Advances in chronic myelomonocytic leukemia and future prospects: Lessons learned from precision genomics

ELN iMDS flow working group validation of the monocyte assay for chronic myelomonocytic leukemia diagnosis by flow cytometry

BACKGROUND

It was proposed that peripheral blood (PB) monocyte profiles evaluated by flow cytometry, called "monocyte assay," could rapidly and efficiently distinguish chronic myelomonocytic leukemia (CMML) from other causes of monocytosis by highlighting an increase in the classical monocyte (cMo) fraction above 94%. However, the robustness of this assay requires a large multicenter validation and the assessment of its feasibility on bone marrow (BM) samples, as some centers may not have access to PB.

METHODS

PB and/or BM samples from patients displaying monocytosis were assessed with the "monocyte assay" by 10 ELN iMDS Flow working group centers with harmonized protocols. The corresponding files were reanalyzed in a blind fashion and the cMo percentages obtained by both analyses were compared. Confirmed diagnoses were collected when available.

RESULTS

The comparison between cMo percentages from 267 PB files showed a good global significant correlation (r = 0.88) with no bias. Confirmed diagnoses, available for 212 patients, achieved a 94% sensitivity and an 84% specificity. Hence, 95/101 CMML patients displayed cMo ≥94% while cMo <94% was observed in 83/99 patients with reactive monocytosis and in 10/12 patients with myeloproliferative neoplasms (MPN) with monocytosis. The established Receiver Operator Curve again provided a 94% cut-off value of cMo. The 117 BM files reanalysis led to an 87% sensitivity and an 80% specificity, with excellent correlation between the 43 paired samples to PB.

CONCLUSIONS

This ELN multicenter study demonstrates the robustness of the monocyte assay with only limited variability of cMo percentages, validates the 94% cutoff value, confirms its high sensitivity and specificity in PB and finally, also confirms the possibility of its use in BM samples.

Plasmacytoid Dendritic Cells, a Novel Target in Myeloid Neoplasms

Plasmacytoid dendritic cells (pDC) are the main type I interferon producing cells in humans and are able to modulate innate and adaptive immune responses. Tumor infiltration by plasmacytoid dendritic cells is already well described and is associated with poor outcomes in cancers due to the tolerogenic activity of pDC. In hematological diseases, Blastic Plasmacytoid Dendritic Cells Neoplasm (BPDCN), aggressive leukemia derived from pDCs, is well described, but little is known about tumor infiltration by mature pDC described in Myeloid Neoplasms (MN). Recently, mature pDC proliferation (MPDCP) has been described as a differential diagnosis of BPDCN associated with acute myeloid leukemia (pDC-AML), myelodysplastic syndrome (pDC-MDS) and chronic myelomonocytic leukemia (pDC-CMML). Tumor cells are myeloid blasts and/or mature myeloid cells from related myeloid disorders and pDC derived from a clonal proliferation. The poor prognosis associated with MPDCP requires a better understanding of pDC biology, MN oncogenesis and immune response. This review provides a comprehensive overview about the biological aspects of pDCs, the description of pDC proliferation in MN, and an insight into putative therapies in pDC-AML regarding personalized medicine.

Role of the bone marrow immune microenvironment in chronic myelomonocytic leukemia pathogenesis: novel mechanisms and insights into clonal propagation

Recent studies in chronic myelomonocytic leukemia (CMML) involving clonal dendritic cell (DC) aggregates and association with systemic immune dysregulation have highlighted novel and potentially targetable pathways of disease progression. CMML DC aggregates are populated by heterogeneous cell types such as CD123⁺ plasmacytoid dendritic cells (pDCs), CD11c + myeloid-derived DCs (mDCs), myeloid-derived suppressor cells (MDSCs), monocytes, and associate with an immune checkpoint called indoleamine 2,3-dioxygenase (IDO). Systemically, these IDO + DC aggregates are associated with immune tolerance marked by regulatory T cell expansion, likely mediated by aberrant DC-T cell interactions occurring within the bone marrow (BM) microenvironment. Somatic mutational events in CMML such as ASXL1 and NRAS mutations cooperate to induce T cell exhaustion and contribute toward disease progression to acute myeloid leukemia (AML). In this review, we explore the role of aging-induced alterations in the BM immune microenvironment, aberrant innate immune and proinflammatory signaling, and the adaptive immune system in CMML.

Mouse Models of CMML

Chronic myelomonocytic leukemia (CMML) is a rare and challenging type of myeloproliferative neoplasm. Poor prognosis and high mortality, associated predominantly with progression to secondary acute myeloid leukemia (sAML), is still an unsolved problem. Despite a growing body of knowledge about the molecular repertoire of this disease, at present, the prognostic significance of CMML-associated mutations is controversial. The absence of available CMML cell lines and the small number of patients with CMML make pre-clinical testing and clinical trials complicated. Currently, specific therapy for CMML has not been approved; most of the currently available therapeutic approaches are based on myelodysplastic syndrome (MDS) and other myeloproliferative neoplasm (MNP) studies. In this regard, the development of the robust CMML animal models is currently the focus of interest. This review describes important studies concerning animal models of CMML, examples of methodological approaches, and the obtained hematologic phenotypes.

Mutational landscape of chronic myelomonocytic leukemia and its potential clinical significance

We evaluated the mutational landscape of chronic myelomonocytic leukemia (CMML) and its potential clinical significance. We analyzed 47 samples with a panel of 112 genes using next-generation sequencing. Forty-five of the 47 patients (95.74%) had at least one mutation identified, with an average of 3.7 (range 0-9) per patient. The most common mutation was NRAS, followed by ASXL1, TET2, SRSF2, RUNX1, KRAS, and SETBP1. Patients 60 years and older more frequently had mutations in TET2 (56% vs. 9.09%, P = 0.001) and ASXL1 (48% vs. 18.18%, P = 0.031) than patients younger than 60 years. Median overall survival (OS) in patients with CMML was 22.0 months (95% CI 19.7-24.3 months). ASXL1 (18 vs. 22 months, P = 0.012), RUNX1 (17 vs. 22 months, P = 0.001), and SETBP1 (20 vs. 27 months, P = 0.032) mutations predicted inferior OS. However, only RUNX1 mutation was significantly associated with inferior acute myeloid leukemia (AML)-free survival. Our data showed that mutation profile differed significantly between CMML patients aged 60 years and older versus those younger than 60 years, and some of these mutations impact the progression and prognosis of the disease to a certain extent.

Differential Diagnosis and Workup of Monocytosis: A Systematic Approach to a Common Hematologic Finding

Purpose of Review

Monocytosis is a frequently encountered clinical condition that needs appropriate investigation due to a broad range of differential diagnoses. This review is meant to summarize the latest literature in the diagnostic testing and interpretation and offer a stepwise diagnostic approach for a patient presenting with monocytosis.

Recent Findings

Basic studies have highlighted the phenotypic and functional heterogeneity in the monocyte compartment. Studies, both translational and clinical, have provided insights into why monocytosis occurs and how to distinguish the different etiologies. Flow cytometry studies have illustrated that monocyte repartitioning can distinguish chronic myelomonocytic leukemia, a prototypical neoplasm with monocytosis from other reactive or neoplastic causes.

Summary

In summary, we provide an algorithmic approach to the diagnosis of a patient presenting with monocytosis and expect this document to serve as a reference guide for clinicians.

How I investigate chronic myelomonocytic leukemia

The 2016 revised 4th edition of the World Health Organization classification of hematopoietic neoplasms updated the diagnostic criteria for chronic myelomonocytic leukemia (CMML). Persistent peripheral blood monocytosis of at least 1 × 10⁹ /L and a percentage of monocytes ≥10% of the circulating white blood cell count (WBC) are both prerequisite criteria for this diagnosis. CMML represents the prototype of "overlapping" myeloid neoplasms with concurrent myeloproliferative and myelodysplastic features. However, clinical presentation is heterogeneous, with cases showing prevailing "dysplastic" features and others a predominant "proliferative" phenotype. Accounting for this diversity, two variants of CMML are recognized: "dysplastic" CMML defined by WBC < 13 × 10⁹ /L and "proliferative" CMML with WBC ≥ 13 × 10⁹ /L often showing features mimicking a myeloproliferative neoplasm. Although not an official WHO category, the "oligomonocytic" variant of CMML is defined by relative monocytosis with an absolute monocyte count of 0.5-0.9 × 10⁹ /L. It can be considered a "pre-phase," as it frequently anticipates the development of an overt, classic CMML. In an attempt at improving disease prognostication, the blast count based grading system for CMML of the WHO 2008 Classification has been expanded in 2016 to include a new "CMML-0" category. Lastly, the large body of knowledge on the molecular events occurring in CMML has been used to assist diagnosis and assess prognosis. Despite the step forwards, diagnosis of CMML still remains one of exclusion as no clinical, pathologic or molecular findings are specific for this disease. The current review brings insight into the spectrum of CMML and provides practical advice to approach suspected cases of CMML.

Incorporating flow cytometry and next-generation sequencing in the diagnosis of CMML. Are we ready for prime?

In the last version of the WHO classification of myeloid malignancies, flow cytometry and molecular investigation are listed as potentially useful, yet non-essential diagnostic tools in hard-to-recognize chronic myelomonocytic leukemias (CMML). Flow recognition of CMML was initially based on an increase in the fraction of peripheral blood, CD14⁺,CD16^- classical monocytes ≥94% of total monocytes. An associated inflammatory disease can preclude the detection of classical monocyte fraction increase by inducing accumulation of CD14⁺,CD16⁺ intermediate monocytes. In such a situation, decrease in the Slan⁺,CD14^low,CD16⁺ non-classical monocyte fraction below 1.7% still supports CMML diagnosis. This robust, two-step flow cytometry assay identifies CMML with a very high sensitivity. Otherwise, detection of one or several acquired gene mutations with high variant allele frequency supports the diagnosis of CMML, oligomonocytic CMML or clonal monocytosis of clinical significance. Together, recent investigations support integration of flow cytometry analysis of peripheral blood monocyte subsets and new generation sequencing of a panel of 20-30 recurrently mutated genes in the diagnostic work-up of CMML.