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

LAF4, an AF4-related gene, is fused to MLL in infant acute lymphoblastic leukemia

Infant acute lymphoblastic leukemia (ALL) with MLL gene rearrangements is characterized by a proB phenotype and a poor clinical outcome. We analyzed an infant proB ALL with t(2;11)(p15;p14) and an MLL rearrangement on Southern blot analysis. Rapid amplification of cDNA ends-polymerase chain reaction (PCR) and reverse transcriptase-PCR identified the LAF4 gene mapped on chromosome region 2q11.2-q12 as a fusion partner of the MLL gene. The LAF4 gene was identified previously by its high sequence homology to the AF4 protein and encodes a protein of 1,227 amino acids. The t(4;11)(q21;q23), creating the MLL-AF4 chimeric transcripts, is the predominant 11q23 chromosome translocation in infant ALL and is associated with an extremely poor prognosis. Our findings further suggest that fusion of MLL to one of the AF4 family members (AF4/LAF4/AF5Q31) might determine a proB-cell phenotype in infant leukemia.

MLL-SEPTIN6 fusion recurs in novel translocation of chromosomes 3, X, and 11 in infant acute myelomonocytic leukaemia and in t(X;11) in infant acute myeloid leukaemia, and MLL genomic breakpoint in complex MLL-SEPTIN6 rearrangement is a DNA topoisomerase II cleavage site

We examined the MLL translocation in two cases of infant AML with X chromosome disruption. The G-banded karyotype in the first case suggested t(X;3)(q22;p21)ins(X;11)(q22;q13q25). Southern blot analysis showed one MLL rearrangement. Panhandle PCR approaches were used to identify the MLL fusion transcript and MLL genomic breakpoint junction. SEPTIN6 from chromosome band Xq24 was the partner gene of MLL. MLL exon 7 was joined in-frame to SEPTIN6 exon 2 in the fusion transcript. The MLL genomic breakpoint was in intron 7; the SEPTIN6 genomic breakpoint was in intron 1. Spectral karyotyping revealed a complex rearrangement disrupting band 11q23. FISH with a probe for MLL confirmed MLL involvement and showed that the MLL-SEPTIN6 junction was on the der(X). The MLL genomic breakpoint was a functional DNA topoisomerase II cleavage site in an in vitro assay. In the second case, the karyotype revealed t(X;11)(q22;q23). Southern blot analysis showed two MLL rearrangements. cDNA panhandle PCR detected a transcript fusing MLL exon 8 in-frame to SEPTIN6 exon 2. MLL and SEPTIN6 are vulnerable to damage to form recurrent translocations in infant AML. Identification of SEPTIN6 and the SEPTIN family members hCDCrel and MSF as partner genes of MLL suggests a common pathway to leukaemogenesis.

Dynamin is a minibrain kinase/dual specificity Yak1-related kinase 1A substrate

The minibrain kinase (Mnbk)/dual specificity Yak 1-related kinase 1A (Dyrk1A) gene is implicated in the mental retardation associated with Down's syndrome. It encodes a proline-directed serine/threonine kinase whose function has yet to be defined. We have used a solid-phase Mnbk/Dyrk1A kinase assay to aid in the search for the cellular Mnbk/Dyrk1A substrates. The assay revealed that rat brain contains two cytosolic proteins, one with a molecular mass of 100 kDa and one with a molecular mass of 140 kDa, that were prominently phosphorylated by Mnbk/Dyrk1A. The 100-kDa protein was purified and identified as dynamin 1. The conclusion was further supported by evidence that a recombinant glutathione S-transferase fusion protein containing dynamin isoform 1aa was phosphorylated by Mnbk/Dyrk1A. In addition to isoform 1aa, Mnbk/Dyrk1A also phosphorylated isoforms 1ab and 2aa but not human MxA protein when analyzed by the solid-phase kinase assay. Upon Mnbk/Dyrk1A phosphorylation, the interaction of dynamin 1 with the Src homology 3 domain of amphiphysin 1 was reduced. However, when Mnbk/Dyrk1A phosphorylation was allowed to proceed more extensively, the phosphorylation enhanced rather than reduced the binding of dynamin 1 to amphiphysin 1. The result suggests that Mnbk/Dyrk1A can play a dual role in regulating the interaction of dynamin 1 with amphiphysin 1. Mnbk/Dyrk1A phosphorylation also reduced the interaction of dynamin with endophilin 1, whereas the same phosphorylation enhanced the binding of dynamin 1 to Grb2. Nevertheless, the dual function of Mnbk/Dyrk1A phosphorylation was not observed for the interaction of dynamin 1 with endophilin 1 or Grb2. The interactions of dynamin with amphiphysin and endophilin are essential for the formation of endocytic complexes; our results suggest that Mnbk/Dyrk1A may function as a regulator controlling the assembly of endocytic apparatus.

Differential expression of endophilin 1 and 2 dimers at central nervous system synapses

Endophilin 1 is proposed to participate in synaptic vesicle biogenesis through SH3 domain-mediated interactions with the polyphosphoinositide phosphatase synaptojanin and the GTPase dynamin. Endophilin family members have also been identified as binding partners for a number of diverse cellular proteins. We define here the endophilin 1-binding site within synaptojanin 1 and show that this sequence independently and selectively purifies from brain extracts endophilin 1 and a closely related protein, endophilin 2. Endophilin 2, like endophilin 1, is highly expressed in brain, concentrated in nerve terminals, and found in complexes with synaptojanin and dynamin. Although a fraction of endophilins 1 and 2 coexist in the same complex, the distribution of these endophilin isoforms among central synapses only partially overlaps. Endophilins 1 and 2 are found predominantly as stable dimers through a predicted coiled-coil domain in their conserved NH2-terminal moiety. Dimerization may allow endophilins to link a number of different cellular targets to the endocytic machinery.

Rabaptin-5 is a novel fusion partner to platelet-derived growth factor beta receptor in chronic myelomonocytic leukemia

Chromosomal translocations involving the platelet-derived growth factor beta receptor (PDGFbetaR) gene have been reported in some patients with chronic myelomonocytic leukemia (CMML). The resultant fusion proteins have constitutive PDGFbetaR tyrosine kinase activity, but the partner genes previously reported (tel, Huntingtin interacting protein 1 [HIP-1], H4/D10S170) have poorly understood roles in the oncogenic activity of the fusion proteins. A novel PDGFbetaR fusion protein has been characterized in a patient with CMML and an acquired t(5;17)(q33;p13). Southern blot analysis on patient leukemia cells demonstrated involvement of the PDGFbetaR gene. Using 5' rapid amplification of complementary DNA ends-polymerase chain reaction (RACE-PCR) on patient RNA, rabaptin-5 was identified as a novel partner fused in-frame to the PDGFbetaR gene. The new fusion protein includes more than 85% of the native Rabaptin-5 fused to the transmembrane and intracellular tyrosine kinase domains of the PDGFbetaR. Transduction with a retroviral vector expressing rabaptin-5/PDGFbetaR transformed the hematopoietic cell line Ba/F3 to growth factor independence and caused a fatal myeloproliferative disease in mice. Rabaptin-5 is a well-studied protein shown to be an essential and rate-limiting component of early endosomal fusion through interaction with the Ras family GTPases Rab5 and Rab4. The fusion protein includes 3 of 4 coiled-coil domains (involved in homodimerization of native rabaptin-5), 2 caspase-3 cleavage sites, and a binding site for the tumor suppressor gene tuberin (tuberous sclerosis complex-2). Early endosomal transport is critical in regulation of various growth factor receptors, through ligand-induced clathrin-mediated endocytosis, and thus this new fusion protein links together 2 important pathways of growth regulation.

Mitogenesis and endocytosis: What's at the INTERSECTIoN?

Endocytosis is a regulated physiological process by which cell surface proteins are internalized along with extracellular factors such as nutrients, pathogens, peptides, toxins, etc. The process begins with the invagination of small regions of the plasma membrane which ultimately form intracellullar vesicles. These internalized vesicles may shuttle back to the plasma membrane to recycle the membrane components or they may be targeted for degradation. One role for endocytosis is in the attenuation of receptor signaling. For example, desensitization of activated membrane bound receptors such as G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs) occurs, in part, through endocytosis of the activated receptor. However, accumulating evidence suggests that endocytosis also mediates intracellular signaling. In this review, we discuss the experimental data that implicate endocytosis as a critical component in cellular signal transduction, both in the initiation of a signal as well as in the termination of a signal. Furthermore, we focus our attention on a recently described adaptor protein, intersectin (ITSN), which provides a link to both the endocytic and the mitogenic machinery of a cell. Thus, ITSN functions at a crossroad in the biochemical regulation of cell function.

Common themes in the pathogenesis of acute myeloid leukemia

The pathogenesis of acute myeloid leukemia is associated with the appearance of oncogenic fusion proteins generated as a consequence of specific chromosome translocations. Of the two components of each fusion protein, one is generally a transcription factor, whereas the other partner is more variable in function, but often involved in the control of cell survival and apoptosis. As a consequence, AML-associated fusion proteins function as aberrant transcriptional regulators that interfere with the process of myeloid differentiation, determine a stage-specific arrest of maturation and enhance cell survival in a cell-type specific manner. The abnormal regulation of transcriptional networks occurs through common mechanisms that include recruitment of aberrant co-repressor complexes, alterations in chromatin remodeling, and disruption of specific subnuclear compartments. The identification and analysis of common and specific target genes regulated by AML fusion proteins will be of fundamental importance for the full understanding of acute myeloid leukemogenesis and for the implementation of disease-specific drug design.

Endophilin regulates JNK activation through its interaction with the germinal center kinase-like kinase

The endophilin family of proteins function in clathrin-mediated endocytosis. Here, we have identified and cloned the rat germinal center kinase-like kinase (rGLK), a member of the GCK (germinal center kinase) family of c-Jun N-terminal kinase (JNK) activating enzymes, as a novel endophilin I-binding partner. The interaction occurs both in vitro and in cells and is mediated by the Src homology 3 domain of endophilin I and a region of rGLK containing the endophilin consensus-binding sequence PPRPPPPR. Overlay analysis of rat brain extracts demonstrates that endophilin I is a major Src homology 3 domain-binding partner for rGLK. Overexpression of full-length endophilin I activates rGLK-mediated JNK activation, whereas N- and C-terminal fragments of endophilin I block JNK activation. Thus, endophilin I appears to have a novel function in JNK activation.

Pharmacogenomics: unlocking the human genome for better drug therapy

There is great heterogeneity in the way humans respond to medications, often requiring empirical strategies to find the appropriate drug therapy for each patient (the "art" of medicine). Over the past 50 years, there has been great progress in understanding the molecular basis of drug action and in elucidating genetic determinants of disease pathogenesis and drug response. Pharmacogenomics is the burgeoning field of investigation that aims to further elucidate the inherited nature of interindividual differences in drug disposition and effects, with the ultimate goal of providing a stronger scientific basis for selecting the optimal drug therapy and dosages for each patient. These genetic insights should also lead to mechanism-based approaches to the discovery and development of new medications. This review highlights the current status of work in this field and addresses strategies that hold promise for future advances in pharmacogenomics.

Genomic organization, tissue expression, and cellular localization of AF3p21, a fusion partner of MLL in therapy-related leukemia

We previously identified the AF3p21 gene, a novel fusion partner of the MLL gene, in a patient who had developed therapy-related leukemia with t(3;11)(p21;q23). The AF3p21 gene encodes a protein consisting of 722 amino acids, which has an SH3 (Src homology 3) domain, a proline-rich domain, and a bipartite nuclear localization signal. The protein's SH3 domain has high homology with that of FYN. Analysis of the DNA from the patient's leukemic cells revealed that intron 6 of the MLL gene was fused at a point upstream of exon 1 in the AF3p21 gene, and that the der(11) chromosome formed an MLL-AF3p21 fusion transcript in leukemic cells, whereas the der(3) chromosome did not form any fusion transcript. The AF3p21 gene on chromosome band 3p21 is 19 kb long and consists of 13 exons. The size of the mRNA of the AF3p21 gene is approximately 3.5 kb. The AF3p21 gene is widely expressed in normal human tissues including the bone marrow, brain, liver, thymus, lung, and skeletal muscle. Western blot and immunocytochemical analyses showed that AF3p21 protein has an apparent molecular weight of 80 kDa and is localized exclusively in the cell nucleus. These results suggest the possibility that AF3p21 protein plays a role in signal transduction in the nucleus.

Functional analysis of the leukemia protein ELL: evidence for a role in the regulation of cell growth and survival

The ELL gene encodes an RNA polymerase II transcription factor that frequently undergoes translocation with the MLL gene in acute human myeloid leukemia. Here, we report that ELL can regulate cell proliferation and survival. In order to better understand the physiological role of the ELL protein, we have developed an ELL-inducible cell line. Cells expressing ELL were uniformly inhibited for growth by a loss of the G(1) population and an increase in the G(2)/M population. This decrease in cell growth is followed by the condensation of chromosomal DNA, activation of caspase 3, poly(ADP ribose) polymerase cleavage, and an increase in sub-G(1) population, which are all indicators of the process of programmed cell death. In support of the role of ELL in induction of cell death, expression of an ELL antisense RNA or addition of the caspase inhibitor ZVAD-fmk results in a reversal of ELL-mediated death. We have also demonstrated that the C-terminal domain of ELL, which is conserved among the ELL family of proteins that we have cloned (ELL, ELL2, and ELL3), is required for ELL's activity in the regulation of cell growth. These novel results indicate that ELL can regulate cell growth and survival and may explain how ELL translocations result in the development of human malignancies.

Genomic sequence and expression analyses of human chromatin assembly factor 1 p150 gene

Chromatin assembly factor-1 (CAF-1) plays essential roles in eukaryotic chromatin assembly during DNA replication (Smith and Stillman, 1989. Cell 58, 15-25), (Krude, 1999. Eur. J. Biochem. 263, 1-5). Its p150 subunit, involved in interaction with histone H3 and H4, is critical to the CAF-1 nucleosome assembly activity. In this study, we sequenced a 96-kb genomic DNA region that includes a 42.8-kb CAF-1 p150 subunit gene (CHAF1A), and a 41.1-kb EEN gene. A scripted bioinformatics analysis pipeline (research agent) has been set up to annotate the BAC sequence with a set of integrated algorithms. The CAF-1 p150 subunit gene contains 15 exons and 14 introns. The promoter region is characterized by deletional analyses, revealing a potential repressor. Tissue-correlated alternative splicing forms of the transcript was initially identified by EST clustering analysis, then confirmed by RT-PCR which resulted more splicing forms than computational prediction.

Sensitization by 5-aza-2'-deoxycytidine of leukaemia cells with MLL abnormalities to induction of differentiation by all-trans retinoic acid and 1alpha,25-dihydroxyvitamin D3

Most chromosomal abnormalities associated with breakage at 11q23 in acute leukaemia involve the MLL gene, and the presence of this breakage strongly predicts a poor clinical outcome. We assessed the possibility of differentiation-inducing therapy for acute leukaemias with chromosomal translocations involving 11q23. Among the cell lines with MLL translocations that we examined, KOCL48 and KOPN-1 cells were induced to differentiate into granulocytes by all-trans retinoic acid (ATRA) or into monocytes by 1alpha,25-dihydroxyvitamin D3 (VD3). These cells expressed p16 mRNA before treatment with 5-aza-2'-deoxycytidine (5-AZA), an inhibitor of DNA methylation. On the other hand, differentiation was not induced in SN-1, KOCL33, KOCL51 or KOCL44 cells by ATRA or VD3, and these cells did not express mRNA of this gene. However, these cells were effectively induced to differentiate by ATRA or VD3 in the presence of 5-AZA, and concomitantly exhibited p16 gene expression, suggesting an association between DNA demethylation and restoration of sensitivity to differentiation-inducing activity of ATRA or VD3 in leukaemia cells with MLL abnormalities. Based on these findings, combined treatment with ATRA or VD3 plus 5-AZA may be clinically useful in therapy for acute leukaemia with MLL abnormalities.

SH3GLB, a new endophilin-related protein family featuring an SH3 domain

A new cDNA encoding a protein of 362 amino acids designated SH3GLB1, for SH3 domain GRB2-like endophilin B1, was identified in a yeast two-hybrid screen devoted to the identification of new partners interacting with the apoptosis inducer Bax. SH3GLB1 shows strong similarities to the SH3 domain-containing proteins of the endophilin family and presumably represents the human homologue of the potential Caenorhabditis elegans SH3 containing-protein identified by systematic translation of the C. elegans genome (GenBank Accession No. U46675). Reversing prey to bait in the yeast screen, a second protein, SH3GLB2, of 395 amino acids showing 65% identity to SH3GLB1 was identified as an interacting partner of SH3GLB1. The discovery of SH3GLB1 itself in the screening with SH3GLB1 as a bait and further mapping experiments demonstrated that a core coiled-coil-type region is required for the formation of SH3GLB homo- and/or heterodimers, whereas the SH3 domain is not involved in these interactions. Interestingly, the similarities with the endophilin proteins cover the entire sequence of the SH3GLB family, suggesting a common fold and presumably a common mode of action. Furthermore, SH3GLB members colocalize to the cytoplasmic compartment of the cell together with Bax and are excluded from the nucleus. SH3GLB1 and SH3GLB2 do not significantly influence the onset and time course of Bax-mediated apoptosis in HeLa or 293T cells.

Traffic jam: a compendium of human diseases that affect intracellular transport processes

As sequencing of the human genome nears completion, the genes that cause many human diseases are being identified and functionally described. This has revealed that many human diseases are due to defects of intracellular trafficking. This 'Toolbox' catalogs and briefly describes these diseases.

Expression and protein-binding studies of the EEN gene family, new interacting partners for dynamin, synaptojanin and huntingtin proteins

EEN, identified initially as a fusion partner to the mixed-lineage leukaemia gene in human leukaemia, and its related members, EEN-B1 and EEN-B2, have recently been shown to interact with two endocytic molecules, dynamin and synaptojanin, as well as with the huntingtin protein. In the present study, we show that the expression of the EEN gene-family members is differentially regulated. Multiple-spliced variants were identified for EEN-B2. In the brain, EEN-B1 and EEN-B2 mRNA are preferentially expressed in the cerebellar Purkinje and granule cells, dentate gyrus cells, hippocampal pyramidal neurons and cerebral granule cells. The expression patterns of EEN-B1 and EEN-B2 mRNA in the brain overlap with those of dynamin-I/III, synaptojanin-I and huntingtin, whereas the ubiquitous expression of EEN is consistent with that of dynamin-II. In testes, members of the EEN family are co-expressed with testis-type dynamin and huntingtin in Sertoli cells and germ cells respectively. Our results on the overlapping expression patterns are consistent with the proposed interaction of EEN family members with dynamin, synaptojanin and huntingtin protein in vivo. Although all three EEN family members bind to dynamin and synaptojanin, EEN-B1 has the highest affinity for binding, followed by EEN and EEN-B2. We also demonstrate that amphiphysin, a major synaptojanin-binding protein in brain, can compete with the EEN family for binding to synaptojanin and dynamin. We propose that recruitment of the EEN family by dynamin/synaptojanin to clathrin-coated pits can be regulated by amphiphysin.

The interaction between EEN and Abi-1, two MLL fusion partners, and synaptojanin and dynamin: implications for leukaemogenesis

The mixed lineage leukaemia gene, MLL (also called HRX, ALL-1) in acute leukaemia is fused to at least 16 identified partner genes that display diverse structural and biochemical properties. Using GST pull down and the yeast two hybrid system, we show that two different MLL fusion partners with SH3 domains, EEN and Abi-1, interact with dynamin and synaptojanin, both of which are involved in endocytosis. Synaptojanin, a member of the inositol phosphatase family that has recently been shown to regulate cell proliferation and survival, is also known to bind to Eps15, the mouse homologue of AF1p, another fusion partner of MLL. Expression studies show that synaptojanin is strongly expressed in bone marrow and immature leukaemic cell lines, very weakly in peripheral blood leukocytes and absent in Raji, a mature B cell line. We found that the SH3 domains of EEN and Abi-1 interact with different proline-rich domains of synaptojanin while the EH domains of Eps15 interact with the NPF motifs of synaptojanin. In vitro competitive binding assays demonstrate that EEN displays stronger binding affinity than Abi-1 and may compete with it for synaptojanin. These findings suggest a potential link between MLL fusion-mediated leukaemogenesis and the inositol-signalling pathway.

ENL, the MLL fusion partner in t(11;19), binds to the c-Abl interactor protein 1 (ABI1) that is fused to MLL in t(10;11)+

Translocations of the chromosomal locus 11q23 that disrupt the MLL gene (alternatively ALL-1 or HRX) are frequently found in children's leukemias. These events fuse the MLL amino terminus in frame with a variety of unrelated proteins. Up to date, 16 different fusion partners have been characterized and more are likely to exist. No general unifying property could yet be detected amongst these proteins. We show here that the frequent MLL fusion partner ENL at 19p13.1 interacts with the human homologue of the mouse Abl-Interactor 1 (ABI1) protein. ABI1 in turn, is fused to MLL in the t(10;11)(p11.2;q23) translocation. ABI1 was identified as an ENL binding protein by a yeast two-hybrid screen. The interaction of ENL and ABI1 could be verified in vitro by far-Western blot assays and GST-pulldown studies as well as in vivo by co-immunoprecipitation experiments. A structure-function analysis identified an internal region of ENL and a composite motif of ABI1 including an SH3 domain as mutual binding partners. These data introduce novel aspects that might contribute to the understanding of the process of leukemogenesis by MLL fusion proteins.

Identification of a gene at 11q23 encoding a guanine nucleotide exchange factor: evidence for its fusion with MLL in acute myeloid leukemia

We have identified a gene at 11q23, telomeric to MLL, that encodes a guanine nucleotide exchange factor (GEF). This gene is transcribed into a 9.5-kb mRNA containing a 4.6-kb ORF. By Northern analysis, it was found to be expressed in all human tissues examined including peripheral blood leukocytes, spleen, prostate, testis, ovary, small intestine, colon, and minimally in thymus. Analysis of the predicted protein sequence indicates that it has strong homology to several members of the family of Rho GEFs that includes such oncogenes as Dbl, Vav, Tiam, and Bcr. A patient with primary acute myeloid leukemia (AML) and a karyotype of 51,XY,+8,+19,+3mar was found to have the 5' end of MLL at exon 6 fused in-frame with the 3' end of almost the entire ORF of this gene, which we named LARG for leukemia-associated Rho GEF. Transcriptional orientation of both genes at 11q23 is from centromere to telomere, consistent with other data that suggest the MLL-LARG fusion resulted from an interstitial deletion rather than a balanced translocation. LARG does not appear to have any homology with other MLL partner genes reported thus far. Thus, LARG represents an additional member of the GEF family and a novel MLL fusion partner in acute myeloid leukemia.

Novel SH3 protein encoded by the AF3p21 gene is fused to the mixed lineage leukemia protein in a therapy-related leukemia with t(3;11) (p21;q23)

The mixed lineage leukemia (MLL) gene located at chromosome band 11q23 is frequently rearranged in patients with therapy-related acute monocytic leukemia who received topoisomerase II inhibitors. We have identified a novel fusion partner of MLL(FAB M5b) in a patient who developed t-AML 9 years after treatment for acute lymphoblastic leukemia (ALL). The leukemic cells had a sole karyotypic abnormality of t(3;11) (p21;q23). Screening of a genomic DNA library, prepared from leukemic cell DNA, identified rearranged clones composed of MLL and a novel gene on chromosome 3p21 (AF3p21). The AF3p21 gene encodes a protein of 722 amino acids, which contains an Src homology 3 (SH3) domain, a proline-rich domain, and a bipartite nuclear localizing signal (NLS). RNA analysis demonstrated that exon 6 of the MLLgene fused to exon 2 of the AF3p21 gene. The resulting chimeric protein consists of AT-hooks, methyltransferase, and transcription repressor domains of MLL in addition to the AF3p21 proline-rich domain and NLS but not the AF3p21 SH3 domain.

MLL2, the second human homolog of the Drosophila trithorax gene, maps to 19q13.1 and is amplified in solid tumor cell lines

The Mixed Lineage Leukemia (MLL) gene is commonly involved in translocations in infantile leukemia and is amplified in some cases of adult myeloid leukemia. A homolog of MLL denoted MLL2, which represents the second human homolog of the Drosophila trithorax gene, was characterized by assembling ESTs, the KIAA0304 cDNA clone, RT - PCR fragments and a new clone isolated from a cDNA phage library and compared to the available genomic sequence. The MLL2 gene maps to 19q13.1, a region of frequent rearrangement or amplification in solid tumors. MLL2 consists of an 8.5 - 9 kb transcript and spans 20 kb of genomic DNA. The predicted MLL2 protein possesses all of the major domains defined in MLL and the two genes have a similar genomic structure. We find that MLL2 is amplified in two of 14 pancreatic carcinoma cell lines and one of five glioblastoma cell lines and is a likely critical gene in 19q13.1 amplifications. It is also a candidate for chromosomal rearrangements involving this chromosome locus. MLL2 is one additional mammalian trithorax-group gene with involvement in human cancer.

Leukemic HRX fusion proteins inhibit GADD34-induced apoptosis and associate with the GADD34 and hSNF5/INI1 proteins

One of the most common chromosomal abnormalities in acute leukemia is a reciprocal translocation involving the HRX gene (also called MLL, ALL-1, or HTRX) at chromosomal locus 11q23, resulting in the formation of HRX fusion proteins. Using the yeast two-hybrid system and human cell culture coimmunoprecipitation experiments, we show here that HRX proteins interact directly with the GADD34 protein. We have found that transfected cells overexpressing GADD34 display a significant increase in apoptosis after treatment with ionizing radiation, indicating that GADD34 expression not only correlates with apoptosis but also can enhance apoptosis. The amino-terminal third of the GADD34 protein was necessary for this observed increase in apoptosis. Furthermore, coexpression of three different HRX fusion proteins (HRX-ENL, HRX-AF9, and HRX-ELL) had an anti-apoptotic effect, abrogating GADD34-induced apoptosis. In contrast, expression of wild-type HRX gave rise to an increase in apoptosis. The difference observed here between wild-type HRX and the leukemic HRX fusion proteins suggests that inhibition of GADD34-mediated apoptosis may be important to leukemogenesis. We also show here that GADD34 binds the human SNF5/INI1 protein, a member of the SNF/SWI complex that can remodel chromatin and activate transcription. These studies demonstrate, for the first time, a gain of function for leukemic HRX fusion proteins compared to wild-type protein. We propose that the role of HRX fusion proteins as negative regulators of post-DNA-damage-induced apoptosis is important to leukemia progression.

Cytogenetics and molecular genetics of childhood leukemia

Childhood leukemia is the commonest form of childhood cancer and represents clonal proliferation of transformed hemopoietic cells as a result of genetic changes. Molecular characterization of these changes, in particular chromosomal translocations, has yielded a wealth of information on the mechanisms of leukemogenesis. These findings have also allowed the development of sensitive assays for the identification of underlying molecular defects, which is applicable to disease diagnosis and to monitor response to treatment. Genetic alterations in childhood leukemia are powerful prognostic indicators. TEL-AML1 fusion and hyperdiploidy >50 chromosomes are associated with a good prognosis in childhood acute lymphoblastic leukemia, whereas BCR-ABL fusion and MLL rearrangements are associated with a poor prognosis. Hence cytogenetic and molecular genetic classification of childhood leukemia will significantly improve the ability of clinicians to predict therapeutic response and prognosis, which paves the way for risk stratification based on clinical and genetic features. Finally, deciphering of genetic lesions in leukemia has allowed elucidation of the molecular basis of current treatment, as typified by the success of all-trans retinoic treatment in acute promyelocytic leukemia, and has identified targets for novel therapeutic approaches. It is envisaged that efforts in characterization of molecular defects in childhood leukemia will ultimately be translated into better clinical outcome for patients.

MSF (MLL septin-like fusion), a fusion partner gene of MLL, in a therapy-related acute myeloid leukemia with a t(11;17)(q23;q25)

MLL (ALL1, Htrx, HRX), which is located on chromosome band 11q23, frequently is rearranged in patients with therapy-related acute myeloid leukemia who previously were treated with DNA topoisomerase II inhibitors. In this study, we have identified a fusion partner of MLL in a 10-year-old female who developed therapy-related acute myeloid leukemia 17 months after treatment for Hodgkin's disease. Leukemia cells of this patient had a t(11;17)(q23;q25), which involved MLL as demonstrated by Southern blot analysis. The partner gene was cloned from cDNA of the leukemia cells by use of a combination of adapter reverse transcriptase-PCR, rapid amplification of 5' cDNA ends, and BLAST database analysis to identify expressed sequence tags. The full-length cDNA of 2.8 kb was found to be an additional member of the septin family, therefore it was named MSF (MLL septin-like fusion). Members of the septin family conserve the GTP binding domain, localize in the cytoplasm, and interact with cytoskeletal filaments. A major 4-kb transcript of MSF was expressed ubiquitously; a 1.7-kb transcript was found in most tissues. An additional 3-kb transcript was found only in hematopoietic tissues. By amplification with MLL exon 5 forward primer and reverse primers in MSF, the appropriately sized products were obtained. MSF is highly homologous to hCDCrel-1, which is a partner gene of MLL in leukemias with a t(11;22)(q23;q11.2). Further analysis of MSF may help to delineate the function of MLL partner genes in leukemia, particularly in therapy-related leukemia.

Transcriptional Inhibition of p53 by the MLL/MEN Chimeric Protein Found in Myeloid Leukemia

The t(11;19)(q23;p13.1) translocation is frequently found in adult myeloid leukemia. In the MLL/MEN fusion protein generated by this translocation, most of the coding region of the MEN protein, an RNA polymerase II elongation factor, is fused to the N-terminal third of the MLL protein, a possible transcriptional regulator. However, the molecular mechanism of leukemogenesis by the fusion protein remains unclear. We investigated the effects of the fusion protein on p53 function using luciferase assays. Overexpression of the fusion protein suppressed the transactivation ability of p53. This negative effect of the fusion protein on p53 function was dependent on the region derived from MEN. Moreover, p53 coimmunoprecipitated with MLL/MEN as well as MEN, suggesting that the fusion protein binds to p53 through the MEN region. We found that MEN binding to p53 was mediated by its N-terminal region and repression of p53 transcriptional activity was mediated by its C-terminal region. We also found that these two functional regions were essential for the transformation of Rat1 cells mediated by MEN. Although we could not demonstrate a functional difference between MLL/MEN and MEN in this study, these data suggest that the MLL/MEN chimeric transcriptional regulator may exert its oncogenic activity by inhibiting the function of the p53 tumor-suppressor protein by binding to it. Our findings provide a novel insight into the leukemogenic mechanism exerted by the t(11;19)(q23;p13.1) translocation.

Childhood acute lymphoblastic leukemia with the MLL-ENL fusion and t(11;19)(q23;p13.3) translocation

PURPOSE

To determine the molecular characteristics, clinical features, and treatment outcomes of children with acute lymphoblastic leukemia (ALL) and the t(11;19)(q23,p13.3) translocation.

PATIENTS AND METHODS

A retrospective analysis of leukemic cell karyotypes obtained from patients with new diagnoses of ALL who were treated at St. Jude Children's Research Hospital or by the Pediatric Oncology Group was performed to identify cases with the t(11;19)(q23;p13.3) translocation. Molecular analyses were performed on these cases to determine the status of the MLL gene and the presence of the MLL-ENL fusion transcript.

RESULTS

Among 3,578 patients with ALL and successful cytogenetic analysis, we identified 35 patients with the t(11;19)(q23;p13.3) translocation: 13 infants and 11 older children had B-precursor leukemia, whereas 11 patients had a T-cell phenotype. Although all of the cases examined had MLL rearrangements and MLL-ENL fusion transcripts, outcome varied according to age and immunophenotype. Among B-precursor cases, only two of the 13 infants remain in complete remission, compared with six of the 11 older children. Most strikingly, no relapses have occurred among B-precursor patients 1 to 9 years of age or among T-cell patients.

CONCLUSION

Although MLL gene rearrangements are generally associated with a dismal outcome in ALL, two distinct subsets with MLL-ENL fusions have an excellent prognosis. Our results suggest that patients with this genetic abnormality who have T-cell ALL or are 1 to 9 years of age should not be considered candidates for hematopoietic stem-cell transplantation during their first remission.

Expression pattern and cellular distribution of the murine homologue of AF10

We have cloned Af10, the murine homologue of the MLL partner gene AF10. The predicted open reading frame of Af10 contains 1069 aa which are 90% identical to those of AF10. Af10 contains an N-terminal cysteine-rich region with a LAP/PHD finger, a leucine zipper domain and a glutamine-rich region at the C-terminus, features also found in the human proteins AF10 and AF17. A single 5. 5-kb transcript was detected in murine tissues with the highest level of expression in the testes. A polyclonal antibody raised to the cysteine-rich region of AF10 was able to identify a double band of 140 kDa on Western analysis in mouse testicular extracts. After subcellular separation Af10 was identified in both the nuclear and cytoplasmic extracts, again as a double band of 140 kDa in size. In situ hybridisation studies were performed with sense and antisense digoxigenin-labelled oligonucleotides. High levels of expression were noted in postmeiotic germ cells, especially in spermatids from around stage VI to stage VIII. High levels of expression were also seen in the white matter of the cerebellum, extending into the granular layer. The expression in differentiated rather than in proliferating cells suggests that the role of Af10 may lie in the suppression of proliferation rather than in differentiation. Since the LAP/PHD finger domains are lost in the MLL-AF10 fusion, arguably such a function could be carried out by this domain.

An In Vivo Topoisomerase II Cleavage Site and a DNase I Hypersensitive Site Colocalize Near Exon 9 in the MLLBreakpoint Cluster Region

The human myeloid-lymphoid leukemia gene, MLL (also calledALL-1, Htrx, or HRX ), maps to chromosomal band 11q23. MLL is involved in translocations that result in de novo acute lymphoblastic leukemia (ALL), acute myelogenous leukemia (AML), mixed lineage leukemia, and also in therapy AML (t-AML) and therapy ALL (t-ALL) resulting from treatment with DNA topoisomerase II (topo II) targeting drugs. MLL can recombine with more than 30 other chromosomal bands, of which 16 of the partner genes have been cloned. Breaks in MLL occur in an 8.3-kb breakpoint cluster region (BCR) encompassing exons 5 through 11. We recently demonstrated that 75% of de novo patient breakpoints in MLL mapped in the centromeric half of the BCR between two scaffold-associated regions (SAR), whereas 75% of the t-AML patient breakpoints mapped to the telomeric half of the BCR within a strong SAR. We have mapped additional structural elements in the BCR. An in vivo DNA topo II cleavage site (induced with several different drugs that target topo II) mapped near exon 9 in three leukemia cell lines. A strong DNase I hypersensitive site (HS) also mapped near exon 9 in four leukemia cell lines, including two in which MLL was rearranged [a t(6;11) and a t(9;11)], and in two lymphoblastoid cell lines with normalMLL. Two of the leukemia cell lines also showed an in vivo topo II cleavage site. Our results suggest that the chromatin structure of the MLL BCR may influence the location of DNA breaks in both de novo and therapy-related leukemias. We propose that topo II is enriched in the MLL telomeric SAR and that it cleaves the DNase I HS site after treatment with topo II inhibitors. These events may be involved in recombination associated with t-AML/t-ALL breakpoints mapping in the MLL SAR.

An In Vivo Topoisomerase II Cleavage Site and a DNase I Hypersensitive Site Colocalize Near Exon 9 in the MLLBreakpoint Cluster Region

The human myeloid-lymphoid leukemia gene, MLL (also calledALL-1, Htrx, or HRX ), maps to chromosomal band 11q23. MLL is involved in translocations that result in de novo acute lymphoblastic leukemia (ALL), acute myelogenous leukemia (AML), mixed lineage leukemia, and also in therapy AML (t-AML) and therapy ALL (t-ALL) resulting from treatment with DNA topoisomerase II (topo II) targeting drugs. MLL can recombine with more than 30 other chromosomal bands, of which 16 of the partner genes have been cloned. Breaks in MLL occur in an 8.3-kb breakpoint cluster region (BCR) encompassing exons 5 through 11. We recently demonstrated that 75% of de novo patient breakpoints in MLL mapped in the centromeric half of the BCR between two scaffold-associated regions (SAR), whereas 75% of the t-AML patient breakpoints mapped to the telomeric half of the BCR within a strong SAR. We have mapped additional structural elements in the BCR. An in vivo DNA topo II cleavage site (induced with several different drugs that target topo II) mapped near exon 9 in three leukemia cell lines. A strong DNase I hypersensitive site (HS) also mapped near exon 9 in four leukemia cell lines, including two in which MLL was rearranged [a t(6;11) and a t(9;11)], and in two lymphoblastoid cell lines with normalMLL. Two of the leukemia cell lines also showed an in vivo topo II cleavage site. Our results suggest that the chromatin structure of the MLL BCR may influence the location of DNA breaks in both de novo and therapy-related leukemias. We propose that topo II is enriched in the MLL telomeric SAR and that it cleaves the DNase I HS site after treatment with topo II inhibitors. These events may be involved in recombination associated with t-AML/t-ALL breakpoints mapping in the MLL SAR.

Factors regulating the transcriptional elongation activity of RNA polymerase II

The synthesis of mature and functional messenger RNA by eukaryotic RNA polymerase II (Pol II) is a complex, multistage process requiring the cooperative action of many cellular proteins. This process, referred to collectively as the transcription cycle, proceeds via five stages: preinitiation, initiation, promoter clearance, elongation, and termination. During the past few years, fundamental studies of the elongation stage of transcription have demonstrated the existence of several families of Pol II elongation factors governing the activity of Pol II. It is now clear that the elongation stage of transcription is a critical stage for the regulation of gene expression. In fact, two of these elongation factors, ELL and elongin, have been implicated in human cancer. This article will review the proteins involved in the regulation of the elongation stage of transcription by Pol II, describing the recent experimental findings that have propelled vigorous research on the properties and function of the elongating RNA polymerase II. --Shilatifard, A. Factors regulating the transcriptional elongation activity of RNA polymerase II.

Secondary leukemias induced by topoisomerase-targeted drugs

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

Partial Tandem Duplications of the MLL Gene Are Detectable in Peripheral Blood and Bone Marrow of Nearly All Healthy Donors

Partial tandem duplication within the MLL gene has recently been described as a novel genetic alteration in acute myeloid leukemia (AML). It has been associated with trisomy of chromosome 11, but was also identified in AML patients with normal karyotypes. The current study was performed to investigate whether MLL duplications are restricted to AML, and hence whether they may also occur in normal hematopoietic cells. MLL-duplication transcripts were analyzed by nested reverse-transcriptase polymerase chain reaction (RT-PCR) in peripheral blood in two groups of 45 and 20 patients, respectively, as well as in two bone marrow samples from healthy volunteers. Duplications were detected in two independent nested RT-PCR experiments in the peripheral blood samples of 38 of 45 (84%) and 20 of 20 (100%) of the two groups and in both bone marrow samples. On this basis, MLL duplications seem to occur frequently in a subset of cells in normal hematopoiesis. The type of partially duplicated MLL transcripts varied substantially. Three transcripts were identical to those known from AML. In addition, four new transcripts were characterized. Three of these four were in frame and potentially translatable. MLL duplications were also detected by seminested genomic PCR with intron 9– and intron 1–specific primers in 20 of 20 peripheral blood samples studied, indicating that the duplications are genomically fixed at the DNA level and are not an RT-PCR artifact. In summary, MLL duplications are regularly generated by homologous ALU recombination in a small number of hematopoietic cells of most or even all healthy donors. These data suggest that MLL duplications are not implicated in the malignant transformation in AML, or alternatively, that only a few cells will acquire additional oncogenic mutations necessary to establish the malignant phenotype of AML.

© 1998 by The American Society of Hematology.

Partial Tandem Duplications of the MLL Gene Are Detectable in Peripheral Blood and Bone Marrow of Nearly All Healthy Donors

Partial tandem duplication within the MLL gene has recently been described as a novel genetic alteration in acute myeloid leukemia (AML). It has been associated with trisomy of chromosome 11, but was also identified in AML patients with normal karyotypes. The current study was performed to investigate whether MLL duplications are restricted to AML, and hence whether they may also occur in normal hematopoietic cells. MLL-duplication transcripts were analyzed by nested reverse-transcriptase polymerase chain reaction (RT-PCR) in peripheral blood in two groups of 45 and 20 patients, respectively, as well as in two bone marrow samples from healthy volunteers. Duplications were detected in two independent nested RT-PCR experiments in the peripheral blood samples of 38 of 45 (84%) and 20 of 20 (100%) of the two groups and in both bone marrow samples. On this basis, MLL duplications seem to occur frequently in a subset of cells in normal hematopoiesis. The type of partially duplicated MLL transcripts varied substantially. Three transcripts were identical to those known from AML. In addition, four new transcripts were characterized. Three of these four were in frame and potentially translatable. MLL duplications were also detected by seminested genomic PCR with intron 9– and intron 1–specific primers in 20 of 20 peripheral blood samples studied, indicating that the duplications are genomically fixed at the DNA level and are not an RT-PCR artifact. In summary, MLL duplications are regularly generated by homologous ALU recombination in a small number of hematopoietic cells of most or even all healthy donors. These data suggest that MLL duplications are not implicated in the malignant transformation in AML, or alternatively, that only a few cells will acquire additional oncogenic mutations necessary to establish the malignant phenotype of AML.

© 1998 by The American Society of Hematology.

ABI-1, a Human Homolog to Mouse Abl-Interactor 1, Fuses theMLL Gene in Acute Myeloid Leukemia With t(10;11)(p11.2;q23)

Recurrent translocation t(10;11) has been reported to be associated with acute myeloid leukemia (AML). Recently, two types of chimeric transcripts, MLL-AF10 in t(10;11)(p12;q23) andCALM-AF10 in t(10;11)(p13;q14), were isolated. t(10;11) is strongly associated with complex translocations, including invins(10;11) and inv(11)t(10;11), because the direction of transcription of AF10 is telomere to centromere. We analyzed a patient of AML with t(10;11)(p11.2;q23) and identified ABI-1 on chromosome 10p11.2, a human homolog to mouse Abl-interactor 1 (Abi-1), fused with MLL. Whereas the ABI-1 gene bears no homology with the partner genes of MLL previously described, the ABI-1 protein exhibits sequence similarity to protein of homeotic genes, contains several polyproline stretches, and includes asrc homology 3 (SH3) domain at the C-terminus that is required for binding to Abl proteins in mouse Abi-1 protein. Recently, e3B1, an eps8 SH3 binding protein 1, was also isolated as a human homolog to mouse Abi-1. Three types of transcripts of ABI-1 gene were expressed in normal peripheral blood. Although e3B1 was considered to be a full-length ABI-1, the MLL-ABI-1fusion transcript in this patient was formed by an alternatively spliced ABI-1. Others have shown that mouse Abi-1 suppresses v-ABL transforming activity and that e3B1, full-length ABI-1, regulates cell growth. In-frame MLL-ABI-1 fusion transcripts combine the MLL AT-hook motifs and DNA methyltransferase homology region with the homeodomain homologous region, polyproline stretches, and SH3 domain of alternatively spliced transcript of ABI-1. Our results suggest that the ABI-1 gene plays a role in leukemogenesis by translocating to MLL.

© 1998 by The American Society of Hematology.

ABI-1, a Human Homolog to Mouse Abl-Interactor 1, Fuses theMLL Gene in Acute Myeloid Leukemia With t(10;11)(p11.2;q23)

Recurrent translocation t(10;11) has been reported to be associated with acute myeloid leukemia (AML). Recently, two types of chimeric transcripts, MLL-AF10 in t(10;11)(p12;q23) andCALM-AF10 in t(10;11)(p13;q14), were isolated. t(10;11) is strongly associated with complex translocations, including invins(10;11) and inv(11)t(10;11), because the direction of transcription of AF10 is telomere to centromere. We analyzed a patient of AML with t(10;11)(p11.2;q23) and identified ABI-1 on chromosome 10p11.2, a human homolog to mouse Abl-interactor 1 (Abi-1), fused with MLL. Whereas the ABI-1 gene bears no homology with the partner genes of MLL previously described, the ABI-1 protein exhibits sequence similarity to protein of homeotic genes, contains several polyproline stretches, and includes asrc homology 3 (SH3) domain at the C-terminus that is required for binding to Abl proteins in mouse Abi-1 protein. Recently, e3B1, an eps8 SH3 binding protein 1, was also isolated as a human homolog to mouse Abi-1. Three types of transcripts of ABI-1 gene were expressed in normal peripheral blood. Although e3B1 was considered to be a full-length ABI-1, the MLL-ABI-1fusion transcript in this patient was formed by an alternatively spliced ABI-1. Others have shown that mouse Abi-1 suppresses v-ABL transforming activity and that e3B1, full-length ABI-1, regulates cell growth. In-frame MLL-ABI-1 fusion transcripts combine the MLL AT-hook motifs and DNA methyltransferase homology region with the homeodomain homologous region, polyproline stretches, and SH3 domain of alternatively spliced transcript of ABI-1. Our results suggest that the ABI-1 gene plays a role in leukemogenesis by translocating to MLL.

© 1998 by The American Society of Hematology.

Endocytosis proteins and cancer: a potential link?

Recent studies have shown that a variety of proteins participate with clathrin and clathrin adaptors in receptor-mediated endocytosis. The genes encoding some of these proteins are targets of chromosomal rearrangements in human haematopoietic malignancies. In addition, abnormal expression or mutation of some endocytosis proteins has been reported in human cancers. This article discusses these observations and elaborates the potential mechanisms by which the abnormal expression of endocytosis proteins might participate in the biology of cancer.

t(11;22)(q23;q11.2) In acute myeloid leukemia of infant twins fuses MLL with hCDCrel, a cell division cycle gene in the genomic region of deletion in DiGeorge and velocardiofacial syndromes

We examined the MLL genomic translocation breakpoint in acute myeloid leukemia of infant twins. Southern blot analysis in both cases showed two identical MLL gene rearrangements indicating chromosomal translocation. The rearrangements were detectable in the second twin before signs of clinical disease and the intensity relative to the normal fragment indicated that the translocation was not constitutional. Fluorescence in situ hybridization with an MLL-specific probe and karyotype analyses suggested t(11;22)(q23;q11. 2) disrupting MLL. Known 5' sequence from MLL but unknown 3' sequence from chromosome band 22q11.2 formed the breakpoint junction on the der(11) chromosome. We used panhandle variant PCR to clone the translocation breakpoint. By ligating a single-stranded oligonucleotide that was homologous to known 5' MLL genomic sequence to the 5' ends of BamHI-digested DNA through a bridging oligonucleotide, we formed the stem-loop template for panhandle variant PCR which yielded products of 3.9 kb. The MLL genomic breakpoint was in intron 7. The sequence of the partner DNA from band 22q11.2 was identical to the hCDCrel (human cell division cycle related) gene that maps to the region commonly deleted in DiGeorge and velocardiofacial syndromes. Both MLL and hCDCrel contained homologous CT, TTTGTG, and GAA sequences within a few base pairs of their respective breakpoints, which may have been important in uniting these two genes by translocation. Reverse transcriptase-PCR amplified an in-frame fusion of MLL exon 7 to hCDCrel exon 3, indicating that an MLL-hCDCrel chimeric mRNA had been transcribed. Panhandle variant PCR is a powerful strategy for cloning translocation breakpoints where the partner gene is undetermined. This application of the method identified a region of chromosome band 22q11.2 involved in both leukemia and a constitutional disorder.

Exon scrambling of MLL transcripts occur commonly and mimic partial genomic duplication of the gene

The MLL gene is frequently rearranged in acute human leukemia of both the myeloid and lymphoid lineages. Using a sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we identified several abnormally spliced transcripts in which MLL exons were joined in an order different from the genomic orientation (scrambled exons). Mis-splicing of MLL was present in both normal and malignant tissues. Although the majority of these scrambled transcripts were joined accurately at consensus splice sites, there were several examples in which the junctions of exons spliced in aberrant order were at non-consensus sites. A number of features differentiate mis-splicing of MLL from the previously described cases of scrambled exons and circular RNAs. Some scrambled transcripts appear to be present in the polyadenylated fraction of RNA. No correlation of exon scrambling with exon skipping was found, and there was no particular tendency for the exons involved to be near large introns. Our data show that splicing of MLL is extremely complex. The presence of scrambled transcripts in both normal and leukemic cells, indistinguishable from transcripts resulting from genomic MLL rearrangements, precludes the use of nested RT-PCR as a screening method for detection of tandem duplication of tandem duplication of MLL.

HRX leukemic fusion proteins form a heterocomplex with the leukemia-associated protein SET and protein phosphatase 2A

One of the most common chromosomal abnormalities in acute leukemia is a reciprocal translocation involving the HRX gene at chromosome locus 11q23, resulting in HRX fusion proteins. Using the yeast two-hybrid system, in vitro binding studies, and human cell culture coimmunoprecipitation experiments, we show here that a region of the HRX protein that is consistently retained in HRX leukemic fusion proteins interacts directly with SET, another protein implicated in leukemia. We have identified the binding sites on HRX for SET and show that these sequences are clustered near the A.T hooks that have been shown to bind DNA. We also show that carboxyl-terminal SET sequences, possibly the acidic tail of SET, bind to HRX. We have also found serine/threonine-specific protein phosphatase activity in anti-HRX coimmunoprecipitates. Using the phosphatase inhibitor okadaic acid and Western blotting, the phosphatase was identified as protein phosphatase 2A (PP2A). Mutation of a single amino acid in one of the SET binding sites of HRX resulted in lower amounts of both coimmunoprecipitated SET protein and coimmunoprecipitated PP2A. These results suggest that the leukemogenic effects of HRX fusion proteins may be related to interactions with SET and PP2A.