Implication of prior treatment with drug combinations including inhibitors of topoisomerase II in therapy-related monocytic leukemia with a 9;11 translocation

Acute Monoblastic Leukemia with t(9;11) in a Patient Receiving Chemotherapy for Ovarian Cancer: Secondary Leukemia or Fortuitous Association of Two Neoplasias?

We describe the clinical course of a 61 years old patient who developed fulminant acute monoblastic leukemia (MSA, FAB) while still on systemic chemotherapy for an advanced adenocarcinoma of the ovary. The leukemia developed following chemotherapy with Cyclophosphamide and Cisplatin (9 cycles), and then Cyclophosphamide and Carboplatin (6 cycles) resulting in a partial remission of the ovarian tumor. Survival from the onset of acute leukemia was extremely short and the cause of death was intracerebral bleeding. Cytogenetic analysis of the leukemic cells revealed a 9:11 translocation, a pattern usually occurring in de novo leukemias. The literature on the association of solid tumors, particularly ovarian cancer, with acute leukemia is reviewed; while systemic chemotherapy for ovarian cancer definitely increases the risk of secondary leukemia, especially if alkylating agents are administered, the occurrence of leukemia during the administration of chemotherapy for the solid tumor is distinctly unusual. Our patient seems to be an additional example of a recently described therapy-related variant of acute leukemia developing shortly after the onset of chemotherapy and characterized by a rapidly downhill clinical course, a monocytic lineage and a cytogenetic rearrangement t(9;11)(p22;q23). While the number of these newly described cases is still small, this additional example should increase the awareness of this potential association in Cisplatin-treated patients.

A "Double-Edged" Scaffold: Antitumor Power within the Antibacterial Quinolone

In the late 1980s, reports emerged describing experimental antibacterial quinolones having significant potency against eukaryotic Type II topoisomerases (topo II) and showing cytotoxic activity against tumor cell lines. As a result, several pharmaceutical companies initiated quinolone anticancer programs to explore the potential of this class in comparison to conventional human topo II inhibiting antitumor drugs such as doxorubicin and etoposide. In this review, we present a modern re-evaluation of the anticancer potential of the quinolone class in the context of today's predominantly pathway-based (rather than cytotoxicity-based) oncology drug R&D environment. The quinolone eukaryotic SAR is comprehensively discussed, contrasted with the corresponding prokaryotic data, and merged with recent structural biology information which is now beginning to help explain the basis for that SAR. Quinolone topo II inhibitors appear to be much less susceptible to efflux-mediated resistance, a current limitation of therapy with conventional agents. Recent advances in the biological understanding of human topo II isoforms suggest that significant progress might now be made in overcoming two other treatment-limiting disadvantages of conventional topo II inhibitors, namely cardiotoxicity and drug-induced secondary leukemias. We propose that quinolone class topo II inhibitors could have a useful future therapeutic role due to the continued need for effective topo II drugs in many cancer treatment settings, and due to the recent biological and structural advances which can now provide, for the first time, specific guidance for the design of a new class of inhibitors potentially superior to existing agents.

Acute Myeloid Leukemia Diagnosed 5 Years after Adult T-Cell Leukemia/Lymphoma

A case of secondary acute myeloid leukemia (AML) was identified following adult T-cell leukemia/lymphoma (ATL), for which combination chemotherapy had been administered, including epipodophyllotoxin, anthracycline, and alkylating agents. AML with maturation was diagnosed by the cytological findings, cell surface markers, and chromosomal abnormalities. We previously reported two cases of AML accompanied by ATL. In this case of AML after chemotherapy for ATL, we considered that the AML was probably associated with previous chemotherapy for ATL. Although the ATL remained in remission, the therapy-related AML with complex chromosomal abnormalities proved resistant to chemotherapy, and the patient died from complications associated with AML.

TDP2-dependent non-homologous end-joining protects against topoisomerase II-induced DNA breaks and genome instability in cells and in vivo

Anticancer topoisomerase "poisons" exploit the break-and-rejoining mechanism of topoisomerase II (TOP2) to generate TOP2-linked DNA double-strand breaks (DSBs). This characteristic underlies the clinical efficacy of TOP2 poisons, but is also implicated in chromosomal translocations and genome instability associated with secondary, treatment-related, haematological malignancy. Despite this relevance for cancer therapy, the mechanistic aspects governing repair of TOP2-induced DSBs and the physiological consequences that absent or aberrant repair can have are still poorly understood. To address these deficits, we employed cells and mice lacking tyrosyl DNA phosphodiesterase 2 (TDP2), an enzyme that hydrolyses 5'-phosphotyrosyl bonds at TOP2-associated DSBs, and studied their response to TOP2 poisons. Our results demonstrate that TDP2 functions in non-homologous end-joining (NHEJ) and liberates DSB termini that are competent for ligation. Moreover, we show that the absence of TDP2 in cells impairs not only the capacity to repair TOP2-induced DSBs but also the accuracy of the process, thus compromising genome integrity. Most importantly, we find this TDP2-dependent NHEJ mechanism to be physiologically relevant, as Tdp2-deleted mice are sensitive to TOP2-induced damage, displaying marked lymphoid toxicity, severe intestinal damage, and increased genome instability in the bone marrow. Collectively, our data reveal TDP2-mediated error-free NHEJ as an efficient and accurate mechanism to repair TOP2-induced DSBs. Given the widespread use of TOP2 poisons in cancer chemotherapy, this raises the possibility of TDP2 being an important etiological factor in the response of tumours to this type of agent and in the development of treatment-related malignancy.

Therapy-related acute myelogenous leukemia with an 11q23/MLL translocation following adjuvant cisplatin and vinorelbine for non-small-cell lung cancer

Cisplatin-based chemotherapy has become an accepted standard in the adjuvant treatment of non-small-cell lung cancer (NSCLC). We present a case of acute myelogenous leukemia with an 11q23/MLL rearrangement diagnosed 1 year after the completion of 4 cycles of cisplatin and vinorelbine for resected NSCLC. To our knowledge, this is the first case of therapy-related acute myelogenous leukemia (t-AML) associated with this chemotherapy combination. The literature on t-AML with the 11q23/MLL rearrangement is reviewed.

DNA cleavage and Trp53 differentially affect SINE transcription

Among the cellular responses observed following treatment with DNA-damaging agents is the activation of Short Interspersed Elements (SINEs; retrotransposable genetic elements that comprise over 10% of the human genome). By placing a human SINE (the Alu element) into murine cells, we have previously shown that DNA-damaging agents such as etoposide can induce both upregulation of SINE transcript levels and SINE retrotransposition. A similarly cytotoxic (but not genotoxic) exposure to vincristine was not associated with SINE activation. Here we demonstrate that multiple other genotoxic exposures are associated with upregulation of SINE transcript levels. By comparing the effects of similarly cytotoxic doses of the topoisomerase II inhibitors etoposide and merbarone, we confirm that DNA strand breakage is specifically associated with SINE induction. By evaluating transcription rate and RNA stability, we demonstrate that SINE induction by genotoxic exposure is associated with transcriptional induction and not with transcript stabilization. Finally we demonstrate that SINE induction by genotoxic stress is mediated by a Trp53-independent pathway, and in fact that Trp53 plays an inhibitory role in attenuating the transcriptional induction of SINE elements following exposure to a genotoxic agent. Together these data support a model in which initial DNA damage can trigger genomic instability due to SINE activation, a response which may be amplified in cancer cells lacking functional TP53.

Topoisomerase II and the etiology of chromosomal translocations

Acute leukemias with balanced chromosomal translocations, protean morphologic and immunophenotypic presentations but generally shorter latency and absence of myelodysplasia are recognized as a complication of anti-cancer drugs that behave as topoisomerase II poisons. Translocations affecting the breakpoint cluster region of the MLL gene at chromosome band 11q23 are the most common molecular genetic aberrations in leukemias associated with the topoisomerase II poisons. These agents perturb the cleavage-religation equilibrium of topoisomerase II and increase cleavage complexes. One model suggests that this damages the DNA directly and leads to chromosomal breakage, which may result in untoward DNA recombination in the form of translocations. This review will summarize the evidence for topoisomerase II involvement in the genesis of translocations and extension of the model to acute leukemia in infants characterized by similar MLL translocations.

Biochemical mechanisms of chromosomal translocations resulting from DNA double-strand breaks

Exposure of mammalian cells to agents that induce DNA double-strand breaks typically results in both reciprocal and nonreciprocal chromosome translocations. Over the past decade, breakpoint junctions of a significant number of translocations and other genomic rearrangements, both in clinical tumors and in experimental models, have been analyzed at the DNA sequence level. Based on these data, reasonable inferences regarding the biochemical mechanisms involved in translocations can be drawn. In a few cases, breakpoints have been shown to correlate with sites of double-strand cleavage by agents to which the cells or patients have been exposed, including exogenous rare-cutting endonucleases, radiomimetic compounds, and topoisomerase inhibitors. These results confirm that translocations primarily reflect misjoining of the exchanged ends of two or more double-strand breaks. Many junctions show significant loss of DNA sequence at the breakpoints, suggesting exonucleolytic degradation of DNA ends prior to joining. The size and frequency of these deletions varies widely, both between experimental systems, and among individual events in a single system. Homologous recombination between repetitive DNA sequences does not appear to be a major pathway for translocations associated with double-strand breaks. Rather, the general features of the junction sequences, particularly the high frequency small terminal deletions, the apparent splicing of DNA ends at microhomologies, and gap-filling on aligned double-strand break ends, are consistent with the known biochemical properties of the classical nonhomologous end joining pathway involving DNA-dependent protein kinase, XRCC4 and DNA ligase IV. Nevertheless, cells with deficiencies in this pathway still exhibit translocations, with grossly similar junction sequences, suggesting an alternative but less conservative end joining pathway. Although evidence for participation of specific DNA end processing enzymes in formation of translocations is largely circumstantial, likely candidates include DNA polymerases lambda and mu, Artemis nuclease, polynucleotide kinase/phosphatase, tyrosyl-DNA phosphodiesterase, DNase III, Werner syndrome protein, and the Mre11/Rad50/NBS1 complex.

Therapy-related myelodysplastic syndrome in childhood: A retrospective study of 36 patients in Japan

We report here a retrospective analysis of 36 children with therapy-related myelodysplastic syndrome (t-MDS) diagnosed between 1990 and 1999 in Japan. Their median age was 7.7 years and the median latency period for the development of t-MDS was 38.5 months. The primary tumors were hematologic in 15 of the cases and nonhematologic in 21. Chromosomal abnormalities were detected in 32/34(94%) patients: abnormalities of chromosomes 5and/or 7 in 41% and notably, 11q23 abnormalities in 31%. The prognosis of children with t-MDS was very poor as compared to children with primary MDS (5 year survival: 16% versus 54%, p<0.0001).

11q23 balanced chromosome aberrations in treatment-related myelodysplastic syndromes and acute leukemia: report from an international workshop

Among 511 patients with therapy-related myelodysplastic syndrome or acute leukemia (t-MDS/t-AL) and balanced chromosome aberrations, 162 (32%) had translocations involving 11q23. The recurring translocation partners were 9p22 (48%), 19p13.3 (11%), 19p13.1 (10%), 4q21 (9%), 6q27 (6%), 1p32 (2%), 16p13.1 (2%), 10p13 (1%), and 17q25 (1%); in 9%, the translocations were seen only once. The remaining 349 patients were divided into five subgroups based on the balanced aberration: 21q22, inv(16), t(15;17), Rare, and Unique aberrations. Patients in the 11q23 subgroup had a sole cytogenetic abnormality more often than those in the 21q22, inv(16), Rare, and Unique subgroups, and a complex karyotype or -5/del(5q) and/or -7/del(7q) less often than patients in the 21q22, Rare, and Unique subgroups. Clinically, 11q23 patients had acute lymphoblastic leukemia (ALL) more often as their primary disease and a shorter latency from start of treatment for the primary disease to their t-MDS/t-AL diagnosis, except when compared with the inv(16) subgroup. The 11q23 subgroup demonstrated a younger age at t-MDS/t-AL diagnosis, but this finding was not significant when patients with AL as their primary diagnosis were excluded. Survival from the time of diagnosis of t-MDS/t-AL was significantly shorter for the 11q23 subgroup compared with that of the 21q22, inv(16), and t(15;17) subgroups (median 8 vs. 14, 28, and 29 months, respectively). Inferior survival occurred even though 11q23 patients were younger and more often received blood or marrow transplantation (BMT). Even among patients receiving BMT, 11q23 patients had a shorter median survival (9 vs. 12-31 months for the other subgroups). However, among 11q23 patients, those receiving BMT survived longer, with 1- and 5-year survivals of 43% and 18% compared with 23% and 7% for patients not transplanted. With regard to prior therapy, 11q23 patients, compared with other patients, received radiotherapy less often as their sole therapy and chemotherapy more often. They had received VP16, methotrexate, 6MP/6TG, L-asparaginase, daunorubicin, cytarabine, and VM26 more often, likely attributed to the high frequency of AL as their primary disease. More patients in the 11q23 subgroup had received doxorubicin, except in comparison with the 21q22 subgroup; more vincristine, except in comparison with the Rare and Unique subgroups; and more prednisone, except in comparison with the Unique subgroup. Patients in the 11q23 subgroup more often received alkylating agents (AAs) (86% vs. 59-82% for the other subgroups), and topoisomerase II inhibitors (TIs) (84% vs. 49-75%), and they more often reported exposure to AAs plus TIs without radiotherapy (33% vs. 12-21%), except in comparison with the 21q22 subgroup (36%). We performed a multivariate analysis to determine whether the adverse survival of 11q23 patients compared to other Workshop patients was explained by factors other than the presence of the 11q23 abnormality. Covariates in the final model were the five cytogenetic subgroup indicators, where the 11q23 subgroup was the referent (P < 0.0001); age at t-MDS/t-AL (P = 0.0036); previous exposure to lomustine (P < 0.0001) and mitoxantrone (P = 0.0225); BMT for t-MDS/t-AL (P = 0.0006); and karyotype complexity (P = 0.0114). The risk of death for 11q23 patients relative to patients in the 21q22, inv(16), t(15;17), and Unique subgroups was significant, even after adjustment for other risk factors (relative risks 2.3, 3.6, 3.1, and 1.5, respectively; P < 0.0001 for the first three comparisons and P = 0.0125 for the last). When a multivariable model was constructed, excluding patients with AL or MDS as their primary diagnosis, the relative risk of death for 11q23 patients was significantly higher than that of all five other cytogenetic subgroups. We conclude that among t-MDS/t-AL patients with balanced aberrations, 11q23 translocations are an independent adverse risk factor. Although BMT is the current therapy of choice, new treatment is required.

Acute leukemia

This article provides a review of the acute leukemias with updated basic and practical information. The main emphasis is on techniques used to arrive at the correct diagnosis. Although morphology and cytochemistry were the mainstays of diagnosis in the past, new developments in immunophenotyping, cytogenetics, molecular biology, and in vitro assays have improved the understanding of this disease dramatically and enable the identification of new entities with distinct clinicobiologic features.

Leukemias related to treatment with DNA topoisomerase II inhibitors

The epipodophyllotoxins etoposide and teniposide and other DNA topoisomerase II inhibitors including anthracyclines and dactinomycin are highly efficacious anticancer drugs. All are associated with a distinct form of leukemia characterized by chromosomal translocations as a treatment complication. Most of the translocations disrupt a breakpoint cluster region (bcr) of the MLL gene at chromosome band 11q23. Other characteristic translocations also may occur. The normal function of the nuclear enzyme DNA topoisomerase II is to catalyze changes in DNA topology between relaxed and supercoiled states by transiently cleaving and re-ligating both strands of the double helix. Anticancer drugs that are DNA topoisomerase II inhibitors are cytotoxic because they form complexes with DNA and DNA topoisomerase II. The complexes decrease the re-ligation rate, disrupt the cleavage-re-ligation equilibrium, and have a net effect of increasing cleavage. The increased cleavage damages the DNA and leads to chromosomal breakage. Cells with irreparable DNA damage die by apoptosis. The association of DNA topoisomerase II inhibitors with leukemia suggests that the drug-induced, DNA topoisomerase II-mediated chromosomal breakage may be relevant to translocations in addition to this anti-neoplastic, cytotoxic action. Epidemiological studies, genomic translocation breakpoint cloning and in vitro DNA topoisomerase II cleavage assays together lead to a model for treatment-related leukemia in which DNA topoisomerase II causes chromosomal breakage and translocations form when the breakage is repaired.

Treatment-related leukaemia--a clinical and scientific challenge

The development of a second tumour, including treatment-related leukaemia (TRL), is the most devastating complication of intensive cancer chemotherapy. This is especially relevant in the paediatric population as over 70% of children diagnosed with a malignancy will now live at least 5 years. Most TRLs are myeloid leukaemias and carry an overall poor prognosis when compared with their de novo counterparts. Despite the well known association with specific cytotoxic agents, improved understanding of the pathogenesis and risk factors of TRL is ultimately essential if we are to develop successful strategies for prevention and treatment. Here we review these aspects, together with the clinical and diverse biological features of this complication and the efficacy of current therapy.

Loss of drug-stimulated topoisomerase II DNA breaks in living cells is different at two unrelated loci

Topoisomerase II (top2) has been implicated in the initial steps of chromosomal translocations leading to leukemias and lymphomas, since it can generate DNA cleavage. To evaluate the effects of chromatin structure on enzyme-mediated cleavage, we determined the kinetics of loss of double-stranded DNA breaks stimulated by top2 poisons in Drosophila melanogaster Kc cells at two genomic regions that differ in chromatin structure. Moreover, cleavage loss was determined at 25 degrees C as well as after heat shock. Kinetics were dependent on the poison, nevertheless, loss rate overall was slow at the histone gene cluster, an active chromatin domain. At the repressed satellite III DNA, loss of cleavage was much faster and complete after 5 min in drug-free medium. In addition, differences were noted among sites that were closely spaced and equally intense. Following heat shock at 37 degrees C, we observed reduced cleavage levels and faster loss of breaks at the histone gene cluster. In vitro reversal could only partially explain the in vivo kinetics. Thus, the chromatin context of DNA breaks might play a role in the loss of top2 DNA breaks. The present findings suggest that irreversible cuts may more likely occur in active than silent loci.

Mutagenic properties of topoisomerase-targeted drugs

Topoisomerases maintain DNA structure by relieving torsional stress occurring in DNA during transcription, replication and cell division. Topoisomerases are of two main types, causing transient breaks in one (type I) or both (type II) and strands of DNA, and a number of clinical anticancer drugs are thought to act by inhibiting religation of these transient breaks. Topoisomerase II appears to have a close association with the SMC (stable maintenance of chromosomes) family of proteins involved in organisation of the chromatin in a series of loops on the proteinaceous chromosomal scaffold. Inhibition of topoisomerase II function can result in deletions of such loops, probably mediated by reciprocal exchange of topoisomerase subunits. Disruption of topoisomerase I and/or II function during DNA replication results in smaller DNA deletions and other mutations, probably arising from non-homologous recombination. Inhibition of topoisomerase II action during mitosis and meiosis can cause incomplete separation of chromatids and chromosomes, with the consequent production of genomic mutations. Topoisomerase-mediated mutagenicity is important because it can lead not only to drug resistance but also to drug-induced secondary cancers. Mutagenicity of topoisomerase-directed agents has been underestimated in the past, since these drugs are not usually capable of reacting covalently with DNA and usually have low mutagenicity in microbial assays.

Secondary leukemias induced by topoisomerase-targeted drugs

The major established cause of acute myeloid leukemia (AML) in the young is cancer chemotherapy. There are two forms of treatment-related AML (t-AML). Each form has a de novo counterpart. Alkylating agents cause t-AML characterized by antecedent myelodysplasia, a mean latency period of 5-7 years and complete or partial deletion of chromosome 5 or 7. The risk is related to cumulative alkylating agent dose. Germline NF-1 and p53 gene mutations and the GSTT1 null genotype may increase the risk. Epipodophyllotoxins and other DNA topoisomerase II inhibitors cause leukemias with translocations of the MLL gene at chromosome band 11q23 or, less often, t(8;21), t(3;21), inv(16), t(8;16), t(15;17) or t(9;22). The mean latency period is about 2 years. While most cases are of French-American-British (FAB) M4 or FAB M5 morphology, other FAB AML subtypes, myelodysplastic syndrome (MDS), acute lymphoblastic leukemia (ALL) and chronic myelogenous leukemia (CML) occur. Between 2 and 12% of patients who receive epipodophyllotoxin have developed t-AML. There is no relationship with higher cumulative epipodophyllotoxin dose and genetic predisposition has not been identified, but weekly or twice-weekly schedules and preceding l-asparaginase administration may potentiate the risk. The translocation breakpoints in MLL are heterogeneously distributed within a breakpoint cluster region (bcr) and the MLL gene translocations involve one of many partner genes. DNA topoisomerase II cleavage assays demonstrate a correspondence between DNA topoisomerase II cleavage sites and the translocation breakpoints. DNA topoisomerase II catalyzes transient double-stranded DNA cleavage and rejoining. Epipodophyllotoxins form a complex with the DNA and DNA topoisomerase II, decrease DNA rejoining and cause chromosomal breakage. Furthermore, epipodophyllotoxin metabolism generates reactive oxygen species and hydroxyl radicals that could create abasic sites, potent position-specific enhancers of DNA topoisomerase II cleavage. One proposed mechanism for the translocations entails chromosomal breakage by DNA topoisomerase II and recombination of DNA free ends from different chromosomes through DNA repair. With few exceptions, treatment-related leukemias respond less well to either chemotherapy or bone marrow transplantation than their de novo counterparts, necessitating more innovative treatments, a better mechanistic understanding of the pathogenesis, and strategies for prevention.

Secondary haematological neoplasm after treatment of adult acute lymphoblastic leukemia: analysis of 1170 adult ALL patients enrolled in the GIMEMA trials. Gruppo Italiano Malattie Ematologiche Maligne dell'Adulto

Between 1983 and 1994 the incidence of secondary haematological neoplasms (SHM) was evaluated in 1170 new cases of ALL enrolled in the GIMEMA trials. Of the 942 patients who achieved complete remission (CR); seven developed a SHM: four AMLs and three NHLs. The median latency from onset of ALL and of secondary haematological neoplasm was 69 months for AML and 61 months for NHL. Three out of four patients with secondary AML were unresponsive to the new chemotherapy and died, whereas the fourth patient achieved a new CR. Among the three NHL cases, two patients are presently alive in CR, whereas the third patient was refractory to chemotherapy and died. The relative risk of haematological malignancy among the GIMEMA trials population, as compared to that of the Italian Cancer Registries, was 15.25-fold higher, and the actuarial estimated cumulative proportion of ALL patients with a secondary haematological neoplasm at 5 and 10 years were 0.59% and 3.63% respectively. The incidence of adult ALL who developed a SHM, although apparently lower than in the paediatric ALL series, was higher when compared to the normal population. The difference between paediatric and adult ALL is probably due to the lack of craniospinal radiotherapy and to the lower doses of epipodoxiphyllotoxins used in adult trials. The higher percentage of childhood ALL with a prolonged event-free survival could result in an increase of secondary neoplasms in these cases, which suggests that secondary haematological neoplasms in adult ALL patients are real, although rare, events.

Detection of chimeric mRNAs by reverse transcriptase-polymerase chain reaction for diagnosis and monitoring of acute leukemias with 11q23 abnormalities

Recurrent translocations involving chromosome band 11q23 are often found in human acute leukemias. Recently, the MLL gene on 11q23 and 10 partner genes involved in these translocations have been cloned and characterized. We performed a reverse transcriptase-polymerase chain reaction (RT-PCR) to detect the resultant der(11) chimeric mRNAs of the 3 types of 11q23 translocations including t(4;11), t(9;11), or t(11;19), in 14 leukemia patients with MLL gene rearrangements. At diagnosis or relapse, chimeric mRNA could be detected in all of the 4 patients with t(4;11), 2 of 3 with t(9;11), 2 of 3 with t(11;19), and 1 of 4 with unsuccessful karyotype. In 5 patients, we could monitor minimal residual disease (MRD) serially through the clinical course. One patient, in whom chi-meric mRNA was detected during complete remission (CR) just after the induction chemotherapy, relapsed within 2 months and died, while 2 patients in which chimeric mRNA was not detected remained in CR from 10-23 months. These findings suggest that RT-PCR is a useful approach for detecting which partner gene is involved in the translocation and monitoring MRD in patients with MLL gene rearrangement. Nonetheless, the clinical relevance of MRD evaluation by RT-PCR monitoring remains controversial. Long-term and prospective investigation of a larger series of patients is needed to confirm the clinical significance of monitoring MRD by RT-PCR method.

Therapy-related myeloid leukemia

One of the most serious possible consequences of cancer therapy is the development of a second cancer, especially leukemia. Several distinct subsets of therapy-related leukemia can be distinguished currently. These include classic therapy-related myeloid leukemia, leukemia that follows treatment with agents that inhibit topoisomerase II, acute lymphoblastic leukemia, and leukemias with 21q22 rearrangements or inv(16) or t(15;17). These types of leukemia are discussed in detail in this article.

Mechanism of action of DNA topoisomerase inhibitors

DNA topoisomerases are enzymes that regulate DNA topology and are essential for the integrity of the genetic material during transcription, replication and recombination processes. Inhibitors of the mammalian enzymes are widely used antitumor drugs. They stabilize topoisomerase-DNA cleavable complexes by hindering the DNA relegating step of the catalytic reaction, thus resulting in DNA cleavage stimulation. Investigations on the sequence selectivity of DNA cleavage stimulated by chemically unrelated compounds established that specific nucleotides flanking strand cuts are required for drug action. Moreover, structure-activity relationship studies have identified structural determinants of drug sequence specificities, thus eventually allowing the design of new agents targeted at selected genomic regions. The initial cellular lesion, i.e., the drug-stabilized cleavable complex, is a reversible molecular event; however, how it may lead to cell death remains to be fully clarified. Several laboratories focused in past years on molecular and genetic aspects of drug-activated apoptosis. Irreversible double-stranded DNA breaks, generated from collisions between cleavable complexes and advancing replication forks, were suggested to increase p53 protein levels, thus triggering the cell death program. Other genes were also shown to cooperate in modulating the cell response to drug treatments. Recently, several groups have evaluated the possible prognostic value of topoisomerase II levels in solid tumors and hematopoietic neoplasms. Topoisomerase II inhibitors may also have genotoxic effects. Secondary leukemias, characterized by a translocation between chromosomes 11 and 9, have been reported in disease-free patients after treatments with drug regimens that included anti-topoisomerase II agents. It has been proposed that an impairment of topoisomerase activity may be involved in the molecular pathogenesis of secondary leukemias.

Detection of major and minor bcr/abl fusion gene transcripts in a patient with acute undifferentiated leukemia secondary to treatment with an alkylating agent

In this paper we describe a patient with bcr/abl positive acute undifferentiated leukemia (AUL) derived from acquired sideroblastic anemia secondary to ifosphamide treatment given for the preceding non-Hodgkin lymphoma of the lung. Cytogenetically, Philadelphia chromosome was not detected through the whole course in this patient, and multiple chromosomal abnormalities including 5q- and monosomy 7 were found at the stage of sideroblastic anemia. The reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed no bcr/abl fusion transcript at the diagnosis of malignant lymphoma. The mRNA encoding the major bcr/abl fusion protein then appeared in the stage of sideroblastic anemia. Finally, the mRNA encoding both major and minor bcr/abl was detected in the stage of AUL transformation. MLL gene rearrangement was not found by RT-PCR analysis at any stage of the disorder. These results may be direct evidence for the induction of the bcr/abl fusion gene by treatment with an alkylating agent (ifosphamide).

DNA damage and mutagenesis induced by nitrogen mustards

The nitrogen mustards are bifunctional alkylating agents which, although used extensively in cancer chemotherapy, are themselves highly carcinogenic. All nitrogen mustards induce monofunctional guanine-N7 adducts, as well as interstrand N7-N7 crosslinks involving the two guanines in GNC.GNC (5'-->3'/5'-->3') sequences. In addition, the aromatic mustards melphalan and chlorambucil also induce substantial alkylation at adenine N3, while cyclophosphamide forms phosphotriesters with relatively high frequency. Nitrogen mustards are genotoxic in virtually every assay, and produce a wide array of mutations, including base substitutions at both G.C and A.T base pairs, intragenic as well as multilocus deletions, and chromosomal rearrangements. Mutational spectra generated by these agents in various model systems vary widely, and no single lesion has been implicated as being primarily responsible for mustard-induced mutagenesis. On the contrary, adducts of both adenine and guanine, and monofunctional as well as bifunctional adducts, appear to be involved. Further, it is still not known which types of mutation are responsible for mustard-induced cancers, since no genes have yet been identified which are consistently altered in these malignancies.

Apoptosis induced by DNA topoisomerase I and II inhibitors in human leukemic HL-60 cells

The induction of apoptosis following topoisomerase inhibitors proceeds in at least three distinct steps: (1) induction of cleavable complexes (potentially lethal damage), (2) topoisomerase-induced DNA damage, and (3) a presently unknown sequence of events that must either lead to cell cycle arrest (G2-block, differentiation) or apoptosis. DNA degradation provides a convenient way to quantify apoptosis in HL-60 cells. Extensive apoptosis can be induced rapidly in undifferentiated HL-60 cells without prevention by cycloheximide or actinomycin D. Therefore, HL-60 cells appear to express constitutively the apoptotic machinery that may be kept under control of a yet unknown repressor. The absence of the tumor suppressor p53 and the presence of bcl-2 are in contrast with the sensitivity of these cells to apoptosis. Agents that modify chromatin structure (zinc, poly[ADPribose] inhibitors, spermine) can block DNA fragmentation without affecting cell survival. By contrast macrophage-like differentiation by phorbol esters suppresses apoptosis without affecting topoisomerase-induced DNA damage. Better understanding of the apoptotic regulation in the widely used and characterized HL-60 cell line should allow the identification of new mechanisms and parameters of cellular sensitivity and resistance to the cytotoxic activity of anticancer agents.

Topoisomerase II enzymes and mutagenicity

Topoisomerase II (topo II) enzymes maintain DNA structure by relieving torsional stress occurring in double-strand DNA during transcription and replication. Topo II causes transient breaks in both strands of DNA, allowing passage of one double helix through another, and probably acts as a structural protein in interphase cells, playing a role in the organisation of mitotic and meiotic chromosomes. A number of clinical anticancer drugs are thought to act on topo II enzymes to stabilise DNA-drug-topo II ternary complexes known as "cleavable complexes." These complexes may lead to illegitimate recombination events, as well as to the formation of other DNA lesions. Topo II-mediated genotoxicity is strongly dependent on the cell cycle status of the target cells. It is now apparent that some dietary components and environmental chemicals may act on topo II. Since the structural features of chemicals that lead to topo II interaction are not clear, it is currently not possible to predict such activity from chemical structure. For many years, the central dogma of chemical carcinogenesis has been that the most carcinogenic chemicals are those that can form a covalent bond with DNA, either directly or after metabolic activation. Topo II-directed drugs are not usually capable of forming covalent bonds with DNA and tend to have low mutagenicity in microbial assays. However, topo II-directed agents are potent cancerogens, inducing characteristic cytogenetic modifications. It is important to define the most sensitive tests to identify topo II-directed mutagens and to develop appropriate strategies for genotoxicity testing of such chemicals.

DNA topoisomerase I and II as targets for rational design of new anticancer drugs

BACKGROUND

Topoisomerase I and II (topo I and II) are enzymes which alter the topological state of DNA through DNA strand cleavage, strand passage and religation. They participate in most aspects of DNA metabolism and are therefore vital to the cell undergoing division. Only one form of topo I has been identified whereas two isoenzymes of topo II have been described: the alpha form (170 kDa protein) and beta form (180 kDa protein). Both topo II isoenzymes have distinct nuclear localisation, are regulated independently, differ in their responsiveness to inhibitors and are differentially expressed in drug resistant cell lines.

RESULTS

Several clinically active anticancer drugs (e.g., doxorubicin, m-AMSA, VP-16 and camptothecins) poison these enzymes by stabilizing a putative reaction intermediate called the cleavable complex (cc) where the topoisomerase remains covalently attached to either one strand of DNA (topo I) or both strands of double helix (topo II) after strand cleavage. DNA cleavage sites appear unique for different classes of inhibitor, and are probably critical for defining cytotoxicity. Formation of the cc may cause cell death either by colliding with replication forks, by promoting illegitimate genomic-DNA recombination, by arresting cells in the G2-phase of the cell cycle or by inducing apoptosis.

CONCLUSION

New classes of inhibitor have recently been described with novel mechanisms of action including compounds which do not stabilize cleavable complexes or bind significantly to DNA. These may prove to be more selective and less toxic. They may also avoid the possible problem of therapy-related leukemias associated with topo inhibitors which induce DNA cleavage and chromosomal aberrations.

Secondary acute lymphoblastic leukemia with t (4;11): report on two cases and review of the literature

We report two cases of secondary acute lymphoblastic leukemia (ALL) with t (4;11) (q21;q23) translocation occurring after chemotherapy and radiotherapy for a prior cancer. Seven previously published cases of secondary ALL with t (4;11) (q21;q23) are also reviewed. Most patients had received a combination of topoisomerase II inhibitors (anthracyclines, mitoxantrone, or the epipodophillotoxin derivatives VP16 or VM26) and cyclophosphamide, which have also been implicated in the pathogenesis of secondary acute myeloid leukemia (AML) with 11q23 rearrangements. These observations give further support to the existence of a subgroup of secondary acute leukemias with cytogenetic findings "specific" for de novo ALL and AML, especially those with translocations involving the 11q23 region.

Acute leukemias with the t(4;11)(q21;q23)

The t(4;11)(q21;q23) has been associated with marked lineage heterogeneity. Most of the reported cases were classified as acute lymphoblastic leukemia (ALL). The t(4;11) is one of the commonest specific chromosomal translocations in ALL, occurring in 2% of childhood and 5% of adult cases. In childhood ALL, this translocation is associated with female sex, age less than 1 year, hyperleukocytosis, CD10-/CD19+ B-precursor cell immunophenotype, and myeloid-associated antigen (CD15) expression. There also appears to be an age-related difference in treatment outcome. Adults had the worst prognosis, and children aged 1 to 9 years appeared to have a better outcome than infants or adolescents. Reported cases of acute myeloid leukemia (AML) or secondary leukemia with the t(4;11) have not been well characterized. It is intriguing that virtually all of the reported cases with secondary leukemia had received epipodophyllotoxins or doxorubicin, agents that affect topoisomerase II and are associated with secondary AML characterized by 11q23 abnormalities. Identification of the involved gene(s) in the t(4;11) will provide a molecular approach permitting more accurate classification of these cases.

Chromosomal abnormalities in skin following total body or total lymphoid irradiation

Patients undergoing bone marrow transplantation often receive total body or total lymphoid irradiation as part of the conditioning regimen prior to marrow infusion. The cytogenetic effects of this therapy on skin fibroblasts were studied. Fibroblast cultures from eight skin biopsies were harvested in early passages for G-banded chromosome analysis. Four biopsies were from three patients who had high-dose cyclophosphamide and total body radiotherapy; one was from within and one was from outside the radiation field of a patient who had high-dose cyclophosphamide and lymphoid radiotherapy, one was from a patient who had combination chemotherapy alone, and one was from a normal control. No abnormal mitoses were found in the control or the patient who had chemotherapy alone, and only two of 30 mitoses from skin outside the lymphoid radiotherapy field were abnormal. However, most cells (49-88%) from five biopsies within radiotherapy fields were abnormal. Typically, abnormal karyotypes were pseudodiploid and contained multiple balanced rearrangements, of which reciprocal translocations were most common. The data indicate that the radiotherapy used for bone marrow transplantation induces extensive, sustained chromosome abnormalities in vivo in skin fibroblasts.

Therapy-related acute myeloid leukemia secondary to inhibitors of topoisomerase II: from the bedside to the target genes

In the past five years, several groups have reported acute myeloid leukemia (AML) often monoblastic, as a complication of chemotherapy regimens including the epipodophyllotoxins, etoposide and teniposide. This syndrome is distinct clinically, pathologically and cytogenetically from classical therapy-related myelodysplasia and AML. There is also evidence that other topoisomerase II inhibitors, such as the intercalating agents (including doxorubicin, mitoxantrone, and actinomycin D) may be leukemogenic. Furthermore, there may be further interactions from concomitant topoisomerase II inhibitors and alkylating agents. Topoisomerase II inhibitors induce DNA cleavage and other chromosomal aberrations, including sister chromatid exchanges. These clastogenic abnormalities are not fully understood, and may be specific for each cytotoxic agent. Work is in progress to clone breakpoints such as the t(9;11) and t(8;21) and the use of the resultant DNA probes will enhance our understanding of the leukemogenic process. Given the potential diversity in patients with secondary leukemia, cytogenetic studies should be mandatory for both enhancing our knowledge base and guiding treatment in individual patients. Clinicians must also be wary of the leukemogenic potential of 'dose-intense' regimens including agents such as etoposide and doxorubicin.

Cytogenetics of secondary myelodysplasia (sMDS) and acute nonlymphocytic leukemia (sANLL)

76 cases of secondary myelodysplasia (sMDS) and acute non-lymphocytic leukemia (sANLL) were cytogenetically analyzed. Among the 36 sMDS patients, 13 (36%) had only normal karyotypes whereas 23 (64%) displayed clonal chromosomal abnormalities. The most common aberrations were -7, 5q-, -5, and +8. In 10 patients (43% of the cytogenetically aberrant cases), clones with only one anomaly, mostly 5q- or -7, were found. Of the 40 sANLL patients, normal karyotypes were detected in 10 (25%). Among the 30 (75%) abnormal cases, the most frequent aberrations were -7, -5, +8, 7q-, -17, and +21. 12 patients (40%) had clones with single abnormalities, most often -7. In 4 sANLL patients cytogenetically unrelated clones were detected. A survey of all previously published secondary hematologic neoplasias reveals that the most frequent abnormalities in sMDS are -7 (41%), 5q- (28%), and -5 (11%), followed by der(21q), +8, 7q-, der(12p), t(1;7), -12, -17, der(17p), der(3p), der(6p), and -18. Clones with single aberrations have been found in 45% of the cases and cytogenetically unrelated clones have been described in 6%. The most common abnormalities in sANLL are -7 (38%), 5q- (17%), -5 (15%), +8 (13%), and -17 (11%), followed by der(3q), der(11q), der(12p), -21, 7q-, -18, der(3p), der(17p), +21, der(21q), der(6p), and -16. 38% of the sANLL patients have had clones with only one aberration and 3% have had unrelated clones. The frequencies of these nonrandom abnormalities in sMDS and sANLL are thus remarkably similar - the only exception appears to be 5q-, which is more common in sMDS. Also the mean number of abnormalities per case is similar - 5.3 in sMDS and 5.6 in sANLL. When the incidences of characteristic cytogenetic abnormalities were correlated with the type of previous therapy, -7 was found to be more frequent in sMDS and sANLL patients who had been exposed to chemotherapy whereas 5q- was associated with previous exposure to ionizing radiation in sMDS patients.