Chromosomal translocation master genes, mouse models and experimental therapeutics

Human embryonic stem cells carrying an unbalanced translocation demonstrate impaired differentiation into trophoblasts: an in vitro model of human implantation failure

Carriers of the balanced translocation t(11;22), the most common reciprocal translocation in humans, are at high risk of creating gametes with unbalanced translocation, leading to repeated miscarriages. Current research models for studying translocated embryos and the biological basis for their implantation failure are limited. The aim of this study was to elucidate whether human embryonic stem cells (hESCs) carrying the unbalanced chromosomal translocation t(11;22) can provide an explanation for repeated miscarriages of unbalanced translocated embryos. Fluorescent in situ hybridization and karyotype analysis were performed to analyze the t(11;22) in embryos during PGD and in the derived hESC line. The hESC line was characterized by RT-PCR and FACS analysis for pluripotent markers. Directed differentiation to trophoblasts was carried out by bone morphogenetic protein 4 (BMP4). Trophoblast development was analyzed by measuring β-hCG secretion, by β-hCG immunostaining and by gene expression of trophoblastic markers. We derived the first hESC line carrying unbalanced t(11;22), which showed the typical morphological and molecular characteristics of a hESC line. Control hESCs differentiated into trophoblasts secreted increasing levels of β-hCG and concomitantly expressed the trophoblast genes, CDX2, TP63, KRT7, ERVW1, CGA, GCM1, KLF4 and PPARG. In contrast, differentiated translocated hESCs displayed reduced and delayed secretion of β-hCG concomitant with impaired expression of the trophoblastic genes. The reduced activation of trophoblastic genes may be responsible for the impaired trophoblastic differentiation in t(11;22)-hESCs, associated with implantation failure in unbalanced t(11;22) embryos. Our t(11;22) hESCs are presented as a valuable human model for studying the mechanisms underlying implantation failure.

Prognostic Significance of the Lymphoblastic Leukemia-Derived Sequence 1 (LYL1) GeneExpression in Egyptian Patients with AcuteMyeloid Leukemia

Objective: Aberrant activation of transcription factor genes is the most frequent target of genetic alteration in lymphoid malignancies. The lymphoblastic leukemia-derived sequence 1 (LYL1) gene, which encodes a basic helix-loop helix, was first identified with human T-cell acute leukemia. Recent studies suggest its involvement in myeloid malignancies. We aimed to study the expression percent of oncogene LYL1 in primary and secondary high-risk myeloid leukemia and the impact on prognostic significance in those patients.

Materials and Methods: Using quantitative real-time polymerase chain reaction for detection of LYL1 oncogenes, our study was carried out on 39 myeloid leukemia patients including de novo cases, myelodysplastic syndrome (MDS) with transformation, and chronic myelogenous leukemia (CML) in accelerated and blast crisis, in addition to 10 healthy individuals as the reference control.

Results: LYL1 expression was increased at least 2 times compared to the controls. The highest expression of this transcription factor was observed in the MDS cases transformed to acute leukemia at 7.3±3.1, p=0.0011. LYL1 expression was found in 68.2%, 75%, and 77.8% of cases of acute myeloid leukemia, CML crisis, and MDS, respectively. Significant correlation of LYL1 overexpression with some subtypes of French-American-British classification was found. There was, for the first time, significant correlation between the blood count at diagnosis and LYL1 expression (p=0.023, 0.002, and 0.031 for white blood cells, hemoglobin, and platelets, respectively). The rate of complete remission was lower with very high levels of LYL1 expression and the risk of relapse increased with higher levels of LYL1 expression, suggesting an unfavorable prognosis for cases with enhanced expression.

Conclusion: Overexpression of LYL1 is highly associated with acute myeloid leukemia and shows more expression in MDS with unfavorable prognosis in response to induction chemotherapy. These observations could signal a promising tool for a therapeutic target to basic helix–loop helix protein related to transcription factors, which may improve patient outcome in acute myeloid leukemia, MDS, and CML in blast crisis.

Conformational flexibility of the oncogenic protein LMO2 primes the formation of the multi-protein transcription complex

LMO2 was discovered via chromosomal translocations in T-cell leukaemia and shown normally to be essential for haematopoiesis. LMO2 is made up of two LIM only domains (thus it is a LIM-only protein) and forms a bridge in a multi-protein complex. We have studied the mechanism of formation of this complex using a single domain antibody fragment that inhibits LMO2 by sequestering it in a non-functional form. The crystal structure of LMO2 with this antibody fragment has been solved revealing a conformational difference in the positioning and angle between the two LIM domains compared with its normal binding. This contortion occurs by bending at a central helical region of LMO2. This is a unique mechanism for inhibiting an intracellular protein function and the structural contusion implies a model in which newly synthesized, intrinsically disordered LMO2 binds to a partner protein nucleating further interactions and suggests approaches for therapeutic targeting of LMO2.

Gene expression signatures in childhood acute leukemias are largely unique and distinct from those of normal tissues and other malignancies

BACKGROUND

Childhood leukemia is characterized by the presence of balanced chromosomal translocations or by other structural or numerical chromosomal changes. It is well know that leukemias with specific molecular abnormalities display profoundly different global gene expression profiles. However, it is largely unknown whether such subtype-specific leukemic signatures are unique or if they are active also in non-hematopoietic normal tissues or in other human cancer types.

METHODS

Using gene set enrichment analysis, we systematically explored whether the transcriptional programs in childhood acute lymphoblastic leukemia (ALL) and myeloid leukemia (AML) were significantly similar to those in different flow-sorted subpopulations of normal hematopoietic cells (n = 8), normal non-hematopoietic tissues (n = 22) or human cancer tissues (n = 13).

RESULTS

This study revealed that e.g., the t(12;21) [ETV6-RUNX1] subtype of ALL and the t(15;17) [PML-RARA] subtype of AML had transcriptional programs similar to those in normal Pro-B cells and promyelocytes, respectively. Moreover, the 11q23/MLL subtype of ALL showed similarities with non-hematopoietic tissues. Strikingly however, most of the transcriptional programs in the other leukemic subtypes lacked significant similarity to approximately 100 gene sets derived from normal and malignant tissues.

CONCLUSIONS

This study demonstrates, for the first time, that the expression profiles of childhood leukemia are largely unique, with limited similarities to transcriptional programs active in normal hematopoietic cells, non-hematopoietic normal tissues or the most common forms of human cancer. In addition to providing important pathogenetic insights, these findings should facilitate the identification of candidate genes or transcriptional programs that can be used as unique targets in leukemia.

Intrinsic structural disorder confers cellular viability on oncogenic fusion proteins

Chromosomal translocations, which often generate chimeric proteins by fusing segments of two distinct genes, represent the single major genetic aberration leading to cancer. We suggest that the unifying theme of these events is a high level of intrinsic structural disorder, enabling fusion proteins to evade cellular surveillance mechanisms that eliminate misfolded proteins. Predictions in 406 translocation-related human proteins show that they are significantly enriched in disorder (43.3% vs. 20.7% in all human proteins), they have fewer Pfam domains, and their translocation breakpoints tend to avoid domain splitting. The vicinity of the breakpoint is significantly more disordered than the rest of these already highly disordered fusion proteins. In the unlikely event of domain splitting in fusion it usually spares much of the domain or splits at locations where the newly exposed hydrophobic surface area approximates that of an intact domain. The mechanisms of action of fusion proteins suggest that in most cases their structural disorder is also essential to the acquired oncogenic function, enabling the long-range structural communication of remote binding and/or catalytic elements. In this respect, there are three major mechanisms that contribute to generating an oncogenic signal: (i) a phosphorylation site and a tyrosine-kinase domain are fused, and structural disorder of the intervening region enables intramolecular phosphorylation (e.g., BCR-ABL); (ii) a dimerisation domain fuses with a tyrosine kinase domain and disorder enables the two subunits within the homodimer to engage in permanent intermolecular phosphorylations (e.g., TFG-ALK); (iii) the fusion of a DNA-binding element to a transactivator domain results in an aberrant transcription factor that causes severe misregulation of transcription (e.g. EWS-ATF). Our findings also suggest novel strategies of intervention against the ensuing neoplastic transformations.

Chromosomal translocations generate chimeric proteins by fusing segments of two distinct genes and are frequently associated with cancer. The proteins involved are large and fairly heterogeneous in sequence and typically have only a few dispersed structural domains connected by long uncharacterized regions. It has never been studied from a structural perspective how these chimeras survive losing significant portions of the original proteins and acquire new oncogenic functions. By analyzing a collection of 406 human translocation proteins we show here that the answer to both questions lies to a large extent in the high level of structural disorder in the fusion partner proteins (on average, they are twice as disordered as all human proteins). The translocation breakpoints usually avoid globular domains. In rare cases when a globular domain is truncated by the fusion, it happens at a location in the domain where the hydrophobicity exposed by the split is favorable (i.e., not too high). Disorder on average is significantly higher in the vicinity of the breakpoint than in the rest of the fusion proteins. Disorder also plays a pivotal role in the acquired oncogenic function by bringing distant/disparate fusion segments together that enables novel intra- and/or intermolecular interactions.

Recurrent chromosomal rearrangements implicate oncogenes contributing to T-cell lymphomagenesis in Lck-MyrAkt2 transgenic mice

The oncogene v-akt was isolated from a retrovirus that induced naturally occurring thymic lymphomas in AKR mice. We hypothesized that constitutive activation of Akt2 could serve as a first hit for the clonal expansion of malignant T-cells by promoting cell survival and genomic instability, leading to chromosome alterations. Furthermore, genes that cooperate with Akt2 to promote malignant transformation may reside at translocation/inversion junctions found in spontaneous thymic lymphomas from transgenic mice expressing constitutively active Akt2 specifically in T cells. Cytogenetic analysis revealed that thymic tumors from multiple founder lines exhibited either of two recurrent chromosomal rearrangements, inv(6)(A2B1) or t(14;15)(C2;D1). Fluorescence in situ hybridization, array CGH, and PCR analysis were used to delineate the inv(6) and t(14;15) breakpoints. Both rearrangements involved T-cell receptor loci. The inv(6) results in robust upregulation of the homeobox/transcription factor gene Dlx5 because of its relocation near the Tcrb enhancer. The t(14;15) places the Tcra enhancer in the vicinity of the Myc proto-oncogene, resulting in upregulated Myc expression. These findings suggest that activation of the Akt pathway can act as the initial hit to promote cell survival and genomic instability, whereas the acquisition of T-cell-specific overexpression of Dlx5 or Myc leads to lymphomagenesis.

The role of CALM-AF10 gene fusion in acute leukemia

Chromosomal translocations are important genetic perturbations frequently associated with hematologic malignancies; characterization of these events has been a rich source of insights into the mechanisms that lead to malignant transformation. The t(10;11)(p13;q14-21) results in a recently identified rare but recurring chromosomal translocation seen in patients with ALL as well as AML, and results in the production of a CALM-AF10 fusion gene. Although the details by which the CALM-AF10 fusion protein exerts its leukemogenic effect remain unclear, emerging data suggests that the CALM-AF10 fusion impairs differentiation of hematopoietic cells, at least in part via an upregulation of HOXA cluster genes. This review discusses the normal structure and function of CALM and AF10, describes the spectrum of clinical findings seen in patients with CALM-AF10 fusions, summarizes recently published CALM-AF10 mouse models and highlights the role of HOXA cluster gene activation in CALM-AF10 leukemia.

Expression of a CALM-AF10 fusion gene leads to Hoxa cluster overexpression and acute leukemia in transgenic mice

To assess the role of the CALM-AF10 fusion gene in leukemic transformation in vivo, we generated transgenic mice that expressed a CALM-AF10 fusion gene. Depending on the transgenic line, at least 40% to 50% of the F(1) generation mice developed acute leukemia at a median age of 12 months. Leukemic mice typically had enlarged spleens, invasion of parenchymal organs with malignant cells, and tumors with myeloid markers such as myeloperoxidase, Mac1, and Gr1. Although most leukemias were acute myeloid leukemia, many showed lymphoid features, such as CD3 staining, or clonal Tcrb or Igh gene rearrangements. Mice were clinically healthy for the first 9 months of life and had normal peripheral blood hemograms but showed impaired thymocyte differentiation, manifested by decreased CD4(+)/CD8(+) cells and increased immature CD4(-)/CD8(-) cells in the thymus. Hematopoietic tissues from both clinically healthy and leukemic CALM-AF10 mice showed up-regulation of Hoxa cluster genes, suggesting a potential mechanism for the impaired differentiation. The long latency period and incomplete penetrance suggest that additional genetic events are needed to complement the CALM-AF10 transgene and complete the process of leukemic transformation.

Chromosomal rearrangements and microRNAs: a new cancer link with clinical implications

There is widespread aberrant expression of mature and/or precursor microRNAs in cancer cells, as microRNAs are deregulated consequent to chromosomal alterations and other genomic abnormalities. The identification of such abnormalities has a clear diagnostic and prognostic significance, and there are ever increasing examples of links between certain human cancers and modifications at microRNA loci.

MicroRNAs and chromosomal abnormalities in cancer cells

Over the past five decades, a plethora of nonrandom chromosomal abnormalities have been consistently reported in malignant cells facilitating the identification of cancer-associated protein coding oncogenes and tumor suppressors. The genetic dissection of hot spots for chromosomal abnormalities in the age of the sequenced human genome resulted in the discovery that microRNA (miRNA) genes, encoding for a class of small noncoding RNAs, frequently resides in such genomic regions. The combination of nonrandom chromosomal abnormalities and other types of genetic alterations or epigenetic events contribute to downregulation or overexpression of miRNAs. The consequent abnormal expression of miRNAs affect cell cycle, survival and differentiation programs and selective targeting of these noncoding genes could provide novel therapeutic options for killing the malignant cells.

Chromosomal translocations involving the MLL gene: molecular mechanisms

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

CALM-AF10+ T-ALL expression profiles are characterized by overexpression of HOXA and BMI1 oncogenes

The t(10;11)(p13;q14-21) is found in T-ALL and acute myeloid leukemia and fuses CALM (Clathrin-Assembly protein-like Lymphoid-Myeloid leukaemia gene) to AF10. In order to gain insight into the transcriptional consequences of this fusion, microarray-based comparison of CALM-AF10+ vs CALM-AF10- T-ALL was performed. This analysis showed upregulation of HOXA5, HOXA9, HOXA10 and BMI1 in the CALM-AF10+ cases. Microarray results were validated by quantitative RT-PCR on an independent group of T-ALL and compared to mixed lineage leukemia-translocated acute leukemias (MLL-t AL). The overexpression of HOXA genes was associated with overexpression of its cofactor MEIS1 in CALM-AF10+ T-ALL, reaching levels of expression similar to those observed in MLL-t AL. Consequently, CALM-AF10+ T-ALL and MLL-t AL share a specific HOXA overexpression, indicating they activate common oncogenic pathways. In addition, BMI1, located close to AF10 breakpoint, was overexpressed only in CALM-AF10+ T-ALL and not in MLL-t AL. BMI1 controls cellular proliferation through suppression of the tumor suppressors encoded by the CDKN2A locus. This locus, often deleted in T-ALL, was conserved in CALM-AF10+ T-ALL. This suggests that decreased CDKN2A activity, as a result of BMI1 overexpression, contributes to leukemogenesis in CALM-AF10+ T-ALL. We propose to define a HOXA+ leukemia group composed of at least MLL-t, CALM-AF10 and HOXA-t AL, which may benefit from adapted management.

Causes of oncogenic chromosomal translocation

Non-random chromosomal translocations are frequently associated with a variety of cancers, particularly hematologic malignancies and childhood sarcomas. In addition to their diagnostic utility, chromosomal translocations are increasingly being used in the clinic to guide therapeutic decisions. However, the mechanisms that cause these translocations remain poorly understood. Illegitimate V(D)J recombination, class switch recombination, homologous recombination, non-homologous end-joining and genome fragile sites all have potential roles in the production of non-random chromosomal translocations. In addition, mutations in DNA-repair pathways have been implicated in the production of chromosomal translocations in humans, mice and yeast. Although initially surprising, the identification of these same oncogenic chromosomal translocations in peripheral blood from healthy individuals strongly suggests that the translocation is not sufficient to induce malignant transformation, and that complementary mutations are required to produce a frank malignancy.

Molecular Pathogenesis of MLL-Associated Leukemias

Chromosome translocations disrupting the MLL gene are associated with various hematologic malignancies but are particularly common in infant and secondary therapy-related acute leukemias. The normal MLL-encoded protein is an essential component of a supercomplex with chromatin-modulating activity conferred by histone acetylase and methyltransferase activities, and the protein plays a key role in the developmental regulation of gene expression, including Hox gene expression. In leukemia, this function is subverted by breakage, recombination, and the formation of chimeric fusion with one of many alternative partners. Such MLL translocations result in the replacement of the C-terminal functional domains of MLL with those of a fusion partner, yielding a newly formed MLL chimeric protein with an altered function that endows hematopoietic progenitors with self-renewing and leukemogenic activity. This potent impact of the MLL chimera can be attributed to one of 2 kinds of activity of the fusion partner: direct transcriptional transactivation or dimerization/oligomerization. Key unresolved issues currently being addressed include the set of target genes for MLL fusions, the stem cell of origin for the leukemias, the role of additional secondary mutations, and the origins or etiology of the MLL gene fusions themselves. Further elaboration of the biology of MLL gene-associated leukemia should lead to novel and specific therapeutic strategies.

The versatile mixed lineage leukaemia gene MLL and its many associations in leukaemogenesis

The marked association of abnormalities of chromosome 11 long arm, band q23, with human leukaemia led to the identification of the 11q23 gene called MLL (or HTRX, HRX, TRX1, ALL-1). MLL can become fused with one of a remarkable panoply of genes from other chromosome locations in individual leukaemias, leading to either acute myeloid or lymphoid tumours (hence the name MLL for mixed lineage leukaemia). The unusual finding that a single protein could be involved in both myeloid and lymphoid malignancies and that the truncated protein could do so as a fusion with very disparate partners has prompted studies to define the molecular role of MLL-fusions in leukaemogenesis and to the development of MLL-controlled mouse models of leukaemogenesis. These studies have defined MLL-fusion proteins as regulators of gene expression, controlling such elements as HOX genes, and have indicated a variety of mechanisms by which MLL-fusion proteins contribute to leukaemogenesis.

Age-related phenotypic and oncogenic differences in T-cell acute lymphoblastic leukemias may reflect thymic atrophy

Postnatal thymic involution occurs progressively throughout the first 3 decades of life. It predominantly affects T-cell receptor (TCR) alphabeta-lineage precursors, with a consequent proportional increase in multipotent thymic precursors. We show that T-acute lymphoblastic leukemias (T-ALLs) demonstrate a similar shift with age from predominantly TCR expressing to an immature (IM0/delta/gamma) stage of maturation arrest. Half demonstrate HOX11, HOX11L2, SIL-TAL1, or CALM-AF10 deregulation, with each being associated with a specific, age-independent stage of maturation arrest. HOX11 and SIL-TAL represent alphabeta-lineage oncogenes, whereas HOX11L2 expression identifies an intermediate alphabeta/gammadelta-lineage stage of maturation arrest. In keeping with preferential alphabeta-lineage involution, the incidence of SIL-TAL1 and HOX11L2 deregulation decreased with age. In contrast, HOX11 deregulation became more frequent, suggesting longer latency. TAL1/LMO1 deregulation is more frequent in alphabeta-lineage T-ALL, when it is predominantly due to SIL-TAL1 rearrangements in children but to currently unknown mechanisms in adolescents and adults. LMO2 was more frequently coexpressed with LYL1, predominantly in IM0/delta/gamma adult cases, than with TAL1. These age-related changes in phenotype and oncogenic pathways probably reflect progressive changes in the thymic population at risk of malignant transformation.

Etoposide induces chimeric Mll gene fusions

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

LMO2-associated clonal T cell proliferation in two patients after gene therapy for SCID-X1

We have previously shown correction of X-linked severe combined immunodeficiency [SCID-X1, also known as gamma chain (gamma(c)) deficiency] in 9 out of 10 patients by retrovirus-mediated gamma(c) gene transfer into autologous CD34 bone marrow cells. However, almost 3 years after gene therapy, uncontrolled exponential clonal proliferation of mature T cells (with gammadelta+ or alphabeta+ T cell receptors) has occurred in the two youngest patients. Both patients' clones showed retrovirus vector integration in proximity to the LMO2 proto-oncogene promoter, leading to aberrant transcription and expression of LMO2. Thus, retrovirus vector insertion can trigger deregulated premalignant cell proliferation with unexpected frequency, most likely driven by retrovirus enhancer activity on the LMO2 gene promoter.

Formation of trisomies and their parental origin in hyperdiploid childhood acute lymphoblastic leukemia

High hyperdiploidy, common in childhood acute lymphoblastic leukemia (ALL) with a favorable prognosis, is characterized by specific trisomies. Virtually nothing is known about its formation or pathogenetic impact. We evaluated 10 patients with ALL using 38 microsatellite markers mapped to 18 of the 24 human chromosomes to investigate the mechanisms underlying hyperdiploidy and to ascertain the parental origin of the trisomies. Based on the results, doubling of a near-haploid clone and polyploidization with subsequent losses of chromosomes could be excluded. The finding of equal allele dosage for tetrasomy 21 suggests that hyperdiploidy originates in a single aberrant mitosis, though a sequential gain of chromosomes other than 21 in consecutive cell divisions remains a possibility. Our study, the first to address experimentally the parental origin of trisomies in ALL, revealed no preferential duplication of maternally or paternally inherited copies of X, 4, 6, 9, 10, 17, 18, and 21. Trisomy 8 was of paternal origin in 4 of 4 patients (P =.125), and +14 was of maternal origin in 7 of 8 patients (P =.0703). Thus, the present results indicate that imprinting is not pathogenetically important in hyperdiploid childhood ALL, with the possible exception of the observed parental skewness of +8 and +14.

Transcription regulator LMO4 interferes with neuritogenesis in human SH-SY5Y neuroblastoma cells

LMO4 is a transcription regulator interacting with proteins involved, among else, in tumorigenesis. Its function in the nervous system, and particularly in the adult nervous system, has however still to be elucidated. We decided to modify its expression in a neuronal model, human SH-SY5Y neuroblastoma cells, by permanent transfection of sense or anti-sense Lmo4 cDNAs. Generated clones overexpressing the Lmo4 transcript in sense orientation tended to aggregate. They showed significantly reduced average number of neurites per cell and average neuritic length per cell. The opposite was observed with clones overexpressing the anti-sense Lmo4 transcript. Furthermore, selected clones were subjected to 72 h long-term treatments with retinoic acid and phorbol ester (TPA), two biochemicals known to stimulate differentiation of non-transfected SH-SY5Y cells and other neuroblastoma cells. Neuritogenesis occurred after retinoic acid stimulation in all cases. The inhibitory effect of sense Lmo4 RNA overexpression on neuritic outgrowth was indeed prevented. The protein kinase C activator TPA could not induce neuritogenesis in SH-SY5Y cells overexpressing sense Lmo4 RNA. Thus, sense Lmo4 RNA overexpression, not Lmo4 endogenous transcription, overrides the stimulatory effect of TPA upon neuritic outgrowth. We also showed that Lmo4-dependent neuritic retraction and outgrowth correspond to altered phosphorylation of cytoskeletal proteins. Overall, Lmo4 RNA overexpression interferes with neuritic outgrowth, whereas anti-sense Lmo4 RNA expression favors neuritogenesis in SH-SY5Y cells. Consequently, changes in Lmo4 RNA expression levels might alter the rate of neuritic outgrowth in the developing and adult nervous system.

Occurrence of leukaemia following gene therapy of X-linked SCID

Recombinant viral vectors have allowed gene transfer to be developed as a promising approach to the treatment of genetic diseases. Recently, gene therapy of children with X-linked severe combined immune deficiency resulted in impressive levels of immune reconstitution--a triumph that was later overshadowed by the development of leukaemia in two patients. What were the causes of this cancer, and how can the therapeutic benefits of gene therapy be achieved while minimizing risk to the patient?

Collaborative competition mechanism for gene activation in vivo

The mechanism by which gene regulatory proteins gain access to their DNA target sites is not known. In vitro, binding is inherently cooperative between arbitrary DNA binding proteins whose target sites are located within the same nucleosome. We refer to such competition-based cooperativity as collaborative competition. Here we show that arbitrarily chosen foreign DNA binding proteins, LexA and Tet repressor, cooperate with an adjacently binding endogenous activator protein, Gcn4, to coactivate expression of chromosomal reporter genes in Saccharomyces cerevisiae. Coactivation requires that the cooperating target sites be within a nucleosome-length distance; it leads to increased occupancy by Gcn4 at its binding site; and it requires both Gcn5 and Swi/Snf which, at an endogenous Gcn4-dependent promoter, act subsequent to Gcn4 binding. These results imply that collaborative competition contributes to gene regulation in vivo. They further imply that, even in the presence of the cell's full wild-type complement of chromatin remodeling factors, competition of regulatory proteins with histone octamer for access to regulatory target sites remains a quantitative determinant of gene expression levels. We speculate that initial target site recognition and binding may occur via spontaneous nucleosomal site exposure, with remodeling factor action required downstream to lock in higher levels of regulatory protein occupancy.

A pooled analysis of karyotypic patterns, breakpoints and imbalances in 783 cytogenetically abnormal multiple myelomas reveals frequently involved chromosome segments as well as significant age- and sex-related differences

The cytogenetic features (ploidy, complexity, breakpoints, imbalances) were ascertained in 783 abnormal multiple myeloma (MM) cases to identify frequently involved chromosomal regions as well as a possible impact of age/sex. The series included MM patients from the Mitelman Database of Chromosome Aberrations in Cancer and from our own laboratory. Hyperdiploidy was most common, followed by hypodiploidy, pseudodiploidy and tri-/tetraploidy. Most cases were complex, with a median of eight changes per patient. The distribution of modal numbers differed between younger and older patients, but was not related to sex. No sex- or age-related differences regarding the number of anomalies were found. The most frequent genomic breakpoints were 14q32, 11q13, 1q10, 8q24, 1p11, 1q21, 22q11, 1p13, 1q11, 19q13, 1p22, 6q21 and 17p11. Breaks in 1p13, 6q21 and 11q13 were more common in the younger age group. The most frequent imbalances were + 9, - 13, + 15, + 19, + 11 and - Y. Trisomy 11 and monosomy 16 were more common among men, while -X was more frequent among women. Loss of Y as the sole change and + 5 were more common in elderly patients, and - 14 was more frequent in the younger age group. The present findings strongly suggest that some karyotypic features of MM are influenced by endogenous and/or exogenous factors.

[Oncogenes and leukemia: history and perspectives]

Oncogenes involved in the development of hematological malignancies were first discovered through the study of experimental leukemias induced in animals by retroviruses. The discovery that some of these genes were located at the breakpoints of chromosome rearrangements in human malignancies, such as the MYC gene in Burkitt's lymphoma and the ABL gene in chronic myeloid leukemia (CML) has suggested that chromosome abnormalities were causally implicated in the pathogenesis of human diseases. Numerous nonrandom somatically acquired chromosomal translocations or inversions have been identified in human leukemias. The molecular cloning of the genes located at the breakpoints of these rearrangements allowed to identify more than 100 new oncogenes, the products of which affect normal programs of cell proliferation, differentiation and survival. Chromosome translocations can lead to the deregulated expression of a normal gene product, but in most cases of leukemia, chromosome rearrangements result in the expression of a chimeric fusion protein. Oncogene products associated with acute leukemias are often transcription factors while tyrosine kinases and antiapoptotic proteins are more commonly activated or overexpressed in chronic leukemias and in lymphomas. Recent data indicated that gene rearrangements were not the sole gene alterations occurring in human leukemia since point mutations could also affect the function of transcription factors playing a key role in hematopoiesis such as C/EBP alpha, GATA1 and AML1. But the most exciting finding was the discovery of activating point mutations in tyrosine kinase receptors such as FLT3 and c-KIT in acute leukemia. Treatment of leukemia could therefore benefit from new therapeutic approaches targeting the function of specific oncogene products as already demonstrated for CML and acute promyelocytic leukemia.

Overexpression of NPM-ALK induces different types of malignant lymphomas in IL-9 transgenic mice

Anaplastic large-cell lymphoma (ALCL) comprises approximately 25% of all non-Hodgkin lymphomas (NHL) in children and young adults, and up to 15% of high-grade NHL in older patients. Over 50% of these tumours carry the translocation t(2;5)(p23;q35). The result of this translocation is the fusion of the nucleophosmin (NPM) gene to the anaplastic lymphoma kinase (ALK) gene. The resulting hybrid protein contains the ALK catalytic domain that consequently confers transforming potential, which contributes to the pathogenesis of ALCL. To further analyse the transforming activity in an animal model, a cDNA encoding the protein product, NPM-ALK, was inserted into the retrovirus vector pLXSN and transduced into mouse bone marrow progenitors. These cells were subsequently used in a bone marrow transplant with the aim of reconstituting the haematopoietic compartments of lethally irradiated recipients. IL-9 transgenic mice were chosen as the animal model system, because dysregulated expression of the IL-9 gene in transgenic mice results in the sporadic development of spontaneous thymic lymphomas. Moreover, IL-9 is known to be expressed in cases of human ALCL. We used 15 IL-9 transgenic mice and eight corresponding wild-type mice (FVB/N) and transplanted them with NPM/ALK infected bone marrow cells. Eight IL-9 transgenic mice, serving as a control group, received pLXSN (vector only)-infected marrow. Reconstituted mice developed NPM-ALK-positive lymphomas, including lymphoblastic lymphomas of T-cell type (T-LB), mature and immature plasmacytoma (PC), and plasmoblastic/anaplastic diffuse large-B-cell lymphoma after about 19-20 weeks. The combined overexpression of NPM-ALK and IL-9 led to the transformation of murine lymphoid cells with accelerated and enhanced development of T-LB in 46% of the mice, which only very rarely occurs in IL-9 transgenic mice only. Of the 15 animals, five (33%) developed plasmacytic/plasmoblastic neoplasms, of which the most aggressive tumours share many features with anaplastic/plasmoblastic diffuse large-B-cell lymphoma on the basis of morphology, a characteristic growth pattern and ALK expression.

Trisomy 8 as the sole chromosomal aberration in myelocytic malignancies: a multicolor and locus-specific fluorescence in situ hybridization study

Trisomy 8 is the most common chromosomal aberration in myelocytic malignancies, occurring both as a sole change as well as in addition to other abnormalities. In spite of this, next to nothing is known about its pathogenetic importance or its molecular genetic consequences. Possible mechanisms involved in the transformation process include dosage effects of genes mapping to chromosome 8 and presence of specific mutations or cryptic fusion genes on the duplicated chromosome. In the latter case, +8 would be secondary to a cryptic primary rearrangement and not involved in leukemogenesis as such, but rather in tumor evolution. Although hidden genetic changes have been found in some trisomies, for example, mutations in KIT in acute myelocytic leukemia (AML) with +4 and in MET in hereditary papillary kidney carcinoma with trisomy 7, none associated with +8 have so far been discovered. To address this issue, we have investigated a total of 13 cases of AML, myelodysplastic syndromes, and chronic myeloproliferative disorders with trisomy 8 as the sole chromosomal anomaly. All cases were studied by combined binary ratio multicolor fluorescence in situ hybridization (FISH) and with FISH using locus-specific probes for both arms of chromosome 8, the subtelomeric regions of 8p and 8q, and the leukemia-associated genes FGFR1, MOZ, ETO, and MYC. No cryptic changes were detected, thus excluding the possibility of gross genetic rearrangements or aberrations involving these loci on chromosome 8.