Ter is a configuration of insertional mutagenesis most commonly found for lymphoma induction by MLVs. However, in the present case the LTR is located close to the promoter, which might explain the upregulation observed by the LTR3NS as well. For position 9, on the other hand, the cassette in sense orientation (LTR9NS) stimulated expression of Nras mRNA whereas the antisense orientation LTR9NAS reduced it. In case of LTR9NS we detected the normal Nras mRNA in which the LTR was excised as part of intron 1 as well as two types of LTR initiated mRNAs lacking exon 1 of Nras. The two LTR-initiated mRNAs corresponded to those observed in the original tumor 9 harboring a provirus at this position, indicating that the inserted solo-LTR functions similarly to the inserted provirus. LTR9NAS gave rise to RNA species initiated at several sites at the locus, including an antisense Licochalcone-A chemical information promoter in the LTR [8] as well as the normal Nras promoter and a cryptic promoter at the intron 3/exon 4 boundary of Nras. The enhancer of the inserted LTR activated transcription start sites in a window of about 250 bp at the cryptic promoter whereas the transcription start sites at the normal promoter are confined to a much smaller window irrespective of the presence or absence of the LTR cassette. Such enhancer activation of cryptic promoters has previously been reported to use scattered transcription start sites [14]. Endogenous retroviruses are known to be targets for epigenetic silencing of transcription in the early embryo and mouse retroviruses transferred to embryonic stem cells may be subject to such silencing mechanisms [15]. In humans, failure to sustain such silencing in adult tissues has been linked to LTR-driven expression of a neighboring gene as an oncogenic mechanism [6]. A major Z-360 site target for silencing of murine leukemia viruses such as Akv is overlapping with the primer binding site for proline tRNA [16], but there is also evidence of other determinants in the viral genome including the LTR [17]. In the present study the knock-in cassette contains only the LTR, and not the proline primer binding site. Only one of the knock-in alleles, LTR9NAS did result in reduced Nras expression. However, this reduction was only 2? fold and therefore relatively minor compared to the strong repression observed for some endogenous retrovirus. Whether this reduction involves epigenetic mechanisms or altered promoter configurations is not 18325633 clear. We note, however, that Cre-mediated excision of the PGK/neo cassette, previously found to downregulate gene expression [11] causes upregulation of Nras mRNA relative to wt, suggesting that the LTR does not contribute to the reduction of expression of Nras mRNA in LTR9NAS. Moreover, a cryptic promoter further downstream in Nras is induced in LTR9NAS as well as in LTR9AS indicating that the LTR does not cause a general reduction in transcriptional activity of the target locus. The results therefore indicate that the downregulation observed in LTR9AS is unrelated to epigenetic repression targeted to the LTR. In conclusion, we have shown that a gammaretroviral LTR inserted into the mouse germ line is transcriptionally active and mimics a number of features of retroviral insertional mutagenesisLTR-Mediated Nras DeregulationFigure 5. Nras expression in knock-in animals with and without the neomycin selection marker. Nras expression was quantified by qPCR employing two different methods, SYBR green (amplicon covering part of exon 2 and 3) or.Ter is a configuration of insertional mutagenesis most commonly found for lymphoma induction by MLVs. However, in the present case the LTR is located close to the promoter, which might explain the upregulation observed by the LTR3NS as well. For position 9, on the other hand, the cassette in sense orientation (LTR9NS) stimulated expression of Nras mRNA whereas the antisense orientation LTR9NAS reduced it. In case of LTR9NS we detected the normal Nras mRNA in which the LTR was excised as part of intron 1 as well as two types of LTR initiated mRNAs lacking exon 1 of Nras. The two LTR-initiated mRNAs corresponded to those observed in the original tumor 9 harboring a provirus at this position, indicating that the inserted solo-LTR functions similarly to the inserted provirus. LTR9NAS gave rise to RNA species initiated at several sites at the locus, including an antisense promoter in the LTR [8] as well as the normal Nras promoter and a cryptic promoter at the intron 3/exon 4 boundary of Nras. The enhancer of the inserted LTR activated transcription start sites in a window of about 250 bp at the cryptic promoter whereas the transcription start sites at the normal promoter are confined to a much smaller window irrespective of the presence or absence of the LTR cassette. Such enhancer activation of cryptic promoters has previously been reported to use scattered transcription start sites [14]. Endogenous retroviruses are known to be targets for epigenetic silencing of transcription in the early embryo and mouse retroviruses transferred to embryonic stem cells may be subject to such silencing mechanisms [15]. In humans, failure to sustain such silencing in adult tissues has been linked to LTR-driven expression of a neighboring gene as an oncogenic mechanism [6]. A major target for silencing of murine leukemia viruses such as Akv is overlapping with the primer binding site for proline tRNA [16], but there is also evidence of other determinants in the viral genome including the LTR [17]. In the present study the knock-in cassette contains only the LTR, and not the proline primer binding site. Only one of the knock-in alleles, LTR9NAS did result in reduced Nras expression. However, this reduction was only 2? fold and therefore relatively minor compared to the strong repression observed for some endogenous retrovirus. Whether this reduction involves epigenetic mechanisms or altered promoter configurations is not 18325633 clear. We note, however, that Cre-mediated excision of the PGK/neo cassette, previously found to downregulate gene expression [11] causes upregulation of Nras mRNA relative to wt, suggesting that the LTR does not contribute to the reduction of expression of Nras mRNA in LTR9NAS. Moreover, a cryptic promoter further downstream in Nras is induced in LTR9NAS as well as in LTR9AS indicating that the LTR does not cause a general reduction in transcriptional activity of the target locus. The results therefore indicate that the downregulation observed in LTR9AS is unrelated to epigenetic repression targeted to the LTR. In conclusion, we have shown that a gammaretroviral LTR inserted into the mouse germ line is transcriptionally active and mimics a number of features of retroviral insertional mutagenesisLTR-Mediated Nras DeregulationFigure 5. Nras expression in knock-in animals with and without the neomycin selection marker. Nras expression was quantified by qPCR employing two different methods, SYBR green (amplicon covering part of exon 2 and 3) or.
