Accumulation of methotrexate polyglutamates, ploidy and trisomies of both chromosomes 4 and 10 in lymphoblasts from children with B-progenitor cell acute lymphoblastic leukemia: a Pediatric Oncology Group Study

Overview on Aneuploidy in Childhood B-Cell Acute Lymphoblastic Leukemia

Recent years have brought significant progress in the treatment of B-cell acute lymphoblastic leukemia (ALL). This was influenced by both the improved schemes of conventionally used therapy, as well as the development of new forms of treatment. As a consequence, 5-year survival rates have increased and now exceed 90% in pediatric patients. For this reason, it would seem that everything has already been explored in the context of ALL. However, delving into its pathogenesis at the molecular level shows that there are many variations that still need to be analyzed in more detail. One of them is aneuploidy, which is among the most common genetic changes in B-cell ALL. It includes both hyperdiploidy and hypodiploidy. Knowledge of the genetic background is important already at the time of diagnosis, because the first of these forms of aneuploidy is characterized by a good prognosis, in contrast to the second, which is in favor of an unfavorable course. In our work, we will focus on summarizing the current state of knowledge on aneuploidy, along with an indication of all the consequences that may be correlated with it in the context of the treatment of patients with B-cell ALL.

Hyperdiploidy: the longest known, most prevalent, and most enigmatic form of acute lymphoblastic leukemia in children

Hyperdiploidy is the largest genetic entity B-cell precursor acute lymphoblastic leukemia in children. The diagnostic hallmark of its two variants that will be discussed in detail herein is a chromosome count between 52 and 67, respectively. The classical HD form consists of heterozygous di-, tri-, and tetrasomies, whereas the nonclassical one (usually viewed as "duplicated hyperhaploid") contains only disomies and tetrasomies. Despite their apparently different clinical behavior, we show that these two sub-forms can in principle be produced by the same chromosomal maldistribution mechanism. Moreover, their respective array, gene expression, and mutation patterns also indicate that they are biologically more similar than hitherto appreciated. Even though in-depth analyses of the genomic intricacies of classical HD leukemias are indispensable for the elucidation of the disease process, the ensuing results play at present surprisingly little role in treatment stratification, a fact that can be attributed to the overall good prognoses and low relapse rates of the concerned patients and, consequently, their excellent treatment outcome. Irrespective of this underutilization, however, the detailed genetic characterization of HD leukemias may, especially in planned treatment reduction trials, eventually become important for further treatment stratification, patient management, and the clinical elucidation of outcome data. It should therefore become an integral part of all upcoming treatment studies.

Genetic and immunophenotypic diversity of acute leukemias in children

Acute leukemias are the most commonly diagnosed malignancies in children. Acute leukemias constitute a heterogeneous group of cancers resulting from clonal outgrowth and accumulation of immature precursor cells of different hematologic lineages. Cancerous transformation begins with disruption of cell maturation mechanisms triggered by particular environmental or endogenic factors, including innate and acquired immunodeficiencies as well as autoimmune diseases.

Research in the field of acute leukemias has revealed many possible genetic abnormalities in leukemic cells, including both structural and numerical aberrations. The former can produce some particular fusion genes, yielding fusion protein products which can have an oncogenic potential in hematopoietic cells. Some of them, including translocations resulting in fusion product formation BCR-ABL1 and different fusion products involving the KMT2A gene, are markers of adverse prognosis, whereas numerical aberrations with high hyperdiploidy and chromosome number exceeding 51 are markers of favorable prognosis. Detection of these aberrations already has a well-grounded clinical significance in acute lymphoblastic leukemia and plays an important role in patient risk stratification. The appearance of particular genetic changes often correlates with the expression of certain markers on the surface of leukemic cells. Determination of expression or lack of specific antigens, that is, immunophenotyping, is possible with the use of the flow cytometry technique. Flow cytometry is currently considered as a fast and broadly available technique which can provide clinically useful information in a relatively short time after biological specimen collection. Flow cytometry also enables appropriate classification of acute leukemias.

Multicolor flow cytometry immunophenotyping and characterization of aneuploidy in pediatric B-cell precursor acute lymphoblastic leukemia

The aim of this study was to assess the incidence of DNA aneuploidy in Polish children with B-cell precursor acute lymphoblastic leukemia (BCP-ALL) and the relationship between aneuploidy and immunological phenotype, age, leukocyte count, S-phase fraction (SPF) and early response to induction chemotherapy assessed by the percentage of residual blast cells in bone marrow aspirates. The study group consisted of 267 patients. DNA content and immunophenotype were assessed in the bone marrow before treatment using multicolor flow cytometry (FC). DNA aneuploidy was detected in 50/267 (19%) patients. High hyperdiploidy was found to be associated with lower leukocyte count (p = 0.006) and common ALL immunophenotype. Flow cytometry analysis revealed that high hyperdiploid BCP-ALL patients showed significantly higher expression of CD9, CD20, CD22, CD58, CD66c, CD86 and CD123 antigens as compared to other groups of ploidy. In contrast, CD45 showed decreased expression. The percentage of leukemic blasts at diagnosis was lower in high hyperdiploid BCP-ALL cases than in diploid (79% vs. 85.7%, p = 0.001). The difference in minimal residual disease (MRD) levels on day 15 and 33 of induction therapy between analyzed groups was not significant.

This study showed that high hyperdiploidy is associated with lower WBC count and specific immunological phenotype. Flow cytometric evaluation of expression of selected antigens can be used for fast identification of markers of aneuploidy in pediatric BCP-ALL, before genetic tests results are available. Understanding the biological significance of aneuploidy in leukemia can potentially be exploited therapeutically using targeted therapies against specific blast cell subclones.

Between-course targeting of methotrexate exposure using pharmacokinetically guided dosage adjustments

PURPOSE

It is advantageous to individualize high-dose methotrexate (HDMTX) to maintain adequate exposure while minimizing toxicities. Previously, we accomplished this through within-course dose adjustments.

METHODS

In this study, we evaluated a strategy to individualize HDMTX based on clearance of each individual's previous course of HDMTX in 485 patients with newly diagnosed acute lymphoblastic leukemia. Doses were individualized to achieve a steady-state plasma concentration (Cpss) of 33 or 65 μM (approximately 2.5 or 5 g/m(2)/day) for low- and standard-/high-risk patients, respectively.

RESULTS

Individualized doses resulted in 70 and 63 % of courses being within 20 % of the targeted Cpss in the low- and standard-/high-risk arms, respectively, compared to 60 % (p < 0.001) and 61 % (p = 0.43) with conventionally dosed therapy. Only 1.3 % of the individualized courses in the standard-/high-risk arm had a Cpss greater than 50 % above the target compared to 7.3 % (p < 0.001) in conventionally dosed therapy. We observed a low rate (8.5 % of courses) of grade 3-4 toxicities. The odds of gastrointestinal toxicity were related to methotrexate plasma concentrations in both the low (p = 0.021)- and standard-/high-risk groups (p = 0.003).

CONCLUSIONS

Individualizing HDMTX based on the clearance from the prior course resulted in fewer extreme Cpss values and less delayed excretion compared to conventional dosing.

Folylpolyglutamate synthetase gene transcription is regulated by a multiprotein complex that binds the TEL-AML1 fusion in acute lymphoblastic leukemia

Acute Lymphoblastic Leukemia (ALL) non-random fusions influence clinical outcome and alter the accumulation of MTX-PGs in vivo. Analysis of primary ALL samples uncovered subtype-specific patterns of folate gene expression. Using an FPGS-luciferase reporter gene assay, we determined that E2A-PBX1 and TEL-AML1 expression decreased FPGS transcription. ChIP assays uncovered HDAC1, AML1, mSin3A, E2F, and Rb interactions with the FPGS promoter region. We demonstrate that FPGS expression is epigenetically regulated through binding of selected ALL fusions to a multiprotein complex, which also controls the cell cycle dependence of FPGS expression. This study provides insights into the pharmacogenomics of MTX in ALL subtypes.

High hyperdiploid childhood acute lymphoblastic leukemia

High hyperdiploidy (51-67 chromosomes) is the most common cytogenetic abnormality pattern in childhood B-cell precursor acute lymphoblastic leukemia (ALL), occurring in 25-30% of such cases. High hyperdiploid ALL is characterized cytogenetically by a nonrandom gain of chromosomes X, 4, 6, 10, 14, 17, 18, and 21 and clinically by a favorable prognosis. Despite the high frequency of this karyotypic subgroup, many questions remain regarding the epidemiology, etiology, presence of other genetic changes, the time and cell of origin, and the formation and pathogenetic consequences of high hyperdiploidy. However, during the last few years, several studies have addressed some of these important issues, and these, as well as previous reports on high hyperdiploid childhood ALL, are reviewed herein.

Acquired variation outweighs inherited variation in whole genome analysis of methotrexate polyglutamate accumulation in leukemia

Methotrexate polyglutamates (MTXPGs) determine in vivo efficacy in acute lymphoblastic leukemia (ALL). MTXPG accumulation differs by leukemic subtypes, but genomic determinants of MTXPG variation in ALL remain unclear. We analyzed 3 types of whole genome variation: leukemia cell gene expression and somatic copy number variation, and inherited single nucleotide polymorphism (SNP) genotypes and determined their association with MTXPGs in leukemia cells. Seven genes (FHOD3, IMPA2, ME2, RASSF4, SLC39A6, SMAD2, and SMAD4) displayed all 3 types of genomic variation associated with MTXPGs (P < .05 for gene expression, P < .01 for copy number variation and SNPs): 6 on chromosome 18 and 1 on chromosome 10. Increased chromosome 18 (P = .002) or 10 (P = .036) copy number was associated with MTXPGs even after adjusting for ALL subtype. The expression of the top 7 genes in leukemia cells accounted for more variation in MTXPGs (46%) than did the expression of the top 7 genes in normal HapMap cell lines (20%). The top 7 inherited SNPs in patients accounted for approximately the same degree of variation (17%) in MTXPGs as did the top 7 SNP genotypes in HapMap cell lines (20%). We conclude that acquired genetic variation in leukemia cells has a stronger influence on MTXPG accumulation than inherited genetic variation.

High leucovorin doses during high-dose methotrexate treatment may reduce the cure rate in childhood acute lymphoblastic leukemia

We explored the relationship between time to relapse and different exposure variables (serum methotrexate (S-MTX) 23, 36 and 42 h after start of administration, MTX elimination time and leucovorin (LV) dose) during high-dose MTX (HDM) treatment of 445 children with acute lymphoblastic leukemia. MTX was infused at 5 g/m2 (non-high risk) or 8 g/m2 (high risk) over 24 h, 2-9 times per patient. LV rescue dose was adjusted according to the S-MTX concentration. Time from end of the last HDM to relapse was analyzed by Cox regression analysis with the logarithms of S-MTX and LV dose as exposures. The combined results from all risk groups suggest that high LV dose is related to higher risk for relapse. Doubling of the LV dose increased the relapse risk by 22% (95% confidence interval 1-49%, P = 0.037). High LV doses correlated with high MTX levels at 23, 36 and 42 h and longer elimination time. The results suggest that high doses of LV increase the risk for relapse despite the fact that they were correlated with high MTX levels and longer MTX elimination time. The choice of MTX and LV doses may be regarded as an intricate balance between effect and counter-effect.

5-amino-4-imidazolecarboxamide riboside potentiates both transport of reduced folates and antifolates by the human reduced folate carrier and their subsequent metabolism

Transport is required before reduced folates and anticancer antifolates [e.g., methotrexate (MTX)] exert their physiologic functions or cytotoxic effects. The folate/antifolate transporter with the widest tissue distribution and greatest activity is the reduced folate carrier (RFC). There is little evidence that RFC-mediated influx is posttranscriptionally regulated. We show that [(3)H]MTX influx in CCRF-CEM human childhood T-leukemia cells is potentiated up to 6-fold by exogenous 5-amino-4-imidazolecarboxamide riboside (AICAr) in a AICAr and MTX concentration-dependent manner. Metabolism to more biologically active polyglutamate forms is also potentiated for MTX and other antifolates. That potentiation of influx by AICAr is mediated by effects on the RFC is supported by analyses +/-AICAr showing (a) similarity and magnitude of kinetic constants for [(3)H]MTX influx; (b) similarity of inhibitory potency of known RFC substrates; (c) lack of potentiation in a CCRF-CEM subline that does not express the RFC; and (d) similarity of time and temperature dependence. Potentiation occurs rapidly and does not require new protein synthesis. Effects of specific inhibitors of folate metabolism and the time and sequence of AICAr incubation with cells suggest that both dihydrofolate reductase inhibition and metabolism of AICAr are essential for potentiation. Acute folate deficiency or incubation of CCRF-CEM with AICAr-related metabolites (e.g., adenosine) does not initiate potentiation. AICAr increases growth inhibitory potency of MTX and aminopterin against CCRF-CEM cells when both AICAr and antifolate are present for the first 24 hours of a 120-hour growth period. AICAr is the first small molecule that regulates RFC activity.

Prospective analysis of TEL/AML1-positive patients treated on Dana-Farber Cancer Institute Consortium Protocol 95-01

In a retrospective analysis, we previously reported that children whose leukemia cells harbored the TEL/AML1 gene rearrangement have excellent outcomes. From 1996 to 2000, we conducted a prospective study to determine the incidence and outcomes of children with TEL/AML1-positive acute lymphoblastic leukemia (ALL). Children with newly diagnosed ALL were treated on DFCI ALL Consortium Protocol 95-01. Patients were risk stratified primarily by current National Cancer Institute (NCI)-Rome risk criteria. With a median follow-up of 5.2 years, the 5-year event-free survival for TEL/AML1-positive patients was 89% compared with 80% for TEL/AML1-negative B-precursor patients (P = .05). The 5-year overall survival rate was 97% among TEL/AML-positive patients compared with 89% among TEL/AML1-negative patients (P = .03). However, in a multivariable analysis, risk group (age and leukocyte count at diagnosis) and asparaginase treatment group, but not TEL/AML1 status, were found to be independent predictors of outcome. We conclude that TEL/AML1-positive patients have excellent outcomes, confirming our previous findings. However, factors such as age at diagnosis and presenting leukocyte count should be taken into consideration when treating this group of patients.

Accumulation of methotrexate and methotrexate polyglutamates in lymphoblasts and treatment outcome in children with B-progenitor-cell acute lymphoblastic leukemia: a Pediatric Oncology Group study

We reported that children with B-progenitor-cell acute lymphoblastic leukemia (BpALL) treated in the early 1980s whose lymphoblasts accumulated high levels of methotrexate (MTX) and of methotrexate polyglutamates (MTXPGs) in vitro had an improved 5-year event-free survival (EFS) (65% (standard error (s.e.) 12%) vs 22% (s.e. 9%)). We repeated this study in children with BpALL treated in the early 1990s. The major change in treatment was the addition of 12 24-h infusions of 1 g/M2 MTX with leucovorin rescue (IDMTX). In 87 children treated on Pediatric Oncology Group (POG) study 9005 and POG 9006, the 5-year EFS for those whose lymphoblasts accumulated high levels of MTX and MTXPGs (79.2%, s.e. 8.3%) was not significantly different from that of patients with lesser accumulation of MTX and MTXPGs (77.7%, s.e. 5.4%). These findings support the notion that higher dose MTX therapy has contributed to increased cure, particularly for patients whose lymphoblasts accumulate the drug less well.

Folate pathway gene expression differs in subtypes of acute lymphoblastic leukemia and influences methotrexate pharmacodynamics

The ability of leukemia cells to accumulate methotrexate polyglutamate (MTXPG) is an important determinant of the antileukemic effects of methotrexate (MTX). We measured in vivo MTXPG accumulation in leukemia cells from 101 children with acute lymphoblastic leukemia (ALL) and established that B-lineage ALL with either TEL-AML1 or E2A-PBX1 gene fusion, or T-lineage ALL, accumulates significantly lower MTXPG compared with B-lineage ALL without these genetic abnormalities or compared with hyperdiploid (fewer than 50 chromosomes) ALL. To elucidate mechanisms underlying these differences in MTXPG accumulation, we used oligonucleotide microarrays to analyze expression of 32 folate pathway genes in diagnostic leukemia cells from 197 children. This revealed ALL subtype-specific patterns of folate pathway gene expression that were significantly related to MTXPG accumulation. We found significantly lower expression of the reduced folate carrier (SLC19A1, an MTX uptake transporter) in E2A-PBX1 ALL, significantly higher expression of breast cancer resistance protein (ABCG2, an MTX efflux transporter) in TEL-AML1 ALL, and lower expression of FPGS (which catalyzes formation of MTXPG) in T-lineage ALL, consistent with lower MTXPG accumulation in these ALL subtypes. These findings reveal distinct mechanisms of subtype-specific differences in MTXPG accumulation and point to new strategies to overcome these potential causes of treatment failure in childhood ALL.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

The association of the TEL-AML1 chromosomal translocation with the accumulation of methotrexate polyglutamates in lymphoblasts and with ploidy in childhood B-progenitor cell acute lymphoblastic leukemia: a Pediatric Oncology Group study

Lymphoblasts from children with B-progenitor cell acute lymphoblastic leukemia (BpALL) with chromosomal hyperdiploidy and with translocations affecting chromosome 12p11-13, accumulate high and low levels of methotrexate polyglutamates (MTXPGs), respectively. Recently a cryptic translocation, t(12;21) (p13;q22), has been demonstrated by molecular and fluorescence in situ hybridization techniques in this disease. The chimeric TEL-AML1 transcript, which has been associated with this translocation, can be detected in up to 25% of children with BpALL. We detected the TEL-AML1 and/or the AML1-TEL transcript in 30 (33%) of 91 patients studied. Levels of lymphoblast MTXPGs were lower in those with than in those without the TEL-AML1 translocation (P = 0.004). Hyperdiploidy was rare in lymphoblasts with the TEL-AML1 translocation (P = 0.047). Both ploidy (P= 0.0015) and TEL-AML1 status (P= 0.0043) were independently and significantly correlated with the log of the lymphoblast MTXPG level. However, the presence of TEL-AML1 or of hyperdiploidy accounted for only 22% of the variation of this value. Our results imply that each of 1.16 > or = DI and the presence of the TEL-AML1 translocation confers a 50% decrease in lymphoblast MTXPG level. When planning reduction of therapy for either of the two excellent outcome categories of hyperdiploid or TEL-AML1 BpALL, one should consider the difference between these two subgroups in the ability of lymphoblasts to accumulate MTXPGs.

The investigation of karyotypic instability in the high-hyperdiploidy subgroup of acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) of childhood has been cytogenetically well characterized, and approximately 25% of cases will have a high-hyperdiploid (51-68) chromosome complement. In a 5 year period a consecutive series of 152 presentation ALL's were karyotyped. In all cases a result was obtained and 138 (91%) had a detectable abnormal clone of which 44 (29%) were high-hyperdiploid. Within the high-hyperdiploidy group karyotypic cell to cell variation was observed in many cases. To provide further evidence of this phenomenon a dual-color fluorescence in-situ hybridization (FISH) experiment was performed on stored fixed suspension from 14 ALL's with such a karyotype. In each case 4-6 probes were investigated, employing probes to centromeres of chromosomes X, 4, 6, 8, and 10 and a locus specific probe to chromosome 21q22. It was found that the FISH produced results that were generally in good agreement with the G-banding findings and supported the notion of karyotypic cell to cell variation. FISH further showed that most of cases would have two extra copies of chromosome 21 in the majority of leukemic cells and a single extra copy in the minority. A further finding was that fewer cells contained extra copies of chromosomes 6, 8 and 10 than was expected based on the comparison of the signal number of the other probes investigated. In contrast chromosomes X, 4, and 21 seldom displayed this feature. We have demonstrated that karyotypic instability as defined by karyotypic cell to cell variation is a feature of the high-hyperdiploid subgroup in childhood ALL. It is questioned whether the underlying defect resulting in the observed karyotypic instability of this subgroup is one of the primary causative events in the formation of the leukemia.

Identification of numerical and structural chromosome aberrations in 15 high hyperdiploid childhood acute lymphoblastic leukemias using spectral karyotyping

Spectral karyotyping (SKY) on metaphase spreads from 15 high hyperdiploid (>51 chromosomes) childhood acute lymphoblastic leukemias (ALL), which typically display a poor chromosome morphology, was performed in order to investigate the pattern of numerical abnormalities, reveal the chromosomal origin of marker chromosomes, and identify translocations and other interchromosomal rearrangements not detected by G-banding analysis. In all cases the numerical changes could be fully characterized, and a non-random pattern of chromosomal gain was identified, with chromosomes X, 21, 14, 17, 6, 18, 4, and 10 being most frequently gained. The numerical changes had been partly misinterpreted in 12 of the 15 ALL patients using G-banding, and the present study hence emphasizes the importance of SKY in identifying such anomalies, some of which, i.e. +4 and +10, have been suggested to be prognostically important. The chromosomal origin of all marker chromosomes and of seven structural rearrangements, one of which was the prognostically important Philadelphia chromosome, could be identified. Five rearrangements [der(1)t(1;14)(q32;q21), der(2)t(2;8)(q36;?), der(3)t(2;3)(q21;?), der(8)t(8;14)(?;?), and t(9;21)(q12;q22)] have previously not been reported in ALL, emphasizing the value of SKY in identifying novel chromosomal rearrangements.

Preliminary experience with a new chemotherapy regimen for adults with acute lymphoblastic leukemia

We treated 11 consecutive adult patients who presented with acute lymphoblastic leukemia to the University of Chicago with a novel, intensified chemotherapy regimen to evaluate the feasibility and toxicity of this program. Following a 5-drug induction therapy (course A), patients received sequential courses (B and C) of high-dose antimetabolite therapies, in part to replace cranial irradiation for CNS prophylaxis. All patients achieved a complete remission. The regimen was designed with a goal of administering all post-remission therapy in the outpatient setting. With the exception of the initial induction course (A), 3(1/3)8 total patient courses were administered in the outpatient clinic. As expected, the induction and consolidation courses (A and B) of this regimen were associated with grade 4 hematologic toxicity and significant but manageable infectious toxicity. Another novel aspect of the regimen included a combined intravenous and oral dosing schedule designed to sustain methotrexate levels > or = 1.0 microM for 30 hours (course C). There was minimal toxicity with the CNS prophylaxis/high-dose methotrexate course (C). Methotrexate levels at 30 hours ranged from 0.31-4.0 microM, with a median of 1.0 microM (n=37). This study demonstrates that this novel post-remission regimen is feasible in adults and that high concentrations of methotrexate can be sustained in the outpatient setting. The Cancer and Leukemia Group B is presently evaluating the efficacy of this regimen in a large phase II trial (CALGB study 19802).

Insights into the biologic and molecular abnormalities in adult acute lymphocytic leukemia

The last 3 decades have seen much progress in the treatment and outcome of patients with ALL. Unfortunately, the success that has been achieved in children with ALL has not yet been translated into adult patients. Insight into the biologic and molecular abnormalities in ALL may, however, provide the necessary clues that allow a clearer understanding of the crucial differences in the behavior of ALL in different groups of patients. As the molecular basis of the disease is deciphered, new targets are discovered that may prove useful for therapeutic interventions in the future.

Simultaneous measurement of the frequencies of intrachromosomal recombination and chromosome gain using the yeast DEL assay

The yeast DEL assay measures the frequency of intrachromosomal recombination between two partially-deleted his3 alleles on chromosome XV. The his3Delta alleles share approximately 400bp of overlapping homology, and are separated by an intervening LEU2 sequence. Homologous recombination between the his3Delta alleles results in deletion of the intervening LEU2 sequence (DEL), and reversion to histidine prototrophy. In this study we have attempted to further extend the use of the yeast DEL assay to measure the frequency of chromosome XV gain events. Reversion to His(+)Leu(+) in the haploid yeast DEL tester strain RSY6 occurs upon non-disjunction of chromosome XV sister chromatids, coupled with a subsequent DEL event. Here we have tested the ability of the yeast DEL assay to accurately predict the aneugenic potential of the diversely-acting, known or suspected aneugens actinomycin D, benomyl, chloral hydrate, ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS), and methotrexate. Actinomycin D and benomyl strongly induced aneuploidy. EMS and methotrexate modestly induced aneuploidy, while chloral hydrate and MMS failed to illicit any significant induction. In addition, by FACS-analysis of DNA content it was shown that the majority of both spontaneous- and chemically-induced His(+)Leu(+) revertants were heterodiploid. Thus, our results indicate endoreduplication of almost entire chromosome sets as a major mechanism of aneuploidy induction in haploid Saccharomyces cerevisiae.