CD34-positive acute promyelocytic leukemia is associated with leukocytosis, microgranular/hypogranular morphology, expression of CD2 and bcr3 isoform

Cellular hierarchy insights reveal leukemic stem-like cells and early death risk in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) represents a paradigm for targeted differentiation therapy, with a minority of patients experiencing treatment failure and even early death. We here report a comprehensive single-cell analysis of 16 APL patients, uncovering cellular compositions and their impact on all-trans retinoic acid (ATRA) response in vivo and early death. We unveil a cellular differentiation hierarchy within APL blasts, rooted in leukemic stem-like cells. The oncogenic PML/RARα fusion protein exerts branch-specific regulation in the APL trajectory, including stem-like cells. APL cohort analysis establishes an association of leukemic stemness with elevated white blood cell counts and FLT3-ITD mutations. Furthermore, we construct an APL-specific stemness score, which proves effective in assessing early death risk. Finally, we show that ATRA induces differentiation of primitive blasts and patients with early death exhibit distinct stemness-associated transcriptional programs. Our work provides a thorough survey of APL cellular hierarchies, offering insights into cellular dynamics during targeted therapy.

PML/RARa leukemia induced murine model for immunotherapy evaluation

Even though leukemia murine models are valuable tools for new drug therapy studies, most of these models consist of immunocompromised mice, which do not exhibit immune responses. In order to obtain an adequate leukemia model, we established an acute promyelocytic leukemia transplantation-based model (PML/RARa) in immunocompetent BALB/c mice, thus making it possible to study drug-induced cellular immune responses in leukemia. The development of PML/RARa leukemia was confirmed by leukocytosis (76.27 ± 21.8 vs. 3.40 ± 1.06; P < 0.0001), anemia (7.46 ± 1.86 vs. 15.10 ± 0.96; P < 0.0001), and thrombocytopenia (131.85 ± 39.32 vs. 839.50 ± 171.20; P < 0.0001), and the presence of blasts in the peripheral blood of mice (approximately 50% blasts; P < 0.0001), 15 days after the transplants. These findings were corroborated through differential counts, flow cytometry, and in vivo imaging, which indicated increased number of immature cells in the bone marrow (15.75 ± 3.30 vs 6.69 ± 0.55; P < 0.001), peripheral blood (7.88 ± 2.67 vs 1.22 ± 0.89; P < 0.001), and spleen (35.21 ± 4.12 vs 1.35 ± 0.86; P < 0.0001), as well as promyelocytes in the bone marrow (41.23 ± 4.80 vs 5.73 ± 1.50; P < 0.0001), peripheral blood (46.08 ± 7.52 vs 1.10 ± 0.59; P < 0.0001) and spleen (35.31 ± 8.26 vs 2.49 ± 0.29; P < 0.0001) of PML/RARa mice. Compared to basal conditions of untransplanted mice, the PML/RARa mice exhibited frequencies of T lymphocytes CD4 helper = 14.85 ± 2.91 vs 20.77 ± 2.9 in the peripheral blood (P < 0.05); 12.75 ± 1.33 vs 45.90 ± 2.02 in the spleen (P < 0.0001); CD8 cytotoxic = 11.27 ± 3.44 vs 11.05 ± 1.22 in the peripheral blood (P > 0.05); 10.48 ± 1.16 vs 30.02 ± 1.80 in the spleen (P < 0.0001); natural killer (NK) cells = 3.68 ± 1.35 vs 6.84 ± 0.52 in the peripheral blood (P < 0.001); 4.43 ± 0.57 vs 6.40 ± 1.14 in the spleen (P < 0.05); B cells 2.50 ± 0.60 vs 15.20 ± 5.34 in the peripheral blood (P < 0.001); 17.77 ± 4.39 vs 46.90 ± 5.92 in the spleen (P < 0.0001); neutrophils = 5.97% ± 1.88 vs 31.57 ± 9.14 (P < 0.0001); and monocytes = 6.45 ± 2.97 vs 15.85 ± 2.57 (P < 0.001), selected as classical (3.33 ± 3.40 vs 57.80 ± 16.51, P < 0.0001), intermediate (57.42 ± 10.61 vs 21.75 ± 5.90, P < 0.0001), and non-classical monocytes (37.51 ± 10.85 vs 18.08 ± 7.13, P < 0.05) in the peripheral blood; and as classically activated (M1) within in the bone marrow (3.70 ± 0.94 vs 1.88 ± 0.39, P < 0.05) and spleen 15.19 ± 3.32 vs 9.47 ± 1.61, P < 0.05), in addition to alternatively activated (M2) macrophages within the bone marrow (23.06 ± 5.25 vs 1.76 ± 0.74, P < 0.0001) and spleen (46.51 ± 11.18 vs 30.58 ± 2.64, P < 0.05) compartments. All-trans retinoic acid (ATRA) treatment of PML/RARa mice reduced blast (immature cells) in the bone marrow (8.62 ± 1.81 vs 15.76 ± 1.25; P < 0.05) and spleen (8.75 ± 1.31 vs 35.21 ± 1.55; P < 0.0001) with no changes in the peripheral blood (10.13 ± 3.33 vs 7.88 ± 1.01; P > 0.05), as well as reduced promyelocytes in the bone marrow (19.79 ± 4.84 vs 41.23 ± 1.81; P < 0.05), peripheral blood (31.65 ± 3.92 vs 46.09 ± 2.84; P < 0.05) and spleen (24.84 ± 2.03 vs 41.46 ± 2.39; P < 0.001), and increased neutrophils of the peripheral blood (35.48 ± 7.24 vs 7.83 ± 1.40; P < 0.05) which was corroborated by reducing of immature cells and increase of neutrophil in the stained smears from PML/RARa mice, thus confirming that this model can be used in drug development studies. Our results show the effective induction of PML/RARa leukemia in BALB/c mice, thus producing a low-priced and reliable tool for investigating cellular immune responses in leukemia.

Obesity accelerates acute promyelocytic leukemia in mice and reduces sex differences in latency and penetrance

OBJECTIVE

Obesity has emerged as a prominent risk factor for multiple serious disease states, including a variety of cancers, and is increasingly recognized as a primary contributor to preventable cancer risk. However, few studies of leukemia have been conducted in animal models of obesity. This study sought to characterize the impact of obesity, diet, and sex in a murine model of acute promyelocytic leukemia (APL).

METHODS

Male and female C57BL/6J.mCG+/PR mice, genetically predisposed to sporadic APL development, and C57BL/6J (wild type) mice were placed on either a high-fat diet (HFD) or a low-fat diet (LFD) for up to 500 days.

RESULTS

Relative to LFD-fed mice, HFD-fed animals displayed increased disease penetrance and shortened disease latency as indicated by accelerated disease onset. In addition, a diet-responsive sex difference in APL penetrance and incidence was identified, with LFD-fed male animals displaying increased penetrance and shortened latency relative to female counterparts. In contrast, both HFD-fed male and female mice displayed 100% disease penetrance and insignificant differences in disease latency, indicating that the sexual dimorphism was reduced through HFD feeding.

CONCLUSIONS

Obesity and obesogenic diet promote the development of APL in vivo, reducing sexual dimorphisms in disease latency and penetrance.

Variant Acute Promyelocytic Leukemia Presenting Without Auer Rods Highlights the Need for Correlation with Cytogenetic Data in Leukemia Diagnosis

Variant acute promyelocytic leukemia (vAPL) is a rare leukemia characterized by rearrangement between RARα and a non-PML partner gene. This type of leukemia can be difficult to recognize by histomorphologic evaluation, particularly in patients with few or no Auer rods, and by flow cytometry, but it can be identified by distinct cytogenetic features. Herein, we report on a patient with vAPL with t(11;17)(q23;q21) who presented an initial diagnostic challenge. Detailed flow cytometry findings are presented for this rare entity. Our case study also presents novel treatment (chemotherapy in combination with venetoclax) chosen based on mechanistic data from preclinical studies.

The curious case of HLA-DR-positive APL

The triad of weak/absent CD34, negative HLA-DR expression, and positivity to CD117 is pathognomonic for the diagnosis of acute promyelocytic leukemia. However, in rare cases, strong positivity to HLD-DR and CD34 may be noted.

Radar plots facilitate differential diagnosis of acute promyelocytic leukemia and NPM1+ acute myeloid leukemia by flow cytometry

BACKGROUND

Acute promyelocytic leukemia (APL) is one of the most life-threatening hematological emergencies and requires a prompt correct diagnosis by cytomorphology and flow cytometry (FCM) with later confirmation by cytogenetics/molecular genetics. However, nucleophosmin 1 muted acute myeloid leukemia (NPM1+ AML) can mimic APL, especially the hypogranular variant of APL. Our study aimed to develop a novel, Radar plot-based FCM strategy to distinguish APLs and NPM1+ AMLs quickly and accurately.

METHOD

Diagnostic samples from 52 APL and 32 NPM1+ AMLs patients were analyzed by a 3-tube panel of 10-color FCM. Radar plots combining all markers were constructed for each tube. Percentages of positive leukemic cells and mean fluorescence intensity were calculated for all the markers.

RESULTS

APL showed significantly higher expression of CD64, CD2, and CD13, whereas more leukemic cells were positive for CD11b, CD11c, CD15, CD36, and HLA-DR in NPM1+ AMLs. Radar plots featured CD2 expression, a lack of a monocytic component, lack of expression of HLA-DR and CD15, and a lack of a prominent CD11c+ population as recurring characteristics of APL. The presence of blasts with low SSC, presence of at least some monocytes, some expression of HLA-DR and/or CD15, and a prominent CD11c population were recurrent characteristics of NPM1+ AMLs. Radar plot analysis could confidently separate all hypergranular APL cases from any NPM1+ AML and in 90% of cases between variant APL and blastic NPM1+ AML.

CONCLUSION

Radar plots can potentially add to differential diagnostics as they exhibit characteristic patterns distinguishing APL and different types of NPM1+ AMLs.

Early mortality in acute promyelocytic leukemia: Potential predictors

Acute promyelocytic leukemia (APL) is a rare leukemia characterized by the balanced reciprocal translocation between the promyelocytic leukemia gene on chromosome 15 and the retinoic acid receptor α (RARα) gene on chromosome 17, and accounts for 10-15% of newly diagnosed acute myeloid leukemia each year. The combined use of all-trans retinoic acid and arsenic trioxide (ATO) as primary therapy has markedly improved the survival rate of patients with APL. Mortality in the first 30 days following therapy remains a major contribution to treatment failure. In the present study, published data was reviewed with a focus on the factors associated with early mortality. When treated with ATO as a primary treatment, the fms-like tyrosine kinase-internal tandem deletion has no impact on early mortality. Low lymphoid enhancer binding factor-1 expression may be a reliable marker for early mortality and the target of therapy if it could be proven by further studies. Cluster of differentiation (CD)56+ and CD34+/CD2+ may be candidates to select high-risk patients. The risk of early mortality in APL still cannot be predicted via the cell surface makers, despite multiple studies on their prognostic significance. Typically, a complex translocation did not alter the survival rate in patients with APL; however, if an abnormal karyotype [e.g., Ide(17), ZBTB16/RARα and STAT5B/RARα] appeared singularly or as part of a complex mutation, there is a high possibility of early mortality if clinicians are unable to identify or monitor it.

The triple-negative (CD34-/HLA-DR-/CD11b-) profile rapidly and specifically identifies an acute promyelocytic leukemia

INTRODUCTION

The genetic testing to confirm or rule out an acute promyelocytic leukemia (APL) typically takes a minimum of 24-72 hours. Flow cytometric immunophenotyping (FCI) on the other hand provides rapid and objective information to differentiate APL from non-APL.

METHODS

FCI features, with single-tube 8-color combination using CD45, CD34, HAL-DR, CD11b, CD13, CD33, and CD117 and CD64, were compared for the 30 consecutive APL and 30 non-APL acute myeloid leukemia (AML) cases which morphologically mimicked an APL. The diagnosis was confirmed by cytogenetic or molecular genetic testing in the form of t (15:17) (q22; q21)/variant translocations or PML-RARA fusion transcript analysis.

RESULTS

The APL cells lacked CD34, HLA-DR, and CD11b in 90%, 90%, and 93.3% cases, respectively. Myeloid antigens such as CD33, CD13, CD117, and CD64 were expressed in 96.7%, 96.7%, 76.7%, and 70% cases, respectively. The dual negative profiles, CD34-/HLA-DR- or HLA-DR-/CD11b-, were noted in 90% and 93.3% cases. The triple-negative (CD34-/HLA-DR-/CD11b-) profile was noted in 90% of the cases. The sensitivity, specificity, and positive predictive value (PPV) of CD34-/HLA-DR- and HLA-DR-/CD11b- profiles for the diagnosis of APL were found to be 90%, 80% & 81.1% and 93.3%, 86.7%& 87.5%, respectively. Combining the above two profiles resulted in a triple-negative profile (CD34-, HLA-DR- and CD11b-), which had a better specificity (93.3%) and positive predictive value (93.1%), with similar sensitivity.

CONCLUSION

FCI is a rapid and reliable modality for the diagnosis of an APL. The triple-negative profile (CD34-/HLA-DR-/CD11b-) rapidly and specifically identifies an APL case.

Microgranular acute promyelocytic leukemia presenting with leukopenia and an unusual immunophenotype

The microgranular variant (M3v) of acute promyelocytic leukemia (APL) is rare, and the diagnosis can be delayed due to variability in how this condition presents. M3v blasts often have folded nuclei, but unlike traditional APL blasts, they often possess faint granules without Auer rods. In addition, microgranular APL often presents with an elevated or normal white blood cell count in contrast with the leukopenia seen in traditional APL. In APL, delayed diagnosis can lead to early death from disseminated intravascular coagulation (DIC), which is the main cause of mortality in an otherwise treatable, and often curable, leukemia. We describe a 19-year-old male with microgranular APL who presented with leukopenia and many blasts resembling non-APL AML blasts with an unexpected immunophenotypic pattern. He was treated for DIC and initiated on all-trans-retinoic acid and arsenic trioxide; he achieved complete molecular remission after induction therapy. Suspicion for APL should always remain high in the presence of clinical manifestations of the disease in order that appropriate treatment can be initiated rapidly to prevent early death.

CD34(+) therapy-related acute promyelocytic leukemia in a patient previously treated for breast cancer

Therapy-related acute myeloid leukemia (AML) is a long term complication of chemotherapy for a variety of cancers. In most cases, the marrow demonstrates high risk cytogenetics and the prognosis is poor. In a minority of patients "good risk" cytogenetics, including t(15;17)(q22;q12), are seen and the patient's prognosis is similar to those who have de novo disease. Currently we present a patient who developed therapy-related acute promyelocytic leukemia (APL) after chemoradiotherapy for breast cancer. This case was especially atypical because the leukemic cells were CD34(+), which is an unusual immunophenotype for APL. Recognition that this patient had APL, rather than the more common therapy-related MDS or AML, was imperative to initiate chemotherapy in a timely manner.

The diagnostic and clinical impact of genetics and epigenetics in acute myeloid leukemia

Acute myeloid leukemia (AML) is a complex disease, for which our understanding of the role of genetic and epigenetic changes has undergone significant advancements. Newer diagnostic and prognostic classifications have increasingly incorporated such information, and novel therapies have been developed to target specific genes, processes, and pathways based on this growing understanding. Given the rapid evolution of this field, it is critical for physicians and translational researchers to have a more in-depth understanding of this evolving landscape. Here, we review both genetics and epigenetics in acute myeloid leukemia from a practical standpoint.

Persistent Hypoplastic Acute Promyelocytic Leukemia with a Novel Chromosomal Abnormality of 46, XY, t(15;17), t(9;11)(q13;p13)

A diagnosis of acute promyelocytic leukemia (APL) is usually made when normal hematopoietic cells are substituted by APL cells. We encountered a unique APL patient who presented with persistent hypoplastic features of APL. An 84-year-old man presented with leukopenia (2.2 × 10(9)/L) and anemia (Hb 12.5 g/dL). Five months later, the bone marrow (BM) was hypoplastic with a normal proportion of blasts and promyelocytes (5.2%), although the latter cells were hypergranular. The karyotype of BM cells was 46, XY, t(15;17)(q22;q12), t(9;11)(q13;p13). Two months later, the BM remained hypoplastic with 8.5% hypergranular promyelocytes, some of which contained faggot of Auer rods. RT-PCR examination yielded the PML-RARα transcript, and its sequencing revealed the breakpoint of PML to be bcr2. The patient was treated with all-trans retinoic acid under a diagnosis of APL with improvement of the bicytopenia. FISH analysis of BM cells yielded a negative result regarding t(15;17), although RT-PCR was positive for PML-RARα mRNA. Six months later, APL recurred with the same karyotypic abnormalities and therapeutic resistance, and the patient died of pneumonia. A persistent hypoplastic state of APL may be a rare event, and the association of t(15;17) and t(9;11) is novel.

Diagnostic immunophenotype of acute promyelocytic leukemia before and early during therapy with all-trans retinoic acid

OBJECTIVES

To study the immunophenotypic changes of acute promyelocytic leukemia (APL) in patients who recently received all-trans retinoic acid (ATRA) and to assess the diagnostic utility of flow cytometry in this setting.

METHODS

Flow cytometry was performed on 29 newly diagnosed APLs and 93 other acute myeloid leukemias, including 25 HLA-DR- or CD34- cases. Clinical notes from referring institutions were reviewed to assess for recent ATRA administration.

RESULTS

Recent ATRA therapy was documented in 17 (59%) of 29 patients with APL. The main features of untreated APL were preserved with ATRA therapy, including CD34- (83% vs 82%), HLA-DR- (83% vs 100%), and CD117+ (100% vs 77%). CD11b and CD11c were negative in all untreated APLs but positive in 76% and 88% of ATRA-treated APLs, respectively. Optimal diagnostic criteria for untreated APL (CD34- or HLA-DR- and CD11b- and CD11c-) showed 100% sensitivity and 98% specificity but were not useful after ATRA administration. The best interpretative approach to ATRA-treated APL (CD34- or HLA-DR-) showed 100% sensitivity but limited specificity (73%).

CONCLUSIONS

Information about recent ATRA administration is critical for adequate interpretation of the flow cytometric findings in patients with suspected APL.

FLT3/ITD associated with an immature immunophenotype in PML-RARα leukemia

Acute promyelocytic leukemia (APL) is characterized by the specific PML-RARα fusion gene resulting from translocation t(15;17) (q22;q12). Internal tandem duplication (ITD) of the FLT3 gene has been observed in approximately 35% of APLs, and large-scale studies have identified the presence of ITD as an adverse prognostic factor for acute myeloblastic leukemia (AML) patients. Aberrant expressions of surface antigens, such as CD2, CD34, and CD56, have been found in APL, but the implications of this are not well understood. We investigated the incidence of the FLT3/ITD mutation and FLT3/D835 (I836) point mutation in 25 APL patients. Incidence ratios of FLT3/ITD, D835 (I836), and both FLT3/ITD and D835 (I836) were 36%, 36% and 8%, respectively. FLT3/ITD⁺ cases showed a predominance of the bcr3 isoform (P=0.008) and M3v morphology (P<0.001). We found that all FLT3/ITD⁺ cases expressed CD2 (9 of 9) more frequently than that of FLT3/ITD⁻ (1 of 16) (P<0.001), while only one of the CD2⁺ cases (1 of 10, 10%) did not harbor FLT3/ITD, and all CD2⁺CD34⁺ cases (5 of 5, 100%) harbored FLT3/ITD. In addition, quantitative polymerase chain reaction analysis showed that FLT3 mRNA was more abundantly expressed in FLT3/ITD⁺ than that in FLT3/ITD⁻ (P=0.025), while there was no difference between D835(I836) ⁺ and D835(I836)⁻ with regards to aberrant surface-antigen expression, expression levels of FLT3 mRNA, M3v morphology, and the bcr3 isoform of PML-RARα mRNA. This study demonstrates that the presence of FLT3/ITD, but not D835 (I836), is closely related to aberrant CD2 expression and high expression levels of FLT3 mRNA. Our findings also suggest that FLT3/ITD as a secondary genetic event may block differentiation at the immature stage of APL.

Class II-associated invariant chain peptide expression represents a novel parameter for flow cytometric detection of acute promyelocytic leukemia

Because of severe bleeding complications, patients with acute promyelocytic leukemia (APL) have to be treated with all-trans retinoic acid immediately following diagnosis. In addition to morphology, flow cytometry contributes to a rapid detection of APL according to phenotypic characteristics of leukemic cells. In some patients, these analyses are inconclusive or even contradictory to diagnosis. Previously, we showed the clinical and functional impact of class II-associated invariant chain peptide (CLIP) in acute myeloid leukemia (AML). This study focuses on the analysis of CLIP expression on leukemic cells to characterize HLA-DR-negative AML, including APL. We demonstrate exclusive and significant CLIP expression in all cases of typical and variant APL, as compared to other HLA-DR-negative non-APL-type AML. CLIP appears to be a highly sensitive and specific flow cytometric marker, resolving discrepant identification of both genetic subgroups. Our findings show the additive value of CLIP analysis for a fast and unequivocal recognition of APL by flow cytometry in conjunction with morphology.

The biological characteristics of adult CD34+ acute promyelocytic leukemia

We aimed to explore the expression of CD34 and its impact on the disease outcome in patients with APL. The study comprised 40 de novo APL patients. Diagnostic tools included peripheral blood and bone marrow morphology and cytochemistry, immunophenotyping, cytogenetic studies, and PML/RARα fusion gene detection using RT-PCR. CD34 was expressed in 13 (32.5%) of cases with higher expression in M3v compared to M3 subtype. All M3v cases were CD34+, while only 7.4% of M3 cases were CD34+. CD34+ cases were associated with significant higher white blood cell count and peripheral blood promyelocytes. No significant association was found between PML/RAR-α isoform and molecular remission. CD34+ expression was significantly associated with decreased incidence of molecular remission and increased incidence of early death. The overall survival of patients with WBC count >11 × 103/μl was inferior to patients with WBC count <11 × 103/μl, but no significant differences were observed in overall survival between CD34- and CD34+ or between bcr1 and bcr3 groups. Immunophenotypic analysis for CD34 could distinguish an APL subset with different biological characteristics and adverse prognostic outcome.

Flow cytometry rapidly identifies all acute promyelocytic leukemias with high specificity independent of underlying cytogenetic abnormalities

Acute promyelocytic leukemia (APL) is a highly aggressive disease requiring prompt diagnosis and specific early intervention. Immunophenotyping by flow cytometry (FCM) facilitates a rapid diagnosis, but commonly used criteria are neither sufficiently sensitive nor specific. With an antibody panel for diagnostic screening in routine practice, we found all 149 APL cases in this study exhibited a unique immunophenotypic profile, ie, a characteristic CD11b- myeloid population and absent CD11c expression in all myeloid populations; 96.6% of cases also lacked HLA-DR expression. These distinctive features allowed recognition of all unusual cases phenotypically resembling the regular myeloblasts (CD34+/HLA-DR+) or granulocytes (CD117-/CD34-/HLA-DR-). FCM effectively identified all 19 APL cases with variant translocations, including cases with a normal karyotype due to a cryptic submicroscopic t(15;17)(q22;q21), t(11;17)(q23;q21) that escaped the detection by fluorescence in situ hybridization for t(15;17) and der(15)ider(17)(q10) that lacked a simple reciprocal t(15;17). When APL-associated profiles were validated against 107 AML cases of non-APL subtypes, including 51 HLA-DR- cases, the diagnostic specificity and positive predictive value were 98%. FCM effectively provides independent detection of APL during diagnostic workup and harmonizes with the subsequent molecular cytogenetic diagnosis.

Does microgranular variant morphology of acute promyelocytic leukemia independently predict a less favorable outcome compared with classical M3 APL? A joint study of the North American Intergroup and the PETHEMA Group

Few studies have examined the outcome of large numbers of patients with the microgranular variant (M3V) of acute promyelocytic leukemia (APL) in the all-trans retinoic acid era. Here, the outcome of 155 patients treated with all-trans retinoic acid-based therapy on 3 clinical trials, North American Intergroup protocol I0129 and Programa para el Estudio de la Terapéutica en Hemopatía Maligna protocols LPA96 and LPA99, are reported. The complete remission rate for all 155 patients was 82%, compared with 89% for 748 patients with classical M3 disease. The incidence of the APL differentiation syndrome was 26%, compared with 25% for classical M3 patients, and the early death rate was 13.6% compared with 8.4% for patients with classical M3 morphology. With a median follow-up time among survivors of 7.6 years (range 3.6-14.5), the 5-year overall survival, disease-free survival, and cumulative incidence of relapse for patients with M3V were 70%, 73%, and 24%, respectively. With a median follow-up time among survivors of 7.6 years (range 0.6-14.3), the 5-year overall survival, disease-free survival, and cumulative incidence of relapse among patients with classical M3 morphology were 80% (P = .006 compared with M3V), 81% (P = .07), and 15% (P = .005), respectively. When outcomes were adjusted for the white blood cell count or the relapse risk score, none of these outcomes were significantly different between patients with M3V and classical M3 APL.

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.

Frequent but nonrandom expression of lymphoid markers on de novo childhood acute myeloid leukemia

Lymphoid marker expression in 59 cases of de novo childhood acute myeloid leukemia (AML) was as follows: CD2 (15.5%), CD4 (73.8%), CD7 (25.8%), CD19 (22%) and CD56 (28.9%). Individual marker expression, as well as co-expression with other lymphoid markers, could be correlated with the FAB subtype of leukemia and the presence and type of certain leukemia fusion gene transcripts. The data showed that the expression of lymphoid markers in childhood de novo AML was common but nonrandom and was likely a reflection of the biological differences between various types of leukemia.

Asynchronous expression of myeloid antigens in leukemic cells in a PML/RARalpha transgenic mouse model

Acute promyelocytic leukemia (APL) is characterized by the expansion of blasts that resemble morphologically promyelocytes and harbor a chromosomal translocation involving the retinoic acid receptor alpha (RARalpha) and the promyelocytic leukemia (PML) genes on chromosomes 17 and 15, respectively. The expression of the PML/RARalpha fusion gene is essential for APL genesis. In fact, transgenic mice (TM) expressing PML/RARalpha develop a form of leukemia that mimics the hematological findings of human APL. Leukemia is diagnosed after a long latency (approximately 12 months) during which no hematological abnormality is detected in peripheral blood (pre-leukemic phase). In humans, immunophenotypic analysis of APL blasts revealed distinct features; however, the precise immunophenotype of leukemic cells in the TM model has not been established. Our aim was to characterize the expression of myeloid antigens by leukemic cells from hCG-PML/RARalpha TM. In this study, TM (N = 12) developed leukemia at the mean age of 13.1 months. Morphological analysis of bone marrow revealed an increase of the percentage of immature myeloid cells in leukemic TM compared to pre-leukemic TM and wild-type controls (48.63 +/- 16.68, 10.83 +/- 8.11, 7.4 +/- 5.46%, respectively; P < 0.05). Flow cytometry analysis of bone marrow and spleen from leukemic TM identified the asynchronous co-expression of CD34, CD117, and CD11b. This abnormal phenotype was rarely detected prior to the diagnosis of leukemia and was present at similar frequencies in hematologically normal TM and wild-type controls of different ages. The present results demonstrate that, similarly to human APL, leukemic cells from hCG-PML/RARalpha TM present a specific immunophenotype.

CD2+ acute promyelocytic leukemia is associated with leukocytosis, variant morphology and poorer prognosis

The T cell-lineage marker CD2 is sometimes expressed in acute promyelocytic leukemia (APL), and CD2 expression is reported to correlate with some clinical characteristics. However, the significance of CD2 expression in APL has not been fully elucidated. We evaluated CD2 expression in APL treated by the same treatment strategy in a single institute, and whether it had any special characteristics. Among 29 APL, 6 were positive for CD2. Patients with CD2+ APL tended to have a higher leukocyte count than CD2- APL (34.5 +/- 13.1/l vs. 6.8 +/- 2.1/l), morphological characteristics as variant-APL (50 vs. 0%). They also showed poor clinical prognosis. The CR rate of CD2- APL was 87.0% while that of CD2+ APL was 50 %. The mortality was 13.0 and 66.7%, respectively, and the survival rate was significantly lower in CD2+ APL. CD2 expression was proven to be a risk factor associated with death in addition to the morphological characteristics of variant-APL and leukocytosis. These results indicated that CD2 expression might have a significant impact on the prognosis of APL. Whether CD2+ APL should be characterized as a special clinical entity should be discussed in a larger patient population.

CD2+ tetraploid acute promyelocytic leukemia variant with double (15;17) translocations

We report a patient with a variant form of CD2+ acute promyelocytic leukemia (APL) who had double translocations (15;17) in a single leukemic cell. The patient presented with severe neutropenia, thrombocytopenia, and disseminated intravascular coagulation. The bone marrow showed marked hyperplasia with large leukemic cells that had bizarre nuclear configuration and basophilic, hypogranular cytoplasm. Leukemic cells were positive for CD2, 13, 33, 34, and 56 and negative for HLA-DR. The karyotype of the abnormal clone was characterized as 92,XXYY, t(15;17)(q22;q21)x2. No other additional abnormal clone was found, and the patient's condition was diagnosed as tetraploid APL variant. Fluorescence in situ hybridization assay revealed 2 promyelocytic leukemia and retinoic acid receptor alpha (PML/RARA) fusion signals, and reverse transcription-polymerase chain reaction assay revealed short-form PML/RARA fusion transcript. Tetraploidy in APL is a very rare abnormality. Double translocations were an additional abnormality in this case, and this patient's karyotype might have had some influence on morphological characteristics, expression of CD2, and poor clinical outcome.

AML M3 and AML M3 variant each have a distinct gene expression signature but also share patterns different from other genetically defined AML subtypes

Acute promyelocytic leukemia (APL) with t(15;17) appears in two phenotypes: AML M3, with abnormal promyelocytes showing heavy granulation and bundles of Auer rods, and AML M3 variant (M3v), with non- or hypogranular cytoplasm and a bilobed nucleus. We investigated the global gene expression profiles of 35 APL patients (19 AML M3, 16 AML M3v) by using high-density DNA-oligonucleotide microarrays. First, an unsupervised approach clearly separated APL samples from other AMLs characterized genetically as t(8;21) (n = 35), inv(16) (n = 35), or t(11q23)/MLL (n = 35) or as having a normal karyotype (n = 50). Second, we found genes with functional relevance for blood coagulation that were differentially expressed between APL and other AMLs. Furthermore, a supervised pairwise comparison between M3 and M3v revealed differential expression of genes that encode for biological functions and pathways such as granulation and maturation of hematologic cells, explaining morphologic and clinical differences. Discrimination between M3 and M3v based on gene signatures showed a median classification accuracy of 90% by use of 10-fold CV and support vector machines. Additional molecular mutations such as FLT3-LM, which were significantly more frequent in M3v than in M3 (P < 0.0001), may partly contribute to the different phenotypes. However, linear regression analysis demonstrated that genes differentially expressed between M3 and M3v did not correlate with FLT3-LM.

A surrogate marker profile for PML/RAR alpha expressing acute promyelocytic leukemia and the association of immunophenotypic markers with morphologic and molecular subtypes

BACKGROUND

The availability of genotype-specific therapy for PML/RAR alpha(pos) acute promyelocytic leukemia (APL) requires that this disease be precisely diagnosed. Immunophenotypic characteristics heretofore proclaimed as reliably characterizing APL (HLA-DR(low), CD34(low), P-glycoprotein(low) myeloid phenotype) do not differentiate from APL-like immune profiles unassociated with the PML/RAR alpha fusion transcript.

METHODS

To establish a surrogate marker profile for APL, we explored 19 potentially predictive markers compared with differentiated acute myeloid leukemia using the classification tree approach with recursive partitioning.

RESULTS

In a test group of 58 APL patients, the most predictive immune profile was HLA-DR(low), CD11a(low) (alpha(L) subunit of the leukocyte integrin LFA-1), CD18(low) (beta(2) subunit of LFA-1). APL cells always expressed CD117 (c-kit) but lacked the progenitor antigen CD133 and the more mature myeloid antigen, CD11b (alpha(M) leukocyte integrin). This antigen pattern was validated in 90 additional APL patients. M3v APLs (n = 30) had more leukemic promyelocytes expressing the T-cell antigen, CD2 (P < 0.0001) or the stem cell marker, CD34 (P = 0.0003) and demonstrated higher fluorescence intensity for the binding of antibody to the common leukocyte antigen, CD45 (P = 0.0008) than M3 (n = 102). S-form APL (n = 45) had a higher percent of cells expressing CD2 or CD34 (P < 0.0001 for both) or the neural cell adhesion molecule CD56 (P = 0.001) than L-form APL (n = 66).

CONCLUSIONS

PML/RAR alpha(pos) APL cells typically lack leukocyte integrins. HLA-DR(low), CD11a(low), CD18(low) is a reliable surrogate antigen expression profile for PML/RAR alpha(pos) APL, irrespective of morphology and transcript isoform.

Acute promyelocytic leukemia: where does it stem from?

A fundamental issue in cancer biology is the identification of the target cell in which the causative molecular lesion arises. Acute myeloid leukemia (AML) is thought to reflect the transformation of a primitive stem cell compartment. The resultant 'cancer stem cells' comprise only a minor portion of the leukemic clone but give rise through differentiation to more committed progenitors as well as differentiated blasts that constitute the bulk of the tumor. The maintenance of the leukemic clone is dependent on the self-renewal capacity of the cancer stem cell compartment, which is revealed by its ability to re-initiate leukemia in a transplant setting. The cellular basis of acute promyelocytic leukemia (APL) is however less clear. APL has traditionally been considered to be the most differentiated form of AML and to arise from a committed myeloid progenitor. Here we review apparently conflicting evidence pertaining to the cellular origins of APL and propose that this leukemia may originate in more than one cellular compartment. This view could account for many apparent inconsistencies in the literature to date. An understanding of the nature of the target cell involved in transformation of APL has important implications for biological mechanism and for clinical treatment.

Secondary near-tetraploidy with double der(15)t(15;17) in acute promyelocytic leukemia in relapse

Tetraploidy or near-tetraploidy is a rare cytogenetic abnormality in acute myelocytic leukemia. We report here a case of acute promyelocytic leukemia that showed near-tetraploidy with double der(15)t(15;17) the leukemia relapsed. At diagnosis, cytogenetic analysis failed to reveal any karyotypic abnormality; however, a promyelocytic leukemia-retinoic acid receptor alpha (PML/RARA) fusion transcript of the bcr3-type was detected with reverse transcriptase-polymerase chain reaction analysis, and a single PML/RARA fusion signal was observed with fluorescence in situ hybridization analysis. At the first relapse, the majority of leukemic cells showed pseudodiploid karyotypes with der(15)t(15;17), as well as additional chromosomal abnormalities, and exhibited a single PML/RARA fusion signal. A small fraction of leukemic cells, however, showed near-tetraploid karyotypes with double der(15)t(15;17), as well as some additional chromosomal abnormalities in common with the pseudodiploid clones, and exhibited double PML/RARA fusion signals. At the second and third relapses, leukemic cells with near-tetraploidy and double PML/RARA fusion signals became predominant. The PML/RARA fusion transcript of the bcr3 type was also observed at each relapse. In addition, Southern blot analysis of the RARA gene at diagnosis and at the second relapse showed a common rearranged band. Notably, giant, bizarre, and hypogranular promyelocytes expressing CD2, CD34, and HLA-DR appeared at the first relapse and became predominant at the second and third relapses. These observations indicate that the APL cells with near-tetraploidy and double der(15)t(15;17) clonally evolved from the pseudodiploid leukemic cells and exhibited the bizarre morphology and aberrant surface immunophenotypes.

Expression of cell-surface antigens in acute promyelocytic leukaemia

Acute promyelocytic leukaemia (APL) with M3 (or M3v) morphology is the only AML subtype to date for which morphology and immunophenotype agree. In other words, FAB M3 is interchangeable with a unique marker profile. More precisely, we have finally recognized a surrogate marker profile for leukaemia derived from the (15;17) translocation and expressing PML/RARalpha transcripts. To present this as a new development may come as a surprise to many. After all, the antigen expression pattern of AML-M3 was well recognized for many years: absence or weak expression of HLA-DR, CD117, CD15, CD11b and CD34 in the context of a myeloid phenotype (CD33 and CD13 expression) and frequently associated with moderate to high side-scatter appearance upon flow cytometric evaluation, depending upon the degree of granularity of the leukaemic cells. While partially correct, this established APL phenotype is both flawed and limited in its ability to distinguish APL from other AML subtypes, such as natural-killer-cell AML. Given the availability of phenotype-specific therapy for APL, such as all-trans retinoic acid or arsenic trioxide, failing to diagnose APL or misdiagnosing a case of AML with an APL-like phenotype will result in serious clinical consequences. Faced with this dilemma, we have recently performed a comprehensive immunophenotypic analysis of APL patients entered on Eastern Cooperative Oncology Group trials. Our results give diagnostic power to only three antigens, HLA-DR, CD11a and CD18, all of which are characteristically expressed at low levels by APL cells. Despite some significant antigenic differences (e.g. in CD34 expression), this surrogate marker profile for t(15;17) APL applies to both the M3 and the M3v FAB phenotypes and to all three isoforms of the PML/RARalpha transcript.

Morphological and cytochemical characteristics of leukaemic promyelocytes

Evaluation of cell morphology is usually sufficient to diagnose acute promyelocytic leukaemia (APL). In this chapter we discuss the features of classical hypergranular APL, the APL variant, hyperbasophilic promyelocytic leukaemia, APL with basophil-like granules, acute eosinophilic leukaemia with PML/RARalpha positivity and the morphology of APL cells lacking t(15;17). In addition to morphological examination, cytochemical investigations (peroxidase chloroacetate-esterase, etc.) may help further in defining the cytology of leukaemic cells in APL.

CD34 expression is associated with poor clinical outcome in patients with acute promyelocytic leukemia

This study investigated the clinical characteristics and prognostic relevance of CD34 expression in 47 patients with acute promyelocytic leukemia (APL), showing t(15;17) or PML/RARalpha. Ten (21.3%) of the APL patients were CD34(+). CD34 expression was associated with hypogranular morphology (P = 0.002) and hyperleukocytosis (P = 0.015). However, there were no statistically significant differences in age, sex, hemoglobin level, platelet count, or percentage of blasts between the CD34(+) and CD34(-) APL groups. Multiplex RT-PCR analysis showed that the L-form (BCR1) and S-form (BCR3) were correlated with CD34(-) APL and CD34(+) APL, respectively. Despite the lack of a difference in the complete remission rate, overall survival (OS) and disease-free survival (DFS) were significantly lower in the CD34(+) group than in the CD34(-) group (P = 0.012 and P = 0.0051, respectively). By multivariate analysis, the CD34(+) group showed a significant independent variable in DFS compared with the CD34(-) group, but this was not demonstrated for OS. In conclusion, CD34 expression in APL is a unique clinical feature associated with leukocytosis and atypical morphology with hypogranular pattern and is associated with a poor clinical outcome.

HLA-DR antigen-negative acute myeloid leukemia

Human leukocyte antigen (HLA) Class II antigens are variably expressed on acute myeloid leukemia (AML) blasts. The biological and clinical significance of HLA Class II antigen expression by AML cells is not known. Therefore, we sought to characterize cases of AML without detectable HLA-DR expression. Samples from 248 consecutive adult AML patients were immunophenotyped by multiparameter flow cytometry at diagnosis. HLA-DR antigens were not detected on AML cells from 43 patients, including 20 with acute promyelocytic leukemia (APL), and 23 with other subtypes of AML. All APL cases had t(15;17), but there were no characteristic chromosome abnormalities in non-APL cases. No direct expression of other antigens was identified in HLA-DR-negative APL and non-APL cases. Interestingly, cells from three HLA-DR-negative non-APL patients had similar morphology to that of the hypogranular variant of APL. This morphology, however, was not present in any HLA-DR-positive AML cases. Treatment response was similar in the 23 HLA-DR-negative non-APL and the 205 HLA-DR-positive patients. Finally, relapse was infrequently associated with changes in HLA-DR antigen expression, as the HLA-DR antigen was lost at relapse in only 4% of HLA-DR-positive cases, and was gained at relapse in only 17% of HLA-DR-negative cases. We conclude that HLA-DR-negative AML includes approximately equal numbers of APL and non-APL cases, and that the morphology of HLA-DR-negative non-APL cases can mimic the hypogranular variant of APL. The diagnosis of APL cannot be based on morphology and lack of HLA-DR antigen expression; rather, it requires cytogenetic or molecular confirmation.

PML-RAR induces promyelocytic leukemias with high efficiency following retroviral gene transfer into purified murine hematopoietic progenitors

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations resulting in fusion proteins of the retinoic acid receptor (RAR). Here, we report a novel murine model system for APL, based on the transduction of purified murine hematopoietic progenitors (lin(-)) using high-titer retroviral vectors encoding promyelocytic leukemia-RAR (PML-RAR), and the green fluorescent protein (GFP) as a marker. PML-RAR-expressing lin(-) cells were impaired in their ability to undergo terminal myeloid differentiation and showed increased proliferative potential in vitro. Inoculation of transduced lin(-) cells into syngeneic, irradiated mice resulted in the development of retinoic acid-sensitive promyelocytic leukemias at high frequency (> 80%) and short latency (approximately 4 months). Morphologic and immunophenotypic analysis revealed no gross abnormalities of the preleukemic bone marrows. However, hematopoietic progenitors from PML-RAR preleukemic mice showed a severe impairment in their ability to undergo myeloid differentiation in vitro. This result, together with the monoclonality or oligoclonality of the leukemic blasts, supports a "multiple-hit" model, where the fusion protein causes a "preleukemic" phase, and leukemia occurs after additional genetic lesions. This model system faithfully reproduces the main characteristics of human APL and represents a versatile tool for the in vitro and in vivo study of mechanisms of leukemogenesis and the design of protocols for differentiation treatment.

Established practice in the treatment of patients with acute promyleocytic leukemia and the introduction of arsenic trioxide as a novel therapy

First-line therapy for patients with newly diagnosed acute promyelocytic leukemia (APL) has been established in a series of randomized clinical trials. Remission induction and consolidation are based on the differentiation agent, all-trans retinoic acid (ATRA), and anthracycline-based chemotherapy. Maintenance therapy is also based on ATRA and may involve additional chemotherapy. Established protocols are associated with a high rate of complete responses (CRs) (87% to 97%), and long-term follow-up has indicated a 4-year disease-free survival of greater than 60%. Therapy for patients who relapse or are refractory to ATRA-based regimens is not standardized and there is a need for new approaches. Arsenic trioxide (ATO) has recently been licensed for use in patients with relapsed/refractory APL. Controlled clinical trials have indicated that ATO is associated with a CR rate of 87% in this population. This agent has a manageable toxicity profile and presents a welcome option for patients with relapsed disease for whom other, more debilitating therapies are unsuitable. Several prognostic factors have been defined in patients with APL, and it is possible that novel treatments such as ATO should be differentially applied to specific prognostic groups.

Leukemias resembling acute promyelocytic leukemia, microgranular variant

Acute promyelocytic leukemia (APL) should be distinguished from other subtypes of acute myeloid leukemia (AML) because of the increased risk of disseminated intravascular coagulation (DIC) and its response to arsenic compounds and retinoids. Some cases of AML seem morphologically similar to the microgranular variant of APL (French-American-British [FAB] AML-M3v) but lack the t(15;17). We evaluated 8 cases of APL-like leukemias for subtle morphologic, cytochemical, immunophenotypic, and cytogenetic differences compared with 5 cases of promyelocytic leukemia/retinoic receptor alpha (PML/RARalpha)-positive APL (FAB AML-M3v). We also evaluated both groups for the presence of DIC. No differences among the groups were noted in blast size, chromatin pattern, nuclear morphologic features, intensity of myeloperoxidase staining, or presence of Auer rods. Immunophenotypes were similar; both types of cases lacked CD34 and HLA-DR and were CD13+ and CD33+. Two cases of APL-like leukemias also were CD56+. DIC was present in 2 patients with M3v. Our study shows that there are no definitive morphologic, cytochemical, or immunophenotypic findings that can distinguish these cases from PML/RARalpha-positive APL.

Biological features of primary APL blasts: their relevance to the understanding of granulopoiesis, leukemogenesis and patient management

In recent years, discovery of the in vitro and in vivo differentiation of APL blasts by all-trans retinoic acid (ATRA) has modified the therapeutic approach of APL and lead to important advances in understanding the biology of APL. Since it became apparent that differentiation therapy of APL with ATRA was indeed a true model of targetted therapy, evidencing the molecular targets of retinoic acid efficacy became crucial. These molecular targets are closely related to the biological features of APL cells, some of which are well-known and have contributed to the morphological and cytogenetic definition of the leukemia, others have just been defined or re-discovered in the light of a better understanding of molecular controls of cell growth and differentiation. The aims of characterizing the biological features of APL cells should allow a better management of APL therapy and the identification of potential markers for differentiation therapies in other leukemias or solid tumors.