Search

The histological similarities between pleomorphic adenomas (PAs) and cutaneous mixed tumors (CMTs) found in certain facial regions can create a diagnostic challenge. Molecular findings reveal common genetic profiles, particularly PLAG1 rearrangements in both PA and CMT. Although molecular distinctions have received limited attention, our observations indicate multiple cases of CMTs carrying the TRPS1::PLAG1 fusion. This clinical experience has driven our investigation into the potential diagnostic utility of TRPS1::PLAG1 fusions for determining tumor origin. Two cohorts consisting of 46 cases of CMT and 45 cases of PA of the salivary glands were obtained from French institutions and reviewed by specialists in each subspecialty. RNA sequencing analysis was conducted to identify the molecular features of cases harboring PLAG1. Clinical, pathological, and molecular data were collected. In this study, cases of CMT exhibited recurrent gene fusions, primarily TRPS1::PLAG1 (74%). These tumors shared characteristic histological features, including tubuloductal differentiation in 55% of cases and squamous metaplasia in varying proportions. In contrast, cases of PA had gene fusions involving PLAG1 with various gene partners, with only one case in which TRPS1::PLAG1 was identified. This disparity was also observed at the transcriptomic level between TRPS1::PLAG1 CMTs and other tumors. However, TRPS1 immunostaining did not correlate with TRPS1::PLAG1 fusion. In conclusion, we report that recurrent TRPS1::PLAG1 fusion CMTs exhibit similar characteristic histological features, including tubuloductal differentiation that is associated with squamous metaplasia in around half of cases. Detection of this fusion could be valuable in correctly identifying the origin of these tumors. © 2024 The Pathological Society of Great Britain and Ireland. [ABSTRACT FROM AUTHOR]

Aims: Porocarcinoma is a malignant sweat gland tumour differentiated toward the upper part of the sweat duct and may arise from the transformation of a preexisting benign poroma. In 2019, Sekine et al. demonstrated the presence of YAP1::MAML2 and YAP1::NUTM1 fusions in most poromas and porocarcinomas. Recently, our group identified PAK2‐fusions in a subset of benign poromas. Herein we report a series of 12 porocarcinoma cases harbouring PAK1/2/3 fusions. Methods and Results: Five patients were male and the median age was 79 years (ranges: 59–95). Tumours were located on the trunk (n = 7), on the thigh (n = 3), neck (n = 1), or groin area (n = 1). Four patients developed distant metastases. Microscopically, seven cases harboured a benign poroma component and a malignant invasive part. Ductal formations were observed in all, while infundibular/horn cysts and cells with vacuolated cytoplasm were detected in seven and six tumours, respectively. In three cases, the invasive component consisted of a proliferation of elongated cells, some of which formed pseudovascular spaces, whereas the others harboured a predominant solid or trabecular growth pattern. Immunohistochemical staining for CEA and EMA confirmed the presence of ducts. Focal androgen receptor expression was detected in three specimens. Whole RNA sequencing evidenced LAMTOR1::PAK1 (n = 2), ZDHHC5::PAK1 (n = 2), DLG1::PAK2, CTDSP1::PAK1, CTNND1::PAK1, SSR1::PAK3, CTNNA1::PAK2, RNF13::PAK2, ROBO1::PAK2, and CD47::PAK2. Activating mutation of HRAS (G13V, n = 3, G13R, n = 1, Q61L, n = 2) was present in six cases. Conclusion: Our study suggests that PAK1/2/3 fusions is the oncogenic driver of a subset of porocarcinomas lacking YAP1 rearrangement. [ABSTRACT FROM AUTHOR]

Context: Merkel cell carcinoma diagnosis is often based on microscopic examination by pathologists. While histopathologic diagnosis primarily hinges on conscious and analytical cognition, the pathologist's decision‐making process is also influenced by a rapid "gist" or "gestalt" approach. In this study, using cases of Merkel cell carcinoma as a model, we aim to assess how pathologists' viewing short videos containing conceptual clues and visual aids, in conjunction with reading an original article as a reference, may enhance their diagnostic performance. Method: Sixteen pathologists were included in the present work. After participants had read the original article, their ability to distinguish Merkel cell polyomavirus (MCPyV)+ and MCPyV− Merkel cell carcinoma cases was evaluated on a first preliminary series of 20 cases. Following this test, the participants watched the video and then evaluated a second "experimental" series of 20 independent cases. Results: After reading the original article, for each case, a median number of 12 participants (75%, Q1–Q3: 10–13) classified the specimen in the correct category (92 incorrect answers in the whole series). An important interobserver variability was observed in this setting (Kappa coefficient = 0.465). By contrast, following the video, all cases were correctly classified by most of the participants, with only 12 incorrect answers on the whole series and excellent interobserver reproducibility (Kappa coefficient = 0.846). Conclusion: Our study demonstrated that providing a short video together with an original article may enhance pathologists' performance in diagnosing Merkel cell carcinoma. [ABSTRACT FROM AUTHOR]

Merkel cell carcinoma (MCC) is an aggressive skin cancer frequently caused by genomic integration of the Merkel cell polyomavirus (MCPyV). MCPyV‐negative cases often present as combined MCCs, which represent a distinctive subset of tumors characterized by association of an MCC with a second tumor component, mostly squamous cell carcinoma. Up to now, only exceptional cases of combined MCC with neuroblastic differentiation have been reported. Herein we describe two additional combined MCCs with neuroblastic differentiation and provide comprehensive morphologic, immunohistochemical, transcriptomic, genetic and epigenetic characterization of these tumors, which both arose in elderly men and appeared as an isolated inguinal adenopathy. Microscopic examination revealed biphasic tumors combining a poorly differentiated high‐grade carcinoma with a poorly differentiated neuroblastic component lacking signs of proliferation. Immunohistochemical investigation revealed keratin 20 and MCPyV T antigen (TA) in the MCC parts, while neuroblastic differentiation was confirmed in the other component in both cases. A clonal relation of the two components can be deduced from 20 and 14 shared acquired point mutations detected by whole exome analysis in both combined tumors, respectively. Spatial transcriptomics demonstrated a lower expression of stem cell marker genes such as SOX2 and MCM2 in the neuroblastic component. Interestingly, although the neuroblastic part lacked TA expression, the same genomic MCPyV integration and the same large T‐truncating mutations were observed in both tumor parts. Given that neuronal transdifferentiation upon TA repression has been reported for MCC cell lines, the most likely scenario for the two combined MCC/neuroblastic tumors is that neuroblastic transdifferentiation resulted from loss of TA expression in a subset of MCC cells. Indeed, DNA methylation profiling suggests an MCC‐typical cellular origin for the combined MCC/neuroblastomas. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland. [ABSTRACT FROM AUTHOR]

Merkel cell carcinoma (MCC) is a highly aggressive skin cancer associated with integration of Merkel cell polyomavirus (MCPyV). MCPyV‐encoded T‐antigens (TAs) are pivotal for sustaining MCC's oncogenic phenotype, i.e., repression of TAs results in reactivation of the RB pathway and subsequent cell cycle arrest. However, the MCC cell line LoKe, characterized by a homozygous loss of the RB1 gene, exhibits uninterrupted cell cycle progression after shRNA‐mediated TA repression. This unique feature allows an in‐depth analysis of the effects of TAs beyond inhibition of the RB pathway, revealing the decrease in expression of stem cell‐related genes upon panTA‐knockdown. Analysis of gene regulatory networks identified members of the E2F family (E2F1, E2F8, TFDP1) as key transcriptional regulators that maintain stem cell properties in TA‐expressing MCC cells. Furthermore, minichromosome maintenance (MCM) genes, which encodes DNA‐binding licensing proteins essential for stem cell maintenance, were suppressed upon panTA‐knockdown. The decline in stemness occurred simultaneously with neural differentiation, marked by the increased expression of neurogenesis‐related genes such as neurexins, BTG2, and MYT1L. This upregulation can be attributed to heightened activity of PBX1 and BPTF, crucial regulators of neurogenesis pathways. The observations in LoKe were confirmed in an additional MCPyV‐positive MCC cell line in which RB1 was silenced before panTA‐knockdown. Moreover, spatially resolved transcriptomics demonstrated reduced TA expression in situ in a part of a MCC tumor characterized by neural differentiation. In summary, TAs are critical for maintaining stemness of MCC cells and suppressing neural differentiation, irrespective of their impact on the RB‐signaling pathway. [ABSTRACT FROM AUTHOR]

Background Merkel cell carcinoma (MCC) is an aggressive malignant neuroendocrine tumour. There are two subsets of MCC, one related to Merkel cell polyomavirus (MCPyV) and the other to ultraviolet radiation (UVR). MCPyV-positive and MCPyV-negative MCCs have been considered to be different tumours, as the former harbour few DNA mutations and are not related to UVR, and the latter usually arise in sun-exposed areas and may be found in conjunction with other keratinocytic tumours, mostly squamous cell carcinomas. Two viral oncoproteins, large T antigen (LT; coded by MCPyV_gp3) and small T antigen (sT; coded by MCPyV_gp4), promote different carcinogenic pathways. Objectives To determine which genes are differentially expressed in MCPyV-positive and MCPyV-negative MCC; to describe the mutational burden and the most frequently mutated genes in both MCC subtypes; and to identify the clinical and molecular factors that may be related to patient survival. Methods Ninety-two patients with a diagnosis of MCC were identified from the medical databases of participating centres. To study gene expression, a customized panel of 172 genes was developed. Gene expression profiling was performed with nCounter technology. For mutational studies, a customized panel of 26 genes was designed. Somatic single nucleotide variants (SNVs) were identified following the GATK Best Practices workflow for somatic mutations. Results The expression of LT enabled the series to be divided into two groups (LT positive, n = 55; LT negative, n = 37). Genes differentially expressed in LT- negative patients were related to epithelial differentiation, especially SOX9 , or proliferation and the cell cycle (MYC , CDK6), among others. Congruently, LT displayed lower expression in SOX9 -positive patients, and differentially expressed genes in SOX9 -positive patients were related to epithelial/squamous differentiation. In LT -positive patients, the mean SNV frequency was 4.3; in LT -negative patients it was 10 (P = 0.03). On multivariate survival analysis, the expression of SNAI1 [hazard ratio (HR) 1.046, 95% confidence interval (CI) 1.007–1.086; P = 0.02] and CDK6 (HR 1.049, 95% CI 1.020–1.080; P = 0.001) were identified as risk factors. Conclusions Tumours with weak LT expression tend to co-express genes related to squamous differentiation and the cell cycle, and to have a higher mutational burden. These findings are congruent with those of earlier studies. [ABSTRACT FROM AUTHOR]

Aims: Merkel cell carcinoma (MCC) is frequently caused by the Merkel cell polyomavirus (MCPyV). Characteristic for these virus‐positive (VP) MCC is MCPyV integration into the host genome and truncation of the viral oncogene Large T antigen (LT), with full‐length LT expression considered as incompatible with MCC growth. Genetic analysis of a VP‐MCC/trichoblastoma combined tumour demonstrated that virus‐driven MCC can arise from an epithelial cell. Here we describe two further cases of VP‐MCC combined with an adnexal tumour, i.e. one trichoblastoma and one poroma. Methods and results: Whole‐genome sequencing of MCC/trichoblastoma again provided evidence of a trichoblastoma‐derived MCC. Although an MCC‐typical LT‐truncating mutation was detected, we could not determine an integration site and we additionally detected a wildtype sequence encoding full‐length LT. Similarly, Sanger sequencing of the combined MCC/poroma revealed coding sequences for both truncated and full‐length LT. Moreover, in situ RNA hybridization demonstrated expression of a late region mRNA encoding the viral capsid protein VP1 in both combined as well as in a few cases of pure MCC. Conclusion: The data presented here suggest the presence of wildtype MCPyV genomes and VP1 transcription in a subset of MCC. [ABSTRACT FROM AUTHOR]

Poroma is a benign sweat gland tumour showing morphological features recapitulating the superficial portion of the eccrine sweat coil. A subset of poromas may transform into porocarcinoma, its malignant counterpart. Poroma and porocarcinoma are characterised by recurrent gene fusions involving YAP1, a transcriptional co‐activator, which is controlled by the Hippo signalling pathway. The fusion genes frequently involve MAML2 and NUTM1, which are also rearranged in other cutaneous and extracutaneous neoplasms. We aimed to review the clinical, morphological and molecular features of this category of adnexal neoplasms with a special focus upon emerging differential diagnoses, and discuss how their systematic molecular characterisation may contribute to a standardisation of diagnosis, more accurate classification and, ultimately, refinement of their prognosis and therapeutic modalities. [ABSTRACT FROM AUTHOR]