Anoxia-inducible rat VL30 elements and their relationship to ras-containing sarcoma viruses

Abstract

VL30 elements are associated with cancer by their overexpression in rodent malignancies, their induction in a fibroblast response to anoxia which shares features with the malignant phenotype, and their presence recombined into Harvey murine sarcoma virus (HaSV) and Kirsten murine sarcoma virus. These sarcoma viruses contain ras oncogenes flanked on both sides by retrotransposon VL30 element sequences, in turn flanked by mouse leukemia virus sequences. Three very basic questions have existed about the VL30 element sequences found in sarcoma viruses: (i) how did they become recombined, (ii) what are their exact boundaries, and (iii) why are they there? To help decipher the nature of VL30 elements in sarcoma viruses, we examined VL30 clones isolated from an anoxic fibroblast cDNA library and independently by polymerase chain reaction cloning from rat cell DNA. Sequence comparisons with HaSV revealed that HaSV was formed by the substitution of 0.7 kb of VL30 sequences by 0.9 kb of c-Ha-ras sequences, with this event possibly facilitated by the presence of an identical Alu-like repeat found upstream of the 5' recombination point in both the VL30 element and c-Ha-ras. Recombination occurred 42 bases beyond the Alu-like sequences in VL30 and 1596 bases beyond them in c-Ha-ras, at position 926 of HaSV. The 3' ras-VL30 recombination event in HaSV occurred within a seven-base region of shared sequence identity, between HaSV bases 1825 and 1825 and 1831. Recombination between Moloney leukemia virus (MoLV) and VL30 appears to have occurred at a point corresponding to base 218 or 219 of MoLV and was near a TAR-like VL30 sequence; such recombination at the 3' end was between positions 7445 and 7456 of MoLV (HaSV positions 4694 to 4703). Kirsten murine sarcoma virus was found to be closely analogous to HaSV, and limited similar features were also seen with Rasheed sarcoma virus.

Genetic Analysis of the Rat Leukemia Virus: Influence of Viral Sequences in Transduction of the c- ras Proto-Oncogene and Expression of Its Transforming Activity

“…A role for VL30 elements in the formation of both the Ha-MSV and Ki-MSV genomes has been suggested since their v-ras genes are bracketed by VL30 sequences (7,14,32). Prior to the availability of DNA sequence data for both the RaLV and RaSV genomes, it had been speculated that the VL30 elements play a role in the transduction of c-ras by RaLV (14,32). Our data clearly demonstrate that the putative VL30like elements previously noted in the RaSV genome (EMBL accession no.…”

“…The precise mechanisms involved in the generation of highly oncogenic viruses are not well understood. However, based on nucleotide sequence comparisons of the genomes of Ha-MSV, Ki-MSV, and the malignant histocytosis sarcoma virus of the mouse, it has been proposed that virus-like 30S RNA (VL30) elements that flank the 5Ј and 3Ј ends of normal c-ras protooncogenes of both the rat and mouse genomes are responsible for the recombination and transduction of ras oncogenes by the respective helper MuLV strains associated with these sarcoma viruses (14,32). The presence of VL30 sequences in Ha-MSV, Ki-MSV, and malignant histocytosis sarcoma virus of the mouse has also been considered necessary for the expression of transforming activities by these viruses (14).…”

A splicing enhancer in the 3'-terminal c-H-ras exon influences mRNA abundance and transforming activity

“…The mouse sequence includes 6 nt at its end that are the same in the transducing virus (21) and the hamster sequence (ACCAAG), suggesting this as the 3Ј recombination region between NS.C58-ras and the transducing virus, but it is unknown if the cellular mouse sequence at these positions is the same. For Ha-MSV, the 3Ј recombination point was probably within a heptanucleotide (AGGGAGG, positions 189 to 195) shared by rat c-H-ras and the transducing virus (20,43). At the end of the sequences, the rat poly(A) addition site was inferred from similarity to the human sequence.…”

“…H-ras is one of the classical oncogenes first identified as a foreign sequence with oncogenic activity in acutely transforming retroviruses (for reviews, see references 4, 37, and 76). The viral H-ras (v-H-ras) gene of the Harvey murine sarcoma virus (Ha-MSV) differs from its normal cellular counterpart, the rat H-ras proto-oncogene or rat c-H-ras, by slight truncation of the 5Ј-and 3Ј-untranslated regions, the absence of introns, and oncogenic mutations at codons 12 and 59 (7,12,20,62). Steps involved in transfer of the cellular gene into the viral genome almost certainly included complex recombination/integration events upstream of the c-H-ras coding sequence in infected rat cells to bring c-H-ras transcription under the control of the 5Ј long terminal repeat (5ЈLTR) of the parental murine leukemia virus (MLV), splicing to generate chimeric RNAs with the 5Ј virus-cell junction, and template switching during reverse transcription between an MLV genome and chimeric RNA to produce the 3Ј junction (20,43,72,85).…”

Dynamic and selective HERV RNA expression in neuroblastoma cells subjected to variation in oxygen tension and demethylation

“…In rodents (in vivo and in vitro), ERV RNA expression can be selectively modified by certain chemical and physical treatments (15,(25)(26)(27)(28)(29). RNA expression of the mouse ERV VL30 was induced by treatment with anoxia or phorbol esters and diacylglycerol (25,26,28,(30)(31)(32). RNA expression of syncytin-1, the envelope gene of HERVW at chromosome 7q21, was downregulated in pre-eclampsia, a condition leading to fetal hypoxia (33).…”