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Examination of red blood cell (RBC) morphology in peripheral blood smears can help diagnose hematologic disease, even in resource-limited settings, but this analysis remains subjective and semi-quantitative with low throughput. Prior attempts to develop automated tools have been hampered by poor reproducibility and limited clinical validation. Here, we present a novel, open-source machine-learning approach (denoted the 'RBC-diff') to quantify abnormal RBCs in peripheral smear images and generate an RBC morphology differential. RBC-diff cell counts showed high accuracy for single-cell classification (mean AUC: 0.93) and quantitation across smears (mean R2: 0.76 compared to experts, inter-experts R2: 0.75). RBC-diff counts were concordant with clinical morphology grading for 300,000+ images and recovered expected pathophysiologic signals in diverse clinical cohorts. Criteria using RBC-diff counts distinguished thrombotic thrombocytopenic purpura and hemolytic uremic syndrome from other thrombotic microangiopathies, providing greater specificity than clinical morphology grading (72% vs. 41%, p < 0.001), while maintaining high sensitivity (94-100%). Elevated RBC-diff schistocyte counts were associated with increased 6-month all-cause mortality in a cohort of 58,950 inpatients (9.5% mortality for schist. > 1%, vs. 4.7% for schist. < 0.5%, p < 0.001) after controlling for comorbidities, demographics, clinical morphology grading, and blood count indices. The RBC-diff also enabled estimation of single-cell volume-morphology distributions, providing insight into morphology influences on routine blood count measures. Our codebase and expert-annotated images are included here to spur further advancements. These results illustrate that computer vision can enable rapid and accurate RBC morphology quantitation, which may provide value in both clinical and research contexts.

Supplementary Figures S1-S8 with legends.

Supplementary Tables 1-6

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive leukemia of plasmacytoid dendritic cells (pDC). BPDCN occurs at least three times more frequently in men than in women, but the reasons for this sex bias are unknown. Here, studying genomics of primary BPDCN and modeling disease-associated mutations, we link acquired alterations in RNA splicing to abnormal pDC development and inflammatory response through Toll-like receptors. Loss-of-function mutations in ZRSR2, an X chromosome gene encoding a splicing factor, are enriched in BPDCN, and nearly all mutations occur in males. ZRSR2 mutation impairs pDC activation and apoptosis after inflammatory stimuli, associated with intron retention and inability to upregulate the transcription factor IRF7. In vivo, BPDCN-associated mutations promote pDC expansion and signatures of decreased activation. These data support a model in which male-biased mutations in hematopoietic progenitors alter pDC function and confer protection from apoptosis, which may impair immunity and predispose to leukemic transformation.Significance:Sex bias in cancer is well recognized, but the underlying mechanisms are incompletely defined. We connect X chromosome mutations in ZRSR2 to an extremely male-predominant leukemia. Aberrant RNA splicing induced by ZRSR2 mutation impairs dendritic cell inflammatory signaling, interferon production, and apoptosis, revealing a sex- and lineage-related tumor suppressor pathway.This article is highlighted in the In This Issue feature, p. 275

DNA sequences

Background Histiocytic and dendritic cell neoplasms are a diverse group of tumors arising from monocytic or dendritic cell lineage. Whereas the genomic features for Langerhans cell histiocytosis and Erdheim‐Chester disease have been well described, other less common and often aggressive tumors in this broad category remain poorly characterized, and comparison studies across the World Health Organization diagnostic categories are lacking. Methods Tumor samples from a total of 102 patient cases within four major subtypes of malignant histiocytic and dendritic cell neoplasms, including 44 follicular dendritic cell sarcomas (FDCSs), 41 histiocytic sarcomas (HSs), 7 interdigitating dendritic cell sarcomas (IDCSs), and 10 Langerhans cell sarcomas (LCSs), underwent hybridization capture with analysis of up to 406 cancer‐related genes. Results Among the entire cohort of 102 patients, CDKN2A mutations were most frequent across subtypes and made up 32% of cases, followed by TP53 mutations (22%). Mitogen‐activated protein kinase (MAPK) pathway mutations were present and enriched among the malignant histiocytosis (M) group (HS, IDCS, and LCS) but absent in FDCS (72% vs. 0%; p, Histiocytic and dendritic cell neoplasms are a diverse group of tumors arising from the monocytic or dendritic cell lineage. This article presents the molecular characteristics of the four major subtypes of malignant histiocytic and dendritic cell neoplasms, focusing on genomic alterations that could represent therapeutic targets.

Recurrent chromosomal rearrangements are a hallmark of hematologic malignancies and play critical roles in pathogenesis. The TP53 analog TP63 is rearranged in 5-10% of diverse subtypes of both aggressive T- and B-cell lymphomas. Patients with TP63-rearranged lymphomas have dismal outcomes, with 5-year overall survival rates between 0-17%, depending on cohorts. The function and mechanisms of TP63 rearrangements and TP63 fusion proteins in tumorigenesis are poorly understood. As a result, attempts to treat these patients to date have been largely empiric. Thus, there is an urgent need to understand how TP63 fusions contribute to tumorigenesis and to translate the findings into novel therapeutic options for these patients. Here, we demonstrated that TP63 fusions are essential for the propagation of T-cell lymphomas (TCLs). Knockdown of TP63 fusions with specific shRNAs in TCL cell lines harboring TP63 fusions suppressed both cell growth in vitro and tumor growth in vivo. Retroviral expression of TBL1XR1-TP63, the most common TP63 fusion, conferred cytokine independence in Ba/F3 cells, consistent with its role as an oncogene. To investigate the role of TP63 fusions in T- and B-cell lymphomagenesis, we engineered a CAG-Loxp-Stop-Loxp-TBL1XR1-TP63 conditional knock-in mouse model and crossed with hCD2-Cre mice. This results in expression beginning during early lymphoid development. As observed in patients, transgenic mice developed multiple subtypes of both T- and B-cell lymphoma. To define the effects and mechanisms of TP63 fusions within T cells, we performed CRISPR scanning, transcriptomic, epigenomic, and proteomic analyses. Our data showed that domains within both the N-terminal TBL1XR1 and C-terminal TP63 portions contribute to the function of this fusion. We found that the N-terminal component of TP63 fusions interacts with components of the NCOR/SMRT complex. At the same time, the C-terminal portion of TP63 (which recapitulates the deltaN-p63 isoform expressed in some carcinomas) interacts with the enhancer modifier KMT2D and its complex members. TBL1XR1-TP63 binds to a novel distal enhancer to drive MYC expression, and thus upregulates the expression of the histone H3K27 methylase EZH2. Finally, we assessed whether EZH2 is a vulnerability of TP63-rearranged lymphomas. We found that knockdown of EZH2 in TP63-rearranged lines significantly impaired cell growth, as did treatment with the EZH2 and 1 dual inhibitor valemetostat. Valemetostat, which is now being tested in patients with lymphoma, counteracted the oncogenic effects of TP63 fusions in multiple preclinical models in vivo. Together, our results identify the TP63 fusion as a highly unique oncogenic driver in lymphomagenesis capable of recruiting multiple epigenetic modifier complexes and inducing a targetable dependence on EZH2. Citation Format: Gongwei Wu, Noriaki Yoshida, Jihe Liu, Xiaoyang Zhang, Yuan Xiong, Tayla Heavican-Foral, Huiyun Liu, Geoffrey Nelson, Lu Yang, Renee Chen, Katherine Donovan, Marcus Jones, Mikhail Roshal, Yanming Zhang, Ran Xu, Ajit Nirmal, Salvia Jain, Catharine Leahy, Kristen Jones, Kristen Stevenson, Natasha Galasso, Nivetha Ganesan, Tiffany Chang, Wen-Chao Wu, Abner Louissaint, Lydie Debaize, Hojong Yoon, Paola Dal Cin, Wing Chan Chan, Shannan Ho Sui Ho Sui, Samuel Ng, Andrew Feldman, Steven M. Horwitz, Mathew Meyerson, Karen Adelman, Eric Fischer, Chun-Wei Chen, David Weinstock, Myles Brown. TP63 fusions drive enhancer rewiring, lymphomagenesis, and dependence on EZH2. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5755.

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.

PURPOSE Thymomas are epithelial neoplasms that represent the most common thymic tumors in adults. These tumors have been shown to harbor a relatively low mutational burden. As a result, there is a lack of genetic alterations that may be used prognostically or targeted therapeutically for this disease. Here, we describe a recurrent gene rearrangement in type B2 + B3 thymomas. PATIENTS AND METHODS A single index case of thymoma was evaluated by an RNA-based solid fusion assay. Separately, tissues from 255,008 unique advanced cancers, including 242 thymomas, were sequenced by hybrid capture–based next-generation DNA sequencing/comprehensive genomic profiling of 186 to 406 genes, including lysine methyltransferase 2A ( KMT2A) rearrangements, and a portion were evaluated for RNA of 265 genes. We characterized molecular and clinicopathologic features of the pertinent fusion-positive patient cases. RESULTS We identified 11 patients with thymomas harboring a gene fusion of KMT2A and mastermind-like transcriptional coactivator 2 ( MAML2). Fusion breakpoints were identified between exon 8, 9, 10, or 11 of KMT2A and exon 2 of MAML2. Fifty-five percent were men, with a median age of 48 years at surgery (range, 29-69 years). Concurrent genomic alterations were infrequent. The 11 thymomas were of B2 or B3 type histology, with 1 case showing foci of thymic carcinoma. The frequency of KMT2A- MAML2 fusion was 4% of all thymomas (10 of 242) and 6% of thymomas of B2 or B3 histology (10 of 169). CONCLUSION KMT2A- MAML2 represents the first recurrent fusion described in type B thymoma. The fusion seems to be specific to type B2 and B3 thymomas, the most aggressive histologic subtypes. The identification of this fusion offers insights into the biology of thymoma and may have clinical relevance for patients with disease refractory to conventional therapeutic modalities.

Primary cutaneous γδ T cell lymphomas (PCGDTLs) represent a heterogeneous group of uncommon but aggressive cancers. Herein, we perform genome-wide DNA, RNA, and T cell receptor (TCR) sequencing on 29 cutaneous γδ lymphomas. We find that PCGDTLs are not uniformly derived from Vδ2 cells. Instead, the cell-of-origin depends on the tissue compartment from which the lymphomas are derived. Lymphomas arising from the outer layer of skin are derived from Vδ1 cells, the predominant γδ cell in the epidermis and dermis. In contrast, panniculitic lymphomas arise from Vδ2 cells, the predominant γδ T cell in the fat. We also show that TCR chain usage is non-random, suggesting common antigens for Vδ1 and Vδ2 lymphomas respectively. In addition, Vδ1 and Vδ2 PCGDTLs harbor similar genomic landscapes with potentially targetable oncogenic mutations in the JAK/STAT, MAPK, MYC, and chromatin modification pathways. Collectively, these findings suggest a paradigm for classifying, staging, and treating these diseases., Cutaneous gamma-delta T cell lymphomas are rare but aggressive cancers. Here, by DNA, RNA and T cell receptor sequencing, the authors elucidate the molecular ontogeny by revising the cell of origin and identifying potentially targetable mutations.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive leukemia of plasmacytoid dendritic cells (pDC). BPDCN occurs at least three times more frequently in men than in women, but the reasons for this sex bias are unknown. Here, studying genomics of primary BPDCN and modeling disease-associated mutations, we link acquired alterations in RNA splicing to abnormal pDC development and inflammatory response through Toll-like receptors. Loss-of-function mutations in ZRSR2, an X chromosome gene encoding a splicing factor, are enriched in BPDCN, and nearly all mutations occur in males. ZRSR2 mutation impairs pDC activation and apoptosis after inflammatory stimuli, associated with intron retention and inability to upregulate the transcription factor IRF7. In vivo, BPDCN-associated mutations promote pDC expansion and signatures of decreased activation. These data support a model in which male-biased mutations in hematopoietic progenitors alter pDC function and confer protection from apoptosis, which may impair immunity and predispose to leukemic transformation. Significance: Sex bias in cancer is well recognized, but the underlying mechanisms are incompletely defined. We connect X chromosome mutations in ZRSR2 to an extremely male-predominant leukemia. Aberrant RNA splicing induced by ZRSR2 mutation impairs dendritic cell inflammatory signaling, interferon production, and apoptosis, revealing a sex- and lineage-related tumor suppressor pathway. This article is highlighted in the In This Issue feature, p. 275

BACKGROUND: Thymic cysts can change in volume, CT attenuation, and MRI signal over time, raising possibility that spontaneous hemorrhage and resorption could contribute to these changes. A previous study showed some of their CT features to prompt misinterpretation as thymomas. We studied the pathological features of thymic cysts and correlated them with preoperative imaging findings. METHODS: The MGH Pathology archives were searched to identify thymic cyst resections between April 2000 and May 2020. Exclusion criteria included non-prevascular mediastinal localization, location within an enhancing mass (imaging), and/or presence of solid-cystic mass identified on gross examination. Cases without available imaging were also excluded. Various macroscopic/microscopic pathological parameters were evaluated and correlated with cyst imaging characteristics seen on CT or MRI performed closest to the time of surgery. RESULTS: Upon application of exclusion criteria, we identified 18 thymic cysts from the initial 84 mediastinal cystic specimens. The median age at resection was 60.5 with a range of 45–77 years. Most cysts were unilocular (11/18; 61%), with a gross maximum diameter ranging from 1.5 to 11.2 cm (mean ± SD: 4.2±2.7 cm). Microscopic review showed that most cysts had mixed patterns (8/16; 50%) of cuboidal, flat, squamous and/or pseudostratified epithelia. Common findings included hemosiderin deposition in the cyst wall (5/18; 28%), calcifications (6/18; 33%), chronic inflammation (6/18; 33%), hyalinosis (11/18; 61%), and significant fibrosis (12/18; 67%). Other findings included cyst walls with denuded epithelium (6/18; 33%); adjacent cholesterol clefts (3/18; 17%); and granulation tissue (2/18; 11%). The adjacent thymic tissue was involuted in 16/18 (89%) cysts, and showed fat necrosis in 11/18 (61%), microcystic Hassall’s corpuscles change in 4/18 (22%), lymphoid follicular hyperplasia in 3/18 (17%), and true thymic hyperplasia in 1/18 (6%). A total of 17/18 cysts were imaged by CT, and 4/18 imaged by MRI, including 3 imaged by CT. On CT, 6/17 (%) cysts demonstrated wall calcification, 11/17 (%) had attenuation values ≥20 HU, and mean/median wall thickness on CT [solely perceivable and measurable in 5/17 (%)] was 3 mm/3 mm ±1.5 (1 SD). Of the 4 cysts imaged by MRI, three were T1-isointense and the other T1-hypointense to muscle. CONCLUSIONS: Thymic cysts encompass a series of morphologically diverse lesions, which often show histological features suggestive of microbleeding, inflammation, and fibrosis, which may explain their variable CT and MRI appearance.

Objectives Peripheral T-cell lymphomas (PTCLs) are heterogeneous, clinically aggressive, and rare. Subtype distribution varies by geographic location; however, data from sub-Saharan Africa (SSA) are lacking. We sought to elucidate clinicopathologic features of PTCL in SSA. Methods We reviewed PTCL consultation cases from three SSA countries. PTCL subtype was determined per 2017 World Health Organization classification. Cases with sufficient material were evaluated by polymerase chain reaction for human T-cell leukemia virus type 1 (HTLV-1) and T-cell receptor γ (TCRG) rearrangement. Results Among 32 cases, median age was 45 years and male-to-female ratio was 1.7. Thirty (94%) of 32 cases required additional workup for subclassification. PTCL, not otherwise specified (PTCL-NOS) was the most common subtype (13/32, 41%), followed by PTCL with T-follicular helper phenotype (6/32, 19%) and systemic anaplastic large cell lymphoma (6/32, 19%). Four (16%) of 25 cases were Epstein-Barr virus positive (EBV+) (2/2 extranodal natural killer/T-cell lymphoma, 1/13 PTCL-NOS, and 1/4 angioimmunoblastic T-cell lymphoma with EBV+ immunoblasts). Two (15%) of 13 patients with PTCL-NOS were human immunodeficiency virus positive. No cases with evaluable DNA (0/15) were HTLV-1 positive, and 9 of 10 showed clonal TCRG rearrangements. Conclusions In comparison to Western studies, PTCLs from SSA show similar subtype distribution and male predominance but a younger age at diagnosis. Appropriate diagnosis of PTCL requires extensive ancillary testing not readily available in low-income countries, including much of SSA.

Relapsed follicular lymphoma (FL) can arise from common progenitor cells (CPCs). Conceptually, CPC-defining mutations are somatic alterations shared by the initial and relapsed tumours, mostly B-cell leukaemia/lymphoma 2 (BCL2)/immunoglobulin heavy locus (IGH) translocations and other recurrent gene mutations. Through complementary approaches for highly sensitive mutation detection, we do not find CPC-defining mutations in highly purified BCL2/IGH-negative haematopoietic progenitor cells in clinical remission samples from three patients with relapsed FL. Instead, we find cells harbouring the same BCL2/IGH translocation but lacking CREB binding protein (CREBBP), lysine methyltransferase 2D (KMT2D) and other recurrent gene mutations. Thus, (i) the BCL2/IGH translocation can precede CPC-defining mutations in human FL, and (ii) BCL2/IGH-translocated cells can persist in clinical remission.

Objectives Diffuse large B-cell lymphoma (DLBCL) is an aggressive non-Hodgkin lymphoma with a heterogenous genetic landscape that can require multiple assays to characterize. We reviewed a 1-step RNA-based assay to determine cell of origin (COO), detect translocations, and identify mutations and to assess the role of the assay in diagnosis. Methods Using a single custom Archer FusionPlex Lymphoma panel, we performed anchored multiplex polymerase chain reaction–based RNA sequencing on 41 cases of de novo DLBCL. Each case was subclassified by COO, and gene fusions and hotspot mutations were identified. The findings were then compared with COO classification by the Hans immunohistochemical algorithm and NanoString technology, cytogenetics, and fluorescence in situ hybridization results. Results Concordant COO classification by the FusionPlex panel and NanoString was observed in 35 of 41 cases (85.3%), with NanoString and Hans concordant in 33 of 41 cases (80.5%) and FusionPlex and Hans concordant in 33 of 41 cases (80.5%). The FusionPlex assay also detected 6 of 11 BCL6 translocations (4 cryptic), 2 of 3 BCL2 translocations, and 2 of 4 MYC translocations. Mutations were detected in lymphoma-related genes in 24 of 41 cases. Conclusion This FusionPlex assay offers a single method for COO classification, mutation detection, and identification of important translocations in DLBCL. Although not replacing traditional testing, it could offer useful data when limited tissue is available

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive leukemia of plasmacytoid dendritic cells (pDCs). BPDCN occurs at least three times more frequently in men than women, but the reasons for this sex bias are unknown. Here, studying genomics of primary BPDCN and modeling disease-associated mutations, we link acquired alterations in RNA splicing to abnormal pDC development and inflammatory response through Toll-like receptors. Loss-of-function mutations in ZRSR2, an X chromosome gene encoding a splicing factor, are enriched in BPDCN and nearly all mutations occur in males. ZRSR2 mutation impairs pDC activation and apoptosis after inflammatory stimuli, associated with intron retention and inability to upregulate the transcription factor IRF7. In vivo, BPDCN-associated mutations promote pDC expansion and signatures of decreased activation. These data support a model in which male-biased mutations in hematopoietic progenitors alter pDC function and confer protection from apoptosis, which may impair immunity and predispose to leukemic transformation.STATEMENT OF SIGNIFICANCESex bias in cancer is well recognized but the underlying mechanisms are incompletely defined. We connect X chromosome mutations in ZRSR2 to an extremely male-predominant leukemia. Aberrant RNA splicing induced by ZRSR2 mutation impairs dendritic cell inflammatory signaling, interferon production, and apoptosis, revealing a sex- and lineage-related tumor suppressor pathway.

Cutaneous T-cell lymphomas (CTCLs) are a clinically heterogeneous collection of lymphomas of the skin-homing T cell. To identify molecular drivers of disease phenotypes, we assembled representative samples of CTCLs from patients with diverse disease subtypes and stages. Via DNA/RNA-sequencing, immunophenotyping, and ex vivo functional assays, we identified the landscape of putative driver genes, elucidated genetic relationships between CTCLs across disease stages, and inferred molecular subtypes in patients with stage-matched leukemic disease. Collectively, our analysis identified 86 putative driver genes, including 19 genes not previously implicated in this disease. Two mutations have never been described in any cancer. Functionally, multiple mutations augment T-cell receptor–dependent proliferation, highlighting the importance of this pathway in lymphomagenesis. To identify putative genetic causes of disease heterogeneity, we examined the distribution of driver genes across clinical cohorts. There are broad similarities across disease stages. Many driver genes are shared by mycosis fungoides (MF) and Sezary syndrome (SS). However, there are significantly more structural variants in leukemic disease, leading to highly recurrent deletions of putative tumor suppressors that are uncommon in early-stage skin-centered MF. For example, TP53 is deleted in 7% and 87% of MF and SS, respectively. In both human and mouse samples, PD1 mutations drive aggressive behavior. PD1 wild-type lymphomas show features of T-cell exhaustion. PD1 deletions are sufficient to reverse the exhaustion phenotype, promote a FOXM1-driven transcriptional signature, and predict significantly worse survival. Collectively, our findings clarify CTCL genetics and provide novel insights into pathways that drive diverse disease phenotypes.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive leukemia of plasmacytoid dendritic cells (pDCs). BPDCN occurs at least three times more frequently in men than women, but the reasons for this sex bias are unknown. Here, studying genomics of primary BPDCN and modeling disease-associated mutations, we link acquired alterations in RNA splicing to abnormal pDC development and inflammatory response through Toll-like receptors. Loss-of-function mutations in ZRSR2, an X chromosome gene encoding a splicing factor, are enriched in BPDCN and nearly all mutations occur in males. ZRSR2 mutation impairs pDC activation and apoptosis after inflammatory stimuli, associated with intron retention and inability to upregulate the transcription factor IRF7. In vivo, BPDCN-associated mutations promote pDC expansion and signatures of decreased activation. These data support a model in which male-biased mutations in hematopoietic progenitors alter pDC function and confer protection from apoptosis, which may impair immunity and predispose to leukemic transformation.

Primary cutaneous follicle center lymphomas (PCFCLs) are indolent B-cell lymphomas that predominantly remain skin restricted and manageable with skin-directed therapy. Conversely, secondary cutaneous involvement by usual systemic follicular lymphoma (secondary cutaneous follicular lymphoma [SCFL]) has a worse prognosis and often necessitates systemic therapy. Unfortunately, no histopathologic or genetic features reliably differentiate PCFCL from SCFL at diagnosis. Imaging may miss low-burden internal disease in some cases of SCFLs, leading to misclassification as PCFCL. Whereas usual systemic FL is well characterized genetically, the genomic landscapes of PCFCL and SCFL are unknown. Herein, we analyzed clinicopathologic and immunophenotypic data from 30 cases of PCFCL and 10 of SCFL and performed whole-exome sequencing on 18 specimens of PCFCL and 6 of SCFL. During a median follow-up of 7 years, 26 (87%) of the PCFCLs remained skin restricted. In the remaining 4 cases, systemic disease developed within 3 years of diagnosis. Although the SCFLs universally expressed BCL2 and had BCL2 rearrangements, 73% of the PCFCLs lacked BCL2 expression, and only 8% of skin-restricted PCFCLs had BCL2 rearrangements. SCFLs showed low proliferation fractions, whereas 75% of PCFCLs had proliferation fractions >30%. Of the SCFLs, 67% had characteristic loss-of-function CREBBP or KMT2D mutations vs none in skin-restricted PCFCL. Both SCFL and skin-restricted PCFCL showed frequent TNFRSF14 loss-of-function mutations and copy number loss at chromosome 1p36. These data together establish PCFCL as a unique entity with biological features distinct from usual systemic FL and SCFL. We propose 3 criteria based on BCL2 rearrangement, chromatin-modifying gene mutations (CREBBP, KMT2D, EZH2, and EP300), and proliferation index to classify cutaneous FL specimens based on the likelihood of concurrent or future systemic spread.

Summary Tumor cells orchestrate their microenvironment. Here, we provide biochemical, structural, functional, and clinical evidence that Cathepsin S (CTSS) alterations induce a tumor-promoting immune microenvironment in follicular lymphoma (FL). We found CTSS mutations at Y132 in 6% of FL (19/305). Another 13% (37/286) had CTSS amplification, which was associated with higher CTSS expression. CTSS Y132 mutations lead to accelerated autocatalytic conversion from an enzymatically inactive profrom to active CTSS and increased substrate cleavage, including CD74, which regulates major histocompatibility complex class II (MHC class II)-restricted antigen presentation. Lymphoma cells with hyperactive CTSS more efficiently activated antigen-specific CD4+ T cells in vitro. Tumors with hyperactive CTSS showed increased CD4+ T cell infiltration and proinflammatory cytokine perturbation in a mouse model and in human FLs. In mice, this CTSS-induced immune microenvironment promoted tumor growth. Clinically, patients with CTSS-hyperactive FL had better treatment outcomes with standard immunochemotherapies, indicating that these immunosuppressive regimens target both the lymphoma cells and the tumor-promoting immune microenvironment.

Duodenal-type follicular lymphoma (DTFL) is a rare and highly indolent follicular lymphoma (FL) variant. It is morphologically and immunophenotypically indistinguishable from typical FL, characterized by restricted involvement of intestinal mucosa, and lacks extraintestinal manifestations. The molecular determinants of this distinct clinical behavior are largely unknown. Thirty-eight diagnostic biopsies from patients with DTFL were evaluated. The 10-year overall survival rate was 100% in clinically evaluable patients (n = 19). We compared the targeted mutation profile of DTFL (n = 31), limited-stage typical FL (LSTFL; n = 17), and advanced-stage typical FL (ASTFL; n = 241). The mutation frequencies of recurrently mutated genes, including CREBBP, TNFRSF14/HVEM, and EZH2 were not significantly different. However, KMT2D was less commonly mutated in DTFL (52%) and LSTFL (24%) as compared with ASTFL (79%). In ASTFL, 41% of KMT2D-mutated cases harbored multiple mutations in KMT2D, as compared with only 12% in LSTFL (P = .019) and 0% in DTFL (P < .0001). Whole exome and targeted sequencing of DTFL revealed high mutation frequencies of EEF1A1 (35%) and HVCN1 (22%). We compared the immune microenvironment gene expression signatures of DTFL (n = 8) and LSTFL (n = 7). DTFL clearly separated from LSTFL by unsupervised, hierarchical clustering of 147 chemokines and cytokines and was enriched for a chronic inflammation signature. In conclusion, the mutational landscape of DTFL is highly related to typical FL. The lower frequency of multiple mutations in KMT2D in DTFL and LSTFL indicates an increasing selection pressure for complete KMT2D loss in ASTFL pathogenesis. The highly dissimilar immune microenvironment of DTFL suggests a central role in the biology of this disease.

Objectives Therapy-related chronic myeloid leukemia (CML) has been reported, but its clinical presentation and pathologic features have not yet been well characterized. Methods Twenty-one cases of CML following treatment for primary diseases were collected and retrospectively analyzed. Results The clinical presentation, pathologic features, and cytogenetic profile were similar to de novo CML. In particular, those with an isolated Philadelphia chromosome constituted 88.9% of our cases, and additional aberrations characteristic of therapy-related acute myeloid leukemia/myelodysplastic syndrome (AML/MDS) were not identified in this study. The patients responded to imatinib/derivatives and survived with limited follow-up. Conclusions Therapy-related CML has a clinical presentation, pathologic features, and cytogenetic profile akin to de novo CML. Absence of additional significant aberrations seems to suggest a pathogenesis different from therapy-related AML/MDS. Therapy-related CML exhibits a robust therapeutic response to imatinib/derivatives and favorable clinical outcomes similar to de novo CML.

T- and NK-cell lymphomas (TCL) are a heterogenous group of lymphoid malignancies with poor prognosis. In contrast to B-cell and myeloid malignancies, there are few preclinical models of TCLs, which has hampered the development of effective therapeutics. Here we establish and characterize preclinical models of TCL. We identify multiple vulnerabilities that are targetable with currently available agents (e.g., inhibitors of JAK2 or IKZF1) and demonstrate proof-of-principle for biomarker-driven therapies using patient-derived xenografts (PDXs). We show that MDM2 and MDMX are targetable vulnerabilities within TP53-wild-type TCLs. ALRN-6924, a stapled peptide that blocks interactions between p53 and both MDM2 and MDMX has potent in vitro activity and superior in vivo activity across 8 different PDX models compared to the standard-of-care agent romidepsin. ALRN-6924 induced a complete remission in a patient with TP53-wild-type angioimmunoblastic T-cell lymphoma, demonstrating the potential for rapid translation of discoveries from subtype-specific preclinical models., T- and NK-cell lymphomas (TCL) are a group of lymphoid malignancies characterized by poor prognosis, but the absence of appropriate pre-clinical models has hampered the development of effective therapies. Here the authors establish several pre-clinical models and identify vulnerabilities that could be further exploited to treat patients afflicted by these diseases.

A Man with HIV Infection and a Facial Mass A 55-year-old man with HIV infection and a CD4+ T-cell count of 65 per cubic millimeter presented to the hospital 1 week after he started antiretroviral therapy because of a rapidly enlarging mass at the angle of the right mandible. A diagnostic procedure was performed.

Introduction ALK-negative anaplastic large cell lymphoma (ALK- ALCL) is an uncommon type of T-cell non-Hodgkin lymphoma (T-NHL) with worse prognosis compared to ALK-positive (ALK+) ALCLs. Most published studies on the genomics of T-NHL have focused on peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS), and previous studies of ALCL described rearrangements in DUSP22 and TP63 and mutations in genes comprising the JAK/STAT pathway as common genetic drivers in ALK- ALCL. The degree to which these drivers affect survival or other molecular features of ALK- ALCL remains unknown. Here, we describe novel subgroups of ALK- ALCL that exhibit distinct survival. One subgroup appears molecularly similar to ALK+ALCLs and is associated with favorable survival while the second subgroup is quite distinct from ALK- ALCLs and associated with poor outcomes. Methods and Results Eighty-two ALK- ALCL patients were recruited to the Atlas of Blood Cancer (ABC) genomes project, a worldwide consortium established to define the molecular origins of blood cancers. Tumor biopsies from these patients, as well as 10 ALK+ ALCL samples for comparison were obtained from participating institutions. Each case was subjected to centralized pathology review by an experienced panel of hematopathologists to ensure the accuracy of the diagnosis. All cases, along with paired normal tissues, were subjected to DNA and RNA (whole exome-level) sequencing on the Illumina platform to identify mutations and expression changes for each of these cases using methods well established in our group and described previously. We first examined the genetic alterations in ALK- ALCLs. In addition to frequently described genetic alterations such as TP63 and DUSP22 rearrangements, as well as mutations in JAK1, STAT3 and TP53, we also detected mutations in ERBB4, SETD2 and KMT2D, which may serve as potential novel drivers and have not been described previously to our knowledge. We next performed comparative gene expression analysis of the ALK- and ALK+ ALCLs. Surprisingly, a proportion of ALK- ALCL cases (38%) clustered together with ALK+ ALCLs and had a signature resembling ALK+ cases, which we designated as "ALK-like ALCL" here. Both the ALK-like ALCLs and the other ALK- ALCL cases showed decreased ALK expression compared to the ALK+ ALCLs by gene expression analysis. These results point to downstream pathways that are common among ALK+ALCLs and ALK-like ALCLs, but different from the other ALK- ALCLs. Gene set enrichment analysis revealed that the ALK-like ALCLs overexpressed genes in pathways related to monocyte and fibroblast activation, whereas the remaining ALK- ALCLs overexpressed genes in the T follicular helper cells, memory T cells and adaptive immune response-related pathways (P Conclusion Our data indicate that ALK- ALCLs represent a heterogeneous group of diseases and comprise at least two distinct subgroups that can be identified based on their similarity to the ALK+ ALCLs. The ALK-like ALCLs demonstrated distinct molecular features and favorable outcomes. Our results provide a potentially new approach to patient risk-stratification and pathological classification of this disease. Disclosures Kwong: Celgene: Consultancy, Honoraria, Research Funding; Bayer: Consultancy, Honoraria, Research Funding; Astellas: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Consultancy, Honoraria, Research Funding; BeiGene: Consultancy, Honoraria, Research Funding; Gilead: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Merck: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Jaye: Stemline Therapeutics: Honoraria. Behdad: Roche/Foundation Medicine: Speakers Bureau; Thermo Fisher: Speakers Bureau; Lilly: Speakers Bureau. Hsi: AbbVie Inc, Eli Lilly: Research Funding. Dave: Data Driven Bioscience: Current equity holder in publicly-traded company.

Introduction Blood cancers are collectively common and strikingly heterogeneous diseases both clinically and molecularly. According to the WHO taxonomy, there are over 100 distinct myeloid and lymphoid neoplasms. Genomic profiling of blood cancers has been applied in a somewhat ad hoc fashion using diverse sequencing approaches including the use of targeted panels, whole exome sequencing, whole genome sequencing, RNA sequencing, etc. The lack of data uniformity has made it difficult to comprehensively understand the clinical and molecular spectrum within and across diseases. Systematic genomic approaches can address the central challenges in the diagnosis and treatment of blood cancers. For the diagnosis of blood cancers, the incorporation of genomics could greatly enhance the accuracy and speed of clinical diagnostics. Genomics could also inform their pathology classification. However, these applications must be preceded by a clear understanding of the particular genetic aberrations and expression profiles that unite and distinguish different leukemias and lymphomas. Therapeutic development can also be aided by genomic approaches through identification of new targets and establishing the relevance of existing targets and treatments. Targeted therapies including those directed at specific surface markers (e.g. CD19, CD30 and CD123) or molecular targets (e.g. BCR-ABL fusions, IDH1 mutations and EZH2 mutations) are rarely restricted to a single disease, with most occurring in multiple blood cancers. A systematic understanding of the presence or overlap of these targets within or across blood cancers would significantly expand the therapeutic possibilities and better enable the use of existing therapies in both common and rare cancers. However, such therapeutic possibilities need to be established through a rigorous, data-driven approach. We initiated the Atlas of Blood Cancers Genomes (ABCG) project to systematically elucidate the molecular basis of all leukemias and lymphomas by building upon advances in genomic technologies, our capabilities for data analysis and economies of scale. Using a uniform approach to systematically profile all blood cancers through DNA and RNA sequencing at the whole exome/whole transcriptome level, we aim to link genomic events with clinical outcomes, disease categories and subcategories, thereby providing a complete molecular blueprint of blood cancers. Methods/Results The ABCG project consists of collaborators from 25 institutions around the world who have collectively contributed samples from 10,481 patients comprising every type of blood cancer in the current WHO classification. The samples include thousands of myeloid leukemias and mature B cell lymphomas, hundreds of Hodgkin lymphoma and plasma cell myeloma, as well as every rare type of hematologic malignancy (along with case-matched normal tissue). All cases were de-identified and their associated pathology and detailed clinical information entered into a purpose-built web-based system that included disease-specific data templates. All cases were subjected to centralized pathology review and clinical data review by experienced hematopathologists and oncologists. All 10,481cases are being sequenced at the DNA and RNA level, and are being profiled to define the genetic alterations and expression changes that are characteristic of each disease. Analysis will include translocations, copy number alterations, and viral status. These molecular features will be examined in conjunction with genetic events, pathologic factors, and the clinical features. We have already generated results for ALK-negative anaplastic large B cell lymphoma and primary mediastinal B cell lymphomas (N=210). These data demonstrate novel subgroup and molecular discoveries that are enabled by integrative DNA and RNA sequencing analysis and the examination of molecular features across different diseases as well as within individual entities. In addition, other disease entities and the collective data will be presented in the meeting. Conclusion The ABCG project will comprehensively study the genetic and clinicopathological features of all blood cancers using systematic genomic approaches. We anticipate our data, approaches and results will serve as a lasting resource for the molecular classification and therapeutic development for leukemias and lymphomas. Disclosures McKinney: Novartis: Research Funding; Nordic Nanovector: Research Funding; Molecular Templates: Consultancy, Research Funding; Kite/Gilead: Honoraria, Speakers Bureau; Incyte: Research Funding; Genetech: Consultancy, Honoraria, Research Funding; Epizyme: Consultancy; Celgene: Consultancy, Research Funding; BTG: Consultancy; Beigene: Research Funding; ADC Therapeutics: Consultancy, Speakers Bureau; Pharmacyclics: Consultancy; Verastem: Consultancy. Behdad: Lilly: Speakers Bureau; Roche/Foundation Medicine: Speakers Bureau; Thermo Fisher: Speakers Bureau.

Introduction: Primary mediastinal large B-cell lymphoma (PMBL) is a rare non-Hodgkin lymphoma subtype that occurs predominantly in young adults, with an overall favorable prognosis. The cell of origin is presumed to be thymic medullary B-cells and the gene expression profile of PMBL is similar to classic Hodgkin lymphoma. Recent studies have begun unravelling the genomic alterations underlying PMBL. Frequent, recurrent mutations (e.g. B2M, TNFAIP3, SOCS1, STAT6, GNA13) have been reported, but most of the studies have analyzed a small number of cases. To gain further insights into disease biology, we recruited 63 cases of PMBL as part of the Atlas of Blood Cancer Genomes (ABC-G) initiative, a consortium consisting of 25 institutions. Methods: Formalin-fixed paraffin-embedded (FFPE) biopsies and clinical data were collected. All cases were subjected to centralized review by an experienced panel of hematopathologists to ensure accurate diagnosis. Whole-exome DNA and RNA sequencing was performed using the Illumina platform and the DNA and RNA reads aligned to the GRCh38 genome and transcriptome respectively. Exonic variants were filtered using a set of paired normal samples and population-based databases to identify putative driver mutations, which were then aggregated at the gene level. Mutational analysis was performed on 56 samples that passed quality filtering and expression analysis on 45 samples. RNAseq data was normalized using DESeq2. Results: The cohort included samples from 16 males and 24 females, with a median age of 33 years (range 16 - 72) at the time of diagnosis. The majority of patients were treated with R-CHOP (47%) or R-EPOCH (43%), with 93% of patients surviving through the end of follow-up (median follow-up: 60.1 months). Besides the known recurrent mutations involving the JAK-STAT (STAT6 -21%, SOCS1 - 26%), NFKB (TNFAIP3 - 27%, NFKB1A - 7%), immune escape (B2M - 20%, LTB - 11%, IRF8 - 9%, IRF4 -9%), and chromatin modification (ZNF217 - 16%, CREBBP - 11%, KMT2D -11%) pathways , we discovered recurrent somatic variants in novel candidate driver genes in this disease, including NOTCH4 (7%), DICER1 (11%), MCL1 (7%), amongst others. EZH2, EP300, and XPO1 mutations were not detected. CIITA mutations and fusions were observed in 14% and 11% of cases, respectively, with novel partner genes (IGHA2, IGHG1, CDC6) detected in 67% of the fusion positive cases. Copy number alterations included gains at 2p16.1 (REL - 20%) and 9p24.2 (JAK2/PDL1/PDL2 - 24%), as well as loci not previously implicated in PMBL, 8q24.3 and 9q34.3 (each in 20%). Of note, CIITA alterations and 9p24 gains were virtually mutually exclusive, highlighting diverse mechanisms of immune escape in this entity. The transcriptomes of cases harboring CIITA alterations demonstrated differential enrichment of genes involved in protein glycosylation. The PMBLs in our series showed significant enrichment of the reported PMBL genetic classifier score, compared to nodal diffuse large B cell lymphoma (DLBCL) (p=0.0003). Finally, the gene expression profile of thymic B cells was more similar to that of PMBL than nodal DLBCL (p=0.0144). Conclusions: Our study, representing one of the largest comprehensive genomic and transcriptomic analyses of PMBL, expands the mutational landscape of PMBL, provides evidence for biologically distinct disease subsets and suggests an origin of PMBLs from thymic B-cells. Disclosures Hsi: AbbVie: Research Funding; Eli Lilly: Research Funding; Cytomx: Honoraria; Seattle Genetics: Honoraria. McKinney: BTG: Consultancy; Celgene: Consultancy, Research Funding; Epizyme: Consultancy; Genetech: Consultancy, Honoraria, Research Funding; Incyte: Research Funding; Kite/Gilead: Honoraria, Speakers Bureau; Molecular Templates: Consultancy, Research Funding; Nordic Nanovector: Research Funding; Novartis: Research Funding; Pharmacyclics: Consultancy; Verastem: Consultancy; Beigene: Research Funding; ADC Therapeutics: Consultancy, Speakers Bureau. Jaye: Stemline Therapeutics: Honoraria. Cohen: Genentech, Takeda, BMS/Celgene, BioInvent, LAM, Astra Zeneca, Novartis, Loxo/Lilly: Research Funding; Janssen, Adaptive, Aptitude Health, BeiGene, Cellectar, Adicet, Loxo/Lilly, AStra ZenecaKite/Gilead: Consultancy. Behdad: Lilly: Speakers Bureau; Roche/Foundation Medicine: Speakers Bureau; Thermo Fisher: Speakers Bureau. Dave: Data Driven Bioscience: Current equity holder in publicly-traded company.

Follicular dendritic cell sarcoma is a rare malignant neoplasm of dendritic cell origin that is currently poorly characterized by genetic studies. To investigate whether recurrent genomic alterations may underlie the biology of follicular dendritic cell sarcoma and to identify potential contributory regions and genes, molecular inversion probe array analysis was performed on 14 independent formalin-fixed, paraffin-embedded samples. Abnormal genomic profiles were observed in 11 out of 14 (79%) cases. The majority showed extensive genomic complexity that was predominantly represented by hemizygous losses affecting multiple chromosomes. Alterations of chromosomal regions 1p (55%), 2p (55%), 3p (82%), 3q (45%), 6q (55%), 7q (73%), 8p (45%), 9p (64%), 11q (64%), 13q (91%), 14q (82%), 15q (64%), 17p (55%), 18q (64%), and 22q (55%) were recurrent across the 11 samples showing abnormal genomic profiles. Many recurrent genomic alterations in follicular dendritic cell sarcoma overlap deletions that are frequently observed across human cancers, suggesting selection, or an active role for these alterations in follicular dendritic cell sarcoma pathogenesis. In support of a tumor suppressor-driven biology, homozygous deletions involving tumor suppressor genes CDKN2A, RB1, BIRC3, and CYLD were also observed. Neither recurrent gains nor amplifications were observed. This genomic characterization provides new information regarding follicular dendritic cell sarcoma biology that may improve understanding about the underlying pathophysiology, provide better prognostication, and identify potential therapeutic markers for this rare disease.

Mantle cell lymphoma (MCL) is an uncommon B-cell lymphoma that prototypically expresses CD5, but a small subset is CD5 negative. The clinical significance of CD5 negativity is not yet well-elucidated. This case series aims to contribute to the understanding of CD5-negative MCL by looking at specific markers and outcome in our cases with long-term follow-up. Eight cases of CD5-negative MCL were identified in the case files at the Massachusetts General Hospital from 1978 to 2016. Clinicopathological characteristics were evaluated, including immunohistochemical stains for cyclin D1, SOX11, Ki67, and p53. Patients initially presented with involvement of lymph nodes and spleen (n = 4), sinonasal or oral mucosa (n = 2), orbital soft tissue (n = 1), and salivary gland (n = 1). On histology, the cases all showed classic MCL morphology, with a monotonous population of medium-sized cells with irregular nuclear contours. The cases were positive by immunohistochemistry for cyclinD1 (8/8 cases), essentially negative for p53 staining (8/8 cases), and mostly positive for SOX11 (5/8 cases). All cases had a low Ki67 proliferation rate (

The advance of high resolution digital scans of pathology slides allowed development of computer based image analysis algorithms that may help pathologists in IHC stains quantification. While very promising, these methods require further refinement before they are implemented in routine clinical setting. Particularly critical is to evaluate algorithm performance in a setting similar to current clinical practice. In this article, we present a pilot study that evaluates the use of a computerized cell quantification method in the clinical estimation of CD3 positive (CD3+) T cells in follicular lymphoma (FL). Our goal is to demonstrate the degree to which computerized quantification is comparable to the practice of estimation by a panel of expert pathologists. The computerized quantification method uses entropy based histogram thresholding to separate brown (CD3+) and blue (CD3−) regions after a color space transformation. A panel of four board-certified hematopathologists evaluated a database of 20 FL images using two different reading methods: visual estimation and manual marking of each CD3+ cell in the images. These image data and the readings provided a reference standard and the range of variability among readers. Sensitivity and specificity measures of the computer's segmentation of CD3+ and CD− T cell are recorded. For all four pathologists, mean sensitivity and specificity measures are 90.97 and 88.38%, respectively. The computerized quantification method agrees more with the manual cell marking as compared to the visual estimations. Statistical comparison between the computerized quantification method and the pathologist readings demonstrated good agreement with correlation coefficient values of 0.81 and 0.96 in terms of Lin's concordance correlation and Spearman's correlation coefficient, respectively. These values are higher than most of those calculated among the pathologists. In the future, the computerized quantification method may be used to investigate the relationship between the overall architectural pattern (i.e., interfollicular vs. follicular) and outcome measures (e.g., overall survival, and time to treatment). © 2017 International Society for Advancement of Cytometry

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive, male-biased (>3:1 M:F) hematologic malignancy in which some patients have bone marrow involvement at diagnosis (50%) and most have tumor formation in the skin (~90%), often preceding marrow disease. The prognosis is poor (median survival of 12-24 months) and there is unmet need for biological insight. TET2, ASXL1, and RNA splicing genes (SRSF2, SF3B1, and ZRSR2) are recurrently mutated in BPDCN. The X chromosome gene ZRSR2 was the most frequently mutated spliceosome gene reported in a prior BPDCN cohort (7 of 24, 29.2%; Taylor, ASH 2013). Our goal was to define the functional consequences of ZRSR2 mutations in BPDCN. First, we confirmed the frequency of ZRSR2 mutations in a larger cohort from the US and Europe; we found ZRSR2 mutations in 24 of 93 (25.8%). Notably, ZRSR2 mutations were almost exclusively in males (23/73 males vs 1/20 females, P=0.019). Next, we compared the global mutation pattern to 30 predefined signatures from >7000 cancers in COSMIC. Analysis of all somatic single nucleotide variants in 11 tumor-normal pairs using whole exome sequencing (tumor was sorted BPDCN cells from marrow) revealed that BPDCN had an ultraviolet (UV)-induced mutation signature (score >0.25 in 6/11 or 55%; Figure 1A). For comparison, we detected the UV signature in melanoma but not in AML from The Cancer Genome Atlas. These data suggest that mutations acquired in the skin stage of BPDCN evolution are retained in subsequent leukemic disease. Next, we performed RNA-sequencing from sorted BPDCN and normal plasmacytoid dendritic cells (pDCs). Differentially expressed genes between BPDCN and pDCs (BCL2, MYB, IRF4, CEP70, IGLL1, GZMB) were similar to those that distinguish BPDCNs from pDCs by bulk and single cell RNA-sequencing. By gene set enrichment analysis (GSEA), BPDCNs were enriched for overexpression of MYC/E2F targets and PI3K/AKT/MTORC1 signaling pathway-associated genes. BPDCNs transcriptomes were also enriched for gene sets associated with RNA splicing machinery and RNA nonsense mediated decay (NMD). To link RNA splicing with functional consequences of ZRSR2 mutations, we generated ZRSR2-knockout BPDCN cells (CAL1) using CRISPR/Cas9. This models primary tumors because ZRSR2-mutant BPDCNs have complete loss of ZRSR2 protein. Activation marker (CD80) upregulation and type 1 interferon secretion after Toll-like receptor (TLR) stimulation with lipopolysaccharide (LPS) or R848 were reduced in ZRSR2-deficient cells. We found similar defective cytokine production in stimulated primary BPDCN cells compared to normal pDCs. After activation, normal pDCs undergo apoptosis in a negative feedback process. In contrast, ZRSR2-knockout, but not control cells, were protected from TLR activation-induced apoptosis. Reexpression of wild-type ZRSR2 in knockout cells restored activation-induced apoptosis (Figure 1B). These data suggested that ZRSR2-mutant BPDCNs have defects downstream of TLR stimulation. By RNA-sequencing, we found that IRF7 mRNA was mis-spliced in all ZRSR2- (2/2), SRSF2- (4/4), and SF3B1- (1/1) mutant BPDCNs compared to those with no mutated splicing gene (4/4). IRF7 (interferon regulatory factor 7) is a transcription factor activated by TLR signaling that is important for pDC activation and apoptosis. The IRF7 mRNA transcript contains a "weak intron" (intron 4) that is subject to intron retention, which leads to NMD and reduced IRF7 protein level in stimulated dendritic cells (Luke, Mol Cell 2019). IRF7 intron 4 was mis-spliced in ZRSR2-, SRSF2-, and SF3B1-mutant BPDCNs. ZRSR2-knockout CAL1 cells had severely impaired ability to upregulate IRF7 after LPS stimulation, which was partially rescued by reepxression of wild-type ZRSR2 (Figure 1C). Expression of constitutively activated IRF7 inhibited growth of both ZRSR2-knockout and control cells, confirming that the inability to activate IRF7 is important for the effect of ZRSR2 loss on TLR agonist-induced growth inhibition. In conclusion, male-biased ZRSR2 mutations are frequent in BPDCN and impair pDC activation and apoptosis, at least in part via TLR-IRF7. These data may explain why BPDCNs have an impaired activation state (Bierd, BCJ 2019). They also suggest that splicing factor mutations affect cell type-specific pathways to promote transformation, underscoring the importance of studying cancer genes in relevant contexts. Figure Disclosures Griffin: Moderna Therapeutics: Consultancy. Ghandi:Monte Rosa Therapeutics: Consultancy; Cambridge Data Science LLC: Current Employment, Current equity holder in private company. Seiler:Remix Therapeutics: Current Employment. Konopleva:Reata Pharmaceutical Inc.;: Patents & Royalties: patents and royalties with patent US 7,795,305 B2 on CDDO-compounds and combination therapies, licensed to Reata Pharmaceutical; Eli Lilly: Research Funding; Genentech: Consultancy, Research Funding; Agios: Research Funding; Rafael Pharmaceutical: Research Funding; Sanofi: Research Funding; AbbVie: Consultancy, Research Funding; Forty-Seven: Consultancy, Research Funding; AstraZeneca: Research Funding; Ascentage: Research Funding; Calithera: Research Funding; Amgen: Consultancy; F. Hoffmann La-Roche: Consultancy, Research Funding; Cellectis: Research Funding; Ablynx: Research Funding; Kisoji: Consultancy; Stemline Therapeutics: Consultancy, Research Funding. Pemmaraju:Cellectis: Research Funding; Daiichi Sankyo: Research Funding; DAVA Oncology: Honoraria; Plexxikon: Research Funding; Blueprint Medicines: Honoraria; Incyte Corporation: Honoraria; SagerStrong Foundation: Other: Grant Support; Celgene: Honoraria; Pacylex Pharmaceuticals: Consultancy; Affymetrix: Other: Grant Support, Research Funding; MustangBio: Honoraria; Roche Diagnostics: Honoraria; Novartis: Honoraria, Research Funding; LFB Biotechnologies: Honoraria; Stemline Therapeutics: Honoraria, Research Funding; AbbVie: Honoraria, Research Funding; Samus Therapeutics: Research Funding. Abdel-Wahab:H3 Biomedicine Inc.: Consultancy, Research Funding; Merck: Consultancy; Janssen: Consultancy; Envisagenics Inc.: Current equity holder in private company. Lane:Qiagen: Consultancy; Abbvie: Research Funding; Stemline Therapeutics: Research Funding.

BACKGROUND Cutaneous T-cell lymphoma (CTCL) is a group of T-cell lymphomas occurring primarily in the skin, without evidence of disease elsewhere. Central nervous system (CNS) involvement of CTCL is extremely rare, with a reported incidence of 5–9%. Due to its infrequency and radiographic appearance which mimics other CNS processes, CNS CTCL may be misdiagnosed. Here, we describe 8 cases of CNS CTCL at a single institution. METHODS The electronic medical records at our institution were queried for cases of CNS involvement by CTCL, between 2004–19. Clinical data, imaging and surgical pathology were reviewed. RESULTS Eight patients were diagnosed with CNS CTCL with median age of 68 and median time to development of CNS disease of 17 months. 1 patient had synchronous presentation of cutaneous and CNS disease identified at autopsy. Seven out of eight patients presented neurologically with behavioral and mental status changes and 1 patient had concurrent intraocular disease. Imaging showed bilateral non-enhancing T2/FLAIR signal changes in three patients; multifocal enhancement was noted in the others. Malignancy, inflammatory processes and infections including progressive multifocal leukoencephalopathy (PML) were often included in the radiographic differential diagnosis. Prior systemic therapies were variable, including palliative radiation, phototherapy, myeloablative chemotherapy followed by allogeneic stem cell transplant and oral methotrexate. 5 of 8 patients had active cutaneous disease at the time of CNS relapse. Six patients received initial treatment for CNS disease with high-dose methotrexate (HD-MTX). Comprehensive genomic analysis revealed recurrent alterations in genes associated with regulatory T-cell differentiation (SOCS1, NOTCH1), tumor suppressors (CDKN2A, PTEN, TP53); one case was hypermutated. CONCLUSIONS CNS involvement of CTCL is rare, occurring more than a year after initial cutaneous disease. Clinical and radiographic findings may be variable, necessitating high clinical suspicion. Most patients respond to HD-MTX. Further molecular analysis is ongoing.