Nuclear factors that bind to the U3 region of two murine myeloid leukemia-inducing retroviruses, Cas-Br-E and Graffi

Retroviral insertion mutagenesis in mice as a comparative oncogenomics tool to identify disease genes in human leukemia

Retroviral insertion mutagenesis has recently received much attention because of its adverse effects in the application of retroviral vector-based gene therapy, resulting in leukemia in certain patients. At the same time, retroviral mutagenesis in mice is being considered a powerful forward genetic strategy to identify disease genes involved in cancer. The publication of the mouse genome sequence and the development of high-throughput genomic approaches have given a further boost to this rapidly evolving field. The increasing numbers of new potential oncogenes identified in retroviral screens have given a valuable basis for a better understanding of cancer related pathways in mice. Important challenges that now lie ahead of us are (i) to determine the relevance and causal relationship of these genes with various types of human cancer (ii) to develop strategies to identify tumor suppressor genes on a large scale, (iii) to place the disease genes into regulatory networks to better understand their role in the complex pathogenesis of cancer, and (iv) to determine their value for diagnosis refinement and therapeutic target intervention in human disease. In this review, we will give a brief update of the current state-of-the-art and thoughts concerning these issues. We will specifically focus on the value of employing retroviral insertion mutagenesis in mice and gene expression profiling in man in the context of acute myeloid leukemia.

Importance of receptor usage, Fli1 activation, and mouse strain for the stem cell specificity of 10A1 murine leukemia virus leukemogenicity

Murine leukemia viruses (MuLV) induce leukemia through a multistage process, a critical step being the activation of oncogenes through provirus integration. Transcription elements within the long terminal repeats (LTR) are prime determinants of cell lineage specificity; however, the influence of other factors, including the Env protein that modulates cell tropism through receptor recognition, has not been rigorously addressed. The ability of 10A1-MuLV to use both PiT1 and PiT2 receptors has been implicated in its induction of blast cell leukemia. Here we show that restricting receptor usage of 10A1-MuLV to PiT2 results in loss of blast cell transformation capacity. However, the pathogenicity was unaltered when the env gene is exchanged with Moloney MuLV, which uses the Cat1 receptor. Significantly, the leukemic blasts express erythroid markers and consistently contain proviral integrations in the Fli1 locus, a target of Friend MuLV (F-MuLV) during erythroleukemia induction. Furthermore, an NB-tropic variant of 10A1 was unable to induce blast cell leukemia in C57BL/6 mice, which are also resistant to F-MuLV transformation. We propose that 10A1- and F-MuLV actually induce identical (erythro)blastic leukemia by a mechanism involving Fli1 activation and cooperation with inherent genetic mutations in susceptible mouse strains. Furthermore, we demonstrate that deletion of the Icsbp tumor suppressor gene in C57BL/6 mice is sufficient to confer susceptibility to 10A1-MuLV leukemia induction but with altered specificity. In summary, we validate the significance of the env gene in leukemia specificity and underline the importance of a complex interplay of cooperating oncogenes and/or tumor suppressors in determining the pathogenicity of MuLV variants.

Novel insights into the pathogenesis of the Graffi murine leukemia retrovirus

The Graffi murine leukemia virus (MuLV) was isolated in 1954 by Arnold Graffi, who characterized it as a myeloid leukemia-inducing retrovirus. He and his team, however, soon observed the intriguing phenomenon of hematological diversification, which corresponded to a decrease of myeloid leukemias and an increase of other types of leukemias. Recently, we derived two different molecular clones corresponding to ecotropic nondefective genomes that were named GV-1.2 and GV-1.4. The induced leukemias were classified as myeloid based on morphological analysis of blood smears. In this study, we further characterized the two variants of the Graffi murine retrovirus, GV-1.2 and GV-1.4, in three different strains of mice. We show that the Graffi MuLV is a multipotent retrovirus capable of inducing both lymphoid (T- and B-cell) and nonlymphoid (myeloid, erythroid, megakaryocytic) leukemia. Many of these are very complex with concomitant expression of different hematopoietic lineages. Interestingly, a high percentage of megakaryocytic leukemias, a type of leukemia rarely observed with MuLVs, arise in the FVB/n strain of mice. The genetic backgrounds of the different strains of mice influence greatly the results. Furthermore, the enhancer region, different for GV-1.2 and GV-1.4, plays a pivotal role in the disease specificity: GV-1.2 induces more lymphoid leukemias, and GV-1.4 induces more nonlymphoid ones.

Significance of murine retroviral mutagenesis for identification of disease genes in human acute myeloid leukemia

Retroviral insertion mutagenesis is considered a powerful tool to identify cancer genes in mice, but its significance for human cancer has remained elusive. Moreover, it has recently been debated whether common virus integrations are always a hallmark of tumor cells and contribute to the oncogenic process. Acute myeloid leukemia (AML) is a heterogeneous disease with a variable response to treatment. Recurrent cytogenetic defects and acquired mutations in regulatory genes are associated with AML subtypes and prognosis. Recently, gene expression profiling (GEP) has been applied to further risk stratify AML. Here, we show that mouse leukemia genes identified by retroviral insertion mutagenesis are more frequently differentially expressed in distinct subclasses of adult and pediatric AML than randomly selected genes or genes located more distantly from a virus integration site. The candidate proto-oncogenes showing discriminative expression in primary AML could be placed in regulatory networks mainly involved in signal transduction and transcriptional control. Our data support the validity of retroviral insertion mutagenesis in mice for human disease and indicate that combining these murine screens for potential proto-oncogenes with GEP in human AML may help to identify critical disease genes and novel pathogenetic networks in leukemia.

Oncogene activation in myeloid leukemias by Graffi murine leukemia virus proviral integration

The Graffi murine leukemia virus (MuLV) is a nondefective retrovirus that induces granulocytic leukemia in BALB/c and NFS mice. To identify genes involved in Graffi MuLV-induced granulocytic leukemia, tumor cell DNAs were examined for genetic alterations at loci described as common proviral integration sites in MuLV-induced myeloid, lymphoid, and erythroid leukemias. Southern blot analysis revealed rearrangements in c-myc, Fli-1, Pim-1, and Spi-1/PU.1 genes in 20, 10, 3.3, and 3.3% of the tumors tested, respectively. These results demonstrate for the first time the involvement of those genes in granulocytic leukemia.