ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 13, No. 2 Copyright 1983, Institute for Clinical Science, Inc. COMMENTARY Oncogenes: An Overview DANIEL LEIGH WEISS, M.D. National Research Council, Washington, DC 20418 T he id e a th at a p o rtio n o f a c e ll s deoxyribonucleic acid (DNA) is responsible for the transformation of a normal cell under growth control into a freely reproducing cell w ithout growth control, or m alig n an t transform ation, m ust have been in the minds of those who recognized that genes are responsible for cell function. As evidence was adduced that the probability of gene mutation is correlated with the probability of tumor developm ent,15 the concept of the genetumor relationship became strengthened. A combination of studies brought the concept of the oncogene to the attention of biologists. First, specific animal virus infections were related to an increased risk of tum or developm ent. O f particular significance is work of Ludwig Gross on rodent leukem ia.11 Subsequently, studies in h um ans le d to th e asso ciatio n of Epstein-B arr virus (EBV) w ith Burkitt lymphoma.14 A second set of observations established natural tumor viruses as ribonucleic acid (RNA). There is a large body of evidence that supports an etiologic role for these oncornaviruses in animal neoplasia.9'10 To bring the RNA virus into an association with cellular DNA required a third set of studies that characterized RNAdependent DNA polymerase, or reverse transcriptase, as a system w hereby viral RNA is transcribed, or read into, the host cell DNA and becomes part of the cell s genom e. The identification of reverse transcriptase almost sim ultaneously by Tem in and M izutani at the University of Wisconsin and by Baltimore at the Massachusetts Institute of Technology was a fu n d am en tal b iological d isco v ery.1,24 Once incorporated into the cell s DNA, the transcribed genome may be quiescent or active, depending upon the specific sequences that are transcribed and their relationships. If capable of transform ing the cell, the transcribed viral genom e fulfills the original, som ewhat primitive concept of an oncogene. Many studies have dem onstrated hom ologies betw een norm al cell genomes and oncornaviruses. This concept may be regarded as cellular DNA driving both normal host cell activity and viral rep licatio n. G allo refers to th ese v i ruses as products of a cell gene with some as y et u n sp e c ifie d norm al fu n ctio n.9 W hen fully expressed, the product is a C-type virus. R epeated cellular infections, together with genetic changes both in the host cell and in the virus, seem to increase the chance for tumor transform ation of infected cells. Feline leukem ia virus, Rauscher leukem ia virus, and some m urine sarcoma viruses probably fit this pattern of development.9 Type C viruses may be infectious for primates. Among these is the simian sarcom a virus (SSV), w hich was derived from a spontaneous fibro-sarcoma in a pet animal and was found to be highly re- 163 0091-7370/83/0300-0163 $00.90 Institute for Clinical Science, Inc.
164 W E ISS lated to the gibbon ape leukem ia virus (GaLV). T h ese viruses seem to have originated in rodents and to have entered primates as infectious agents.17,27 This part of the viral-oncogene story was tied more directly to humans by several demonstrations of hum an leukemia cells containing reverse transcriptase, some show ing an im m unological relationship to the reverse transcriptase of the SSV-GaLV virus group.8,25 Reverse transcriptases isolated from a hum an spleen of a patient with myelofibrosis, a child s granulocytic sarcoma associated w ith acute m yelom onocytic leukem ia, and hum an prim ary m elanom a tissue w ere also im m unologically related to SSV and GaLV polymerases.3 Such observations suggest that oncornaviruses are probably common infectious agents in humans. Under the appropriate conditions, the incorporation of the viral transcript, or a portion thereof, into hum an DNA may lead to the developm ent of transformed cell and a tumor, the transcript thereby fulfilling the criteria of an oncogene. How do oncogenes work? The answer to this question w ill probably develop along several different lines of activity, at least one of w hich is reasonably clear. The sarcoma viruses transform cells in vitro by the action of a gene product, a transform ing p ro tein. It is a p ro tein kinase that attaches phosphate groups to other proteins, thereby altering cellular proteins that probably play an im portant part in the regulation of cell growth.4 This protein kinase is unique in that it attaches phosphate groups to tyrosine instead of serine or threonine, w hich are the more usual receptors of phosphate governed by normal cell kinases.12 It appears that the transforming segment, the oncogene proper, is not necessary for the m ultiplication of the virus. The gene can be altered or elim inated w ithout affecting viral m ultiplication. The conclusion must be that the transforming segment is not of intrinsic importance to the virus. From this derives the critical question, why does the virus contain the oncogene? The plausible answer is that the virus has somehow captured a normal cell gene that performs as an oncogene.19 One can visualize a nontransforming virus recombining w ith a host cell DNA segm ent and reproducing with the recom bined DNA that is now capable of inducing transformation. This unusual event has led to an appreciation of the probability that the cells of all animals contain genes that are capable of inducing cancer under the proper conditions. W hat these oncogenes are doing in normal cells is not known, but it is assum ed that they play a role in early tissue developm ent. The fact that normal m ature cells contain oncogenes im m ediately leads to the conclusion that they are mostly inactive. There m ust be a norm al cellu lar control system that inhibits the oncogene s transforming potential. It seems that when a virus captures a normal cell s oncogene, it removes it from its controlled environm ent and, in a sense, releases it. The increase in oncogene n u m b ers re su ltin g from rapid viral replication may also play a role in the eventual efficacy of the oncogene in producing a transformation. The role of the virus in converting an unexpressed oncogene (proto-oncogene) to a transform ing oncogene has been studied by Oskazsson and co-workers.20 A proto-oncogene (the m urine cellular src gene) did not cause transformation w hen introduced into normal host cells with its norm al prom oter. T ransform ation was effected when the viral promoter was attached to the cellular src gene. The leukem ia viruses act slowly, in co n trast to th e ra p id tran sfo rm atio n caused by the sarcoma viruses. Unlike the sarcoma viruses, w hich capture the oncogene, the leukem ia viruses do not contain the oncogene. Rather, they insert them selves next to or near the normal cell
oncogene and activate it. The inserted viral promoter segment may be of different lengths, usually only a fragment of the viral genom e.18 The delayed tumor induction is probably related to the random insertion of the viral fragment into the cell genome. A very large num ber of cells would have to be infected before the appropriate integration of promoter and oncogene took place. The structural point of integration of the prom oter into the cellular genome for effective activation of the oncogene may vary; sometimes it may be a considerable distance from the oncogene.5 This suggests that a promoter can act at a distance from its target oncogene, and not necessarily at one or the other end of the oncogene. To this point, viral oncogenesis has b een the focus of our attention. Viral m ediation of tum or production, however, is only part of the oncogene story. We must also consider that there is a nonviral m echanism for activating oncogenes, now that their normal cellular residence has been proven. Chemical oncogenesis bears no relationship to viral infection; yet, chemicals can cause tumors, and extracted DNA from chem ically induced tumors can be introduced into other cells and cause transformation.23 Most chemicals cause tumors over very long periods. W hereas some may cause mutations and gene transpositions, others (such as horm ones or asbestos) are not m utagens. Since m any chem ical carcinogens act through the multistage process of initiation and promotion, the change probably begins by a m utation or transposition event in the genome, followed by modifications of other cellular genes resulting in new structural changes. At some point in this process, an oncogene that was originally inactive becom es activated. Oncogene expression as cellular transformation may then proceed, modified or restricted only by the action of other gene products. This model suggests that there are two actions: activation of an O N C O G E N E S : AN O V E R V IE W 165 oncogene and modification of the activity of other cellular genes, which enables the activated oncogene to transform the cell.7 The nature and variety of cellular oncogenes are also a matter of intense interest. There is a striking similarity betw een oncogenes in w idely different anim al species, such as chickens and cats.22 Although different oncogenes are transferred by the DNA of different tum or types, the same oncogene is transferred within all tumors of the same type, regardless of species of origin. Furthermore, different oncogenes are activated in tumors derived from the same line of cells depending on the stage of differentiation of the cell line.16 About 15 proto-oncogenes have now b een identified, and a m ethod of notational description has evolved. A threeletter symbol represents the name of the virus in which the proto-oncogene was first defined, e.g., abl = transform ing gene of the A belson m urine leukem ia virus, fes = feline sarcoma virus, and myc = avian myelocytomatosis virus. A prefix v- indicates the gene is carried by the retrovirus, c- designates the homologue in normal cellular DNA. Studies of these oncogenes have shown that there is a startling homology among oncogenes from different tum ors. For example, the EJ hum an bladder tum or oncogene is homologous to c-ha-ras 1, the cellular equivalent of the transforming gene of Harvey murine sarcoma virus.21 Another example is the DNA of the LX-1 hum an lung carcinoma line, which is hom ologous to v-ki-ras, the transforming genes of the Kirsten murine sarcoma virus. Although homology does not require identity, some studies come close to proving that some oncogenes of different origins are in fact identical. Furthermore, by analyzing the protein encoded by v-ras by monoclonal antibody studies, it has b e e n ch aracterized as a p h o s pholipid of m olecular w eight 21,000
166 W E ISS (p21). Cells transformed by the human bladder oncogene also contain increases of p21.6 The conclusion may be drawn that the bladder tum or oncogene is an allele of the normal hum an homologue of ras that has been altered to an active state in a m anner thus far unknown. In its activated state, the formerly normal human gene can act as a transform ing agent either because the gene itself has been subtly altered or its control sequences have been changed, all this probably happening during the period of tumor induction. T he q u an titativ e increase in protein product, e.g., p21, is probably the m eans w h ereb y tran sfo rm atio n takes place. The cu rren t excitem ent about oncogenes and their role in carcinogenesis raises several difficult questions. W here in the chain of oncogenetic events does the oncogene stand? Is the active oncogene an end-product of a m ultiple step process? Is transform ation by an oncogene a one-step activity? Are different cells affected differently by transfection w ith the same oncogene? W hat is the nature of the control segm ent of DNA that turns oncogene activity on and off? The fact that we can ask these questions and address them intelligently represents an enormous advance in our understanding of the m alignant process. Please refer to general reviews.2,13,26 References 1. B a l t im o r e, D.: RNA-dependent DNA polymerase in virions of RNA tumor viruses. Nature 226:1209-1211, 1970. 2. B is h o p, J. M.: The molecular biology of RNA tumor viruses: A physician s guide. New Eng. J. Med. 303:675-682, 1980. 3. C h a n d r a, P.: Immunological characterization of RNA-dependent DNA polymerase (reverse transcriptase) from human tumors: An approach towards establishing the expression of oncornavirus information in man. Advances in Medical Oncology Research and Education, vol. I, Carcinogenesis. Margison, G. P., ed. Proceedings of the 12th International Cancer Congress, 1978, Buenos Aires. Oxford, Pergamon Press, pp. 29-41. 4. C o l l e t t, M. S. and E r ik s o n, R. L.: Protein kinase activity associated with the avian sarcoma virus src gen product. Proc. Nat. Acad. Sci. 75:2021-2024, 1978. 5. C o o p e r, G. M. and N e im a n, P. E.: Transforming genes of neoplasms induced by avian lymphoid leukosis viruses. Nature 287:656 659, 1980. 6. D e r, C. J., K r o n t i r u s, T. G., and C o o p e r, G. M.: Transforming genes of human bladder and lung carcinoma cell lines are homologous to the ras genes of Harvey and Kirsten sarcoma viruses. Proc. Nat. Acad. Sci. 79:3637-3640, 1982. 7. D u l b e c c o, R.: The nature of cancer. Endeavor 6:59-65, 1982. 8. G a l l a g h e r, R. E., T o d a r o, G. H., S m i t h, R. G., L iv in g s t o n, D. M., and G a l l o, R. C.: Relationship between RNA-directed DNA polymerase (reverse transcriptase) from hum an acute leukemia blood cells and primate type-c viruses. Proc. Nat. Acad. Sci. 71:1309-1313, 1974. 9. G a l l o, R. C.: RNA viruses, genes, and cancer. Genetics of Human Cancer. Mulvihill, J. J., Miller, R. W., and Fraumeni J. F., Jr., eds. New York. Raven Press, 1977, pp. 455-463. 10. G a l l o, R. C. and T in g, R. C.: Cancer viruses. CRC Crit. Rev. Clin. Lab. Sci.3:403-449,1972. 11. G r o s s, L.: The role of viruses in the etiology of cancer and leukem ia. J. Amer. Med. Assoc. 230:1029-1032, 1974. 12. H u n t e r, T. and S u t t o n, B. M.: Transforming gene product of Rous sarcoma virus phosphorylates tyrosine. Proc. Nat. Acad. Sci. 77:1311-1315, 1980. 13. K a r p a s, A.: Viruses and leukemia. Amer. Scient. 70:277-285, 1982. 14. K l e in, G.: The Epstein-Barr virus and neoplasia. New Engl. J. Med. 293:1353 1357, 1975. 15. K n u d s o n, A. G.: Mutation and cancer. Statistical study of retinoblastoma. Proc. Nat. Acad. Sci. 68:820-823, 1971. 16. L a n e, M. A., S a i n t e n, A., and C o o p e r, G. M.: Stage-specific transforming genes of human and mouse B- and T-lymphocyte neoplasms. Cell 28:873-880, 1982. 17. L i e b e r, M. M., S h e r r, C. J., T o d a r o, G. J., B e n v e n is t e, R. E., C a l l a h a n, R., and C o o n, H. G.: Isolation from the Asian mouse Mus caroli of an endogenous type C virus related to infectious primate type C viruses. Proc. Nat. Acad. Sci. 72:2315-2319, 1975. 18. N e e l, B. G., H a y w a r d, W. S., R o b i n s o n, H. L., F a n g, J., and A s t r i n, S. M.: Avian leukosis virus-induced tumors have common proviral integration sites and synthesize discrete new RNAs: Oncogenesis by promoter insertion. Cell 23:323-334, 1981. 19. O p p e r m a n, H., L e v in s o n, A. D., V a r m u s, H. E., L e v in t o w, L., and B i s h o p, J. M.: Uninfected vertebrate cells contain a protein that is closely related to the product of the avian sarcoma virus transforming gene (src). Proc. Nat. Acad. Sci. 76:1804-1808, 1979.
20. O s k a z s s o n, M., M c C l e m e n t s, W. L., B l a ir, D. G., M a iz e l, J. V., and V a n d e W o n d e, G. F.: Properties of a normal mouse cell DNA sequence (sarc) homologous to the src sequence of Maloney sarcoma virus. Science 207:1222 1224, 1980. 21. P a r a d a, L. F., T a b i n, C. J., S h i h, C., and W e i n b e r g, R. A.: Human EJ bladder carcinoma oncogene is homologue of Harvey sarcoma virus ras gene. Nature 297:474 478, 1982. 22. S h ib u y a, M., H a n a f u s a, T., H a n a f u s a, H., a n d S t e p h e n s o n, J. R.: H o m o lo g y e x is ts a m o n g t h e tr a n s f o r m in g s e q u e n c e s o f a v i a n a n d f e l i n e s a r c o m a v i r u s e s. P r o c. Nat. Acad. S c i. USA 77:6536-6540, 1980. 23. S h i h, C., S h i l o, B-Z., G o l d f a r b, M. P., D a n - n e n b e r g, A., and W e i n b e r g, R. A.: Passage o f O N C O G E N E S : AN O V E R V IE W 167 phenotypes of chemically transformed cells via transfection of DNA and chromatin. Proc. Nat. Acad. Sci. 76:5714-5718, 1979. 24. T e m i n, H. and M iz u t a n i, S.: RNA-dependent DNA polymerase in virions of Rous sarcoma virus. Nature 226:1211-1213, 1970. 25. T o d a r o, G. J. and G a l l o, R. C.: Immunological relationship of DNA polymerase from human acute leukemia cells and primate and mouse leukemia virus reverse transcriptase. Nature 244:206-209, 1973. 26. V a r m u s, H. E.: Form and function of retroviral proviruses. Science 2i6:812-820, 1980. 27. W o n g -S t a a l, F., G a l l o, R. C., and G il l e s p ie, D.: G enetic relationship of a prim ate RNA tumor virus genome to genes to normal mice. Nature 256:670-672, 1975.