thymoma whole sections were negative. On the microarrays, the conventional mesothelial markers calretinin, WT1, D2-40, and CK5/6 had sensitivities for epithelioid mesothelioma of 94%, 90%, 96%, and 91%, respectively. We conclude that HEG1 SKM9-2 antibody offers sensitivity comparable to conventional markers for epithelioid mesotheliomas, but provides considerably better specificity, such that the diagnosis of epithelioid mesothelioma versus NSCLC potentially could be confirmed with a combination of HEG1 and a suitable broad spectrum carcinoma marker such as claudin-4. HEG1 is specific but insensitive for separating sarcomatoid mesotheliomas from sarcomatoid lung carcinomas.

Comparative analysis of prognostic histopathologic parameters in subtypes of epithelioid pleural mesothelioma

AIMS

Malignant pleural mesothelioma (MPM) is a rare malignancy with a dismal prognosis. While the epithelioid type is associated with a more favourable outcome, additional factors are needed to further stratify prognosis and to identify patients who can benefit from multimodal treatment. As epithelioid MPM shows remarkable morphological variability, the prognostic role of the five defined morphologies, the impact of the nuclear grading system and the mitosis-necrosis score were investigated in this study.

METHODS AND RESULTS

Tumour specimens of 192 patients with epithelioid MPM from five European centres were histologically subtyped. Nuclear grading and mitosis-necrosis score were determined and correlated with clinicopathological parameters and overall survival (OS). Digital slides of 55 independent cases from The Cancer Genome Atlas (TCGA) database were evaluated for external validation. Histological subtypes were collapsed into three groups based on their overlapping survival curves. The tubulopapillary/microcystic group had a significantly longer OS than the solid/trabecular group (732 days versus 397 days, P = 0.0013). Pleomorphic tumours had the shortest OS (173 days). The solid/trabecular variants showed a significant association with high nuclear grade and mitosis-necrosis score. The mitosis-necrosis score was a robust and independent prognostic factor in our patient cohort. The prognostic significance of all three parameters was externally validated in the TCGA cohort. Patients with tubulopapillary or microcystic tumours showed a greater improvement in OS after receiving multimodal therapy than those with solid or trabecular tumours.

CONCLUSIONS

Histological subtypes of epithelioid MPM have a prognostic impact, and might help to select patients for intensive multimodal treatment approaches.

Update on the pathologic diagnosis of malignant mesothelioma

Malignant mesothelioma is an uncommon tumor that may be difficult to diagnose. The International Mesothelioma Interest Group has been writing guidelines for pathological diagnosis that are periodically updated. The guidelines are being updated based on published literature in the last 3 years, and experience of more than 20 leading international pathologists in the field who will be co-authors. Updates were discussed by attendees of the Pulmonary Pathology Society Biennial Meeting (Dubrovnik, Croatia, June 2019). Areas with significant advancements/changes include utilization of immunohistochemistry (establishing mesothelial lineage and benign versus malignant), prognosis and nuclear grading, biphasic malignant mesothelioma, transitional pattern, malignant mesothelioma in situ, and therapeutic/molecular targets.

An integrative histopathologic clustering model based on immuno-matrix elements to predict the risk of death in malignant mesothelioma

Objective

Previous studies have reported a close relationship between malignant mesothelioma (MM) and the immune matricial microenvironment (IMM). One of the major problems in these studies is the lack of adequate adjustment for potential confounders. Therefore, the aim of this study was to identify and quantify risk factors such as IMM and various tumor characteristics and their association with the subtype of MM and survival.

Methods

We examined IMM and other tumor markers in tumor tissues from 82 patients with MM. These markers were evaluated by histochemistry, immunohistochemistry, immunofluorescence, and morphometry. Logistic regression analysis, cluster analysis, and Cox regression analysis were performed.

Results

Hierarchical cluster analysis revealed two clusters of MM that were independent of clinicopathologic features. The high‐risk cluster included MM with high tumor cellularity, high type V collagen (Col V) fiber density, and low CD8⁺ T lymphocyte density in the IMM. Our results showed that the risk of death was increased for patients with MM with high tumor cellularity (OR = 1.63, 95% CI = 1.29‐2.89, P = .02), overexpression of Col V (OR = 2.60, 95% CI = 0.98‐6.84, P = .04), and decreased CD8 T lymphocytes (OR = 1.001, 95% CI = 0.995‐1.007, P = .008). The hazard ratio for the high‐risk cluster was 2.19 (95% CI = 0.54‐3.03, P < .01) for mortality from MM at 40 months.

Conclusion

Morphometric analysis of Col V, CD8⁺ T lymphocytes, and tumor cellularity can be used to identify patients with high risk of death from MM.

HEG1-responsive microRNA-23b regulates cell proliferation in malignant mesothelioma cells

Malignant mesothelioma (MM) is a fatal tumor, and the absence of a specific diagnostic marker and/or a pathogenic molecule-targeting drug is a major issue for its pathological diagnosis and for targeting therapy. The molecular target of MM has not been elucidated because of unknown survival, death, and cytotoxic signals in MM. HEG homolog 1 (HEG1) is a mucin-like membrane protein that contains epidermal growth factor-like domains, and it plays an important role in cancers through aberrant signaling, including that during cell adhesion, as well as through protection from invasion of tumor cells. HEG1 expression supports the survival and proliferation of MM cells. In this study, functional analysis of HEG1 and microRNAs using MM cell lines (H226, MESO4, H2052) was performed. The MTS assay revealed that cell proliferation was significantly reduced upon transient transfection with microRNA-23b (miR-23b) inhibitor and/or HEG1 siRNA. The Annexin V assay revealed that apoptosis was induced upon suppression of miR-23b and/or HEG1. Western blotting showed that the autophagy-related protein LC3-II was induced upon suppression of miR-23b and/or HEG1. These results revealed that miR-23b contributes to HEG1-dependent cell proliferation through evasion of cytotoxicity induced by apoptosis and autophagy in MM cells. HEG1-dependent/mediated miR-23b signaling may therefore be a potential target for MM diagnosis and therapy.

Application of immunohistochemistry in diagnosis and management of malignant mesothelioma

Immunohistochemistry plays an indispensable role in accurate diagnosis of malignant mesothelioma, particularly in morphologically challenging cases and in biopsy and cytology specimens, where tumor architecture is difficult or impossible to evaluate. Application of a targeted panel of mesothelial- and epithelial-specific markers permits correct identification of tumor lineage in the vast majority of cases. An immunopanel including two mesothelial markers (calretinin, CK5/6, WT-1, or D2-40) and two epithelial markers (MOC-31 and claudin-4) offers good sensitivity and specificity, with adjustments as appropriate for the differential diagnosis. Once mesothelial lineage is established, malignancy-specific studies can help verify a diagnosis of malignant mesothelioma. BAP1 loss, CDKN2A homozygous deletion, and MTAP loss are highly specific markers of malignancy in a mesothelial lesion, and they attain acceptable diagnostic sensitivity when applied as a diagnostic panel. Novel markers of malignancy, such as 5-hmC loss and increased EZH2 expression, are promising, but have not yet achieved widespread clinical adoption. Some diagnostic markers also have prognostic significance, and PD-L1 immunohistochemistry may predict tumor response to immunotherapy. Application and interpretation of these immnuomarkers should always be guided by clinical history, radiographic findings, and above all histomorphology.

Diagnosis of Mesothelioma

Mesothelioma is a rare neoplasm that arises from mesothelial cells lining body cavities including the pleura, pericardium, peritoneum, and tunica vaginalis. Most malignant mesotheliomas occur in the chest and are frequently associated with a history of asbestos exposure. The diagnosis of malignant mesothelioma is challenging and fraught with pitfalls, particularly in small biopsies. This article highlights what the pathologist needs to know regarding the clinical and radiographic presentation of mesothelioma, histologic features including subtypes and variants, and recent advances in immunohistochemical markers and molecular testing.