Gris1, a new common integration site in Graffi murine leukemia virus-induced leukemias: overexpression of a truncated cyclin D2 due to alternative splicing

Co-expressed Cyclin D variants cooperate to regulate proliferation of germline nuclei in a syncytium

The role of the G1-phase Cyclin D-CDK 4/6 regulatory module in linking germline stem cell (GSC) proliferation to nutrition is evolutionarily variable. In invertebrate Drosophila and C. elegans GSC models, G1 is nearly absent and Cyclin E is expressed throughout the cell cycle, whereas vertebrate spermatogonial stem cells have a distinct G1 and Cyclin D1 plays an important role in GSC renewal. In the invertebrate, chordate, Oikopleura, where germline nuclei proliferate asynchronously in a syncytium, we show a distinct G1-phase in which 2 Cyclin D variants are co-expressed. Cyclin Dd, present in both somatic endocycling cells and the germline, localized to germline nuclei during G1 before declining at G1/S. Cyclin Db, restricted to the germline, remained cytoplasmic, co-localizing in foci with the Cyclin-dependent Kinase Inhibitor, CKIa. These foci showed a preferential spatial distribution adjacent to syncytial germline nuclei at G1/S. During nutrient-restricted growth arrest, upregulated CKIa accumulated in arrested somatic endoreduplicative nuclei but did not do so in germline nuclei. In the latter context, Cyclin Dd levels gradually decreased. In contrast, the Cyclin Dbβ splice variant, lacking the Rb-interaction domain and phosphodegron, was specifically upregulated and the number of cytoplasmic foci containing this variant increased. This upregulation was dependent on stress response MAPK p38 signaling. We conclude that under favorable conditions, Cyclin Dbβ-CDK6 sequesters CKIa in the cytoplasm to cooperate with Cyclin Dd-CDK6 in promoting germline nuclear proliferation. Under nutrient-restriction, this sequestration function is enhanced to permit continued, though reduced, cycling of the germline during somatic growth arrest.

Identification of GPM6A and GPM6B as potential new human lymphoid leukemia-associated oncogenes

BACKGROUND

Previously, we found that the Graffi murine leukemia virus (MuLV) is able to induce a wide spectrum of hematologic malignancies in vivo. Using high-density oligonucleotide microarrays, we established the gene expression profiles of several of these malignancies, thereby specifically focusing on genes deregulated in the lymphoid sub-types. We observed over-expression of a variety of genes, including Arntl2, Bfsp2, Gfra2, Gpm6a, Gpm6b, Nln, Fbln1, Bmp7, Etv5 and Celsr1 and, in addition, provided evidence that Fmn2 and Parm-1 may act as novel oncogenes. In the present study, we assessed the expression patterns of eight selected human homologs of these genes in primary human B-cell malignancies, and explored the putative oncogenic potential of GPM6A and GPM6B.

METHODS

The gene expression levels of the selected human homologs were tested in human B-cell malignancies by semi-quantitative RT-PCR. The protein expression profiles of human GPM6A and GPM6B were analyzed by Western blotting. The localization and the effect of GPM6A and GPM6B on the cytoskeleton were determined using confocal and indirect immunofluorescence microscopy. To confirm the oncogenic potential of GPM6A and GPM6B, classical colony formation assays in soft agar and focus forming assays were used. The effects of these proteins on the cell cycle were assessed by flow cytometry analysis.

RESULTS

Using semi-quantitative RT-PCR, we found that most of the primary B-cell malignancies assessed showed altered expression patterns of the genes tested, including GPM6A and GPM6B. Using confocal microscopy, we found that the GPM6A protein (isoform 3) exhibits a punctate cytoplasmic localization and that the GPM6B protein (isoform 4) exhibits a peri-nuclear and punctate cytoplasmic localization. Interestingly, we found that exogenous over-expression of both proteins in NIH/3T3 cells alters the actin and microtubule networks and induces the formation of long filopodia-like protrusions. Additionally, we found that these over-expressing NIH/3T3 cells exhibit anchorage-independent growth and enhanced proliferation rates. Cellular transformation (i.e., loss of contact inhibition) was, however, only observed after exogenous over-expression of GPM6B.

CONCLUSIONS

Our results indicate that several human homologs of the genes found to be deregulated in Graffi MuLV experimental mouse models may serve as candidate biomarkers for human B-cell malignancies. In addition, we found that GPM6A and GPM6B may act as novel oncogenes in the development of these malignancies.

Characterization of growth suppressive functions of a splice variant of cyclin D2

We have recently cloned a novel splice variant of cyclin D2 termed as cycD2SV. CycD2SV overexpression in several immortalized cell lines led to formation of ubiquitinated protein aggregates accompanied by a significant decrease in cell proliferation. Based on immuno co-localization and ultrastructural analysis experiments, cycD2SV protein aggregates were frequently found in various subcellular compartments such as endosomes, autophagosomes, lysosomes and the microtubule organizing centre. Secondary structure analysis revealed that the amino terminal α-helix in cycD2SV is not tightly packed with the cyclin box suggesting a misfolded conformation compared to other cyclins. Deletion analysis suggests that 1–53 amino acid region of cycD2SV may be required for protein aggregation and 54–136 amino acid region may mediate cell cycle inhibition. Based on co-immunoprecipitation experiments, we have shown that cycD2SV binds to cycD2 as well as CDK4. In addition, gene expression analysis demonstrated an upregulation in GADD45α and dynamin 2 mRNA levels in cycD2SV overexpressing cells. These two proteins are known to play critical roles in the DNA damage response and apoptosis pathways. TUNEL experiments were negative for apoptosis, however, cycD2SV expressing cells were more sensitive to cell death induced by external stressors such as trypsinization. Collectively our results suggest that cycD2SV mediates cell cycle inhibition by sequestering endogenous cell cycle proteins, such as cycD2 and CDK4, and possibly targeting them for ubiquitin mediated protein degradation.

Cyclin D2 in the basal process of neural progenitors is linked to non-equivalent cell fates

Asymmetric cell division plays an indispensable role during corticogenesis for producing new neurons while maintaining a self-renewing pool of apical progenitors. The cellular and molecular determinants favouring asymmetric division are not completely understood. Here, we identify a novel mechanism for generating cellular asymmetry through the active transportation and local translation of Cyclin D2 mRNA in the basal process. This process is regulated by a unique cis-regulatory sequence found in the 3' untranslated region (3'UTR) of the mRNA. Unequal inheritance of Cyclin D2 protein to the basally positioned daughter cell with the basal process confers renewal of the apical progenitor after asymmetric division. Conversely, depletion of Cyclin D2 in the apically positioned daughter cell results in terminal neuronal differentiation. We demonstrate that Cyclin D2 is also expressed in the developing human cortex within similar domains, thus indicating that its role as a fate determinant is ancient and conserved.

Gene profiling of Graffi murine leukemia virus-induced lymphoid leukemias: identification of leukemia markers and Fmn2 as a potential oncogene

The Graffi murine leukemia virus induces a large spectrum of leukemias in mice and thus provides a good model to compare the transcriptome of all types of leukemias. We analyzed the gene expression profiles of both T and B leukemias induced by the virus with DNA microarrays. Given that we considered that a 4-fold change in expression level was significant, 388 probe sets were associated to B, to T, or common to both leukemias. Several of them were not yet associated with lymphoid leukemia. We confirmed specific deregulation of Fmn2, Arntl2, Bfsp2, Gfra2, Gpm6a, and Gpm6b in B leukemia, of Nln, Fbln1, and Bmp7 in T leukemias, and of Etv5 in both leukemias. More importantly, we show that the mouse Fmn2 induced an anchorage-independent growth, a drastic modification in cell shape with a concomitant disruption of the actin cytoskeleton. Interestingly, we found that human FMN2 is overexpressed in approximately 95% of pre-B acute lymphoblastic leukemia with the highest expression levels in patients with a TEL/AML1 rearrangement. These results, surely related to the role of FMN2 in meiotic spindle maintenance, suggest its important role in leukemogenesis. Finally, we propose a new panel of genes potentially involved in T and/or B leukemias.

A splice variant of cyclin D2 regulates cardiomyocyte cell cycle through a novel protein aggregation pathway

The mammalian heart lacks intrinsic ability to replace diseased myocardium with newly divided myocytes. There is scant information on mechanisms regulating cell cycle exit in cardiomyocytes. We cloned a splice variant of cyclin D2 (D2SV) from the mouse heart and found a novel role for this protein in cardiomyocyte cell cycle exit. We report that D2SV is highly expressed in embryonic myocardium compared with the adult heart. Localization studies indicate that D2SV is retained in the endoplasmic reticulum (ER), Golgi and lysosomal compartments and subjected to ER-stress-associated protein aggregation. D2SV aggregation relies on the motor activities of dynein and is blocked by ER stress modulators. The ability of D2SV to sequester other cell cycle proteins provides a mechanistic explanation for its effects on cardiomyocyte cell cycle. We show that D2SV-induced cell cycle exit can be rescued by overexpression of D-type and B-type cyclins. We suggest that protein aggregation may be a major block for cardiomyocyte cell cycle reactivation.

Human and mouse cyclin D2 splice variants: transforming activity and subcellular localization

We have previously reported the identification of a novel 17 kDa truncated isoform of the cyclin D2 activated in 13% of the leukemias induced by the Graffi murine leukemia retrovirus. Retroviral integration in the Gris1 locus causes an alternative splicing of the mouse cyclin D2 gene and expression of a truncated protein of 159 amino acids that is detected at high levels in the Gris1 tumors and also in normal mouse tissues mainly the brain and ovaries. A truncated form of the cyclin D2 was also found in human. We show here that both mouse- and human-truncated cyclin D2 are able to transform primary mouse embryo fibroblasts (MEF) when co-expressed with an activated Ras protein. The truncated cyclin D2 localizes only to the cytoplasm of transfected cells. It has retained the ability to interact with cyclin-dependent kinases (CDKs), although it is a poor catalyst of pRb phosphorylation. Interestingly, the presence of a similar, alternatively spliced cyclin D2 mRNA was also detected in some human brain tumors.

Complete genome sequences of the two viral variants of the Graffi MuLV: Phylogenetic relationship with other murine leukemia retroviruses

A detailed phylogenetic analysis of two variants of the Graffi murine retrovirus, GV-1.2 and GV-1.4, showed that they are closely related to SRS 19-6 and Moloney MuLVs. Two stretches of sequence testify to the divergence between Graffi and SRS 19-6 MuLVs, one corresponding to a recombination event of Graffi MuLV with a xenotropic virus. Moloney MuLV was found more distant, particularly in the GAG region. Our study encompasses every class of MuLVs (ecotropic, amphotropic, xenotropic, polytropic) with some focus on exogenous ecotropic viruses and further adds to previous phylogenetic studies. Graffi, SRS 19-6, Moloney, Friend and Rauscher MuLVs form a cluster that appears to share a common ancestor with the Casitas-amphotropic and -ecotropic MuLVs but are more distant to the Akv-type and xenotropic MuLVs. The analysis also revealed that the ENV region of HEMV, the prototype of the MuLV ancestor, was closely related to the corresponding region of Cas-Br-E.

Pre-mRNA Missplicing as a Cause of Human Disease

Regulated alternative splice site selection emerges as one of the most important mechanisms to control the expression of genetic information in humans. It is therefore not surprising that a growing number of diseases are either associated with or caused by changes in alternative splicing. These diseases can be caused by mutation in regulatory sequences of the pre-mRNA or by changes in the concentration of trans-acting factors. The pathological expression of mRNA isoforms can be treated by transferring nucleic acids derivatives into cells that interfere with sequence elements on the pre-mRNA, which results in the desired splice site selection. Recently, a growing number of low molecular weight drugs have been discovered that influence splice site selection in vivo. These findings prove the principle that diseases caused by missplicing events could eventually be cured.

Radiation leukemia virus common integration at the Kis2 locus: simultaneous overexpression of a novel noncoding RNA and of the proximal Phf6 gene

Retroviral tagging has been used extensively and successfully to identify genes implicated in cancer pathways. In order to find oncogenes implicated in T-cell leukemia, we used the highly leukemogenic radiation leukemia retrovirus VL3 (RadLV/VL3). We applied the inverted PCR technique to isolate and analyze sequences flanking proviral integrations in RadLV/VL3-induced T lymphomas. We found retroviral integrations in c-myc and Pim1 as already reported but we also identified for the first time Notch1 as a RadLV common integration site. More interestingly, we found a new RadLV common integration site that is situated on mouse chromosome X (XA4 region, bp 45091000). This site has also been reported as an SL3-3 and Moloney murine leukemia virus integration site, which strengthens its implication in murine leukemia virus-induced T lymphomas. This locus, named Kis2 (Kaplan Integration Site 2), was found rearranged in 11% of the tumors analyzed. In this article, we report not only the alteration of the Kis2 gene located nearby in response to RadLV integration but also the induction of the expression of Phf6, situated about 250 kbp from the integration site. The Kis2 gene encodes five different alternatively spliced noncoding RNAs and the Phf6 gene codes for a 365-amino-acid protein which contains two plant homology domain fingers, recently implicated in the Börjeson-Forssman-Lehmann syndrome in humans. With the recent release of the mouse genome sequence, high-throughput retroviral tagging emerges as a powerful tool in the quest for oncogenes. It also allows the analysis of large DNA regions surrounding the integration locus.

Animal models of acute myelogenous leukaemia - development, application and future perspectives

From the early inception of the transplant models through to contemporary genetic and xenograft models, evolution of murine leukaemic model systems have been critical to our general comprehension and treatment of cancer, and, more specifically, disease states such as acute myelogenous leukaemia (AML). However, even with modern advances in therapeutics and molecular diagnostics, the majority of AML patients die from their disease. Thus, in the absence of definitive in vitro models which precisely recapitulate the in vivo setting of human AMLs and failure of significant numbers of new drugs late in clinical trials, it is essential that murine AML models are developed to exploit more specific, targeted therapeutics. While various model systems are described and discussed in the literature from initial transplant models such as BNML and spontaneous murine leukaemia virus models, to the more definitive genetic and clinically significant NOD/SCID xenograft models, there exists no single compendium which directly assesses, reviews or compares the relevance of these models. Thus, the function of this article is to provide clinicians and experimentalists a chronological, comprehensive appraisal of all AML model systems, critical discussion on the elucidation of their roles in our understanding of AML and consideration to their efficacy in the development of AML chemotherapeutics.