Some biological properties of two new avian sarcoma viruses

The rapidly changing field of predictive biomarkers of non-small cell lung cancer

Lung cancer is a leading cause of cancer-related death worldwide in both men and women, however mortality in the US and EU are recently declining in parallel with the gradual cut of smoking prevalence. Consequently, the relative frequency of adenocarcinoma increased while that of squamous and small cell carcinomas declined. During the last two decades a plethora of targeted drug therapies have appeared for the treatment of metastasizing non-small cell lung carcinomas (NSCLC). Personalized oncology aims to precisely match patients to treatments with the highest potential of success. Extensive research is done to introduce biomarkers which can predict the effectiveness of a specific targeted therapeutic approach. The EGFR signaling pathway includes several sufficient targets for the treatment of human cancers including NSCLC. Lung adenocarcinoma may harbor both activating and resistance mutations of the EGFR gene, and further, mutations of KRAS and BRAF oncogenes. Less frequent but targetable genetic alterations include ALK, ROS1, RET gene rearrangements, and various alterations of MET proto-oncogene. In addition, the importance of anti-tumor immunity and of tumor microenvironment has become evident recently. Accumulation of mutations generally trigger tumor specific immune defense, but immune protection may be upregulated as an aggressive feature. The blockade of immune checkpoints results in potential reactivation of tumor cell killing and induces significant tumor regression in various tumor types, such as lung carcinoma. Therapeutic responses to anti PD1-PD-L1 treatment may correlate with the expression of PD-L1 by tumor cells. Due to the wide range of diagnostic and predictive features in lung cancer a plenty of tests are required from a single small biopsy or cytology specimen, which is challenged by major issues of sample quantity and quality. Thus, the efficacy of biomarker testing should be warranted by standardized policy and optimal material usage. In this review we aim to discuss major targeted therapy-related biomarkers in NSCLC and testing possibilities comprehensively.

Progress of non-small-cell lung cancer with ROS1 rearrangement

ROS1 rearrangement is found in 0.9%-2.6% of people with non-small-cell lung cancers (NSCLCs). Tyrosine kinase inhibitors (TKIs) target ROS1 and can block tumor growth and provide clinical benefits to patients. This review summarizes the current knowledge on ROS1 rearrangements in NSCLCs, including the mechanisms of ROS1 oncogenicity, epidemiology of ROS1-positive tumors, methods for detecting rearrangements, molecular characteristics, therapeutic agents, and mechanisms of drug resistance.

ROS-1 Fusions in Non-Small-Cell Lung Cancer: Evidence to Date

The ROS-1 gene plays a major role in the oncogenesis of numerous tumors. ROS-1 rearrangement is found in 0.9–2.6% of non-small-cell lung cancers (NSCLCs), mostly lung adenocarcinomas, with a significantly higher rate of women, non-smokers, and a tendency to a younger age. It has been demonstrated that ROS-1 is a true oncogenic driver, and tyrosine kinase inhibitors (TKIs) targeting ROS-1 can block tumor growth and provide clinical benefit for the patient. Since 2016, crizotinib has been the first-line reference therapy, with two-thirds of the patients’ tumors responding and progression-free survival lasting ~20 months. More recently developed are ROS-1-targeting TKIs that are active against resistance mechanisms appearing under crizotinib and have better brain penetration. This review summarizes current knowledge on ROS-1 rearrangement in NSCLCs, including the mechanisms responsible for ROS-1 oncogenicity, epidemiology of ROS-1-positive tumors, methods for detecting rearrangement, phenotypic, histological, and molecular characteristics, and their therapeutic management. Much of this work is devoted to resistance mechanisms and the development of promising new molecules.

ROS1 protein-tyrosine kinase inhibitors in the treatment of ROS1 fusion protein-driven non-small cell lung cancers

ROS1 protein-tyrosine kinase fusion proteins are expressed in 1-2% of non-small cell lung cancers. The ROS1 fusion partners include CD74, CCDC6, EZR, FIG, KDELR2, LRIG3, MSN, SDC4, SLC34A2, TMEM106B, TMP3, and TPD52L1. Physiological ROS1 is closely related to the ALK, LTK, and insulin receptor protein-tyrosine kinases. ROS1 is a so-called orphan receptor because the identity of its activating ligand, if any, is unknown. The receptor is expressed during development, but little is expressed in adults and its physiological function is unknown. The human ROS1 gene encodes 2347 amino acid residues and ROS1 is the largest protein-tyrosine kinase receptor protein. Unlike the ALK fusion proteins that are activated by the dimerization induced by their amino-terminal portions, the amino-terminal domains of several of its fusion proteins including CD74 apparently lack the ability to induce dimerization so that the mechanism of constitutive protein kinase activation is unknown. Downstream signaling from the ROS1 fusion protein leads to the activation of the Ras/Raf/MEK/ERK1/2 cell proliferation module, the phosphatidyl inositol 3-kinase cell survival pathway, and the Vav3 cell migration pathway. Moreover, several of the ROS1 fusion proteins are implicated in the pathogenesis of a very small proportion of other cancers including glioblastoma, angiosarcoma, and cholangiocarcinoma as well as ovarian, gastric, and colorectal carcinomas. The occurrence of oncogenic ROS1 fusion proteins, particularly in non-small cell lung cancer, has fostered considerable interest in the development of ROS1 inhibitors. Although the percentage of lung cancers driven by ROS1 fusion proteins is low, owing to the large number of new cases of non-small cell lung cancer per year, the number of new cases of ROS1-positive lung cancers is significant and ranges from 2000 to 4000 per year in the United States and 10,000-15,000 worldwide. Crizotinib was the first inhibitor approved by the US Food and Drug Administration for the treatment of ROS1-positive non-small cell lung cancer in 2016. Other drugs that are in clinical trials for the treatment of these lung cancers include ceritinib, cabozantinib, entrectinib, and lorlatinib. Crizotinib forms a complex within the front cleft between the small and large lobes of an active ROS1 protein-kinase domain and it is classified as type I inhibitor.

Screening for ROS1 gene rearrangements in non-small-cell lung cancers using immunohistochemistry with FISH confirmation is an effective method to identify this rare target

AIMS

To assess the prevalence of ROS1 rearrangements in a retrospective and prospective diagnostic Australian cohort and evaluate the effectiveness of immunohistochemical screening.

METHODS AND RESULTS

A retrospective cohort of 278 early stage lung adenocarcinomas and an additional 104 prospective non-small-cell lung cancer (NSCLC) cases referred for routine molecular testing were evaluated. ROS1 immunohistochemistry (IHC) was performed (D4D6 clone, Cell Signaling Technology) on all cases as well as fluorescence in-situ hybridization (FISH) using the ZytoVision and Abbott Molecular ROS1 FISH probes, with ≥15% of cells with split signals considered positive for rearrangement. Eighty-eight cases (32%) from the retrospective cohort showed staining by ROS1 IHC, and one case (0.4%) showed ROS1 rearrangement by FISH. Nineteen of the prospective diagnostic cases showed ROS1 IHC staining, 12 (12%) cases of which were confirmed as ROS1 rearranged by FISH. There were no ROS1 rearranged cases that showed no expression of ROS1 with IHC. The ROS1 rearranged cases in the prospective cohort were all EGFR wild-type and anaplastic lymphoma kinase (ALK) rearrangement-negative. The sensitivity of ROS1 IHC in the retrospective cohort was 100% and specificity was 76%.

CONCLUSIONS

ROS1 rearrangements are rare events in lung adenocarcinomas. Selection of cases for ROS1 FISH testing, by excluding EGFR/ALK-positive cases and use of IHC to screen for potentially positive cases, can be used to enrich for the likelihood of identifying a ROS1 rearranged lung cancer and prevent the need to undertake expensive and time-consuming FISH testing in all cases.

ROS receptor tyrosine kinase: a new potential target for anticancer drugs

ROS kinase is one of the last two remaining orphan receptor tyrosine kinases with an as yet unidentified ligand. The normal functions of human ROS kinase in different body tissues have not been fully identified so far. However, the ectopic expression, as well as the production of variable mutant forms of ROS kinase has been reported in a number of cancers, such as glioblastoma multiforme, and non-small cell lung cancer, suggesting a role for ROS kinase in deriving such tumors. It is thought also that c-ROS gene may have a role in some cardiovascular diseases, and the fact that homozygous male mice targeted against c-ROS gene are healthy but infertile, has inspired researchers to think about ROS inhibition as a method for development of new male contraceptives. The recent discovery of new selective and potent inhibitors for ROS kinase, along with the development of new specific diagnostic methods for the detection of ROS fusion proteins, raises the importance of using these selective inhibitors for targeting ROS mutations as a new method for treatment of cancers harboring such genes. This review focuses on the ectopic expression of ROS and its fusion proteins in different cancer types and highlights the importance of targeting these proteins for treatment of substantial cancers. It describes also the recent advances in the field of ROS kinase inhibition, and the potential clinical applications of ROS kinase inhibitors.

The multifaceted roles of the receptor tyrosine kinase ROS in development and cancer

The proto-oncogene receptor tyrosine kinase ROS was originally discovered through the identification of oncogenic variants isolated from tumors. These discoveries spearheaded a body of work aimed at elucidating the function of this evolutionarily conserved receptor in development and cancer. Through genetic and biochemical approaches, progress in the characterization of ROS points to distinctive roles in the program of epithelial cell differentiation during the development of a variety of organs. Although substantial, these advances remain hampered by the absence of an identified ligand, making ROS one of the last two remaining orphan receptor tyrosine kinases. Recent studies on the oncogenic activation of ROS as a result of different chromosomal rearrangements found in brain and lung cancers have shed light on the molecular mechanisms underlying ROS transforming activities. ROS and its oncogenic variants therefore constitute clinically relevant targets for cancer therapeutic intervention. This review highlights the various roles that this receptor plays in multiple system networks in normalcy and disease and points to future directions towards the elucidation of ROS function in the context of ligand identification, signaling pathways and clinical applications.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Ala-->Gly mutation in the putative catalytic loop confers temperature sensitivity on Ros, insulin receptor, and insulin-like growth factor I receptor protein-tyrosine kinases

Temperature-sensitive mutations in the avian sarcoma virus UR2 oncogene ros, encoding a receptor protein-tyrosine kinase (PTK), were identified. The Ala385-->Gly change mapping within the highly conserved RDLAARN motif in the Ros kinase domain was responsible for the temperature-sensitive phenotype. Based on the sequence homology of all known protein kinases and the crystalline structure of the cAMP-dependent protein kinase, this conserved region probably represents the PTK catalytic loop. The same mutation when introduced into the human insulin and insulin-like growth factor I receptors made these PTKs temperature sensitive in both biological function and kinase activity. Our results support the presumed catalytic role of this highly conserved sequence in PTKs. Due to its highly conserved nature, we predict that the same mutation would probably confer temperature sensitivity on other PTKs.

Modulating effects of the extracellular sequence of the human insulinlike growth factor I receptor on its transforming and tumorigenic potential

We reported previously that an N-terminally truncated insulinlike growth factor I receptor (IGFR) fused to avian sarcoma virus UR2 gag p19 had a greater transforming potential than did the native IGFR, but it failed to cause tumors in vivo. To investigate whether the 36 amino acids (aa) of the IGFR extracellular (EC) sequence in the gag-IGFR fusion protein encoded by the retrovirus UIGFR have a modulatory effect on the biological and biochemical properties of the protein, four mutants, NM1, NM2, NM3, and NM4 of the EC sequence were constructed. NM1 lacks the entire 36 aa residues; NM2 lacks the N-terminal 16 aa residues (aa 870 to 885), including two potential N-linked glycosylation sites of the EC sequence; NM3 contains a deletion of the C-terminal 20 aa residues (aa 886 to 905) of the EC sequence; and NM4 contains N-to-Q substitutions at both N-linked glycosylation sites. NM1 was the strongest of the four mutants in promoting anchorage-independent growth of transfected chicken embryo fibroblasts, while NM2 and NM4 had weaker transforming potential than did the original UIGFR virus. Only NM1 and NM3 were able to induce sarcomas in chickens. The four NM mutant-transformed cells expressed the expected proteins with comparable steady-state levels. The in vitro tyrosine kinase activity of P53NM1 was about fourfold higher than that of the parental P57-75UIGFR, whereas NM2 and NM4 proteins exhibited four- to fivefold-lower kinase activities. Despite lacking the IGFR EC sequence, P53NM1 formed covalent dimers similar to those formed by the parental P57-75UIGFR. Increased phosphatidylinositol (PI) 3-kinase activity was found to be associated with the mutant IGFR proteins. Among NM4 proteins. Elevated tyrosine phosphorylation of cellular proteins of 35, 120, 140, 160, and 170 kDa was detected in all mutant IGFR-transformed cells. We conclude that the EC 36-aa sequence of IGFR in the gag-IGFR fusion protein exerts intricate modulatory effects on the protein's transforming and tumorigenic potential. The 20 aa residues immediately upstream of the transmembrane domain have an inhibitory effect on the tumorigenic potential of gag-IGFR, whereas N-linked glycosylation within the EC sequence appears to have a positive effect on the transforming potential of UIGFR. Increased in vitro kinase activity and, to a lesser extent, in vivo tyrosine phosphorylation as well as the elevated association of PI 3-kinase activity with IGFR proteins seem to be correlated with the transforming potential of IGFR mutant proteins.

Transforming properties and substrate specificities of the protein tyrosine kinase oncogenes ros and src and their recombinants

To determine the sequences of the oncogenes src (encoded by Rous sarcoma virus [RSV]) and ros (encoded by UR2) that are responsible for causing different transformation phenotypes and to correlate those sequences with differences in substrate recognition, we constructed recombinants of the two transforming protein tyrosine kinases (PTKs) and studied their biological and biochemical properties. A recombinant with a 5' end from src and a 3' end from ros, called SRC x ROS, transformed chicken embryo fibroblasts (CEF) to a spindle shape morphology, mimicking that of UR2. Neither of the two reverse constructs, ROS x SRC I and ROS x SRC II, could transform CEF. However, a transforming variant of ROS x SRC II appeared during passages of the transfected cells and was called ROS x SRC (R). ROS x SRC (R) contains a 16-amino-acid deletion that includes the 3' half of the transmembrane domain of ros. Unlike RSV, ROS x SRC (R) also transformed CEF to an elongated shape similar to that of UR2. We conclude that distinct phenotypic changes of RSV- and UR2-infected cells do not depend solely on the kinase domains of their oncogenes. We next examined cellular proteins phosphorylated by the tyrosine kinases of UR2, RSV, and their recombinants as well as a number of other avian sarcoma viruses including Fujinami sarcoma virus Y73, and some ros-derived variants. Our results indicate that the UR2-encoded receptorlike PTK P68gag-ros and its derivatives have a very restricted substrate specificity in comparison with the nonreceptor PTKs encoded by the rest of the avian sarcoma viruses. Data from ros and src recombinants indicate that sequences both inside and outside the catalytic domains of ros and src exert a significant effect on the substrate specificity of the two recombinant proteins. Phosphorylation of most of the proteins in the 100- to 200-kDa range correlated with the presence of the 5' src domain, including the SH2 region, but not with the kinase domain in the recombinants. This corroborates the conclusion given above that the kinase domain of src or ros per se is not sufficient to dictate the transforming morphology of these two oncogenes. High-level tyrosyl phosphorylation of most of the prominent substrates of src is not sufficient to cause a round-shape transformation morphology.

Enhancement of transforming potential of human insulinlike growth factor 1 receptor by N-terminal truncation and fusion to avian sarcoma virus UR2 gag sequence

The human insulinlike growth factor 1 (hIGF-1) receptor (hIGFR) is a transmembrane protein tyrosine kinase (PTK) molecule which shares high sequence homology in the PTK domain with the insulin receptor and, to a lesser degree, the ros transforming protein of avian sarcoma virus UR2. To assess the transforming potential of hIGFR, we introduced the intact and altered hIGFR into chicken embryo fibroblasts (CEF). The full-length hIGFR cDNA (fIGFR) was cloned into a UR2 retroviral vector, replacing the original oncogene v-ros. fIGFR was able to promote the growth of CEF in soft agar and cause morphological alteration in the absence of added hIGF-1 to medium containing 11% calf and 1% chicken serum. The transforming ability of hIGFR was not further increased in the presence of 10 nM exogenous hIGF-1. The 180-kDa protein precursor of hIGFR was synthesized and processed into alpha and beta subunits. The overexpressed hIGFR in CEF bound hIGF-1 with high affinity (Kd = 5.4 x 10(-9) M) and responded to ligand stimulation with increased tyrosine autophosphorylation. The cDNA sequence coding for part of the beta subunit of hIGFR, including 36 amino acids of the extracellular domain and the entire transmembrane and cytoplasmic domains, was fused to the 5' portion of the gag gene in the UR2 vector to form an avian retrovirus. The resulting virus, named UIGFR, was able to induce morphological transformation and promote colony formation of CEF with a stronger potency than did fIGFR. The UIGFR genome encodes a membrane-associated, glycosylated gag-IGFR fusion protein. The specific tyrosine phosphorylation of the mature form of the fusion protein, P75, is sixfold higher in vitro and threefold higher in vivo than that of the native IGFR beta subunit, P95. In conclusion, overexpression of the native or an altered hIGFR can induce transformation of CEF with the gag-IGFR fusion protein possessing enhanced transforming potential, which is consistent with its increased in vitro and in vivo tyrosine phosphorylation.

Tissue-specific expression of rat c-ros-1 gene and partial structural similarity of its predicted products with sev protein of Drosophila melanogaster

The expression and predicted products of rat c-ros-1 gene, the proto-oncogene of v-ros in UR2 sarcoma virus, were characterized. The c-ros-1 gene was found to be expressed in a tissue-specific manner, and the sizes of its transcripts were heterogeneous: 8.2 kilobases (kb) long in lung and kidney tissues, 6.9 kb in heart tissue, and 2.4 kb and 1.9 kb in testis tissue. The c-ros-1 cDNAs were isolated from lung and heart tissues. The predicted product of the c-ros-1 gene in lung tissue was a receptor-type tyrosine kinase 2,317 amino acids long (including a very large extracellular domain of approximately 1,800 amino acids) which showed a partial but significant structural homology with the sev gene product of Drosophila melanogaster. An alternatively sliced lung transcript was found to encode a protein with external and transmembrane domains but not a tyrosine kinase catalytic domain. The predicted product in heart tissue was essentially identical to that in lung tissue except for a shorter amino-terminal region and a 21-amino-acid insertion in the extracellular domain. On the basis of these results, the c-ros-1 gene appears to be active in the lungs and kidneys and probably in the hearts of rats.

Avian sarcoma viruses

Twelve independent isolates of avian sarcoma viruses (ASVs) can be divided into four groups according to the transforming genes harbored in the viral genomes. The first group is represented by viruses containing the transforming sequence, src, inserted in the viral genome as an independent gene; the other three groups of viruses contain transforming genes fps, yes or ros fused to various length of the truncated structural gene gag. These transforming sequences have been obtained by avian retroviruses from chicken cellular DNA by recombination. The src-containing viruses code for an independent polypeptide, p60src; and the representative fps, yes and ros-containing ASVs code for P140/130gag-fps, P90gag-yes and P68gag-ros fusion polypeptides respectively. All of these transforming proteins are associated with the tyrosine-specific protein kinase activity capable of autophosphorylation and phosphorylating certain foreign substrates. p60src and P68gag-ros are integral cellular membrane proteins and P140/130gag-fps and P90gag-yes are only loosely associated with the plasma membrane. Cells transformed by ASVs contain many newly phosphorylated proteins and in most cases have an elevated level of total phosphotyrosine. However, no definitive correlation between phosphorylation of a particular substrate and transformation has been established except that a marked increase of the tyrosine phosphorylation of a 34,000 to 37,000 dalton protein is observed in most ASV transformed cells. The kinase activity of ASV transforming proteins appears to be essential, but not sufficient for transformation. The N-terminal domain of p60src required for myristylation and membrane binding is also crucial for transformation. By contrast, the gag portion of the FSV P130gag-fps is dispensable for in vitro transformation and removal of it has only an attenuating effect on in vivo tumorigenicity. The products of cellular src, fps and yes proto-oncogenes have been identified and shown to also have tyrosine-specific protein kinase activity. The transforming potential of c-src and c-fps has been studied and shown that certain structural changes are necessary to convert them into transforming genes. Among the cellular proto-oncogenes related to the four ASV transforming genes, c-ros most likely codes for a growth factor receptor-like molecule. It is possible that the oncogene products of ASVs act through certain membrane receptor(s) or enzyme(s), such as protein kinase C, in the process of cell transformation.

Infection of neuroretinal cells in vitro by avian sarcoma viruses UR1 and UR2: transformation, cell growth stimulation, and changes in transducin levels

Infection in vitro of differentiating chick embryo neuroretinal cells with avian sarcoma viruses UR1 and UR2 results in mitogenic stimulation and morphologic conversion of both support neuronal cells. This was shown by the continuous propagation of transformed cells for over 4 months and growth of reaggregated colonies in liquid medium as well as in soft agar. Production of the transforming proteins p 150 gag-fps and p68 gag-ros of UR1 and UR2, respectively, was similar to that of transformed chick embryo fibroblasts, as judged from in vitro kinase activity assays. The two protein subunits, T beta and T gamma, but not T alpha of the GTP binding protein transducin, found in the retina of many animal species, were present in control neuroretinal cells. Infection with Rous sarcoma virus or UR2 resulted in an inhibition of T gamma synthesis and enhancement of T beta-like protein production.

Heterologous transmembrane signaling by a human insulin receptor-v-ros hybrid in Chinese hamster ovary cells

A hybrid receptor molecule composed of the extracellular ligand-binding domain of the human insulin receptor and the transmembrane and cytoplasmic (protein-tyrosine kinase) domains of the chicken sarcoma virus UR2 transforming protein p68gag-ros has been constructed and expressed in Chinese hamster ovary (CHO) cells. The hybrid is processed normally into alpha and hybrid beta subunits, is expressed on the cell surface at high levels, and binds insulin with near-wild-type affinity. Furthermore, insulin stimulates the phosphorylation on tyrosine residues of the hybrid beta subunit in vivo and the phosphorylation of an exogenous substrate [poly(Glu,Tyr)] in vitro. Thus the hybrid is capable of heterologous transmembrane signaling. However, the hybrid mediates neither the insulin-activated uptake of 2-deoxyglucose nor the incorporation of [3H]thymidine into DNA, suggesting that the physiological response(s) mediated by ligand-activated protein-tyrosine kinases may utilize distinct intracellular mechanisms for postreceptor signaling.

Proto-oncogene c-ros codes for a molecule with structural features common to those of growth factor receptors and displays tissue specific and developmentally regulated expression

A recombinant DNA clone containing cellular sequences homologous to the transforming sequence, v-ros, of avian sarcoma virus UR2 was isolated from a chicken genomic DNA library. Heteroduplex mapping and nucleotide sequencing reveal that the v-ros sequences are distributed in nine exons ranging from 65 to 204 nucleotides on cellular ros (c-ros) DNA over a range of 11 kilobases. Comparison of the deduced amino acid sequences of c-ros and v-ros shows two differences: v-ros contains a three-amino-acid insertion within the hydrophobic domain presumed to be involved in membrane association, and (ii) the carboxyl 12 amino acids of v-ros are completely different from those of the deduced c-ros sequence. The deduced amino acid sequence of c-ros bears striking structural features similar to those of insulin and epidermal growth factor receptors, including the presumed hydrophobic membrane binding domain, amino acids flanking the domain, and the distance between the domain and the catalytic region of the kinase activity. The expression of c-ros appears to be under a very stringent control. When tissues at various stages of chicken development were analyzed, only kidney was found to contain a significant level of c-ros RNA. The level of c-ros RNA in kidney tissue is most abundant in 7- to 14-day-old chickens. Finally, nucleotide sequences of c-ros DNA and UR2-associated helper viral genome at regions corresponding to the gag ros recombination site suggest that the junction has been formed by RNA splicing.

Human c-ros-1 gene homologous to the v-ros sequence of UR2 sarcoma virus encodes for a transmembrane receptorlike molecule

We isolated a human gene (designated c-ros-1) homologous to the v-ros sequence of UR2 sarcoma virus. Ten exons, 1,414 base pairs spanning 26 kilobases, contained a tyrosine kinase domain, a transmembrane domain, and a part of an extracellular domain carrying an N glycosylation site which was not acquired by UR2 sarcoma virus. The predicted structure of c-ros-1 is unique among the src family and clearly distinct from the human insulin receptor.

Specific inhibition of tyrosine kinase activity by an antibody to the v-ros oncogene product

Antibodies present in two peritoneal exudates of rats bearing abdominal tumors induced by UR2-transformed rat cells were characterized. The ability to immunoprecipitate p68gag-ros and to inhibit the protein and phospholipid kinase activities of this protein was investigated. One of the exudates specifically inhibited tyrosyl phosphorylation by p68gag-ros but not the activity of other known tyrosyl kinases, such as p150gag-fps of UR1 avian sarcoma virus, p60src, and the insulin receptor. It precipitated p68gag-ros but not Pr76 or other gag-related proteins from UR2-infected cells. Phosphorylation of phosphatidylinositol was not affected by this exudate, suggesting that this activity is not intrinsic to p68gag-ros. Another exudate precipitated p68gag-ros but not gag-related proteins from UR2-infected cells or p140gag-fps from Fujinami sarcoma virus-infected cells. These results demonstrated that the antibodies in these exudates recognized epitopes present in the ros portion of the fused protein p68gag-ros, but only one of the two exudates inhibited the intrinsic tyrosyl kinase of p68gag-ros.

Transfection and recombination with molecularly cloned derivatives of avian sarcoma virus UR2

A cloned version of avian sarcoma virus UR2, plasmid pKD6, which includes the full, nonpermuted proviral sequence between two LTR regions, has been prepared. The plasmid is biologically active in transfection experiments, even when intact. Two transformation-defective mutants with nonoverlapping deletions within the transforming gene ros were constructed from pKD6. These mutants recombine to produce transforming virus when mixed DNA from both is used to transfect chick embryo fibroblasts along with helper virus DNA. However, recombination was not readily detected when cells were coinfected with fluids harvested from cultures separately transfected with DNA from each mutant. This, and marker rescue experiments with a temperature-sensitive mutant of UR2 defective in transformation but able to replicate, suggest that deletion mutants of UR2 do not propagate efficiently.

The human c-ros gene (ROS) is located at chromosome region 6q16----6q22

The human homolog, c-ros, of the transforming gene, v-ros, of the avian sarcoma virus, UR2, has been isolated from a human genomic library. A single-copy fragment from the human c-ros genomic clone has been used to map the human c-ros homolog (ROS) to human chromosome region 6q16----6q22 by somatic cell hybrid analysis and chromosomal in situ hybridization. Thus, the c-ros gene joins the c-myb oncogene, which is distal to the c-ros gene on the long arm of human chromosome 6, as a candidate for involvement in chromosome 6q deletions and rearrangements seen in various malignancies.

Human c-ros-1 gene homologous to the v-ros sequence of UR2 sarcoma virus encodes for a transmembrane receptorlike molecule

We isolated a human gene (designated c-ros-1) homologous to the v-ros sequence of UR2 sarcoma virus. Ten exons, 1,414 base pairs spanning 26 kilobases, contained a tyrosine kinase domain, a transmembrane domain, and a part of an extracellular domain carrying an N glycosylation site which was not acquired by UR2 sarcoma virus. The predicted structure of c-ros-1 is unique among the src family and clearly distinct from the human insulin receptor.

Proto-oncogene c-ros codes for a molecule with structural features common to those of growth factor receptors and displays tissue specific and developmentally regulated expression

A recombinant DNA clone containing cellular sequences homologous to the transforming sequence, v-ros, of avian sarcoma virus UR2 was isolated from a chicken genomic DNA library. Heteroduplex mapping and nucleotide sequencing reveal that the v-ros sequences are distributed in nine exons ranging from 65 to 204 nucleotides on cellular ros (c-ros) DNA over a range of 11 kilobases. Comparison of the deduced amino acid sequences of c-ros and v-ros shows two differences: v-ros contains a three-amino-acid insertion within the hydrophobic domain presumed to be involved in membrane association, and (ii) the carboxyl 12 amino acids of v-ros are completely different from those of the deduced c-ros sequence. The deduced amino acid sequence of c-ros bears striking structural features similar to those of insulin and epidermal growth factor receptors, including the presumed hydrophobic membrane binding domain, amino acids flanking the domain, and the distance between the domain and the catalytic region of the kinase activity. The expression of c-ros appears to be under a very stringent control. When tissues at various stages of chicken development were analyzed, only kidney was found to contain a significant level of c-ros RNA. The level of c-ros RNA in kidney tissue is most abundant in 7- to 14-day-old chickens. Finally, nucleotide sequences of c-ros DNA and UR2-associated helper viral genome at regions corresponding to the gag ros recombination site suggest that the junction has been formed by RNA splicing.

Membrane association of the transforming protein of avian sarcoma virus UR2 and mutants temperature sensitive for cellular transformation and protein kinase activity

The localization of the transforming protein P68gag-ros of avian sarcoma virus UR2, which has a hydrophobic region at the N terminus of its ros-specific tyrosine kinase-encoding sequence, was examined by subcellular fractionation. P68 behaved as an integral membrane protein associated with the plasma membrane of transformed cells. P68 became membrane associated very rapidly in its biogenesis. Three temperature-sensitive mutants of UR2 were isolated and characterized. Cells infected with the mutants were temperature sensitive for morphological alteration and colony formation. The mutant P68 proteins were membrane associated in mutant-infected cells regardless of the temperature but were active as protein kinases only at the permissive temperature. The results suggest that P68 is a membrane-associated protein whose kinase activity plays a crucial role in UR2-mediated cell transformation.

Construction and biological analysis of deletion mutants of Fujinami sarcoma virus: 5'-fps sequence has a role in the transforming activity

Fujinami sarcoma virus (FSV) genome codes for the gag-fps fusion protein FSV-P130. The amino acid sequence of the 3' one-third portion in v-fps is partially homologous to the 3' half of pp60src, or the kinase domain, but the sequence of the 5' portion is unique to v-fps. To identify a possible domain structure in the v-fps sequence responsible for cell transformation, we constructed various deletion mutants of FSV with molecularly cloned viral DNA. Their transforming activities were assayed by measuring focus formation on chicken embryo fibroblasts and rat 3Y1 cells and tumor formation in chickens. The mutants carrying a deletion at the 3' portion in v-fps, the kinase domain, lost transforming activity. The mutants carrying an approximately 1-kilobase deletion within the 5' portion of the v-fps sequence retained focus-forming activity and tumorigenicity in the chicken system, but the efficiency of focus formation was about 10 times lower than that of the wild type. The morphology of these transformed cells was distinct from that observed in cells infected with wild-type FSV. Furthermore, these mutants could not transform rat 3Y1 cells, although wild-type FSV DNA transformed rat 3Y1 cells at a high frequency. The mutants carrying a larger deletion in the 5' portion of fps completely lacked the transforming activity. These results suggest that the 3' portion of the v-fps sequence is necessary but not sufficient for cell transformation and that the 5' portion of v-fps has a role in the transforming activity.

Increased phosphorylation of tyrosine in vinculin does not occur upon transformation by some avian sarcoma viruses

The level of phosphotyrosine in vinculin was determined in chicken embryo fibroblasts transformed by various strains of avian sarcoma virus. As previously reported (Sefton et al., Cell 24:165-174, 1981), vinculin was phosphorylated at tyrosine residues in most cultures examined, but the level varied greatly and no detectable change was found in cultures infected with Fujinami sarcoma virus or UR2 sarcoma virus. Regardless of the level of vinculin phosphorylation, the number of organized microfilament bundles was found to be decreased in all transformed cells. These results strongly suggest that tyrosine phosphorylation of vinculin is not an obligatory step in cell transformation by this class of oncogenes, nor is it correlated with the associated cytoskeletal disarray.

Nucleotide sequence of avian sarcoma virus UR2 and comparison of its transforming gene with other members of the tyrosine protein kinase oncogene family

The genome of avian sarcoma virus UR2 was completely sequenced and found to have a size of 3,165 nucleotides. The UR2-specific transforming sequence, ros, with a length of 1,273 nucleotides, is inserted between the truncated gag gene coding for p19 and the env gene coding for gp37 of the UR2AV helper virus. The deduced amino acid sequence for the UR2 transforming protein P68 gives a molecular weight of 61,113 and shows that it is closely related to the oncogene family coding for tyrosine protein kinases. P68 contains two distinctive hydrophobic regions that are absent in other tyrosine kinases, and it has unique amino acid changes and insertions within the conserved domain of the kinases. These characteristics may modulate the activity and target specificity of P68.

Increased phosphorylation of tyrosine in vinculin does not occur upon transformation by some avian sarcoma viruses

The level of phosphotyrosine in vinculin was determined in chicken embryo fibroblasts transformed by various strains of avian sarcoma virus. As previously reported (Sefton et al., Cell 24:165-174, 1981), vinculin was phosphorylated at tyrosine residues in most cultures examined, but the level varied greatly and no detectable change was found in cultures infected with Fujinami sarcoma virus or UR2 sarcoma virus. Regardless of the level of vinculin phosphorylation, the number of organized microfilament bundles was found to be decreased in all transformed cells. These results strongly suggest that tyrosine phosphorylation of vinculin is not an obligatory step in cell transformation by this class of oncogenes, nor is it correlated with the associated cytoskeletal disarray.

Cytoskeletal changes induced by two avian sarcoma viruses: UR2 and Rous sarcoma virus

UR2-transformed cells were examined by immunofluorescence and compared to control cells and cells transformed by Rous Sarcoma Virus (RSV). Actin and tubulin which are normally depolymerized in RSV-transformed cells appeared to be unaffected by UR2 transformation. Cell surface fibronectin which is normally lost from RSV-infected cells, appears more abundantly on UR2-transformed cells than on normal cells. Vinculin was shown to be in adhesion plaques in UR2-transformed cells as well as in control fibroblasts but diffuse in the cytoplasm of RSV-transformed cells. Polyacrylamide gel electrophoresis of [35S]methionine-labeled fibronectin and vinculin immunoprecipitated from lysates of normal and transformed cells indicated that cell associated fibronectin synthesized during the labeling period is reduced by 60% in RSV-transformed cells but occurs in normal amounts in UR2-transformed cells. However, immunoprecipitation of radiolabeled fibronectin released in supernatant fluids of normal and transformed cells showed a decreased amount of fibronectin in fluids from UR2-transformed cells, but a considerable increase in the medium from RSV-infected cells as compared to uninfected cultures. These data suggest that more fibronectin binds to the surface of UR2-transformed cells then to normal cells, but is readily released from RSV-transformed cells. Vinculin was reduced by about 50% of normal levels in both RSV- and UR2-transformed cells. Immunofluorescence studies using antibody to virion structural proteins (gag) show that the nuclei of UR2-transformed cells are not fluorescent. This indicates a cytoplasmic location or membrane association for p68ros, the transforming protein of UR2, which contains gag determinants. Overall, these data suggest that changes in the major cytoskeletal proteins of fibroblasts are not essential for the neoplastic properties of cells but are rather a phenotypic expression of transformation, since UR2, which causes tumors in vivo, induces only minor cytoskeletal alterations of cells transformed in vitro.

Molecular cloning and characterization of avian sarcoma virus UR2 and comparison of its transforming sequence with those of other avian sarcoma viruses

Avian sarcoma virus UR2 and its associated helper virus, UR2AV , were molecularly cloned into lambda gtWES X lambda B by using unintegrated viral DNAs. One UR2 and several UR2AV clones were obtained. The UR2 DNA was subsequently cloned into pBR322. Both UR2 and UR2AV DNAs were tested for their biological activity by transfection onto chicken embryo fibroblasts. When cotransfected with UR2AV DNA, UR2 DNA was able to induce transformation of chicken embryo fibroblasts with a morphology similar to that of parental UR2 . UR2 -specific protein with kinase activity and UR2 -specific RNA were detected in the transfected cells. Transforming virus, UR2 ( UR2AV ), was produced from the doubly transfected cells. Five of the six UR2AV clones tested were also shown to be biologically active. The insert of the UR2 DNA clone is 3.4 kilobases in length and contains two copies of the long terminal repeat. Detailed restriction mapping showed that UR2 DNA shared with UR2AV DNA 0.8 kilobases of 5' sequence, including a portion of 5' gag, and 1.4 kilobases of 3' sequence, including a portion of 3' env. The UR2 transforming sequence, ros, is ca. 1.2 kilobases. No significant homology was found between v-ros and the conserved regions of v-src, v-yes, or v- abl . By contrast, a significant homology was found between v-ros and v-fps. The v-fps-related sequence was mapped within a 300-base-pair sequence in the middle of ros.

Transforming protein of avian sarcoma virus UR2 is associated with phosphatidylinositol kinase activity: possible role in tumorigenesis

The transforming protein of avian sarcoma virus UR2, p68v-ros, has an associated tyrosine-specific protein kinase activity similar to that of p60v-src and several other oncogene products. However, this activity has not been linked unequivocally to transformation, and the physiological action of these proteins remains in doubt. We now have found that immunoprecipitated p68v-ros also is associated with phosphatidylinositol (PtdIns) kinase (ATP:PtdIns 4-phosphotransferase, EC 2.7.1.67) activity. PtdIns 4,5-bisphosphate [PtdIns(4,5)P2] specifically inhibits both this activity and the autophosphorylation of p68v-ros. Moreover, cells transformed by UR2 showed significant increases in 32P-labeling of PtdIns 4-phosphate (PtdIns4P) and PtdIns(4,5)P2 and in the formation of their catabolites, inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate, as compared to uninfected cells. These results suggest that a physiologically relevant function of oncogene kinases might be the phosphorylation of PtdIns and that increased turnover of PtdIns4P and PtdIns(4,5)P2 might play a role in transformation by increasing the formation of diacylglycerol, a catabolite of polyphosphoinositides that activates kinase C. This protein copurifies with the phorbol ester receptor, and its activation is likely to be intimately linked with mitogenesis. This hypothesis suggests a mechanism whereby certain oncogene proteins might cause the unrestricted growth typical of transformed cells and could explain why tumor promoters mimic many of the effects of transformation.

The low tumorigenic potential of PRCII, among viruses of the Fujinami sarcoma virus subgroup, corresponds to an internal (fps) deletion of the transforming gene

The avian sarcoma viruses FSV, PRCII, PRCIIp, and PRCIV share a related class of hybrid onc genes (delta gag-fps) defined by a specific nucleotide sequence fps and by delta gag-fps proteins of different sizes. Among these viruses, PRCII appears to have a lower tumorigenic potential than the others. Here we have compared fibroblast-transforming function and onc gene structure of these viruses. The fibroblast transforming ability of PRCII was lower than those of FSV, PRCIIp, and PRCIV. By gel electrophoresis the genomic RNA of PRCII measured 3.5 kb and those of FSV, PRCIIp, and PRCIV 4.5 kb; the delta gag-fps protein of PRCII measured 105 kilodaltons (kd), that of FSV 140 kd, and those of PRCIIp and PRCIV about 150 kd. By fingerprinting viral RNAs hybridized with molecularly cloned viral DNA the delta gag regions of PRCII and PRCIIp were defined to be 1.45 kb and that of FSV to be 1.3 kb. Fingerprint analysis of viral RNA-proto fps DNA hybrids showed the fps regions (approximately 2.8 kb) of FSV and PRCIIp to be isogenic. Compared to FSV and PRCIIp, the fps sequence of PRCII lacked a 1-kb region which maps between 0.3 and 1.3 kb from the 5' end of fps in FSV and PRCIIp. Based on oligonucleotide analysis, the shared fps complements of PRCII and PRCIIp were indistinguishable while that of FSV differed from those of the PRC viruses in scattered point mutations amounting to 1-2% of the RNA. Since all other regions of PRCII are isogenic with those of the highly tumorigenic variants PRCIIp, PRCIV, and FSV, it is concluded that the low fibroblast-transforming and oncogenic potential of PRCII reflects the internal fps deletion. Since the fps deletion reduces but does not eliminate transforming function, we suggest that the complete onc genes of viruses in the FSV subgroup include either several functional, or a regulatory and a functional fibroblast transforming domain. It has been reported that the 3' domains of the onc genes of viruses in the Fujinami subgroup and the onc genes of certain feline sarcoma viruses are distantly related. Since full transforming potential of the avian viruses depends on the 5' fps region not shared with the feline sarcoma viruses, we suggest that despite their structural homology, the avian and feline onc genes must have functionally different domains.

Structural relationship between the chicken DNA locus, proto-fps, and the transforming gene of Fujinami sarcoma virus, delta gag-fps

The avian Fujinami sarcoma virus (FSV) contains a hybrid transforming gene (delta gag-fps) with a 5' 1.3-kb portion derived from the gag gene of avian retroviruses and a 3' 2.8-kb portion (fps) derived from a cellular prototype. A lambda recombinant DNA clone carrying fps sequences within a 16-kb insert of cellular DNA, termed lambda proto-fps clone 12, has been selected from a chicken DNA library for comparison with the viral onc gene. Mapping of endonuclease-resistant proto-fps DNA fragments and hybridization with cloned viral DNA located FSV-related sequences at the 3' end of the insert within a region of about 4.25 kb. Alignment of endonuclease-resistant proto-fps and viral DNA fragments relative to common RNase T1-resistant oligonucleotide sequences of viral RNA, identified by fingerprinting DNA-RNA hybrids, indicated: (i) that proto-fps is colinear with viral fps but is interrupted by 1.75 kb of scattered sequences unrelated to viral fps; (ii) that among the nine endonuclease sites compared, proto-fps and viral fps share one PvuII, one BamHI, and possibly a Kpn1 site at homologous locations, and that they each have unique endonuclease sites and common sites at unique locations; (iii) that within 12 kb upstream from the 5' boundary of overlap with viral fps, proto-fps lacks gag-related sequences; and (iv) that proto-fps clone 12, like several others isolated by us, lacks at the 3' end an equivalent of the 3' 10 to 20% of viral fps. The eight endonuclease site-map coordinates of proto-fps and viral DNA also divided 44 fps oligonucleotides of viral RNA into 9 map segments. We conclude that the onc gene of FSV differs from proto-fps in delta gag and in multiple point mutations, compatible with a transforming function for the viral gene and a normal function for the cellular sequence homolog. Since proto-fps is unrelated to essential virion genes, the onc gene of FSV must have originated from cellular proto-fps by rare, illegitimate recombination.

Isolation of 16L virus: a rapidly transforming sarcoma virus from an avian leukosis virus-induced sarcoma

We have isolated a replication-defective rapidly transforming sarcoma virus (designated 16L virus) from a fibro-sarcoma in a chicken infected with td107A, a transformation-defective deletion mutant of subgroup A Schmidt-Ruppin Rous sarcoma virus. 16L virus transforms fibroblasts and causes sarcomas in infected chickens within 2 wk. Its genomic RNA is 6.0 kilobases and contains sequences homologous to the transforming gene (fps) of Fujinami sarcoma virus (FSV). RNase T1 oligonucleotide analysis shows that the 5' and 3' terminal sequences of 16L virus are indistinguishable from (and presumably derived from) td107A RNA. The central part of 16L viral RNA consists of fps-related sequences. These oligonucleotides fall into four classes: (i) oligonucleotides common to the putative transforming regions of FSV and another fps-containing avian sarcoma virus, UR1; (ii) an oligonucleotide also present in FSV but not in UR1; (iii) an oligonucleotide also present in UR1 but not in FSV; and (iv) an oligonucleotide not present in either FSV, UR1, or td107A. Cells infected with 16L virus synthesize a protein of Mr 142,000 that is immunoprecipitated with anti-gag antiserum. This protein has protein kinase activity. These results suggest that 16L virus arose by recombination between td107A and the cellular fps gene.

Molecular cloning of the Fujinami sarcoma virus genome and its comparison with sequences of other related transforming viruses

Full-length proviral DNA of Fujinami sarcoma virus (FSV) of chickens was molecularly cloned and characterized. An analysis of FSV DNA integrated in mammalian cells showed that restriction endonuclease SacI has a single cleavage site on FSV DNA. Unintegrated closed circular FSV DNA obtained from newly infected cells was linearized by digestion with SacI and cloned into lambdagtWES.lambdaB. The following three different molecules were isolated: FSV-1 (4.4 kilobases [kb]) and FSV-2 (4.7 kb), which appeared to be full-length FSV DNA molecules containing either one or two copies of the long terminal repeat structure, and FSV-3 (6 kb), which consisted of part FSV DNA and part DNA of unknown origin. An analysis of the structure of cloned FSV-1 and FSV-2 DNA molecules by restriction endonuclease mapping and hybridization with appropriate probes showed that about 2.6 kb of the FSV-unique sequence called FSV-fps is located in the middle of the FSV genome and is flanked by helper virus-derived sequences of about 1.3 kb at the 5' end and 0.5 kb at the 3' end. The long terminal repeats of FSV were found to have no cleavage site for either EcoRI or PvuI. Upon transfection, both FSV-1 DNA and FSV-2 DNA were able to transform mammalian fibroblasts. Four (32)P-labeled DNA fragments derived from different portions of the FSV-fps sequence were used for hybridization to viral RNAs. We found that sequences within the 3' half of the FSV-fps gene are homologous to RNAs of PRCII avian sarcoma virus and the Snyder-Theilen strain of feline sarcoma virus, both of which were previously shown to contain transforming genes related to FSV-fps. These results suggest that the 3' portion of the FSV-fps sequence may be crucial for the transforming activity of fps-related oncogenic sequences.

Avian sarcoma virus UR2 encodes a transforming protein which is associated with a unique protein kinase activity

UR2 is a newly characterized avian sarcoma virus whose genome contains a unique sequence that is not related to the sequences of other avian sarcoma virus transforming genes thus far identified. This unique sequence, termed ros, is fused to part of the viral gag gene. The product of the fused gag-ros gene of UR2 is a protein of 68,000 daltons (P68) immunoprecipitable by antiserum against viral gag proteins. In vitro translation of viral RNA and in vivo pulse-chase experiments showed that P68 is not synthesized as a large precursor and that it is the only protein product encoded in the UR2 genome, suggesting that it is involved in cell transformation by UR2. In vivo, P68 was phosphorylated at both serine and tyrosine residues. Immunoprecipitates of P68 with anti-gag antisera had a cyclic nucleotide-independent protein kinase activity that phosphorylated P68, rabbit immunoglobulin G in the immune complex, and alpha-casein. The phosphorylation by P68 was specific to tyrosine of the substrate proteins. P68 was phosphorylated in vitro at only one tyrosine site, and the tryptic phosphopeptide of in vitro-labeled P68 was different from those of Fujinami sarcoma virus P140 and avian sarcoma virus Y73-P90. A comparison of the protein kinases encoded by UR2, Rous sarcoma virus, Fujinami sarcoma virus, and avian sarcoma virus Y73 revealed that UR2-P68 protein kinase is distinct from the protein kinases encoded by those viruses by several criteria. Our results suggest that several different protein kinases encoded by viral transforming genes have the same functional specificity and cause essentially the same cellular alterations.

Cellular sequences related to three new onc genes of avian sarcoma virus (fps, yes, and ros) and their expression in normal and transformed cells

Two onc genes of avian sarcoma viruses unrelated to the src gene have recently been identified: fps of Fujinami sarcoma virus/PRCII/UR1 and yes of Y73/Esh sarcoma virus. In the first part of this study we demonstrated that UR2, the most recently isolated avian sarcoma virus, contains in its genome a unique sequence, ros, nonhomologous to src, fps, and yes sequences or to transforming genes of avian acute leukemia viruses. Using cDNAs specific to the inserts of avian sarcoma virus genomes, we examined the existence and the transcription of cellular nucleotide sequences related to the three new onc genes of avian sarcoma virus (fps, yes and ros) in various cells. The progenitor cellular sequences for these onc genes (c-onc) were present in uninfected chicken DNA in one or few copies per haploid genome. These c-onc sequences were detectable in cellular DNA of a wide variety of vertebrates, and the homology between viral and cellular onc was inversely related to the phylogenetic distance of animal species. The pattern of expression of these c-onc genes in different tissues of chickens was found to be unique to each gene. The expression of c-fps and c-ros genes was generally repressed in many tissues, but c-fps was expressed at higher levels in bone marrow (2.5 copies per cell) and lung (1.1 copies per cell), whereas c-ros was mainly transcribed in kidney (2.5 copies per cell). On the other hand, c-yes transcripts were easily detectable in all tissues analyzed and were found at high levels in kidney (26 copies per cell). These c-onc expressions were unaffected by infection with avian sarcoma viruses that contained other onc genes. In a few cultures of chicken and quail transformed cells derived from tumors induced by chemical carcinogens, we found that the levels of transcription of the four c-onc genes remained unaltered, compared with that in normal tissues.

Genetic structure and transforming sequence of avian sarcoma virus UR2

We have recently shown that a newly isolated avian sarcoma virus, UR2, is defective in replication and contains no sequences homologous to the src gene of Rous sarcoma virus. In this study, we analyzed the genetic structure and transforming sequence of UR2 by oligonucleotide fingerprinting. The sizes of the genomic RNAs of UR2 and its associated helper virus, UR2AV, were determined to be 24S and 35S, respectively, by sucrose gradient sedimentation. The molecular weight of the 24S UR2 genomic RNA was estimated to be 1.1 x 10(6), corresponding to 3,300 nucleotides, by gel electrophoresis under the native and denatured conditions. RNase T1 oligonucleotide mapping indicated that UR2 RNA contains seven unique oligonucleotides in the middle of the genome and shares eight 5'- and six 3'-terminal oligonucleotides with UR2AV RNA. From these data, we estimated that UR2 RNA contains a unique sequence of about 12 kilobases in the middle of the genome, and contains 1.4 and 0.7 kilobases of sequences shared with UR2AV RNA at the 5' and 3' ends, respectively. Partial sequence analysis of the UR2-specific oligonucleotides by RNase A digestion revealed that there are no homologous counterparts to these oligonucleotides in the RNAs of other avian sarcoma and acute leukemia viruses studied to date. UR2-transformed non-virus-producing cells contain a single 24S viral RNA which is most likely the message coding for the transforming protein of UR2. On the basis of the uniqueness of the transforming sequence, we concluded that UR2 is a new member of the defective avian sarcoma viruses.

Proposal for naming host cell-derived inserts in retrovirus genomes

We propose a system for naming inserted sequences in transforming retroviruses (i.e., onc genes), based on using trivial names derived from a prototype strain of virus.

Genetic structure, transforming sequence, and gene product of avian sarcoma virus UR1

We analyzed the genetic structure and gene products of the newly isolated avian sarcoma virus UR1, which recently has been shown to be replication defective and to contain no sequences homologous to the src gene of Rous sarcoma virus. The sizes of the genomic RNAs of UR1 and its associated helper virus, UR1AV, were determined to be 29S and 35S (5.9 and 8.5 kilobases), respectively, by gel electrophoresis and sucrose gradient sedimentation. RNase T1 oligonucleotide mapping of purified viral RNAs indicated that UR1 RNA contains eight unique oligonucleotides in the middle of the genome and shares four 5'-terminal and three 3'-terminal oligonucleotides with UR1AV RNA. The unique sequences of UR1 and Fujinami sarcoma virus were found to be closely related to each other by molecular hybridization of UR1 RNA with DNA complementary to the unique sequence of Fujinami sarcoma virus RNA, but minor differences were found by oligonucleotides fingerprinting. In the regions flanking the unique sequences, UR1 and Fujinami sarcoma viral RNAs contain distinct oligonucleotides, which are shared with oligonucleotides of the respective helper viral RNAs. Cell transformed with UR1 produce a single 29S RNA species which contains a UR1 unique sequence; this species is most likely the mRNA coding for the transforming protein. In UR1-transformed cells, a phosphoprotein fo 150,000 daltons (p150) was detected by immunoprecipitation with antiserum against gag proteins. p150 was associated with a protein kinase activity that was capable of phosphorylating p150 itself, immunoglobulin G of antiserum, and a soluble substrate, alpha-casein. This enzyme transferred phosphate exclusively to tyrosine residues of substrates in vitro, but p 150 labeled in vivo with 32P contained both phosphoserine and phosphotyrosine. The in vitro kinase reaction was not affected by the presence of cyclic AMP or cyclic GMP and strongly preferred Mn2+ over Mg2+. Thus, the properties of UR1 protein are almost identical to those of Fujinami sarcoma virus protein.