Genome-Based Identification of Cancer Genes by Proviral Tagging in Mouse Retrovirus-Induced T-Cell Lymphomas

Abstract

The identification of tumor-inducing genes is a driving force for elucidating the molecular mechanisms underlying cancer. Many retroviruses induce tumors by insertion of viral DNA adjacent to cellular oncogenes, resulting in altered expression and/or structure of the encoded proteins. The availability of the mouse genome sequence now allows analysis of retroviral common integration sites in murine tumors to be used as a genetic screen for identification of large numbers of candidate cancer genes. By positioning the sequences of inverse PCR-amplified, virus-host junction fragments within the mouse genome, 19 target genes were identified in T-cell lymphomas induced by the retrovirus SL3-3. The candidate cancer genes included transcription factors (Fos, Gfi1, Lef1, Myb, Myc, Runx3, and Sox3), all three D cyclins, Ras signaling pathway components (Rras2/ TC21 and Rasgrp1), and Cmkbr7/CCR7. The most frequent target was Rras2. Insertions as far as 57 kb away from the transcribed portion were associated with substantially increased transcription of Rras2, and no coding sequence mutations, including those typically involved in Ras activation, were detected. These studies demonstrate the power of genome-based analysis of retroviral insertion sites for cancer gene discovery, identify several new genes worth examining for a role in human cancer, and implicate the pathways in which those genes act in lymphomagenesis. They also provide strong genetic evidence that overexpression of unmutated Rras2 contributes to tumorigenesis, thus suggesting that it may also do so if it is inappropriately expressed in human tumors.

Dysregulated expression of mitotic regulators is associated with B-cell lymphomagenesis in HOX11-transgenic mice

“…In contrast to other studies using the MMLV or in the AKXD or BXH2 inbred mouse lines to accelerate pre-T and pre-B lymphomas or myeloid leukemias (Li et al, 1999;Hwang et al, 2002;Kim et al, 2003;Erkeland et al, 2004;Iwasaki et al, 2004;Shin et al, 2004), this approach exploited the preferential tropism of the mAIDS virus for mature B lymphocytes and successfully accelerated cancer progression in IgHm-HOX11 Tg mice. The relatively low number of integrations observed (B1.9 per tumor) is consistent with reports that the Du5H mAIDS virus yields fewer (between 2 and 4) integration events when compared to MMLV (between 6 and 10) or with other models of recombinant inbred mouse strains characterized by laterally transmitted retroviruses (Huang et al, 1995).…”

“…Retroviral-derived DNA sequences that integrate adjacent to cellular oncogenes or within coding regions of tumor suppressors, can result in altered host gene expression contributing, ultimately, to clonal expansion of the infected cell. While the proviral tagging approach has resulted in the discovery of many genes involved in lymphoid and myeloid leukemias (Li et al, 1999;Hwang et al, 2002;Kim et al, 2003;Erkeland et al, 2004;Iwasaki et al, 2004;Shin et al, 2004), less work has been conducted on mature B-cell lymphomas.…”

MicroRNAs and genomic instability

“…14) are found in close proximity to miRNA clusters. Gpc5 is located 10.1 kb proximal to the mmu-miR-17 cluster; disruption in the 5′ region of Gpc5 results in T cell lymphomas [78,82]. Integration into a region 5′ of Gpc3 also generates T cell lymphomas [78,82].…”

Deregulated expression of RasGRP1 initiates thymic lymphomagenesis independently of T-cell receptors

“…RasGRP1 is highly expressed in most cases of human acute T lymphocytic leukemias (Yeoh et al, 2002), which originate predominantly from transformed thymocytes (Asnafi et al, 2002). More direct evidence indicating that RasGRP1 has the potential to act as a dominant oncogene in T cells has been obtained from analyses of murine lymphomas which arose as a consequence of retroviral insertional mutagenesis (Li et al, 1999;Kim et al, 2003). Of 48 T-cell lymphomas occurring after infection of neonatal mice with the SL3 retrovirus, five had proviral insertions near the RasGRP1 gene (Kim et al, 2003).…”

“…More direct evidence indicating that RasGRP1 has the potential to act as a dominant oncogene in T cells has been obtained from analyses of murine lymphomas which arose as a consequence of retroviral insertional mutagenesis (Li et al, 1999;Kim et al, 2003). Of 48 T-cell lymphomas occurring after infection of neonatal mice with the SL3 retrovirus, five had proviral insertions near the RasGRP1 gene (Kim et al, 2003). Although these lymphomas have not been characterized in terms of cell phenotypes or RasGRP1 expression, it is expected that they arose due to the imposition of the proviral pattern of expression on the adjacent RasGRP1 gene.…”