Moloney murine sarcoma virus MuSVts110 DNA: cloning, nucleotide sequence, and gene expression

Adult human sarcomas. I. Basic science

When connective tissue undergoes malignant transformation, glioblastomas and sarcomas arise. However, the ancient biochemical mechanisms, which are now operational in sarcomas distorted by mutations and gene fusions in misaligned chromosomes, were originally acquired by those cells that emerged during the Cambrian explosion. Preserved throughout evolution up to the genus Homo, these mechanisms dictate the apoptosis- and senescence-resistant immortality of malignant cells. A 'retroviral paradox' distinguishes human sarcomas from those of the animal world. In contrast to the retrovirally induced sarcomatous transformation of animal (avian, murine, feline and simian) cells, human sarcomas have so far failed to yield a causative retroviral isolate. However, the proto-oncogenes/oncogenes transduced from their host cells by retroviruses of animals are the same that are active in human sarcomas. Since the encoded oncoproteins arise after birth, they are recognized frequently by the immune system of the host. Immune lymphocytes that kill autologous sarcoma cells in vitro commonly fail to do so in vivo. Sarcoma vaccines generate immune T- and natural killer cell reactions; even when vaccinated patients do not show a clinical response, their tumors become more sensitive to chemotherapy. The aim of this review is to lay a solid molecular biological foundation for the conclusion that targeting the sarcoma oncogenes will result in regression of the disease.

R7, a spontaneous mutant of Moloney murine sarcoma virus 124 with three direct repeats and an in-frame truncated gag-mos gene, induces brain lesions

We have isolated Recombinant 7 (R7), a spontaneous mutant of SV7, a molecular clone of MoMuSV124. Like SV7, R7 induces subcutaneous fibrosarcomas, spleen tumors, and mesentery tumors infiltrated by proliferating vessels lined by transformed endothelial cells. However, it also induces brain lesions. We have molecularly cloned and sequenced the R7 proviral DNA and shown that the R7 genome consists of 3401 bp. It has three direct repeats in each enhancer. Its coding sequence consists of only 176 bp of p15, 263 bp of p30, a 7-bp insertion, and 853 bp of an N-terminally truncated mos gene. From the sequence of R7 we have deduced that the truncated mos sequence is in-frame with all of the gag sequence and the 7-bp insertion. The incorporation of the 3' end of the p15 sequence further suggests that the R7 Gag-Mos is myristylated. We have also shown that the molecularly cloned R7 virus transformed NIH/3T3 fibroblasts about sevenfold better than the parental SV7. We have also confirmed that molecularly cloned R7 induces the same disease phenotype as that induced by the nonmolecularly cloned R7.

The Moloney murine sarcoma virus ts110 5' splice site signal contributes to the regulation of splicing efficiency and thermosensitivity

The 5' splice site signal (5'ss) in Moloney murine sarcoma virus ts110 (MuSVts110) RNA was found to participate in the regulation of its splicing phenotype. This 5'ss (CAG/GUAGGA) departs from the mammalian consensus (CAG/GURAGU) at positions +4 and +6, both of which base pair with U1 and U6 small nuclear RNAs during splicing. A doubling in splicing efficiency and near elimination of the splicing thermosensitivity characteristic of MuSVts110 were observed in 5'ss mutants containing a U at position +6 (termed 5' A6U), even in those in which U1-5'ss complementarity had been reduced. At the permissive temperature (28 degrees C), the 5' A6U mutation increased the efficiency of the second splicing reaction, while at the nonpermissive temperature (39 degrees C), both splicing reactions were positively affected.

Possible role of splice acceptor site in expression of unspliced gag-containing message of Moloney murine leukemia virus

Moloney murine leukemia virus (MLV) having the gag coding region alone, G3.6, produced a low level of mRNA (1/10 of the wild-type level). Ligation of 441 nucleotides (nt) containing a splice acceptor (SA) site to the downstream portion of the remaining gag region restored the level of the unspliced message, simultaneously activating a cryptic splice donor (SD) site in the middle of the p30 coding region (between nt 1596 and 1597). Ligation of the 441 nt in the same site in the inverted orientation also increased the level of the unspliced message, activating the same SD site (between nt 1596 and 1597) and a new SA site just in front of the inserted 441 nt (between nt 4770 and 4771). Deletion or inversion of the 441-nt SA sequence from the wild-type MLV or from int in-frame deletion or int frameshift mutant MLVs of nearly full size resulted in the loss of spliced mRNA and concomitantly in a severe reduction of the unspliced mRNA, particularly at 37 degrees C. Deletion of the 5' SD site did not result in the reduction of the unspliced-mRNA level. When the gag region in G3.6 was replaced with a Neo(r) coding region, the level of expression was high. The data taken together suggest that the presence of an SA signal is necessary for high-level expression of unspliced mRNA encoding Gag or Gag-Pol.

Branchpoint and polypyrimidine tract mutations mediating the loss and partial recovery of the Moloney murine sarcoma virus MuSVts110 thermosensitive splicing phenotype

Balanced splicing of retroviral RNAs is mediated by weak signals at the 3' splice site (ss) acting in concert with other cis elements. Moloney murine sarcoma virus MuSVts110 shows a similar balance between unspliced and spliced RNAs, differing only in that the splicing of its RNA is, in addition, growth temperature sensitive. We have generated N-nitroso-N-methylurea (NMU)-treated MuSVts110 revertants in which splicing was virtually complete at all temperatures and have investigated the molecular basis of this reversion on the assumption that the findings would reveal cis-acting elements controlling MuSVts110 splicing thermosensitivity. In a representative revertant (NMU-20), we found that complete splicing was conferred by a G-to-A substitution generating a consensus branchpoint (BP) signal (-CCCUGGC- to -CCCUGAC- [termed G(-25)A]) at -25 relative to the 3' ss. Weakening this BP to -CCCGAC- [G(-25)A,U(-27)C] moderately reduced splicing at the permissive temperature and sharply inhibited splicing at the originally nonpermissive temperature, arguing that MuSVts110 splicing thermosensitivity depends on a suboptimal BP-U2 small nuclear RNA interaction. This conclusion was supported by results indicating that lengthening the short MuSVts110 polypyrimidine tract and altering its uridine content doubled splicing efficiency at permissive temperatures and nearly abrogated splicing thermosensitivity. In vitro splicing experiments showed that MuSVts110 G(-25)A RNA intermediates were far more efficiently ligated than RNAs carrying the wild-type BP, the G(-25)A,U (-27)C BP, or the extended polypyrimidine tract. The efficiency of ligation in vitro roughly paralleled splicing efficiency in vivo [G(-25)A BP > extended polypyrimidine tract > G(-25)A,U(-27)C BP > wild-type BP]. These results suggest that MuSVts110 RNA splicing is balanced by cis elements similar to those operating in other retroviruses and, in addition, that its splicing thermosensitivity is a response to the presence of multiple suboptimal splicing signals.