Prognostic significance of minimal residual disease detection and PML/RAR-alpha isoform type: long-term follow-up in acute promyelocytic leukemia

SOHO State of the Art Updates and Next Questions: Pre-emptive Therapy at Molecular Measurable Residual Disease Failure in Acute Myeloid Leukemia

Molecular measurable residual disease (MRD, eg, by real-time quantitative polymerase chain reaction, RT-qPCR), is an integral part of response assessment in acute myeloid leukemia (AML) with established prognostic and evolving therapeutic significance. MRD failure can occur through several pathways (namely MRD persistence at the end of treatment at a high level, MRD progression from a low level or MRD re-emergence during follow up; the latter two constitute MRD relapse as defined by the European Leukemia Net) and is clinically actionable, with survival benefit reported in AML subgroups. Selection of pre-emptive therapy at MRD failure relies upon an integrated clinico-molecular assessment and is subset-specific. In acute promyelocytic leukemia, arsenic trioxide-based regimen for MRD failure following frontline treatment with all-trans-retinoic acid plus chemotherapy represents standard of care, while hypomethylating agents (eg, azacitidine), salvage chemotherapy (eg, FLAG-IDA) and venetoclax-based regimens are effective in NPM1-mutated AML. Specific inhibitors of FLT3 have emerging use in FLT3-mutated AML and are associated with minimal toxicity. Furthermore, immunotherapeutic approaches such as donor lymphocyte infusions and interferon-⍺ are efficacious options in the post-allogeneic-HSCT settings. Enrollment into clinical trials with genomic-guided assignment of pre-emptive therapy at MRD failure should be prioritized. Finally, with the emergence of novel agents (eg, menin inhibitors) and approaches (eg, adoptive cellular and immunological therapy), an exciting future lies ahead where a broad array of highly active pre-emptive therapeutic options will likely be clinically applicable to a wide range of AML subsets.

Familial Acute Promyelocytic Leukemia: A Case Report and Review of the Literature

Acute promyelocytic leukemia (APL) is characterized by a reciprocal translocation t (15;17) (q24;q21), which leads to the fusion of PML and RARα genes known as PML-RARα fusion. A few cases of potentially hereditary leukemia-related genes in APL have been reported, but no instances of familial aggregation of APL have been documented. Here, we describe a family in whom two members successively affected by APL。The potential familial association observed in these two cases of APL highlights the need for further investigation and more definitive genetic lineage tracing in order to understand the genetic basis of this disease.

Fatal intracranial haemorrhage in acute promyelocytic leukemia patients with short isoform of PML-RARα: Review of molecular and radiological data

Three major PML-RARα fusion gene transcripts (long [bcr1], variant [bcr2], and short [bcr3]) are currently used in clinical laboratories for the diagnosis and treatment monitoring of APL patients. Despite highly improved outcome, relapse and intracranial haemorrhage that may lead to early death is still an unsolved complication in APL. We reviewed APL patients confirmed by qPCR for the presence of PML-RARα transcripts (n = 27) and studied their outcome in relation to the isoform expression at diagnosis and follow-up in King Fahad Medical City. Eight in twenty-seven patients showed bcr3 and nineteen patients with bcr1 as major isoforms at diagnosis. Half of the bcr3 patients (n = 4/8) showed early mortality, prolonged qPCR positivity, 4-fold higher neutrophil/lymphocyte ratio, higher creatinine levels, and significantly reduced relapse free and overall survival time compared with bcr1 patients. Radiological findings in bcr3 patients revealed CNS involvement in the form of intracranial haemorrhage and periventricular microangiopathy and no CNS involvement in bcr1 patients. In conclusion, PML-RARα isoform expression at diagnosis in selective patients influences disease course over time and may even lead to early mortality due to haemorrhage. Thus, timely reporting of the specific PML-RARα isoform by clinical laboratories and CNS assessment by radiology can prevent complications leading to death in some APL patients.

Clinical significance of increased PML-RARa transcripts after induction therapy for acute promyelocytic leukaemia

Objective: To analyze the clinical and biological significance of the acute promyelocytic leukemia (APL) whose PML-RARa transcripts increased after induction therapy.Methods: We analyzed 9 cases of APL whose PML-RARa transcripts increased after induction treatment and compare them with APL whose PML-RARa transcripts decreased.Results: The only factor affecting increased PML-RARa transcripts was the induction protocol. The cases of increased PML-RARa transcripts received induction treatment mainly based on ATRA and ATO. The evaluation of bone marrow aspirate cytology showed that the cell percentage from myelocyte to segmented neutrophil of the patients with increased PML-RARa transcripts was significantly higher than that of the patients with decreased PML-RARa transcripts. In the follow-up, MRD in 9 cases was consistently negative.Conclusions: Our studies showed the increased PML-RARa transcripts after induction treatment had different clinical significance from the decreased PML-RARa transcripts.

Acute Promyelocytic Leukemia: A Constellation of Molecular Events around a Single PML-RARA Fusion Gene

Although acute promyelocytic leukemia (APL) is one of the most characterized forms of acute myeloid leukemia (AML), the molecular mechanisms involved in the development and progression of this disease are still a matter of study. APL is defined by the PML-RARA rearrangement as a consequence of the translocation t(15;17)(q24;q21). However, this abnormality alone is not able to trigger the whole leukemic phenotype and secondary cooperating events might contribute to APL pathogenesis. Additional somatic mutations are known to occur recurrently in several genes, such as FLT3, WT1, NRAS and KRAS, whereas mutations in other common AML genes are rarely detected, resulting in a different molecular profile compared to other AML subtypes. How this mutational spectrum, including point mutations in the PML-RARA fusion gene, could contribute to the 10%–15% of relapsed or resistant APL patients is still unknown. Moreover, due to the uncertain impact of additional mutations on prognosis, the identification of the APL-specific genetic lesion is still the only method recommended in the routine evaluation/screening at diagnosis and for minimal residual disease (MRD) assessment. However, the gene expression profile of genes, such as ID1, BAALC, ERG, and KMT2E, once combined with the molecular events, might improve future prognostic models, allowing us to predict clinical outcomes and to categorize APL patients in different risk subsets, as recently reported. In this review, we will focus on the molecular characterization of APL patients at diagnosis, relapse and resistance, in both children and adults. We will also describe different standardized molecular approaches to study MRD, including those recently developed. Finally, we will discuss how novel molecular findings can improve the management of this disease.

Molecular remission as a therapeutic objective in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a subtype of acute leukemia characterized by a unique t(15;17) translocation generating the PML/RARA fusion gene and hybrid oncoprotein. Besides its critical role in leukemogenesis, this genetic aberration serves as a disease-specific biomarker for rapid diagnosis and monitoring of minimal residual disease (MRD). Moreover, PML/RARA is specifically targeted by All-trans retinoic acid (ATRA) and arsenic trioxide (ATO), two agents that synergistically act to induce degradation of the oncoprotein. Large clinical studies including two randomized trials conducted in newly diagnosed APL patients have shown that the ATRA-ATO combination is superior to conventional ATRA and chemotherapy both in terms of efficacy and safety. Preliminary studies using oral formulations of arsenic and ATRA suggest that oral arsenic is as effective and manageable as intravenous ATO. Following early retrospective studies indicating the prognostic relevance of PML/RARA monitoring, several prospective studies were conducted in large cohorts of APL patients enrolled in clinical trials with the aim of better assessing the prognostic value of longitudinal PCR testing. The results consistently showed that molecular remission (defined as negativization of the PCR test for PML/RARA) correlates with a significantly decreased risk of relapse, whereas persistence of PCR positivity for PML/RARA after consolidation or conversion from negative to positive during follow-up is strongly associated with hematologic relapse. Based on these data, various groups started using pre-emptive salvage therapy for patients who persisted PCR-positive after frontline consolidation or converted from negative to positive PCR during follow-up. Finally, several expert panels have recommended that molecular remission should be considered a therapeutic objective in APL, and molecular response has been adopted as a study endpoint in modern clinical trials.

Can Minimal Residual Disease Determination in Acute Myeloid Leukemia Be Used in Clinical Practice?

In acute myeloid leukemia (AML) that is in complete remission, minimal residual disease (MRD) is presumed to be present, though not morphologically evident. Advances in diagnostics now permit the detection and quantification of MRD in AML by several techniques. The level of MRD after induction and consolidation therapy correlates with disease sensitivity to chemotherapy and has greater power to predict long-term survival than patient and disease characteristics that are available at diagnosis, including genetic information. A unique advantage of MRD is that it is an integrated measure of the impact and interaction of genetics, epigenetics, host immune milieu, bone marrow environment, and drug sensitivity on disease response to treatment. Here, we review the main techniques for MRD assessment in AML, including polymerase chain reaction, multiparameter flow cytometry, and next-generation sequencing, with a focus on method-specific and general limitations to the optimal employment of MRD techniques for the determination of AML prognosis. We also review the data that establish the prognostic and predictive value of MRD assessment in AML. Finally, we provide recommendations for the use of MRD in the care of patients with AML in clinical practice today, including whether it should influence treatment decisions.

Acute WT1-positive promyelocytic leukemia with hypogranular variant morphology, bcr-3 isoform of PML-RARα and Flt3-ITD mutation: a rare case report

CONTEXT:

Acute promyelocytic leukemia (APL) accounts for 8% to 10% of cases of acute myeloid leukemia (AML). Remission in cases of high-risk APL is still difficult to achieve, and relapses occur readily.

CASE REPORT:

Here, we describe a case of APL with high white blood cell counts in blood tests and hypogranular variant morphology in bone marrow, together with fms-like tyrosine kinase-3 with internal tandem duplication mutations (FLT3-ITD), and bcr-3 isoform of PML-RARα. Most importantly, we detected high level of Wilms' tumor gene (WT1) in marrow blasts, through the reverse transcription polymerase chain reaction (RT-PCR). To date, no clear conclusions about an association between WT1 expression levels and APL have been reached. This patient successively received a combined treatment regimen consisting of hydroxycarbamide, arsenic trioxide and idarubicin plus cytarabine, which ultimately enabled complete remission. Unfortunately, he subsequently died of sudden massive hemoptysis because of pulmonary infection.

CONCLUSION:

Based on our findings and a review of the literature, abnormal functioning of WT1 may be a high-risk factor in cases of APL. Further studies aimed towards evaluating the impact of WT1 expression on the prognosis for APL patients are of interest.

Molecular Monitoring as a Path to Cure Acute Promyelocytic Leukemia

Acute promyelocytic leukemia (APL) is a molecularly well-defined disease, characterized by a specific chromosomal translocation; the improvement in biologic and clinical knowledge and subsequent introduction of molecularly targeted therapies have transformed the management of APL, with survival rates now exceeding 80%. Minimal residual disease (MRD) assessment in APL is the most important tool for its treatment; the prognostic role of the molecular detection of promyelocytic leukemia retinoic acid receptor α (PML-RARα) transcript after consolidation therapy in the early identification of the following hematologic relapse is now well established and guides preemptive therapy. First experiences performed with a qualitative polymerase chain reaction (PCR) approach were replaced with more accurate real-time quantitative PCR (RQ-PCR), which guarantees a numeric quantification of MRD. The identification of arsenic trioxide (ATO) as a valid therapy not only in relapsed patients but also as an alternative to standard therapy alone or in association with all-trans-retinoic acid enlarges the setting of validation of MRD evaluation in APL patients, considering a possible different clearance of PML-RARα with innovative therapy different from the standard ones. MRD monitoring demonstrated its validity also in the setting of relapsed patients with interesting results in the autologous and allogeneic stem cell transplantation setting or with the use of other biological agents. The aim of this review is to report and discuss the actual state of the art of MRD in APL.

Simple, rapid and accurate molecular diagnosis of acute promyelocytic leukemia by loop mediated amplification technology

The diagnostic work-up of acute promyelocytic leukemia (APL) includes the cytogenetic demonstration of the t(15;17) translocation and/or the PML-RARA chimeric transcript by RQ-PCR or RT-PCR. This latter assays provide suitable results in 3-6 hours. We describe here two new, rapid and specific assays that detect PML-RARA transcripts, based on the RT-QLAMP (Reverse Transcription-Quenching Loop-mediated Isothermal Amplification) technology in which RNA retrotranscription and cDNA amplification are carried out in a single tube with one enzyme at one temperature, in fluorescence and real time format. A single tube triplex assay detects bcr1 and bcr3 PML-RARA transcripts along with GUS housekeeping gene. A single tube duplex assay detects bcr2 and GUSB. In 73 APL cases, these assays detected in 16 minutes bcr1, bcr2 and bcr3 transcripts. All 81 non-APL samples were negative by RT-QLAMP for chimeric transcripts whereas GUSB was detectable. In 11 APL patients in which RT-PCR yielded equivocal breakpoint type results, RT-QLAMP assays unequivocally and accurately defined the breakpoint type (as confirmed by sequencing). Furthermore, RT-QLAMP could amplify two bcr2 transcripts with particularly extended PML exon 6 deletions not amplified by RQ-PCR. RT-QLAMP reproducible sensitivity is 10⁻³ for bcr1 and bcr3 and 10⁻2 for bcr2 thus making this assay particularly attractive at diagnosis and leaving RQ-PCR for the molecular monitoring of minimal residual disease during the follow up. In conclusion, PML-RARA RT-QLAMP compared to RT-PCR or RQ-PCR is a valid improvement to perform rapid, simple and accurate molecular diagnosis of APL.

Autologous is superior to allogeneic hematopoietic cell transplantation for acute promyelocytic leukemia in second complete remission

To identify favored choice of transplantation in patients with acute promyelocytic leukemia (APL) in second complete remission, we studied 294 patients with APL in second complete remission (CR2) receiving allogeneic (n = 232) or autologous (n = 62) hematopoietic cell transplantation (HCT) reported to the Center for International Blood and Marrow Transplantation Research (CIBMTR) from 1995 to 2006, including 155 with pre-HCT PML/RAR∝ status (49% of allogeneic and 66% of autologous). Patient characteristics and transplantation characteristics, including treatment-related mortality, overall survival (OS), and disease-free survival, were collected and analyzed for both univariate and multivariate outcomes. With median follow-up of 115 (allogeneic) and 72 months (autologous), 5-year disease-free survival (DFS) favored autologous with 63% (49% to 75%), compared with allogeneic at 50% (44% to 57%) (P = .10). OS was 75% (63% to 85%) versus 54% (48% to 61%) (P = .002), for autologous and allogeneic transplantation, respectively. Multivariate analysis showed significantly worse DFS after allogeneic HCT (hazard ratio [HR], 1.88; 95% confidence interval [CI], 1.16 to 3.06; P = .011) and age > 40 years (HR, 2.30; 95% CI, 1.44 to 3.67; P = .0005). OS was significantly worse after allogeneic HCT (HR, 2.66; 95% CI, 1.52 to 4.65; P= .0006); age > 40 (HR, 3.29; 95% CI, 1.95 to 5.54; P < .001), and first complete remission < 12 months (HR, 1.56; 95% CI, 1.07 to 2.26; P = .021). Positive pre-HCT PML-RAR∝ status in 17 of 114 allogeneic and 6 of 41 receiving autologous transplantation did not influence relapse, treatment failure, or survival in either group. The survival advantage for autografting was attributable to increased treatment-related mortality (TRM) in the allogeneic group of 30% compared to 2% in the autologous group, in addition to the added mortality associated with GVHD. We conclude that autologous HCT yields superior OS for APL in CR2. Long-term DFS in autologous recipients, even with minimal residual disease-positive grafts, remains an important subject for further study.

Treatment of an acute promyelocytic leukemia relapse using arsenic trioxide and all-trans-retinoic in a 6-year-old child

In adult therapy, arsenic trioxide (ATO) and all-trans-retinoic acid (ATRA) are recognized as active treatment of relapsed acute promyelocytic leukemia (APL). The efficacy of this combination in pediatric APL has not yet been well established. We report the case of a 6-year-old girl with relapsed APL, with a PML-RARα mutation, treated with a combination of ATO and ATRA. Over a period of 5 months, she received in total, 75 doses of intravenous ATO and 40 doses of oral ATRA. Currently, 22 months after relapse, she is still in complete remission. Here, we describe treatment of a relapsed APL in a child with limited treatment of ATO and ATRA and review the literature.

Children's Oncology Group's 2013 blueprint for research: acute myeloid leukemia

For the 365 children diagnosed with acute myeloid leukemia in the US annually, 5-year survival for patients on COG trials with low, intermediate, and high risk disease is 83%, 62%, and 23%, respectively. Recent advances include improved therapeutic stratification, improved survival with dose intensification, and further elucidation of the heterogeneity specific to childhood AML. These discoveries now guide current strategy incorporating targeted agents to pathways specific to childhood AML as well as evaluating methods to increase the sensitivity of the leukemic stem cell, first in Phase II feasibility trials followed by Phase III efficacy trials of the most promising agents. Acute myeloid leukemia in children, though with similar subgroups to adults, remains uniquely different based upon quite different prevalence of subtypes as well as overall response to therapy. The Children's Oncology Group's research agenda builds upon earlier efforts to better elucidate the leukemogenic steps distinct to childhood AML in order to more scientifically develop and test novel therapeutic approaches to the treatment and ultimate cure for children with this disorder. Pediatr Blood Cancer 2013; 60: 964-971. © 2012 Wiley Periodicals, Inc.

Early Death in Two Patients with Acute Promyelocytic Leukemia Presenting the bcr3 Isoform, FLT3-ITD Mutation, and Elevated WT1 Level

Despite major advances in the treatment of acute promyelocytic leukemia (APL), the problem of early death (ED) remains unsolved. Alongside the currently known clinical and hematological risk factors, prognostic significance has been attributed to internal tandem duplication mutations of the fms-like tyrosine kinase-3 (FLT3-ITD), hypogranular variant morphology, and the bcr-3 isoform of PML-RARα. We describe premature death of two patients with the hypogranular variant of APL who presented remarkably high expression levels of Wilms' tumor gene (WT1). Our results point to WT1 as an important prognostic factor of ED that needs to be promptly evaluated in all newly diagnosed cases of APL.

Promyelocytic leukemia nuclear bodies support a late step in DNA double-strand break repair by homologous recombination

The PML protein and PML nuclear bodies (PML-NB) are implicated in multiple cellular functions relevant to tumor suppression, including DNA damage response. In most cases of acute promyelocytic leukemia, the PML and retinoic acid receptor alpha (RARA) genes are translocated, resulting in expression of oncogenic PML-RARα fusion proteins. PML-NB fail to form normally, and promyelocytes remain in an undifferentiated, abnormally proliferative state. We examined the involvement of PML protein and PML-NB in homologous recombinational repair (HRR) of chromosomal DNA double-strand breaks. Transient overexpression of wild-type PML protein isoforms produced hugely enlarged or aggregated PML-NB and reduced HRR by ~2-fold, suggesting that HRR depends to some extent upon normal PML-NB structure. Knockdown of PML by RNA interference sharply attenuated formation of PML-NB and reduced HRR by up to 20-fold. However, PML-knockdown cells showed apparently normal induction of H2AX phosphorylation and RAD51 foci after DNA damage by ionizing radiation. These findings indicate that early steps in HRR, including recognition of DNA double-strand breaks, initial processing of ends, and assembly of single-stranded DNA/RAD51 nucleoprotein filaments, do not depend upon PML-NB. The HRR deficit in PML-depleted cells thus reflects inhibition of later steps in the repair pathway. Expression of PML-RARα fusion proteins disrupted PML-NB structure and reduced HRR by up to 10-fold, raising the possibility that defective HRR and resulting genomic instability may figure in the pathogenesis, progression and relapse of acute promyelocytic leukemia.

What happened to anti-CD33 therapy for acute myeloid leukemia?

CD33, a 67-kDa glycoprotein expressed on the majority of myeloid leukemia cells as well as on normal myeloid and monocytic precursors, has been an attractive target for monoclonal antibody (mAb)-based therapy of acute myeloid leukemia (AML). Lintuzumab, an unconjugated, humanized anti-CD33 mAb, has modest single-agent activity against AML but failed to improve patient outcomes in two randomized trials when combined with conventional chemotherapy. Gemtuzumab ozogamicin, an anti-CD33 mAb conjugated to the antitumor antibiotic calicheamicin, improved survival in a subset of AML patients when combined with standard chemotherapy, but safety concerns led to US marketing withdrawal. The activity of these agents confirms that CD33 remains a viable therapeutic target for AML. Strategies to improve the results of mAb-based therapies for AML include antibody engineering to enhance effector function, use of alternative drugs and chemical linkers to develop safer and more effective drug conjugates, and radioimmunotherapeutic approaches.

Ab therapy of AML: native anti-CD33 Ab and drug conjugates

MAb have become an important treatment modality in cancer therapy.Genetically engineered chimeric and humanized Ab have demonstrated activity against a variety of tumors. While the humanized anti-CD33MAb lintuzumab has only modest single-agent activity against overt AML, it can eliminate minimal residual disease detectable by reverse transcription-PCR in acute promyelocytic leukemia. Targeted chemotherapy with the anti−CD33−calicheamicin construct gemtuzumab ozogamicin has produced remissions in patients with relapsed AML and appears promising when used in combination with standard chemotherapy in the treatment of newly diagnosed AML.

Acute promyelocytic leukemia: an experience on 95 greek patients treated in the all-trans-retinoic Acid era

Acute promyelocytic leukemia (APL) is highly curable with the combination of all-transretinoic acid (ATRA) and anthracycline based chemotherapy, but the percentage of early deaths remains high. In the present study, we report the clinical, immunophenotypic, cytogenetic and molecular characteristics and outcome of APL patients diagnosed and treated in various Hospitals of Greece and Cyprus.We describe the data of ninety-five APL patients who were diagnosed during the last 15 years. Seven (7.4%) newly diagnosed APL patients died due to intracranial hemorrhage within 72 hours of presentation. All but two patients were induced with ATRA alone or ATRA plus chemotherapy. The early death rate was 14.9%. After induction all 80 evaluable patients achieved complete hematologic remission. The cumulative incidence of relapse was 18.3%. Eight of the ten relapsed patients were successfully salvaged, while both patients with molecularly resistant disease died during salvage treatment. Overall survival (OS) at 5 years was 78.4% and disease free survival (DFS) 73.6%. In multivariate analysis of OS age over 60 years, DIC at diagnosis and marginally major hemorrhage at presentation were identified as adverse prognostic factors. In the subgroup of patients with available data on FLT3 mutation status (49 out of 94), ITD positivity also remained as an independent prognostic factor in the final model of OS, together with major hemorrhage and marginally high Sanz score. We found a close correlation between the CD2 expression and the development of the differentiation syndrome (DS). In conclusion, the main problem in managing patients with APL is still the high early death rate.

Development of real-time quantitative polymerase chain reaction assays to track treatment response in retinoid resistant acute promyelocytic leukemia

Molecular detection of minimal residual disease (MRD) has become established to assess remission status and guide therapy in patients with ProMyelocytic Leukemia-RARA+ acute promyelocytic leukemia (APL). However, there are few data on tracking disease response in patients with rarer retinoid resistant subtypes of APL, characterized by PLZF-RARA and STAT5b-RARA. Despite their rarity (<1% of APL) we identified 6 cases (PLZF-RARA, n = 5; STAT5b-RARA, n = 1), established the respective breakpoint junction regions and designed reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) assays to detect leukemic transcripts. The relative level of fusion gene expression in diagnostic samples was comparable to that observed in t(15;17) - associated APL, affording assay sensitivities of ∼1 in 10(4)-10(5). Serial samples were available from two PLZF-RARA APL patients. One showed persistent polymerase chain reaction positivity, predicting subsequent relapse, and remains in CR2, ∼11 years post-autograft. The other, achieved molecular remission (CRm) with combination chemotherapy, remaining in CR1 at 6 years. The STAT5b-RARA patient failed to achieve CRm following frontline combination chemotherapy and ultimately proceeded to allogeneic transplant on the basis of a steadily rising fusion transcript level. These data highlight the potential of RT-qPCR detection of MRD to facilitate development of more individualized approaches to the management of rarer molecularly defined subsets of acute leukemia.

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.

Long-term survey of outcome in acute promyelocytic leukemia: a single center experience in 340 patients

The aims of this study are to investigate the outcome and prognostic factors influencing long-term survival on patients with acute promyelocytic leukemia (APL). A total of 340 APL patients admitted to the Department of Hematology from January 1988 to December 2009 were enrolled in this study. All patients received all-trans retinoic acid (ATRA) and/or arsenic trioxide (ATO) with anthracycline-based induction therapy. After three courses of consolidation chemotherapy, 279 patients received 2 years of maintenance therapy. Survival analyses were carried out using the Kaplan-Meier method and the Cox regression model. In total, 288 achieved CR with the CR rate of 84.7%, and 50 patients died during induction therapy. Univariate analysis identified the following three risk factors for hemorrhagic mortality: fibrinogen level (<1.0 g/l) (P = 0.0007), initial peripheral WBC count(>4 × 10(9)/l) (P = 0.0001), as well as the presence of coagulopathy(P < 0.0001). With a median follow-up of 49 (6-255) months, the estimated 5-year overall survival (OS) and relapse-free survival (RFS) were (89.0 ± 2.4)% and (83.7 ± 2.6)%, respectively. Cox regression analysis of the 290 patients showed initial WBC count, years of diagnosis, and the status of PML-RARα in remission seemed to be independent prognostic indicators for OS and RFS (P = 0.03, P < 0.01 and P = 0.0001, respectively). Cytogenetics in addition to above three variables remained significant for RFS (P = 0.01). Our retrospective observations suggest that the combination of ATRA and/or ATO with anthracycline-based therapy may have useful implications in the perspective of long-term prognosis for adult APL.

Utilization of molecular phenotypes to detect relapse and optimize the management of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is characterized by a unique genetic aberration, the t(15;17) chromosome translocation. Translocation breakpoints are located within the promyelocytic leukemia (PML) locus on chromosome 15 and the retinoic acid receptor alpha (RARA) locus on chromosome 17. In the past 2 decades, critical advances have been made in understanding the molecular pathogenesis of APL. APL represents a paradigm for molecularly targeted therapy in cancer and an extraordinary model for translational research in medicine. In fact, the release of differentiation block upon treatment of APL with all-trans-retinoic acid (ATRA) has represented the first example of targeted therapy in human cancer. More recently, the advent of arsenic trioxide (ATO) has allowed further progress in the management of this disease through improved outcomes in patients receiving this agent in combination with ATRA. Finally, optimization of therapy and minimization of toxicity is feasible in this disease through careful monitoring of residual disease using polymerase chain reaction-based approaches targeting the PML-RARA fusion gene.

Quantitative detection of PML-RARalpha fusion transcript by real-time PCR with a single primer pair

Quantitative detection of minimal residual disease has prognostic value for some leukemias. Acute promyelocytic leukemia (APL) is characterized by the specific PML-RARalpha fusion gene from t(15;17). Added to three PML-RARalpha isoforms, alternative spliced forms of PML exons give rise to multiple isoforms even within a single patient. To date, multiple primer pairs for the detection of the various PML-RARalpha transcripts have been designed, potentially generating some nonspecific amplification products. Here, we established a real-time quantitative PCR (RQ-PCR) strategy with a single primer pair using LightCycler (sp-RQ-PCR), which could simultaneously detect three isoforms with equal specificity and sensitivity as well as alternative spliced forms. Results obtained with sp-RQ-PCR for 39 samples from 15 APL patients and 31 non-APL samples were compared with those with TaqMan assay with three primer pairs. In two of the APL samples, PML-RARalpha was detected in the TM, but not in the sp-RQ-PCR or nested PCR. Furthermore, the sp-RQ-PCR showed no positive results for the 31 non-APL samples, whereas the TM identified 13% (4/31) as positive. Electrophoresis detected some artifacts in the TM, which do not correspond to PML-RARalpha. We conclude that our sp-RQ-PCR is specific enough to identify various forms of PML-RARalpha and yields no false-positive results.

Minimal residual disease levels assessed by NPM1 mutation-specific RQ-PCR provide important prognostic information in AML

Nucleophosmin (NPM1)-mutated acute myeloid leukemia (AML), which is recognized as a provisional entity in the World Health Organization 2008 classification of myeloid neoplasms, accounts for 30% of AML. We analyzed 1227 diagnostic and follow-up samples in 252 NPM1-mutated AML patients with 17 different NPM1 mutation-specific real-time quantitative polymerase chain reaction (RQ-PCR) assays. Paired diagnostic/relapse samples of 84 patients revealed stable NPM1 mutations in all cases, suggesting that they are pathogenetically early events and thus applicable for minimal residual disease detection. A total of 47 relapses were predictable because of an NPM1 mutation level (%NPM1/ABL1) increase of at least 1 log or in 15 cases because of NPM1 mutation levels not decreasing less than 3 log ranges. A high prognostic value of NPM1 levels was shown for 4 different intervals after therapy was initiated. Furthermore, thresholds of 0.1 and 0.01%NPM1/ABL1 during/after treatment discriminated between prognostic subgroups. Univariate analyses, including age, white blood cell count, blast count, CD34 positivity, FLT3 mutations status, FAB type, karyotype, NPM1 mutation type, and pretreatment NPM1 mutational level, showed that, besides NPM1 mutation level, only age and FLT3-LM mutation status were prognostically significant for EFS. Multivariate analysis, including age, FLT3-LM status, and NPM1 mutation level at different time points, demonstrated that NPM1 level was the most relevant prognostic factor during first-line treatment. Similar results were obtained in patients undergoing second-line chemotherapy or allogeneic stem cell transplantation.

Acute promyelocytic leukemia: from highly fatal to highly curable

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia. Morphologically, it is identified as the M3 subtype of acute myeloid leukemia by the French-American-British classification and cytogenetically is characterized by a balanced reciprocal translocation between chromosomes 15 and 17, which results in the fusion between promyelocytic leukemia (PML) gene and retinoic acid receptor alpha (RARalpha). It seems that the disease is the most malignant form of acute leukemia with a severe bleeding tendency and a fatal course of only weeks. Chemotherapy (CT; daunorubicin, idarubicin and cytosine arabinoside) was the front-line treatment of APL with a complete remission (CR) rate of 75% to 80% in newly diagnosed patients. Despite all these progresses, the median duration of remission ranged from 11 to 25 months and only 35% to 45% of the patients could be cured by CT. Since the introduction of all-trans retinoic acid (ATRA) in the treatment and optimization of the ATRA-based regimens, the CR rate was raised up to 90% to 95% and 5-year disease free survival (DFS) to 74%. The use of arsenic trioxide (ATO) since early 1990s further improved the clinical outcome of refractory or relapsed as well as newly diagnosed APL. In this article, we review the history of introduction of ATRA and ATO into clinical use and the mechanistic studies in understanding this model of cancer targeted therapy.

A comprehensive review of acute promyelocytic leukemia in children

The outcome of patients with acute promyelocytic leukemia (APL) has substantially improved since the successful introduction of tretinoin, and nowadays combining tretinoin with chemotherapy is potentially curative for at least 70-75% of patients with newly diagnosed APL. In most pediatric series, APL represents < or = 10% of childhood acute myelogenous leukemia. APL in children is more common in girls and in obese children. It is characterized by a higher incidence of hyperleukocytosis, an increased incidence of microgranular morphology and by more frequent occurrence of the PML/RARalpha isoforms bcr 2 and bcr 3 compared to adults. Tretinoin-based therapy is curative for the majority of children with APL. Recent data indicate that > or = 2 negative RT-PCR assays for PML/RARalpha on bone marrow performed at least 1 month apart after completing therapy are strongly associated with long-term remissions, while conversion to PCR positivity for PML/RARalpha during remission is highly predictive of impending relapse. Data from recent studies in adults and limited data from children show that arsenic trioxide is the single most effective agent in APL and deserves immediate study in newly diagnosed children in an effort to further improve prognosis and to limit exposure to conventional cytotoxic chemotherapy.

Diagnosis and treatment of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL), characterized by a translocation between the promyelocytic leukemia gene (PML) on chromosome 15 and the retinoic acid receptor-alpha (RARalpha) gene on chromosome 17, has become a model for targeted treatment of cancer. Advances in our understanding of the fundamental biology of this disease have led to the development of tools for diagnosis, monitoring of minimal residual disease, and detection of early relapse. Differentiation therapy with all-trans retinoic acid in combination with chemotherapy has significantly improved survival in patients with APL. Moreover, arsenic trioxide, which induces differentiation and apoptosis of APL cells, has become standard treatment for relapsed disease, and its role in the treatment of newly diagnosed APL is under active investigation. The lessons learned from APL have broad applications to other forms of leukemia and to cancer in general, whereby molecularly targeted therapy is directed to specifically defined subgroups.

Clinical implications of minimal residual disease monitoring by quantitative polymerase chain reaction in acute myeloid leukemia patients bearing nucleophosmin (NPM1) mutations

To explore the validity and prognostic significance of minimal residual disease detection by quantitative polymerase chain reaction (qPCR) in patients of acute myeloid leukemia (AML) bearing Nucleophosmin (NPM1) mutations, we quantified mutants in 194 bone marrow samples from 38 patients with a median follow-up time of 20.6 months. Following induction chemotherapy, a median of 2.78 log decline in mutant copy number was observed. Relapse was always accompanied by significant increase of mutant numbers (P<0.001). After achieving complete remission (CR), the mutant copy number was significantly higher in patients with subsequent relapse than in those remaining in continuous CR (P<0.001). Presence of detectable mutants after treatment predicted relapse if no further chemotherapy was administered. Furthermore, the patients with any rise of mutant signals during serial follow-up had 3.2-fold increase of relapse risk compared to those with persistently low or undetectable signals (P<0.001). Patients who could achieve mutant reduction to <0.1% of internal control had significantly longer overall survival (OS) (P=0.004) and relapse-free survival (RFS) (P<0.001). Failure to achieve 2 logs of reduction after consolidation predicted shorter OS (P=0.01) and RFS (P=0.001). In conclusion, qPCR monitoring may have prognostic impact in AML patients with NPM1 mutations.

Monoclonal antibody therapy of APL

Acute promyelocytic leukemia (APL) is a rare subtype of acute myeloid leukemia (AML) for which a number of targeted therapies have been developed. The "targets" have included both genotypic and phenotypic features of the disease. The application of monoclonal antibodies (MAbs) to this disease to date have been limited to a relatively small number of studies where this therapy has been used to supplement effective approaches to the disease. The preliminary results have been promising, and further development of this modality as an effective adjunct to existing treatment regimens will most certainly occur in the near future.

Outcome of patients with acute promyelocytic leukemia failing to front-line treatment with all-trans retinoic acid and anthracycline-based chemotherapy (PETHEMA protocols LPA96 and LPA99): benefit of an early intervention

To determine prognosis of acute promyelocytic leukemia (APL) failing to front-line therapy with all-trans retinoic acid (ATRA) and anthracyclines, outcome of 52 patients (32 M/20 F; age: 37, 3-72) included in PETHEMA trials LPA96 and LPA99 who presented with either molecular failure (MOLrel, n=16) or hematological relapse (HEMrel, n=36) was analyzed. Salvage therapy consisted of ATRA and high-dose ara-C-based chemotherapy (HDAC) in most cases (83%), followed by stem-cell transplantation (autologous, 18; allogeneic, 10; syngeneic, 1). Fourteen patients with MOLrel (88%) achieved second molecular complete response (molCR), whereas 81% HEMrel patients responded to second-line treatment, with 58% molCR. After median follow-up of 45 months, four MOLrel and 18 HEMrel patients, respectively, experienced a second relapse. Outcome after MOLrel compared favorably to HEMrel, with longer survival (5-year survival: 64+/-14 vs 24+/-8%, P=0.01) and lower relapse risk (5-year relapse risk: 30+/-13 vs 64+/-9%; P=0.044). Additionally, age <or=40 and male gender were favorable variables for survival, whereas molecular response predicted longer leukemia-free survival. In conclusion, early institution of salvage therapy at molecular failure, before onset of hematological relapse, is beneficial in APL. Moreover, given the poor outcome of HEMrel managed with ATRA and HDAC, use of alternative therapeutic strategies in this setting is warranted.

Front line clinical trials and minimal residual disease monitoring in acute promyelocytic leukemia

In spite of the very high cure rate (70%-80%) achieved in APL with combinatorial all-trans retinoic acid (ATRA) and anthracycline-based chemotherapy regimens, a number of issues are still open for investigation in front-line therapy of this disease. These include, among others, improvements in early death rate, the role of arsenic trioxide (ATO) and maintenance treatment, and, finally, optimization of molecular monitoring to better identify patients at increased risk of relapse. The current consensus on the most appropriate induction therapy consists of the concomitant administration of ATRA and anthracycline-based chemotherapy. Although the antileukemic benefit provided by the addition of ATRA to consolidation therapy has not been demonstrated in randomized studies, historical comparisons of consecutive studies carried out by Spanish and Italian cooperative groups suggest that the combination of ATRA and chemotherapy for consolidation may also contribute to improving therapeutic results. While a variety of distinct treatments are being investigated for front-line therapy, most experts agree that a risk-adapted therapy represents the optimal approach, through the use of more intensive therapy in patients with initial hyperleukocytosis. Longitudinal RT-PCR of PML/RARalpha allows sensitive assessment of response to treatment and minimal residual disease (MRD) monitoring in APL. Achievement of negative PCR status or molecular remission at the end of consolidation is now universally accepted and recommended as a therapeutic objective in this disease. On the other hand, persistence of, or conversion to, PCR positive in the marrow during follow-up is associated with impending relapse. Preliminary studies on therapy of molecular relapse indicate a survival advantage as compared to administering salvage treatment at time of hematologic relapse. The more accurate and reproducible real-time PCR method to detect at quantitative levels the PML/RARalpha hybrid will likely provide better inter-laboratory standardization and trial results comparison in the near future.

Treatment of acute promyelocytic leukemia by retinoids

We review the role of all-trans retinoic acid (ATRA) in the treatment of acute promyelocytic leukemia (APL). The combination of ATRA and conventional anthracycline-ARA-C chemotherapy (CT) has clearly demonstrated its superiority over CT alone (in terms of relapse and survival) in newly diagnosed APL. Combination treatment probably also reduces the incidence of initial failures, and complete remission (CR) rates greater than 90% are now regularly reported in large multicenter trials. Some randomized studies strongly suggest that prolonged maintenance treatment (for 1 or 2 years) with ATRA and low-dose CT, and possibly very early introduction of anthracycline CT during induction treatment, may reduce the incidence of relapse. With those treatments, the relapse risk appears to be only 10%-15%, although it remains greater in patients who initially have high white blood cell counts (often associated with variant M3 morphology, short bcr3 isoform, etc.) and patients with residual disease detectable by RT-PCR at the end of consolidation courses. In those patients, addition of arsenic derivatives to induction or consolidation treatment (or both treatments together) may prove useful and is currently being tested. ATRA syndrome (now generally called APL differentiation syndrome, as it is also seen with arsenic derivatives) remains the major side effect of ATRA treatment. It occurs in 10%-15% of patients and is currently fatal in at least 10% of them. Rapid onset of CT or high dose steroids (or both) should improve its outcome. A sizeable proportion of APL patients who relapse after ATRA and CT can be durably salvaged by the same treatment followed by allogeneic or autologous stem cell transplantation, provided the transplant (in the autologous setting) is RT-PCR-negative. However, in relapsing APL arsenic derivatives (mainly arsenic trioxide) are now considered to be the reference treatment. Some of the current issues with ATRA treatment in newly diagnosed APL include whether ATRA has a role during consolidation treatment and whether arabinoside (AraC) is required in addition to anthracyclines in the chemotherapy combined to ATRA.

Relapse of acute promyelocytic leukemia with PML-RARalpha mutant subclones independent of proximate all-trans retinoic acid selection pressure

Relapse of acute promyelocytic leukemia (APL) following all-trans retinoic acid (ATRA) therapy has been associated with the acquisition of mutations in the high-affinity ATRA binding site in PML-RARalpha, but little information is available about the selection dynamics of the mutation-harboring subclones. In this study, 6/18 patients treated with sequential ATRA and chemotherapy on protocol INT0129 relapsed with complete replacement of the nonmutant pretreatment APL cell population by a PML-RARalpha mutant subclone. Two patients relapsed in proximity of ATRA treatment; however, in four patients there was a 6-48 month hiatus between the last ATRA treatment and relapse. The mutant subclones were not detectable in samples tested > or = 3 months before relapse at > or = 1 in 10(2) (10(-2)) sensitivity. In one patient, a functionally weak mutation was detected at 10(-4) sensitivity before therapy but only limited pre-relapse enrichment of the mutant subclone was observed on subsequent ATRA therapy. These results indicate that proximate ATRA selection pressure is frequently not the main determinant for the emergence of strongly dominant PML-RARalpha mutant subclones and suggest that APL subclones harboring PML-RARalpha mutations are predisposed to the acquisition of secondary genetic/epigenetic alterations that result in a growth/survival advantage.

PML-RARalpha fusion gene transcripts and biological features in acute promyelocytic leukemia patients

Acute promyelocytic leukemia (APL) is characterized by the presence of rearrangements involving the retinoic acid receptor alpha (RARalpha) gene and a variable incidence in different populations. The hybrid gene PML-RARalpha, present in 98% of cases, encodes a fusion protein essential to the pathogenesis of the disease. Depending of the PML's gene breakpoint in chromosome 15, the transcript subtypes bcr1, bcr2 and bcr3 may be formed. The correlation between these transcript subtypes and clinical parameters is still controversial. The objective of this study was to determine the frequencies of the PML-RARalpha transcripts and subtypes in a series of 32 APL patients from Northeast Brazil and to evaluate the association of these subtypes to different parameters. The method used was RT-PCR. The frequency of our APL cases is approximately 28% of the acute leukemias. The results showed the presence of PML-RARalpha isoform in all patients and a higher frequency of the bcr1/2 subtype. No significant statistical association was found between molecular subtypes and age, sex, French-American-British (FAB) classification, leukocyte and platelet count, hemoglobin level or coagulation tests. In conclusion, these data suggest similar molecular and biological features for our APL patients at diagnosis in comparison with those reported in current scientific literature.

Molecular response in acute promyelocytic leukemia: a direct comparison of regular and real-time RT-PCR

Evaluation of molecular response is important for the diagnosis and monitoring of minimal residual disease in patients with acute promyelocytic leukemia (APL). In this study, we analyzed the molecular response by regular reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR in 31 newly diagnosed patients. The real-time RT-PCR results are reported as normalized DoseN and log-reduction (3.0-4.9 log-reduction as minor and > or =5.0 log-reduction as major molecular response). After induction therapy and completion of consolidation, minor molecular response was documented in 35.5 and 96.8% patients, respectively, which was equivalent to the regular RT-PCR (22.6 and 96.8%), whereas the major molecular response rate was significantly lower (12.9 and 90.3%, respectively). All patients achieved major molecular response during and after maintenance therapy. During the follow-up study, loss of major molecular response was observed in two patients, which was associated with subsequent loss of minor molecular response, positive RT-PCR and then documentation of central nervous system leukemia or clinical relapse in 3-6 months. For summary, we demonstrated that the real-time RT-PCR is potentially superior to regular RT-PCR in evaluation of molecular response in APL patients and that reporting real-time RT-PCR data by log-reduction is feasible and clinically relevant.

Adult acute myeloid leukemia

Acute myeloid leukemia (AML) is a group of several different diseases, the treatment and outcome of which depend on several factors, including leukemia karyotype, patient age, and comorbid conditions. Despite advances in understanding the molecular biology of AML, its treatment remains challenging. Standard regimens using cytarabine and anthracyclines for induction followed by some form of postremission therapy produce response rates of 60% to 70%, with less than 20% of all patients achieving long-term disease-free survival. New therapies are emerging based on the definition of specific cytogenetic-molecular abnormalities. Such targeted therapies offer the promise of better antileukemic activity in adult AML.

Pro-proliferative function of the long isoform of PML-RARalpha involved in acute promyelocytic leukemia

The promyelocytic leukemia (PML) gene codes for a tumor suppressor protein that is associated with distinct subnuclear macromolecular structures called the PML bodies. The PML gene is frequently involved in the t(15;17) chromosomal translocation of acute promyelocytic leukemia (APL). The translocation results in a fusion gene product, PML-RARalpha, in which the PML gene fuses to the retinoic acid receptor alpha (RARalpha) gene. PML-RARalpha has been shown to promote transcriptional repression of genes involved in myeloid terminal differentiation and to disrupt the architecture of PML bodies, a phenotype reversed by treatment with all trans retinoic acid (ATRA). However, there are several alternatively spliced isoforms of PML-RARalpha. Here, we addressed the differences between the short and the long isoforms of PML-RARalpha (L and S) since both are associated with APL. We demonstrate that PML-RARalphaL, but not PML-RARalphaS, can directly promote cell growth by transcriptionally activating the pro-proliferative gene, c-fos, in response to mitogenic stimulation. The activity of the PML-RARalphaL is completely sensitive to ATRA. We further show that this activation is not via direct recruitment of the protein to the c-fos promoter but indirectly by altering the chromosomal environment of the c-fos gene, thereby rendering it more accessible to the signal induced transcriptional activators. Our results suggest that in addition to antagonizing the PML-tumor suppressor or the PML-pro-apoptotic activity, PML-RARalpha proteins can also directly promote cell growth by activating c-fos.

Latent acute promyelocytic leukemia t(15;17)(q22;q12-21) and sarcoidosis: long-term cohabitation

The association of sarcoidosis with hematological malignancies is a well-known phenomenon. To our knowledge, we report the first case involving sarcoidosis and acute promyelocytic leukemia (APL) t(15;17)(q22;q12-21). The major interest lies in the chronology of the two diseases: the APL demonstrated an unusual smoldering evolution, suggesting that pre-existing sarcoidosis may have a non-fortuitous immunological impact on leukemic clone proliferation.

Monitoring of minimal residual disease in acute myeloid leukemia

Monitoring minimal residual disease (MRD) becomes increasingly important in the risk-adapted management of patients with acute myeloid leukemia (AML). The two most sensitive and quantitative methods for MRD detection are multiparameter flow cytometry (MFC) and real-time polymerase chain reaction (QRT-PCR). Fusion gene-specific PCR in AML is based on the RNA level, and thus in contrast to MFC expression levels rather than cell counts are assessed. For both methods independent prognostic values have been shown. The strong power of MFC has been shown mainly in the assessment of early clearance of the malignant clone. MRD levels in AML with fusion genes have the strongest prognostic power after the end of consolidation therapy. In addition, with QRT-PCR highly predictive initial expression levels can be assessed. With both methods early detection of relapse is possible. So far, validated PCR-based MRD was done with fusion genes that are detectable in only 20-25% of all AML MFC is superior since it is applicable for most AML. However, QRT-PCR is still more sensitive in most cases. Thus, it is desirable to establish new molecular markers for PCR-based studies. Large clinical trials will determine the role and place of immunologic and PCR-based monitoring in the prognostic stratification of patients with AML.

Tricks of the trade for the appropriate management of newly diagnosed acute promyelocytic leukemia

Most reviews on the state-of-the-art treatment in acute promyelocytic leukemia (APL) have focused mainly on the comparison of therapeutic approaches, including all-trans retinoic acid (ATRA) and chemotherapy. However, outcome of individual patients also depends on appropriate knowledge of several aspects related to APL management that are less appreciated and/or are underestimated in the literature. These aspects include appropriate diagnostic strategy, use of supportive care, early recognition and treatment of life-threatening complications typically associated with APL and its specific treatment, tools and timing for adequate evaluation of response, and, finally, management of the disease in special conditions such as older patients and pregnant women. Besides reviewing current consensus and controversies on the use of ATRA and chemotherapy in the distinct treatment phases (eg, induction, consolidation, maintenance), this article addresses the aforementioned issues on APL management (“tricks of the trade”) with special emphasis on several peculiar aspects that distinguish APL from other acute myeloid leukemias.

Identification of cell lineages involved by t(15;17) in acute promyelocytic leukemia by combined fluorescence activated cell sorting and FISH

Bone marrow cells from five patients with acute promyelocytic leukemia (APL) with t(15;17) were studied by a combination of fluorescence activated cell sorting and fluorescence in situ hybridization (FISH) to establish the cell lineage involvement of t(15;17). Interphase FISH demonstrated that the fusion gene (PML/RARA) was present in almost all abnormal promyelocytes. In one case, the translocation was demonstrated in both CD34+ and CD34- APL cells. The t(15;17) abnormality was not detectable in erythroblasts nor in T- or B-lymphoid cells. These results suggest that lymphocytes and erythroblasts are not clonally involved in APL, and that malignant transformation in some cases of APL may occur at the level of CD34+ cells.