Acute leukemias of different lineages have similar MLL gene fusions encoding related chimeric proteins resulting from chromosomal translocation

The YEATS domain epigenetic reader proteins ENL and AF9 and their therapeutic value in leukemia

Recent studies have uncovered similarities and differences between 2 highly homologous epigenetic reading proteins, namely, ENL (MLLT1) and AF9 (MLLT3) with therapeutic implications. The importance of these proteins has traditionally been exemplified by their involvement in chromosomal translocations with the mixed-lineage leukemia gene (MLL; aka KMT2a). MLL rearrangements occur in a subset of acute leukemias and generate potent oncogenic MLL-fusion proteins that impact epigenetic and transcriptional regulation. Leukemic patients with MLL rearrangements display intermediate-to-poor prognoses, necessitating further mechanistic research. Several protein complexes involved in regulating RNA polymerase II transcription and the epigenetic landscape are hijacked in MLL-r leukemia, which include ENL and AF9. Recent biochemical studies have defined a highly homologous YEATS domain in ENL and AF9 that binds acylated histones, which aids in the localization and retention of these proteins to transcriptional targets. In addition, detailed characterization of the homologous ANC-1 homology domain (AHD) on ENL and AF9 revealed differential association with transcriptional activating and repressing complexes. Importantly, CRISPR knockout screens have demonstrated a unique role for wild-type ENL in leukemic stem cell function, whereas AF9 appears important for normal hematopoietic stem cells. In this perspective, we examine the ENL and AF9 proteins with attention to recent work characterizing the epigenetic reading YEATS domains and AHD on both wild-type proteins and when fused to MLL. We summarized the drug development efforts and their therapeutic potential and assess ongoing research that has refined our understanding of how these proteins function, which continues to reveal new therapeutic avenues.

Rare KMT2A-ELL and Novel ZNF56-KMT2A Fusion Genes in Pediatric T-cell Acute Lymphoblastic Leukemia

BACKGROUND/AIM

Previous reports have associated the KMT2A-ELL fusion gene, generated by t(11;19)(q23;p13.1), with acute myeloid leukemia (AML). We herein report a KMT2A-ELL and a novel ZNF56-KMT2A fusion genes in a pediatric T-lineage acute lymphoblastic leukemia (T-ALL).

MATERIALS AND METHODS

Genetic investigations were performed on bone marrow of a 13-year-old boy diagnosed with T-ALL.

RESULTS

A KMT2A-ELL and a novel ZNF56-KMT2A fusion genes were generated on der(11)t(11;19)(q23;p13.1) and der(19)t(11;19)(q23;p13.1), respectively. Exon 20 of KMT2A fused to exon 2 of ELL in KMT2A-ELL chimeric transcript whereas exon 1 of ZNF56 fused to exon 21 of KMT2A in ZNF56-KMT2A transcript. A literature search revealed four more T-ALL patients carrying a KMT2A-ELL fusion. All of them were males aged 11, 11, 17, and 20 years.

CONCLUSION

KMT2A-ELL fusion is a rare recurrent genetic event in T-ALL with uncertain prognostic implications. The frequency and impact of ZNF56-KMT2A in T-ALL are unknown.

Keeping RNA polymerase II on the run: Functions of MLL fusion partners in transcriptional regulation

Since the identification of key MLL fusion partners as transcription elongation factors regulating expression of HOX cluster genes during hematopoiesis, extensive work from the last decade has resulted in significant progress in our overall mechanistic understanding of role of MLL fusion partner proteins in transcriptional regulation of diverse set of genes beyond just the HOX cluster. In this review, we are going to detail overall understanding of role of MLL fusion partner proteins in transcriptional regulation and thus provide mechanistic insights into possible MLL fusion protein-mediated transcriptional misregulation leading to aberrant hematopoiesis and leukemogenesis.

The FOXO's Advantages of Being a Family: Considerations on Function and Evolution

The nematode Caenorhabditis elegans possesses a unique (with various isoforms) FOXO transcription factor DAF-16, which is notorious for its role in aging and its regulation by the insulin-PI3K-AKT pathway. In humans, five genes (including a protein-coding pseudogene) encode for FOXO transcription factors that are targeted by the PI3K-AKT axis, such as in C. elegans. This common regulation and highly conserved DNA-binding domain are the pillars of this family. In this review, I will discuss the possible meaning of possessing a group of very similar proteins and how it can generate additional functionality to more complex organisms. I frame this discussion in relation to the much larger super family of Forkhead proteins to which they belong. FOXO members are very often co-expressed in the same cell type. The overlap of function and expression creates a certain redundancy that might be a safeguard against the accidental loss of FOXO function, which could otherwise lead to disease, particularly, cancer. This is one of the points that will be examined in this "family affair" report.

Tissue-Specific Metabolic Regulation of FOXO-Binding Protein: FOXO Does Not Act Alone

The transcription factor forkhead box (FOXO) controls important biological responses, including proliferation, apoptosis, differentiation, metabolism, and oxidative stress resistance. The transcriptional activity of FOXO is tightly regulated in a variety of cellular processes. FOXO can convert the external stimuli of insulin, growth factors, nutrients, cytokines, and oxidative stress into cell-specific biological responses by regulating the transcriptional activity of target genes. However, how a single transcription factor regulates a large set of target genes in various tissues in response to a variety of external stimuli remains to be clarified. Evidence indicates that FOXO-binding proteins synergistically function to achieve tightly controlled processes. Here, we review the elaborate mechanism of FOXO-binding proteins, focusing on adipogenesis, glucose homeostasis, and other metabolic regulations in order to deepen our understanding and to identify a novel therapeutic target for the prevention and treatment of metabolic disorders.

FOXO family in regulating cancer and metabolism

FOXO proteins are a sub-group of a superfamily of forkhead box (FOX)-containing transcription factors (TFs). FOXOs play an important role in regulating a plethora of biological activities ranging from development, cell signaling, and tumorigenesis to cell metabolism. Here we mainly focus on reviewing the role of FOXOs in regulating tumor and metabolism. Moreover, how crosstalk among various pathways influences the function of FOXOs will be reviewed. Further, the paradoxical role for FOXOs in controlling the fate of cancer and especially resistance/sensitivity of cancer to the class of drugs that target PI3K/AKT will also be reviewed. Finally, how FOXOs regulate crosstalk between common cancer pathways and cell metabolism pathways, and how these crosstalks affect the fate of the cancer will be discussed.

Molecular analysis of more than 140 gene fusion variants and aberrant activation of EVI1 and TLX1 in hematological malignancies

Gene fusions are observed in abnormal chromosomal rearrangements such as translocations in hematopoietic malignancies, especially leukemia subtypes. Hence, it is critical to obtain correct information about these rearrangements in order to apply proper treatment techniques. To identify abnormal molecular changes in patients with leukemia, we developed a multiplex reverse transcriptase polymerase chain reaction (MRT-PCR) protocol and investigated more than 140 gene fusions resulting from variations of 29 prevalent chromosomal rearrangements along with EVI1 and TLX1 oncogenic expression in the presence of optimized primers. The potential of the MRT-PCR method was approved by evaluating the available cell lines as positive control and confirmed by sequencing. Samples from 53 patients afflicted with hematopoiesis malignancies were analyzed. Results revealed at least one chromosomal rearrangement in 69% of acute myeloid leukemia subjects, 64% of acute lymphoblastic leukemia subjects, and 81% of chronic myeloid leukemia subjects, as well as a subject with hypereosinophilic syndrome. Also, five novel fusion variants were detected. Results of this study also showed that chromosomal rearrangements, both alone and in conjunction with other rearrangements, are involved in leukemogenesis. Moreover, it was found that EVI1 is a suitable hallmark for hematopoietic malignancies.

Mouse models of MLL leukemia: recapitulating the human disease

Chromosome translocations involving the mixed lineage leukemia (MLL) gene fuse it in frame with multiple partner genes creating novel fusion proteins (MLL-FPs) that cause aggressive acute leukemias in humans. Animal models of human disease are important for the exploration of underlying disease mechanisms as well as for testing novel therapeutic approaches. Patients carrying MLL-FPs have very few cooperating mutations, making MLL-FP driven leukemias ideal for animal modeling. The fact that the MLL-FP is the main driver mutation has allowed for a wide range of different experimental model systems designed to explore different aspects of MLL-FP leukemogenesis. In addition, MLL-FP driven acute myeloid leukemia (AML) in mice is often used as a general model for AML. This review provides an overview of different MLL-FP mouse model systems and discusses how well they have recapitulated aspects of the human disease as well as highlights the biological insights each model has provided into MLL-FP leukemogenesis. Many promising new drugs fail in the early stages of clinical trials. Lessons learned from past and present MLL-FP models may serve as a paradigm for designing more flexible and dynamic preclinical models for these as well as other acute leukemias.

Pediatric T-cell acute lymphoblastic leukemia

The most common pediatric malignancy is acute lymphoblastic leukemia (ALL), of which T-cell ALL (T-ALL) comprises 10-15% of cases. T-ALL arises in the thymus from an immature thymocyte as a consequence of a stepwise accumulation of genetic and epigenetic aberrations. Crucial biological processes, such as differentiation, self-renewal capacity, proliferation, and apoptosis, are targeted and deranged by several types of neoplasia-associated genetic alteration, for example, translocations, deletions, and mutations of genes that code for proteins involved in signaling transduction, epigenetic regulation, and transcription. Epigenetically, T-ALL is characterized by gene expression changes caused by hypermethylation of tumor suppressor genes, histone modifications, and miRNA and lncRNA abnormalities. Although some genetic and gene expression patterns have been associated with certain clinical features, such as immunophenotypic subtype and outcome, none has of yet generally been implemented in clinical routine for treatment decisions. The recent advent of massive parallel sequencing technologies has dramatically increased our knowledge of the genetic blueprint of T-ALL, revealing numerous fusion genes as well as novel gene mutations. The challenges now are to integrate all genetic and epigenetic data into a coherent understanding of the pathogenesis of T-ALL and to translate the wealth of information gained in the last few years into clinical use in the form of improved risk stratification and targeted therapies. Here, we provide an overview of pediatric T-ALL with an emphasis on the acquired genetic alterations that result in this disease. © 2016 Wiley Periodicals, Inc.

A cryptic three-way translocation t(10;19;11)(p12.31;q13.31;q23.3) with a derivative Y-chromosome in an infant with acute myeloblastic leukemia (M5b)

Acute myeloid leukemia (AML) is a heterogeneous disease characterized by the malignant transformation of hematopoietic precursors to a pathogenic cell clone. Chromosomal band 11q23 harboring MLL (=mixed lineage leukemia) gene is known to be involved in rearrangements with variety of genes as activating partners of MLL in different AML subtypes. Overall, an unfavorable prognosis is associated with MLL abnormalities. Here we investigated an 11-month-old male presenting with hyperleukocytosis being diagnosed with AML subtype FAB-M5b. In banding cytogenetics a der(19)t(19;?)(q13.3;?) and del(Y)(q11.23) were found as sole aberrations. Molecular cytogenetics revealed that the MLL gene was disrupted and even partially lost due to a t(10;19;11)(p12.31;q13.31;q23.3), an MLL/MLLT10 fusion appeared, and the der(Y) was an asymmetric inverted duplication with breakpoints in Yp11.2 and Yq11.23. The patient got hematopoietic stem cell transplantation from his haploidentical mother. Still three months afterwards 15% of blasts were detected in bone marrow and later the patient was lost during follow-up. The present case highlights the necessity to exclude MLL rearrangements, even when there seems to be no actual hint from banding cytogenetics.

FISH analysis of MLL gene rearrangements: detection of the concurrent loss or gain of the 3' signal and its prognostic significance

INTRODUCTION

The rearrangement of the mixed-lineage leukemia (MLL) gene occurs through translocations and insertions involving a variety of partner chromosome genes. However, there are few studies on aberrant MLL signal patterns such as concurrent 3' MLL deletion.

METHODS

A total of 84 patients with acute leukemia (AL) who had MLL rearrangements detected by florescence in situ hybridization (FISH) were enrolled in the study. The distribution of MLL fusion partner genes was analyzed, and aberrant MLL signals were evaluated.

RESULTS

Seventy-seven (91.7%) patients had MLL rearrangements, involving previously described translocation partner genes (TPGs). Among these TPGs, the frequencies of MLLT3, AFF1, MLLT4, and ELL were 29.8%, 17.9%, 15.5%, and 13.1%, respectively. A high frequency of MLLT4 in our study was due to the high proportion of acute myeloid leukemia cases in pediatric and adult patients. Aberrant MLL signals were found in 18 patients: 11 (61.1%) with 3' MLL signal loss and 7 with 3' MLL signal gain. All cases with 3' MLL signal gain were due to an extra derivative partner chromosome. The median overall survival period of patients with 3' MLL gain was shorter than that in patients without aberrant MLL signal patterns.

CONCLUSION

Aberrant MLL signals were frequently detected by FISH analysis. The 3' MLL gain was associated with poor prognosis in patients with AL. Therefore, it is important to detect aberrant MLL signal patterns using FISH analysis.

FoxO3a is activated and executes neuron death via Bim in response to β-amyloid

The molecules that mediate death of selective neurons in Alzheimer's disease (AD) are mostly unknown. The Forkhead transcription factor FoxO3a has emerged as an important mediator of cell fate including apoptosis. When phosphorylated by Akt, it is localized in the cytosol as an inactive complex bound with 14-3-3 protein. For activation and localization of FoxO3a in the nucleus, further modifications are required, such as phosphorylation by mammalian sterile 20-like kinase 1 (MST1) and arginine methylation by protein arginine methyltransferase1. We report here that Akt-mediated phosphorylation of FoxO3a is diminished in neurons exposed to oligomeric β-amyloid (Aβ), in vitro and in vivo. We also find that oligomeric Aβ activates FoxO3a by MST1 phosphorylation and arginine methylation in primary cultures of hippocampal and cortical neurons. Moreover, FoxO3a translocates from the cytosol to nucleus in cultured neurons in response to Aβ. Most importantly, the nuclear redistribution of FoxO3a is significantly increased in Aβ-overexpressing AβPPswe-PS1dE9 mice and Aβ-infused rat brains. We further find that FoxO3a is essential for loss of neurons and neural networks in response to Aβ. Recent reports implicate Bim, a pro-apoptotic member of Bcl-2 family, in neuron death in AD, as a key target of this transcription factor. We show that Bim is a direct target of FoxO3a in Aβ-treated neurons. Our findings thus indicate that FoxO3a is activated, translocated to the nucleus and mediates neuron death via Bim in response to Aβ toxicity.

Identification of T-lymphocytic leukemia-initiating stem cells residing in a small subset of patients with acute myeloid leukemic disease

Xenotransplantation of acute myeloid leukemia (AML) into immunodeficient mice has been critical for understanding leukemogenesis in vivo and defining self-renewing leukemia-initiating cell subfractions (LICs). Although AML-engraftment capacity is considered an inherent property of LICs, substrains of NOD/SCID mice that possess additional deletions such as the IL2Rγc(null) (NSG) have been described as a more sensitive recipient to assay human LIC function. Using 23 AML-patient samples, 39% demonstrated no detectable engraftment in NOD/SCID and were categorized as AMLs devoid of LICs. However, 33% of AML patients lacking AML-LICs were capable of engrafting NSG recipients, but produced a monoclonal T-cell proliferative disorder similar to T-ALL. These grafts demonstrated self-renewal capacity as measured by in vivo serial passage and were restricted to CD34-positive fraction, and were defined as LICs. Molecular analysis for translocations in MLL genes indicated that these AML patient-derived LICs all expressed the MLL-AFX1 fusion product. Our results reveal that the in vivo human versus xenograft host microenvironment dictates the developmental capacity of human LICs residing in a small subset of patients diagnosed with AML harboring MLL mutations. These findings have implications both for the basic biology of CSC function, and for the use of in vivo models of the leukemogenic process in preclinical or diagnostic studies.

Challenges and strategies for generating therapeutic patient-specific hemangioblasts and hematopoietic stem cells from human pluripotent stem cells

Recent characterization of hemangioblasts differentiated from human embryonic stem cells (hESC) has further confirmed evidence from murine, zebrafish and avian experimental systems that hematopoietic and endothelial lineages arise from a common progenitor. Such progenitors may provide a valuable resource for delineating the initial developmental steps of human hemato-endotheliogenesis, which is a process normally difficult to study due to the very limited accessibility of early human embryonic/fetal tissues. Moreover, efficient hemangioblast and hematopoietic stem cell (HSC) generation from patient-specific pluripotent stem cells has enormous potential for regenerative medicine, since it could lead to strategies for treating a multitude of hematologic and vascular disorders. However, significant scientific challenges remain in achieving these goals, and the generation of transplantable hemangioblasts and HSC derived from hESC currently remains elusive. Our previous work has suggested that the failure to derive engraftable HSC from hESC is due to the fact that current methodologies for differentiating hESC produce hematopoietic progenitors developmentally similar to those found in the human yolk sac, and are therefore too immature to provide adult-type hematopoietic reconstitution. Herein, we outline the nature of this challenge and propose targeted strategies for generating engraftable human pluripotent stem cell-derived HSC from primitive hemangioblasts using a developmental approach. We also focus on methods by which reprogrammed somatic cells could be used to derive autologous pluripotent stem cells, which in turn could provide unlimited sources of patient-specific hemangioblasts and HSC.

Development of a biochip-based assay integrated in a global strategy for identification of fusion transcripts in acute myeloid leukemia: a work flow for acute myeloid leukemia diagnosis

Three major types of rearrangements are involved in acute myeloid leukemias (AML): t(8;21)(q22;q22), inv(16)(p13q22), and 11q23/MLL abnormalities. Their precise identification becomes essential for diagnosis, prognosis, and therapeutic choices. Resulting fusion transcripts (FT) are also powerful markers for monitoring the efficacy of treatment, the minimal residual disease (MRD) and could become therapeutic targets. Today, the challenge is to propose an individual follow-up for each patient even for those with a rare fusion event. In this study, we propose a biochip-based assay integrated in a global strategy for identification of rare FT in AML, after fluorescence in situ hybridization detection, as described by the World Health Organization classification. Using cell lines, we developed and validated a biochip-based assay called the AMLFusionChip that identifies every FT of AML1-ETO, CBFbeta-MYH11 as well as MLL-AF9, MLL-ENL, MLL-AF6, and MLL-AF10. The original design of our AMLFusionChip.v01 enables the identification of these FT wherever the breakpoint on the partner gene may be. In case of biochip negative result, our 3'RACE amplification strategy enables to clone and then sequence the new translocation partner. This AMLFusionChip strategy fits into the concept of personalized medicine for the largest number of patients.

Transcription factor Foxo3 controls the magnitude of T cell immune responses by modulating the function of dendritic cells

Foxo transcription factors regulate cell cycle progression, survival, and DNA repair pathways. Here, we demonstrate that a deficiency in Foxo3 resulted in increased expansion of T cell populations after viral infection. This exaggerated expansion was not T cell intrinsic. Rather, it was caused by the enhanced capacity of Foxo3-deficient dendritic cells to sustain T cell viability by producing increased amounts of interleukin 6 (IL-6). CTLA-4-mediated stimulation of dendritic cells induced nuclear localization of Foxo3, which in turn inhibited IL-6 and tumor necrosis factor production. Thus, Foxo3 acts to constrain dendritic cell production of key inflammatory cytokines and control T cell survival.

FOXOs, cancer and regulation of apoptosis

Forkhead box O (FOXO) transcription factors are involved in multiple signaling pathways and play critical roles in a number of physiological and pathological processes including cancer. The importance of FOXO factors ascribes them under multiple levels of regulation including phosphorylation, acetylation/deacetylation, ubiquitination and protein-protein interactions. As FOXO factors play a pivotal role in cell fate decision, mounting evidence suggests that FOXO factors function as tumor suppressors in a variety of cancers. FOXOs are actively involved in promoting apoptosis in a mitochondria-independent and -dependent manner by inducing the expression of death receptor ligands, including Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand, and Bcl-2 family members, such as Bim, bNIP3 and Bcl-X(L), respectively. An understanding of FOXO proteins and their biology will provide new opportunities for developing more effective therapeutic approaches to treat cancer.

Forkhead transcription factors and cardiovascular biology

The Forkhead family of transcription factors modulates a wide variety of cellular functions in cardiovascular tissues. In this review article, we discuss recent advances in our understanding of regulation provided by the forkhead factors in cardiac myocytes and vascular cells.

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.

Dynamic FoxO transcription factors

Forkhead box O (FoxO) transcription factors FoxO1, FoxO3a, FoxO4 and FoxO6, the mammalian orthologs of Caenorhabditis elegans DAF-16, are emerging as an important family of proteins that modulate the expression of genes involved in apoptosis, the cell cycle, DNA damage repair, oxidative stress, cell differentiation, glucose metabolism and other cellular functions. FoxO proteins are regulated by multiple mechanisms. They undergo inhibitory phosphorylation by protein kinases such as Akt, SGK, IKK and CDK2 in response to external and internal stimuli. By contrast, they are activated by upstream regulators such as JNK and MST1 under stress conditions. Their activities are counterbalanced by the acetylases CBP and p300 and the deacetylase SIRT1. Also, whereas polyubiquitylation of FoxO1 and FoxO3a leads to their degradation by the proteasome, monoubiquitylation of FoxO4 facilitates its nuclear localization and augments its transcriptional activity. Thus, the potent functions of FoxO proteins are tightly controlled by complex signaling pathways under physiological conditions; dysregulation of these proteins may ultimately lead to disease such as cancer.

A case of acute myelogenous leukemia with MLL–AF10 fusion caused by insertion of 5′ MLL into 10p12, with concurrent 3′ MLL deletion

Structural abnormalities involving the mixed-lineage leukemia (MLL) gene on 11q23 have been associated with hematological malignancies. The rearrangement of MLL occurs during translocations and insertions involving a variety of genes on the partner chromosome. We report a rare case of acute myelogenous leukemia (AML-M2) with 11q23 abnormalities. Fluorescence in situ hybridization (FISH) using a commercial dual-color MLL probe detected an atypical signal pattern: one fusion signal, two green signals smaller than those usually detected, and no orange signals. Spectral karyotyping (SKY) analysis indicated that one green signal was detected on the short arm of derivative chromosome 10, and the other green signal on the long arm of a derivative chromosome 11, on which no orange signal was detected. A long-distance inverse polymerase chain reaction (LDI-PCR) identified the fusion partner gene, in which intron 6 of MLL was fused with intron 8 of AF10 on 10p12 in the 5' to 3' direction. Our observations indicated that the MLL-AF10 fusion gene resulted from the insertion of part of the region that included the 5' MLL insertion into 10p12; this was concurrent with the deletion of 3' MLL.

FOXO factors: a matter of life and death

Forkhead box O-class (FOXO) transcription factors, including FOXO1, FOXO3a and FOXO4, function as tumor-suppressor proteins by inhibiting cell proliferation, promoting apoptotic cell death and protecting cells from DNA damage and oxidative stress. The potency of these functions is regulated tightly by phosphorylation, acetylation and ubiquitination. Emerging evidence indicates that protein levels of FOXO1 are under dual regulation by Ak-mediated phosphorylation and Skp2-mediated ubiquitination. Given that Akt and Skp2 proteins are highly activated in human cancers due to the loss of phosphatase and tensin homolog (PTEN), deregulation of the FOXO1 protein appears to be a promising target for future drug discovery and cancer therapy.

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.

3′ CBFβ deletion associated with inv(16) in acute myeloid leukemia

Recent reports have shown that concomitant submicroscopic deletions can occur in association with chromosomal translocations/inversions in several leukemia subtypes. Detectable by fluorescence in situ hybridization (FISH), these losses of sequence include deletion of the 5' region of the ABL gene and the 3' region of BCR in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL), as well as the 5' region of ETO in acute myeloid leukemia (AML) French-American-British type M2 associated with t(8;21), 3'MLL in AML and ALL, and 3' core-binding factor beta (CBFbeta) in AML associated with inv(16). While it has been widely reported that submicroscopic deletions of the derivative 9 in CML have an adverse prognostic impact, the clinical significance, if any, of deletions associated with t(8;21), inv(16)/t(16;16), or MLL rearrangement is yet to be determined. We analyzed a series of 39 patients diagnosed with AML who had cytogenetically detectable inv(16)/t(16;16) by using a FISH probe for the CBFbeta region to determine the incidence of the 3'CBFbeta deletion. Deletions were detected in three patients (8%), all associated with inv(16), bringing the number of cases reported so far to seven. The prognostic significance of this finding remains unclear.

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.

Constitutively active FOXO4 inhibits Akt activity, regulates p27 Kip1 stability, and suppresses HER2-mediated tumorigenicity

The FOXO family of Forkhead transcription factors, regulated by the phosphoinositide-3-kinase-Akt pathway, is involved in cell cycle regulation and apoptosis. Strong expression of HER2, a receptor tyrosine kinase oncogene, in cancers has been associated with a poor prognosis. Recently, FOXO4 was shown to regulate the transcription of the cyclin-dependent kinase inhibitor p27 Kip1 gene directly. Also, we have shown that HER2 promotes mitogenic growth and transformation of cancer cells by downregulation of p27 Kip1. Given the fact that FOXO4 mediates p27 transcription, we hypothesize that an Akt phosphorylation mutant of FOXO4 (FOXO4A3), which maintains the activity to transactivate p27 Kip1, may be used as an anticancer agent for HER2-overexpressing cancers. Here, we applied the FOXO4 gene as a novel anticancer agent for HER2-overexpressing cells under the control of a tetracycline (tet)-regulated gene expression system. Overexpression of FOXO4A3 inhibits HER2-activated cell growth. We found that FOXO4A3 inhibited the kinase activity of protein kinase B/Akt and reversed HER2-mediated p27 mislocation in the cytoplasm. FOXO4A3 expression also led to decreased levels of CSN5, a protein involved in p27 degradation. These data suggest that FOXO4A3 also can regulate p27 post-transcriptionally. In addition, we found that FOXO4A3 sensitized cells to apoptosis induced by the chemotherapeutic agent 2-methoxyestradiol. Most significantly for clinical application, FOXO4A3 expression in HER2-overexpressing cells can be regulated in vivo and reduces the tumor volume in a tumor model. These findings indicate the applicability of employing FOXO4 regulation as a therapeutic intervention in HER2-overexpressing cancers.

Granulocytic differentiation of leukemic cells with t(9;11)(p22;q23) induced by all-trans-retinoic acid

Acute leukemia patients with MLL (mixed linage leukemia) rearrangements tend to respond poorly to conventional therapies. We examined differentiation of human myeloid leukemia cells displaying the MLL-AF9 gene, using several differentiation agents. When MOLM-14 cells were treated with all-trans retinoic acid (ATRA) or 1beta,25-dihydroxyvitamin D3, significant induced differentiation was observed. Trichostatin A (TSA), an inhibitor of histone deacetylase, demonstrated enhance effects with ATRA in regard to growth inhibition and differentiation induction in MOLM-14 cells. Pretreatment with TSA before exposure to ATRA displayed increased effect. Based on these findings, combined treatment with ATRA and TSA may be clinically useful in therapy for acute leukemia displaying MLL-AF9 fusion gene.

X chromosome insertion in the MLL gene in a case of childhood acute myeloblastic leukemia

Band 11q23 is known to be involved in translocations and insertions with a variety of partner chromosomes. These lead to MLL rearrangement, resulting in a fusion with numerous genes. We report here the case of a 5-month-old boy presenting with hemianopsia and severe diffuse intravascular coagulopathy in whom a diagnosis of acute myeloblastic leukemia (AML) French-American-British M4 classification was made. Conventional cytogenetic techniques showed an ins(11;X) (q23;q28q12). Fluorescent in situ hybridization (FISH) with whole chromosome paints confirmed this finding. Using a specific probe, the MLL gene was found to be disrupted, a portion of the X chromosome being inserted between the 5' and 3' regions of the MLL gene. Although some cases of insertion involving chromosomes X and 11 have been reported in AML, this appears to be the first case involving band Xq28. We postulate that this chromosomal rearrangement led to the fusion of the 5' region of the MLL gene with a yet unidentified gene located in band Xq28.

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.

Absence of leukaemic fusion gene transcripts in preterm infants exposed to diagnostic x rays

BACKGROUND

Childhood leukaemias express novel, clonotypic fusion genes that may already be present at birth before the clinical manifestation of leukaemia. Exposure of the fetus to diagnostic x rays is reported to increase the risk of childhood leukaemia, and may do so by generating leukaemic fusion genes. Advances in neonatal medicine in the past decade that have extended the limits of viability of preterm babies down to 23 weeks of gestation have resulted in the increased use of diagnostic x rays to monitor neonatal progress.

AIM

To investigate whether exposure of very preterm infants to diagnostic x rays in the neonatal period leads to the development of leukaemic fusion genes.

METHODS

Peripheral blood samples were collected at birth from very preterm infants (23-30 weeks gestation) and following exposure to diagnostic x rays at intervals of two weeks, until discharge. Cord blood samples from normal full term infants served as controls. Total RNA was extracted from the blood and the expression of the fusion genes TEL-AML1, MLL-AF4, and BCR-ABL, characteristic of three subtypes of childhood leukaemia, was investigated in the preterm and full term infant samples using a nested reverse transcriptase polymerase chain reaction method. Serial pre- and post-x ray samples from 42 preterm babies, pre-x ray samples from an additional 46 preterm infants, and cord blood samples from 100 normal full term infants were screened for fusion gene transcripts.

RESULTS

No leukaemic fusion gene transcripts were detected in preterm infants following exposure to diagnostic x rays. A BCR-ABL transcript was identified in a single preterm infant prior to x ray exposure. TEL-AML1 transcripts were detected in cord blood samples from two full term infants. MLL-AF4 transcripts were not detected in any of the pre- or full term infants tested.

CONCLUSIONS

Exposure of the preterm infants to x rays in this small series and at the doses used for diagnostic purposes did not induce leukaemic fusion gene expression, but we cannot exclude the possibility that a small proportion of preterm infants may be unusually sensitive to x rays.

Engineering de novo reciprocal chromosomal translocations associated with Mll to replicate primary events of human cancer

The etiology of human tumors often involves chromosomal translocations. Models that emulate translocations are essential to understanding the determinants of frank malignancy, those dictating the restriction of translocations to specific lineages, and as a basis for development of rational therapeutic methods. We demonstrate that developmentally regulated Cre-loxP-mediated interchromosomal recombination between the Mll gene, whose human counterpart is involved in a spectrum of leukemias, and the Enl gene creates reciprocal chromosomal translocations that cause myeloid tumors. There is a rapid onset and high penetrance of leukemogenesis in these translocator mice, and high proportions of cells carrying chromosomal translocations can be found in bone marrow as early as 12 days after birth. This de novo strategy is a direct recapitulation of naturally occurring human cancer-associated translocations.

The C-terminal region of cellular Qin oligomerizes: correlation with oncogenic transformation and transcriptional repression

Expression of the oncoprotein Qin induces tumors in chickens and oncogenic transformation of chicken embryo fibroblasts in culture. We performed a detailed deletion analysis of the C-terminal region of Qin (amino acids 246-451, extending from the winged helix domain to the C-terminus) and identified amino acids 246-379 as important for transformation. The same region mediates homo-oligomerization of Qin as documented in vitro by GST pulldowns and in vivo by coimmunoprecipitation. A 60 amino-acid region within the oligomerization domain is necessary and sufficient for transcriptional repression induced by Qin.

The many forks in FOXO's road

The FOXO family of transcription factors constitute an evolutionarily conserved subgroup within the larger family known as winged helix or Forkhead transcriptional regulators. Building upon work in the nematode, researchers have uncovered a role for these proteins in a diverse set of cellular responses that include glucose metabolism, stress response, cell cycle regulation, and apoptosis. At the organismal level, FOXO transcription factors are believed to function in various pathological processes ranging from cancer and diabetes to organismal aging. A number of studies have also shed light on the signaling pathways that regulate FOXO activity in response to external stimuli and have identified multiple FOXO target genes that mediate this varied set of biological responses.

Large deletions 5' to the ETO breakpoint are recurrent events in patients with t(8;21) acute myeloid leukemia

Recurrent chromosomal rearrangements are observed in many leukemia subtypes. Recently, it has been shown that several of these translocations/inversions were associated with the loss of sequences located in the vicinity of the chromosomal breakpoints. So far, such deletions have not been described for the t(8;21) translocation. We have analyzed a series of 65 patients with t(8;21) using several probes specific for the ETO and AML1 regions. We have found six patients (9%) with deletion of the region 5' to ETO. In all six patients, the deletion encompassed at least 260 kb, and was even larger in two patients (up to 2 Mb). A similar analysis of the 21q22 region did not reveal any deletion of the 3'AML1 region. In conclusion, cytogenetically undetectable small deletions located immediately 5' to the ETO breakpoint were found to accompany the t(8;21) translocation in a significant percentage of cases. The clinical significance, if any, of these deletions remains to be determined.

The leukemia-associated gene Mllt1/ENL: characterization of a murine homolog and demonstration of an essential role in embryonic development

MLLT1 (ENL/LTG19) is one of a number of fusion gene partners with the MLL oncogene involved in 11q23 translocations in human leukemia and encodes a transcriptional regulator of unknown function. Leukemias bearing MLL translocations may be myeloid or lymphoid or bear mixed lineage properties; however, those bearing MLL/MLLT1 translocations are predominantly lymphoid, suggesting that MLLT1 may influence the leukemic phenotype. The murine homolog Mllt1 exhibits 86% amino acid sequence identity with the human gene and is broadly expressed in murine tissues and cell lines, with the exception of liver and myeloid cell lines. We have mapped Mllt1 to mouse chromosome 17 band E2 using FISH analysis. The genomic structure and 5' regulatory sequence of Mllt1 are highly conserved between mouse and human. There is also conservation of the genomic structure, but not the promoter, between MLLT1 and MLLT3/AF9, a homologous gene that is also an MLL translocation partner in human leukemias with a predominant myeloid phenotype. Targeted disruption of Mllt1 in mice leads to embryonic lethality prior to 8.5 dpc. These studies indicate that MLLT1 is involved in essential developmental processes and suggest that expression patterns of MLL fusion partners may influence the lineage of MLL-associated leukemias.

An mRNA splice variant of the AFX gene with altered transcriptional activity

Several studies indicate that FKHR and AFX, mammalian homologues of the Caenorhabditis elegans forkhead transcription factor DAF-16, function in the insulin signaling pathway. Here we describe the discovery of a novel AFX isoform, which we designated AFX zeta, in which the first 16 amino acids of the forkhead domain are not present. PCR analysis showed that this isoform is most abundant in the liver, kidney, and pancreas. In HepG2 cells, overexpressed AFX zeta induced reporter gene activity through the insulin-responsive sequences of the phosphoenolpyruvate carboxykinase (PEPCK), IGFBP-1, and G6Pase promoters. AFX zeta-mediated stimulation was repressed by insulin treatment, by bisperoxovanadate treatment, and by overexpression of constitutively active protein kinase B (PKB). Insulin treatment and PKB overexpression resulted in phosphorylation of AFX zeta. Furthermore, 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR), an AMP-activated protein kinase activator, repressed AFX zeta-dependent reporter activation. Taken together, these findings suggest that AFX zeta is a downstream target of both the phosphatidylinositol 3-kinase/PKB insulin signaling pathway and an AMP-activated protein kinase-dependent pathway.

Novel der(1)t(1;19) in two patients with myeloid neoplasias

Cytogenetic studies can be useful in the clinical management of patients with leukemia. They may also give a clue to leukemogenesis and/or pathogenesis. Numerous disease-specific chromosomal aberrations have been and continue to be identified. Translocation (1;19)(q21 through q23;p13.3) involving the long arm of chromosome 1 and the short arm of chromosome 19 is usually associated with acute lymphoblastic leukemia. We found a new translocation involving one virtually identical breakpoint 19p13 and one distinct 1p13 in two cases of myeloid neoplasms. Studies of bone marrow and peripheral blood specimens specified in one of our patients acute myeloid leukemia and in an other myelodysplastic syndrome. Conventional cytogenetics was supplemented by spectral karyotyping (SKY), microdissection, and fluorescence in situ hybridization. Our first case showed a der(1)t(1;19)(p13;p13.1) as the sole chromosomal change. In addition to this translocation, a pericentric inversion within chromosome 10 and with a cryptic t(10;11) were detected by SKY in the second case. Translocation (1;19)(p13;p13.1) may play a role in the leukemogenesis of myeloid diseases.

An ins(X;11)(q24;q23) fuses the MLL and the Septin 6/KIAA0128 gene in an infant with AML-M2

The MLL (HRX, ALL-1 HTRX) gene at chromosome band 11q23 frequently is rearranged in acute lymphoblastic and myeloblastic leukemia. To date, more than 40 different 11q23 abnormalities have been described on the cytogenetic level, and at least 25 of the respective fusion partner genes are cloned. The vast majority of the respective reciprocal translocations generate a chimeric 5'-MLL/partner-3' gene on the derivative 11q23. In this work, we report a unique ins(X;11)(q24;q23) in an infant with acute myeloid leukemia (AML-M2) that fuses the human KIAA0128 gene at Xq24 with MLL. In contrast to the typical reciprocal MLL translocations, however, we provide evidence that the 5'-MLL/KIAA0128-3' fusion resides on Xq24 rather than on 11q23. The KIAA0128 gene encodes the human Septin 6 protein, which contains an ATP-GTP binding motif and three nuclear targeting sequences in its carboxy terminus. The maintenance of the reading frame of the 5'-MLL/KIAA0128-3' mRNA fusion allows for the formation of a novel chimeric protein. Septin 6 is the third member of the Septins that is fused to the MLL protein; the other two are hCDCrel at 22q11 and MSF at 17q25.

Characterization of t(11;19)(q23;p13.3) by fluorescence in situ hybridization analysis in a pediatric patient with therapy-related acute myelogenous leukemia

This case presents a Caucasian girl diagnosed with early pre-B cell acute lymphoblastic leukemia at age 2 years. The only chromosomal anomaly detected in her bone marrow cells at this time was an add(12p). By age 4 years, she had a bone marrow and central nervous system (CNS) relapse of ALL and was treated with chemotherapy that included etoposide. She was in complete remission for 2 years following chemotherapy with etoposide, but later developed therapy-related acute myeloid leukemia (t-AML). At this time, a t(11;19)(q23;p13.3) rearrangement was detected in her bone marrow cells. The AML relapsed again 1 year after allogeneic bone marrow transplant (BMT). The presence of a chromosome 11 abnormality involving band 11q23 in this patient suggests that the transformation from ALL to t-AML was a consequence of etoposide included in her chemotherapy. Studies have shown that the 11q23 breakpoint in the t(11;19) rearrangement is consistent, and involves the MLL gene in t-AML patients. However, the breakpoint in 19p is variable in that it could be located either at 19p13.1 or 19p13.3 and thus could involve either of two genes: ELL (11-19 lysine-rich leukemia gene) on 19p13.1 or ENL (11-19 leukemia gene) on 19p13.3. In this study, the t(11;19)(q23;p13.3) was further characterized and the breakpoint regions were defined by fluorescence in situ hybridization (FISH) analysis.

Primary chromosomal rearrangements of leukemia are frequently accompanied by extensive submicroscopic deletions and may lead to altered prognosis

BCR/ABL fluorescent in situ hybridization study of chronic myeloid leukemia (CML) and Philadelphia(+) (Ph(+)) acute lymphoid leukemia (ALL) indicated that approximately 9% of patients exhibited an atypical hybridization pattern consistent with a submicroscopic deletion of the 5' region of ABL and the 3' region of the BCR genes on the 9q(+) chromosome. The CML patients with deletions had a shorter survival time and a high relapse rate following bone marrow transplant. Since deletions are associated with both Ph(+) CML and ALL, it seemed probable that other leukemia-associated genomic rearrangements may also have submicroscopic deletions. This hypothesis was confirmed by the detection of deletions of the 3' regions of the CBFB and the MLL genes in AML M4 patients with inv(16) and in patients with ALL and AML associated with MLL gene translocations, respectively. In contrast, analysis of the AML M3 group of patients and AML M2 showed that similar large deletions were not frequently associated with the t(15;17) or t(8;21) translocations. Analysis of sequence data from each of the breakpoint regions suggested that large submicroscopic deletions occur in regions with a high overall density of Alu sequence repeats. These findings are the first to show that the process of deletion formation is not disease specific in leukemia and also implicate that the presence of repetitive DNA in the vicinity of breakpoint regions may facilitate the generation of submicroscopic deletions. Such deletions could lead to the loss of one or more genes, and the associated haploinsufficiency may result in the observed differences in clinical behavior. (Blood. 2001;97:3581-3588)

Inhibition of nuclear import by protein kinase B (Akt) regulates the subcellular distribution and activity of the forkhead transcription factor AFX

AFX belongs to a subfamily of Forkhead transcription factors that are phosphorylated by protein kinase B (PKB), also known as Akt. Phosphorylation inhibits the transcriptional activity of AFX and changes the steady-state localization of the protein from the nucleus to the cytoplasm. Our goal was threefold: to identify the cellular compartment in which PKB phosphorylates AFX, to determine whether the nuclear localization of AFX plays a role in regulating its transcriptional activity, and to elucidate the mechanism by which phosphorylation alters the localization of AFX. We show that phosphorylation of AFX by PKB occurs in the nucleus. In addition, nuclear export mediated by the export receptor, Crm1, is required for the inhibition of AFX transcriptional activity. Both phosphorylated and unphosphorylated AFX, however, bind Crm1 and can be exported from the nucleus. These results suggest that export is unregulated and that phosphorylation by PKB is not required for the nuclear export of AFX. We show that AFX enters the nucleus by an active, Ran-dependent mechanism. Amino acids 180 to 221 of AFX comprise a nonclassical nuclear localization signal (NLS). S193, contained within this atypical NLS, is a PKB-dependent phosphorylation site on AFX. Addition of a negative charge at S193 by mutating the residue to glutamate reduces nuclear accumulation. PKB-mediated phosphorylation of AFX, therefore, attenuates the import of the transcription factor, which shifts the localization of the protein from the nucleus to the cytoplasm and results in the inhibition of AFX transcriptional activity.

11q23 abnormalities in patients with acute myelogenous leukemia and myelodysplastic syndrome as detected by molecular and cytogenetic analyses

11q23 chromosomal abnormalities and rearrangement of the mixed lineage leukemia (MLL) gene are important prognostic factors in acute myelogenous leukemia (AML) and myelodysplastic syndrome (MDS). However, the presence of 11q23 abnormalities does not always correlate with that of MLL gene rearrangement. We retrospectively compared the occurrence of 11q23 abnormalities (measured by karyotyping) and MLL gene rearrangement (measured by Southern blotting) in bone marrow from 311 consecutive adult patients with AML or MDS. 11q23 abnormalities were found in 18 patients (5.8%), of whom 7 (39%) did not have the MLL gene rearrangement. MLL gene rearrangement was detected in 35 patients (11.2%). Of these 35 patients, only 11 (31%) had cytogenetic evidence of 11q23 abnormalities. None of the 21 patients with chronic myelomonocytic leukemia had 11q23 abnormalities or MLL gene rearrangement. 11q23 abnormalities were associated with shorter survival than was a diploid karyotype. Both cytogenetic and molecular studies should be performed to detect 11q23 abnormalities in patients with AML or MDS.

Treatment-related leukaemia--a clinical and scientific challenge

The development of a second tumour, including treatment-related leukaemia (TRL), is the most devastating complication of intensive cancer chemotherapy. This is especially relevant in the paediatric population as over 70% of children diagnosed with a malignancy will now live at least 5 years. Most TRLs are myeloid leukaemias and carry an overall poor prognosis when compared with their de novo counterparts. Despite the well known association with specific cytotoxic agents, improved understanding of the pathogenesis and risk factors of TRL is ultimately essential if we are to develop successful strategies for prevention and treatment. Here we review these aspects, together with the clinical and diverse biological features of this complication and the efficacy of current therapy.

Monitoring of minimal residual disease in patients with MLL-AF6-positive acute myeloid leukaemia by reverse transcriptase polymerase chain reaction

We studied 210 unselected patients with acute myeloid leukaemia (AML) for MLL abnormalities. Twenty-seven patients (13%) with rearranged MLL genes were identified by means of Southern blot analysis. An MLL-AF6 fusion transcript was detected in six patients by a reverse transcriptase polymerase chain reaction (RT-PCR) for the MLL-AF6 translocation. Sequence analysis showed fusion of MLL exon 7 as well as exon 6 (two patients) or MLL exon 6 as well as exon 5 (four patients) to AF6 exon 2. In only three patients could the t(6;11) also be identified by cytogenetic and/or fluorescence in situ hybridization (FISH) analysis. The MLL-AF6-positive patients were monitored by RT-PCR for a period of 6-33 months. Complete haematological remission (CR) was achieved in all six cases, but was short in 5/6 patients (range 2.6-8.3 months). In these five patients, the MLL-AF6 transcripts were detected in every sample tested after induction and consolidation chemotherapy. One patient received autologous bone marrow transplantation (BMT) which also did not lead to PCR negativity. Intensive salvage therapy was unable to induce a second remission in the relapsed patients. One of the six MLL-AF6-positive patients achieved a molecular CR. He is still in CR at 33 months after diagnosis. Survival analysis indicates a poor prognosis in MLL-AF6-positive patients. The median event-free survival was 6.8 months, the median overall survival 15 months. Persistent PCR positivity was consistently associated with relapse. Thus, RT-PCR provides a valuable and sensitive tool for the identification of t(6;11)-positive AML and the monitoring of response to treatment in these patients. The results of RT-PCR may be useful to evaluate therapeutic procedures and to make treatment decisions, which will enable molecular remissions to be achieved and improve the clinical outcome in this group of patients.