Molecular Pathogenesis of MLL-Associated Leukemias

Parental alcohol consumption and risk of leukemia in the offspring: a systematic review and meta-analysis

Parental alcohol consumption before and during pregnancy has been linked to adverse outcomes in the offspring including leukemogenesis. We, therefore, aimed to systematically assess and quantitatively synthesize published data on the association of paternal consumption during preconception and maternal consumption during pregnancy with leukemia risk in childhood (0-14 years). Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, we searched PubMed (until February 2016) and the reference lists of the relevant studies. Observational studies examining the association between parental alcohol consumption and childhood leukemia were considered eligible. Data extracted from 39 case-control studies (over 16 000 leukemia cases and 30 000 controls) were pooled and summary-effect estimates were calculated. Subgroup analyses were carried out by main acute leukemia type [lymphoblastic or myeloid), cytogenetics/genetic polymorphisms, and specific alcohol beverages. We found a statistically significant dose-response association of any level of maternal alcohol consumption compared with nondrinking during pregnancy exclusively with acute myeloid leukemia (AML) [odds ratio (OR)moderate consumption: 1.64, 95% confidence intervals (CIs): 1.23-2.17 and ORhigh consumption: 2.36, 95% CI: 1.60-3.49]. In contrast, no association of paternal preconception consumption with any leukemia type was noted. In beverage-specific analyses, only a positive association of maternal wine drinking with childhood AML was found, which was more pronounced in analyses including only studies on infant leukemia (ORwine: 2.12, 95% CI: 1.16-3.90). The largest ever meta-analysis shows a sizeable, statistically significant dose-response association of maternal alcohol consumption during index pregnancy with AML risk. Future research exploring the role of genetic polymorphisms is anticipated to shed light on the underlying pathophysiology.

History of Maternal Fetal Loss and Childhood Leukaemia Risk in Subsequent Offspring: Differentials by Miscarriage or Stillbirth History and Disease Subtype

BACKGROUND

Despite the putative intrauterine origins of childhood (0-14 years) leukaemia, it is complex to assess the impact of perinatal factors on disease onset. Results on the association of maternal history of fetal loss (miscarriage/stillbirth) with specific disease subtypes in the subsequent offspring are in conflict. We sought to investigate whether miscarriage and stillbirth may have different impacts on the risk of acute lymphoblastic leukaemia (ALL) and of its main immunophenotypes (B-cell and T-cell ALL), as contrasted to acute myeloid leukaemia (AML).

METHODS

One thousand ninety-nine ALL incidents (957 B-ALL) and 131 AML cases along with 1:1 age and gender-matched controls derived from the Nationwide Registry for Childhood Hematological Malignancies and Brain Tumors (1996-2013) were studied. Multivariable regression models were used to assess the roles of previous miscarriage(s) and stillbirth(s) on ALL (overall, B-, T-ALL) and AML, controlling for potential confounders.

RESULTS

Statistically significant exposure and disease subtype-specific associations of previous miscarriage(s) exclusively with AML [odds ratio (OR) 1.67, 95% confidence interval (CI) 1.00, 2.81] and stillbirth(s) with ALL [OR 4.82, 95% CI 1.63, 14.24] and B-ALL particularly, emerged.

CONCLUSION

Differential pathophysiological pathways pertaining to genetic polymorphisms or cytogenetic aberrations are likely to create hostile environments leading either to fetal loss or the development of specific leukaemia subtypes in subsequent offspring, notably distinct associations of maternal miscarriage history confined to AML and stillbirth history confined to ALL (specifically B-ALL). If confirmed and further supported by studies revealing underlying mechanisms, these results may shed light on the divergent leukemogenesis processes.

Congenital Acute Myeloid Leukemia with Unique Translocation t(11;19)(q23;p13.3)

Congenital leukemia is rarely encountered in clinical practice, even in tertiary children's hospitals. Leukemia may cause significant coagulopathy, putting the patient at risk of intracranial hemorrhage. In this case, the authors present a female infant with a unique mixed phenotypic congenital acute myeloid leukemia showing mixed-lineage leukemia (MLL) rearrangement and severe coagulopathy resulting in a large subdural hematoma. Despite the fatal outcome in this case, neurosurgical treatment of patients with acute myeloid leukemia should be considered if coagulopathy and the clinical scenario allow.

Cytogenetic as an important tool for diagnosis and prognosis for patients with hypocellular primary myelodysplastic syndrome

We analyzed cytogenetically 105 patients with hypocellular primary MDS and their clinical implications. The main chromosomal abnormalities found were del(5q)/−5, del(6q)/+6, del(7q)/−7, del(11q), and del(17p). Pediatric patients had a higher frequency of abnormal karyotypes compared with adult patients (P < 0,05). From our patients, 18% showed evolution of the disease. The chromosomal abnormalities presented in the diagnosis of patients who evolved to AML included numerical (−7, +8) and structural del(6q), del(7q), i(7q), t(7;9), i(9q), and del(11q) abnormalities and complex karyotypes. Although the frequency of evolution from hypocellular MDS to AML is low, our results suggest that some chromosomal alterations may play a critical role during this process. We applied the IPSS in our patients because this score system has been proved to be useful for predicting evolution of disease. When we considered the patients according to group 1 (intermediate-1) and group 2 (intermediate-2 and high risk), we showed that group 2 had a high association with respect to the frequency of abnormal karyotypes (P < 0,0001), evolution of disease (P < 0,0001), and mortality (P < 0,001). In fact, the cytogenetic analysis for patients with hypocellular primary MDS is an important tool for diagnosis, prognosis, in clinical decision-making and in follow-up.

Genomic characterization of acute leukemias

Over the past two decades, hematologic malignancies have been extensively evaluated due to the introduction of powerful technologies, such as conventional karyotyping, FISH analysis, gene and microRNA expression profiling, array comparative genomic hybridization and SNP arrays, and next-generation sequencing (including whole-exome sequencing and RNA-seq). These analyses have allowed for the refinement of the mechanisms underlying the leukemic transformation in several oncohematologic disorders and, more importantly, they have permitted the definition of novel prognostic algorithms aimed at stratifying patients at the onset of disease and, consequently, treating them in the most appropriate manner. Furthermore, the identification of specific molecular markers is opening the door to targeted and personalized medicine. The most important findings on novel acquisitions in the context of acute lymphoblastic leukemia of both B and T lineage and de novo acute myeloid leukemia are described in this review.

Classifying leukemia types with chromatin conformation data

Background

Although genetic or epigenetic alterations have been shown to affect the three-dimensional organization of genomes, the utility of chromatin conformation in the classification of human disease has never been addressed.

Results

Here, we explore whether chromatin conformation can be used to classify human leukemia. We map the conformation of the HOXA gene cluster in a panel of cell lines with 5C chromosome conformation capture technology, and use the data to train and test a support vector machine classifier named 3D-SP. We show that 3D-SP is able to accurately distinguish leukemias expressing MLL-fusion proteins from those expressing only wild-type MLL, and that it can also classify leukemia subtypes according to MLL fusion partner, based solely on 5C data.

Conclusions

Our study provides the first proof-of-principle demonstration that chromatin conformation contains the information value necessary for classification of leukemia subtypes.

SMYD2 is highly expressed in pediatric acute lymphoblastic leukemia and constitutes a bad prognostic factor

Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy. Although several clinical characteristics can be associated with worse prognosis, more robust biological markers still remains uncovered. SMYD2, a member of SMYD protein family, regulates the activity of several proteins through methylation. In this study, we performed quantitative real time PCR to compare the expression of SMYD2 in 83 pediatric ALL patients and non-neoplastic bone marrow samples (BMS). The study revealed that SMYD2 expression is altered in ALL BMS and its high expression was correlated with a bad prognosis. Moreover, we also revealed that SMYD2 expression level significantly decreases in patients that respond to chemotherapy treatment.

Ligand-independent FLT3 activation does not cooperate with MLL-AF4 to immortalize/transform cord blood CD34+ cells

MLL-AF4 fusion is hallmark in high-risk infant pro-B-acute lymphoblastic leukemia (pro-B-ALL). Our limited understanding of MLL-AF4-mediated transformation reflects the absence of human models reproducing this leukemia. Hematopoietic stem/progenitor cells (HSPCs) constitute likely targets for transformation. We previously reported that MLL-AF4 enhanced hematopoietic engraftment and clonogenic potential in cord blood (CB)-derived CD34+ HSPCs but was not sufficient for leukemogenesis, suggesting that additional oncogenic lesions are required for MLL-AF4-mediated transformation. MLL-AF4+ pro-B-ALL display enormous levels of FLT3, and occasionally FLT3-activating mutations, thus representing a candidate cooperating event in MLL-AF4+ pro-B-ALL. We have explored whether FLT3.TKD (tyrosine kinase domain) mutation or increased expression of FLT3.WT (wild type) cooperates with MLL-AF4 to immortalize/transform CB-CD34+ HSPCs. In vivo, FLT3.TKD/FLT3.WT alone, or in combination with MLL-AF4, enhances hematopoietic repopulating function of CB-CD34+ HSPCs without impairing migration or hematopoietic differentiation. None of the animals transplanted with MLL-AF4+FLT3.TKD/WT-CD34+ HSPCs showed any sign of disease after 16 weeks. In vitro, enforced expression of FLT3.TKD/FLT3.WT conveys a transient overexpansion of MLL-AF4-expressing CD34+ HSPCs associated to higher proportion of cycling cells coupled to lower apoptotic levels, but does not augment clonogenic potential nor confer stable replating. Together, FLT3 activation does not suffice to immortalize/transform MLL-AF4-expressing CB-CD34+ HSPCs, suggesting the need of alternative (epi)-genetic cooperating oncogenic lesions.

The silent mutational landscape of infant MLL-AF4 pro-B acute lymphoblastic leukemia

Over 90% of infants (< 1-year-old) diagnosed with leukemia have pro-B acute lymphoblastic leukemia (ALL) containing the MLL-AF4 fusion. When compared with other forms of paediatric ALL affecting later B-cell differentiation, MLL-AF4 pro-B is associated with a dismal prognosis with a typical 5-year disease-free survival of <20%. MLL-AF4 may be sufficient on its own for leukemogenesis or the gene-fusion product may alternatively predispose transformed cells to global genetic instability, enhancing the acquisition of additional key mutations. To gain insight into the genomic landscape of infant MLL-AF4 pro-B ALL we performed whole genome sequencing of diagnostic leukemic blasts and matched germline samples from three MLL-AF4 pro-B ALL infants. Our analysis revealed few somatic changes (copy number abnormalities, loss of heterozygosity, or single nucleotide variants), demonstrating that only a very small number of mutations are necessary to generate infant MLL-leukemia.

Histone modification profiling in normal and transformed human embryonic stem cells using micro chromatin immunoprecipitation, scalable to genome-wide microarray analyses

Comparing normal human embryonic stem cells (hESCs) to those that have acquired cellular properties of neoplasm provides a unique opportunity to study the distinguishing molecular features of human cellular transformation. As global alterations in the epigenetic landscape are a common feature of cancer, we sought to investigate the loci-specific and global differences between normal and transformed hESCs using ChIP-PCR and ChIP-microarray (also known as ChIP-chip). Here, specific emphasis was placed on optimizing ChIP for low cell numbers (termed micro-ChIP; μChIP) towards applications where the target population is rare, such as the case for somatic human tumors containing a low frequency of cancer stem cell populations and for single-colony analysis of embryonic and induced pluripotent stem cells emerging from initial derivation. Using these methods, we suggest that μChIP-PCR and microarray analysis is thus a powerful technology for epigenetic profiling of cell populations relevant to developmental biology, cancer, and regenerative medicine where target populations regulating the biological process can only be isolated in small numbers.

Intraclonal heterogeneity and distinct molecular mechanisms characterize the development of t(4;14) and t(11;14) myeloma

We have used whole exome sequencing to compare a group of presentation t(4;14) with t(11;14) cases of myeloma to define the mutational landscape. Each case was characterized by a median of 24.5 exonic nonsynonymous single-nucleotide variations, and there was a consistently higher number of mutations in the t(4;14) group, but this number did not reach statistical significance. We show that the transition and transversion rates in the 2 subgroups are similar, suggesting that there was no specific mechanism leading to mutation differentiating the 2 groups. Only 3% of mutations were seen in both groups, and recurrently mutated genes include NRAS, KRAS, BRAF, and DIS3 as well as DNAH5, a member of the axonemal dynein family. The pattern of mutation in each group was distinct, with the t(4;14) group being characterized by deregulation of chromatin organization, actin filament, and microfilament movement. Recurrent RAS pathway mutations identified subclonal heterogeneity at a mutational level in both groups, with mutations being present as either dominant or minor subclones. The presence of subclonal diversity was confirmed at a single-cell level using other tumor-acquired mutations. These results are consistent with a distinct molecular pathogenesis underlying each subgroup and have important impacts on targeted treatment strategies. The Medical Research Council Myeloma IX trial is registered under ISRCTN68454111.

Genetic abnormalities and challenges in the treatment of acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematopoietic disorder in which there are too many immature blood-forming cells accumulating in the bone marrow and interfering with the production of normal blood cells. It has long been recognized that AML is a clinically heterogeneous disease characterized by a multitude of chromosomal abnormalities and gene mutations, which translate to marked differences in responses and survival following chemotherapy. The cytogenetic and molecular genetic aberrations associated with AML are not mutually exclusive and often coexist in the leukemic cells. AML is a disease of the elderly, with a mean age of diagnosis of 70 years. Adverse cytogenetic abnormalities increase with age, and within each cytogenetic group, prognosis with standard treatment worsens with age. In the past 20 years, there has been little improvement in chemotherapeutic regimens and hence the overall survival for patients with AML. A huge unmet need exists for efficacious targeted therapies for elderly patients that are less toxic than available chemotherapy regimens. The multitude of chromosomal and genetic abnormalities makes the treatment of AML a challenging prospect. A detailed understanding of the molecular changes associated with the chromosomal and genetic abnormalities in AML is likely to provide a rationale for therapy design and biomarker development. This review summarizes the variety of cytogenetic and genetic changes observed in AML and gives an overview of the clinical status of new drugs in development.

Enforced expression of MLL-AF4 fusion in cord blood CD34+ cells enhances the hematopoietic repopulating cell function and clonogenic potential but is not sufficient to initiate leukemia

Infant acute lymphoblastic leukemia harboring the fusion mixed-lineage leukemia (MLL)-AF4 is associated with a dismal prognosis and very brief latency. Our limited understanding of transformation by MLL-AF4 is reflected in murine models, which do not accurately recapitulate the human disease. Human models for MLL-AF4 disease do not exist. Hematopoietic stem or progenitor cells (HSPCs) represent probable targets for transformation. Here, we explored in vitro and in vivo the impact of the enforced expression of MLL-AF4 in human cord blood-derived CD34(+) HSPCs. Intrabone marrow transplantation into NOD/SCID-IL2Rγ(-/-) mice revealed an enhanced multilineage hematopoietic engraftment, efficiency, and homing to other hematopoietic sites on enforced expression of MLL-AF4. Lentiviral transduction of MLL-AF4 into CD34(+) HSPCs increased the in vitro clonogenic potential of CD34(+) progenitors and promoted their proliferation. Consequently, cell cycle and apoptosis analyses suggest that MLL-AF4 conveys a selective proliferation coupled to a survival advantage, which correlates with changes in the expression of genes involved in apoptosis, sensing DNA damage and DNA repair. However, MLL-AF4 expression was insufficient to initiate leukemogenesis on its own, indicating that either additional hits (or reciprocal AF4-MLL product) may be required to initiate ALL or that cord blood-derived CD34(+) HSPCs are not the appropriate cellular target for MLL-AF4-mediated ALL.

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.

Insights into the cellular origin and etiology of the infant pro-B acute lymphoblastic leukemia with MLL-AF4 rearrangement

Infant acute lymphoblastic leukemia (ALL) involving mixed-lineage leukemia (MLL) fusions has attracted a huge interest in basic and clinical research because of its prenatal origin, mixed-lineage phenotype, dismal prognosis and extremely short latency. Over 90% of infant ALLs are pro-B ALL harboring the leukemic fusion MLL-AF4. Despite the fact that major achievements have provided a better understanding about the etiology of infant MLL-AF4+ ALL over the last two decades, key questions remain unanswered. Epidemiological and genetic studies suggest that the in utero origin of MLL rearrangements in infant leukemia may be the result of prenatal exposure to genotoxic compounds. In fact, chronic exposure of human embryonic stem cells (hESCs) to etoposide induces MLL rearrangements and makes hESC more prone to acquire subsequent chromosomal abnormalities than postnatal CD34(+) cells, linking embryonic exposure to topoisomerase II inhibitors to genomic instability and MLL rearrangements. Unfortunately, very little is known about the nature of the target cell for transformation. Neuron-glial antigen 2 expression was initially claimed to be specifically associated with MLL rearrangements and was recently shown to be readily expressed in CD34+CD38+, but not CD34+CD38- cells suggesting that progenitors rather than stem cells may be the target cell for transformation. Importantly, the recent findings showing that MLL-AF4 rearrangement is present and expressed in mesenchymal stem cells from infant patients with MLLAF4+ ALL challenged our current view of the etiology and cellular origin of this leukemia. It becomes therefore crucial to determine where the leukemia relapses come from and how the tumor-stroma relationship is defined at the molecular level. Finally, MLL-AF4 leukemogenesis has been particularly difficult to model and bona fide MLL-AF4 disease models do not exist so far. It is likely that the current disease models are missing some essential ingredients of leukemogenesis in the human embryo/fetus. We thus propose modeling MLL-AF4+ infant pro-B ALL using prenatal hESCs.

E2F-1 directly regulates thrombospondin 1 expression

Thrombospondin 1 (TSP1) has been shown to play a critical role in inhibiting angiogenesis, resulting in inhibition of tumor growth and metastases. To figure out TSP1's regulators will lead to reveal its biological function mechanistically. In this study, we show that E2F-1 could activate the transcription of TSP1 by both promoter assays and Northern blot. Analysis of various TSP1 promoter mutant constructs showed that a sequence located −144/−137 up-stream of the transcriptional initiation site, related to the consensus E2F-responsive sequence, is necessary for the activation. In consistence with up-regulation of TSP-1 activity by over-expression of E2F-1, the knockdown of endogenous E2F-1 inhibited TSP-1 promoter activity significantly, implying that E2F-1 mediated regulation of TSP-1 is relevant in vivo. In addition, E2F-1 could also directly bind to the TSP1 promoter region covering −144/−137 region as revealed by ChIP assays. Furthermore, the E2F-1-induced activation of TSP1 gene transcription is suppressed by pRB1 in a dose-dependent manner. Taken together, the results demonstrate that TSP1 is a novel target for E2F1, which might imply that E2F-1 can affect angiogenesis by modulating TSP1 expression.

t(4;11) leukemias display addiction to MLL-AF4 but not to AF4-MLL

The most frequent MLL-gene rearrangement found in leukemia is a reciprocal translocation with AF4 on chromosome 4 resulting in the formation of the MLL-AF4 and the AF4-MLL fusion genes. The oncogenic role of MLL-AF4 is documented but the significance of the reciprocal product - AF4-MLL in leukemia is less clear. In the human leukemia cell lines - RS4;11 and SEMK2-M1, both of which express MLL-AF4 and AF4-MLL, we knocked down the expression of AF4-MLL using siRNA. Loss of AF4-MLL had no effect on the growth of either RS4;11 or SEMK2-M1 cells. Furthermore, in SEMK2-M1 cells there were no changes in cell cycle or apoptosis with loss of AF4-MLL. In contrast, knockdown of MLL-AF4 significantly inhibited growth of both RS4;11 and SEMK2-M1. Additionally, in SEMK2-M1 cells, loss of MLL-AF4 led to G2/M cell cycle arrest and increased apoptosis. Overall, these results demonstrate that in t(4;11) leukemia, the MLL-AF4 fusion protein is critical for leukemia cell proliferation and survival while the AF4-MLL fusion product is dispensable.

MLL fusions: pathways to leukemia

Human leukemias with chromosomal band 11q23 aberrations that disrupt the MLL/HRX/ALL-1 gene portend poor prognosis. MLL associated leukemias account for the majority of infant leukemia, approximately 10% of adult de novo leukemia and approximately 33% of therapy related acute leukemia with a balanced chromosome translocation. The 500 kD MLL precursor is processed by Taspase1 to generate mature MLL(N320/C180), which orchestrates many aspects of biology such as embryogenesis, cell cycle, cell fate and stem cell maintenance. Leukemogenic MLL translocations fuse the common MLL N-terminus (approximately 1,400 aa) in frame with more than 60 translocation partner genes (TPGs). Recent studies on MLL and MLL leukemia have greatly advanced our knowledge concerning the normal function of MLL and its deregulation in leukemogenesis. Here, we summarize the critical biological and pathological activities of MLL and MLL fusions, and discuss available models and potential therapeutic targets of MLL associated leukemias.

The molecular signature of oncofusion proteins in acute myeloid leukemia

Acute myeloid leukemia (AML) associated translocations often cause gene fusions that encode oncofusion proteins. Although many of the breakpoints involved in chromosomal translocations have been cloned, in most cases the role of the chimeric proteins in tumorigenesis is not elucidated. Here we will discuss the fusion proteins of the 4 most common translocations associated with AML as well as the common molecular mechanisms that these four and other fusion proteins utilize to transform progenitor cells. Intriguingly, although the individual partners within the fusion proteins represent a wide variety of cellular functions, at the molecular level many commodities can be found.

Diagnostic microarrays in hematologic oncology: applications of high- and low-density arrays

Microarrays have become important tools for high-throughput analysis of gene expression, chromosome aberrations, and gene mutations in cancer cells. In addition to high-density experimental microarrays, low-density, gel-based biochip technology represents a versatile platform for translation of research into clinical practice. Gel-based microarrays (biochips) consist of nanoliter gel drops on a hydrophobic surface with different immobilized biopolymers (primarily nucleic acids and proteins). Because of the high immobilization capacity of the gel, such biochips have a high probe concentration and high levels of fluorescence signals after hybridization, which allow the use of simple, portable detection systems. The notable accuracy of the analysis is reached as a result of the high level of discrimination between positive and negative gel-bound probes. Different applications of biochips in the field of hematologic oncology include analysis of chromosomal translocations in leukemias, diagnostics of T-cell lymphomas, and pharmacogenetics.

Elongation factor ELL (Eleven-Nineteen Lysine-rich Leukemia) acts as a transcription factor for direct thrombospondin-1 regulation

The eleven-nineteen lysine-rich leukemia (ELL) gene undergoes translocation and fuses in-frame to the multiple lineage leukemia gene in a substantial proportion of patients suffering from acute forms of leukemia. Studies show that ELL indirectly modulates transcription by serving as a regulator for transcriptional elongation as well as for p53, U19/Eaf2, and steroid receptor activities. Our in vitro and in vivo data demonstrate that ELL could also serve as a transcriptional factor to directly induce transcription of the thrombospondin-1 (TSP-1) gene. Experiments using ELL deletion mutants established that full-length ELL is required for the TSP-1 up-regulation and that the transactivation domain likely resides in the carboxyl terminus. Moreover, the DNA binding domain may localize to the first 45 amino acids of ELL. Not surprisingly, multiple lineage leukemia-ELL, which lacks these amino acids, did not induce expression from the TSP-1 promoter. In addition, the ELL core-response element appears to localize in the -1426 to -1418 region of the TSP-1 promoter. Finally, studies using zebrafish confirmed that ELL regulates TSP-1 mRNA expression in vivo, and ELL could inhibit zebrafish vasculogenesis, at least in part, through up-regulating TSP-1. Given the importance of TSP-1 as an anti-angiogenic protein, our findings may have important ramifications for better understanding cancer.

Aberrant chromatin at genes encoding stem cell regulators in human mixed-lineage leukemia

Mixed-lineage leukemia (MLL) fusion proteins are potent inducers of leukemia, but how these proteins generate aberrant gene expression programs is poorly understood. Here we show that the MLL-AF4 fusion protein occupies developmental regulatory genes important for hematopoietic stem cell identity and self-renewal in human leukemia cells. These MLL-AF4-bound regions have grossly altered chromatin structure, with histone modifications catalyzed by trithorax group proteins and DOT1 extending across large domains. Our results define direct targets of the MLL fusion protein, reveal the global role of epigenetic misregulation in leukemia, and identify new targets for therapeutic intervention in cancer.

RAS oncogene suppression induces apoptosis followed by more differentiated and less myelosuppressive disease upon relapse of acute myeloid leukemia

To study the oncogenic role of the NRAS oncogene (NRAS(G12V)) in the context of acute myeloid leukemia (AML), we used a Vav promoter-tetracycline transactivator (Vav-tTA)-driven repressible TRE-NRAS(G12V) transgene system in Mll-AF9 knock-in mice developing AML. Conditional repression of NRAS(G12V) expression greatly reduced peripheral white blood cell (WBC) counts in leukemia recipient mice and induced apoptosis in the transplanted AML cells correlated with reduced Ras/Erk signaling. After marked decrease of AML blast cells, myeloproliferative disease (MPD)-like AML relapsed characterized by cells that did not express NRAS(G12V). In comparison with primary AML, the MPD-like AML showed significantly reduced aggressiveness, reduced myelosuppression, and a more differentiated phenotype. We conclude that, in AML induced by an Mll-AF9 transgene, NRAS(G12V) expression contributes to acute leukemia maintenance by suppressing apoptosis and reducing differentiation of leukemia cells. Moreover, NRAS(G12V) oncogene has a cell nonautonomous role in suppressing erythropoiesis that results in the MPD-like AML show significantly reduced ability to induce anemia. Our results imply that targeting NRAS or RAS oncogene-activated pathways is a good therapeutic strategy for AML and attenuating aggressiveness of relapsed AML.

Brief historical sketch of chromosomal translocations and tumors

The discovery of chromosomes emerged from the cytological analysis of mitosis in the 1870s. At the turn of the 20th century, cytologists and geneticists established that chromosomes carried the hereditary material. In the early 20th century, Theodore Boveri, recognizing the nonequivalence of individual chromosomes, began thinking about the biological consequences of imbalances of chromosomal compositions in somatic cells and how these might explain the origin of cancer. Many of his predictions would have to wait for confirmation until the 1950--1960s, when mammalian cytogenetics became feasible with the use of ascites tumors as sources of metaphases. This advance coupled with the discovery of G banding by Caspersson and his associates led to finding characteristic recurring chromosomal abnormalities in certain kinds of tumors. Chromosomal translocations that were associated with promoter deregulations or the formation of novel fusion genes were the prime models. This continuing progress combined with dramatic advances in DNA structure, transcription, and repair have provided new insights into the role of this class of mutations in neoplastic development.

A variant-type MLL/SEPT9 fusion transcript in adult de novo acute monocytic leukemia (M5b) with t(11;17)(q23;q25)

As a result of recurrent chromosomal translocations in acute leukemias, the mixed-lineage-leukemia (MLL) gene fuses with a variety of partner genes, which include several members of the septin gene family. SEPT9 is a very rare but recurrent fusion partner of MLL, and has recently been implicated in the oncogenesis of various malignancies. Herein, we report a case of de novo acute monocytic leukemia (M5b) with t(11;17)(q23;q25). MLL involvement was revealed by fluorescent in situ hybridization (FISH) analysis, and an MLL/SEP9 fusion transcript was detected by RT-PCR. Sequencing analysis further showed that, in contrast to originally reported cases, MLL exon 8 was fused not with SEPT9 exon 3 but with exon 2, which codes for the unique N-terminal region of the SEPT9_v1 isoform, the region implicated in the regulation of gene expression and cell proliferation. We did not detect any mutation of FLT3, which was expressed at a relatively low level in the leukemic cells. Relapsing after a very short complete remission, the leukemia progressed rapidly and became fatal in spite of intensive therapies including hematopoietic stem cell transplantation. It is thus suggested that, in common with the original MLL/SEPT9 cases, monocytic differentiation and a poor prognosis may also be associated with acute myeloid leukemia with the variant MLL/SEPT9 fusion transcript.

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.

Acute leukemia in early childhood

Acute leukemia in early childhood is biologically and clinically distinct. The particular characteristics of this malignancy diagnosed during the first months of life have provided remarkable insights into the etiology of the disease. The pro-B, CD10 negative immunophenotype is typically found in infant acute leukemia, and the most common genetic alterations are the rearrangements of the MLL gene. In addition, the TEL/AML1 fusion gene is most frequently found in children older than 24 months. A molecular study on a Brazilian cohort (age range 0-23 months) has detected TEL/AML1+ve (N = 9), E2A/PBX1+ve (N = 4), PML/RARA+ve (N = 4), and AML1/ETO+ve (N = 2) cases. Undoubtedly, the great majority of genetic events occurring in these patients arise prenatally. The environmental exposure to damaging agents that give rise to genetic changes prenatally may be accurately determined in infants since the window of exposure is limited and known. Several studies have shown maternal exposures that may give rise to leukemogenic changes. The Brazilian Collaborative Study Group of Infant Acute Leukemia has found that mothers exposed to dipyrone, pesticides and hormones had an increased chance to give birth to babies with infant acute leukemia [OR = 1.48 (95%CI = 1.05-2.07), OR = 2.27 (95%CI = 1.56-3.31) and OR = 9.08 (95%CI = 2.95-27.96)], respectively. This review aims to summarize recent clues that have facilitated the elucidation of the biology of early childhood leukemias, with emphasis on infant acute leukemia in the Brazilian population.

The mixed-lineage leukemia fusion partner AF4 stimulates RNA polymerase II transcriptional elongation and mediates coordinated chromatin remodeling

AF4 gene, frequently translocated with mixed-lineage leukemia (MLL) in childhood acute leukemia, encodes a putative transcriptional activator of the AF4/LAF4/FMR2 (ALF) protein family previously implicated in lymphopoiesis and Purkinje cell function in the cerebellum. Here, we provide the first evidence for a direct role of AF4 in the regulation of transcriptional elongation by RNA polymerase II (Pol II). We demonstrate that mouse Af4 functions as a positive regulator of Pol II transcription elongation factor b (P-TEFb) kinase and, in complex with MLL fusion partners Af9, Enl and Af10, as a mediator of histone H3-K79 methylation by recruiting Dot1 to elongating Pol II. These pathways are interconnected and tightly regulated by the P-TEFb-dependent phosphorylation of Af4, Af9 and Enl which controls their transactivation activity and/or protein stability. Consistently, increased levels of phosphorylated Pol II and methylated H3-K79 are observed in the ataxic mouse mutant robotic, an over-expression model of Af4. Finally, we confirm the functional relevance of Af4, Enl and Af9 to the regulation of gene transcription as their over-expression strongly stimulates P-TEFb-dependent transcription of a luciferase reporter gene. Our findings uncover a central role for these proteins in the regulation of transcriptional elongation and coordinated histone methylation, providing valuable insight into their contribution to leukemogenesis and neurodegeneration. Since these activities likely extend to the entire ALF protein family, this study also significantly inputs our understanding of the molecular basis of FRAXE mental retardation syndrome in which FMR2 expression is silenced.

Pharmacogenetics of alkylator-associated acute myeloid leukemia

Therapy-related acute myeloid leukemia (t-AML) is a lethal late complication of alkylator chemotherapy. The genetic basis of susceptibility to t-AML is poorly understood. Both t-AML and de novo AML are complex genetic diseases, requiring cooperating mutations in interacting pathways for disease initiation and progression. Germline variants of these ‘leukemia pathway’ genes may cooperate with somatic mutations to induce both de novo and therapy-related AML. Several cancer susceptibility syndromes have been identified that cause an inherited predisposition to de novo and t-AML. The genes responsible for these syndromes are also somatically mutated in sporadic AML. We reason that germline polymorphism in any gene somatically mutated in AML could contribute to t-AML risk in the general population. Identification of these susceptibility alleles should help clinicians develop tailored therapies that reduce the relative risk of t-AML.

MLL chimeric protein activation renders cells vulnerable to chromosomal damage: An explanation for the very short latency of infant leukemia

MLL fusion genes are a predominant feature of acute leukemias in infants and in secondary acute myeloid leukemia (AML) associated with prior chemotherapy with topo-II poisons. The former is considered to possibly arise in utero via transplacental chemical exposure. A striking feature of these leukemias is their malignancy and remarkably brief latencies implying the rapid acquisition of any necessary additional mutations. We have suggested that these coupled features might be explained if MLL fusion gene encoded proteins rendered cells more vulnerable to further DNA damage and mutation in the presence of chronic exposure to the agent(s) that induced the MLL fusion itself. We have tested this idea by exploiting a hormone regulated MLL-ENL (MLLT1) activation system and show that MLL-ENL function in normal murine progenitor cells substantially increases the incidence of chromosomal abnormalities in proliferating cells that survive exposure to etoposide VP-16. This phenotype is associated with an altered pattern of cell cycle arrest and/or apoptosis.