Background: Hodgkin lymphoma (HL) is predominantly composed of reactive, non-neoplastic cells surrounding scarcely distributed tumor cells, that is, so-called Hodgkin and Reed-Sternberg (HRS) or lymphocyte predominant (LP) cells. This scarcity impeded the analysis of the tumor cell genomes for a long time, but recently developed methods (especially laser capture microdissection, flow cytometry/fluorescence-activated cell sorting) facilitated molecular investigation, elucidating the pathophysiological principles of “Hodgkin lymphomagenesis”. Methods: We reviewed the relevant literature of the last three decades focusing on the genomic landscape of classic and nodular lymphocyte predominant HL (NLPHL) and summarized molecular cornerstones. Results: Firstly, the malignant cells of HL evade the immune system by altered expression of PDL1/2, B2M and MHC class I and II due to various genetic alterations. Secondly, tumor growth is promoted by permanently activated JAK/STAT signaling due to pervasive mutations of multiple genes involved in the pathway. Thirdly, apoptosis of neoplastic cells is prevented by alterations of NF-κB compounds and the PI3K/AKT/mTOR axis. Additionally, Epstein-Barr virus infection can simultaneously activate JAK/STAT and NF-κB, similarly leading to enhanced survival and evasion of apoptosis. Finally, epigenetic phenomena such as promoter hypermethylation lead to the downregulation of B-lineage-specific, tumor-suppressor and immune regulation genes. Conclusion: The blueprint of HL genomics has been laid, paving the way for future investigations into its complex pathophysiology.

S-methyl-5'-thioadenosine phosphorylase (MTAP) deficiency is an emerging biomarker in non-small cell lung cancer (NSCLC) and beyond. The MTAP gene is located in the chromosomal region 9p21.3, which shows one of the most common homozygous deletions across all human cancers (9p21 loss). Loss of 9p21 is found in the majority of pleural mesotheliomas, where it serves as an established diagnostic marker. Until recently, fluorescence in situ hybridization (FISH) was the gold standard for the detection of 9p21 losses, but loss of MTAP expression by immunohistochemistry (IHC) gains increasing importance as an easy to apply and cost-effective diagnostic surrogate marker. Besides, MTAP loss, which has been reported in 13% of NSCLC, is becoming an emerging predictive biomarker in two different scenarios in NSCLC and other cancer types: 1) MTAP loss seems to negatively predict the response to immune checkpoint inhibitor (ICI) treatment via silencing of the tumor microenvironment, and 2) MTAP loss serves as a predictive biomarker for novel targeted treatment strategies. MTAP deficiency leads to an impaired function of the protein arginine methyltransferase 5 (PRMT5) due to its partial inhibition by MTAP's accumulating substrate methylthioadenosine (MTA). This process leaves MTAP deficient tumor cells heavily dependent on the remaining function of PRMT5, making it a perfect target for synthetic lethality. Indeed, MTA-cooperative PRMT5-inhibitors are now tested in several clinical trials with promising early results in solid malignancies. With its emergence as a predictive biomarker, the implementation of MTAP IHC into diagnostic routine for NSCLC and other tumors is likely to take place soon. In this review article, we summarize the current literature on the role of MTAP in thoracic tumors and evaluate different testing methods, including IHC, FISH and next generation sequencing.

Loss of MTAP serves as a potential predictive marker of response to cooperative PRMT5 inhibitors and as negative predictor of response to immune-checkpoint inhibitors. We investigated the prevalence of MTAP deficiency by immunohistochemistry (IHC) in NSCLC as a surrogate for MTAP loss. MTAP IHC was performed on 698 NSCLC samples. Data from routine next-generation sequencing, analyzed with the Oncomine^TM Precision Assay (OPA-NGS, Thermo Fisher), were available in 426 cases, including CDKN2A copy number variation (CNV) data in 411 cases. Our findings were compared to data from The Cancer Genome Atlas (TCGA). MTAP deficiency by IHC was found in 18.2% of NSCLC. CDKN2A loss by OPA-NGS - used as a surrogate for MTAP loss - was significantly associated with MTAP deficiency by IHC, but was found in only 28.4% of the MTAP deficient NSCLC analyzed for CDKN2A CNV. In the TCGA cohort, only 72.9% of NSCLC with CDKN2A loss had a concurrent MTAP loss defined by CNV. MTAP IHC seems to be better suited than OPA-NGS to assess the MTAP status in NSCLC, especially as the MTAP gene is not specifically covered within this panel. CDKN2A loss is not a reliable MTAP loss surrogate, as it overestimated MTAP loss in more than 25% of the cases in the TCGA cohort.

Myeloproliferative neoplasms (MPN) and myelodysplastic/myeloproliferative neoplasms (MDS/MPN) both harbor the potential to undergo myelodysplastic progression or acceleration and can transform into blast-phase MPN or MDS/MPN, a form of secondary acute myeloid leukemia (AML). Although the initiating transforming events are yet to be determined, current concepts suggest a stepwise acquisition of (additional) somatic mutations—apart from the initial driver mutations—that trigger disease evolution. In this study we molecularly analyzed paired bone marrow samples of MPN and MDS/MPN patients with known progression and compared them to a control cohort of patients with stable disease course. Cases with progression displayed from the very beginning a higher number of mutations compared to stable ones, of which mutations in five (ASXL1, DNMT3A, NRAS, SRSF2 and TP53) strongly correlated with progression and/or transformation, even if only one of these genes was mutated, and this particularly applied to MPN. TET2 mutations were found to have a higher allelic frequency than the putative driver mutation in three progressing cases (“TET2-first”), whereas two stable cases displayed a TET2-positive subclone (“TET2-second”), supporting the hypothesis that not only the sum of mutations but also their order of appearance matters in the course of disease. Our data emphasize the importance of genetic testing in MPN and MDS/MPN patients in terms of risk stratification and identification of imminent disease progression.

The World Health Organization's (WHO) Reporting System for Lung Cytopathology intends to standardise the diagnosing and reporting of cytology specimens from the lung and aims at enhancing the communication between clinicians and (cyto)pathologists. It is closely connected to the 5th edition of the WHO Classification of Thoracic Tumours. The system includes five diagnostic categories, among them the ‘Malignant’ diagnostic category that incorporates both primary malignant tumours and metastases. Advancements in bronchoscopy have notably improved the diagnostic capacity of cytological specimens that represent the sole source of tumour material in approximately 40% of all lung carcinoma cases. An accurate diagnosis of malignancy and treatment‐guiding classification into specific tumour types and subtypes can reliably be achieved with cytology specimens. They additionally serve as an excellent source for predictive immunocytochemistry (ICC) and molecular testing for targetable oncogenic alterations. This review article provides an overview of the key cytopathological features defining the ‘Malignant’ category of the WHO Reporting System for Lung Cytopathology for non‐small cell carcinomas, neuroendocrine neoplasms and other specific carcinomas and malignancies which can be encountered in cytological specimens of the lung. It further describes the application of ancillary techniques, such as ICC and molecular testing, that have been successfully incorporated into different cytological sample types using various preparation methods.

Clonal hematopoiesis of indeterminate potential (CHIP) is a predisposing condition to lymphoma development. CHIP carrying mutations that are recurrently found in lymphomas are designated as L‐CHIP. We presume that bone marrow‐derived L‐CHIP populations are able to expand and manifest in peripheral lymphoid tissues, where they could hence be called L‐CHIP tissue‐equivalents . There, they may proliferate and foster unexplained follicular hyperplasias, and, thus, potentially represent an early precursor of lymphoma. Analogously, we hypothesize that certain germline‐derived mutations lead to lymphoproliferations ( germline‐derived lymphoproliferations ) in otherwise healthy individuals. We collected seven exceptional cases of symptomatic nodal and extranodal lymphoid hyperplasias, which were all morphologically suspicious and displayed somatic and/or germline‐derived mutations recurrently found in B‐cell lymphomas. One patient developed follicular lymphoma after 8 years carrying the same non‐productive immunoglobulin rearrangement detected in the initial biopsy. L‐CHIP tissue‐equivalents and germline‐derived lymphoproliferations potentially represent first steps in lymphomagenesis and knowledge about their existence might be of diagnostic utility in challenging cases of (atypical) lymphoproliferations. With histology, immunohistochemistry, and molecular testing, such lesions can be identified in situ.