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01. ONCOGENES

Bell J.

Department of Biochemistry, MC Gill University, 3655

Drummond, Montreal, Quebec, Canada, H3G ly6.

Malignant tumours arise as a result of a complex series of genetic and epigenetic events. It seems logical to suggest that the abberations of normal growth and differentiation programs that occur during oncogenesia could arise in part from mutations in genes coding for regulatory proteins. The recent identificaclon of oncogenes and the elucidation of the biochemical and biological properties of their protein products indicates that this is indeed the case. Proto-oncogenea and ocogenes, their mutated counterparts, code for proteins which play integral roles in signal transduction, mRNA biogenesis and DNA replication.

ACTIVATION OF PROTO-ONCOGENES

Conversion of normal genes to their transforming counterparts appears to occur by a discrete series of mechanisms: point mutation, amplification, insertional muta- genesis and chromosomal translocation. Specific examples of each of these mechanisms will be presented.

ONCOGENES AND THEIR PROTEIN PRODUCTS

Oncogenes were originally defined by the analysis of acutely transforming retro- viruses, however it is now apparent that the genomes of all eukaryotic species contain a set of genes which upon activation are capable of transforming cells to the malignant state. The protein products of these oncogenes can be loosely categorized as falling into one of the following four classes:

1) GTP/CDP binding proteins: The ras oncogene family codes for a series of

related proteins of molecular weight 21,000 that all share the biochemical property of binding and hydrolyaing GTP. This observation coupled with the fact that ras proteins are localized on the inside surface of the plasma membrane suggests that these polypeptides may play a role in signal transduction. Indeed genetic evidence from the study of yeast raa proteins suggests that they may be components of the adenylate cyclase system that couples nutritional information to intracellular CAMP levels. A mutated ras protein may therefore contribute to the generation of malignancies by inappropriate signal transduction. In my talk I will consider the evidence which supports this hypothesis as well as discuss the mechanism of ras proto-oncogene activation to the oncogenic state.

2) Nuclear proteins: A number of oncogene products including the polypeptides

coded for by myc, myb, fos and ski oncogenes are associated wiith the nucleus and I will review the evidence that they may play some role in the regulation of mRNA

transcription.

3) Growth factors: Sequence analysis has revealed that the ais oncogene codes for a protein very similar to platelet derived growth factor. It seems likely that this oncogene and analogous growth factors such as TGF-a may contribute to malignancies by auCocrine stimulation of target cells.

4) Protein kinaaes: Protein phosphorylation/dephosphorylation is one of the major mechanisms by which growth and differentiation programs appear to be regulated. Not surprisingly then, kinases (and possibly phosphatases) appear to be one of the major targets of genetic lesions vhich lead to the generation of

malignancies. Serine/threonine kinases coded for by the v-mos and v-raf onco- genes have been identified although their biological role has yet to be identified.

Phosphotyrosine is a rare modification in the normal cell (0.02% of total phosphoamino acids) but it is implicated as having an important role in the regulation of cell growth and differentiation. Indeed, a large proportion of all oncogenes appear to be members of the unique group of kinases that phosphorylate tyrosine (PTK). Included in these are the oncogenes fms, erb, and nau which code for proteins representing mutated forms of grovth and differentiation factor receptors. Another group of oncogenes including abl, src and fps are PTK that are located on the inside surface of the plasma membrane. To understand how protein tyrosine kinases regulate normal and abnormal cell growth and differen- tiation, it is necessary to identify their substrates and to this end we have used antibodies to phosphotyrosine to attempt to identify phosphotyrosyl proteins in normal and transformed cells. I will conclude my talk by presenting evidence which suggests that one way PTK may control cell growth is through direct phos- phorylation of sequence specific DNA binding proteins.