Gerrit Jan Schuurhuis

High Stem Cell Frequency in Acute Myeloid Leukemia at Diagnosis Predicts High Minimal Residual Disease and Poor Survival

Purpose: In CD34-positive acute myeloid leukemia (AML), the leukemia-initiating event originates from the CD34+CD38− stem cell compartment. Survival of these cells after chemotherapy may lead to minimal residual disease (MRD) and subsequently to relapse. Therefore, the prognostic impact of stem cell frequency in CD34-positive AML was investigated. Experimental Design: First, the leukemogenic potential of unpurified CD34+CD38− cells, present among other cells, was investigated in vivo using nonobese diabetic/severe combined immunodeficient mice transplantation experiments. Second, we analyzed whether the CD34+CD38− compartment at diagnosis correlates with MRD frequency after chemotherapy and clinical outcome in 92 AML patients. Results:In vivo data showed that engraftment of AML blasts in nonobese diabetic/severe combined immunodeficient mice directly correlated with stem cell frequency of the graft. In patients, a high percentage of CD34+CD38− stem cells at diagnosis significantly correlated with a high MRD frequency, especially after the third course of chemotherapy. Also, it directly correlated with poor survival. In contrast, total CD34+ percentage showed no such correlations. Conclusions: Both in vivo data, as well as the correlation studies, show that AML stem cell frequency at diagnosis offers a new prognostic factor. From our data, it is tempting to hypothesize that a large CD34+CD38− population at diagnosis reflects a higher percentage of chemotherapy-resistant cells that will lead to the outgrowth of MRD, thereby affecting clinical outcome. Ultimately, future therapies should be directed toward malignant stem cells.

High Prognostic Impact of Flow Cytometric Minimal Residual Disease Detection in Acute Myeloid Leukemia: Data From the HOVON/SAKK AML 42A Study

PURPOSE Half the patients with acute myeloid leukemia (AML) who achieve complete remission (CR), ultimately relapse. Residual treatment-surviving leukemia is considered responsible for the outgrowth of AML. In many retrospective studies, detection of minimal residual disease (MRD) has been shown to enable identification of these poor-outcome patients by showing its independent prognostic impact. Most studies focus on molecular markers or analyze data in retrospect. This study establishes the value of immunophenotypically assessed MRD in the context of a multicenter clinical trial in adult AML with sample collection and analysis performed in a few specialized centers. PATIENTS AND METHODS In adults (younger than age 60 years) with AML enrolled onto the Dutch-Belgian Hemato-Oncology Cooperative Group/Swiss Group for Clinical Cancer Research Acute Myeloid Leukemia 42A study, MRD was evaluated in bone marrow samples in CR (164 after induction cycle 1, 183 after cycle 2, 124 after consolidation therapy). RESULTS After all courses of therapy, low MRD values distinguished patients with relatively favorable outcome from those with high relapse rate and adverse relapse-free and overall survival. In the whole patient group and in the subgroup with intermediate-risk cytogenetics, MRD was an independent prognostic factor. Multivariate analysis after cycle 2, when decisions about consolidation treatment have to be made, confirmed that high MRD values (> 0.1% of WBC) were associated with a higher risk of relapse after adjustment for consolidation treatment time-dependent covariate risk score and early or later CR. CONCLUSION In future treatment studies, risk stratification should be based not only on risk estimation assessed at diagnosis but also on MRD as a therapy-dependent prognostic factor.

Standardization of flow cytometry in myelodysplastic syndromes: report from the first European LeukemiaNet working conference on flow cytometry in myelodysplastic syndromes

This article decribes the results of the first European LeukemiaNet working conference on flow cytometry immunophenotyping in myelodysplastic syndrome. This report is a very comprehensive analysis of the topic, and provides detailed information on what is currently known in the field. See related perspective article on page 1041. The myelodysplastic syndromes are a group of clonal hematopoietic stem cell diseases characterized by cytopenia(s), dysplasia in one or more cell lineages and increased risk of evolution to acute myeloid leukemia (AML). Recent advances in immunophenotyping of hematopoietic progenitor and maturing cells in dysplastic bone marrow point to a useful role for multiparameter flow cytometry (FCM) in the diagnosis and prognostication of myelodysplastic syndromes. In March 2008, representatives from 18 European institutes participated in a European LeukemiaNet (ELN) workshop held in Amsterdam as a first step towards standardization of FCM in myelodysplastic syndromes. Consensus was reached regarding standard methods for cell sampling, handling and processing. The group also defined minimal combinations of antibodies to analyze aberrant immunophenotypes and thus dysplasia. Examples are altered numbers of CD34+ precursors, aberrant expression of markers on myeloblasts, maturing myeloid cells, monocytes or erythroid precursors and the expression of lineage infidelity markers. When applied in practice, aberrant FCM patterns correlate well with morphology, the subclassification of myelodysplastic syndromes, and prognostic scoring systems. However, the group also concluded that despite strong evidence for an impact of FCM in myelodysplastic syndromes, further (prospective) validation of markers and immunophenotypic patterns are required against control patient groups as well as further standardization in multi-center studies. Standardization of FCM in myelodysplastic syndromes may thus contribute to improved diagnosis and prognostication of myelodysplastic syndromes in the future.

Stability and prognostic influence of FLT3 mutations in paired initial and relapsed AML samples

In acute myeloid leukemia (AML), activating mutations in the fms-like tyrosine kinase 3 (FLT3) gene predict poor prognosis. We determined FLT3 internal tandem duplications (FLT3/ITD) and D835 point mutations in paired initial and relapse samples from 80 pediatric and adult AML patients. One D835 point mutation was found in an initial pediatric AML sample. Fms-like tyrosine kinase 3/ITDs were present in 21 initial and 22 relapse samples (26.3 and 27.5%, respectively). Interestingly, FLT3/ITD positivity was related to a significantly shorter time to relapse, most pronounced when the ITD-positive status was found at relapse (P<0.001). However, FLT3/ITD status changed between diagnosis and relapse in 14 cases. In four patients, the FLT3/ITD became undetectable at relapse in five patients FLT3/ITDs were only detected at relapse, and in five patients the length or number of FLT3/ITDs changed. Gain of FLT3/ITDs may suggest oligoclonality with selective outgrowth of the FLT3/ITD-positive clone, whereas losses may reflect ITDs in the more mature leukemic cells rather than in the leukemic stem cell, or, alternatively, that other genetic aberrations provided a greater selective advantage. Studying FLT3/ITD kinetics in minimal residual disease setting may provide some answers for the changes we observed. Fms-like tyrosine kinase 3/ITD is a relevant marker for prognosis, and remains an important target for therapeutic inhibition.

MRD parameters using immunophenotypic detection methods are highly reliable in predicting survival in acute myeloid leukaemia

Outgrowth of minimal residual disease (MRD) in acute myeloid leukaemia (AML) is responsible for the occurrence of relapses. MRD can be quantified by immunophenotyping on a flow cytometer using the expression of leukaemia-associated phenotypes. MRD was monitored in follow-up samples taken from bone marrow (BM) of 72 patients after three different cycles of chemotherapy and from autologous peripheral blood stem cell (PBSC) products. The MRD% in BM after the first cycle (n=51), second cycle (n=52) and third cycle (n=30), as well as in PBSC products (n=39) strongly correlated with relapse-free survival. At a cutoff level of 1% after the first cycle and median cutoff levels of 0.14% after the second, 0.11% after the third cycle and 0.13% for PBSC products, the relative risk of relapse was a factor 6.1, 3.4, 7.2 and 5.7, respectively, higher for patients in the high MRD group. Also, absolute MRD cell number/ml was highly predictive of the clinical outcome. After the treatment has ended, an increase of MRD% predicted forthcoming relapses, with MRD assessment intervals of ⩽3 months. In conclusion, MRD parameter assessment at different stages of disease is highly reliable in predicting survival and forthcoming relapses in AML.

C-Type Lectin-Like Molecule-1 A Novel Myeloid Cell Surface Marker Associated with Acute Myeloid Leukemia

Acute myeloid leukemia (AML) has a poor prognosis due to treatment-resistant relapses. A humanized anti-CD33 antibody (Mylotarg) showed a limited response rate in relapsed AML. To discover novel AML antibody targets, we selected a panel of single chain Fv fragments using phage display technology combined with flow cytometry on AML tumor samples. One selected single chain Fv fragment broadly reacted with AML samples and with myeloid cell lineages within peripheral blood. Expression cloning identified the antigen recognized as C-type lectin-like molecule-1 (CLL-1), a previously undescribed transmembrane glycoprotein. CLL-1 expression was analyzed with a human anti-CLL-1 antibody that was generated from the single chain Fv fragment. CLL-1 is restricted to the hematopoietic lineage, in particular to myeloid cells present in peripheral blood and bone marrow. CLL-1 is absent on uncommitted CD34+/CD38− or CD34+/CD33− stem cells and present on subsets of CD34+/CD38+ or CD34+/CD33+ progenitor cells. CLL-1 is not expressed in any other tissue. In contrast, analysis of primary AMLs demonstrated CLL-1 expression in 92% (68 of 74) of the samples. As an AML marker, CLL-1 was able to complement CD33, because 67% (8 of 12) of the CD33− AMLs expressed CLL-1. CLL-1 showed variable expression (10–60%) in CD34+ cells in chronic myelogenous leukemia and myelodysplastic syndrome but was absent in 12 of 13 cases of acute lymphoblastic leukemia. The AML reactivity combined with the restricted expression on normal cells identifies CLL-1 as a novel potential target for AML treatment.

Clinical significance of flowcytometric minimal residual disease detection in pediatric acute myeloid leukemia patients treated according to the DCOG ANLL97/MRC AML12 protocol.

Analysis of minimal residual disease (MRD) in childhood acute myeloid leukemia (AML) may predict for clinical outcome. MRD levels were assessed by flowcytometric immunophenotyping in 94 children with AML enrolled into a single trial (United Kingdom Medical Research Council AML12 and similar Dutch Childhood Oncology Group ANLL97). An aberrant immunophenotype could be detected in 94% of patients. MRD levels after the first course of chemotherapy predicted for clinical outcome: 3-year relapse-free survival was 85%±8% (s.e.) for MRD-negative patients (MRD<0.1%), 64%±10% for MRD-low-positive patients (0.1%⩽MRD<0.5%) and only 14±9% for MRD-high-positive patients (MRD⩾0.5%; P<0.001), whereas overall survival was 95%±5%, 70%±10% and 40%±13%, respectively, (P<0.001). Multivariate analysis allowing for age, karyotype, FLT3-internal tandem duplications and white blood cell count at diagnosis showed that MRD after the first course of chemotherapy was an independent prognostic factor. Although comparison of paired diagnosis-relapse samples (n=23) showed immunophenotypic shifts in 91% of cases, this did not hamper MRD analysis. In conclusion, flowcytometric MRD detection is possible in children with AML. The level of MRD after the first course of chemotherapy provides prognostic information that may be used to guide therapy.

Identification of a small subpopulation of candidate leukemia-initiating cells in the side population of patients with acute myeloid leukemia.

In acute myeloid leukemia (AML), apart from the CD34+CD38− compartment, the side population (SP) compartment contains leukemic stem cells (LSCs). We have previously shown that CD34+CD38− LSCs can be identified using stem cell‐associated cell surface markers, including C‐type lectin‐like molecule‐1 (CLL‐1), and lineage markers, such as CD7, CD19, and CD56. A similar study was performed for AML SP to further characterize the SP cells with the aim of narrowing down the putatively very low stem cell fraction. Fluorescence‐activated cell sorting (FACS) analysis of 48 bone marrow and peripheral blood samples at diagnosis showed SP cells in 41 of 48 cases that were partly or completely positive for the markers, including CD123. SP cells in normal bone marrow (NBM) were completely negative for markers, except CD123. Further analysis revealed that the SP fraction contains different subpopulations: (a) three small lymphoid subpopulations (with T‐, B‐, or natural killer‐cell markers); (b) a differentiated myeloid population with high forward scatter (FSChigh) and high sideward scatter (SSChigh), high CD38 expression, and usually with aberrant marker expression; (c) a more primitive FSClow/SSClow, CD38low, marker‐negative myeloid fraction; and (d) a more primitive FSClow/SSClow, CD38low, marker‐positive myeloid fraction. NBM contained the first three populations, although the aberrant markers were absent in the second population. Suspension culture assay showed that FSClow/SSClow SP cells were highly enriched for primitive cells. Fluorescence in situ hybridization (FISH) analyses showed that cytogenetically abnormal colonies originated from sorted marker positive cells, whereas the cytogenetically normal colonies originated from sorted marker‐negative cells. In conclusion, AML SP cells could be discriminated from normal SP cells at diagnosis on the basis of expression of CLL‐1 and lineage markers. This reveals the presence of a low‐frequency (median, 0.0016%) SP subfraction as a likely candidate to be enriched for leukemia stem cells.

Review of the relevance of aberrant antigen expression by flow cytometry in myeloid neoplasms

This article reviews the use of aberrant antigen expression detected by flow cytometry in the diagnosis and clinical handling of acute myeloid leukaemia (AML) and the myelodysplastic syndromes (MDS). Such aberrancies offer a valuable tool for the proper classification of these myeloid malignancies according the World Health Organization 2008 classification. Aberrant antigen expression by flow cytometry is also important for prognostification. This review supports the view, that minimal residual disease detection methods that make use of such aberrancies should be part of the routine management of AML patients to guide therapy, but also suggests the introduction of flow cytometry in MDS for diagnosis and treatment decisions in the near future.

P-glycoprotein drug efflux pump involved in the mechanisms of intrinsic drug resistance in various colon cancer cell lines. Evidence for a saturation of active daunorubicin transport.

We studied the resistance of colon tumors to anticancer agents in vitro. Using daunorubicin (DN), a number of cellular parameters which normally indicate acquired or multidrug resistance (MDR), were compared for several human wild-type colon cell lines, i.e. HT29, SW1116 and COLO 320, and the murine colon cell line C-26. The sensitive/MDR human ovarian cancer cell line couple A2780/2780AD was used as a reference. The amount of P-glycoprotein (P-gp) was in the order HT29, A2780 less than or equal to SW1116 less than C26 less than or equal to COLO 320 less than 2780AD. The MDR modifiers verapamil, Cremophor EL, cyclosporin A and Ro 11-2933/001 had significant effects on DN cytotoxicity, total DN accumulation and efflux, only if P-gp was present. A flow-through system was used to study the mechanism of DN transport. For the first time, evidence for saturation of an active transport of DN from the cells is reported. We discussed the possible presence of cooperative activity between at least two binding sites on the protein responsible for DN efflux, likely to be P-gp.