SOX10-Internal Tandem Duplications and PLAG1 or HMGA2 Fusions Segregate Eccrine-Type and Apocrine-Type Cutaneous Mixed Tumors.

Cutaneous mixed tumors exhibit a wide morphologic diversity and are currently classified into apocrine and eccrine types based on their morphologic differentiation. Some cases of apocrine-type cutaneous mixed tumors (ACMT), namely, hyaline cell-rich apocrine cutaneous mixed tumors (HCR-ACMT) show a prominent or exclusive plasmacytoid myoepithelial component. Although recurrent fusions of PLAG1 have been observed in ACMT, the oncogenic driver of eccrine-type cutaneous mixed tumors (ECMT) is still unknown. The aim of the study was to provide a comprehensive morphologic, immunohistochemical, and molecular characterization of these tumors. Forty-one cases were included in this study: 28 cases of ACMT/HCR-ACMT and 13 cases of ECMT. After morphologic and immunohistochemical characterization, all specimens were analyzed by RNA sequencing. By immunohistochemistry, all cases showed expression of SOX10, but only ACMT/HCR-ACMT showed expression of PLAG1 and HMGA2. RNA sequencing confirmed the presence of recurrent fusion of PLAG1 or HMGA2 in all cases of ACMT/HCR-ACMT, with a perfect correlation with PLAG1/HMGA2 immunohistochemical status, and revealed internal tandem duplications of SOX10 (SOX10-ITD) in all cases of ECMT. Although TRPS1::PLAG1 was the most frequent fusion, HMGA2::WIF1 and HMGA2::NFIB were detected in ACMT cases. Clustering analysis based on gene expression profiling of 110 tumors, including numerous histotypes, showed that ECMT formed a distinct group compared with all other tumors. ACMT, HCR-ACMT, and salivary gland pleomorphic adenoma clustered together, whereas myoepithelioma with fusions of EWSR1, FUS, PBX1, PBX3, POU5F1, and KLF17 formed another cluster. Follow-up showed no evidence of disease in 23 cases across all 3 tumor types. In conclusion, our study demonstrated for the first time SOX10-ITD in ECMT and HMGA2 fusions in ACMT and further refined the prevalence of PLAG1 fusions in ACMT. Clustering analyses revealed the transcriptomic distance between these different tumors, especially in the heterogenous group of myoepitheliomas.

Detection of wildtype Merkel cell polyomavirus genomic sequence and VP1 transcription in a subset of Merkel cell carcinoma.

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.

Sweat Gland Tumors Arising on Acral Sites: A Molecular Survey.

Recurrent oncogenic drivers have been identified in a variety of sweat gland tumors. Recently, integration of human papillomavirus type 42 (HPV42) has been reported in digital papillary adenocarcinoma (DPA). The main objectives of the present study were (i) to provide an overview of the prevalence of previously identified oncogenic drivers in acral sweat gland tumors and (ii) to genetically characterize tumors in which no recurrent genetic alteration has been identified yet. Cases of acral sweat gland tumors were identified from the database of the French network CARADERM. After histologic review, the presence of previously identified genetic alterations was investigated in the entire cohort (n=79) using a combination of immunohistochemistry and targeted DNA and RNA sequencing. Tumor entities with no recurrent genetic alterations were submitted to whole-transcriptome sequencing. CRTC1::MAML2 fusion was identified in cases of hidradenoma and hidradenocarcinoma (n=9/12 and n=9/12). A p.V600E mutation of BRAF was observed in all cases of tubular adenoma (n=4). YAP1:MAML2 and YAP1::NUTM1 fusions were observed in poroid tumors (n=15/25). ETV6::NTRK3 and TRPS1::PLAG1 fusion transcripts were identified in secretory carcinoma (n=1/1) and cutaneous mixed tumors (n=3/4), respectively. The HPV42 genome was detected in most cases of DPA (n=10/11) and in 1 adnexal adenocarcinoma not otherwise specified. Finally, whole-transcriptome analysis revealed BRD3::NUTM1 or NSD3::NUTM1 fusions in 2 cases of NUT adnexal carcinoma and NCOA4::RET and CCDC6::RET fusion transcripts in 2 cystadenoma/hidrocystoma-like tumors. Our study confirms distinctive cytogenetic abnormalities in a wide number of acral adnexal neoplasms and supports the use of molecular analysis as a valuable aid in the diagnosis of these rare and often difficult to diagnose group of neoplasms.

Distinct regulations driving YAP1 expression loss in poroma, porocarcinoma and RB1-deficient skin carcinoma.

AIMS: Recently, YAP1 fusion genes have been demonstrated in eccrine poroma and porocarcinoma, and the diagnostic use of YAP1 immunohistochemistry has been highlighted in this setting. In other organs, loss of YAP1 expression can reflect YAP1 rearrangement or transcriptional repression, notably through RB1 inactivation. In this context, our objective was to re-evaluate the performance of YAP1 immunohistochemistry for the diagnosis of poroma and porocarcinoma. METHODS AND RESULTS: The expression of the C-terminal part of the YAP1 protein was evaluated by immunohistochemistry in 543 cutaneous epithelial tumours, including 27 poromas, 14 porocarcinomas and 502 other cutaneous tumours. Tumours that showed a lack of expression of YAP1 were further investigated for Rb by immunohistochemistry and for fusion transcripts by real-time PCR (YAP1::MAML2 and YAP1::NUTM1). The absence of YAP1 expression was observed in 24 cases of poroma (89%), 10 porocarcinoma (72%), 162 Merkel cell carcinoma (98%), 14 squamous cell carcinoma (SCC) (15%), one trichoblastoma and one sebaceoma. Fusions of YAP1 were detected in only 16 cases of poroma (n = 66%), 10 porocarcinoma (71%) all lacking YAP1 expression, and in one sebaceoma. The loss of Rb expression was detected in all cases except one of YAP1-deficient SCC (n = 14), such tumours showing significant morphological overlap with porocarcinoma. In-vitro experiments in HaCat cells showed that RB1 knockdown resulted in repression of YAP1 protein expression. CONCLUSION: In addition to gene fusion, we report that transcriptional repression of YAP1 can be observed in skin tumours with RB1 inactivation, including MCC and a subset of SCC.

Sweet-like syndrome and multiple COVID arm syndrome following COVID-19 vaccines: 'specific' patterns in a series of 192 patients.

The two clinico-pathological patterns are 'Sweet-like syndrome' and 'Multiple COVID-Arm'. 'Sweet-like syndrome' presents clinically as erythematous and oedematous papules or plaques, sometimes developing vesiculation or bullae. Histology shows classical Sweet syndrome with a diffuse dermal neutrophilic infiltrate, or an infiltrate of histiocyte-like immature myeloid cells consistent with a histiocytoid Sweet syndrome. 'Multiple COVID-arm' is characterized by multiple large inflammatory plaques with histological analyses showing a perivascular and interstitial inflammatory infiltrate with eosinophils.

Investigation of the RB1-SOX2 axis constitutes a tool for viral status determination and diagnosis in Merkel cell carcinoma.

MCC (Merkel cell carcinoma) is an aggressive neuroendocrine cutaneous neoplasm. Integration of the Merkel cell polyomavirus (MCPyV) is observed in about 80% of the cases, while the remaining 20% are related to UV exposure. Both MCPyV-positive and -negative MCCs-albeit by different mechanisms-are associated with RB1 inactivation leading to overexpression of SOX2, a major contributor to MCC biology. Moreover, although controversial, loss of RB1 expression seems to be restricted to MCPyV-negative cases.The aim of the present study was to assess the performances of RB1 loss and SOX2 expression detected by immunohistochemistry to determine MCPyV status and to diagnose MCC, respectively.Overall, 196 MCC tumors, 233 non-neuroendocrine skin neoplasms and 70 extra-cutaneous neuroendocrine carcinomas (NEC) were included. SOX2 and RB1 expressions were assessed by immunohistochemistry in a tissue micro-array. Diagnostic performances were determined using the likelihood ratio (LHR).RB1 expression loss was evidenced in 27% of the MCC cases, 12% of non-neuroendocrine skin tumors and 63% of extra-cutaneous NEC. Importantly, among MCC cases, RB1 loss was detected in all MCPyV(-) MCCs, while MCPyV( +) cases were consistently RB1-positive (p < 0.001). SOX2 diffuse expression was observed in 92% of the MCC cases and almost never observed in non-neuroendocrine skin epithelial neoplasms (2%, p < 0.0001, LHR + = 59). Furthermore, SOX2 diffuse staining was more frequently observed in MCCs than in extra-cutaneous NECs (30%, p < 0.001, LHR + = 3.1).These results confirm RB1 as a robust predictor of MCC viral status and further suggest SOX2 to be a relevant diagnostic marker of MCC.

Recent Advances on Immunohistochemistry and Molecular Biology for the Diagnosis of Adnexal Sweat Gland Tumors.

Cutaneous sweat gland tumors are a subset of adnexal neoplasms that derive or differentiate into the sweat apparatus. Their great diversity, rarity, and complex terminology make their pathological diagnosis challenging. Recent findings have revealed a wide spectrum of oncogenic drivers, several of which are of diagnostic interest for pathologists. Most of these molecular alterations are represented by gene fusions, which are shared with other homologous neoplasms occurring in organs containing exocrine glands, such as salivary and breast glands, which show similarities to the sweat apparatus. This review aims to provide a synthesis of the most recent immunohistochemical and molecular markers used for the diagnosis of sweat gland tumors and to highlight their relationship with similar tumors in other organs. It will cover adenoid cystic carcinoma (NFIB, MYB, and MYBL1 fusion), cutaneous mixed tumor (PLAG1 fusion), cylindroma and spiradenoma and their carcinomas thereof (NF-κB activation through CYLD inactivation or ALKP1 hotspot mutation), hidradenoma and hidradenocarcinoma (MAML2 fusion), myoepithelioma (EWSR1 and FUS fusion), poroma and porocarcinoma (YAP1, MAML2, and NUTM1 fusion), secretory carcinoma (ETV6, NTRK3 fusion), tubular adenoma and syringo-cystadenoma papilliferum (HRAS and BRAF activating mutations). Sweat gland tumors for which there are no known molecular abnormalities will also be briefly discussed, as well as potential future developments.

Peripheral neuropathy and livedoid vasculopathy.

BACKGROUND: Livedoid vasculopathy (LV) is a chronic dermatosis associated with micro-thrombosis of the vessels of the dermis, leading to ischemic lesions and painful skin ulcerations of the lower limbs. This thrombosing occlusive vasculopathy, clearly distinct from 'classical vasculitis' (not related to alteration of vessel walls), may lead to peripheral neuropathy. OBJECTIVE: To clarify the main clinical, electrophysiological and pathological characteristics of peripheral neuropathy linked to LV. METHOD: We presented a series of personal cases of peripheral neuropathy due to LV. We also conducted a review of the literature (since the first description of LV in 1974) using multiple combinations of keywords from 'PubMed', 'Google Scholar' and 'ScienceDirect' databases according to the 'Preferred Reporting Items for Systematic reviews and Meta-Analyses' guidelines. RESULTS: We identified 16 patients (6 personal cases and 10 cases from the medical literature). Our personal cases were five females and one male, with a median age (at the onset of cutaneous signs of LV) of 38 (range 25-62). Several types of skin lesions of the lower limbs were observed. Median age at the onset of peripheral neuropathy symptoms was 48 years (range 29-66), with a main clinical and electrophysiological pattern of mononeuropathy multiplex. DISCUSSION: We observed a typical pattern of peripheral neuropathy, mostly mononeuropathy multiplex, whose pathophysiology might be related to occlusions of the small vessels of the nerves, as seen in the dermis. Moreover, LV may also be associated with other types of peripheral neuropathies (sometimes of autoimmune etiology) not directly related to the skin lesions. CONCLUSION: The 'ischemic form' of peripheral neuropathy linked to LV is mainly responsible for sensory disturbances (with multifocal distribution), sometimes for motor disturbances. This type of peripheral neuropathy has to be distinguished from 'classical vasculitic neuropathies' which are usually treated with antithrombotic therapies.

Impact of expert pathology review in skin adnexal carcinoma diagnosis: Analysis of 2573 patients from the French CARADERM network.

PURPOSE: To prospectively assess the impact of expert pathological review of skin adnexal carcinoma diagnosis in France. METHODS: From 2014 to 2019, 2573 samples from patients with newly diagnosed or suspected skin adnexal carcinomas were reviewed prospectively by expert pathologists through the national CARADERM (CAncers RAres DERMatologiques) network. Changes in diagnosis between referral and expert review were analysed regarding their potential impact on patient care or prognosis. RESULTS: The samples comprised 2205 newly diagnosed adnexal carcinomas, 129 benign adnexal tumours, 136 basal cell carcinomas, 74 squamous cell carcinomas, six cutaneous metastases and 13 other malignancies. There were 930 (42%) sweat gland carcinomas, of which porocarcinoma (261; 11.8%), microcystic adnexal carcinoma (125; 5.7%) and hidradenocarcinoma (109; 4.9%) were the most frequent subtypes; 778 (35%) hair follicle carcinomas, 238 (11%) sebaceous carcinomas and 212 (10%) extramammary Paget diseases/mammary-like anogenital gland adenocarcinomas. A diagnostic change between referral and expert review occurred in 503 (21.3%) patients, significantly higher for cases sent with a provisional diagnosis seeking an expert second opinion (45.7%) than for cases sent with a formal diagnosis (2.8%) (p < .0001). Sweat gland carcinomas were more prone to diagnostic discrepancies than other tumours (p < .0001), including 1.8% of patients with sweat gland carcinoma subtype misclassification with predicted clinical impact. Changes between benign and malignant conditions occurred in 117 samples (5% of patients). CONCLUSION: The study provides a unique description of the distribution of skin adnexal carcinomas and highlights the importance of expert review for these rare cancers. Optimal clinical management was impacted in a significant proportion of patients.

Recurrent FOXK1::GRHL and GPS2::GRHL fusions in trichogerminoma.

We report 21 cases of trichogerminoma harbouring previously undescribed FOXK1::GRHL1/2 or GPS2::GRHL1/2/3 in-frame fusion transcripts. Microscopic examination of a preliminary set of five cases revealed well-delimitated tumours located in the dermis with frequent extension to the subcutaneous tissue. Tumours presented a massive and nodular architecture and consisted of a proliferation of basaloid cells. A biphasic pattern sometime resulting in tumour cell nests ('cell balls') was present. Immunohistochemistry demonstrated the expression of cytokeratins (CKs) 15, 17, and PHLDA1. In addition, numerous CK20-positive Merkel cells were detected. RNA sequencing (RNA-seq) revealed a FOXK1::GRHL1 chimeric transcript in three cases and a FOXK1::GRHL2 fusion in two cases. In a second series for validation (n = 88), FOXK1::GRHL1/2 fusion transcripts were detected by RT-qPCR or FISH in an additional 12 trichogerminomas and not in any other follicular tumour entities or basal cell carcinoma cases (n = 66). Additional RNA-seq analysis in trichogerminoma cases without detected FOXK1::GRHL1/2 rearrangements revealed GPS2::GRHL1 fusion transcripts in two cases, GPS2::GRHL2 in one case, and GPS2::GRHL3 fusion transcript in one case. Therefore, our study strongly suggests that GRHL1/2/3 gene rearrangements might represent the oncogenic driver in trichogerminoma, a subset of follicular tumours characterized by immature features and numerous Merkel cells. © 2022 The Pathological Society of Great Britain and Ireland.

ALK-positive histiocytosis: a new clinicopathologic spectrum highlighting neurologic involvement and responses to ALK inhibition.

ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 in 3 infants with multisystemic disease involving the liver and hematopoietic system. This entity has subsequently been documented in case reports and series to occupy a wider clinicopathologic spectrum with recurrent KIF5B-ALK fusions. The full clinicopathologic and molecular spectra of ALK-positive histiocytosis remain, however, poorly characterized. Here, we describe the largest study of ALK-positive histiocytosis to date, with detailed clinicopathologic data of 39 cases, including 37 cases with confirmed ALK rearrangements. The clinical spectrum comprised distinct clinical phenotypic groups: infants with multisystemic disease with liver and hematopoietic involvement, as originally described (Group 1A: 6/39), other patients with multisystemic disease (Group 1B: 10/39), and patients with single-system disease (Group 2: 23/39). Nineteen patients of the entire cohort (49%) had neurologic involvement (7 and 12 from Groups 1B and 2, respectively). Histology included classic xanthogranuloma features in almost one-third of cases, whereas the majority displayed a more densely cellular, monomorphic appearance without lipidized histiocytes but sometimes more spindled or epithelioid morphology. Neoplastic histiocytes were positive for macrophage markers and often conferred strong expression of phosphorylated extracellular signal-regulated kinase, confirming MAPK pathway activation. KIF5B-ALK fusions were detected in 27 patients, whereas CLTC-ALK, TPM3-ALK, TFG-ALK, EML4-ALK, and DCTN1-ALK fusions were identified in single cases. Robust and durable responses were observed in 11/11 patients treated with ALK inhibition, 10 with neurologic involvement. This study presents the existing clinicopathologic and molecular landscape of ALK-positive histiocytosis and provides guidance for the clinical management of this emerging histiocytic entity.

Merkel Cell Polyomavirus‒Negative Merkel Cell Carcinoma Originating from In Situ Squamous Cell Carcinoma: A Keratinocytic Tumor with Neuroendocrine Differentiation.

Although virus-negative Merkel cell carcinoma (MCC) is characterized by a high frequency of UV-induced mutations, the expression of two viral oncoproteins is regarded as a key mechanism driving Merkel cell polyomavirus‒positive MCC. The cells in which these molecular events initiate MCC oncogenesis have yet not been identified for both MCC subsets. A considerable proportion of virus-negative MCC is found in association with squamous cell carcinoma (SCC), suggesting (i) coincidental collision, (ii) one providing a niche for the other, or (iii) one evolving from the other. Whole-exome sequencing of four combined tumors consisting of SCC in situ and Merkel cell polyomavirus‒negative MCC showed many mutations shared between SCC and MCC in all cases, indicating a common ancestry and thereby a keratinocytic origin of these MCCs. Moreover, analyses of the combined cases as well as of pure SCC and MCC suggest that RB1 inactivation in SCC facilitates MCC development and that epigenetic changes may contribute to the SCC/MCC transition.