Coronavirus disease-19 (COVID-19) is caused by a newly discovered coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although SARS-CoV-2 is visualized on electron microscopy, there is an increasing demand for widely applicable techniques to visualize viral components within tissue specimens. Viral protein and RNA can be detected on formalin-fixed paraffin-embedded (FFPE) tissue using immunohistochemistry (IHC) and in situ hybridization (ISH), respectively. Herein, we evaluate the staining performance of ISH for SARS-CoV-2 and an IHC directed at the SARS-CoV nucleocapsid protein and compare these results to a gold standard, tissue quantitative real-time polymerase chain reaction (qRT-PCR). We evaluated FFPE sections from 8 COVID-19 autopsies, including 19 pulmonary and 39 extrapulmonary samples including the heart, liver, kidney, small intestine, skin, adipose tissue, and bone marrow. We performed RNA-ISH for SARS-CoV-2 on all cases with IHC for SARS-CoV and SARS-CoV-2 qRT-PCR performed on selected cases. Lungs from 37 autopsies performed before the COVID-19 pandemic served as negative controls. The ISH and IHC slides were reviewed by 4 observers to record a consensus opinion. Selected ISH and IHC slides were also reviewed by 4 independent observers. Evidence of SARS-CoV-2 was identified on both the IHC and ISH platforms. Within the postmortem lung, detected viral protein and RNA were often extracellular, predominantly within hyaline membranes in patients with diffuse alveolar damage. Among individual cases, there was regional variation in the amount of detectable virus in lung samples. Intracellular viral RNA and protein was localized to pneumocytes and immune cells. Viral RNA was detected on RNA-ISH in 13 of 19 (68%) pulmonary FFPE blocks from patients with COVID-19. Viral protein was detected on IHC in 8 of 9 (88%) pulmonary FFPE blocks from patients with COVID-19, although in 5 cases the stain was interpreted as equivocal. From the control cohort, FFPE blocks from all 37 patients were negative for SARS-CoV-2 RNA-ISH, whereas 5 of 13 cases were positive on IHC. Collectively, when compared with qRT-PCR on individual tissue blocks, the sensitivity and specificity for ISH was 86.7% and 100%, respectively, while those for IHC were 85.7% and 53.3%, respectively. The interobserver variability for ISH ranged from moderate to almost perfect, whereas that for IHC ranged from slight to moderate. All extrapulmonary samples from COVID-19-positive cases were negative for SARS-CoV-2 by ISH, IHC, and qRT-PCR. SARS-CoV-2 is detectable on both RNA-ISH and nucleocapsid IHC. In the lung, viral RNA and nucleocapsid protein is predominantly extracellular and within hyaline membranes in some cases, while intracellular locations are more prominent in others. The intracellular virus is detected within pneumocytes, bronchial epithelial cells, and possibly immune cells. The ISH platform is more specific, easier to analyze and the interpretation is associated with the improved interobserver agreement. ISH, IHC, and qRT-PCR failed to detect the virus in the heart, liver, and kidney.

ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 in three 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 (seven and twelve 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-ERK, confirming MAPK pathway activation. KIF5B-ALK fusions were detected in 27 patients, while 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, ten 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.

Our understanding of the novel coronavirus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for the coronavirus disease 2019 (COVID-19), is rapidly expanding. 1 Peripheral blood count abnormalities are diverse and can include neutrophilia and lymphopenia, or thrombocytopenia as well as thrombocytosis. 2 Although there have been several case reports or

Sarcomas are driven by diverse pathogenic mechanisms, including gene rearrangements in a subset of cases. Rare soft tissue sarcomas containing KMT2A fusions have recently been reported, characterized by a predilection for young adults, sclerosing epithelioid fibrosarcoma-like morphology, and an often aggressive course. Nonetheless, clinicopathologic and molecular descriptions of KMT2A-rearranged sarcomas remain limited. In this study, we identified by targeted next-generation RNA sequencing an index patient with KMT2A fusion-positive soft tissue sarcoma. In addition, we systematically searched for KMT2A structural variants in a comprehensive genomic profiling database of 14,680 sarcomas interrogated by targeted next-generation DNA and/or RNA sequencing. We characterized the clinicopathologic and molecular features of KMT2A fusion-positive sarcomas, including KMT2A breakpoints, rearrangement partners, and concurrent genetic alterations. Collectively, we identified a cohort of 34 sarcomas with KMT2A fusions (0.2%), and YAP1 was the predominant partner (n = 16 [47%]). Notably, a complex rearrangement with YAP1 consistent with YAP1–KMT2A–YAP1 fusion was detected in most cases, with preservation of KMT2A CxxC-binding domain in the YAP1–KMT2A–YAP1 fusion and concurrent deletions of corresponding exons in KMT2A. The tumors often affected younger adults (age 20–66 [median 40] years) and histologically showed variably monomorphic epithelioid-to-spindle shaped cells embedded in a dense collagenous stroma. Ultrastructural evidence of fibroblastic differentiation was noted in one tumor examined. Our cohort also included two sarcomas with VIM–KMT2A fusions, each harboring concurrent mutations in CTNNB1, SMARCB1, and ARID1A and characterized histologically by sheets of spindle-to-round blue cells. The remaining 16 KMT2A-rearranged sarcomas in our cohort exhibited diverse histologic subtypes, each with unique novel fusion partners. In summary, KMT2A-fusion-positive sarcomas most commonly exhibit sclerosing epithelioid fibrosarcoma-like morphology and complex YAP1–KMT2A–YAP1 fusions. Cases also include rare spindle-to-round cell sarcomas with VIM–KMT2A fusions and tumors of diverse histologic subtypes with unique KMT2A fusions to non-YAP1 non-VIM partners.