Specificity of immunophenotyping in acute promyelocytic leukemia

Immunophenotype distinction between acute promyelocytic leukaemia and CD15- CD34- HLA-DR- acute myeloid leukaemia with nucleophosmin mutations

Acute promyelocytic leukaemia (APL) is a unique clinicobiologic entity that can be successfully treated with All-trans Retinoic Acid ATRA-based regimens. Some cases of acute myeloid leukaemia (AML) with nucleophosmin (NPM) mutations have an immunophenotype that is similar to APL. The objective of the study is to compare antigenic expression in a group of APL patients with that in AML patients with NPM mutations and an APL-like immunophenotype (CD15- CD34- HLA-DR-). A consecutive series of 40 APL and 12 NPM patients with an APL-like phenotype were included in the study. Immunophenotypic patterns were investigated by multiparametric flow cytometry. Promyelocytic leukaemia-retinoic acid receptor-α transcript type, NPM and FLT3 mutations were investigated using conventional methods. Statistically significant differences were found between APL and NPM-mutated AML in CD33, CD13, CD2 and CD110 reactivity. CD2 expression was absent in every patient with NPM-mutated AML. In addition, mean fluorescence intensity and the coefficient of variation (cv) of CD33 and CD13 showed statistical differences between the two groups for CD33 (p = 0.007) and a trend to significance for CD13 (p = 0.05). Furthermore, among 45 evaluable patients, CD110 expression statistically differentiates between the two groups: [2/33 (6%) in the APL group and 8/12 (66.6%) in the NPM-mutated AML (p = 0.014)]. However, these traits were subtle, raising the possibility of practical diagnostic challenges. In conclusion, CD110 and CD33 reactivity may be useful to distinguish APL from NPM-mutated AML with CD15, CD34 and HLA-DR negativity. Nevertheless, cytogenetic and molecular characterization is necessary to establish the accurate diagnosis of AML.

HLA-DRneg patients without acute promyelocytic leukemia show distinct immunophenotypic, genetic, molecular, and cytomorphologic characteristics compared to acute promyelocytic leukemia

BACKGROUND

Loss of HLA-DR and CD34 is a well-known characteristic of malignant promyelocytes in acute promyelocytic leukemia (APL). However, this immunophenotype is not specific for APL. The purpose of this study was to investigate whether further biological characterization of the HLA-DR(neg) acute myeloid leukemia patients would allow more clearly define criteria to separate APL from non-APL patients.

METHODS

Immunophenotyping, cytogenetics, molecular analyses, and cytomorphology were prospectively performed within routine leukemia diagnostics of 800 patients included in different prospective acute myeloid leukemia multicenter trials.

RESULTS

Beside 60 APL, an additional 62 HLA-DR(neg) non-APL patients were identified. The main differential characteristics of HLA-DR(neg) non-APL included high CXCR-4 expression in most patients and almost all leukemia cells, a significantly higher proportion of patients presenting with NPM1 mutations as well as the significant association with cup-like nuclear morphology. The biological distinctness of both leukemia subtypes was further emphasized by the complete absence of aberrant CD2 expression and increased leukocyte and platelet counts in HLA-DR(neg) non-APL patients. Even in the CD34(pos) subgroup of HLA-DR(neg) non-APL all those features contributed in at least the same way to the separation from APL.

CONCLUSIONS

The results of the present study show that an immunophenotypic, molecular, and cytomorphologic separation of both HLA-DR(neg) leukemia subgroups is possible indicating that both groups are biologically distinct.

A flow cytometric method to detect apoptosis-related protein expression in minimal residual disease in acute myeloid leukemia

Minimal residual disease (MRD) cells are thought to be responsible for the persistence and relapse of acute myeloid leukemia (AML). Flow cytometric MRD detection by the establishment of a leukemia-associated phenotype (LAP) at diagnosis can be used in 80% of AML patients, allowing detection and functional characterization of MRD in follow-up bone marrow. One of the mechanisms contributing to inefficient chemotherapy is apoptosis resistance. Measuring apoptosis parameters in MRD cells will help to unravel the importance of apoptosis resistance in AML. We therefore developed a four-color flow cytometry method that enables establishment of apoptosis-related protein expression such as Bcl-2, Bcl-x(L), Mcl-1 and Bax at diagnosis and in MRD. Firstly, validation of this assay using Western blot analysis in five leukemia cell lines showed a significant correlation (R=0.70: P<0.0001). Secondly, the influence of the permeabilization procedure on LAP expression was investigated in 38 AML samples at diagnosis and in 42 MRD samples. Quantification of the frequency of LAP+ cells with and without permeabilization showed no significant differences (diagnosis: P= 0.57, follow-up: P= 0.43). The flow cytometric protocol thus enables analysis of apoptosis-related proteins at different stages of the disease, which will lead to a better understanding of the role of apoptosis resistance in the emergence of MRD in AML.

Acute promyelocytic leukemia M3: cytomorphologic, immunophenotypic, cytogenetic, and molecular variants

Acute promyelocytic leukemia (APL) M3 is an acute myeloid leukemia (AML) subtype characterized by proliferation of malignant promyelocytes with mature myeloid immunophenotype and the translocation t(15;17)(q22;q11), which results in the fusion of retinoic acid receptor-alpha (RARalpha) gene on chromosome 17 and the gene PML on chromosome 15. There are three M3 morphologic variants: the typical hypergranular form and the microgranular and basophilic variants. Although most leukemic cells in M3 patients express t(15;17), other cytogenetic abnormalities have also been reported. Also, there are three molecular variants of the PML/RARalpha transcript (bcr1, bcr2, bcr3). Blasts had typical hypergranular appearance (13 patients) with a mature myeloid immunophenotype (HLA-DR(-),CD13(+), and/or CD33(+)) (10 patients) in the majority of patients with M3 followed in this study. The typical translocation [t(15;17)(q22;q11)] was detected by cytogenetic analysis in 5 M3 patients, but PML/RARalpha was positive in 13 out of 15 patients, as assessed by RT-PCR (8 patients with bcr1 and 5 with bcr3 subtype). Cytogenetic diversity was found in three patients (1 with t(17;17), 1 with +8, and 1 with add (7)(q22); -7; +8). According to many studies, leukemic cell heterogeneity in APL influences the clinical outcome of disease. The analysis of certain leukemic cell characteristics on the clinical outcome in our study revealed that patients with bcr3 had shorter medians of first remission and survival in comparison to patients with the bcr1 isoform of PML/RARalpha. Also, the clinical relapse of disease in 4 APL patients with reverted PML/RAR alpha positivity is consistent with the view that detection of PML/RARalpha by RT-RCR in patients in remission implies a poor prognosis. On the contrary, lack of detection of PML/RARalpha by RT-PCR at least three times is a sign of long remission and survival.