Rearrangement of the MLL gene in acute lymphoblastic and acute myeloid leukemias with 11q23 chromosomal translocations

BO-0742, a derivative of AHMA and N-mustard, has selective toxicity to drug sensitive and drug resistant leukemia cells and solid tumors

This is a preclinical study of BO-0742, a derivative of 3-(9-acridinylamino)-5-hydroxymethyl-aniline (AHMA) and N-mustard, as an anti-cancer agent. MTS assays revealed a broad spectrum of anti-cancer activities in vitro, with the greatest cytotoxicity against leukemia and neuroblastoma including those with drug resistant characteristics, and a good therapeutic index with leukemia being 10-40 times more sensitive to BO-0742 than hematopoietic progenitors. Administration of BO-0742 at an optimal dose schedule based on its pharmacokinetics significantly suppressed the growth of xenografts of human breast and ovarian cancers in mice. Thus, BO-0742 is a potent anti-cancer agent worthy of further clinical development.

Chromosomal rearrangements leading to MLL gene fusions: clinical and biological aspects

Rearrangements of the MLL gene located at 11q23 are common chromosomal abnormalities associated with acute leukemia, especially infant and therapy-related leukemias. A variety of chimeric oncoproteins resulting from these rearrangements has been described; all of these include the NH(2)-terminal region of MLL implicated in protein-protein interactions and transcriptional repression. Although the molecular basis for the oncogenic activity of MLL chimeric proteins is incompletely understood, it seems to be derived, at least in part, through activation of clustered homeobox (HOX) genes. Here, we survey MLL gene rearrangements that are associated with acute leukemia and discuss molecular pathways leading to these rearrangements.

GammaH2AX and cancer

Histone H2AX phosphorylation on a serine four residues from the carboxyl terminus (producing gammaH2AX) is a sensitive marker for DNA double-strand breaks (DSBs). DSBs may lead to cancer but, paradoxically, are also used to kill cancer cells. Using gammaH2AX detection to determine the extent of DSB induction may help to detect precancerous cells, to stage cancers, to monitor the effectiveness of cancer therapies and to develop novel anticancer drugs.

A comparison of gene expression profiles between primary human AML cells and AML cell line

In acute myeloid leukemia (AML), hematologic malignancies are characterized by recurring chromosomal abnormalities. Chromosome translocation t(9;11)(p22;q23) is one of the most common genetic aberrations and results in the formation of the MLL-AF9 fusion gene that functions as a facilitator of cell growth directly. In order to study this type of AML, the cell lines with cytogenetically diagnosed t(9;11)(p22;q23), such as Mono Mac 6 (MM6), have been widely used. To examine whether there is any difference in gene expression between the primary human t(9;11) AML cells and MM6 cell line, genome-wide transcriptome analysis was performed on MM6 cell line using SAGE and the results were compared to the profile of primary human t(9;11) AML cells. 884 transcripts which were alternatively expressed between MM6 cells and primary human t(9;11) cells were identified through statistical analysis (P < 0.05) and 4-fold expression change. Of these transcripts, 830 (94%) matched to known genes or EST were classified by functional categories (http://david.abcc.ncifcrf.gov/). The majority of alternatively expressed genes in MM6 were involved in biosynthetic and metabolic processes, but HRAS, a protein that is known to be associated with leukemogenesis, was expressed only in MM6 cells and several other genes involved in Erk1/Erk2 MAPK pathway were also over-expressed in MM6. Therefore, since MM6 cell line has a similar expression profile to primary human t(9;11) AML in general and expresses uniquely a strong Erk1/Erk2 MAPK pathway including HRAS, it can be used as a model for HRAS-positive t(9;11) AML.

Etoposide-initiated MLL rearrangements detected at high frequency in human primitive hematopoietic stem cells with in vitro and in vivo long-term repopulating potential

Rearrangements initiating within the well-characterized break-point cluster region of the mixed lineage leukemia (MLL) gene on 11q23 are a hallmark of therapy-related leukemias following treatment with topoisomerase II poisons including etoposide. Hematopoietic stem cells (HSC) are believed to be the target cell for leukemia-initiating MLL rearrangement events. Although etoposide treatment is sufficient to induce readily detectable MLL rearrangements in primary human CD34+ cells, the majority of cells that gain translocations do not proliferate in culture possibly due to reduced proliferative capacity of most CD34+ cells during normal differentiation [Blood 2005;105:2124]. We characterized the impact of etoposide on primary human long-term repopulating HSC that represent only a minor portion of CD34+ cells. The proliferative capacity of HSC is dramatically increased following both a single and multiple exposures to etoposide as determined by their ability to engraft bone marrow of immune-deficient non-obese diabetic/severe combined immunodeficient mice and to initiate hematopoiesis in long-term initiating cultures. Similar to results in CD34+ cells, a significant proportion of etoposide-treated HSC-derived clones harbored stable MLL rearrangements, including duplications, inversions and translocations. These results indicate HSC are highly susceptible to etoposide-induced and potentially oncogenic rearrangements initiating within MLL, and these HSC are particularly proficient for continued long-term proliferation both in vivo and in vitro.

Human CyP33 binds specifically to mRNA and binding stimulates PPIase activity of hCyP33

Human nuclear cyclophilin 33 (hCyP33) was the first protein which was found to contain an RNA-binding motif and a PPIase domain. It was not known what cellular and physiological roles are played by the RNA-binding activity as well as the PPIase activity of hCyP33. In this paper, we investigated the binding specificity of hCyP33 to different cellular RNA using ion-exchange chromatography and affinity adsorption. Furthermore, the influence of different cellular RNAs to the PPIase activity of hCyP33 was investigated using a protease-coupled method. The results show that hCyP33 binds specifically to mRNA, namely poly(A)(+)RNA, and that binding stimulates the PPIase activity of hCyP33.

Congenital leukemia cutis

We describe a premature neonate who was born with pancytopenia and a single subcutaneous nodule on her right lower extremity. A biopsy specimen from the nodule demonstrated a dense infiltrate of pleomorphic mononuclear cells that extended throughout the dermis and into the subcutaneous tissue. Immunohistochemical stains and bone marrow examination confirmed a diagnosis of acute myelogenous leukemia. Cytogenetic studies on peripheral blood by G-banding analysis revealed an abnormal karyotype of 46, XX, ins[inv(10)(p11.2q22.2);11](q22.2;q13.2q23.2). A split in the mixed lineage leukemia gene was identified by fluorescence in situ hybridization. Induction chemotherapy was started but was complicated by multiorgan failure. The patient died on the eleventh day of life. As leukemia cutis more typically presents as multiple infiltrative papules, nodules, or plaques, we stress the importance of including leukemia in the differential diagnosis of a solitary nodule in a neonate.

Bimodal degradation of MLL by SCFSkp2 and APCCdc20 assures cell cycle execution: a critical regulatory circuit lost in leukemogenic MLL fusions

Human chromosome 11q23 translocations disrupting MLL result in poor prognostic leukemias. It fuses the common MLL N-terminal approximately 1400 amino acids in-frame with >60 different partners without shared characteristics. In addition to the well-characterized activity of MLL in maintaining Hox gene expression, our recent studies established an MLL-E2F axis in orchestrating core cell cycle gene expression including Cyclins. Here, we demonstrate a biphasic expression of MLL conferred by defined windows of degradation mediated by specialized cell cycle E3 ligases. Specifically, SCF(Skp2) and APC(Cdc20) mark MLL for degradation at S phase and late M phase, respectively. Abolished peak expression of MLL incurs corresponding defects in G1/S transition and M-phase progression. Conversely, overexpression of MLL blocks S-phase progression. Remarkably, MLL degradation initiates at its N-terminal approximately 1400 amino acids, and tested prevalent MLL fusions are resistant to degradation. Thus, impaired degradation of MLL fusions likely constitutes the universal mechanism underlying all MLL leukemias. Our data conclude an essential post-translational regulation of MLL by the cell cycle ubiquitin/proteasome system (UPS) assures the temporal necessity of MLL in coordinating cell cycle progression.

Translocation (4;11)(p12;q23) with rearrangement of FRYL and MLL in therapy-related acute myeloid leukemia

Reciprocal chromosomal translocations involving the MLL gene at chromosome region 11q23 are recurring cytogenetic abnormalities in both de novo and therapy-related acute myeloid leukemia (AML) and in acute lymphoblastic leukemia. We report a t(4;11)(p12;q23) with rearrangement of MLL and FRYL (also known as AF4p12), a human homolog to the furry gene of Drosophila, in an adult patient with therapy-related AML after fludarabine and rituximab therapy for small lymphocytic lymphoma and radiation therapy for breast carcinoma. To our knowledge, t(4;11)(p12;q23) has been reported in two previous patients, and MLL and FRYL rearrangement was demonstrated in one of them. Both of the previous patients had therapy-related leukemias after exposure to topoisomerase II inhibitors, whereas our patient had received cytotoxic therapy that did not include a topoisomerase II inhibitor. Thus, t(4;11)(p12;q23) with MLL and FRYL involvement represents a new recurring 11q23 translocation, to date seen only in therapy-related acute leukemias.

C/EBPbeta suppression by interruption of CUGBP1 resulting from a complex rearrangement of MLL

Translocations involving the mixed-lineage leukemia gene (MLL) confer a poor prognosis in acute leukemias. In t(1;11)(q21;q23), MLL is fused reciprocally with AF1q. Here we describe a t(1;11)(q21;q23) with a secondary event involving insertion of the telomeric portion of MLL into the p arm of chromosome 11 (11p11). We show that this latter event interrupts the CUG triplet repeat binding protein-1 (CUGBP1) gene, a translational enhancer of C/EBPbeta. We then showed that these cells have reduced expression of CUGBP1 and C/EBPbeta when compared to other AML blasts. This is the first report to describe insertional disruption of the CUGBP1 gene and to suggest a role for the CUGBP1-C/EBPbeta pathway in leukemogenesis.

Guidance for fluorescence in situ hybridization testing in hematologic disorders

Fluorescence in situ hybridization (FISH) provides an important adjunct to conventional cytogenetics and molecular studies in the evaluation of chromosome abnormalities associated with hematologic malignancies. FISH employs DNA probes and methods that are generally not Food and Drug Administration-approved, and therefore, their use as analyte-specific reagents involves unique pre- and postanalytical requirements. We provide an overview of the technical parameters influencing a reliable FISH result and encourage laboratories to adopt specific procedures and policies in implementing metaphase and interphase FISH testing. A rigorous technologist training program relative to specific types of probes is detailed, as well as guidance for consistent interpretation of findings, including typical and atypical abnormal results. Details are provided on commonly used dual-fusion, extra signal, and break-apart probes, correct FISH nomenclature in the reporting of results, and the use of FISH in relation to other laboratory testing in the ongoing monitoring of disease. This article provides laboratory directors detailed guidance to be used in conjunction with existing regulations to successfully implement a FISH testing program or to assess current practices, allowing for optimal clinical testing for patient care.

Flavonoids and heart health: proceedings of the ILSI North America Flavonoids Workshop, May 31-June 1, 2005, Washington, DC

This article provides an overview of current research on flavonoids as presented during a workshop entitled, "Flavonoids and Heart Health," held by the ILSI North America Project Committee on Flavonoids in Washington, DC, May 31 and June 1, 2005. Because a thorough knowledge and understanding about the science of flavonoids and their effects on health will aid in establishing dietary recommendations for bioactive components such as flavonoids, a systematic review of the science of select flavonoid classes (i.e., flavonols, flavones, flavanones, isoflavones, flavan-3-ols, anthocyanins, and proanthocyanidins) was presented. The objectives of the workshop were to 1) present and discuss current research on flavonoid intake and the relation between flavonoids and heart health; 2) develop information that could lead to expert consensus on the state-of-the-science of dietary intake of flavonoids on heart health; and 3) summarize and prioritize the research needed to establish the relations between specific flavonoids and heart health. Presentations included the basics of the biology of flavonoids, including the types and distribution in foods, analytical methodologies used to determine the amounts in foods, the bioavailability, the consumption patterns and potential biomarkers of intake, risk assessment and safety evaluation, structure/function claims, and the proposed mechanism(s) of the relation between certain flavonoids and heart health endpoints. Data presented support the concept that certain flavonoids in the diet can be associated with significant health benefits, including heart health. Research gaps were identified to help advance the science.

Bromodomain and histone acetyltransferase domain specificities control mixed lineage leukemia phenotype

A critical unanswered question about mixed lineage leukemia (MLL) is how specific MLL fusion partners control leukemia phenotype. The MLL-cyclic AMP-responsive element binding protein-binding protein (CBP) fusion requires both the CBP bromodomain and histone acetyltransferase (HAT) domain for transformation and causes acute myelogenous leukemia (AML), often preceded by a myelodysplastic phase. We did domain-swapping experiments to define whether unique specificities of these CBP domains drive this specific MLL phenotype. Within MLL-CBP, we replaced the CBP bromodomain or HAT domain with P300/CBP-associated factor (P/CAF) or TAF(II)250 bromodomains or the P/CAF or GCN5 HAT domains. HAT, but not bromodomain, substitutions conferred enhanced proliferative capacity in vitro but lacked expression of myeloid cell surface markers normally seen with MLL-CBP. Mice reconstituted with domain-swapped hematopoietic progenitors developed different disease from those with MLL-CBP. This included development of lymphoid disease and lower frequency of the myelodysplastic phase in those mice developing AML. We conclude that both the CBP bromodomain and HAT domain play different but critical roles in determining the phenotype of MLL-CBP leukemia. Our results support an important role for MLL partner genes in determining the leukemia phenotype besides their necessity in leukemogenesis. Here, we find that subtleties in MLL fusion protein domain specificity direct cells toward a specific disease phenotype.

Solution structure of the nonmethyl-CpG-binding CXXC domain of the leukaemia-associated MLL histone methyltransferase

Methylation of CpG dinucleotides is the major epigenetic modification of mammalian genomes, critical for regulating chromatin structure and gene activity. The mixed-lineage leukaemia (MLL) CXXC domain selectively binds nonmethyl-CpG DNA, and is required for transformation by MLL fusion proteins that commonly arise from recurrent chromosomal translocations in infant and secondary treatment-related acute leukaemias. To elucidate the molecular basis of nonmethyl-CpG DNA recognition, we determined the structure of the human MLL CXXC domain by multidimensional NMR spectroscopy. The CXXC domain has a novel fold in which two zinc ions are each coordinated tetrahedrally by four conserved cysteine ligands provided by two CGXCXXC motifs and two distal cysteine residues. We have identified the CXXC domain DNA binding interface by means of chemical shift perturbation analysis, cross-saturation transfer and site-directed mutagenesis. In particular, we have shown that residues in an extended surface loop are in close contact with the DNA. These data provide a template for the design of specifically targeted therapeutics for poor prognosis MLL-associated leukaemias.

Pediatric T-cell acute lymphoblastic leukemia with aberrations of both MLL loci

Translocations involving the MLL gene at 11q23 have been implicated in acute lymphoblastic leukemia (ALL), as well as acute myeloid leukemia (AML). Such translocations result in gain of function fusion proteins that drive cell proliferation. Except in cases of T-cell ALL, MLL rearrangement is typically associated with a poor prognosis. We report a case of T-cell ALL with a t(11;19)(q23;p13.3) and deletion of the other chromosome 11 homolog at band q23. Fluorescence in situ hybridization (FISH) analyses confirmed involvement of the MLL loci in both the translocation and deletion. This case is unique in that deletions of 11q23 reported in ALL generally do not involve MLL. We are unaware of a previous report showing rearrangement of the MLL loci on both chromosome 11 homologues.

Topoisomerase II and the etiology of chromosomal translocations

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

Crucial role of MLL for the maintenance of memory T helper type 2 cell responses

The Mixed-Lineage Leukemia (MLL) gene, a mammalian homolog of the Drosophila trithorax, is implicated in regulating the maintenance of Hox gene expression and hematopoiesis. The physiological functions of MLL in the immune system remain largely unknown. Although MLL(+/-) CD4 T cells differentiate normally into antigen-specific effector Th1/Th2 cells in vitro, the ability of memory Th2 cells to produce Th2 cytokines was selectively reduced. Furthermore, histone modifications at the Th2 cytokine gene loci were not properly maintained in MLL(+/-) memory Th2 cells. The reduced expression of MLL in memory Th2 cells resulted in decreased GATA3 expression accompanied with impaired GATA3 locus histone modifications. The direct association of MLL with the GATA3 locus and the Th2 cytokine gene loci was demonstrated. Memory Th2 cell-dependent allergic airway inflammation was decreased in MLL(+/-) Th2 cell-transferred mice. Thus, a crucial role for MLL in the maintenance of memory Th2 cell function is indicated.

Chromosomal translocations involving the MLL gene: molecular mechanisms

A wide array of recurrent, non-random chromosomal translocations are associated with hematologic malignancies; experimental models have clearly demonstrated that many of these translocations are causal events during malignant transformation. Translocations involving the MLL gene are among the most common of these non-random translocations. Leukemias with MLL translocations have been the topic of intense interest because of the unusual, biphenotypic immunophenotype of these leukemias, because of the unique clinical presentation of some MLL translocations (infant leukemia and therapy-related leukemia), and because of the large number of different chromosomal loci that partner with MLL in these translocations. This review is focused on the potential mechanisms that lead to MLL translocations, and will discuss aberrant VDJ recombination, Alu-mediated recombination, non-homologous end joining, as well as the effect of DNA topoisomerase II poisons and chromatin structure.

K-Ras mutations and N-Ras mutations in childhood acute leukemias with or without mixed-lineage leukemia gene rearrangements

BACKGROUND

It is believed that Ras mutations drive the proliferation of leukemic cells. The objective of this study was to investigate the association of Ras mutations with childhood acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) with special reference to the presence or absence of mixed-lineage leukemia gene (MLL) rearrangements.

METHODS

Bone marrow samples from 313 children with B-precursor ALL and 130 children with de novo AML were studied at diagnosis. Southern blot analysis was used to detect MLL rearrangements, and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis was used to detect common MLL fusion transcripts. Complementary DNA panhandle PCR was used to identify the infrequent or unknown MLL partner genes. DNA PCR or RT-PCR followed by direct sequencing was performed to detect mutations at codons 12, 13, and 61 of the N-Ras and K-Ras genes.

RESULTS

Twenty of 313 patients with B-precursor ALL and 17 of 130 patients with de novo AML had MLL rearrangements. N-Ras mutations were detected in 2 of 20 patients with MLL-positive ALL and in 27 of 293 patients with MLL-negative ALL (P = 1.000). N-Ras mutations were detected in 2 of 17 patients with MLL-positive AML and in 14 of 113 patients with MLL-negative AML (P = 1.000). K-Ras mutations were present in 8 of 20 patients with MLL-positive ALL compared with 32 of 293 patients with MLL-negative ALL (P = 0.001). K-Ras mutations were detected in 3 of 17 patients with MLL-positive AML compared with 5 of 113 patients with MLL-negative AML (P = 0.069).

CONCLUSIONS

Ras mutations were detected in 20.8% of patients with childhood B-precursor ALL and in 17.7% of patients with childhood AML. MLL-positive B-precursor ALL was associated closely with Ras mutations (50%), especially with K-Ras mutations (40%), whereas MLL-positive AML was not associated with Ras mutations.

Characterization of fusion partner genes in 114 patients with de novo acute myeloid leukemia and MLL rearrangement

The fusion transcripts of MLL rearrangement [MLL(+)] in acute myeloid leukemia (AML) and their clinicohematologic correlation have not be well characterized in the previous studies. We used Southern blot analysis to screen MLL(+) in de novo AML. Reverse transcriptase-polymerase chain reaction was used to detect the common MLL fusion transcripts. cDNA panhandle PCR was used to identify infrequent or unknown MLL partner genes. MLL(+) was identified in 114 (98 adults) of 988 AML patients. MLL fusion transcripts comprised of 63 partial tandem duplication of MLL (MLL-PTD), 14 MLL-AF9, 9 MLL-AF10, 9 MLL-ELL, 8 MLL-AF6, 4 MLL-ENL and one each of MLL-AF1, MLL-AF4, MLL-MSF, MLL-LCX, MLL-LARG, MLL-SEPT6 and MLL-CBL. The frequency of MLL-PTD was 7.1% in adults and 0.9% in children (P<0.001). 11q23 abnormalities were detected in 64% of MLL/t11q23 and in none of MLL-PTD by conventional cytogenetics. There were no differences in remission rate, event-free survival and overall survival between adult MLL-PTD and MLL/t11q23 groups. Adult patients had a significantly poorer outcome than children. The present study showed that cDNA panhandle PCR can identify all rare or novel MLL partner genes. MLL-PTD was rare in childhood AML. MLL(+) adults had a poor outcome with no difference in survival between MLL-PTD and MLL/t11q23 groups.

MLL rearrangements are induced by low doses of etoposide in human fetal hematopoietic stem cells

During fetal development, the liver serves as the primary hematopoietic organ in which hematopoietic stem cells (HSC) comprise a large proportion of hepatic cell populations. Because HSC are capable of initiating long-term hematopoiesis, injury to these cells during pregnancy may play a role in the development of hematopoietic disorders manifested after birth. Of interest is the role of genetic injury to fetal HSC in the etiology of the infant acute leukemias, which are characterized by chromosomal rearrangements in the 11q23 region involving the mixed lineage leukemia (MLL) gene. These gene fusions also occur in leukemias in adults following chemotherapy with etoposide and other inhibitors of DNA topoisomerase II. We used etoposide as a model compound to determine the sensitivity of human fetal HSC to DNA damage and to determine whether we could induce MLL rearrangements in cultured human fetal HSC. Exposure of HSC to etoposide resulted in a dose-dependent loss of viability, with effects observed at low nanomolar concentrations. DNA strand breaks were observed on exposure to 140 nM etoposide, and higher etoposide concentrations stimulated an increase in early lymphoid populations and elicited G2/M cell cycle arrest. Immunophenotyping of MLL translocations revealed a significant increase in positive flow cytometry events at low etoposide concentrations and were consistent with MLL recombination. MLL translocations were confirmed using fluorescent in situ hybridization. In vitro inhibition of DNA topoisomerase II was observed at >or=25 microM etoposide, but was not evident at lower etoposide concentrations associated with DNA damage. Our data indicate that low acute doses of etoposide can cause DNA strand breaks and chromosomal rearrangements involving MLL in human fetal HSC. Ultimately, such injury may have ramifications with regards to transplacental exposures to environmental chemicals linked to the etiology of infant acute leukemias.

MLL: how complex does it get?

The mixed lineage leukemia (MLL) gene encodes a very large nuclear protein homologous to Drosophila trithorax (trx). MLL is required for the proper maintenance of HOX gene expression during development and hematopoiesis. The exact regulatory mechanism of HOX gene expression by MLL is poorly understood, but it is believed that MLL functions at the level of chromatin organization. MLL was identified as a common target of chromosomal translocations associated with human acute leukemias. About 50 different MLL fusion partners have been isolated to date, and while similarities exist between groups of partners, there exists no unifying property shared by all the partners. MLL gene rearrangements are found in leukemias with both lymphoid and myeloid phenotypes and are often associated with infant and secondary leukemias. The immature phenotype of the leukemic blasts suggests an important role for MLL in the early stages of hematopoietic development. Mll homozygous mutant mice are embryonic lethal and exhibit deficiencies in yolk sac hematopoiesis. Recently, two different MLL-containing protein complexes have been isolated. These and other gain- and loss-of-function experiments have provided insight into normal MLL function and altered functions of MLL fusion proteins. This article reviews the progress made toward understanding the function of the wild-type MLL protein. While many advances in understanding this multifaceted protein have been made since its discovery, many challenging questions remain to be answered.

Oncogene amplification in transforming myelodysplasia

The MLL gene, located within band 11q23, has been shown to be involved in translocations with a large variety of reciprocal sites in both lymphoid and myeloid leukemia and has also been shown to undergo submicroscopic self-fusion/partial duplication. We report 29 patients with cytogenetic evidence of 11q23 alteration, all of which demonstrate molecular cytogenetic evidence of amplification of the MLL gene by fluorescence in situ hybridization (FISH). In all MLL cases, the patients were clinically classified as having transforming myelodysplasia (RAEB/RAEBT) or AML. An additional patient with AML was found by 24-color and gene-specific FISH to have AML1 oncogene amplification. Four patients had been previously diagnosed with cancer and had received topoisomerase II targeted drug therapy which is known to be associated with fusion transcripts involving the MLL and AML1 genes. MLL amplification appeared in various forms: an atypical banded region that bridges from 11q23 into a dicentric chromosome, expanded regions emanating from band 11q23, chromosome 11 paint-positive rings with "spoke-like" MLL amplification, and expansion at sites other than chromosome 11 (including extra markers) in the absence of one of the 11 homologues. The fluorescence pattern in most cases suggests palindromic duplication with neighboring sequences in the long arm of chromosome 11. As opposed to MYCN amplification in hsrs (homogeneously staining regions) and double minutes in neuroblastoma, amplification of MLL in most cases occurred at the site of the gene. All of our patients rapidly developed refractory AML. The frequency and clinical correlations of MLL gene amplification in leukemia will need careful follow-up, since the frequently cryptic amplification described in these cases may not generally provoke confirmatory FISH studies. The reported MLL cases represented about 1% of the total abnormal MDS/AML cases over 8 years. A common cytogenetic profile of 5 q-, -17/17 p-, -18/18 q-, and a missing or abnormal chromosome 11, may help direct appropriate follow-up studies. The MLL and the AML1 oncogenes appear to be the only oncogenes amplified at the natural site of the gene. Both genes also show a high degree of diversity of pathogenic mechanisms of leukemia evolution, including numerous reciprocal fusion genes in transformation to either AML or ALL and gain of function amplification.

Secondary acute myeloid leukaemia: results of conventional treatments. Experience of GIMEMA trials

BACKGROUND

The aim of the study was to evaluate the outcome of acute myeloid leukaemia (AML) in patients with a previous malignancy (sAML) treated with chemo- and/or radiotherapy, enrolled in conventional trials.

PATIENTS AND METHODS

In a multicentre setting, a prospective non-concurrent analysis was performed on 2513 new AML patients, aged 12-78 years, consecutively enrolled in EORTC-GIMEMA trials between 1987 and 2001. Thirty-eight patients with sAML were identified and compared with a group of 114 de novo AML patients matched according to age, French-American-British criteria, white blood cell count at diagnosis, trial and time of diagnosis of AML. Induction treatment response, disease-free survival (DFS), duration and overall survival (OS) were evaluated in the two groups.

RESULTS

Comparing the complete remission (CR) rate between 38 sAML patients and 114 de novo AML patients, selected according to the previously reported criteria, we observed no difference in the CR rates [25/38 (66%) versus 66/114 (58%); Pearson chi(2) 0.7393, P=0.390] as well as no differences while comparing the DFS and the OS between the two groups.

CONCLUSION

The results of this study suggest that sAML patients are characterised by a good performance status permitting their recruitment in conventional trials without a previous myelodysplastic phase. Similar to de novo AML patients, sAML patients show good response to treatment and the possibility of cure.

MLL is fused to EB1 (MAPRE1), which encodes a microtubule-associated protein, in a patient with acute lymphoblastic leukemia

We have shown that the EB1 (MAPRE1) gene, at 20q11.2, is fused to MLL in an adult patient with pro-B acute lymphoblastic leukemia. Southern blot analysis indicated that a rearrangement of the MLL gene was involved in the chromosomal abnormality. cDNA panhandle polymerase chain reaction (PCR) identified the fusion transcript, in which MLL exon 6 was fused in-frame with EB1 exon 5. The presence of the MLL-EB1 and the reciprocal EB1-MLL fusion transcripts was verified by reverse-transcription PCR. EB1 is the first gene on chromosome 20 found to fuse with MLL. The genomic break junctions of MLL-EB1 and EB1-MLL were amplified by long-distance PCR. Sequencing of the break junctions revealed that multiple DNA breaks had occurred and that the DNA fragments flanked by these breaks had been duplicated, deleted, or inverted. Nontemplate DNA segments of 2 bp also were detected at the breakpoints on derivative chromosomes 11 and 20. These features indicate that this translocation likely resulted from the DNA damage-repair pathway. EB1 is a microtubule-associated protein that interacts with the colorectal adenomatous polyposis coli tumor-suppressor protein and plays important roles in regulating microtubule dynamics, cell polarity, and chromosome stability. Immunofluorescence staining demonstrated that the MLL-EB1 fusion proteins were localized in the nuclei.

Identification of various MLL gene aberrations that lead to MLL gene mutation in patients with acute lymphoblastic leukemia (ALL) and infants with acute leukemia

Studies were done to investigate MLL gene aberrations using Conventional Cytogenetics, Southern blotting as well as FISH using a panel of probes on 218 cases which included 206 cases of pediatric/young adult ALL and 12 cases of infantile acute leukemias from Tata Memorial Hospital, India. The incidence of MLL gene rearrangements in acute lymphoblastic leukemia (ALL) was 9.4% which included infants as well as pediatric/young adults. In infantile group which included ALL as well as AML cases, MLL gene rearrangement was very common (75% frequency). Application of metaphase-FISH helped unravel MLL rearrangements not only as a result of translocations but also inversions, insertions, partial deletion, duplications, partial duplication-->self-fusion. Besides age, MLL gene rearrangements showed significant association with hyperleukocytosis, peripheral blood blast percentage and early Pre-B phenotype. Clinical outcome of patients with MLL gene rearrangements revealed unfavorable prognosis.

Therapy-related acute myeloid leukemia–like MLL rearrangements are induced by etoposide in primary human CD34+ cells and remain stable after clonal expansion

Rearrangements involving the MLL gene on chromosome band 11q23 are a hallmark of therapy-related acute myeloid leukemias following treatment with topoisomerase II poisons including etoposide. Therapy-related and de novo genomic translocation breakpoints cluster within a well-characterized 8.3-kb fragment of MLL. Repair of etoposide-stabilized DNA topoisomerase II covalent complexes may initiate MLL rearrangements observed in patients. We used a culture system of primary human hematopoietic CD34+ cells and inverse polymerase chain reaction to characterize the spectrum of stable genomic rearrangements promoted by etoposide exposure originating within an MLL translocation hotspot in therapy-related leukemia. Alterations to the region were observed at a readily detectable frequency in etoposide-treated cells. Illegitimate repair events after minimal repair included MLL tandem duplications and translocations, with minor populations of deletions or insertions. In stably repaired cells that proliferated for 10 to 14 days, the significant majority of illegitimate events were MLL tandem duplications, and several deletions, inversions, insertions, and translocations. Thus, etoposide promotes specific rearrangements of MLL consistent with the full spectrum of oncogenic events identified in leukemic samples. Although etoposide-initiated rearrangements are frequent, only a small subset of translocations occurs in cells that proliferate significantly.

Phylogenetic roots of Alu-mediated rearrangements leading to cancer

There are over a million Alu repetitive elements dispersed throughout the human genome, and a high level of Alu-sequence similarity ensures a strong propensity for unequal crossover events, some of which have lead to deleterious oncogenic rearrangements. Furthermore, Alu insertions introduce consensus 3' splice sites, which potentially facilitate alternative splicing. Not surprisingly, Alu-mediated defective splicing has also been associated with cancer. To investigate a possible correlation between the expansion of Alu repeats associated with primate divergence and predisposition to cancer, 4 Alu-mediated rearrangements — known to be the basis of cancer — were selected for phylogenetic analysis of the necessary genotype. In these 4 cases, it was determined that the different phylogenetic age of the oncogenic recombination-prone genotype reflected the evolutionary history of Alu repeats spreading to new genomic sites. Our data implies that the evolutionary expansion of Alu repeats to new genomic locations establishes new predispositions to cancer in various primate species.Key words: Alu repeats, evolution, cancer, primates, splicing, DNA recombination.

Long-term follow-up of a patient with idiopathic myelofibrosis associated with chromosome 11 and 13 abnormalities

A case of a leukemic transformation following a 27-year history of idiopathic myelofibrosis (IMF) is presented. The patient had two chromosomal abnormalities: a deletion of chromosome 13, del 13(q12q14), and a deletion of chromosome 11, del 11(q14q23). This patient's final diagnosis was acute micromegakaryocytic leukemia, and she died 1 month after leukemic transformation with an additional chromosomal abnormality, trisomy 8. IMF with myeloid metaplasia associated with deletion of the long arms of chromosomes 11 and 13 has not been previously reported. We speculate that the leukemic transformation in this patient was associated with chromosomal abnormalities del 11 and trisomy 8.

Translocation t(X;11)(q13;q23) in B-cell chronic lymphocytic leukemia disrupts two novel genes

Deletion of chromosome region 11q22-q23 defines a subgroup of patients with B-cell chronic lymphocytic leukemia (B-CLL) characterized by poor survival. Although the tumor-suppressor gene ATM in the consensus deletion region was found to be biallelically inactivated in about one third of B-CLL cases, in the majority of those who have this deletion, inactivation of the remaining ATM allele was not observed. To identify a second disease-associated gene, we investigated two B-CLL cases with translocation breakpoints in the critical 11q23 deletion region. In one case, a t(X;11)(q13;q23) was cloned and two novel genes were isolated. The breakpoint on 11q23 affected the ARHGAP20 gene, which encodes a protein predicted to be involved in the regulation of Rho family GTPases. The breakpoint on Xq13 occurred in BRWD3, which codes for a putative novel transcription factor. The rearrangement of ARHGAP20 and BRWD3 did not result in fusion transcripts, but it disrupted both genes. Mutation analysis of 28 B-CLL samples with monoallelic deletions and two B-CLL samples with 11q23 translocations detected no deleterious mutation in the remaining copy of ARHGAP20. Quantitative expression analysis in 22 B-CLLs revealed significant up-regulation of ARHGAP20 in CLL B cells, whereas BRWD3 was slightly down-regulated. Thus, deregulation of ARHGAP20 by altered gene expression or by gene disruption (but not point mutation) might be a general molecular mechanism of B-CLL leukemogenesis.

Stem cells, aging, and cancer: inevitabilities and outcomes

Given the unique abilities of a stem cell to self-renew, differentiate, and proliferate, it is no wonder that they are critically important to an organism during development and to maintain homeostasis. Likewise, when something goes awry within a stem cell, it is likely to have far-reaching effects, since stem cells persist throughout the lifetime of the individual. Two significant biological phenomena that involve stem cells are the inevitable process of aging and a major health issue whose incidence increases with aging: cancer. In this review, we summarize evidence and theories concerning these two stem cell processes. The inability of stem cells to be passaged indefinitely in mice and the data supporting regular replication of the quiescent stem cell pool are discussed. Further, the current evidence indicating a stem cell origin of acute myeloid leukemia, including examples from both experimental mouse models and human clinical samples, is evaluated. Finally, we propose a model in which aging of the stem cell population of the hematopoietic system in particular can create conditions that are permissive to leukemia development; in fact, we suggest that aging is a secondary event in leukemogenesis.

MLL/GRAFfusion in an infant acute monocytic leukemia (AML M5b) with a cytogenetically cryptic ins(5;11)(q31;q23q23)

More than 30 fusions involving the MLL gene at 11q23 have been reported in acute myeloid leukemia (AML). Some of these chimeras are rather common, such as MLL/MLLT3(AF9), but many are quite rare, with some, for example, MLL/GRAF, described only in a single case. The MLL/GRAF fusion, in which the reciprocal hybrid was not expressed, suggesting that the former transcript was the leukemogenic one, was detected in a juvenile myelomonocytic leukemia with a t(5;11)(q31;q23). Here, we report a second case--an infant acute monocytic leukemia (AML M5b)--with an MLL/GRAF fusion. By conventional G-banding, the karyotype was normal. However, Southern blot and fluorescence in situ hybridization analyses revealed that MLL was rearranged and that the 5' part of the MLL gene was inserted into 5q in the vicinity of 5q31, which harbors GRAF. Reverse-transcriptase polymerase chain reaction (PCR) showed that exon 9 of MLL was fused in-frame with exon 19 of GRAF. Extralong genomic PCR with subsequent sequence analysis demonstrated that the breakpoints occurred in intron 9 of MLL, nine base pairs (bp) downstream from exon 9, and in intron 18 of GRAF, 117 bp downstream from exon 18. A 6-bp insertion (ACACTC) of unknown origin was present at the junction. The putative MLL/GRAF fusion protein would retain the AT-hook DNA-binding domain, the DNA methyl transferase motif, the transcription repression domain of MLL, and the SH3 domain of GRAF. As expected, the reciprocal GRAF/MLL was neither expressed nor generated at the genomic level as a consequence of the ins(5;11)(q31;q23q23). On the basis of the now-reported two cases with MLL/GRAF, we conclude that this transcript--but not the reciprocal one--characterizes a rare genetic subgroup of infant AML.

Uncommon karyotypic abnormality, t(11;19)(q23;p13.3), in a patient with blastic phase of chronic myeloid leukemia

We describe unusual cytogenetic findings in a 33-year-old male with blastic phase of Philadelphia chromosome (Ph)-positive chronic myeloid leukemia. In addition to the t(9;22)(q34;q11), which was detected in all metaphases, a t(11;19)(q23;p13.3) was also identified as an evolutional change in all 20 metaphases. Fluorescence in situ hybridization (FISH) analysis showed that fusion signals of the ABL/BCR probes were found in 95% of blastic cells. Southern blotting and FISH analysis also revealed involvement of the MLL gene on 11q23. Clinical course was aggressive and the patient responded poorly to therapy. These findings suggest an association between Ph and 11q23 with poor prognosis, and that t(11;19)(q23;p13.3) was the essential pathogenic factor in our case.

Modulation of cell cycle by graded expression of MLL-AF4 fusion oncoprotein

Acute lymphoblastic leukemia (ALLs) expressing MLL-AF4, the fusion product of t(4;11)(q21;q23), show marked leucocytosis and extramedullary disease in multiple organs, respond poorly to chemotherapy and have poor prognosis. In vitro, leukemic cells with the t(4;11) show resistance to serum deprivation-induced or interferon gamma-regulated CD95-mediated apoptosis. In addition, t(4;11) cells have prolonged doubling time and lower percentage of cells in cycle compared to non-t(4;11) B lineage cell lines. In this study, we examine the time- and level-dependent effects of MLL-AF4 conditional expression on cell cycle and differentiation of myelomonocytic leukemia cell line U937. By varying the concentration of tetracycline in growth media, we found that increasing levels of MLL-AF4 expression result in a progressive decrease in growth rate and fraction of S phase cells, paralleled by an increase in percentage of cells expressing CD11b. Our results demonstrate a dosage-dependent effect of MLL-AF4 fusion oncoprotein on cell cycle progression, with increasing expression levels resulting in the accumulation in G1, prolonged doubling time, both findings that might be responsible for the increased resistance to etoposide-mediated cytotoxicity. We propose the cell cycle control exerted by MLL-AF4 may be responsible of resistance to cell-death promoting stimuli in leukemia carrying the t(4;11) translocation.

Adult acute myeloid leukaemia

The curability of acute myeloid leukaemia (AML) in a fraction of adult patients was demonstrated a long time ago. Currently, the probability of cure is consistently above fifty per cent in patients with de novo disease expressing favourable-risk associated cytogenetic features. Even better, the cure rate exceeds 75% in the acute promyelocytic subtype since the introduction of retinoic acid-containing regimens. In the meantime, continuing progress in supportive care systems and stem cell transplant procedures is making myeloablative therapies, when needed, somewhat less toxic-and thereby more effective-than in the recent past. Therefore, evidence is accumulating to indicate an improved therapeutic trend over the years, with the notable exception of older (>55 years) patients with adverse-risk chromosomal aberrations and/or leukemia secondary to myelodysplasia or prior cancer-related chemotherapy and/or radiotherapy. This review conveys the many facets of this progress, focusing on diagnostic subsets, risk classes, newer biological issues and conventional as well as innovative therapeutic interventions with or without autologous/allogeneic stem cell transplantation.

FLJ10849, a septin family gene, fuses MLL in a novel leukemia cell line CNLBC1 derived from chronic neutrophilic leukemia in transformation with t(4;11)(q21;q23)

A t(4;11)(q21;q23) has been described in 50-70% of cases of infant acute lymphoblastic leukemia and, less frequently, in cases of pediatric and adult acute lymphoblastic leukemia and acute myeloid leukemia (AML). In t(4;11)(q21;q23) leukemias, the AF4 gene has been cloned as a fusion partner of the MLL gene. A human myeloid leukemia cell line, chronic neutrophilic leukemia (CNL)BC1, was established from a peripheral blood specimen of a patient with CNL in leukemic transformation. As with the original leukemia cells, the established line had a t(4;11)(q21;q23). We showed that the MLL gene on 11q23 was fused to the FLJ10849 gene on 4q21. The protein encoded by FLJ10849 belongs to the septin family, sharing highest homology with human SEPT6, which is one of the fusion partners of MLL in t(X;11)(q13;q23) AML. Our results suggest that FLJ10849 might define a new septin family particularly involved in the pathogenesis of 11q23-associated leukemia. The established cell line, CNLBC1, could provide a useful model for analyzing the pathogenesis of MLL-septin leukemias and chronic neutrophilic leukemia.

Acute Monocytic Leukemia (French-American-British classification M5) Does Not Have a Worse Prognosis Than Other Subtypes of Acute Myeloid Leukemia: A Report From the Eastern Cooperative Oncology Group

Purpose

Acute monocytic leukemia is a distinct subtype of acute myeloid leukemia (AML) with characteristic biologic and clinical features. This study was designed to compare the outcome of patients with M5 to that of other subtypes of AML, and to identify differences in M5a and M5b.

Patients and Methods

We reviewed all patients with AML M5 entered in three clinical trials for newly diagnosed AML conducted by the Eastern Cooperative Oncology Group between 1989 and 1998. Eighty-one patients, 21 with M5a and 60 with M5b, were identified.

Results

The complete remission rate was 62% for all patients with M5; 52% for patients with M5a and 65% for patients with M5b (P = .3), and 60% for the 1,122 patients with non-M5 AML entered on the same clinical trials (P = .8 for M5 v non-M5). The 3-year disease-free survival was 26% for all M5 patients; 18% for M5a and 28% for M5b (P = .31), and 33% for non-M5 patients (P = .13 for M5 v non-M5). The 3-year overall survival was 31% for all M5 patients; 33% for M5a and 30% for M5b (P = .65), and 30% for non-M5 (P = .74 for M5 v non-M5). The karyotypes of patients with AML M5 were heterogeneous. CD11b was the only leukemic cell antigen expressed differently in M5a (53%) compared to M5b (77%) to a significant degree (P = .02).

Conclusion

AML M5 represents an immunologically heterogeneous population similar to non-M5 AML with a prognosis that is not dependent on morphology. The disease-free survival and overall survival of patients with M5a, M5b, and non-M5 appear not to differ with currently available therapy.

Myeloid sarcoma: clinical and morphologic criteria useful for diagnosis

Extramedullary accumulation of myeloblasts or immature myeloid cells form tumors called myeloid sarcoma in the WHO classification. Such tumors develop in lymphoid organs, bone (skull, orbit, etc.), skin, soft tissue, various mucosae and organs, and the CNS. They may precede or occur concurrently with acute myeloid leukemia, or reveal blastic transformation of chronic myeloproliferative disorders or myelodysplastic syndromes. They may also reveal relapses in treated patients. They are constituted by a diffuse infiltrate made up of medium-to-large cells. The cells are difficult to identify. Imprints are very useful. Immunohistochemistry can help diagnose and distinguish four variants: granulocytic myeloperoxidase (MPO+, CD 68+ [KP1+/-, PGM1-] lysozyme+, CD 34+/-), monoblastic (MPO-, CD 68+, [KP1+, PGM1+] lysozyme+, CD 34-), myelomonoblastic (MPO-, CD 68+, [KP1+, PGM1+] lysozyme+, CD 34-), or megakaryoblastic (positivity for factor VIII, CD 61, CD 31). Immunohistochemistry sometimes demonstrates expression of CD 43, CD 7, CD 79a, and CD 56 (particularly the monoblastic variant with t[8;21]). Recently the demonstration of CD 99 and CD 117, which can now be done on paraffin sections, may be useful to identify blasts of granulocytic origin. The diagnosis is missed in about 50% of cases when immunohistochemistry is not used. Patients with myeloid sarcomas should be treated in the same way as patients with acute myeloblastic leukemia. Disease progression and prognosis are similar for the two conditions.

Taspase1: a threonine aspartase required for cleavage of MLL and proper HOX gene expression

The Mixed-Lineage Leukemia gene (MLL/HRX/ALL1) encodes a large nuclear protein homologous to Drosophila trithorax that is required for the maintenance of HOX gene expression. MLL is cleaved at two conserved sites generating N320 and C180 fragments, which heterodimerize to stabilize the complex and confer its subnuclear destination. Here, we purify and clone the protease responsible for cleaving MLL. We entitle it Taspase1 as it initiates a class of endopeptidases that utilize an N-terminal threonine as the active site nucleophile to proteolyze polypeptide substrates following aspartate. Taspase1 proenzyme is intramolecularly proteolyzed generating an active 28 kDa alpha/22 kDa beta heterodimer. RNAi-mediated knockdown of Taspase1 results in the appearance of unprocessed MLL and the loss of proper HOX gene expression. Taspase1 coevolved with MLL/trithorax as Arthropoda and Chordata emerged from Metazoa suggesting that Taspase1 originated to regulate complex segmental body plans in higher organisms.

p53 alterations in myeloid leukemia

Although most solid tumors contain inactivating mutations of the p53 tumor suppressor, hematological malignancies do not contain frequent alterations in the p53 gene (<20%). How these tumors arise in the presence of a super tumor suppressor like p53 remains to be elucidated. Given the number of downstream effectors of p53, it is likely that critical targets of p53 are inactivated in leukemia, bypassing the requirement for p53 gene mutations in these tumors. This review describes new biochemical and transcriptional activities of p53 as well as the status of p53 in acute myelogenous leukemia and chronic myelogenous leukemia.

Inversion of chromosome 12 and lineage promiscuity in hematologic malignancies

Rearrangements of the short arm of chromosome 12 are among the most common aberrations found in hematologic malignancies, including myelodysplastic syndromes, acute myelocytic leukemias, acute lymphoblastic leukemias, and non-Hodgkin lymphomas. We report on a group of 46 patients with a variety of myelocytic and lymphoid malignancies, all with an inversion of chromosome 12. Both pericentric and paracentric inversions occurred. The identified hotspots for breakage were p13 and q24. These correspond to gene-rich areas of known chromosome instability. The inv(12) is difficult to detect and may be misinterpreted as a partial deletion by routine cytogenetics. Fluorescence in situ hybridization studies revised the G-banding interpretations of a deleted 12p in some cases to an inversion. The inv(12) may occur as the sole abnormality in both myelocytic and lymphoid malignancies, suggesting lineage promiscuity as seen with MLL and ETV6 gene disruptions. The majority of patients with the inv(12) had complex karyotypic changes that predicted a poor prognosis. Of the 24 patients with known clinical follow-up, many were refractory to chemotherapy and overall survival was short.

MLL translocations with concurrent 3? deletions: Interpretation of FISH results

Rearrangements involving the MLL gene at 11q23 occur in a clinically relevant subgroup of patients with acute lymphoblastic leukemia (ALL) at all ages, and therefore their accurate identification at diagnosis is important. It has become commonplace to screen ALL patients for rearrangements of MLL using a dual-color fluorescence in situ hybridization (FISH) assay. We report on 12 ALL patients with an unusual FISH result consisting of the following signal pattern: one 5' green, no 3' red, and one/two fusion signals. This configuration is consistent with a MLL translocation and simultaneous deletion of 3' MLL-a well-established phenomenon-which has been interpreted as a positive result. G-banded and complementary metaphase FISH analyses confirmed an 11q23/MLL translocation in 8 of the 12 cases, whereas in one case, the identification of a del(11)(q23) was restricted to G-banded analysis only. In three cases, an MLL rearrangement was excluded by extensive FISH analysis and/or Southern blotting. In conclusion, the loss of the 3' MLL signal should not be assumed to be the result of a concurrent translocation and deletion event, and such aberrant FISH signal patterns should be investigated further by alternative methods for determining their MLL status.

Cryptic insertion ofMLL gene into 9p22 leads toMLL-MLLT3 (AF9) fusion in a case of acute myelogenous leukemia

The formation of a leukemogenic fusion product in hematopoietic malignancies is commonly achieved by chromosomal translocation. Alternate and cytogenetically undetectable mechanisms of fusion transcript generation have been documented for BCR-AB1, AML1-ETO, PML-RARA, NPM/ALK, and MLL-MLLT2 (AF4). Here, we report the investigation of a cryptic rearrangement leading to MLL-MLLT3 transcript formation. Cytogenetic analysis of peripheral blood from a 50-year-old acute myeloid leukemia patient yielded a karyotype of 47,XY,+8,del(11)(q21q23) in all metaphase cells examined. Metaphase fluorescence in situ hybridization analysis using the MLL probe at 11q23 revealed that the 5' portion of the MLL gene was inserted into chromosome 9 at band p22, whereas the 3' region of the MLL gene remained on chromosome 11. Whole-chromosome paint analysis confirmed the cryptic transfer of chromosome 11 material to 9p22. With this information, the karyotype was reassigned as 47,XY,+8,der(9)ins(9;11)(p22;q23q23),del(11)(q21q23). RT-PCR was used to show that the cryptic rearrangement in this patient led to the fusion of the MLL and MLLT3 transcripts on the der(9). The presence of the MLL-MLLT3 transcript is consistent with the clinical findings in this patient.

Etoposide induces chimeric Mll gene fusions

MLL gene fusions are the hallmark of more than 70% of therapy-related leukemias (t-ML) associated with topoisomerase II inhibitors (e.g., etoposide) and cause leukemia in murine transgenic models. To determine whether Mll genomic fusions can occur after exposure to topoisomerase II inhibitors, we developed a long-distance inverse PCR DNA-based assay for chimeric Mll fusions in mouse embryonic stem cells. We detected Mll fusions at a higher frequency following 100 microM etoposide for 8 h (16x10(-6) cell(-1)) than in no-drug controls (1.0x10(-6) cell(-1), P=0.0002) or after treatment with a comparably cytotoxic exposure to the antimicrotubule drug vincristine (1.0x10(-6) cell(-1), P=0.0047). The fusion points in Mll chimeric products induced by etoposide were localized to a 1.5 kb region between exons 9 and 11, analogous to the MLL breakpoint cluster region in human leukemia. All 49 Mll fusion partners analyzed matched known genomic murine sequences, with 40 (82%) matching annotated genes covering eighteen murine autosomes. One partner was Runx1, the murine homologue of the transcription factor AML-1, a target of human translocations in therapy-related leukemia. These findings indicate that etoposide triggers the formation of Mll gene fusions, a critical step for the development of treatment-induced leukemic transformation.

Development of a real-time RT-PCR assay for the quantification of the most frequentMLL/AF9fusion types resulting from translocation t(9;11)(p22;q23) in acute myeloid leukemia

One strategy to predict clinical outcome in patients with acute myeloid leukemia (AML) is detection of minimal residual disease (MRD) after achievement of hematologic complete remission (CR). We established a real-time RT-PCR assay by use of TaqMan technology for the identification of MRD by quantification of the most frequent fusion transcripts resulting from t(9;11)(p22;q23). To achieve comparable PCR efficiencies between the different PCR assays, primers were chosen to obtain amplicons of nearly identical lengths. MLL/AF9 copy numbers were normalized to the housekeeping gene porphobilinogen deaminase (PBGD). The sensitivity of the assay, as determined at the cellular level, was comparable to that of qualitative single-round RT-PCR. Samples from eight patients with t(9;11)-positive AML were analyzed. At diagnosis and relapse, normalized copy numbers were positive and ranged from 490 to 5,558. Samples from two of seven patients collected at the time of CR became negative, whereas five cases still had positive normalized copy numbers with values between 5 and 5,286. The implications of MRD detection by MLL/AF9 fusion transcript quantification for the clinical management of t(9;11)-positive AML have to be determined in further studies.

MLL/SEPTIN6 chimeric transcript from inv ins(X;11)(q24;q23q13) in acute monocytic leukemia: report of a case and review of the literature

Rearrangements of the MLL gene on chromosome 11, band q23, are one of the most common genetic changes in acute leukemia. Reciprocal translocation is the most common form of MLL rearrangement, and the partner genes in MLL translocation are notably diverse. Involvement of the SEPTIN6 gene on Xq24 in MLL rearrangements occurs very rarely, with only six cases having been documented in the literature. Of note, the MLL/SEPTIN6 rearrangements in these cases were cryptic or complex, and it was shown that the 5'-MLL/SEPTIN6-3' transcript resides on the derivative X chromosome rather than on the derivative chromosome 11 as in the majority of cases of MLL translocations. These observations suggested that MLL and SEPTIN6 reside on their respective chromosome loci in reverse orientation, that is, centromere-to-telomere and telomere-to-centromere, respectively. We here report a case of acute monocytic leukemia with inv ins(X;11)(q24;q23q13) in a 29-month-old child. Fluorescence in situ hybridization study revealed the break-apart 5'-MLL segment to be translocated to the derivative X chromosome, and reverse transcriptase-polymerase chain reaction followed by sequencing analysis confirmed the 5'-MLL/SEPTIN6-3' chimeric transcript. This case is the first to provide direct cytogenetic evidence for the salient nature of the MLL/SEPTIN6 rearrangement. We reviewed clinical and cytogenetic features of all cases of 11q23 and Xq22-24 rearrangements reported up to now, including six cases where the involvement of the SEPTIN6 gene was confirmed by molecular techniques.

MLL repression domain interacts with histone deacetylases, the polycomb group proteins HPC2 and BMI-1, and the corepressor C-terminal-binding protein

The MLL (mixed-lineage leukemia) gene is involved in many chromosomal translocations associated with acute myeloid and lymphoid leukemia. We previously identified a transcriptional repression domain in MLL, which contains a region with homology to DNA methyltransferase. In chromosomal translocations, the MLL repression domain is retained in the leukemogenic fusion protein and is required for transforming activity of MLL fusion proteins. We explored the mechanism of action of the MLL repression domain. Histone deacetylase 1 interacts with the MLL repression domain, partially mediating its activity; binding of Cyp33 to the adjacent MLL-PHD domain potentiates this binding. Because the MLL repression domain activity was only partially relieved with the histone deacetylase inhibitor trichostatin A, we explored other protein interactions with this domain. Polycomb group proteins HPC2 and BMI-1 and the corepressor C-terminal-binding protein also bind the MLL repression domain. Expression of exogenous BMI-1 potentiates MLL repression domain activity. Functional antagonism between Mll and Bmi-1 has been shown genetically in murine knockout models for Mll and Bmi-1. Our new data suggest a model whereby recruitment of BMI-1 to the MLL protein may be able to modulate its function. Furthermore, repression mediated by histone deacetylases and that mediated by polycomb group proteins may act either independently or together for MLL function in vivo.