Genomic organization and regulation of the vav proto-oncogene

Cross-species epigenetics identifies a critical role for VAV1 in SHH subgroup medulloblastoma maintenance

The identification of key tumorigenic events in Sonic Hedgehog subgroup medulloblastomas (MB_SHH) will be essential for the development of individualized therapies and improved outcomes. However, beyond confirmation of characteristic SHH-pathway mutations, recent genome-wide sequencing studies have not revealed commonly-mutated genes with widespread relevance as potential therapeutic targets. We therefore examined any role for epigenetic DNA methylation events in MB_SHH using a cross-species approach to candidate identification, prioritization and validation. MB_SHH–associated DNA methylation events were first identified in 216 subgrouped human medulloblastomas (50 MB_SHH, 28 WNT_, 44 Group 3, 94 Group 4) and their conservation then assessed in tumors arising from four independent murine models of Shh medulloblastoma, alongside any role in tumorigenesis using functional assessments in mouse and human models. This strategy identified widespread regional CpG hypo-methylation of VAV1, leading to its elevated expression, as a conserved aberrant epigenetic event which characterizes the majority of MB_SHH tumors in both species, and is associated with a poor outcome in MB_SHH patients. Moreover, direct modulation of VAV1 in mouse and human models revealed a critical role in tumor maintenance, and its abrogation markedly reduced medulloblastoma growth. Further, Vav1 activity regulated granule neuron precursor (GNP) germinal zone exit and migration initiation in an ex vivo model of early post-natal cerebellar development. These findings establish VAV1 as a critical epigenetically-regulated oncogene with a key role in MB_SHH maintenance, and highlight its potential as a validated therapeutic target and prognostic biomarker for the improved therapy of medulloblastoma.

Molecular cloning and characterization of growth factor receptor bound-protein in Clonorchis sinensis

Background

Clonorchis sinensis causes clonorchiasis, a potentially serious disease. Growth factor receptor-bound protein 2 (Grb2) is a cytosolic protein conserved among animals and plays roles in cellular functions such as meiosis, organogenesis and energy metabolism. In the present study, we report first molecular characters of growth factor receptor bound-protein (CsGrb2) from C. sinensis as counter part of Grb2 from animals and its possible functions in development and organogenesis of C. sinensis.

Methodology/Principal Findings

A CsGrb2 cDNA clone retrieved from the C. sinensis transcriptome encoded a polypeptide with a SH3-SH2-SH3 structure. Recombinant CsGrb2 was bacterially produced and purified to homogeneity. Native CsGrb2 with estimated molecular weight was identified from C. sinensis adult extract by western blotting using a mouse immune serum to recombinant CsGrb2. CsGrb2 transcripts was more abundant in the metacercariae than in the adults. Immunohistochemical staining showed that CsGrb2 was localized to the suckers, mesenchymal tissues, sperms in seminal receptacle and ovary in the adults, and abundantly expressed in most organs of the metacercariae. Recombinant CsGrb2 was evaluated to be little useful as a serodiagnostic reagent for C. sinesis human infections.

Conclusion

Grb2 protein found in C. sinensis was conserved among animals and suggested to play a role in the organogenesis, energy metabolism and mitotic spermatogenesis of C. sinensis. These findings from C. sinensis provide wider understanding on diverse function of Grb2 in lower animals such as platyhelminths.

Vav1 haploinsufficiency in a common variable immunodeficiency patient with defective T-cell function

Common variable immunodeficiency (CVID) is a primary immune disorder characterized by impaired antibody production, which is in many instances secondary to defective T cell function (T-CVID). We previously identified a subset of T-CVID patients characterized by defective expression of Vav1, a guanine nucleotide exchanger which couples the T-cell antigen receptor to reorganization of the actin cytoskeleton. Here we have addressed the possibility that an intrinsic defect in the Vav1 gene might underlie the reduction in Vav protein observed in T cells from these patients. We report the identification in one T-CVID patient of a heterozygous deletion in Vav1. The gene deletion, spanning exons 2-27, accounts for the reduction in Vav1 mRNA and protein in T cells from this patient. The disease-related pedigree of this patient suggests a de novo origin of the Vav1 deletion. The findings highlights Vav1 as an autosomal dominant disease gene associated with CVID with defective T-cell function.

Defective Vav expression and impaired F-actin reorganization in a subset of patients with common variable immunodeficiency characterized by T-cell defects

Common variable immunodeficiency (CVID) is a primary immune disorder characterized by impaired antibody production, which is in many instances secondary to defective T-cell function (T-CVID). We have previously identified a subset of patients with T-CVID characterized by defective T-cell receptor (TCR)-dependent protein tyrosine phosphorylation. In these patients, ZAP-70 fails to be recruited to the TCR as the result of impaired CD3zeta phosphorylation, which is, however, not dependent on defective Lck expression or activity. Here we show that neither Fyn nor CD45 is affected in these patients. On the other hand, T-CVID T cells show dramatic defects in the Vav/Rac pathway controlling F-actin dynamics. A significant deficiency in Vav protein was indeed observed; in 3 of 4 patients with T-CVID, it was associated with reduced VAV1 mRNA levels. The impairment in Vav expression correlated with defective F-actin reorganization in response to TCR/CD28 co-engagement. Furthermore, TCR/CD28-dependent up-regulation of lipid rafts at the cell surface, which requires F-actin dynamics, was impaired in these patients. The actin cytoskeleton defect could be reversed by reconstitution of Vav1 expression in the patients' T cells. Results demonstrate an essential role of Vav in human T cells and strongly suggest Vav insufficiency in T-CVID.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

Detection and identification of Vav1 protein in primary cultured murine cerebellar neurons and in neuroblastoma cells (SH-SY5Y and Neuro-2a)

Vav1 was detected in neuronal cells during a screening for 1-methylthiodihydroceramide (1-MSDH-Cer) binding proteins. 1-MSDH-Cer is a metabolically stable analogue of dihydroceramide that was reported to strongly interfere with the formation of ceramide and hence the biosynthesis of all sphingolipids in neuronal cells. To identify target proteins that function as putative mediators of this molecule, a 1-MSDH-Cer affinity chromatography was utilised. When the cytosolic fraction of human neuroblastoma SH-SY5Y cells was subjected to 1-MSDH-Cer affinity chromatography, the sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the eluted protein fraction revealed an about 2-fold enrichment of the 98 kD protein band. Tryptic digestion of the excised band in combination with MALDI mass spectrometry strongly suggested that this band contained Vav1 protein. This was surprising since Vav1 in contrast to the other two isoforms Vav2 and Vav3 is believed to be exclusively expressed in hematopoietic cells and has not been detected in neuronal cells until now. The expression of Vav1 was confirmed in human SH-SY5Y neuroblastoma cells and additionally in murine Neuro-2A neuroblastoma cells as well as in primary cultured murine cerebellar neurons by Western blot analysis and reverse transcription polymerase chain reaction.

Pleiotropic defects in TCR signaling in a Vav-1-null Jurkat T-cell line

The Rac/Rho-specific guanine nucleotide exchange factor, Vav-1, is a key component of the T-cell antigen receptor (TCR)-linked signaling machinery. Here we have used somatic cell gene-targeting technology to generate a Vav-1-deficient Jurkat T-cell line. The J.Vav1 cell line exhibits dramatic defects in TCR-dependent interleukin (IL)-2 promoter activation, accompanied by significant reductions in the activities of the NFAT(IL-2), NFkappaB, AP-1 and REAP transcription factors that bind to the IL-2 promoter region. In contrast, loss of Vav-1 had variable effects on early TCR-stimulated signaling events. J.Vav1 cells display a selective defect in sustained Ca(2+) signaling during TCR stimulation, and complementation of this abnormality by exogenously introduced Vav-1 is dependent on the Vav-1 calponin homology domain. While JNK activation was severely impaired, the stimulation of Ras, ERK and protein kinase C-theta activities, as well as the mobilization of lipid rafts, appeared normal in the J.Vav1 cells. Finally, evidence is presented to suggest that the alternative Vav family members, Vav-2 and Vav-3, are activated during TCR ligation, and partially compensate for the loss of Vav-1 in Jurkat T cells.

PU.1 regulates the expression of the vav proto-oncogene

Vav is a guanine nucleotide exchange factor for the rho/rac GTPases that is upregulated in the embryo during the transition from primitive to definitive hematopoiesis. It is one of several genetic markers that correlates with the differentiation of the intraembryonic definitive hematopoietic stem cell. Subsequently, in the adult, vav is expressed predominantly in cells of the hematopoietic system. A heat-resistant protein complex that bound to a 23-bp segment, which is essential for vav promoter activity, was found to be present in myeloid cells but not T-cells. The complex was absent in non-hematopoietic cells which normally do not express vav. Using a saturation mutagenesis method, Mutex, a "footprint" of the protein binding site (AGAGGAAGT) was obtained that was consistent with the consensus binding site for PU.1. A specific antibody to PU.1 supershifted the complex and identified the presence of PU.1 within the complex. A GST fusion protein of the human PU.1 bound to the same consensus sequence as the heat-resistant complex from myeloid lineages. Specific mutation of the GGAA PU.1 core binding site silenced vav promoter activity and a dominant negative PU.1 inhibited the transactivation of PU.1 at the vav promoter as measured by the expression of the EGFP reporter gene. In addition, PCR analysis of immunoprecipitated chromatin using specific antibodies for PU.1 detected the co-immunoprecipitation of DNA containing the vav promoter. These results suggest that PU.1 is essential for transcriptional activity of the vav promoter in myeloid cells.

Cancer/testis antigen CSAGE is concurrently expressed with MAGE in chondrosarcoma

Differential display-polymerase chain reaction was used to compare gene expression between human chondrosarcoma cell lines and normal cartilage. A new gene, CSAGE, has been cloned and belongs to a gene family that includes the taxol resistance associated gene (TRAG)-3. CSAGE, like TRAG-3, does not confer resistance to taxol when transfected in vitro. Both genes have alternatively spliced variants. CSAGE and TRAG-3 are expressed in chondrosarcoma, melanoma, and cartilage and testis, but not in other normal tissues. TRAG-3 has been reported to be a cancer/testis antigen. Our results suggest that CSAGE belongs to the growing list of cancer/testis antigens as well. In all of the CSAGE positive samples, the melanoma antigen gene family was also expressed. This is the first report on the expression of cancer/testis antigens in chondrosarcoma.

Regulation of the vav proto-oncogene by LKLF

The transcriptional induction of the vav proto-oncogene coincides with the first appearance of the definitive hematopoietic stem cell in the aorta-gonad-mesonephros region. Vav promoter activity was dependent on a previously identified 23 bp DNA segment containing PU.1 and Runx1/AML-1 binding sites and on a newly identified, highly conserved, 12 bp DNA segment (Box B). The sequence of Box B was identical in the human, mouse and rat species. Mutation of the CACCC core sequence led to diminished vav promoter activity. A protein complex which bound to Box B was found in hematopoietic cells but not in cells which did not express vav. A double-stranded oligonucleotide containing a mutation of the CACCC core was less effective in electro-mobility shift assay competitions than the wild-type sequence. UV crosslinking studies identified a 37 kDa DNA binding protein which interacted with Box B in U937 cells. Antibody supershift assays identified this protein as lung Krüppel-like factor (LKLF). LKLF, expressed as a glutathione S-transferase fusion protein, was capable of binding to Box B. A dominant-negative LKLF was able to inhibit the expression of enhanced green fluorescent protein by the vav promoter and chromatin immunoprecipitations detected LKLF bound to the vav promoter in U937 but not HeLa cells. These in vitro results suggest future in vivo experiments to examine the role of LKLF, a gene required for vasculogenesis, in the induction of vav during the genesis of the definitive hematopoietic stem cell from the vascular endothelium.