Infant leukemia, topoisomerase II inhibitors, and the MLL gene

Genetic polymorphisms and susceptibility to childhood acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most common form of pediatric cancer. Although exposure to environmental agents appears to predispose individuals to this disease, little attention has been paid to the role of genetic susceptibility to environmental exposures in the etiology of childhood ALL. The enzymes GSTM1, GSTT1, GSTP1, CYP1A1, and CYP2E1 are involved in the bioactivation and detoxification of a variety of xenobiotics present in food, organic solvents, tobacco smoke, drugs, alcoholic drinks, pesticides, and environmental pollutants. Polymorphisms in the genes coding for these enzymes have been associated with increased susceptibility to different cancers, including hematologic malignancies. To investigate whether these polymorphisms represent risk-modifying factors for childhood ALL, a study was conducted involving 113 Brazilian patients of childhood ALL and 221 controls with similar ethnic backgrounds. The data revealed that carriers of the rare GSTP1 Val allele were at higher risk of ALL (odds ratio [OR] = 2.7; 95% confidence interval [CI] = 1.1-6.8; P = 0.04). No difference was found in the prevalence of the GSTM1 and GSTT1 null genotypes between ALL patients and the controls, and no association was found between CYP1A1*2 and CYP2E1*3 variants and ALL. However, when the mutant CYP1A1 and CYP2E1 alleles were considered together with the GSTM1 and GSTP1 risk-elevating genotypes, the risk of ALL was increased further (OR = 10.3; 95% CI = 1.0-111.8; P = 0.05), suggesting a combined effect. These results imply that genetic variants of xenobiotic metabolizing genes influence the risk of developing childhood ALL.

Reduction of genistein clastogenicity in Chinese hamster V79 cells by daidzein and other flavonoids

A broad spectrum of health benefits has been ascribed to soy products. These products contain soy protein and relatively high levels of polyphenolic compounds known as flavonoids. While they are the most likely candidates for biological activity, flavonoids as a class, and of specific interest, genistein, are well known to be genotoxic due to their ability to "poison" cellular DNA topoisomerase II (topo II) resulting in stable chromosome breakage and mutation and raising questions about the long term health effects associated with chronic flavonoid exposure. Interestingly, some flavonoids, such as biochanin, galangin and daidzein, are catalytic topo II inhibitors (not poisons) and actually antagonize the clastogenicity of topo II poisons. It is shown in the present paper that flavonoids possessing catalytic topo II inhibitory activity, strongly antagonize the clastogenicity of genistein in Chinese hamster V79 cells. Importantly, one of these, daidzein, is a major constituent of marketed soy products. It is conjectured that the potential human clastogenic risk of soy products containing genistein might be mitigated or abolished due to the presence of daidzein or other flavonoids in those products.

Leukemia in twins: lessons in natural history

Identical infant twins with concordant leukemia were first described in 1882, and since that time many such pairs of infants and older children have been described. It has long been recognized that this situation offers a unique opportunity to identify aspects of the developmental timing, natural history, and molecular genetics of pediatric leukemia in general. We reviewed both the older literature and more recent molecular biologic studies that have uncovered the basis of concordance of leukemia. Molecular markers of clonality, including unique, genomic fusion gene sequences, have provided unequivocal evidence that twin pairs of leukemia have a common clonal origin. The only plausible basis for this, first suggested more than 40 years ago, is that following initiation of leukemia in one twin fetus, clonal progeny spread to the co-twin via vascular anastomoses within a single, monochorionic placenta. This explanation has been endorsed by the identification of clonotypic gene fusion sequences in archived neonatal blood spots of individuals who subsequently developed leukemia. These analyses of twin leukemias have thrown considerable light on the natural history of disease. They reveal a frequent prenatal origin and an early or initiating role for chromosome translocations. Further, they provide evidence for a variable and often protracted latency and the need, in childhood acute lymphoblastic leukemia (ALL)/acute myeloblastic leukemia (AML), for further postnatal exposures and/or genetic events to produce clinical disease. We argue that these insights provide a very useful framework for attempts to understand etiologic mechanisms.

Molecular analysis of t(X;11)(q24;q23) in an infant with AML-M4

t(X;11)(q24;q23) is a recurring chromosomal translocation in pediatric acute myeloid leukemia. The rearrangement results in fusion of MLL at 11q23 with SEPT6 at Xq24. Here, we report the identification of an MLL-SEPT6 fusion transcript in an infant with acute myeloid leukemia (AML)-M4. Reverse transcription-polymerase chain reaction confirmed the presence of an MLL-SEPT6 fusion transcript composed of exon 8 of MLL and exon 2 of SEPT6, but the absence of the reciprocal SEPT6-MLL fusion transcript. Sequence analysis of the genomic break junctions in MLL and SEPT6 suggested that the rearrangement in this case was the result of an insertion of the inverted Xq24 segment, which contained the 3' region of SEPT6 intron 1 and up to 950 kb centromeric to SEPT6, into MLL intron 8. This is a novel type of chromosomal rearrangement leading to the MLL-SEPT6 fusion. The presence of deletions, duplications, and non-template DNA sequence at the break junctions suggested that the DNA damage-repair machinery is likely to be involved in the translocation events.

Origins of chromosome translocations in childhood leukaemia

Chromosome translocations are often early or initiating events in leukaemogenesis, occurring prenatally in most cases of childhood leukaemia. Although these genetic changes are necessary, they are usually not sufficient to cause leukaemia. How, when and where do translocations arise? And can these insights aid our understanding of the natural history, pathogenesis and causes of leukaemia?

Recent advances in the treatment of infant acute myeloid leukemia

Infant acute myeloid leukemia (AML) of less than 12 months old is generally characterized by a high incidence of acute monoblastic or myelomonoblastic leukemia with hyperleukocytosis and extramedullary involvement. Most of the leukemic cells have 11q23 translocations, which lead to the MLL gene rearrangements. The MLL gene rearrangements occur at a high frequency in monoblastic subtype, hyperleukocytosis or young age in infant AML. Compared with acute lymphoblastic leukemia, however, it remains unknown whether prenatal origin exists in the pathogenesis of infant AML. Recently, the treatment outcome of infant AML has been clarified by two study groups, which confirmed the effect of intensive chemotherapy including repeated cycles of cytarabine and anthracyclines for infant AML. Presence of the MLL gene rearrangements, gender, age and white blood cell count showed no influence on the outcome of infant AML. The allogeneic hematopoietic stem cell transplantation (HSCT) remains the treatment of choice for infant AML when a matched related donor is available. Monitoring of minimal residual disease by real-time PCR is a useful technique to predict the outcome or efficacy of the treatment in infant AML. Although intensive chemotherapy and/or allogeneic HSCT have cured most AML infants, some still relapse and ultimately die. A need remains for future development by exploiting the unusual biologic properties of leukemic progenitor cells expressing the abnormal MLL gene product.

Lack of significant genotoxicity of purified soy isoflavones (genistein, daidzein, and glycitein) in 20 patients with prostate cancer

BACKGROUND

Genistein may be useful in the prevention or treatment of prostate cancer; however, it causes genetic damage in cultured human cells.

OBJECTIVE

The objective was to assess the potential genotoxicity of a purified soy unconjugated isoflavone mixture in men with prostate cancer.

DESIGN

Twenty patients with prostate cancer were treated with 300 mg genistein/d for 28 d and then with 600 mg/d for another 56 d. In peripheral lymphocytes, DNA strand breaks were assessed as nuclear tail moment, chromosomal damage was assessed as micronucleus frequency (MF), and translocations of the MLL gene (11q23) were assessed by using fluorescence in situ hybridization. Values are also reported for 6 healthy men. The studies were performed under Investigational New Drug application no. 54 137 at a tertiary referral academic medical center.

RESULTS

No changes in group average or individual nuclear tail moment and MF were observed. We observed a single elevated MF value in one subject that exceeded a clinical threshold set before we initiated the study. A significant decrease in average COMET tail moment was observed on day 28 relative to day 0. We detected no genistein-induced rearrangements of the MLL gene in the 3 subjects we studied with this technique. MF increased significantly in lymphocytes exposed in vitro to unconjugated genistein at concentrations > or = 100 micromol/L. Total genistein never exceeded a peak concentration of 27.1 micro mol/L, and unconjugated genistein never exceeded a peak concentration of 0.32 micromol/L.

CONCLUSION

Although isoflavones are capable of inducing genetic damage in vitro, a similar effect was not observed in subjects treated with a purified soy unconjugated isoflavone mixture.

Prenatal presentation supports the in utero development of congenital leukemia: a case report

Congenital leukemia is a rare disease developing within the first 4 to 6 weeks of life. We report a female infant born with facial mass and multiple subcutaneous nodules. The facial mass was discovered by ultrasound during a routine prenatal examination at the 36th week of gestation. Biopsies were consistent with the diagnosis of acute monoblastic leukemia (AML, FAB M5b). Cytogenetic studies showed 46 XX, t(11;19)(q23;p13.1), which is only found in acute monoblastic leukemia and involves the gene. The infant died at 12 days of age and autopsy revealed a large leukemic tumor burden in several body organs. The discovery of the facial mass prenatally and massive extramedullary leukemic burden support the notion of the in utero development of congenital leukemia.

Molecular characterization of genomic AML1-ETO fusions in childhood leukemia

T(8;21) AML1(CBFA2)-ETO(MTG8) is the most common chromosomal translocation in acute myeloid leukemia (AML) in both children and adults. We sought to understand the structure and gain insight into the fusion process between AML1 and ETO by sequencing genomic fusions in 17 primary childhood AMLs and two cell lines with t(8;21). Reciprocal translocations were sequenced for seven of the 19 samples. We assumed a null hypothesis that the translocation breakpoints would be evenly distributed along the intronic breakpoint cluster regions. Testing for multimodality via smoothed bootstrap statistical methods suggested, however, the presence of two separate cluster regions within both the AML1 and ETO breakpoint cluster regions. ETObreakpoints were predominantly located in intron 1B in a defined cluster 5' of exon 1A (scan statistic P value = 0.00001). All patients with available RNA expressed an AML1-ETO mRNA fusion between exon 5 of AML1 and exon 2 of ETO. Since the structural restraints for the fusion protein of AML1-ETO exclude exon 1A, we reason that ETO intron 1B harbors a structural feature with propensity for breakage and/or recombination. Chromosomal breakpoints displayed evidence of fusion by a non-homologous end joining process, with microhomologies and nontemplate nucleotides at some fusion junctions. Breakpoints in general displayed similar complexity of duplications, deletions, and insertions to other common pediatric leukemia translocations (TEL-AML1, MLL-AF4, PML-RARA, CBFB-MYH11) that we and others have analyzed.

Methylenetetrahydrofolate reductase (MTHFR) polymorphisms and risk of molecularly defined subtypes of childhood acute leukemia

Low folate intake as well as alterations in folate metabolism as a result of polymorphisms in the enzyme methylenetetrahydrofolate reductase (MTHFR) have been associated with an increased incidence of neural tube defects, vascular disease, and some cancers. Polymorphic variants of MTHFR lead to enhanced thymidine pools and better quality DNA synthesis that could afford some protection from the development of leukemias, particularly those with translocations. We now report associations of MTHFR polymorphisms in three subgroups of pediatric leukemias: infant lymphoblastic or myeloblastic leukemias with MLL rearrangements and childhood lymphoblastic leukemias with either TEL-AML1 fusions or hyperdiploid karyotypes. Pediatric leukemia patients (n = 253 total) and healthy newborn controls (n = 200) were genotyped for MTHFR polymorphisms at nucleotides 677 (C-->T) and 1,298 (A-->C). A significant association for carriers of C677T was demonstrated for leukemias with MLL translocations (MLL+, n = 37) when compared with controls [adjusted odd ratios (OR) = 0.36 with a 95% confidence interval (CI) of 0.15-0.85; P = 0.017]. This protective effect was not evident for A1298C alleles (OR = 1.14). In contrast, associations for A1298C homozygotes (CC; OR = 0.26 with a 95% CI of 0.07--0.81) and C677T homozygotes (TT; OR = 0.49 with a 95% CI of 0.20--1.17) were observed for hyperdiploid leukemias (n = 138). No significant associations were evident for either polymorphism with TEL-AML1+ leukemias (n = 78). These differences in allelic associations may point to discrete attributes of the two alleles in their ability to alter folate and one-carbon metabolite pools and impact after DNA synthesis and methylation pathways, but should be viewed cautiously pending larger follow-up studies. The data provide evidence that molecularly defined subgroups of pediatric leukemias have different etiologies and also suggest a role of folate in the development of childhood leukemia.

Prognostic factors for hematologic cancers

This article reviews the molecular biology of hematologic cancers and the current understanding of prognostic factors for these cancers. Specific molecular biomarkers that have potential as prognostic factors for various hematologic cancers are discussed. Quantitative and statistical methods of evaluating the usefulness of prognostic factors are presented.

Dietary bioflavonoids induce cleavage in the MLL gene and may contribute to infant leukemia

Chromosomal translocations involving the MLL gene occur in about 80% of infant leukemia. In the search for possible agents inducing infant leukemia, we identified bioflavonoids, natural substances in food as well as in dietary supplements, that cause site-specific DNA cleavage in the MLL breakpoint cluster region (BCR) in vivo. The MLL BCR DNA cleavage was shown in primary progenitor hematopoietic cells from healthy newborns and adults as well as in cell lines; it colocalized with the MLL BCR cleavage site induced by chemotherapeutic agents, such as etoposide (VP16) and doxorubicin (Dox). Both in vivo and additional in vitro experiments demonstrated topoisomerase II (topo II) as the target of bioflavonoids similar to VP16 and Dox. Based on 20 bioflavonoids tested, we identified a common structure essential for topo II-induced DNA cleavage. Reversibility experiments demonstrated a religation of the bioflavonoid as well as the VP16-induced MLL cleavage site. Our observations support a two-stage model of cellular processing of topo II inhibitors: The first and reversible stage of topo II-induced DNA cleavage results in DNA repair, but also rarely in chromosome translocations; whereas the second, nonreversible stage leads to cell death because of an accumulation of DNA damage. These results suggest that maternal ingestion of bioflavonoids may induce MLL breaks and potentially translocations in utero leading to infant and early childhood leukemia.

The United Kingdom Childhood Cancer Study: objectives, materials and methods. UK Childhood Cancer Study Investigators

An investigation into the possible causes of childhood cancer has been carried out throughout England, Scotland and Wales over the period 1991-1998. All children known to be suffering from one or other type of the disease over periods of 4-5 years have been included, and control children matched for sex, age and area of residence have been selected at random from population registers. Information about both groups of children (with and without cancer) has been obtained from parental questionnaires, general practitioners' and hospital records, and from measurement of the extent of exposure to radon gas, terrestrial gamma radiation, and electric and magnetic fields. Samples of blood have also been obtained from the affected children and their parents and stored. Altogether 3,838 children with cancer, including 1,736 with leukaemia, and 7,629 unaffected children have been studied. Detailed accounts are given of the nature of the information obtained in sections describing the general methodology of the study, the measurement of exposure to ionizing and non-ionizing radiation, the classification of solid tumours and leukaemias, and the biological material available for genetic analysis.

Molecular genetics, natural history and the demise of childhood leukaemia

The patterns of genetic change, clonal evolution, natural history and latency are very different in the paediatric leukaemias compared with adult epithelial cancers but are similar to those in other childhood cancers of mesenchymal stem cell origin. This distinction has a biological logic in the context of the selective pressures for clonal emergence in different developmental and cellular contexts and has a major impact on curability. Most childhood leukaemias and some other mesenchymal stem cell tumours are of fetal origin and can metastasize without corruption of restraints on cell proliferation or bypassing apoptosis. In marked contrast to most invasive or metastatic epithelial carcinomas in adults, these former cancers then retain sensitivity to therapeutic apoptosis. Moreover, their abbreviated and less complex evolutionary status is associated with less genetic diversity and instability, minimising opportunity for clonal selection for resistance. A minority of leukaemias in children and a higher fraction in adults do, however, have genetic alterations that bypass cell cycle controls and apoptosis imposition. These are the 'bad news' genotypes. The cellular and molecular diversity of acute leukaemia impacts also on aetiology. Paediatric acute leukaemias can be initiated prenatally by illegitimate recombination and fusion gene formation in fetal haemopoiesis. For acute lymphoblastic leukaemia (ALL) in children, twin studies suggest that a secondary postnatal molecular event is also required. This may be promoted by an abnormal or delayed response to common infections. Even for a classic case of a cancer that is intrinsically curable by systematic chemotherapy i.e. childhood ALL, prevention may turn out to be the preferred option.

Congenital and neonatal leukemia

Congenital and neonatal leukemia occur rarely, yet carry high mortality rates and pose special problems for the perinatologist and hematologist. Although the etiology is unknown, the presence of leukemia at birth suggests genetic abnormalities and possibly intrauterine exposures to drugs or other toxins as contributing factors. Specific chromosomal rearrangements that are common in congenital leukemia have recently been identified and promise to enhance our understanding of these enigmatic diseases. The differential diagnosis is broad and includes many disorders that occur frequently in the neonatal period. Infants diagnosed with congenital or neonatal leukemia require thorough investigative workup and extensive supportive care. Although the prognosis is poor, recent use of high-intensity multiagent chemotherapy regimens has produced promising results.

A novel chromosomal rearrangement associated with therapy-related acute leukemia

We describe a 7-year-old girl with therapy-related acute myeloid leukemia (AML) associated with a single and novel karyotypic abnormality. The patient had been treated with alkylating agents and etoposide for hypothalamic pilocytic astrocytoma at age 17 months, and developed mixed lineage AML. Cytogenetic analysis of the leukemic blasts showed 46,XX,der(7)t(7;11)(q22;q14) in all cells examined. Southern blot analysis revealed three copies of an unrearranged MLL gene on chromosome 11q. This is the first report of a triplicated, unrearranged MLL gene in association with a deletion of 7q anomaly and an unbalanced translocation in therapy-related leukemia.

Molecular genetics, natural history and the demise of childhood leukaemia

The patterns of genetic change, clonal evolution, natural history and latency are very different in the paediatric leukaemias compared with adult epithelial cancers but are similar to those in other childhood cancers of mesenchymal stem cell origin. This distinction has a biological logic in the context of the selective pressures for clonal emergence in different developmental and cellular contexts and has a major impact on curability. Most childhood leukaemias and some other mesenchymal stem cell tumours are of fetal origin and can metastasize without corruption of restraints on cell proliferation or bypassing apoptosis. In marked contrast to most invasive or metastatic epithelial carcinomas in adults, these former cancers then retain sensitivity to therapeutic apoptosis. Moreover, their abbreviated and less complex evolutionary status is associated with less genetic diversity and instability, minimising opportunity for clonal selection for resistance. A minority of leukaemias in children and a higher fraction in adults do, however, have genetic alterations that bypass cell cycle controls and apoptosis imposition. These are the 'bad news' genotypes. The cellular and molecular diversity of acute leukaemia impacts also on aetiology. Paediatric acute leukaemias can be initiated prenatally by illegitimate recombination and fusion gene formation in fetal haemopoiesis. For acute lymphoblastic leukaemia (ALL) in children, twin studies suggest that a secondary postnatal molecular event is also required. This may be promoted by an abnormal or delayed response to common infections. Even for a classic case of a cancer that is intrinsically curable by systematic chemotherapy i.e. childhood ALL, prevention may turn out to be the preferred option.

The causes of acute leukemia

Although impressive biologic advances have increased understanding of leukemogenesis, we know little about the causes of the acute leukemias. Epidemiologic studies have focused primarily on children. Higher birth weight is associated with an increased risk of childhood acute leukemia. Several theories have been advanced that may account for these observations, and additional biologic studies are needed. Some epidemiologic studies suggest that the acute leukemias in children may have an infectious component. Again, further work, especially in the area of specific causative agents, is necessary. Another area for future epidemiologic study includes investigation of exposure to natural and synthetic DNA topoisomerase II inhibitors. Preliminary evidence suggests that exposure to these agents, which are found in certain foods and medications, may be related to the subsequent development of acute leukemia in infants.

Childhood cancer: improved prospects for survival but is prevention possible?

With optimum treatment 65-70% of children diagnosed with cancer should be long term survivors and probably cured. Prevention is better than cure. Recent studies into the causes of childhood malignancy are reviewed. The incidence of childhood acute lymphoblastic leukaemia (ALL) is steadily increasing. The cause for this may be increasing social and economic development. Exposure to electromagnetic fields has been a cause of concern for almost 20 years. A recent large case control study has shown no increased risk of cancer or leukaemia in those who have measurably increased exposure to electromagnetic fields. Leukaemia which has the cytogenetic abnormality 11q23 or MLL gene rearrangement characteristically occurs as a second malignancy after exposure to epipodophyllotoxins which act by inhibiting topoisomerase. Infant leukaemia has the same cytogenetic profile. Mothers of babies who develop infant leukaemia have high exposure to potential dietary inhibitors of topoisomerase during pregnancy. Clusters of leukaemia can probably best be accounted for by population mixing. There may be an increased risk of ALL following vitamin K given to newborns.

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.

Seminars from the University of Minnesota. Chromosome translocations: dangerous liaisons

Many chromosome abnormalities, especially translocations or inversions, are closely associated with a particular morphologic or phenotypic subtype of leukemia, lymphoma, or sarcoma. Cloning the genes at the breakpoints of these rearrangements has provided critical tools for more-precise diagnosis; in some cases the particular diagnosis has prognostic implications. In addition, many of the genes had not been previously identified; their discovery has had a major impact on our understanding of the molecular biology of cancer. One such gene is MLL (myeloid-lymphoid or mixed-lineage leukemia), which is located at chromosome band 11q23. This gene is involved in the 4;11 and 11;19 (p13.3) translocations in acute lymphoblastic leukemia and in the 6;11, 9;11, and 11;19 (p13.1) translocations in acute myeloblastic leukemia. It is also involved in most translocations in infants (under 1 year of age) with acute leukemia and in patients with acute leukemia who were previously treated with drugs that inhibit toposiomerase II. The target gene of MLL is unknown at present, but because of its homology to the trithorax gene in Drosophila, and based on experimental data from mice, it appears to be involved in maintaining the function of some of the homeobox genes. The development of cytogenetic and molecular probes for MLL rearrangements has confirmed that translocations involving MLL are associated with a very poor prognosis. Thus physicians can identify patients with MLL involvement and can institute treatment for these high-risk patients. An increasing understanding of MLL should lead to more-effective targeted therapy.

Inhibitors of topoisomerase II enzymes: a unique group of environmental mutagens and carcinogens

Many inhibitors of topoisomerase II enzymes are potent mutagens, leading to major chromosomal deletions, illegitimate recombination and aneuploidy. There is increasing evidence that they are also human carcinogens. However, their lack of chemical reactivity means that they may give weak or negative results in commonly used mutagenicity tests, or may give data with characteristics quite distinct from chemicals that alkylate DNA. They do not form DNA adducts and assays such as 32P-postlabelling will not detect their presence in the body. They are generally not point mutagens and may fail to provide distinctive fingerprints in mutation spectra. These characteristics may be limiting a realistic evaluation of their role in human carcinogenesis using current methodologies.

Diversity of DNA topoisomerases I and inhibitors

The present review first describes the different type I topoisomerases found in eukaryotic cells: nuclear topoisomerase I (top1), topoisomerase 3 (top3), mitochondrial topoisomerase I and viral topoisomerases I. The second part of the review provides extensive information on the topoisomerase I inhibitors identified to date. These drugs can be grouped in two categories: top1 poisons and top1 suppressors. Both inhibit enzyme catalytic activity but top1 poisons trap the top1 catalytic intermediates ('cleavage complexes') while top1 suppressors prevent or reverse top1 cleavage complexes. The molecular interactions of camptothecin with the top1 cleavage complexes are discussed as well as the mechanisms of selective killing of cancer cells.

Spatial clustering of childhood leukaemia: summary results from the EUROCLUS project

The interpretation of reports of clusters of childhood leukaemia is difficult, first because little is known about the causes of the disease, and second because there is insufficient information on whether cases show a generalized tendency to cluster geographically. The EUROCLUS project is a European collaborative study whose primary objective is to determine whether the residence locations of cases at diagnosis show a general tendency towards spatial clustering. The second objective is to interpret any patterns observed and, in particular, to see if clustering can be explained in terms of either infectious agents or environmental hazards as aetiological agents. The spatial distribution of 13351 cases of childhood leukaemia diagnosed in 17 countries between 1980 and 1989 has been analysed using the Potthoff-Whittinghill method. The overall results show statistically significant evidence of clustering of total childhood leukaemia within small census areas (P=0.03) but the magnitude of the clustering is small (extra-Poisson component of variance (%) = 1.7 with 90% confidence interval 0.2-3.1). The clustering is most marked in areas that have intermediate population density (150-499 persons km[-2]). It cannot be attributed to any specific age group at diagnosis or cell type and involves spatial aggregation of cases of different ages and cell types. The results indicate that intense clusters are a rare phenomenon that merit careful investigation, although aetiological insights are more likely to come from investigation of large numbers of cases. We present a method for detecting clustering that is simple and readily available to cancer registries and similar groups.

Spatial temporal patterns in childhood leukaemia: further evidence for an infectious origin. EUROCLUS project

The EUROCLUS project included information on residence at diagnosis for 13351 cases of childhood leukaemia diagnosed in the period 1980-89 in defined geographical regions in 17 countries. A formal algorithm permits identification of small census areas as containing case excesses. The present analysis examines spatial-temporal patterns of the cases (n = 970) within these clustered areas. The objectives were, first, to compare these results with those from an analysis conducted for UK data for the period 1966-83, and, second, to extend them to consider infant leukaemias. A modification of the Knox test investigates, within the small areas, temporal overlap between cases in a subgroup of interest at a putative critical time and all other cases at any time between birth and diagnosis. Critical times were specified in advance as follows: for cases of acute lymphoblastic leukaemia aged 2-4 years, the 18-month period preceding diagnosis; for cases of total leukaemia aged 5-14 years, 1 year before to 1 year after birth; and for infant cases (diagnosed < 1 year), 1 year before to 6 months after birth. Each of the analyses found evidence of excess space-time overlap compared with that expected; these were 10% (P = 0.005), 15% (P= 0.0002) and 26% (P= 0.03) respectively. The results are interpreted in terms of an infectious origin of childhood leukaemia.

Backtracking leukemia to birth: identification of clonotypic gene fusion sequences in neonatal blood spots

Epidemiological evidence has suggested that some pediatric leukemias may be initiated in utero and, for some pairs of identical twins with concordant leukemia, this possibility has been strongly endorsed by molecular studies of clonality. Direct evidence for a prenatal origin can only be derived by prospective or retrospective detection of leukemia-specific molecular abnormalities in fetal or newborn samples. We report a PCR-based method that has been developed to scrutinize neonatal blood spots (Guthrie cards) for the presence of numerically infrequent leukemic cells at birth in individuals who subsequently developed leukemia. We demonstrate that unique or clonotypic MLL-AF4 genomic fusion sequences are present and detectable in neonatal blood spots from individuals who were diagnosed with acute lymphoblastic leukemia at ages 5 months to 2 years and, therefore, have arisen during fetal hematopoiesis in utero. This result provides unequivocal evidence for a prenatal initiation of acute leukemia in young patients. The method should be applicable to other fusion genes in children with common subtypes of leukemia and will be of value in attempts to unravel the natural history and etiology of this major subtype of pediatric cancer.

Infant cancer in the U.S.: histology-specific incidence and trends, 1973 to 1992

BACKGROUND

Many cancers in infants demonstrate unique epidemiologic, clinical, and genetic characteristics compared with cancers in older children. Few epidemiologic reports, however, have focused on this important age group.

METHODS

Population-based data from the Surveillance, Epidemiology, and End Results (SEER) program were used to estimate relative frequency, incidence rates, and average annual percentage change of rates among children in their first year of life (infants) who were diagnosed with a malignant neoplasm from 1973 to 1992 (N = 1461).

RESULTS

The greatest proportion of cases (12%) was diagnosed during the first month of life, with extracranial neuroblastoma accounting for 35% of this total. Overall, the average annual incidence rate was 223/1,000,000 infants. Extracranial neuroblastoma was the most common infant malignancy (58/1,000,000 infants per year), followed by leukemias (37/1,000,000), brain and central nervous system (CNS) tumors (34/1,000,000), and retinoblastoma (27/1,000,000). White infants had a 32% higher incidence rate than black infants. The average annual percentage increase in rates for all cancer from 1973 to 1992 was 2.9% (95% CI: 1.9%, 3.8%). For neoplasms with at least 100 cases, increasing trends were greatest for retinoblastoma (4.6%), CNS (4.1%), and extracranial neuroblastoma (3.4%).

CONCLUSIONS

Incidence rates increased notably over the study period. Future studies should consider the unique presentation of infants with cancer when developing new hypotheses related to cancer etiology and gene-environment interactions.

MLL rearrangements in infant leukemia: is there a higher frequency in females?

Unlike childhood leukemia diagnosed at later ages, where there is an excess risk in males, there is a notable female predominance in infant leukemia. Since abnormalities involving the MLL gene are frequent in infants with leukemia, we asked whether there are gender differences with respect to MLL gene status. Data were combined from seven published molecular studies that examined MLL abnormalities in infants with leukemia. Of 239 cases with conclusive information, there was a female predominance in cases with an MLL abnormality (female:male ratio 1.3); this was in marked contrast to cases with normal germline MLL (female:male ratio, 0.7). These potential gender-dependent MLL frequencies should be explored in large populations of infant leukemia and, if confirmed, future investigations could be directed toward potential etiologic mechanisms.

Higher Frequency of Glutathione S-Transferase Deletions in Black Children With Acute Lymphoblastic Leukemia

The genetic polymorphisms in human glutathione S-transferases (GST) M1 and T1 have been associated with race, disease risk, and outcome of some adult cancers. Also, there are racial differences in the incidence and characteristics of childhood acute lymphoblastic leukemia (ALL). Our objectives were to compare the frequency of the null genotype for GSTM1, GSTT1, or both in children with ALL to that in healthy controls, and to determine whether GST genotype was associated with treatment outcome and prognostic factors. We studied GSTM1 and GSTT1 genotypes in somatic cell DNA from black children and white children with ALL and in 416 healthy controls, using a polymerase chain reaction technique. Ninety of 163 (55.2%) white ALL patients and 14 of 34 (41.2%) black patients were GSTM1 null, frequencies not significantly different (P = .19) than healthy controls (53.5% in whites and 27.6% in blacks), although there was a trend toward more null genotypes in black ALL patients. Twenty-three of 163 (14.1%) white ALL patients and 12 of 34 (35.3%) black ALL patients were GSTT1 null, not different (P = .34) than the frequencies in healthy controls (15.0% in whites and 24.1% in blacks). However, the frequency of the “double-null” genotype, lacking both GSTM1 and GSTT1, was higher in black patients with ALL (8 of 34 or 23.5%) than in black controls (3.9%) (P = .0005), but this was not the case in white patients with ALL (10 of 163 or 6.1%) compared to white controls (8.0%) (P = .68). In stratified analyses, the GST double-null genotype was not associated with other characteristics that might differ between whites and blacks with ALL, such as age, T-lineage immunophenotype, presenting white blood cell count, DNA index, or insurance status. The null genotype for GSTM1, GSTT1, or both was not found to be a prognostic factor for disease-free survival or probability of hematologic remission; central nervous system relapse tended to be less common in those with the GSTM1 null genotype (P = .054) . The double-null genotype for GSTM1 and GSTT1 is more common among blacks but not whites with childhood ALL. These data suggest that GST genotype, coupled with unidentified additional risk factors, may play a role in risk of childhood ALL in American blacks.

Higher Frequency of Glutathione S-Transferase Deletions in Black Children With Acute Lymphoblastic Leukemia

The genetic polymorphisms in human glutathione S-transferases (GST) M1 and T1 have been associated with race, disease risk, and outcome of some adult cancers. Also, there are racial differences in the incidence and characteristics of childhood acute lymphoblastic leukemia (ALL). Our objectives were to compare the frequency of the null genotype for GSTM1, GSTT1, or both in children with ALL to that in healthy controls, and to determine whether GST genotype was associated with treatment outcome and prognostic factors. We studied GSTM1 and GSTT1 genotypes in somatic cell DNA from black children and white children with ALL and in 416 healthy controls, using a polymerase chain reaction technique. Ninety of 163 (55.2%) white ALL patients and 14 of 34 (41.2%) black patients were GSTM1 null, frequencies not significantly different (P = .19) than healthy controls (53.5% in whites and 27.6% in blacks), although there was a trend toward more null genotypes in black ALL patients. Twenty-three of 163 (14.1%) white ALL patients and 12 of 34 (35.3%) black ALL patients were GSTT1 null, not different (P = .34) than the frequencies in healthy controls (15.0% in whites and 24.1% in blacks). However, the frequency of the “double-null” genotype, lacking both GSTM1 and GSTT1, was higher in black patients with ALL (8 of 34 or 23.5%) than in black controls (3.9%) (P = .0005), but this was not the case in white patients with ALL (10 of 163 or 6.1%) compared to white controls (8.0%) (P = .68). In stratified analyses, the GST double-null genotype was not associated with other characteristics that might differ between whites and blacks with ALL, such as age, T-lineage immunophenotype, presenting white blood cell count, DNA index, or insurance status. The null genotype for GSTM1, GSTT1, or both was not found to be a prognostic factor for disease-free survival or probability of hematologic remission; central nervous system relapse tended to be less common in those with the GSTM1 null genotype (P = .054) . The double-null genotype for GSTM1 and GSTT1 is more common among blacks but not whites with childhood ALL. These data suggest that GST genotype, coupled with unidentified additional risk factors, may play a role in risk of childhood ALL in American blacks.

Aetiology of acute leukaemia

Ionising radiation, in high dose and with acute exposure, is a factor that has been implicated in leukaemogenesis, but what is the evidence for leukaemogenesis and exposure to diagnostic X-rays, to natural terrestrial or cosmic ionising radiation, to electromagnetic fields, or to nuclear energy? Why is population mixing and infection a possible explanation for the clusters of childhood acute leukaemias around the nuclear processing plants of Sellafield and Dounreay? These questions, as well as how chemical agents, including therapeutic substances, might contribute to leukaemogenesis, are discussed in this last article in the leukaemia series.

Apurinic sites are position-specific topoisomerase II poisons

Many anticancer drugs "poison" topoisomerase II by enhancing its double-stranded DNA cleavage activity. To determine whether DNA lesions act as endogenous topoisomerase II poisons, we characterized the effects of position-specific apurinic sites on enzyme activity. Lesions located within the 4-base overhang generated by enzyme-mediated DNA scission stimulated cleavage approximately 10-18-fold without altering the specificity of topoisomerase II. DNA breaks were double-stranded in nature, protein-linked, and readily reversible. In contrast, apurinic sites located immediately outside the cleavage overhang were inhibitory. Thus, apurinic sites, which are the most commonly formed lesion in DNA, are position-specific topoisomerase II poisons. A model is proposed that encompasses the actions of endogenous and exogenous topoisomerase II poisons and provides a pre-existing pathway for the cellular actions of topoisomerase II-targeted anticancer drugs.

Clustering of childhood leukaemia in Hong Kong: association with the childhood peak and common acute lymphoblastic leukaemia and with population mixing

Incidence data of childhood leukaemia (CL) in Hong Kong (1984-90) have been analysed for evidence of variation between small areas. All cases (n=261) were classified by morphological cell type, with the majority (n=205) being acute lymphoblastic leukaemia (ALL), and haematological review has permitted immunophenotypic classification for 73% of these. The data have been examined for evidence of spatial clustering within small census areas (TPUs) and for association with population mixing, with attention focused on those subgroups (especially the childhood peak of ALL--taken here to be diagnoses in children from 24 months up to the seventh birthday--and common ALL) which, it has been hypothesized, may be caused by unusual patterns of exposure and response to common infections. For the whole of Hong Kong, there was evidence of spatial clustering of ALL at ages 0-4 years (P = 0.09) and in the childhood peak (P<0.05). When these analyses were restricted to TPUs where extreme population mixing may have occurred, overall incidence was elevated and significant evidence of clustering was found for ALL (P<0.007) at these ages and for the common ALL in the childhood peak (P = 0.032). Replication of the analyses for subsets of leukaemia that were not dominated by the childhood peak of ALL found no evidence of clustering. This is the first investigation of an association between population mixing and childhood leukaemia in Asia and the first to include clustering and to consider particular subsets. The results are supportive of the 'infectious' aetiology hypothesis for subsets of childhood leukaemia, specifically common ALL in the childhood peak.

Maternal exposure to potential inhibitors of DNA topoisomerase II and infant leukemia (United States): a report from the Children's Cancer Group

Nearly 80 percent of infant leukemias present with an abnormality involving the MLL gene at 11q23. Moreover, secondary acute myeloid leukemias (AML) that occur as the result of chemotherapy agents, which are known to inhibit DNA topoisomerase II, often manifest the same MLL abnormalities. It has been hypothesized that de novo infant leukemias may occur as a result of maternal exposure to agents in diet and medications that inhibit DNA topoisomerase II. Three epidemiologic studies of childhood leukemia with similar methodologies were conducted in the United States and Canada over the past 10 years by the Children's Cancer Group (CCG). Of the total 771 mothers of infants diagnosed at one year of age or less (< 12.5 months) who originally were interviewed (303 infant cases and 468 matched controls) across the three studies, follow-up questionnaire data on maternal exposure to potential DNA topoisomerase II inhibitors during pregnancy were available on 84 cases and 97 matched controls in the US. For maternal diet, a composite variable was created that consisted of 10 foods identified alpha priori as containing DNA topoisomerase II inhibitors. There were no significant trends with increasing maternal consumption for either the overall group, or the acute lymphoblastic leukemia (ALL) stratum. However, within the AML stratum, there was a statistically significant positive association (P trend = 0.04) with increasing consumption of DNA topoisomerase II-inhibitor containing foods (odds ratio [OR] = 9.8, 95 percent confidence interval [CI] = 1.1-84.8; OR = 10.2, CI = 1.1-96.4; for medium and high consumption, respectively). Other potential topoisomerase II inhibitors were explored; no significant findings were found. Results of this preliminary study, in combination with molecular data, should be used in future investigations of childhood leukemia (particularly, infant) to justify the incorporation of a detailed dietary history.

Second primary cancers after childhood cancer

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Effects of beta- and gamma-carboline derivatives of DNA topoisomerase activities

beta-Carbolines, harman (1-methyl-9H-pyrido[3,4-b]indole) and norharman (9H-pyrido[3,4-b]indole) and gamma-carbolines, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 3-amino-4-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), are present in cooked foods and cigarette smoke. We studied the effects of these heterocyclic amines on the activity of DNA topoisomerases. Trp-P-1 and Trp-P-2 inhibited topoisomerase I (topo I) activity with ED50 values of 1.48 and 1.55 micrograms/ml, respectively, in a relaxation assay. Harman and norharman inhibited topo I activity but with much higher ED50 values, 23.8 and 34.4 micrograms/ml, respectively. Trp-P-1 and Trp-P-2 also inhibited topoisomerase II (topo II) activity at about 50 micrograms/ml, in a decatenation assay. Harman and norharman showed a much lower inhibitory effect on topo II activity. None of these compounds stabilized the cleavable complex mediated by topo II. Trp-P-1 and Trp-P-2 intercalated into DNA at concentrations inhibitory to topoisomerases. We considered that the intercalation with DNA and the inhibition of DNA topoisomerases by heterocyclic amines might be partly related to their inhibition of DNA excision repair and their enhancing effect on UV- or chemically induced mutagenic activity.

Abasic sites stimulate double-stranded DNA cleavage mediated by topoisomerase II. DNA lesions as endogenous topoisomerase II poisons

Several clinically relevant anticancer drugs induce genomic mutations and cell death by increasing topoisomerase II-mediated DNA breakage. To determine whether endogenous DNA damage also affects this cleavage event, the effects of abasic sites (the most commonly formed spontaneous DNA lesion) on topoisomerase II activity were investigated. The presence of 3 abasic sites/plasmid stimulated enzyme-mediated DNA breakage > 6-fold, primarily by enhancing the forward rate of cleavage. This corresponds to a potency that is > 2000-fold higher than that of the anticancer drug, etoposide. These findings suggest that abasic sites represent endogenous topoisomerase II poisons and imply that anticancer drugs mimic the cleavage-enhancing actions of naturally occurring DNA lesions.