Retroviral transduction of the human c-Ha-ras-1 oncogene into midgestation mouse embryos promotes rapid epithelial hyperplasia

Mutant RAS inhibits neutrophil but not macrophage differentiation and allows continued growth of neutrophil precursors

Mutational activation of RAS is the most common molecular abnormality in myeloid leukemias. In order to better understand its role in leukemogenesis, we have devised a model based on the multipotent cell line, FDCP-mix. We show that expression of mutant RAS in FDCP-mix strongly inhibits terminal neutrophil differentiation under the influence of G-CSF plus GM-CSF at the metamyelocyte stage, whereas macrophage differentiation was unaffected. In addition, whereas control cultures differentiated and became postmitotic under these conditions, FDCP-mix cells expressing mutant RAS continued to proliferate indefinitely while maintaining a metamyelocytic phenotype. Labeling of these cultures with the fluorescent tracking dye, PKH26, showed that this extended proliferative capacity resulted from continued division of metamyelocytes in the culture. Dissection of the growth factor response of these cells demonstrated that GM-CSF was critical in maintaining proliferation and inhibiting the differentiation of these cells. We further show the block in neutrophil differentiation could be partially overcome by treatment with low-dose Ara C, suggesting that maintenance of cell cycle progression may be partly responsible for the anti-differentiation effect of this oncogene. These findings suggest that activation of RAS is able to specifically inhibit terminal neutrophil differentiation and in so doing promotes continued division of metamyelocyte cells.

Oncogenic RAS genes impair erythroid differentiation of erythroleukaemia cells

RAS mutations occur frequently in acute myeloid leukaemia and myelodysplasia, suggesting a functional role for this oncogene in leukaemogenesis. We show here, for the first time, that both N-RAS and H-RAS can impair erythroid differentiation of erythroleukaemia cells induced with hexamethylene bisacetamide. Transformation by RAS allowed extended proliferation in the presence of inducer and also inhibited maturation as measured by impaired haemoglobinization and reduction in cell size. These data provide an interesting counterpoint to the effect of mutant RAS on monocytic cells, where it has a potentiating effect on differentiation and may indicate a causal link between the activation of RAS and erythroid lineage dysplasia in preleukaemia.

Recovery of a rare clone from a population of unstable retroviral vector-expressing mammalian cells using a new RNA extraction and slot-blot protocol

Although a useful and important method of gene transfer, retroviral vectors can be genetically unstable. In the course of experiments using DOEJS, a retroviral vector able to confer expression of a H-ras oncogene and a neomycin resistance gene (neo) on mammalian cells (Compere et al., 1989), it was found that the vast majority of infected rat embryo fibroblasts, recovered on the basis of neo activity (i.e., G418 resistance), did not express ras mRNA. It was subsequently observed that most cells in the psi 2 cell line used to propagate DOEJS failed to produce virus capable of expressing both ras and neo in primary rat embryo fibroblasts. A simplified RNA extraction and slot-blot technique was developed to screen mRNA from several hundred fibroblast clones and, in doing so, infected fibroblast clones producing both neo and ras mRNA were identified at low frequency. The DOEJS/psi 2 packaging line was subsequently subcloned and individual clones screened for their ability to confer appropriate gene expression on target cells. Subclone DOEJS/psi 2-B6 was eventually isolated after screening 24 DOEJS subclones and 240 infected rat embryo fibroblast colonies. DOEJS/psi 2-B6 was shown to induce reliably phenotypic transformation, G418 resistance, and ras and neo mRNA expression in primary rat embryo fibroblasts. The RNA extraction and screening procedure was thus useful for recovering an infrequent subclone producing a retrovirus with the original properties.

Distribution of p21ras in postimplantation rat embryos

Expression of ras cellular oncogenes during the early postimplantation period in the rat was investigated using immunohistochemistry to p21ras. A broad spectrum polyclonal antibody recognizing N-, Ha- and Ki- forms of p21ras was used in an indirect avidin-biotin-peroxidase (ABC) technique. Positive staining indicating the presence of p21ras was found in embryos from 6.5 to 12 days embryonic age. In early egg cylinders (6.5 days), positive staining for p21ras was observed on the ectoplacental cone, primitive ectoderm and trophectoderm, while primitive endoderm and parietal endoderm appeared paler. In later egg cylinder stages (7.5 days), strong positive staining was observed in the primitive embryonic ectoderm and ectoplacental cone, but parietal and visceral endoderm still appeared to be devoid of positive staining. As development proceeded during primitive streak stages, the visceral and parietal endoderm became positively stained. By 10 days, all tissues appeared to be positive for p21ras, with strong staining appearing in the heart and neural elements. Therefore, p21ras does not appear to be ubiquitous in the rat conceptus prior to gastrulation, but shows differential distribution, appearing later in endodermal derivatives. Possibly p21ras is involved in determination of the ectodermal and endodermal lineages.

Stepwise transformation of primary thyroid epithelial cells by a mutant Ha-ras oncogene: an in vitro model of tumor progression

Activating mutations of the ras oncogene family occur at high frequency in all stages of thyroid tumorigenesis, both human and experimental. To test the causal nature of this association, and to investigate the biological role of ras mutation, we introduced a mutant c-Ha-ras gene into normal rat thyroid follicular cells using an ecotropic retroviral vector. The major immediate effect was to greatly extend the proliferative lifespan of these cells in culture from less than 3 to more than 15 doublings, without any observable loss of growth-factor dependence or differentiated functions. This in vitro phenotype strongly supports an initiating role for ras mutation in the genesis of benign thyroid tumors (adenomas) in vivo. Spontaneous transformation was observed at low frequency on continuous culture of mutant ras-expressing cells, giving rise to fully immortalized, growth factor-independent, highly tumorigenic lines. Transformation was associated with (i) loss of responsiveness to the growth inhibitor TGF-beta 1, and (ii) greatly increased nuclear levels of p53 protein, which unexpectedly was not due to point mutation in the conserved regions of the p53-coding sequence. We postulate that these two phenomena are causally related to each other and to the transformed phenotype.

Effect of serum growth factors and phorbol ester on growth and survival of human thyroid epithelial cells expressing mutant ras

We previously demonstrated a high incidence of ras mutation in thyroid follicular (epithelial) cell neoplasms and showed that expression of mutant ras is a potent mitogenic stimulus for normal human follicular cells in culture. Here we show that induction of cell proliferation in primary follicular cells by a mutant human Ha-ras (val 12) expressed from a retroviral vector was absolutely dependent on the presence of serum growth factors. Induction of DNA synthesis showed partial dependence. Mutant ras-induced growth was also inhibited by exposure to phorbol ester at concentrations sufficient to downregulate protein kinase C. More importantly, we observed an unexpected toxic effect of phorbol ester in this system that was specific to cells expressing mutant ras. This has potential significance both for elucidating the basic mechanism of ras action in epithelial cells and also as a pointer to a novel therapeutic strategy.

Role of abortive retroviral infection of neurons in spongiform CNS degeneration

Retroviruses are involved in several human neurological diseases with varying pathological features. Whether these diseases are due to a direct effect of the virus on nervous system cells is unknown. To gain insight into the pathogenesis of one retroviral neurological disease, we are studying the murine neurotropic retrovirus, Cas-Br-E, which causes lower motor neuron disease associated with spongiform degenerative changes in brain and spinal cord. Central nervous system (CNS) injury seems to be due to direct viral action, but the precise target cells of the virus are uncertain. After blood-borne virus enters the CNS it is found in capillary endothelial cells. No microscopic evidence for virus within glia or neurons has been found in some studies, whereas virus or incomplete particles have been observed in CNS cells in other studies. Here we identify the neuron as a major target for Cas-Br-E in the CNS, suggesting that this disease may be a direct result of viral infection of neurons. We also show that envelope protein (Env, encoded by the env gene), a major determinant of neurovirulence, cannot be detected in neurons but is present in non-neuronal cells, although spliced env messenger RNA is synthesized in CNS tissue. This suggests that a post-transcriptional step in Cas-Br-E Env protein synthesis is impaired and that the neurological disease may be a consequence of abortive replication of virus in neurons. This may explain the failure to find neuronal infection in other neurological diseases by conventional methods of virus detection.