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In this article, we describe three broad pathologic presentations of plasmacytoid dendritic cells (pDCs) that may be encountered in clinical practice, in which an association between pDCs and myeloid neoplasms is identified: (1) myeloid neoplasms with mature pDC expansion, most commonly seen in chronic myelomonocytic leukaemia (CMML); (2) myeloid neoplasms with pDC differentiation, in which pDCs show a spectrum of maturation from early immature pDCs to mature forms, most commonly seen in acute myeloid leukaemia (AML); (3) myeloid neoplasms associated with blastic plasmacytoid dendritic cell neoplasm (BPDCN), either stemming from the same precursor or representing an independent clonal process. Additionally, we also discuss AML with pDC-like phenotype, in which myeloblasts show immunophenotypic features that may mimic those seen in pDCs. Using these presentations, we provide a diagnostic algorithm for appropriate pathologic classification, while attempting to clarify and homogenize nomenclatures pertaining to different biologic states of pDCs.

BCL11B is an essential transcription factor for T-cell lineage commitment and differentiation, and its dysregulation has been shown to be associated with T-cell tumourigenesis. In this study, we investigated BCL11B expression by immunohistochemical analysis in 120 cases of mature T-cell lymphoma, 34 B-cell lymphomas, 11 NK-cell neoplasms and 17 reactive cutaneous conditions. All cases of mycosis fungoides (n=23), primary cutaneous CD4+ small/medium T-cell lymphoproliferative disorder (n=8) and T-prolymphocytic leukaemia (n=6) were positive for BCL11B and the staining intensity was higher than that of reactive T-cells. Fourteen of 15 (93%) cases of angioimmunoblastic T-cell lymphoma, 10 of 12 (83%) T-large granular lymphocytic leukaemia and 14 of 20 (70%) peripheral T-cell lymphoma, not otherwise specified, were also positive for BCL11B with an intensity comparable to reactive T-cells. Other T-cell neoplasms were uncommonly positive including one of three (33%) cases of primary cutaneous gamma delta T-cell lymphoma, one of four (25%) cases of subcutaneous panniculitis-like T-cell lymphoma, one of four (25%) cases of hepatosplenic T-cell lymphoma, and one of 20 (5%) cases of anaplastic large cell lymphoma (8 ALK-positive, 12 ALK-negative). T-cells in reactive cutaneous infiltrates were also positive for BCL11B, but staining intensity was much weaker than in mycosis fungoides. All NK-cell (n=11) and B-cell neoplasms (n=34) were negative for BCL11B. In conclusion, BCL11B shows a distinct expression pattern in various T-cell neoplasms. BCL11B appears to have utility as another T-cell marker and may be useful in the differential diagnosis of lymphoid neoplasms.

The immunophenotype of pure erythroid leukemia (PEL) as determined by flow cytometry immunophenotypic analysis is not well characterized. The immunophenotypic difference between PEL and reactive conditions is under-explored.We assessed and compared the immunophenotype of 24 PEL cases and 28 reactive cases containing early erythroid precursors by flow cytometry.The neoplastic erythroid cells in all PEL cases were positive for CD36 and CD71. CD45 was also positive in all cases, but the expression level was often dimmer than granulocytes. CD117 expression ranged from partial to uniform, and CD235a was often only positive in the CD117-dim to negative cells, corresponding to more differentiated subset. PEL cases frequently (87%) showed decreased or negative CD38 expression, contrasting to reactive early erythroid precursors that showed bright CD38 (p 0.0001). CD7 (25%) and CD13 (29%) aberrant expressions were only observed in PEL but not in the reactive erythroid cells. Normal early erythroid precursors in all reactive bone marrows showed partial expression of CD4; In contrast, aberrant CD4 expression was detected in 71% PEL cases, either uniformly positive (50%) or completely negative (21%). While normal/reactive bone marrows almost always contained a small subset of CD34-positive early erythroid precursors, the neoplastic pronormoblasts in all PEL cases were CD34 negative. Although not increased in number, CD34-positive myeloblasts were frequently detected in PEL and demonstrated an aberrant immunophenotype in 90% PEL cases.PEL shows a distinctive immunophenotype which can be distinguished from reactive erythroid precursors by flow cytometry immunophenotyping.

T-prolymphocytic leukemia (T-PLL) is a rare mature T-cell neoplasm defined by rearrangements involving TCL1 or MTCP1. Cases showing some overlapping features with T-PLL but lacking TCL1 and MTCP1 rearrangements have been rarely reported but are not well characterized. Whether these neoplasms belong within the category of T-PLL or represent a distinct entity is unknown. Here, we fully characterize 20 such cases. The median survival for this cohort was 34.7 months. Clinically, 40% of patients were diagnosed incidentally and 65% of patients presents with an indolent phase that was associated with a better survival. Leukemic cells were small to medium sized with a mature morphology. They were CD4-positive with TCRαβ subtype and maintained the expression of other pan-T antigens. A complex karyotype, 11q22.3/ATM deletion and chromosome 8q abnormalities were common, present in 70%, 45% and 35% of patients, respectively. The most common mutations involved ATM and JAK/STAT pathway genes, identified in 40% and 38% of patients, respectively. When this cohort was compared to 42 cases of prototypical T-PLL, they shared many overlapping clinicopathological features and had a similarly poor prognosis. We therefore propose that the neoplasms in this cohort are best classified as TCL1-family negative T-PLL.

SUMMARYMesenchymal stem/stromal cells (MSCs) within the bone marrow microenvironment (BMME) support normal hematopoietic stem and progenitor cells (HSPCs). However, the heterogeneity of human MSCs has limited the understanding of their contribution to clonal dynamics and evolution to myelodysplastic syndromes (MDS). We combined three MSC cell surface markers, CD271, VCAM-1 (Vascular Cell Adhesion Molecule-1) and CD146, to isolate distinct subsets of human MSCs from bone marrow aspirates of healthy controls (Control BM). Based on transcriptional and functional analysis, CD271+CD106+CD146+(NGFR+/VCAM1+/MCAM+/Lin-;NVML) cells display stem cell characteristics, are compatible with murine BM- derived Leptin receptor positive MSCs and provide superior support for normal HSPCs. MSC subsets from 17 patients with MDS demonstrated shared transcriptional changes in spite of mutational heterogeneity in the MDS clones, with loss of preferential support of normal HSPCs by MDS-derived NVML cells. Our data provide a new approach to dissect microenvironment-dependent mechanisms regulating clonal dynamics and progression to MDS.

Context.— Unicentric Castleman disease (UCD) is a dynamic entity with a wide spectrum of morphologic findings. UCD can be further subdivided into hyaline-vascular and mixed/plasmacytic variants. Hyaline-vascular UCD has both follicular and interfollicular (stromal) changes, and occasionally these lesions show a skewed representation of either the follicular or stromal compartments. Plasmacytosis is usually minimal in hyaline-vascular variant. Mixed/plasmacytic variant of UCD is composed of sheets of plasma cells often associated with a variable number of follicles with regressive changes. Objective.— To illustrate the differential diagnosis of UCD, as it is quite broad and includes lymphomas, plasma cell neoplasms and stromal neoplasms such as follicular dendritic cell sarcoma and vascular neoplasms, immunoglobulin G4–related disease, infections, and other rare lesions. An additional objective is to enhance awareness of the morphologic features of UCD in excisional and in small core-needle biopsy specimens, the latter of which may inadvertently target follicle- or stroma-rich areas, causing diagnostic challenges. Data Sources.— In this review, we provide the readership a concise illustration of the morphologic spectrum of UCD that we have encountered in our practice and a brief discussion of entities in the differential diagnosis. Conclusions.— UCD exhibits a broad spectrum of morphologic changes, and awareness of these morphologic variations is key to avoid misdiagnosis.

Plasma cell neoplasms are notorious for having diverse morphological presentations, and less frequently, unusual immunophenotypical profiles. This unexpected immunomorphological variability could lead to erroneous impressions upon initial assessment, potentially delaying the generation of a final accurate diagnosis. In this review, we present a concise, yet comprehensive summary of both morphological and immunophenotypical variants of plasma cell neoplasms from the archives of MD Anderson Hematopathology Department, with emphasis on possible diagnostic pitfalls precluding a timely and accurate assessment.

Artificial intelligence-based tools designed to assist in the diagnosis of lymphoid neoplasms remain limited. The development of such tools can add value as a diagnostic aid in the evaluation of tissue samples involved by lymphoma. A common diagnostic question is the determination of chronic lymphocytic leukemia (CLL) progression to accelerated CLL (aCLL) or transformation to diffuse large B-cell lymphoma (Richter transformation; RT) in patients who develop progressive disease. The morphologic assessment of CLL, aCLL, and RT can be diagnostically challenging. Using established diagnostic criteria of CLL progression/transformation, we designed four artificial intelligence-constructed biomarkers based on cytologic (nuclear size and nuclear intensity) and architectural (cellular density and cell to nearest-neighbor distance) features. We analyzed the predictive value of implementing these biomarkers individually and then in an iterative sequential manner to distinguish tissue samples with CLL, aCLL, and RT. Our model, based on these four morphologic biomarker attributes, achieved a robust analytic accuracy. This study suggests that biomarkers identified using artificial intelligence-based tools can be used to assist in the diagnostic evaluation of tissue samples from patients with CLL who develop aggressive disease features. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

B-cell lymphoma/leukaemia 11B (BCL11B) is an essential transcription factor for T-cell lineage commitment and maturation. We investigated BCL11B expression by immunohistochemistry in T-lymphoblastic leukaemia/lymphoma (T-ALL/LBL) (n = 115). The majority (83%) of early T-cell precursor T-ALL/LBL (ETP-ALL) cases showed negative BCL11B expression, while most (84%) of non-ETP-ALL/LBL were positive for BCL11B. A simplified three-marker [BCL11B, cluster of differentiation 5 (CD5), CD13] immunophenotypic score discriminated reliably between ETP-ALL and non-ETP-ALL/LBL. In ETP-ALL, patients with positive BCL11B expression had a better overall survival than those with negative BCL11B (P = 0·009). In summary, BCL11B is a valuable marker for T-ALL/LBL subtyping and serves as a potential prognostic marker in patients with ETP-ALL.

Until recently, conventional prognostication of myelodysplastic neoplasms (MDS) was performed using the revised International Prognostic Scoring System (IPSS-R), with additional adverse prognoses conferred by select mutations. Nonetheless, the clonal diversity and dynamics of coexisting mutations have been shown to alter the prognosis and treatment response in patients with MDS. Often in the process of clonal evolution, various initial hits are preferentially followed by a specific spectrum of secondary alterations, shaping the phenotypic and biologic features of MDS. Our ability to recapitulate the clonal ontology of MDS is a necessary step toward personalized therapy and the conceptualization of a better classification system, which ideally would take into consideration all genomic aberrations and their inferred clonal architecture in individual cases. In this review, we summarize our current understanding of the molecular landscape of MDS and the role of mutational combinations, clonal burden, and clonal hierarchy in defining the clinical fate of the disease.

Autologous and allogeneic hematopoietic stem cell transplantation (HSCT) has revolutionized the therapy of hematolymphoid malignancies. Yet, how to best detect or predict the emergence of HSCT-related complications remain unresolved. Here, we describe a case of donor-derived, transient Alpha Beta (αβ) T-cell large granular clonal lymphocytosis and cytopenia that emerged post-HSCT in a patient with a history of gamma delta (γδ) T-cell large granular lymphocytic leukemia (T-LGLL). Clonal unrelatedness of post-transplant T-LGL lymphocytosis to the patient's pretransplant T-LGLL was first identified by T-cell receptor (TCR) PCR showing different sized fragments of rearranged gamma chains, in addition to shift from γδ to αβ TCR expression by flow cytometry analyses. Donor-derivation of the patient's post-transplant clonal lymphocytosis was confirmed by serial chimerism analyses of recipient's blood specimens demonstrating 100% donor DNA. Moreover, oncogenicDNMT3AandRUNX1mutations were detected by next-generation sequencing (NGS) only in post-transplant specimens. Intriguingly, despite continued increase inDNMT3AandRUNX1mutation load, the patient's clonal lymphocytosis and anemia eventually largely resolved; yet, the observed mutation profile with persistent thrombocytopenia indicated secondary clonal cytopenia of undetermined significance (CCUS) in the absence of overt morphologic evidence of myeloid neoplasm in the marrow. This case illustrates the utility of longitudinal chimerism analysis and NGS testing combined with flow cytometric immunophenotyping to evaluate emerging donor-derived hematolymphoid processes and to properly interpret partial functional engraftment. It may also support the notion that driver mutation-induced microenvironmental changes may paradoxically contribute to reestablishing tissue homeostasis.

Our understanding of chronic myelomonocytic leukemia (CMML) has evolved tremendously over the past decade. Large-scale sequencing studies have led to increased insight into the genomic landscape of CMML and clinical implications of these changes. This in turn has resulted in refined and improved risk stratification models, which to date remain versatile and subject to remodeling, as new and evolving studies continue to refine our understanding of this disease. In this article, we present an up-to-date review of CMML from a hematopathology perspective, while providing a clinically practical summary that sheds light on the constant evolution of our understanding of this disease.

Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) with unusual features, including some that can overlap morphologically with classic Hodgkin lymphoma (CHL), have been described. Herein, we describe 12 cases of NLPHL with fibrous bands and capsular fibrosis resembling, in part, nodular sclerosis (NS) CHL. Seven of 12 cases harbored Reed-Sternberg-like cells, further suggestive of CHL, but all cases lacked associated eosinophils and/or plasma cells in the background. In this cohort, all cases had areas of so-called pattern D (nodular T-cell rich) as a sole component in 7 (58%) cases or as a hybrid pattern along with pattern E (diffuse T-cell/histiocyte-rich) in 5 (42%) cases. The immunophenotype of the large neoplastic cells in these cases supported their being lymphocyte predominant cells of NLPHL, positive for CD20, CD79a, and OCT2, and negative for CD15 and CD30. However, PAX5 was weak in 9 of 11 cases similar to Hodgkin/Reed-Sternberg cells in CHL. We conclude that some cases of NLPHL are associated with fibrous bands and capsular fibrosis and resemble, in part, NS CHL. In our experience, NLPHL with NS-like features occurs in 10% to 15% of cases of NLPHL and is associated with a variant pattern (D and/or E). In addition, all patients in this cohort were not treated before biopsy, suggesting that the prominent sclerosis in these cases is inherent to disease biology. Recognition of NLPHL with NS-like features further expands the morphologic spectrum of NLPHL and helps avoid potential misdiagnosis as CHL.

Prompt diagnosis of acute promyelocytic leukemia (APL) is critical for patient care. In this study, we aimed to characterize the immunophenotype of APL and explore immunophenotypic difference between APL and its mimics using flow cytometric analysis.Eighty-five cases were collected, including 47 APL, 26 NPM1-mutated acute myeloid leukemia (AML) and 12 KMT2A-rearranged AML with an APL-like immunophenotype. Immunophenotypes were analyzed using flow cytometric analysis.APL showed four distinct patterns (designated a-d) based on CD45/SSC plots. Blasts in patterns a-c showed high side scatter, whereas blasts in pattern d had low side scatter and were located in the traditional blast gate. Compared with patterns a-c, pattern d of APL (APL-D) was more often positive for CD2 (p = 0.0005) and CD34 (p = 0.0002) in blasts. All NPM1-mutated AML and KMT2A-rearranged AML cases with an APL-like immunophenotype had blasts in the traditional blast gate on CD45/SSC, mimicking APL-D. In comparison, uniform CD13 and positive CD64 were seen in 100% (n = 13) APL-D cases and in only 2 of 26 (8%) NPM1-mutated AML cases (p 0.0001). In addition, APL-D cases were more likely to be positive for CD2 and/or CD34 than NPM1-mutated AML (p 0.0001 and p = 0.0007, respectively). In comparison with APL-D, KMT2A-rearranged AML cases were less often positive for myeloperoxidase (MPO) (p = 0.001), with none being strongly positive. Similar to NPM1-mutated AML and different from APL-D, KMT2A-rearranged AML cases were rarely positive for CD34 and all negative for CD2.APL and its immunophenotypic mimics share some immunophenotypic similarities but can be distinguished by CD2, CD13, CD34, CD64, and MPO.

Early T-cell precursor acute lymphoblastic leukemia/lymphoma (ETP-ALL/LBL) is characterized by a distinct immunophenotype (CD1a-negative, CD8-negative, CD5-negative or weak-positive

Myeloproliferative neoplasms (MPNs) are frequently associated with classic driver mutations involving JAK2, MPL or CALR. SRSF2 is among the most frequently mutated splicing genes in myeloid neoplasms and SRSF2 mutations are known to confer a poor prognosis in patients with MPNs. In this study, we sought to evaluate the clinicopathologic spectrum of myeloid neoplasms harboring concurrent MPN-driver mutations and SRSF2 mutations. The study cohort included 27 patients, 22 (82%) men and five (19%) women, with a median age of 71 years (range, 51-84). These patients presented commonly with organomegaly (n = 15; 56%), monocytosis (n = 13; 48%), morphologic dysplasia (n = 11; 41%), megakaryocytic hyperplasia and/or clustering (n = 10; 37%) and bone marrow fibrosisMF-1 (17/22; 77%). About one third of patients either initially presented with acute myeloid leukemia (AML) or eventually progressed to AML. Eighteen (68%) patients had a dominant clone with SRSF2 mutation and nine (33%) patients had a dominant clone with a classic MPN-associated driver mutation. Our data suggest that the presence of an SRSF2 mutation preceding the acquisition of a MPN driver mutations is not a disease-defining alteration nor is it restricted to any specific disease entity within the spectrum of myeloid neoplasms. In summary, patients with myeloid neoplasms associated with concurrent SRSF2 and classic MPN driver mutations have clinical and morphologic features close to that of classic MPNs often with frequent dysplasia and monocytosis.

The development of clonally related hematologic neoplasms in the setting of primary mediastinal germ cell tumors (PMGCTs) has been recognized previously and is associated with a dismal prognosis. However, the presentation of hematologic neoplasms as chronic myelomonocytic leukemia (CMML) and hemophagocytic lymphohistiocytosis (HLH) has been rarely reported. Here we report two patients with PMGCTs and hematologic neoplasms. The PMGCT was composed mostly of yolk sac tumor whereas the hematologic neoplasms had morphologic features that resembled CMML and HLH. The hematologic neoplasms from both patients harbored isochromosome 12p [i(12p)] and TP53 mutations, supporting a clonal relationship between these tumors. This association represents a unique clinical syndrome that likely contributes to the poor clinical outcome of these patients.

Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL) is characterized morphologically by numerous small lymphocytes and pale nodules composed of prolymphocytes and paraimmunoblasts known as proliferation centers (PCs). Patients with CLL can undergo transformation to a more aggressive lymphoma, most often diffuse large B-cell lymphoma (DLBCL), known as Richter transformation (RT). An accelerated phase of CLL (aCLL) also may be observed which correlates with subsequent transformation to DLBCL, and may represent an early stage of transformation. Distinguishing PCs in CLL from aCLL or RT can be diagnostically challenging, particularly in small needle biopsy specimens. Available guidelines pertaining to distinguishing CLL from its' progressive forms are limited, subject to the morphologist's experience and are often not completely helpful in the assessment of scant biopsy specimens. To objectively assess the extent of PCs in aCLL and RT, and enhance diagnostic accuracy, we sought to design an artificial intelligence (AI)-based tool to identify and delineate PCs based on feature analysis of the combined individual nuclear size and intensity, designated here as the heat value. Using the mean heat value from the generated heat value image of all cases, we were able to reliably separate CLL, aCLL and RT with sensitive diagnostic predictive values.

AIMS Kikuchi-Fujimoto disease (KFD) is a self-limited disease characterised by destruction of the lymph node parenchyma. Few studies have assessed the immunohistological features of KFD, and most employed limited antibody panels that lacked many of the novel immunohistochemistry markers currently available. METHODS AND RESULTS We used immunohistochemistry to reappraise the microanatomical distribution of plasmacytoid dendritic cells (pDCs), follicular helper T cells and cytotoxic T cells, B cells, follicular dendritic cell (FDC) meshworks, and histiocytes in lymph nodes involved by KFD. The study group consisted of 138 KFD patients (89 women; 64.5%) with a median age of 27 years (range, 3-50 years). Cervical lymph nodes were most commonly involved, in 108 (78.3%) patients. The numbers of pDCs were increased, predominantly around and within apoptotic areas and the paracortex, and tapering off within xanthomatous areas. pDCs formed sizeable tight clusters, most notably around apoptotic/necrotic areas. T cells consisted mostly of CD8-positive cells with predominant expression of T-cell receptor-β. There were notable increases in the numbers of CD8-positive T cells within lymphoid follicles, and their numbers correlated with alterations in FDC meshworks (P

Aggressive natural killer-cell leukemia (ANKL) is a rare, lethal disease with pathologic features that are underdescribed in the literature, particularly in Western nations. In addition, although data on the molecular pathogenesis of ANKL has been reported, evaluation of such data in a clinicopathologic context remains limited. Patients diagnosed with ANKL were identified retrospectively. Detailed demographic and clinicopathologic data were analyzed. We assessed novel markers by immunohistochemistry and performed targeted next-generation sequencing analysis. The study group included 9 men and 3 women with a median age at diagnosis of 47.5 years (range, 20 to 75 y). Two distinct patterns of bone marrow involvement were identified: interstitial and sinusoidal. The neoplastic cells were positive for CD56 and CD94, and negative for surface CD3, CD5, and CD57 in all cases assessed. They were also positive for CD2 (10/12), c-MYC (6/8), BCL2 (6/8), CD16 (5/7), EBER (9/12), CD7 (6/11), pSTAT3 (3/8), CD8 (2/6), PD-L1 (2/8), CD4 (2/11), CD8 (2/6), and CD158 (1/5). Aberrant p53 expression was identified in most (7/8) cases; p53 was strongly expressed in 4 cases. Conventional cytogenetic analysis showed clonal abnormalities in 5 of 12 cases. TP53 mutations were detected in 3 of 6 cases, whereas ASXL1 and TET2 mutations were each detected in 2 of 6 cases. Patients had very poor outcomes despite intensive chemotherapy, with a median survival of 2 months. ANKL exhibits 2 distinct patterns of tissue involvement. Neoplastic cells in ANKL are commonly positive for c-MYC and EBER, and they have a high frequency of p53 overexpression, frequently with corresponding TP53 mutations.

Primary signet ring cell carcinoma of the pancreas (PSRCCP) is an extremely rare diagnosis that has not been extensively studied in literature. Primary and metastatic neoplasms to the pancreas may exhibit cytomorphological similarities to signet ring cells, posing diagnostic challenges. In this article, we review PSRCCP and provide a study of several primary pancreatic neoplasms that may mimic the appearance of PSRCCP upon cytopathology evaluation, shedding light on the existence of this dilemma, and helping cytopathologists in navigating similar scenarios in their practice.

Microsatellite instability (MSI) is caused by defects in DNA mismatch repair (MMR) components. Inactivation of any MMR gene(s), including hMLH1, hMSH2, hMSH6, and hPMS2, can result in MSI. Immunohistochemistry (IHC) is a sensitive and specific screening tool for MSI that can detect loss of expression of one or more MMR components. Of the four MMR markers, hMLH1 and hMSH2 are considered most informative of MSI status. There has been renewed interest in MSI status in view of its favorable association with response to immune checkpoint inhibitors in some cancers. MMR expression patterns in acute myeloid leukemia (AML) have not been evaluated systematically.We used clinically-validated IHC assays to assess the expression of hMLH1, hMSH2, hMSH6, and/or hPMS2 in formalin-fixed paraffin-embedded tissue sections of bone marrow core biopsies from patients diagnosed with AML. Mutation profiling was performed using next-generation sequencing to assess for mutations in MMR genes.The study group included 236 patients with AML, including a cohort treated on a clinical trial of azacitidine and nivolumab (NCT02397720). In addition, hMSH6, and/or hPMS2 expression was assessed in 99 AML patients with diploid karyotype. All patients, except two, had retained expression of all MMR markers assessed: One patient from the azacytidine+nivolumab group had zonal patchy loss of staining of hMLH1 and, to a lesser extent, a similar staining pattern of hMSH2; and one patient from the AML with diploid karyotype group had loss of hMSH2 but retained expression of hMLH1, hMSH6 and hPMS2. In addition, a retrospective analysis on a separate cohort of 139 patients with primary AML, on which next generation sequencing profiling was performed, identified 14 cases with alterations in MMR genes.MMR loss is a rare event in AML, thus does not appear to underlie response patterns to anti-PD1 therapy.

Well-differentiated systemic mastocytosis (WDSM) is a rare unique variant of systemic mastocytosis that has been described in few studies [1–11]. This entity has unique features that can cause pote...

The role of WT1 protein in hematopoiesis and leukemogenesisis incompletely elucidated. WT1 overexpression is common in acute myeloid leukemia (AML); however, WT1 mutations occur in only about 10% of cases, with increasing incidence in the setting of relapse. In this study, we investigated the clinical and molecular characteristics of WT1 mutations in NPM1-mutated AML, to enhance our understanding of the biology and potential therapeutic implications of WT1 mutations. Our study cohort included 67 patients with NPM1 mutated AML and a median follow-up of 13.7 months. WT1 mutations were identified in 7% (n = 5) of patients at the time of initial diagnosis. WT1 mutant clones were presumed to be present as co-dominant clones in 3/5 and in subclonal populations in 2/5 cases based on variant allelic frequency (VAF) when compared with NPM1 mutation VAF. All WT1 mutations became undetectable at time of MRD-negative (NPM1-wild type) remission. None of these patients experienced relapse at the time of last follow-up (median, 15 months; range, 4.5-20.2 months). A total of 15/67 (22%) patients relapsed; among these patient, four (27%) relapsed with WT1 mutant AML. Three of four patients had undergone allogeneic hematopoietic stem cell transplantation (HSCT). None of these patients had detectable WT1 mutations at the time of initial diagnosis. WT1 mutations were presumed clonal in two cases and subclonal in the other two cases, based on VAF. Our results indicate that WT1 mutations contribute to relapse in NPM1 mutated AML, especially in the setting of HSCT. These findings suggest that emerging WT1 mutations may serve as a conduit for relapse in NPM1-mutated AML, and that sequential molecular profiling to evaluate potential emergent WT1 mutations during surveillance and particularly at relapse likely has prognostic value in patients with NPM1 mutated AML.