Molecular genetics of 11q23 chromosome translocations

Neonatal leukaemia cutis

Neonatal leukaemia cutis is a significant neoplasm that may represent a cutaneous manifestation of systemic leukaemia, usually of myeloblastic type. Rarely, it may be or appear to be limited to skin, in which case it is called neonatal aleukaemic leukaemia cutis. By definition, it presents within the first 4 weeks of life and often has a 'blueberry muffin baby' appearance of magenta coloured nodules affecting almost any area of the skin, usually sparing mucous membranes, palms and soles. This clinical pattern is more commonly associated with neonatal infections such rubella and toxoplasmosis, and may be evident with other neonatal neoplasms such as neuroblastoma. Due to the morbidity associated with chemotherapy and reported cases of spontaneous remission without systemic progression in those with neonatal aleukaemic leukaemia cutis without 11q23 translocation, the authors not treating the child with chemotherapy, but to simply monitor for fading of the violaceous nodules, and watch for possible signs of systemic leukaemia.

Lymphohematopoietic cancers induced by chemicals and other agents and their implications for risk evaluation: An overview

Lymphohematopoietic neoplasia are one of the most common types of cancer induced by therapeutic and environmental agents. Of the more than 100 human carcinogens identified by the International Agency for Research on Cancer, approximately 25% induce leukemias or lymphomas. The objective of this review is to provide an introduction into the origins and mechanisms underlying lymphohematopoietic cancers induced by xenobiotics in humans with an emphasis on acute myeloid leukemia, and discuss the implications of this information for risk assessment. Among the agents causing lymphohematopoietic cancers, a number of patterns were observed. Most physical and chemical leukemia-inducing agents such as the therapeutic alkylating agents, topoisomerase II inhibitors, and ionizing radiation induce mainly acute myeloid leukemia through DNA-damaging mechanisms that result in either gene or chromosomal mutations. In contrast, biological agents and a few immunosuppressive chemicals induce primarily lymphoid neoplasms through mechanisms that involve alterations in immune response. Among the environmental agents examined, benzene was clearly associated with acute myeloid leukemia in humans, with increasing but still limited evidence for an association with lymphoid neoplasms. Ethylene oxide and 1,3-butadiene were linked primarily to lymphoid cancers. Although the association between formaldehyde and leukemia remains controversial, several recent evaluations have indicated a potential link between formaldehyde and acute myeloid leukemia. The four environmental agents examined in detail were all genotoxic, inducing gene mutations, chromosomal alterations, and/or micronuclei in vivo. Although it is clear that rapid progress has been made in recent years in our understanding of leukemogenesis, many questions remain for future research regarding chemically induced leukemias and lymphomas, including the mechanisms by which the environmental agents reviewed here induce these diseases and the risks associated with exposures to such agents.

Dietary flavonoids induce MLL translocations in primary human CD34+ cells

Genetic abnormalities leading to infant leukemias already occur during fetal development and often involve rearrangements of the mixed-lineage leukemia (MLL) gene. These rearrangements resemble the aberrations observed in therapy-related leukemias following treatment with topoisomerase II (topoII)-inhibiting agents such as etoposide. Since flavonoids are potent topoII inhibitors, we examined the role of three widely consumed dietary flavonoids (quercetin, genistein and kaempferol) on the development of MLL rearrangements in primary human CD34(+) cells. Using the neutral Comet assay, we demonstrated a dose-dependent double-strand break (DSB) formation after exposure to flavonoids. An incorrect repair of these DSBs resulted in chromosomal translocations that co-localized with those identified in infant leukemias. Most of these translocations were formed by microhomology-mediated end joining. Moreover, in all but one translocation, SINE/Alu or LINE/L1 repetitive elements were present in at least one side of the breakpoint junction. Beside MLL translocations, fluorescence in situ hybridization analysis demonstrated monosomy or trisomy of MLL in 8-10% of the quercetin-exposed CD34(+) cells. Our study demonstrates that biologically relevant concentrations of flavonoids can induce MLL abnormalities in primary hematopoietic progenitor cells. This is particularly alarming knowing that the differences in metabolism and excretion rate between mother and fetus can lead to a higher flavonoid concentration on the fetal side. Therefore, it is important to raise public awareness and set guidelines for marketing flavonoid supplements to reduce the risk of infant leukemias.

Cleavage of the MLL gene by activators of apoptosis is independent of topoisomerase II activity

Exposure to topoisomerase II inhibitors is linked to the generation of leukemia involving translocations of the MLL gene, normally restricted to an 8.3 kbp tract, the breakpoint cluster region (BCR). Using an in vitro assay, apoptotic activators, including radiation and anti-CD95 antibody, trigger site-specific cleavage adjacent to exon 12 within the MLL BCR and promote translocation of the MLL gene in cells that can survive. To explore the mechanism of cleavage and rearrangement in more detail, the entire MLL BCR was placed into the pREP4 episomal vector and transfected into human lymphoblastoid TK6 cells. Episomes containing either the MLL BCR, or deletion constructs of 367 bp or larger, were cleaved at the same position as genomic MLL after exposure to apoptotic stimuli. Further analysis of sequence motifs surrounding the cleaved region of MLL showed the presence of both a predicted nuclear matrix attachment sequence and a potential strong binding site for topoisomerase II, flanking the site of cleavage. Inactivation of topoisomerase II by the catalytic inhibitor merbarone did not inhibit MLL cleavage, suggesting that the initial cleavage step for MLL rearrangement is not mediated by topoisomerase II.

Infertility with defective spermiogenesis in mice lacking AF5q31, the target of chromosomal translocation in human infant leukemia

AF5q31 (also called MCEF) was identified by its involvement in chromosomal translocation with the gene MLL (mixed lineage leukemia), which is associated with infant acute lymphoblastic leukemia. Several potential roles have been proposed for AF5q31 and other family genes, but the specific requirements of AF5q31 during development remain unclear. Here, we show that AF5q31 is essential for spermatogenesis. Although most AF5q31-deficient mice died in utero and neonatally with impaired embryonic development and shrunken alveoli, respectively, 13% of AF5q31-deficient mice thrived as wild-type mice did. However, the male mice were sterile with azoospermia. Histological examinations revealed the arrest of germ cell development at the stage of spermiogenesis, and virtually no spermatozoa were seen in the epididymis. AF5q31 was found to be preferentially expressed in Sertoli cells. Furthermore, mutant mice displayed severely impaired expression of protamine 1, protamine 2, and transition protein 2, which are indispensable to compact the haploid genome within the sperm head, and an increase of apoptotic cells in seminiferous tubules. Thus, AF5q31 seems to function as a transcriptional regulator in testicular somatic cells and is essential for male germ cell differentiation and survival. These results may have clinical implications in the understanding of human male infertility.

Expression of leukemic MLL fusion proteins in Drosophila affects cell cycle control and chromosome morphology

The Mixed Lineage Leukemia (MLL) gene is involved in lymphoblastic and myeloid leukemia through chromosome translocations leading to fusion of MLL to partner genes, or through internal MLL rearrangements. MLL is the mammalian counterpart of the Drosophila trithorax (trx) gene, involved in maintaining active gene expression states. We have used transgenic Drosophila to assess the molecular targets and cellular processes affected by MLL and two of its leukemic fusion proteins. We find that whereas expression of normal human MLL in flies does not result in phenotypic alterations, overexpressing the human MLL-AF9 and MLL-AF4 proteins causes larval to pupal lethality, which interestingly resembles the phenotypes displayed by certain Drosophila trx mutant alleles. MLL-AF9 and MLL-AF4 transgenic flies exhibit antagonistic alterations in cell cycle progression. Additionally, flies expressing MLL-AF9 display impairment in higher order chromatin integrity, evidenced in decondensation of mitotic figures. The effects of MLL fusion proteins in Drosophila suggest that alteration of chromatin structure by MLL fusion proteins may contribute to the lethal phenotype. Our results indicate that the mode(s) of action of MLL-AF9 in Drosophila varies from that of MLL-AF4. Taken together, the expression of MLL fusion proteins in Drosophila provides a new and powerful system to reveal and characterize biological activities associated with MLL fusion proteins.

Analysis of t(9;11) chromosomal breakpoint sequences in childhood acute leukemia: almost identical MLL breakpoints in therapy-related AML after treatment without etoposides

The translocation t(9;11)(p22;q23) is a recurring chromosomal abnormality in acute myeloid leukemia (AML) fusing two genes designated as MLL and AF9. Within MLL, almost all rearrangements cluster in an 8.3-kb restricted region and fuse 5' portions of MLL to a variety of heterologous genes in various 11q23 translocations. AF9 is one of the most common fusion partners of MLL. It spans more than 100 kb, and two breakpoint cluster regions (BCRs) have been identified in a telomeric region of intron 4 (BCR1) and within introns 7 and 8 (BCR2). We investigated 11 children's bone marrow or peripheral blood samples (3 AML, 5 t-AML, 2 ALL, 1 ALL relapse) and two cell lines (THP-1 and Mono-Mac-6) with cytogenetically diagnosed translocations t(9;11). By use of an optimized multiplex nested long-range PCR assay, a breakpoint-spanning DNA fragment from each sample was amplified and directly sequenced. In four patients and two cell lines, the AF9 breakpoints were located within BCR1 and in two patients within BCR2, respectively. However, in five patients the AF9 breakpoints were found outside the previously described BCRs within the centromeric region of intron 4 and even within intron 3 in one case. All five patients with a secondary AML, who had not received etoposides during treatment of the primary malignant disease, revealed almost identical MLL breakpoints very close to a breakage hot spot inducible by topoisomerase II inhibitors or apoptotic triggers in vitro. Sequence patterns around the breakpoints indicated involvement of a "damage-repair mechanism" in the development of t(9;11) similar to t(4;11) in infants' acute leukemia.

ALL-1 is a histone methyltransferase that assembles a supercomplex of proteins involved in transcriptional regulation

ALL-1 is a member of the human trithorax/Polycomb gene family and is also involved in acute leukemia. ALL-1 is present within a stable, very large multiprotein supercomplex composed of > or =29 proteins. The majority of the latter are components of the human transcription complexes TFIID (including TBP), SWI/SNF, NuRD, hSNF2H, and Sin3A. Other components are involved in RNA processing or in histone methylation. The complex remodels, acetylates, deacetylates, and methylates nucleosomes and/or free histones. The complex's H3-K4 methylation activity is conferred by the ALL-1 SET domain. Chromatin immunoprecipitations show that ALL-1 and other complex components examined are bound at the promoter of an active ALL-1-dependent Hox a9 gene. In parallel, H3-K4 is methylated, and histones H3 and H4 are acetylated at this promoter.

MLL5, a homolog of Drosophila trithorax located within a segment of chromosome band 7q22 implicated in myeloid leukemia

Proteins encoded by Polycomb and Trithorax-group (Pc-G and Trx-G) genes regulate developmental fates by maintaining or repressing HOX gene expression, respectively. In a search for candidate myeloid leukemia tumor suppressor genes from a approximately 2.5 Mb commonly-deleted segment within chromosome band 7q22, we identified a novel human Trithorax (Trx) family member named MLL5. Trx-G genes encode proteins that modulate transcriptional programs through protein-protein interactions that are mediated by PHD and SET domains, and by binding to DNA via A-T hooks and methyltransferase homology motifs. MLL5 is a homolog of the Drosophila gene CG9007; it encodes a 6.5 kb mRNA that is expressed widely. MLL5 includes a SET domain and a single PHD finger, but lacks A-T hooks and methyltransferase homology domains that are found in MLL. The leukemia cell line RCV-ACV-A carries a heterozygous missense mutation within the PHD domain; however, no mutations within the MLL5 coding region were detected in primary leukemias. MLL5 is a novel mammalian Trx-G gene that might modulate transcription by protein association.

A novel selection system for chromosome translocations in Saccharomyces cerevisiae

Chromosomal translocations are common genetic abnormalities found in both leukemias and solid tumors. While much has been learned about the effects of specific translocations on cell proliferation, much less is known about what causes these chromosome rearrangements. This article describes the development and use of a system that genetically selects for rare translocation events using the yeast Saccharomyces cerevisiae. A translocation YAC was created that contains the breakpoint cluster region from the human MLL gene, a gene frequently involved in translocations in leukemia patients, flanked by positive and negative selection markers. A translocation between the YAC and a yeast chromosome, whose breakpoint falls within the MLL DNA, physically separates the markers and forms the basis for the selection. When RAD52 is deleted, essentially all of the selected and screened cells contain simple translocations. The detectable translocation rates are the same in haploids and diploids, although the mechanisms involved and true translocation rates may be distinct. A unique double-strand break induced within the MLL sequences increases the number of detectable translocation events 100- to 1000-fold. This novel system provides a tractable assay for answering basic mechanistic questions about the development of chromosomal translocations.

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.

A DNA damage repair mechanism is involved in the origin of chromosomal translocations t(4;11) in primary leukemic cells

Some chromosomal translocations involved in the origin of leukemias and lymphomas are due to malfunctions of the recombinatorial machinery of immunoglobulin and T-cell receptor-genes. This mechanism has also been proposed for translocations t(4;11)(q21;q23), which are regularly associated with acute pro-B cell leukemias in early childhood. Here, reciprocal chromosomal breakpoints in primary biopsy material of fourteen t(4;11)-leukemia patients were analysed. In all cases, duplications, deletions and inversions of less than a few hundred nucleotides indicative of malfunctioning DNA repair mechanisms were observed. We concluded that these translocation events were initiated by several DNA strand breaks on both participating chromosomes and subsequent DNA repair by 'error-prone-repair' mechanisms, but not by the action of recombinases of the immune system.

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.

Identification and characterization of the ARP1 gene, a target for the human acute leukemia ALL1 gene

ALL1, the human homologue of Drosophila trithorax, is directly involved in human acute leukemias associated with abnormalities at 11q23. Using the differential display method, we isolated a gene that is down-regulated in All1 double-knockout mouse embryonic stem (ES) cells. The gene, designated ARP1 (also termed RIEG, Ptx2, or Otlx2), is a member of a family of homeotic genes containing a short motif shared with several homeobox genes. Using a bacterially synthesized All1 polypeptide encompassing the AT-hook motifs, we identified a 0.5-kb ARP1 DNA fragment that preferentially bound to the polypeptide. Within this DNA, a region of approximately 100 bp was protected by the polypeptide from digestion with ExoIII and DNase I. Whole-mount in situ hybridization to early mouse embryos of 9.5-10.5 days indicated a complex pattern of Arp1 expression spatially overlapping with the expression of All1. Although the ARP1 gene is expressed strongly in bone marrow cells, no transcripts were detected in six leukemia cell lines with 11q23 translocations. These results suggest that ARP1 is up-regulated by the All1 protein, possibly through direct interaction with an upstream DNA sequence of the former. The results are also consistent with the suggestion that ALL1 chimeric proteins resulting from 11q23 abnormalities act in a dominant negative fashion.

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.

Characterization of the gene encoding human sarcolipin (SLN), a proteolipid associated with SERCA1: absence of structural mutations in five patients with Brody disease

Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). Genomic DNA and cDNA encoding human sarcolipin (SLN) were isolated and characterized and the SLN gene was mapped to chromosome 11q22-q23. Human, rabbit, and mouse cDNAs encode a protein of 31 amino acids. Homology of SLN with phospholamban (PLN) suggests that the first 7 hydrophilic amino acids are cytoplasmic, the next 19 hydrophobic amino acids form a single transmembrane helix, and the last 5 hydrophilic amino acids are lumenal. The cytoplasmic and transmembrane sequences are not well conserved among the three species, but the lumenal sequence is highly conserved. Like SERCA1, SLN is highly expressed in rabbit fast-twitch skeletal muscle, but it is expressed to a lower extent in slow-twitch muscle and to an even lower extent in cardiac muscle, where SERCA2a and PLN are highly expressed. It is expressed in only trace amounts in pancreas and prostate. SLN and PLN genes resemble each other in having two small exons, with their entire coding sequences lying in exon 2 and a large intron separating the two segments. Brody disease is an inherited disorder of skeletal muscle function, characterized by exercise-induced impairment of muscle relaxation. Mutations in the ATP2A1 gene encoding SERCA1 have been associated with the autosomal recessive inheritance of Brody disease in three families, but not with autosomal dominant inheritance of the disease. A search for mutations in the SLN gene in five Brody families, four of which were not linked to ATP2A1, did not reveal any alterations in coding, splice junction or promoter sequences. The homozygous deletion of C438 in the coding sequence of ATP2A1 in Brody disease family 3, leading to a frameshift and truncation following Pro147 in SERCA1, is the fourth ATP2A1 mutation to be associated with autosomal recessive Brody disease.

Chromosome changes in human cancer--are they pointers to mechanisms of initiation?

The identification of tumour-specific chromosome abnormalities in a wide range of malignancies together with an understanding of their role in the activation of proto-oncogenes and loss of tumour suppressor genes has resulted in chromosome analysis of malignant cells beginning to provide information on the etiology of different malignancies. Two principle types of translocation have been identified. In the first, a protooncogene becomes juxtaposed to the enhancing or controlling elements of an immunoglobulin or T-cell receptor gene, resulting in deregulation of proto-oncogene expression. The second type results when breaks occur within a gene on each chromosome, creating a fusion gene that encodes for a chimaeric protein, usually a transcription factor. Homozygosity for loss of tumour suppressor gene function is frequently achieved by the inherited or somatic mutation of one allele together with a chromosome deletion of the region containing the other allele. The increasing knowledge of the mechanisms involved in the formation of cancer-specific chromosome abnormalities should help elucidate the causative role of environmental mutagens such as radiation.

Nuclear punctate distribution of ALL-1 is conferred by distinct elements at the N terminus of the protein

The ALL-1 gene positioned at 11q23 is directly involved in human acute leukemia either through a variety of chromosome translocations or by partial tandem duplications. ALL-1 is the human homologue of Drosophila trithorax which plays a critical role in maintaining proper spatial and temporal expression of the Antennapedia-bithorax homeotic genes determining the fruit fly's body pattern. Utilizing specific antibodies, we found that the ALL-1 protein distributes in cultured cells in a nuclear punctate pattern. Several chimeric ALL-1 proteins encoded by products of the chromosome translocations and expressed in transfected cells showed similar speckles. Dissection of the ALL-1 protein identified within its approximately 1,100 N-terminal residues three polypeptides directing nuclear localization and at least two main domains conferring distribution in dots. The latter spanned two short sequences conserved with TRITHORAX. Enforced nuclear expression of other domains of ALL-1, such as the PHD (zinc) fingers and the SET motif, resulted in uniform nonpunctate patterns. This indicates that positioning of the ALL-1 protein in subnuclear structures is mediated via interactions of ALL-1 N-terminal elements. We suggest that the speckles represent protein complexes which contain multiple copies of the ALL-1 protein and are positioned at ALL-1 target sites on the chromatin. Therefore, the role of the N-terminal portion of ALL-1 is to direct the protein to its target genes.

Molecular epidemiology of hematological neoplasms--present status and future directions

The field of molecular epidemiology, using modern epidemiological approaches and taking the advantage of the advances in molecular biology can provide new tools for the exploration of etiological determinants, either environmental or hereditary, in the development of hematological neoplasms. It is now possible to identify some host susceptibility characteristics, to measure the effective dose of exposure, and to identify early, pre-clinical biological effects, using sensitive and specific biomarkers. The significant variation in the incidence of hematological neoplasms in different geographical areas, races, and age groups, the high rates of familial aggregation in certain populations, the involvement of protooncogenes and tumor suppressor genes in the development of hematological neoplasms, as well as of many environmental agents such as chemicals, radiation, and viruses, support the important role of molecular epidemiology in the investigation of the development of hematological neoplasms.

EEN encodes for a member of a new family of proteins containing an Src homology 3 domain and is the third gene located on chromosome 19p13 that fuses to MLL in human leukemia

The MLL gene, the closest human homologue to the Drosophila trithorax gene, undergoes chromosomal translocation with a large number of different partner genes in both acute lymphoid and acute myeloid leukemias. We have identified a new partner gene, EEN, fused to MLL in a case of acute myeloid leukemia. The gene is located on chromosome 19p13, where two other MLL partner genes, ENL and ELL/MEN have also been identified. The deduced protein of 368 aa contains a central alpha-helical region and a C-terminal Src homology 3 (SH3) domain most similar to the C-terminal SH3 domain found in the Grb2/Sem-5/Drk family of genes. Sequence analysis of the fusion MLL/EEN transcript in our patient reveals that exon 6 of MLL is fused to the N-terminal end of EEN, a fusion that would create a chimeric protein that includes the major functional domain of EEN. EEN is expressed in a variety of tissue types and encodes a protein of approximately 46 kDa. The EEN protein is the human homologue of a member of a recently described murine SH3 domain-containing protein family. It is also highly related to a putative gene identified in Caenorhabditis elegans, and a number of similar sequences are present in the EST databases of several species.

Infant leukaemia after in utero exposure to radiation from Chernobyl

There has been no documented increase in childhood leukaemia following the Chernobyl accident. However, different forms of childhood leukaemia may not be equally susceptible to radiation carcinogenesis. Infant leukaemia is a distinct form associated with a specific genetic abnormality. Outside the former Soviet Union, contamination resulting from the Chernobyl accident has been highest in Greece and Austria and high also in the Scandinavian countries. All childhood leukaemia cases diagnosed throughout Greece since 1 January 1980 have been recorded. Here we report that infants exposed in utero to ionizing radiation from the Chernobyl accident had 2.6 times the incidence of leukaemia compared to unexposed children (95% confidence interval, 1.4 to 5.1; P approximately 0.003), and those born to mothers residing in regions with high radioactive fallout were at higher risk of developing infant leukaemia. No significant difference in leukaemia incidence was found among children aged 12 to 47 months. Preconceptional irradiation had no demonstrable effect on leukaemia risk at any of the studied age groups.

Domains with transcriptional regulatory activity within the ALL1 and AF4 proteins involved in acute leukemia

The ALLI gene, located at chromosome band 11q23, is involved in acute leukemia through a series of chromosome translocations and fusion to a variety of genes, most frequently to A4 and AF9. The fused genes encode chimeric proteins proteins. Because the Drosophila homologue of ALL1, trithorax, is a positive regulator of homeotic genes and acts at the level of transcription, it is conceivable that alterations in ALL1 transcriptional activity may underlie its action in malignant transformation. To begin studying this, we examined the All1, AF4, AF9, and AF17 proteins for the presence of potential transcriptional regulatory domains. This was done by fusing regions of the proteins to the yeast GAL4 DNA binding domain and assaying their effect on transcription of a reporter gene. A domain of 55 residues positioned at amino acids 2829-2883 of ALL1 was identified as a very strong activator. Further analysis of this domain by in vitro mutagenesis pointed to a core of hydrophobic and acidic residues as critical for the activity. An ALL1 domain that repressed transcription of the reporter gene coincided with the sequence homologous to a segment of DNA methyltransferase. An AF4 polypeptide containing residues 480-560 showed strong activation potential. The C-terminal segment of AF9 spanning amino acids 478-568 transactivated transcription of the reporter gene in HeLa but not in NIH 3T3 cells. These results suggest that ALL1, AF4, and probably AF9 interact with the transcriptional machinery of the cell.