Durham, Benjamin H.

Abstract Malignant histiocytic neoplasms (MHNs) are rare tumors derived from the mononuclear phagocyte system (MPS), encompassing histiocytic sarcoma, Langerhans cell sarcoma, interdigitating dendritic cell sarcoma, and other high-grade MPS lineage tumors. Despite advances in understanding histiocytic neoplasms, MHNs remain diagnostically challenging and lack standardized treatment algorithms. Current classification systems differ in lineage framing and fail to address mixed or ambiguous phenotypes, contributing to diagnostic uncertainty and inconsistent care. To address these gaps, the Histiocyte Society convened an international working group of pathologists and oncologists, including World Health Organization and International Consensus Classification contributors, to harmonize nomenclature, define minimum diagnostic criteria, and develop pragmatic treatment recommendations. Using a modified Delphi process and case-based review, the group formulated >40 consensus statements spanning classification, pathology, molecular testing, clinical evaluation, and therapeutic strategies. Key recommendations include the adoption of a unified MHN designation, use of a minimum immunophenotypic panel, integration of broad molecular profiling, and the documentation of previous hematopoietic malignancy. Treatment algorithms emphasize surgical resection for unifocal disease and targeted therapy or immune checkpoint inhibition for multifocal disease when actionable mutations or programmed death ligand 1 expression are present. These consensus recommendations aim to reduce diagnostic ambiguity, standardize reporting, and improve outcomes for both pediatric and adult patients. Future priorities include international registries to refine risk stratification and biomarker-driven trials exploring targeted therapy, immunotherapy, and combination approaches.

Summary Histiocytic neoplasms are diverse clonal haematopoietic disorders, and clinical disease is mediated by tumorous infiltration as well as uncontrolled systemic inflammation. Individual subtypes include Langerhans cell histiocytosis (LCH), Rosai–Dorfman–Destombes disease (RDD) and Erdheim–Chester disease (ECD), and these have been characterized with respect to clinical phenotypes, driver mutations and treatment paradigms. Less is known about patients with mixed histiocytic neoplasms (MXH), that is two or more coexisting disorders. This international collaboration examined patients with biopsy‐proven MXH with respect to component disease subtypes, oncogenic driver mutations and responses to conventional (chemotherapeutic or immunosuppressive) versus targeted (BRAF or MEK inhibitor) therapies. Twenty‐seven patients were studied with ECD/LCH (19/27), ECD/RDD (6/27), RDD/LCH (1/27) and ECD/RDD/LCH (1/27). Mutations previously undescribed in MXH were identified, including KRAS, MAP2K2, MAPK3, non‐V600‐BRAF, RAF1 and a BICD2‐BRAF fusion. A repeated‐measure generalized estimating equation demonstrated that targeted treatment was statistically significantly (1) more likely to result in a complete response (CR), partial response (PR) or stable disease (SD) (odds ratio [OR]: 17.34, 95% CI: 2.19–137.00, p = 0.007), and (2) less likely to result in progression (OR: 0.08, 95% CI: 0.03–0.23, p < 0.0001). Histiocytic neoplasms represent an entity with underappreciated clinical and molecular diversity, poor responsiveness to conventional therapy and exquisite sensitivity to targeted therapy.

Purpose: The discovery of MAPK/ERK pathway mutations has led to the approval of MEK-inhibitor (MEKi) cobimetinib in histiocytic neoplasms. However, the pivotal clinical trial that led to the approval of cobimetinib included only 4 patients with Langerhans cell histiocytosis (LCH). Therefore, there is a need to define outcomes with MEKi therapy in patients with LCH. We undertook a multi-center collaboration through the Adult LCH Working Group for the Histiocyte Society to address this gap in knowledge. Methods: Adults with LCH who received MEKi therapy were included from five institutions. Objective response rate (ORR) was evaluated using PET/CT criteria, and CT/MRI if PET scan was not available; response categories were categorized into complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD). Progression-free (PFS) and overall survival (OS) were calculated from the time of targeted therapy initiation. Risk organ disease was defined as liver, spleen, or bone marrow involvement. Results: 25 adults with LCH were included in this study; median age at MEKi initiation 43y (range, 18-85). Most common systems involved at diagnosis were bone (52%), pituitary (44%), and lung (36%). Risk organ involvement was observed in 7 (28%, 6 liver, 1 liver and spleen) and 2 (8%) patients had intra-axial CNS involvement. Median lines of prior therapy received were 2 (range: 1-5), and 4 (16%) patients received MEKi in first-line. MAPK/ERK pathway status was assessed via target capture next generation sequencing (NGS) in 22 (96%) cases. Amongst those who underwent NGS, alterations identified included: BRAF V600E (2/22, 9%), BRAF deletions (11/22 50%), MAP2K1 or MEK mutations (4/22, 18%), and MAPK1 or ERK2 (n=1, 5%). Overall, 5 patients also received BRAF inhibitors (4 patients as first therapy prior to MEKi and 1 concomitantly due to BRAF V600E positive melanoma). Specific MEKi agents utilized included cobimetinib (n=13) and trametinib (n=12). Among the response evaluable patients (n=23), ORR to MEKi treatment was 78% (9 CR, 9 PR, 4 SD, 1 PD). Among the 7 patients with liver involvement, responses were seen in 2 (29%) cases. ORR did not significantly differ according to mutation status among those with NGS data (78% MAPK mutated vs. 100% unmutated; p=1.0). In this subcohort, all of the 4 cases that did not respond to MEKi had liver involvement; 3 had BRAF deletions (2 SD and 1 PD), and one had MAPK1 mutation (SD). Median follow-up duration after MEKi initiation in the entire cohort was 18 months (95% CI 6 - 30). The median PFS and OS were not reached; 2-year PFS and OS were 78.1% and 79.9%, respectively (Figure 1A). The median PFS was much shorter in those with liver involvement (7 months [95% CI 0 - 14]) compared with those without liver disease (not reached [95% CI 19-not reached] P=0.047; Figure 1B). PFS did not differ significantly between MAPK pathway mutated and unmutated patients (p=0.34) Adverse events (AEs) were reported in 36% (n=9) and most commonly included rash (n=5) and fatigue (n=3). Four patients experienced grade 3-4 AEs including rash, fatigue, and thrombocytopenia. Common causes of MEKi discontinuation included AEs (n=3), progression (n=2), and drug holiday (n=2). At last follow-up, 5 patients died and 18 remained on MEKi. Death was attributable to LCH (n=3), concomitant malignancy (n=1), or unknown causes (n=1). Conclusions: In our multi-institutional study of adults with LCH, we found MEKi therapy to be highly efficacious. However, liver involvement correlated with a lack of response and worse PFS, while mutational status did not. Future studies are needed to evaluate optimum treatment for high-risk subgroups of LCH like liver disease.

ObjectiveErdheim‐Chester disease (ECD) is rare histiocytosis with a wide range of clinical manifestations. Somatic mutations are key to the pathogenesis of the disease; however, the relationship between germline genetic variants and ECD has not been examined so far. The present study aims to explore the inherited genetic component of ECD by performing the first genome‐wide association study.MethodsAfter quality controls, a cohort of 255 patients with ECD and 7,471 healthy donors was included in this study. Afterward, a logistic regression followed by in silico functional annotation was performed.ResultsA signal at the 18q12.3 genomic region was identified as a new susceptibility locus for ECD (P = 2.75 × 10−11; Odds Ratio = 2.09). This association was annotated to the SETBP1 gene, which is involved in clonal haematopoiesis. Functional annotation of this region and of the identified suggestive signals revealed additional genes that could be potentially involved in the pathogenesis of the disease.ConclusionOverall, this work demonstrates that germline genetic variants can impact on the development of ECD and suggests new pathways with a potential pathogenic role.image

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

Abstract Erdheim-Chester disease (ECD) is a rare histiocytosis that was recently recognized as a neoplastic disorder owing to the discovery of recurrent activating MAPK (RAS-RAF-MEK-ERK) pathway mutations. Typical findings of ECD include central diabetes insipidus, restrictive pericarditis, perinephric fibrosis, and sclerotic bone lesions. The histopathologic diagnosis of ECD is often challenging due to nonspecific inflammatory and fibrotic findings on histopathologic review of tissue specimens. Additionally, the association of ECD with unusual tissue tropism and an insidious onset often results in diagnostic errors and delays. Most patients with ECD require treatment, except for a minority of patients with minimally symptomatic single-organ disease. The first ECD consensus guidelines were published in 2014 on behalf of the physicians and researchers within the Erdheim-Chester Disease Global Alliance. With the recent molecular discoveries and the approval of the first targeted therapy (vemurafenib) for BRAF-V600–mutant ECD, there is a need for updated clinical practice guidelines to optimize the diagnosis and treatment of this disease. This document presents consensus recommendations that resulted from the International Medical Symposia on ECD in 2017 and 2019. Herein, we include the guidelines for the clinical, laboratory, histologic, and radiographic evaluation of ECD patients along with treatment recommendations based on our clinical experience and review of literature in the molecular era.

Abstract The identification of mutations in BRAFV600E and other kinases in systemic histiocytoses changed our understanding of the pathophysiology of these disorders from inflammatory non-malignant conditions to clonal disorders driven by altered MAP kinase signaling. In addition, BRAFV600E mutations have recently been noted in bone marrow (BM) CD34+ cells of patients with Langerhans Cell Histiocytosis (LCH), suggesting that LCH represents a clonal disorder arising from hematopoietic stem/progenitor cells (HSPCs). However, it is unknown if HSPCs from LCH or other histiocytoses patients can functionally give rise to these disorders. Moreover, whether or not patients with histiocytosis have additional hematological disorders related to the presence of HSPC-based mutations is unclear. Here we attempted to understand the spectrum of hematological disorders in patients with histiocytoses and the cell-of-origin of histiocytic neoplasms by functionally analyzing HSPCs from histiocytoses patients. We first analyzed a multicenter cohort of 190 adult histiocytoses patients. This unexpectedly identified a high frequency of patients with a co-existing myeloid malignancy. Of the 190 patients, 9.5% (18/190) were diagnosed with Erdheim-Chester Disease (ECD) and/or LCH concomitantly with a classic MPN (3.7%; 7/190), MPN/MDS overlap (4.7%; 9/190), or MDS (1.1%; 2/190) (Figure). This clinical observation was confirmed by detection of co-existing histiocytoses-associated kinase mutations (including BRAF, MAP2K1, NRAS, or KRAS mutations) with JAK2 or CALR mutations, as well as other mutations recurrently found in myeloid malignancies (such as IDH1/2, ASXL1, and TET2 mutations (Figure)). These data therefore reveal a heretofore-unrecognized clinical overlap of adult-onset systemic histiocytoses and WHO-classified myeloid malignancies while further highlighting the possibility of an aberrant HSPC origin for histiocytic neoplasms. In most cases, the BRAF/MAP2K1/RAS mutation predominated in the histiocytic lesion while the JAK2/CALR mutation predominated in the BM/peripheral blood suggesting likely distinct clones driving each clinical condition. Next, to test the self-renewal and differentiation capacity of HSPCs from histiocytoses patients, CD34+ cells were purified from 8 patients including 6 with ECD or LCH plus a concomitant myeloid neoplasm, and 2 with ECD alone. 0.1-0.8x106 CD34+ cells from each patient were transplanted by intrafemoral injection into sublethally irradiated NOD/SCID IL2Rγ transgenic human GM-CSF, IL-3, and SCF expressing (NSG-SGM3) mice. Human engraftment was monitored by monthly flow cytometric analysis of peripheral blood until mice became moribund. Thereafter, animals were sacrificed and tissues were analyzed using flow cytometry, histology, immunohistochemistry, and sequencing analyses to detect driver mutations identified in the histiocytosis/co-occurring myeloid neoplasm. Successful engraftment of human myeloid cells was verified in grafts generated from 2/8 patients after a mean of 90 days (range: 60-120 days). This included lethal engraftment of KRAS-mutated ECD patient cells at 90 days manifested by infiltration of human hCD45+ cells co-expressing human myeloid and monocyte lineage markers hCD33 and hCD14 in the bone marrow, spleen, liver, lung, and kidney (Figure). Immunostains revealed that tissues were infiltrated by an hCD45+hCD163+hCD68+ population of foamy histiocytes, characteristic of ECD (Figure). Furthermore, genomic analysis of DNA from the engrafted animals BM and spleen revealed the same KRASG12S mutation found in the donor patients ECD lesion. Engraftment of a patient diagnosed with concomitant BRAFV600E-mutant ECD and ASXL1/TET2-mutant CMML was seen at 60 days. In this case, however, only engraftment of human CMML was detected without evidence of the BRAFV600E-mutant ECD component. This study suggests that adult histiocytic neoplasms frequently co-occur with other myeloid neoplasms. Moreover, histiocytosis disease-driver mutations are present in the CD34+ HSPCs, which we show for the first time can initiate disease that resembles human histiocytosis in a patient-derived xenograft. Importantly, however, even though the CD34+ compartment gives rise to both the histiocytic and myeloid neoplasm disease component, each disease is likely initiated by distinct precursors. Figure. Figure. Disclosures No relevant conflicts of interest to declare.

Abstract Histiocytic neoplasms are clonal, hematopoietic disorders characterized by an accumulation of abnormal monocyte-derived dendritic cells or macrophages in Langerhans Cell (LCH) and non-Langerhans (non-LCH) histiocytoses, respectively. The discovery of the BRAF V600E mutation in ~50% of patients with LCH and the non-LCH Erdheim-Chester Disease (ECD) provided the first molecular target in these patients and novel insights into the pathogenesis of these disorders. However, recurrent mutations in the majority of the ~50% of BRAF V600E-wild type patients with non-LCH are unknown. Moreover, recurrent mutations outside of the MAP kinase pathway are undefined throughout histiocytic neoplasms. To address these issues, we performed whole exome sequencing (WES) of frozen biopsies from 24 patients with LCH (n=10) or ECD (n=14) paired with peripheral blood mononuclear cells. 13/24 patients also underwent RNA sequencing (RNA-seq). All mutations in activating kinases were validated by droplet-digital PCR, while targeted-capture next-generation sequencing validated all others. Both adult (n=18; n=2 with LCH) and pediatric cases (n=9; n=8 with LCH) were included. Using combined WES/RNA-seq, activating kinase alterations were identified in 100% of patients. In LCH, 60% and 40% had BRAF V600E and MAP2K1 mutations, respectively. In non-LCH 51%, 14%, 14%, and 7% were BRAFV600E, ARAF, MAP2K1, and NRAS mutant (Fig1A). Overall, a mean of 7 non-synonymous mutations per adult patient was identified (range 1-22) compared with 5 mutations per pediatric patient (range 4-9; p =ns). Mutations affecting diverse cellular processes were found to co-exist with kinase mutations including mutations in epigenetic modifiers and the p38/MAPK pathway. In addition to kinase point mutations, RNA-seq identified recurrent, in-frame kinase fusions-a first for these disorders. All identified fusions were validated using FISH and RT-PCR. This includes novel fusions in BRAF (RNF11-BRAF and CLIP2-BRAF), as well as therapeutically important fusions in ALK (2 separate KIF5B-ALK fusions) and NTRK1 (LMNA-NTRK1;Fig1B). Expression of each fusion in Ba/F3 cells conferred cytokine-independent growth. Importantly, the BRAF fusions were found to be sensitive to MEK inhibition but resistant to vemurafenib while the ALK fusions conferred sensitivity to the ALK inhibitors crizotinib or alectinib. We next interrogated a validation cohort of 37 BRAF V600E-wild type, non-LCH, formalin-fixed, paraffin-embedded tissue samples using targeted mutational profiling for MAP2K1, ARAF, NRAS, KRAS, and PIK3CA. This revealed activating mutations in MAP2K1 (32%; n=12), NRAS (16%; n=6), KRAS (11%; n=4), PIK3CA (8%; n=3), and ARAF (3%; n=1). Three of the investigated non-LCH patients with refractory disease and progressive organ dysfunction were treated with targeted therapies based on the discovery of novel kinase alterations described above. Treatment of 2 refractory MAP2K1- mutant, non-LCH patients with MEK inhibitors (trametinib or cobimetinib) resulted in dramatic clinical improvement (Fig1C). Both patients have been maintained on MEK inhibitor single-agent therapy with a sustained clinical response for >100 days. Further evidence of effective targeted inhibition was found in a refractory ECD patient carrying an ARAF S214A mutation. This patient failed to respond to 3 lines of prior therapies and suffered near blindness due to disease infiltration in the retina and optic nerves. Given a recent report of complete response to sorafenib in a lung cancer patient with an ARAF S214C mutation, we initiated sorafenib. Within 12 weeks, there was improvement in the patientÕs eyesight and decreased infiltrative disease, coinciding with >50% decrease in mutant ARAF DNA in plasma cell-free DNA. Whole exome and transcriptome sequencing identified activating kinase mutations or translocations in all patients with the common downstream effect of activating the MAPK pathway. The preliminary, dramatic, clinical efficacy observed with use of MEK and RAF inhibitors in MAP2K1 - and ARAF-mutated, non-LCH patients further supports the central role of targeting the MAPK pathway in these tumors. The discovery of the discussed mutations and fusions in diverse kinases provides critical new insights into the genetic events central to a spectrum of adult and pediatric histiocytic neoplasms. Figure 1. Figure 1. Disclosures Off Label Use: This abstract describes use of MEK inhibitors (both tremetinib and cobimetinib) as well as sorafenib for MEK1 and ARAF mutant histiocytosis. . Stephens:Foundation Medicine, Inc.: Employment, Equity Ownership. Miller:Foundation Medicine, Inc.: Employment, Equity Ownership. Ross:Foundation Medicine Inc.: Employment. Ali:Foundation Medicine Inc.: Employment. Hyman:Chugai Pharma: Consultancy; Biotherapeutics: Consultancy; Atara: Consultancy, Honoraria.

Key Points KMT2C mutations occur in 15% and 25% of patients with cHCL and vHCL, respectively, along with CCND3 and U2AF1 mutations each in 13% of vHCLs. NF1, NF2, N/KRAS, and IRS1 alterations contribute to clinical resistance to vemurafenib treatment in patients with cHCL.

Abstract Histiocytic neoplasms are clonal, hematopoietic disorders characterized by an accumulation of abnormal, monocyte-derived dendritic cells or macrophages in Langerhans cell histiocytosis (LCH) and non-Langerhans cell histiocytosis (non-LCH), respectively. The discovery of BRAFV600E mutations in approximately 50% of these patients provided the first molecular therapeutic target in histiocytosis. However, recurrent driving mutations in the majority of patients with BRAFV600E–wild-type non-LCH are unknown, and recurrent cooperating mutations in non-MAP kinase pathways are undefined for the histiocytic neoplasms. Through combined whole-exome and transcriptome sequencing, we identified recurrent kinase fusions involving BRAF, ALK, and NTRK1, as well as recurrent, activating MAP2K1 and ARAF mutations in patients with BRAFV600E–wild-type non-LCH. In addition to MAP kinase pathway lesions, recurrently altered genes involving diverse cellular pathways were identified. Treatment of patients with MAP2K1- and ARAF-mutated non-LCH using MEK and RAF inhibitors, respectively, resulted in clinical efficacy, demonstrating the importance of detecting and targeting diverse kinase alterations in these disorders. Significance: We provide the first description of kinase fusions in systemic histiocytic neoplasms and activating ARAF and MAP2K1 mutations in non-Langerhans histiocytic neoplasms. Refractory patients with MAP2K1- and ARAF-mutant histiocytoses had clinical responses to MEK inhibition and sorafenib, respectively, highlighting the importance of comprehensive genomic analysis of these disorders. Cancer Discov; 6(2); 154–65. ©2015 AACR. This article is highlighted in the In This Issue feature, p. 109