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. 2019 Dec 17;36(2):309–315. doi: 10.1007/s12288-019-01232-4

Higher Level of Peripheral Blood CD34 Positive Cells Presented with Unfavorable Prognosis in Intermediate-Low Risk Acute Promyelocytic Leukemia

Cuiling Zhang 1, Haibo Dong 1, Yipeng Lin 1, Peipei Xu 1, Rongfu Zhou 1, Hui Zeng 1,2,✉
PMCID: PMC7229103  PMID: 32425382

Abstract

Our previous work has demonstrated that some acute promyelocytic leukemia (APL) patients had significantly elevated circulating CD34+ cell count (≥ 10 × 106/L), and these patients with higher CD34+ cell level usually presented with high-risk disease (WBC > 10,000/μL). The aim of this study was to investigate whether circulating CD34+ cell count is a prognostic marker in intermediate-low risk APL patients. In this study, 76 intermediate-low risk APL patients and 56 age-adjusted healthy volunteers were evaluated. Enumeration of CD34+ cells was investigated before the treatment. A cut-off value of 10 × 106/L CD34+ cells could just distinguish APL patients with adverse prognostic factors from others and may have the power to predict shorter progression-free survival (PFS) and poor prognosis. Higher count of CD34+ cells was usually associated with nonclassical chromosomal translocation, PML/RARα gene complex fusion, APL history, chemotherapy-related APL, disease progression, second tumor, extramedullary infiltration, FLT3-ITD positive mutation, atypical morphology, BM promyelocyte CD56/CD34 positive expression, myelofibrosis, PCR-positive PML/RARa gene fusion but FISH-negative, marrow necrosis and shorter PFS. Our results suggest that the level of CD34+ cells can be further the stratification of disease risk, a higher CD34+ cell count may be indicative of inferior survival and serve as an adverse biomarker for intermediate-low risk APL.

Keywords: Circulating CD34+ cell count, Peripheral blood, Acute promyelocytic leukemia, Prognostic factor, Progression-free survival

Introduction

Acute promyelocytic leukemia (APL) is a subtype of myeloid leukemia which is characterized by leukemic cells blocked at the promyelocytic stage of granulocytic differentiation and is associated with the presence of balanced t(15;17) and PML/RARa fusion transcripts [1]. CD34 antigen is transmembrane glycophosphoprotein expressed on early hematopoietic cells [2]. In steady state, there is little exchange of hematopoietic stem cells from the marrow through the blood and into the marrow under basal conditions, while many factors can facilitate bone marrow CD34+ cells influx into peripheral blood [3].

Our previous study has elucidated the distribution regularity of peripheral blood circulating CD34+ cell enumeration in APL patients and found some patients had significantly elevated counts (≥ 10 × 106/L), and these patients with higher CD34+ cell level usually presented with high-risk disease (WBC > 10,000/μL) [4]. With our further work, interestingly, we found intermediate-low risk APL, although removing higher WBC, some patients still had markedly increased CD34+ cell counts and presented clinical features. These patients were usually accompanied by adverse prognostic factors like FLT3-ITD positive mutation, non classical abnormalities of chromosome karyotype, more immature and aggressive immunophenotype CD34+/CD56+ and had poor survival. Our aim in the present study was to fully investigate the enumeration of CD34+ cells in intermediate-low risk APL patients.

Materials and Methods

Risk Stratification

Low risk defined patients with WBC ≤ 10 × 109/L and platelet count > 40 × 109/L, intermediate risk patients had WBC < 10 × 109/L and platelet count ≤ 40 × 109/L. All samples were collected after obtaining informed consents.

Patients

Between February 2011 and September 2018, peripheral blood samples from 76 patients with intermediate-low APL (28 with low risk, 48 with intermediate risk; 27 males and 49 females; median age, 44.5 years; age range 18–85 years) and 56 age-adjusted healthy volunteers (29 males and 27 females; median age, 31 years; age range 22–38 years) were recruited. Diagnoses of all patients were made according to World Health Organization (WHO) criteria.

Enumeration of Circulating CD34+ Cells

The peripheral blood samples were drawn into EDTA anticoagulated tubes for complete blood count and enumeration of CD34+ cells by flow cytometry. Cells (50 μL of blood) were incubated for 15 min at room temperature with fluorescein isothiocyanate (FITC)-conjugated CD45 monoclonal antibody (Becton Dickinson Immunocytometry Systems) and phycoerythrin (PE)-conjugated CD34 monoclonal antibody (Becton Dickinson) or PE-conjugated isotype matched mouse IgG1 (Becton Dickinson) which was chosen as the negative control. FACS Lysing Solution (Becton Dickinson) was then added to lyse the red blood cells (RBCs). After a wash, the cells were resuspended in PBS and analyses were performed by FACS Calibur flow cytometer (Becton Dickinson).The cell Quest Pro COUNT program was used to acquire CD34+ cells. The turn around time of doing CD34+ cell counts was about 2 h.

Cytogenetic Analysis

Bone marrow samples for cytogenetic analyses were performed after short-term culture (24 or 48 h) following standard procedures. The chromosomes were stained by G-banding and the karyotypes were reported according to International System for Human Cytogenetic Nomenclature (ISCN,1995) recommendation. Based on the cytogenetic analyses, the subgroup was made as follows: (1) classical chromosome translocation, namely t(15;17); (2) non classical chromosome translocation, all other karyotypes excluding classical t(15;17) and normal karyotype.

Immunophenotypic Analysis

Immunophenotypic analysis was carried out using bone marrow samples taken at diagnosis and analyzed in the reference laboratory by standard immunofluorescence methods. ≥ 20% was positive.

Leukemic-Related Gene Mutation Detection

The bone marrow (2 mL) was drawn into EDTA anticoagulated tubes, the gene mutation of FLT3-ITD, PML/RARα and other leukemic-related gene mutation were determined by polymerase chain reaction.

Treatment

All patients received induction, consolidation and maintenance treatment according to guidelines set by the Hematological Society of the Chinese Medical Association. Once a diagnosis of APL was suspected, all-transretinoic acid (ATRA) 25 mg/m2/day was given as induction treatment, as early as possible, until complete remission (CR) was achieve. Of all 76 patients, 35 patients experienced ATRA and anthracycline-based chemotherapy, 23 patients were given ATRA and arsenic trioxide, 18 patients received ATRA, arsenic trioxide and anthracycline-based chemotherapy. Remission induction was followed by two–three consolidation cycles with anthracycline-based regimens. Maintenance treatment continued for 2 years and comprised at least five cycles of three months each. Each cycle comprised intermittent ATRA (25 mg/m2/day for 14 days), arsenic trioxide (0.16 mg/kg/day for 14 days).

Statistics

By Graphpad prism7 Statistical software, Multiple Linear Regression was employed, differences in distribution of variables among subsets of patients were analyzed using Fisher's exact tests. Survival curves were drawn using the Kaplan–Meier estimate and compared by the log-rank tests. P values less than 0.05 were considered statistically significant.

Results

The Expression Patterns of CD34+ Cells Among Low Risk and Intermediate Risk APL Patients

The median absolute count of CD34+ cells were 2.06 × 106/L (range 0.54–5.28 × 106/L) in normal subjects and 1.47 × 106/L (range 0.15–5675.67 × 106/L) in APL patients. Compared with normal subjects, APL patients had no obviously increased CD34+ cell count (P = 0.420, Mann–Whitney Test). (Fig. 1).

Fig. 1.

Fig. 1

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Circulating CD34+ cell counts in subgroups. Each circle or square or triangle represents one study and the Bars represent the median count of each subgroup. APL compared with normal, P = 0.420; intermediate compared with low, P = 0.381; intermediate compared with normal, P = 0.957; low compared with normal, P = 0.070

Intermediate risk APL patients had median CD34+ cell levels of 2.04 × 106/L (range 0.15–5676 × 106/L), the median values of CD34+ cells in low risk patients were 1.18 × 106/L (range 0.24–1081 × 106/L). (Fig. 1). Compared with normal subjects, intermediate and low risk patients all had no obviously increased CD34+ cell count (P = 0.957 intermediate risk with normal, P = 0.070 low risk with normal, Mann–Whitney Test). (Fig. 1). And there was no obvious difference of count was found between low risk and intermediate risk subgroups (P = 0.381, Mann–Whitney Test). (Fig. 1). However, all normal subjects had count of CD34+ cells lower than 10 × 106/L, 21 of all 76 intermediate-low patients had higher CD34+ cell counts (above 10 × 106/L). Among 21 patients with higher CD34+ cell counts, 19 patients were in intermediate risk subgroup, low risk subgroup had 2 patients.

The Main Characteristics of APL Patients at Initial Diagnosis According to CD34+ Cell Count

CD34+ cell percentage and absolute count were related to WBC count(P < 0.001, P < 0.001). There was no significant correlation between CD34+ cell percentage and count and the following parameters in the studied cohort: patients' age (P = 0.795, P = 0.725), sex (P = 0.315, P = 0.923), BM promyelocyte percentage (P = 0.528, P = 0.404) and different PML/RARα bcr isoforms(P = 0.204, P = 0.058). CD34+ cell count above 10 × 106/L was observed in 21 patients, of particular interest were clinical features of these patients: (1) 9 patients with FLT3-ITD positive mutation; (2) 1 patient with PML/RARa gene complex fusion; (3) 1 patient with myelofibrosis and marrow necrosis; (4) 9 patients with BM promyelocyte CD56/CD34 antigen positive expression; (5) 2 patient with chemotherapy-related APL; (6) 7 patients with nonclassical chromosomal translocation; (7) 1 patient with 12 years APL history; (8) 1 patient with second tumor (gastric cancer); (9) 1 patient with extramedullary infiltration (spinal cord); (10) 2 patient with atypical morphology; (11) 1 patient with PCR- positive PML/RARa gene fusion but FISH-negative; (12) 1 patient with myelofibrosis; (13) 1 patient with disease progression 49 months later (Table 1 and Fig. 2).

Table 1.

Clinical features of the patients whose CD34+ cell counts above 10 × 106/L

Patients (NO) Karyotype abnormality CD56/CD34 antigen expression FLT3-ITD mutation Other indexes CD34+ cells percent (%)/absolute count (×106/L) PFS (months) Survival (months)
No. 1 Classical chromosomal translocation CD34+ CD56+ + Myelofibrosis and marrow necrosis; atypical morphology 54.1/3031.3 0.5
No. 2 46, Y, X p+, ? t(2;9)(q34;q21), 13 p+, t(15;17)(q22;q12) CD34+ CD56− − PML/RARa gene complex fusion 60.4/5130.6 8
No. 3 Classical chromosomal translocation − + − 0.32/ 13.76 0.3
No. 4 Classical chromosomal translocation − − Disease relapse 21.75/1749 0.3
No. 5 45, X, −Y, 9q−, ?, t (11;21) (p13;q22), t(15;17)(q22;q12) − − − 16.09/193.1 27
No. 6 47, XY, t(15;17)(q22;q21), +8 − − Disease progression 49 months later 4.02/140.7 49 69
No. 7 Classical chromosomal translocation − − Chemotherapy-related APL 2.03/38.57 25
No. 8 Classical chromosomal translocation CD34+ CD56+ − − 12.17/73.02 31
No. 9 Classical chromosomal translocation CD34+ CD56− − Myelofibrosis, second tumor (gastric cancer) 40.3/1411 33
No. 10 Classical chromosomal translocation CD34+ CD56− + − 38.61/5676 30
No. 11 45, XX, der(13;14)(q10;q10), t(15;17)(q22:q21) − − − 3.89/101.1 24
No. 12 Classical chromosomal translocation CD34− CD56+ − − 3.59/43.08 53
No. 13 46, XX, 3p+, −12, t(15, 17)(q22;q12), +mar [6]/46, XX − − − 1.02/10.2 48
No. 14 Classical chromosomal translocation CD34+ CD56− + − 13.92/125.28 40
No. 15 Classical chromosomal translocation − + − 6.21/310.5 1
No. 16 Classical chromosomal translocation − + Extramedullay infiltration (spinal cord) 31.8/1081.2 3
No. 17 Classical chromosomal translocation CD34+ CD56− + − 26.6/611.8 3
No. 18 Classical chromosomal translocation CD34+ CD56− − Atypical morphology; chemotherapy-related APL 4.46/98.12 3
No. 19 47, XX, t(15;17)(q22;q21), +8 − − PCR-positive PML/RARa gene fusion but FISH- negative 5.82/285.18 5
No. 20 Classical chromosomal translocation − + − 3.29/32.9 12
No. 21 46, XX, t(7;19)(q21;q13);t(15;17) − + − 9.99/59.94 43
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Fig. 2.

Fig. 2

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Correlations between CD34+ cells and poor clinical indexes. Each circle or filled circle represents one study. Circle represents poor clinical indexes, respectively

Association Between CD34+ Cells and Remission, Disease Progression and Survival

By follow-up, 4 (19.0%) of 21 patients with higher CD34+ cells had died early (namely No.1, No.2, No.3 and No.4 patient). No.1 and No.3 patient died from brain hemorrhage and pneumorrhagia, No.2 patient died from gastrointestinal hemorrhage, No.4 patient died from brain hemorrhage. The remaining 17 patients and 55 others with normal CD34+ cells achieved complete cytological or molecular remission after induction treatment, and the CD34+ cell counts in all patients reached below 10 × 106/L. Complete remission (CR) rate was respectively 81.0% and 100% in higher CD34+ cells and normal subgroup, significant difference was observed (P < 0.001). Follow-ups of CD34+ cells were available from all 72 patients receiving regular therapy. With a median follow-up of 35.5 months (range 0.3–108 months), 1 patient had relapse after 49 months, whose CD34+ cell level was significantly elevated (140.7 × 106/L) at initial diagnosis. The remaining 71 patients were in complete molecular remission, the counts in all patients were less than 10 × 106/L. The 5-year overall survival (OS) and progression-free survival (PFS) rates of in higher CD34+ cell subgroup were significantly lower than that in normal subgroup (P < 0.001) (Fig. 3 and Fig. 4).

Fig. 3.

Fig. 3

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Kaplan–Meier estimates of overall survival of 76 patients

Fig. 4.

Fig. 4

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Kaplan–Meier estimates of progression-free survival of 76 patients

Discussion

Acute promyelocytic leukemia (APL) is a special subtype in acute myeloid leukemia, which can be divided into high risk subgroup and intermediate-low risk subgroup according to the peripheral WBC and platelet count of newly diagnosed patients. High risk APL patients are often associated with severe hemorrhagic manifestations, intracranial hemorrhage either at presentation or during the first days of remission induction, resulting in increased early mortality. By comparison with high risk patients, intermediate-low risk patients frequently have better overall survival(OS) and progression-free survival (PFS) [5–7]. Initial WBC and platelet count have been recognized as significant prognostic factors, however, although not independent from WBC count, FLT3-ITD positive mutation, non classical abnormalities of chromosome karyotype, PML/RARa gene complex fusion are all adverse prognostic candidates in APL [8, 9].

Although with favourable prognosis, clinically some intermediate-low risk APL patients sometimes still presented life threatening events especially fatal hemorrhage, higher rate of induction death and relapse, these patients usually demonstrated aggressive clinical features, namely higher occurrence of fever, hemorrhage, elevated blast counts, microgranular morphology, aberrant expression of CD56/CD34 surface antigens, FLT3-ITD positive mutation [10]. It is very imperative to quickly recognize this subgroup and to initiate intensive supportive measures as soon as possible.

Various prognostic factors predicting outcome are being continuously analyzed, a good prognostic index can provide reliable information for patients and allow physicians to tailor therapy according to risk stratification. Flow cytometry enumeration of CD34+ cell is non-invasive, fast, practicable and can be easily performed at initial diagnosis. Previously published studies have showed elevated counts of CD34+ cells in patients with myelodysplastic syndrome (MDS) correlated with disease grade-related, cytogenetic abnormalities and leukemia free survival and have found its usefulness as a clinical marker for disease progression and prognosis [11–14]. Similar to previous conclusion, in this study, our results also demonstrated that some intermediate-low risk APL patients had significantly elevated level of CD34+ cells and higher CD34+ cells always associated with some poor clinical indexes and had inferior influence on PFS and OS. These indexes included nonclassical chromosomal translocation, PML/RARa gene complex fusion, FLT3-ITD positive mutation, APL history, BM promyelocyte CD56/CD34 antigen positive expression, second tumor (gastric cancer), PCR-positive PML/RARa gene fusion but FISH-negative, atypical morphology, extramedullary infiltration, disease progression, chemotherapy-related APL, myelofibrosis and marrow necrosis, however these poor indexes were not appeared in patients with normal CD34+ cells. It is well documented that higher CD34+ cell level indicates inferior prognosis in MDS patients [15], Knipp S et al. and our study have reported that a cut-off of ≥ 10 × 106/L could better distinguish low risk MDS patients from high risk subjects [16, 17]. As for intermediate-low risk APL patients, our observation also indicated that a threshold of ≥ 10 × 106/L could have the power to distinguish patients with adverse clinical prognostic factors from others.

Four early-death patients had significantly elevated CD34+ cells and correspondingly had more poor indexes: like PML/RARa gene complex fusion, complex karyotype abnormalities, APL history, CD56/CD34 antigen positive expression, FLT3-ITD positive mutation, atypical morphology and myelofibrosis and marrow necrosis. The patients with higher CD34+ cells had usually one or more poor prognostic indexes, fatal hemorrhage frequently occurred during induction treatment, therefore the count above 10 × 106/L at initial diagnosis might have indication for early-death and could be a candidate biomarker for poor prognosis, clinically intensive transfusion support should be initiated as soon as possible to avoid early death.

During follow-up, the counts of CD34+ cells in all patients were in normal level and remained stable, but 1 patient with higher CD34+ cells had disease progression after 49 months later. The elevated CD34+ cells at initial diagnosis might have indication for disease progression, clinically close monitoring and follow-up of these patients should be done to alert disease progression.

In conclusion, we have showed significant elevation of CD34+ cell counts in intermediate-low risk APL patients and elevated counts help to reliably identify these patients with adverse clinical factors and could be a candidate biomarker for shorter PFS and worse prognosis. The monitoring of CD34+ cells is practicable, non-invasive and can be easily performed at initial diagnosis and during following-up. The enumeration of CD34+ cells should be included in the work-up of APL patients.

Abbreviations

APL

Acute promyelocytic leukemia

PFS

Progression-free survival

OS

Overall survival

ATRA

All-transretinoic acid

CR

Complete remission

Compliance with Ethical Standards

Conflict of interest

The authors declare no conflict of interest.

Ethical statement

The study protocol received institutional review board approval and that the participants provided informed consent. All patient’s personal data have been secured.

Footnotes

Cuiling Zhang and Haibo Dong are co-first author.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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