Exosomes in Chronic Myeloid Leukemia: Are We Reading a New Reliable Message?

Extracellular vesicles in the Chronic Myeloid Leukemia scenario: an update about the shuttling of disease markers and therapeutic molecules

Extracellular vesicles (EVs) are various sets of cell-derived membranous structures containing lipids, nucleic acids, and proteins secreted by both eukaryotic and prokaryotic cells. It is now well recognized that EVs are key intercellular communication mediators, allowing the functional transfer of bioactive chemicals from one cell to another in both healthy and pathological pathways. It is evident that the condition of the producer cells heavily influences the composition of EVs. Hence, phenotypic changes in the parent cells are mirrored in the design of the secreted EVs. As a result, EVs have been investigated for a wide range of medicinal and diagnostic uses in different hematological diseases. EVs have only recently been studied in the context of Chronic Myeloid Leukemia (CML), a blood malignancy defined by the chromosomal rearrangement t(9;22) and the fusion gene BCR-ABL1. The findings range from the impact on pathogenesis to the possible use of EVs as medicinal chemical carriers. This review aims to provide for the first time an update on our understanding of EVs as carriers of CML biomarkers for minimal residual disease monitoring, therapy response, and its management, as well as the limited reports on the use of EVs as therapeutic shuttles for innovative treatment approaches.

Twenty years of evolution of CML therapy: how the treatment goal is moving from disease to patient

The introduction of imatinib in 2000 opened the era of tyrosine kinase inhibitors (TKIs) for CML therapy and has revolutionized the life expectancy of CML patients, which is now quite like the one of the healthy aged population. Over the last 20 years, both the TKI therapy itself and the objectives have undergone evolutions highlighted and discussed in this review. The main objective of the CML therapy in the first 10 years after TKI introduction was to abolish the disease progression from the chronic to the blastic phase and guarantee the long-term survival of the great majority of patients. In the second 10 years (from 2010 to the present), the main objective of CML therapy moved from survival, considered achieved as a goal, to treatment-free remission (TFR). Two phenomena emerged: no more than 50-60% of CML patients could be candidates for discontinuation and over 50% of them molecularly relapse. The increased cumulative incidence of specific TKI off-target side effects was such relevant to compel to discontinue or reduce the TKI administration in a significant proportion of patients and to avoid a specific TKI in particular settings of patients. Therefore, the treatment strategy must be adapted to each category of patients. What about the patients who do not get or fail the TFR? Should they be compelled to continue the TKIs at the maximum tolerated dose? Alternative strategies based on the principle of minimal effective dose have been tested with success and they are now re-evaluated with more attention, since they guarantee survival and probably a better quality of life, too. Moving from treating the disease to treating the patient is an important change of paradigm. We can say that we are entering a personalized CML therapy, which considers the patients' age, their comorbidities, tolerability, and specific objectives. In this scenario, the new techniques supporting the monitoring of the patients, such as the digital PCR, must be considered. In the present review, we present in deep this evolution and comment on the future perspectives of CML therapy.

Liquid biopsies and minimal residual disease in lymphoid malignancies

Minimal residual disease (MRD) assessment using peripheral blood instead of bone marrow aspirate/biopsy specimen or the biopsy of the cancerous infiltrated by lymphoid malignancies is an emerging technique with enormous interest of research and technological innovation at the current time. In some lymphoid malignancies (particularly ALL), Studies have shown that MRD monitoring of the peripheral blood may be an adequate alternative to frequent BM aspirations. However, additional studies investigating the biology of liquid biopsies in ALL and its potential as an MRD marker in larger patient cohorts in treatment protocols are warranted. Despite the promising data, there are still limitations in liquid biopsies in lymphoid malignancies, such as standardization of the sample collection and processing, determination of timing and duration for liquid biopsy analysis, and definition of the biological characteristics and specificity of the techniques evaluated such as flow cytometry, molecular techniques, and next generation sequencies. The use of liquid biopsy for detection of minimal residual disease in T-cell lymphoma is still experimental but it has made significant progress in multiple myeloma for example. Recent attempt to use artificial intelligence may help simplify the algorithm for testing and may help avoid inter-observer variation and operator dependency in these highly technically demanding testing process.

Digital PCR as a New Method for Minimal Residual Disease Monitoring and Treatment Free Remission Management in Chronic Myeloid Leukemia Patients: Is It Reliable?

The effective and sensitive monitoring of Minimal Residual Disease or Measurable Residual Disease (MRD) is a very important aspect in the management of patients affected by hematologic malignancies. The recent availability of new technologies has opened to the improvement of MRD monitoring. It is particularly relevant in patients affected by Chronic Myeloid Leukemia (CML). MRD monitoring is key in the management of CML patients thanks to the efficacy of TKIs therapy. Moreover, the policies of TKIs discontinuation aimed at treatment free remission are strongly based on the good selection of patients eligible for stopping TKIs therapy. The recently described application of digital PCR in CML patients monitoring seems to improve the accuracy and precision in the identification of optimal responders. The present review reports an overview on the application of digital PCR in the monitoring of MRD in CML and its impact on TKIs discontinuation trials and, consequently, on TFR success.

Feasibility of Leukemia-Derived Exosome Enrichment and Co-isolated dsDNA Sequencing in Acute Myeloid Leukemia Patients: A Proof of Concept for New Leukemia Biomarkers Detection

Simple Summary

The present pilot study aimed at investigating the feasibility of a leukemia-derived exosome enrichment approach followed by exosomal dsDNA target re-sequencing for adult Acute Myeloid Leukemias (AML) marker detection. To our knowledge, this is the first time that a proof-of-concept combining a leukemia-derived exosome enrichment strategy based on a commercial CE-IVD kit and next-generation sequencing was applied in a cohort of adult AML patients. The reported approach is easy, quick and user friendly and gives the possibility of obtaining a good quantity of exosomal dsDNA (composed of exosomal cargo and surrounding DNA) suitable for further analysis. The time-effective procedure opens up future effective clinical applications. This pilot study presents the potential of a proof-of-concept based on exosome analysis to be applied in clinical practice, as well as the feasibility of this kind of investigations using a certified kit, avoiding many additional analyses. It may encourage further studies regarding extracellular vesicles in myeloid neoplasia.

Abstract

Exosomes are extracellular vesicles playing a pivotal role in the intercellular communication. They shuttle different cargoes, including nucleic acids from their cell of origin. For this reason, they have been studied as carriers of tumor markers in different liquid biopsy approaches, in particular for solid tumors. Few data are available concerning exosomes as markers of myeloid neoplasia. To better understand their real potential and the best approach to investigate leukemic exosomes, we present the results of a pilot feasibility study evaluating the application of next-generation sequencing analysis of dsDNA derived from exosomes isolated in 14 adult patients affected by acute myeloid leukemias. In particular, leukemia-derived exosome fractions have been analyzed. The concentration of dsDNA co-extracted with exosomes and the number and types of mutations detected were considered and compared with ones identified in the Bone Marrow (BM) and Peripheral Blood (PB) cells. Exosomal DNA concentration, both considering the cargo and the DNA surrounding the lipid membrane resulted in a linear correlation with leukemic burden. Moreover, exosomal DNA mutation status presented 86.5% of homology with BM and 75% with PB. The results of this pilot study confirmed the feasibility of a leukemia-derived exosome enrichment approach followed by exosomal dsDNA NGS analysis for AML biomarker detection. These data point to the use of liquid biopsy in myeloid neoplasia for the detection of active leukemic cells resident in the BM via a painless procedure.

Exosomes and Extracellular Vesicles in Myeloid Neoplasia: The Multiple and Complex Roles Played by These "Magic Bullets"

Simple Summary

Extracellular vesicles (EVs) are released by the majority of cell types and can be isolated from both cell cultures and body fluids. They are involved in cell-to-cell communication and may shuttle different messages (RNA, DNA, and proteins). These messages are known to influence the microenvironment of cells and their behavior. In recent years, some evidence about the involvement of EVs and exosomes, an EV subgroup, in immunomodulation, the transfer of disease markers, and the treatment of myeloid malignancies have been reported. Little is known about these vesicles in this particular setting of hematologic neoplasia; here, we summarize and critically review the available results, aiming to encourage further investigations.

Abstract

Extracellular vesicles (exosomes, in particular) are essential in multicellular organisms because they mediate cell-to-cell communication via the transfer of secreted molecules. They are able to shuttle different cargo, from nucleic acids to proteins. The role of exosomes has been widely investigated in solid tumors, which gave us surprising results about their potential involvement in pathogenesis and created an opening for liquid biopsies. Less is known about exosomes in oncohematology, particularly concerning the malignancies deriving from myeloid lineage. In this review, we aim to present an overview of immunomodulation and the microenvironment alteration mediated by exosomes released by malicious myeloid cells. Afterwards, we review the studies reporting the use of exosomes as disease biomarkers and their influence in response to treatment, together with the recent experiences that have focused on the use of exosomes as therapeutic tools. The further development of new technologies and the increased knowledge of biological (exosomes) and clinical (myeloid neoplasia) aspects are expected to change the future approaches to these malignancies.

Molecular Testing in CML between Old and New Methods: Are We at a Turning Point?

Molecular monitoring of minimal residual disease (MRD) and BCR-ABL1 kinase domain (KD) mutation testing have a well consolidated role in the routine management of chronic myeloid leukemia (CML) patients, as they provide precious information for therapeutic decision-making. Molecular response levels are used to define whether a patient has an "optimal", "warning", or "failure" response to tyrosine kinase inhibitor (TKI) therapy. Mutation status may be useful to decide whether TKI therapy should be changed and which alternative TKI (or TKIs) are most likely to be effective. Real-time quantitative polymerase chain reaction (RQ-qPCR) and Sanger sequencing are currently the gold standard for molecular response monitoring and mutation testing, respectively. However, in recent years, novel technologies such as digital PCR (dPCR) and next-generation sequencing (NGS) have been evaluated. Here, we critically describe the main features of these old and novel technologies, provide an overview of the recently published studies assessing the potential clinical value of dPCR and NGS, and discuss how the state of the art might evolve in the next years.

RT-qPCR versus Digital PCR: How Do They Impact Differently on Clinical Management of Chronic Myeloid Leukemia Patients?

Real-time quantitative PCR (RT-qPCR) is the gold standard to quantify the BCR-ABL1 transcript for molecular response monitoring in chronic myeloid leukemia (CML) patients, and it plays a pivotal role in clinical decision-making process, even if it presents technical limits. Increasing data suggest that digital PCR (dPCR) is more accurate and reliable than RT-qPCR in CML minimal residual disease monitoring and in patients' selection for treatment discontinuation. But what about the identification of treatment discontinuation failures? We present the case of a CML patient enrolled both in a study aiming to comparatively assess molecular response by RT-qPCR and dPCR and in the progressive arm of the OPTkIMA trial. This is a phase III trial including CML patients randomized to receive a fixed versus a progressive intermittent tyrosine kinase inhibitor regimen. At 24 months, because of two consecutive detections of MR^2.0 by RT-qPCR, the patient resumed daily treatment. Conversely, dPCR revealed a stability of molecular response and even a slight decreasing of transcript over time. An additional specimen was sampled one month after the first MR^2.0 detection because of clinical decision: RT-qPCR resulted MR^3.0 and dPCR confirmed the transcript's stability. Nowadays, the resumption of therapy is RT-qPCR-driven despite its limits in detection and robustness. In this case, according to dPCR, the patient could have continued intermittent treatment and the stability of response was then confirmed by RT-qPCR. So, dPCR could be able to better identify peculiar clinical response to therapy.

Isolation of extracellular vesicles improves the detection of mutant DNA from plasma of metastatic melanoma patients

Detection of BRAFV600E within cell free tumor DNA (ctDNA) is emerging as a promising means to improve patients' stratification or enable BRAF inhibitor (BRAFi) therapeutic monitoring in a minimally invasive manner. Here, we investigated whether extracellular vesicle-(EV)-associated-DNA (EV-DNA) has value as an alternative source of circulating BRAFV600E. To do so, we identified a clinical practice-compatible protocol for the isolation of EV-DNA and assessed BRAF gene status on plasma samples from metastatic melanoma patients at the beginning and during BRAFi therapy. This protocol uses a peptide with high affinity for EVs and it has been found to recover more mutant DNA from plasma than standard ultracentrifugation. Molecular analyses revealed that mutant DNA is largely unprotected from nuclease digestion, interacting with the outer side of the EV membrane or directly with the peptide. When used on clinical samples, we found that the protocol improves the detection of BRAFV600E gene copies in comparison to the reference protocol for ctDNA isolation. Taken together, these findings indicate that EVs are a promising source of mutant DNA and should be considered for the development of next-generation liquid biopsy approaches.

Extracellular Vesicles: From Biomarkers to Therapeutic Tools

Intercellular communication is an essential hallmark of multicellular organisms and can be mediated through direct cell–cell contact or transfer of secreted molecules. In the last two decades, a third mechanism for intercellular communication has emerged that involves intercellular transfer of extracellular vesicles (EVs). EVs are membranous vesicles of 30–5000 nm in size. Based on their dimension and biogenesis, EVs can be divided into different categories, such as microvesicles, apoptotic bodies, ectosomes, and exosomes. It has already been demonstrated that protein changes, expressed on the surfaces or in the content of these vesicles, may reflect the status of producing cells. For this reason, EVs, and exosomes in particular, are considered ideal biomarkers in several types of disease—from cancer diagnosis to heart rejection. This aspect opens different opportunities in EVs clinical application, considering the importance given to liquid biopsy in the recent years. Furthermore, extracellular vesicles can be natural or engineered carriers of cytoprotective or cytotoxic factors and applied, as a therapeutic tool, from regenerative medicine to target cancer therapy. This is of pivotal importance in the so called “era of the 4P medicine”. This Editorial focuses on recent findings pertaining to EVs in different medical areas, from biomarkers to therapeutic applications.