Infection of developing mouse embryos with murine leukemia virus: tissue specificity and genetic transmission of the virus

The tissue specificity of Moloney leukemia virus (M-MuLV) was studied by infecting mice at two different stages of development. Either newborn mice which can be considered as essentially fully differentiated animals were infected with M-MuLV or preimplantation mouse embryos were infected in vitro at the 4-8 cell stage, a stage of development before any differentiation has taken place. After surgical transfer to the uteri of pseudopregnant surrogate mothers, the latter developed to term and adult mice. In both cases, animals were obtained that had developed an M-MuLV induced leukemia. Molecular hybridization tests for the presence of M-MuLV-specific sequences were conducted on DNA extracted from different tissues of leukemic animals to determine which tissues were successfully infected by the virus. Mice which were infected as newborns carried M-MuLV-specific DNA sequences in "target tissues" only, i. e., thymus, spleen, lymph nodes or in organs infiltrated by tumor cells, whereas "non-target tissues" did not carry virus-specific sequences. In contrast, when leukemic animals derived from M-MuLV-infected preimplantation embryos were analyzed, virus-specific sequences were detected in target tissues as well as in non-target tissues, such as liver, kidney, brain, testes and the germ line. To study the expression of the viral DNA integrated in target and non-target organs, RNA was extracted from different tissues of an animal infected at the preimplantation stage. Fifty to 100 times more M-MuLV-specific RNA was detected in tumor tissues than was found in non-target organs. Since all organs contained the same amount of virus-specific DNA, these results indicate that the integrated virus genome can be differentially expressed in different tissues. The organ-tropism of RNA tumor viruses is discussed in view of these findings. Mice that were infected at the preimplantation stage were found to have M-MuLV integrated into their germ line. Virus transmission from the father to the offspring occurred according to simple Mendelian expectations. Molecular hybridization tests revealed that in the animals studied, the virus was integrated into the germ line at only one out of two or three possible integration sites. During the development of leukemia amplification of this virus copy was observed in the target tissues only, but not in the non-target tissues.

Infection of preimplantation mouse embryos and of newborn mice with leukemia virus: tissue distribution of viral DNA and RNA and leukemogenesis in the adult animal

Explanted mouse embryos derived from low leukemia incidence strains were infected with Moloney murine leukemia virus (M-MuLV) at the 4-8 cell stage of development. After cultivation in vitro to the blastocyst stage, the embryos were surgically transferred to the uteri of pseudo-pregnant surrogate mothers. Of 15 animals born, one developed a leukemia at 8 weeks of age. When autopsied, this leukemia was found to be of the lymphatic type, as is typical for the M-MuLV-induced disease. In addition, infectious M-MuLV virus was isolated from the serum. Molecular hybridization tests for the presence of M-MuLV-specific sequences were conducted on DNA and RNA extracted from eight different organs. The DNA-DNA reannealing experiments revealed the presence of two classes of M-MuLV-specific sequences in equal concentrations in all tissues tested. The less abundant class of M-MuLV-specific sequences was not detected in tissues from uninfected animals or in non-target tissues of leukemic animals infected at birth. The results are consistent with the working hypothesis that the virus was integrated in all cells of the animal, possibly including the germ line. Fifty to 100 times more M-MuLV-specific RNA was detected in tumor tissues than was found in non-target organs such as liver, brain, and testes. Since all organs contained the same amount of virus-specific DNA, these results indicate that the M-MuLV-specific DNA can be differentially expressed in different tissues.

RNA metabolism of murine leukemia virus II. Endogenous virus-specific RNA in the uninfected BALB/c cell line JLS-V9

Type C virus-specific RNA sequences of BALB/c endogenous virus were detected in JLS-V9 cells (an uninfected BALB/c derived line) by annealing cell RNA with 3-H-labeled virus-specific DNA. Endogenous viruses used in preparing the 3-H-labeled DNA (mostly xenotropic) was prepared from JLS-V9 cells induced to produce virus with iododeoxyuridine. In whole-cell extracts, two virus-specific RNA species, 38S and 27S, were detected. No 60 to 70S virus-specific RNA was found. The same two species of virus-specific RNA were observed in isolated cytoplasmic RNA and in cytoplasmic RNA selected for polyadenylic acid-containing species by binding and elution from oligo(dT) cellulose. Very little, if any, of the virus-specific RNA was active as messenger RNA on polyribosomes. No virus-specific RNA transcribed from genes coding for the BALB/c endogenous N-tropic virus was detected, since 3-H-labeled DNA prepared from endogenous N-tropic virus did not hybridize measurably with JLS-V9 RNA.

Measurement of the sequence complexity of cloned Moloney murine leukemia virus 60 to 70S RNA: evidence for a haploid genome

The sequence complexity of the 60-70S RNA complex from Moloney murine leukemia virus (M-MuLV) was determined by measuring the annealing rate of radioactively labeled virus-specific DNA with M-MuLV 60-70S RNA in conditions of vast RNA excess. The M-MuLV RNA annealing rate, characterized by the quantity C(r)t((1/2)), was compared with the C(r)t((1/2)) values for annealing of poliovirus 35S RNA (2.6 x 10(6) molecular weight) with poliovirus-specific DNA and Sindbis virus 42S RNA (4.3 x 10(6) molecular weight) with Sindbis-specific DNA. M-MuLV-specific DNA was prepared in vitro by the endogenous DNA polymerase reaction of M-MuLV virions, and poliovirus and Sindbis virus DNAs were prepared by incubation of viral RNA and DNA polymerase purified from avian myeloblastosis virus and an oligo deoxynucleotide primer. The poliovirus and Sindbis virus DNAs were sedimented through alkaline sucrose gradients, and those portions of the DNA with sizes similar to the M-MuLV DNA were selected out for the annealing measurements. M-MuLV was cloned on NIH-3T3 cells because it appeared possible that the standard source of M-MuLV for these experiments was a mixture of viruses. The annealing measurements indicated a sequence complexity of approximately 9 x 10(6) daltons for the cloned M-MuLV 60-70S RNA when standardized to poliovirus and Sindbis virus RNAs. This value supports the hypothesis that each of the 35S RNA subunits of M-MuLV 60-70S RNA has a different base sequence.

Hamster leukemia virus: lack of endogenous DNA synthesis and unique structure of its DNA polymerase

Infectious hamster leukemia virus (HaLV) contains a DNA polymerase different from those of murine and avian viruses. No endogenous reaction directed by the 60 to 70S RNA of HaLV could be demonstrated in detergenttreated HaLV virions, nor could the purified DNA polymerase copy added viral RNA. The virion RNA could, however, act as template for added avian myeloblastosis virus DNA polymerase and the HaLV DNA polymerase could efficiently utilize homopolymers as templates. The HaLV enzyme was like other reverse transcriptases in that certain ribohomopolymers were much better templates than the homologous deoxyribohomopolymers. No ribonuclease H activity could be shown in the HaLV enzyme, but neither could activity be found in the murine leukemia virus DNA polymerase. The hamster enzyme was unique in that poly(A) .oligo(dT) was a poor template, and globin mRNA primed with oligo(dT) was totally inactive as a template. Its uniqueness was also indicated by its subunit composition; electrophoresis of the HaLV DNA polymerase in sodium dodecyl sulfate-containing polyacrylamide gels revealed equimolar amounts of two polypeptides of molecular weight 68,000 and 53,000. The sedimentation rate of the enzyme in glycerol gradients was consistent with a structure containing one each of the two polypeptides. The enzyme thus appears to be structurally distinct from other known virion DNA polymerases. Its inability to carry out an endogenous reaction in vitro might result from an inability to utilize certain primers.

Mechanism of induction of RNA tumor viruses by halogenated pyrimidines

Frome these studies on JLS V-9 cells, a number of conclusions can be drawn about the mechanism of MuLV induction by halogenated pyrimidines. The compounds can induce virus from otherwise healthy cells as long as deoxycytidine is present along with the inducing agent. The compounds must be present during the S phase of the cell cycle and must be incorporated into DNA in order to induce virus (Teich et al. 1973). Only one strand of DNA need be substituted by BrdU or IdU in order to induce virus, because a one-hour period of incorporation leads to induction. From these results it is possible to construct a model for how halogenated pyrimidines are able to induce viruses from otherwise uninfected cells. Because the critical period for the incorporation of the compound is a restricted segment of the S phase of the cell, there would appear to be a critical segment of the genetic information of the cell which, when substituted with BrdU or IdU, leads to a transcriptional derepression. Presumably the critical segment of DNA is either a controlling element of the integrated provirus or it is a separate gene which controls the expression of the integrated provirus. Whichever is true, these results strongly imply that the search for specific repressors of the segments of mammalian DNA is likely to be successful and that RNA tumor viruses may offer a system in which such repression systems can be identified and investigated.

Mitochondrial RNA synthesis during mitosis

HeLa cells arrested in metaphase synthesized relatively normal amounts of mitochondrial RNA, while little RNA synthesis associated with the nucleus was detected. The RNA synthesized resembled the portion of mitochondrial RNA sensitive to ethidium bromide in interphase cells, with major peaks at 21, 12, and 4S. Unlike that in interphase cells, RNA synthesis in the mitoclhonidrial fraction of mitotic cells was completely inhibited by ethidium bromide.