Oncogenes Of Retroviruses Research Articles

Role of the retinoblastoma gene in the oncogenesis of human breast carcinoma.

The notion that genetic alterations are involved in cancer has gained support from many studies. Certain genetic alterations in tumors have been precisely defined. First, the activated proto-oncogenes were isolated from tumors by their abilities to transform nonneoplastic cells in culture [1]. To date many genes potentially involved in cancer have been isolated based on the sequence homology to oncogenes of retroviruses [for review see 2]. In contrast to the activated oncogenes in tumor cells, a different class of cancer genes has been suggested by somatic cell hybrid studies. Fusion cells made between tumor cells and normal fibroblasts, lymphocytes, or kerati- nocytes are often nontumorigenic, an effect contributed by chromosomes [1–5]. A correlation exists between suppression of tumorigenicity and the retention of certain chromosomes in the fused cells [6]. For example, the introduction of chromosome 11, but not chromosome X nor 13, suppresses the tumorigenic phenotype of a Wilm’s tumor cell line, indicating the tumor suppressor gene for pediatric nephroblastoma is located on chromosome 11 [7]. These genes are actively expressed in normal cells and are mutationally inactivated in tumor cells. They are termed ’tumor suppressor genes’, since the presence of one or more normal alleles is thought to prevent the tumor phenotype [for review see 8, 9]. A complementary line of evidence comes from cytogenetic observations in which a specific chromosomal deletion occurs in somatic or tumor cells from patients with retinoblastoma, Wilm’s tumor, and bilateral acoustic neurofibromatosis [10–12]. This suggests the existence of a tumor suppressor gene in normal cells in specific chromosomal regions. Additional support for tumor suppressor genes is derived from the isolation of DNA fragments capable of reverting the transformed phenotypes of cultured cells [13]. A recent study led to the identification of the Krev—1 gene that converts the H-ras transformed cells into flat revertants upon transfection. Sequence analysis indicates that significant amino acid homology exists between Krev-1 and H-ras [14].

Analysis of c-raf oncogene expression in gastrointestinal tumor cells.

Oncogenes of retroviruses are responsible for the induction of tumors in animals and for the malignant transformation of cells in culture. They are derived from normal cellular proto-oncogenes. Comparison of differences between viral oncogenes and proto-oncogenes has indicated that they are never identical. In human tumors that are not induced by retroviruses, activated proto-oncogenes have been discovered which resemble the viral oncogenes in properties such as mutations, deletions, or amplified expression [1]. Activated oncogenes have been described in many different tumors or tumor cell lines [2]. Often, more than one oncogene seems to be activated, e.g., in HL60 promyelocyte leukemic cells myc, myb and mil/raf are amplified ([3] and unpublished observation). We are interested in the expression of the activated proto-oncogene c-mil/raf in human tumors. It is the homologue to the oncogene v-mil of the avian retrovirus MH2[4] While v-mil is expressed in virus- transformed cells as pl00gag-mil fusion protein, the cellular homologue exhibits a molecular weight of 74 kD, p74hu-c-raf [5] The v-mil protein is located predominantly in the cytoplasm and exhibits a virus-coded protein kinase activity in autophosphorylation reactions specific for the amino acids serine and threonine [6, 7]. It renders the cells independent of growth factors and therefore is involved in alteration of the signal transduction characteristic of many tumor cells.

Characterization of two new human oncogenes.

The first oncogenes discovered were the transforming genes of the oncogenic viruses (reviewed by Bishop 1985). The subsequent discovery that the oncogenes of retroviruses were derived from normal host cellular genes provided the first direct evidence that cellular genomes contain genes with transforming potential. More recently, the development of techniques for DNA transfer in eukaryotic cells led to the discovery of cellular transforming genes in tumor cells by their ability to induce foci of transformed NIH-3T3 cells (reviewed by Land et al. 1983). Several new oncogenes have been discovered this way, including N-ras (Shimizu et al. 1983), met (Cooper et al. 1984), neu (Bargmann et al. 1986), and possible others (Goubin et al. 1983; Lane et al. 1984; Takahashi et al. 1985).

Molecular cloning of the chromosomal breakpoint of B-cell lymphomas and leukemias with the t(11;14) chromosome translocation.

The chromosomal breakpoint of chronic lymphocytic leukemia (CLL) cells of the B-cell type carrying the translocated long arms of chromosomes 11 and 14 [t(11;14) (q13;q32)] was cloned. The breakpoint was found to be within the joining segment of the human heavy chain locus on the translocated long arm of chromosome 14. A probe that is specific for chromosome 11 and that maps immediately 5' to the breakpoint on the 14q+ chromosome was isolated. The probe detected a rearrangement of the homologous genomic DNA segment in the parental CLL cells and also in DNA from a diffuse large cell lymphoma with the t(11;14) translocation. This rearranged DNA segment was not present in Burkitt lymphoma cells with the t(8;14) translocation or in nonneoplastic human lymphoblastoid cells. The probe can thus be used to identify and characterize a gene located on band q13 of chromosome 11 that appears to be involved in the malignant transformation of human B cells carrying the t(11;14) translocation. This gene, named bcl -1, appears to be unrelated to any of the known retrovirus oncogenes described to date.

Nucleotide sequence of avian retroviral oncogene v-mil: homologue of murine retroviral oncogene v-raf.

Eukaryotic cells contain genes termed proto-oncogenes (c-onc) which have the potential to transform cells in culture and induce tumours in vivo. Most of these genes have been identified by their occasional incorporation into retroviral genomes which can act as natural transducing vectors for these and perhaps other cellular genes. Cell-derived oncogenes of retroviruses (v-onc) are associated mostly with the induction of mesenchymal tumours whereas carcinoma induction is rare. One of these rare carcinoma-inducing viruses is the acutely transforming avian retrovirus MH2 (refs 3-5). Recently we and others have shown that this virus carries a novel putative oncogene, v- mil , in addition to the known oncogene v-myc. While the transforming ability of v- mil has not been directly established, we have recently discovered by hybridization analysis that v- mil is homologous to v-raf (ref. 9), the transforming gene of the murine retrovirus 3611 MSV (ref. 10). Both viruses express the mil /raf oncogene product as a gag-fusion polyprotein, while the myc oncogene of MH2 is expressed via a subgenomic mRNA. Here we report the complete nucleotide sequence of v- mil and compare it with that of v-raf. The 80% homology between the nucleotide sequences and the 94% homology between the predicted amino acid sequences of the two viral genes clearly indicate that these are the avian and murine forms of the same gene. Comparison of the two sequences with that of the human cellular homologue (T. I. Bonner et al., manuscript in preparation) indicates that v-raf has more 3' untranslated sequences while v- mil has additional sequences from two 5' exons of the cellular homologue. Although the mil /raf amino acid sequences reveal partial homology to that of the v-src product, no tyrosine-specific protein kinase activity is detected for the gag- mil and the gag-raf hybrid proteins.

Evolution of oncogenes: From c-abl to v-abl

The discovery that the oncogenes of retroviruses are derived from cellular genes has led to a search for differences between the viral gene and its cellular counterpart that might explain the oncogenic activity of the former. Two oncogenes, the abl and the src genes, have recently come under such scrutiny. The src gene will be the subject of a later article: here, Jean Yin Jen Wang discusses the abl gene.

Cancer genes come of age.

In 1866, Paul Broca sketched the pedigree of his wife’s family and discerned a hereditary diathesis to cancer. The insight seems to have attracted little attention in Broca’s time (indeed, this child of Broca’s brain went unmentioned by Carl Sagan in our time). During the century that followed, however, biologists began to seek genetic explanations for tumorigenesis. Now the quest has reached fruition: the long-imagined “cancer genes” have been brought into view. They were unearthed first by the predilection of retroviruses to transduce cellular genes-it was the search for retroviral oncogenes in the DNA of normal cells that incarnated cancer genes (Bishop, Ann. Rev. Biothem., in press). Two further strategies have since added fuel to the fire: integration of retroviral DNA can finger cellular genes whose activation apparently initiates tumorigenesis (Varmus, Cancer Surv. 7, 30931 9, 1982); and DNA taken from a variety of tumors can convert rodent cells to neoplastic growth, as if the DNA contained an active oncogene-of the sort we were once accustomed to finding only in viruses (Weinberg, Adv. Cancer Res. 36, 149-163, 1982; Cooper, Science 2 7 7, 801-806, 1982). Oncogenes of retroviruses have shown us that dominant genetic loci can convert cells to neoplastic growth. The experimental simplicity offered by viral oncogenes may be misleading, since natural tumorigenesis is thought to arise from several discrete events within the emerging tumor cell. It is nevertheless from tumor viruses that we have gained our most promising glimpse of biochemical mechanisms that may lead to neoplastic growth. Eighteen different oncogenes have now been found in retroviruses, causing different tumors by a variety of mechanisms. The proteins encoded by these genes display a provocative diversity (Bishop, op. cit.): some are tyrosinespecific protein kinases located on the plasma membrane; some are nuclear proteins that may bind to DNA within chromatin; some are glycoproteins that linger inexplicably on intracellular membranes throughout their life span; some possess structural features and biochemical properties evocative of electrogenic nucleoside triphosphatases (Gay and Walker, Nature 307, 262-264, 1983); and some remain wholly uncharacterized, offering promise of even further diversity. What does this diversity connote? That there is more than one way to create a cancer cell, of course. But a greater truth may lie beyond. It seems likely that the growth and division of cells are regulated by an interdigitating network spanning from the surface of the cell to the heart of the nucleus. If that Minireviews

A transforming gene present in human sarcoma cell lines.

Morphological transformation of NIH/3T3 cells by transfection with DNA has been used to identify transforming sequences in human tumours. Transforming activity has been reported for DNAs isolated from bladder, mammary, colon and lung carcinomas, neuroblastoma, lymphoid and myeloid tumours. Each of these tissues seems to contain different transforming sequences except for the colon and lung tumours where the same sequence seems to be involved. We now report that in two different human sarcoma cell lines, a fibrosarcoma and an embryonal rhabdomyosarcoma, the DNAs have transforming activity. The transforming gene is the same in both sarcomas but differs from the activated sequences detected in other tumours. We have also found that the transforming gene has no detectable homology to eight retrovirus oncogenes tested.

Retroviruses and Cancer Genes

Publisher Summary The oncogenic potential of many animal viruses is now well established, and the use of tumor viruses now dominates efforts to dissect the mechanisms of tumorigenesis. Although several themes appear to unite oncogenesis by diverse viral agents, and each family of tumor viruses has important distinctive features, it is the retroviruses that have provided the most coherent and penetrating view of tumorigenesis. Three features of retroviruses account for this sentiment. First, the oncogenes of retroviruses have proved exceptionally accessible to definition and study, and as a consequence, they have provided first glimpse of enzymatic mechanisms responsible for neoplastic transformation. Second, the diversity of retrovirus oncogenes has provided a rich set of oncogenic agents whose versatility far exceeds that of DNA tumor virus oncogenes, and whose tumorigenic capacities provide separate experimental models for most major forms of malignancy. Finally, oncogenes appear not to be indigenous components of retrovirus genomes, but instead have been transduced from normal genetic loci of the vertebrate hosts in which retroviruses replicate.

Isolation and characterization of chicken DNA homologous to the two putative oncogenes of avian erythroblastosis virus

The genome of avian erythroblastosis virus contains two independently expressed genetic loci (v- erbA and v- erbB) whose activities are probably responsible for oncogenesis by the virus. Both loci are closely related to nucleotide sequences found in the DNA and RNA of chickens and other vertebrates. We have isolated and characterized chicken DNA homologous to v- erbA and v- erbB. The two viral genes are represented by separate domains within chicken DNA ( c-erbA and c-erbB), which are separated by a minimum of 12 kilobases (kb) of DNA and may not be linked at all. The nucleotide sequences shared by the viral and cellular erb loci are colinear, but the cellular loci are interrupted by multiple intervening sequences of various lengths. Polyribosomes prepared from normal chicken embryos contain two polyadenylated RNAs transcribed from c-erbA and two transcribed from c-erbB. The evident coding regions of these RNAs represent an unusually small fraction of the lengths of the RNAs, as if the 3′ untranslated domains of the RNAs might be exceptionally large (3–11 kb). These findings indicate that the c-erb loci are normal vertebrate genes rather than genes of cryptic endogenous retroviruses, and that they may have a role in the metabolism of normal cells. It appears that the viral erb genes, like most other retrovirus oncogenes, have been copied from cellular genes. In the viral genome, the two genes are devoid of introns, but they remain independently expressed loci, and they remain colinear with the coding domains of their cellular progenitors.