JOURNAL OF PATHOLOGY, VOL. 143: 1-10 (1984)

ONCOGENES JOHN PAUL

Beatson Institute for Cancer Research, Garscube Estate, Switchback Road, Bearsden, Glasgow G61 I BD, U. K .

Received 16 January 1984 Accepted 17 January 1984

KEY woms-Cancer, onogenesis, oncogenetic retroviruses.

INTRODUCTION

The idea of oncogenes was proposed by Huebner and Todaro’ in 1969 in the context of tumorigenesis by tumour viruses. They proposed that viral genes could be integrated into the cellular genome where they might remain dormant or become ‘activated’, thus causing cancer. Although not absolutely correct in detail, this has proved to be a prophetic hypothesis.

The present model (Fig. l), the evidence for which will be discussed, is as Eukaryotic cells contain certain genes (proto- oncogenes) concerned with the control (possibly cell-type specific) of growth and differentiation. Quantitative or qualitative anomalies of expression of these genes can give rise to malignant transformation. Anomalies may arise from the action of mutagens (carcinogens, radiations), viruses or random events on the genes themselves. Moreover. the gene sequences may be picked up by certain viruses (in which they occur as viral oncogenes) and transduced to other cells. Viruses which have acquired cellular sequences of this kind are recognized as oncogenic retroviruses.

ONCOGENIC RETROVIRUSES5

The background discoveries that led to this model arose mainly from the study of oncornaviruses, now usually called oncogenic retroviruses. In these viruses (Fig. 2) the genome is composed of RNA. It is present as two identical copies enclosed within a capsid which is itself enclosed within a glycoprotein envelope. On entry to a cell, the envelope and capsid are first removed (uncoating). The RNA is copied by the enzyme

reverse transcriptase, also carried by the virus, into a DNA molecule, called a provirus, which becomes integrated into the cellular genome. The provirus is then treated as a cellular gene and transcribed by RNA polymerase into new RNA molecules identical with the original viral genome. These can have two different fates. Some act as messenger RNA molecules for the synthesis of new capsid and envelope proteins and viral reverse transcriptase. Others form the genomes for new viruses which assemble spontaneously from the different components. Finally the new virus particles bud off from the surface of the infected cells.

Two different classes of oncogenic retroviruses have been recognized. They are distinguished by their ability to induce tumours slowly or rapidly. The archetypic slow retrovirus contains only three genes, called Gag, Pol and Env. Gag (group specific antigen) genes carry the information for synthesis of capsid proteins; Pol (polymerase) genes carry information for reverse transcriptase (RNA dependent DNA polymerase); and Env (envelope) genes carry information for specific glycoproteins which form a component of the viral envelope, the rest of which is derived from cell membrane. Viruses which contain these three genes have all the components needed to proceed through the complete replicative cycle as described above. Typical examples are Avian Leukosis Virus (ALV), Murine Leukaemia Virus (MuLV) and Feline Leukaemia Virus (FeLV). They occur as natural wild-type viruses infecting populations of fowls, mice or cats in which they typically produce lymphomas or leukaemias. It is characteristic of these viruses that the incubation time of the disease is long, some months following inoculation.

0022-341 7/84/050001-1O$01 .OO 0 1984 by John Wiley & Sons, Ltd.

2

r Mu togenesis

J. PAUL

- lronsduction

PROTO - ONCOGENE - 4

Growlh/Differentiotion peptide

ACTIVATED CELLULAR ONCOGENE

Abnormal or Increased

PROVIRAL ONCOGENE

Abnormal peplide

8. TRANSFORMED CELL C. TRANSFORMED CELL

Fig. 1-A model to explain the possible role of oncogenes in carcinogenesis. See text for explanation

The second group, the rapidly transforming retroviruses, are closely related to the first but differ in important respects. They do not occur widely in wild populations but have nearly always been isolated from individual tumours. They do not readily infect animals by natural routes and have usually to be inoculated by injection. They produce tumours very rapidly, in days or weeks.

Only one rapidly transforming virus, the Rous Sarcoma Virus (RSV) is ‘complete’ (non- defective), all others being replication defective as described below. The Rous Sarcoma Virus contains four instead of three genes. Three of these are similar to the genes of ALV, the Gag, Pol and Env genes (Fig. 3). The fourth, called v-src, is particularly interesting because it can be shown that this gene is necessary and sufficient for the production of cellular transformation. The evidence for this statement is as follows (Fig. 4). RSV causes cellular transformation in cultured chick fibroblasts. The normal cells usually grow as sheets of fusiform cells orientated in parallel arrays. On infection with RSV this orderly appearance is lost. The cells round up and pile up on each other to form foci or colonies where individual infectious particles have entered cells. Fibroblastic transformation gives rise to cells which cause tumours when they are reinoculated into a chicken. Variants of RSV in which the v-src gene has been inactivated by deletion or muta- tion do not cause fibroblastic transformation. Moreover, temperature-sensitive mutants of the v-src gene have been produced and cells infected with the corresponding RSV show a transformed

morphology at low (permissive) temperature but a normal morphology at a higher (non-permissive) temperature. Finally, by genetic engineering methods, it has proved possible to isolate the DNA corresponding to the v-src gene, to insert it into a bacterial plasmid and by this means to obtain quite large amounts of the pure gene as DNA. This by itself can be shown to transform fibroblasts (Fig.

The v-src gene is now known to be one of the small family (about 20) of tranforming genes known together as viral oncogenes (v-oncs) each of which is typical of a particular acute transforming retrovirus. However, whereas v-src exists in RSV as an extra gene added to Gag, Pol and Env, all the other viral oncogenes are components of defective viruses in which part of the Gag, Pol and Env genes are deleted and replaced by the v-onc. Commonly, the Pol gene is completely absent from these viruses and also part of either the Gag gene or the Env gene. Typical genetic configurations of viruses of this kind are shown in Fig. 3. These viruses are ‘defective’; they do not have the capacity to go through a replicative cycle by themselves but if cells are co-infected with them and a ‘helper’ virus such as ALV which provides the missing functions then they can be propagated and will produce tumours in infected animals. Individual viruses give rise to specific neoplasms, for example MuSV (Murine Sarcoma Virus) causes mainly sarcomas, AEV (Avian Erythroblastosis Virus) causes mainly erythroblastosis and so on. They are not completely specific for although the tumour most commonly produced is characteristic of each virus

4).

ONCOGENES 3

Among the oncogenes which code for tyrosine kinase activities are v-src, v-fps, v-fes, v-yes, v-abl and v-ros but not all oncogenes carry this kind of information and several quite different kinds of oncogene-related activities have so far been described.

Virus @ Cell Membrane

/ Uncootong 1 \

Reverse Transcription RNA

Viral DNA

Nuclear Envelope

-Viral

Messe ngcr --: 1 1 1 RNA I tenant Vim --- ,gag pol e n v ,

1 Virus @

Fig. 2-A scheme summarizing the life cycle of a typical non-defective retrovirus. See text for explanation

and is directly correlated with the v-onc it contains, most will also give a low incidence of different tumours. Table I lists some known acute transforming retroviruses and their oncogenes.

A good deal has been learned about the protein products of v-oncs. Many of them, related to v-src, possess a particular enzymic activity, the ability to phosphorylate tyrosine in certain proteins. This tyrosine kinase is usually associated with the cell membrane and there are parallels with normal cellular functions. One of the most interesting of these relates to the receptor for epidermal growth factor which is apparently linked to a tyrosine kinase activity.

PROTO-ONCOGENES

Current interest in cellular oncogenes (c-oncs) arose when it was found that DNA from normal tissues of all multicellular eukaryotes examined contains genes similar to v-oncs. This discovery6 arose from the application of the technique of DNA hybridization and particularly a method known as Southern blotting.

DNA hybridization depends on the base-pairing relationship in double-stranded DNA. Adenine (A) in one DNA strand, always pairs with thymine (T) in the other strand and cytosine (C) in one strand with guanine (G) in the other. Consequent- ly, the sequence of bases (such as CAGTACGCT) in one strand dictates the sequence of bases in the other strand (in this instance it would be GTCATGCGA according to the rules just stated). This relationship is very highly specific, almost exactly as specific as the relationship between a Xerox copy of a page of a book and the original. Moreover, if DNA is heated the two strands separate and, if cooled, the complementary strands reassociate-but only if they are perfectly or nearly perfectly matched. It follows that a radioactively-labelled purified strand of DNA can be used as a tracer to recognize complementary sequences. If such a probe is added to a very complex mixture of DNA molecules and then put through the annealing procedure, radioactive double-stranded DNA will form only if a com- plementary (matching) strand is present in the mixture and there are techniques for identifying this.

Using DNA copied from v-onc RNA by reverse transcriptase as a probe, it became possible to determine whether DNA from normal animals contains matching sequences. This turns out to be the case and proves the existence of normal cellular sequences closely related to v-oncs which are referred to as proto-oncogenes.

The specially sensitive hybridization technique used to demonstrate these is known as the Southern blot. This exploits special endonucleases called restriction endonucleases which have the

4 J . PAUL

ROUS SARCOMA VIRUS Onc 3'

0 Gog 1 Pol I Env 5'

AVIAN LEUKOSIS VIRUS / MURINE LEUKAEMIA VIRUS 3 '

0 Go 9 Po l Env 5'

I I

DEFECTIVE RECOMB IN ANT VIR USES

5 ' 1 E n v - o n c 3 '

5' Gog - o n c - env 31 0

Fig. 3-Diagrams illustrating the arrangements of the genes in some retroviruses. The horizontal lines represent the RNA molecules which form the viral chromosomes. Filled in blocks represent viral oncogenes. Open blocks are special 5' and 3' sequences carrying transcriptional signals. The RNA is transcribed from the provirus (see text) from 5' end to 3' end

property of cleaving DNA molecules at unique sites defined by a sequence of bases (usually 4 or 6) which the enzyme can distinguish from other sequences. There are a large number of these restriction endonucleases and each one cleaves at a different site. A typical restriction enzyme, called EcoRl, cuts DNA wherever the sequence GAATTC occurs. This sequence occurs randomly in DNA but, on the average, roughly every 4000 base pairs. Consequently, if a piece of DNA is completely digested with the enzyme, it gives rise to many fragments varying from a few base pairs to many thousands of base pairs in length but averaging this size. A simple piece of DNA such as DNA from a bacteriophage 40 000-50 000 base pairs long may be cut into a small number of discrete pieces, roughly about 10. These can be separated into discrete fragments of specific size by electrophoresing them in an agarose gel. A more complex DNA preparation, such as that from human cells, also gives rise to fragments of unique size but with such complex material there may be more than 100 000 of them. Nevertheless, these can also be separated on a gel. The problem is that there are so many that they cannot readily be discriminated and if one simply looks at the distribution of all the DNA fragment they appear as a smear. The Southern technique solves this problem in an ingenious way. First, the DNA is transferred from the agarose to a sheet of cellulose acetate filter paper (this is the blotting component of the Southern blot technique). Unique fragments of DNA can then be identified by hybridizing a

specific probe to this cellulose acetate sheet. For example, a probe for an oncogene may hybridize with only one of the 100 000 or so bands present on the paper. By the application of this method it has been shown that every single v-onc is related to sequences which occur in specific restriction fragments in every animal DNA tested, even in species as disparate as the human and Drosophila. These normal genes, related to v-oncs are called proto-oncogenes.

VIRAL ONCOGENES ARE DERIVED FROM NORMAL CELLULAR GENES

These are remarkable findings and their implica- tions have had a profound impact on recent thinking about cancer. The first implication is that, since they are detected in DNA from a wide range of species the genes with which the v-onc DNAs hybridize must have been very rigorously con- served during evolution because the ability to hybridize depends on rather close similarity between the two DNAs used. The second implica- tion is that either proto-oncogenes are derived from v-oncs or v-oncs are derived from proto-oncogenes. In view of the universal preva- lence of these sequences in vertebrate DNAs it would seem much more likely that the v-oncs are derived from proto-oncogenes than the other way round. There is convincing independent evidence of this. By genetic engineering methods it is possible to clone individual genes from the genome

ONCOGENES 5 R U5

Rous sarcoma Virus RNA

U 3 R U 5 I U 3 R U 5

gag 7 Proviral DNA

amplified by clonlng in bacteriophage

Restrict~on Endonucleose - ‘-v-src A

I f7I)---- gag pol env

\ Ligated into plasmid

Amplified in

bacteria 8 purified J ~

v -src (Pure and in high yield 1 I

oncogene the information is split up, pieces of protein-coding sequences (exons) alternating with intervening sequences (IVS or introns). It is known that this is the typical structure of vertebrate genes.

The alternating exon-intron structure of a normal cellular gene is transcribed into a high molecular weight RNA molecule which is then ‘processed’ to remove the RNA corresponding to the introns to yield a cellular messenger RNA which is very similar indeed to the viral messenger RNA. These studies reveal, therefore, that proto-oncogenes have the structure of typical cellular genes and strongly suggest that the viral oncogenes arose by reverse transcription of a messenger RNA which was originally derived by processing of the transcript from a proto- oncogene.

Information about the kinds of proteins pro- duced by v-oncs and their similarity to proto- oncogenes stimulated the speculation that the proteins coded by proto-oncogenes are in some way concerned with growth regulation. This idea has been very much strengthened by the recent finding that the v-sis gene codes for a protein very similar to a well-known growth factor, the Platelet Derived Growth Factor (PDGF).’.’

‘ACTIVATED’ CELLULAR ONCOGENES’

1

Transformed cells

Fig. +Diagram of an experiment to test the function of v-src. The gene is isolated by genetic engineering methods which cnable i t to be purified in amounts which can be used to transform cells

of a vertebrate and in this way some of the proto-oncogenes from mice, humans and other species have been isolated, purified and analysed. It is these analyses which have provided revealing information about the nature of proto-oncogenes for not only are there striking resemblances between v-oncs and proto-oncogenes but there are also striking differences. In particular, in v-oncs the nucleotide sequences which code for a protein usually occur in one continuous block, a situation fairly typical of viruses. However, in proto-

Since viral oncogenes have the ability to transform cells very rapidly, the question naturally arises-can cancer arise by inappropriate express- ion of the related proto-oncogenes? Much in- formation has accumulated recently to show that inappropriate expression of these genes is, indeed, frequently associated with cancer and this will now be discussed.

The first evidence’’ arose from study of bursa1 lymphomatosis in chickens, a condition produced by infection with the Avian Leukosis Virus (ALV). It will be recollected that ALV does not contain an oncogene but that it can give rise to lymphomatosis after a long incubation period. When DNA from the lymphomas was analysed by the Southern blotting technique, it was discovered that restriction fragments in the avian genome which contained the ALV provirus also contained sequences which hybridized with a viral oncogene, v-myc. The startling implication was that in these particular tumours the ALV provirus must have integrated very near the proto-oncogene, c-myc, in

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Table I-Viral oncogenes. Some are related, e.g. v-fps and v-fes appear to be the same oncogene from different species

Virus

Rous sarcoma Fujinama sarcoma Yamaguchi sarcoma Rochester-2 sarcoma Myelocytomatosis MCZY

Avian erythroblastosis Myeloblastosis Avian SKV770 Reticulo-endotheliosis Moloney murine sarcoma Harvey murine sarcoma Kirsten murine sarcoma 3611 murine sarcoma Abelson murine leukaemia FBJ murine osteosarcoma Feline sarcoma Gardner-Pasheed feline sarcoma McDonough feline sarcoma Simian sarcoma

Species of origin

Fowl Fowl Fowl Fowl Fowl

Fowl Fowl Fowl Turkey Mouse Rat Rat Mouse Mouse Mouse Cat Cat Cat Monkey

. ~~

v-onc

v-src v-fps v-yes v-ros v-myc

v-erbB v-myb v-ski v-re1 v-mos v - H - r a s v-Ki-ras v-rab v-abl v-fos v-fes v-fgr v-fms v-sis

Type of tumour

Sarcoma Sarcoma Sarcoma Sarcoma Carcinoma, sarcoma,

Leukaemia, sarcoma Leukaemia

__ ~~

leukaemia

Leukaemia Sarcoma Sarcoma Sarcoma Sarcoma Leukaemia Sarcoma Sarcoma Sarcoma Sarcoma Sarcoma

Proto-oncogene

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

the cellular genome and this had somehow led to an alteration of the behaviour of the cell such that it became neoplastic. It was soon shown that at least part of this story was correct in that, in most lymphomas examined, ALV sequences were indeed integrated adjacent to the proto-oncogene, c-myc. Moreover, there was evidence that, in the infected cells, c-myc was expressed at a high level. This mechanism for the ‘activation’ of c-myc was further studied in the light of the structure of ALV.

The RNA genome of ALV, like all related retroviruses, has a short sequence (R) which is repeated at both end (see Fig. 4). At the 5‘ end of the RNA the short repeat sequence is followed by a special unique sequence, U5, and at the 3’ end the short repeat is preceded by a unique sequence, called U3. When the RNA is transcribed into DNA and integrated into the host genome, the end sequences are duplicated in such a way that the proviral DNA sequences become flanked by identical sequences called long terminal repeats (LTR) at either end, consisting of the U3 sequence linked to an R sequence and followed by the U5 sequence. This long terminal repeat sequence with the U3RU.5 structure has been found to have some extremely interesting properties. For one thing, it contains the promoter sequences from which

transcription of RNA starts. (In the orientation shown in Fig. 4 the genes are usually transcribed from left-‘upstream’-to right-‘downstream’). It also contains a termination sequence where the RNA transcript stops. Besides these, it contains another kind of sequence called an enhancer which has the remarkable property of increasing the rate of expression of most DNA sequences within a few thousand base pairs in either direction.

In the first studies on bursa1 lymphomas. evidence was found that ALV sequences were inserted in front of the c-myc gene and this gave rise to the ‘promoter insertion’ model of carci- nogenesis (Fig. 5 ) . It was postulated that transcrip- tion could start not only at the ‘upstream’ LTR. giving rise to transcription of the viral genome, but also from the ‘downstream’ LTR, giving rise to a transcript which ran into the host genome and could account for transcription of the adjacent c-myc. This mechanism may indeed operate in certain cases but it soon turned out that it could not account for all because the LTR was active even when it was inserted downstream of the c-myc gene! It is now thought, therefore, that viral activation of a c-onc may only require proximity of an enhancer sequence such as that contained in the viral LTR.

ONCOGENES 7

7 Normal Prolo- oncogene A t + ~ _ .

pre-rn RNA

8. t

b ---* ---------------- Mutat ions

Fig. S-Modes of activation of proto-oncogenes. (A) Represents a normal proto-oncogenc with start and stop signals. The dotted line represents the transcript from it. (B) Represents ;I similar gene in which an LTR. usually associated with a proviral sequence. has been insertcd in front of the gene causing an increased rate of transcription. (C) Illustrates another examplc of insertional mutagenesis in which an enhancer. usually associated with a rctrovirus has been inserted near the gene. (D) Illustrates two possible mutations. one a deletion and the other ii point mutation

TRANSFORMATION BY CELLULAR DNA

The next important observation was based on an assay for cell transformation using a special cell line called NIH 3T3. This cell line had been used for some years to study transformation by retroviruses and in the course of this work experiments were designed to investigate whether proviruses, i.e. integrated retrovirus sequences, could transform. These experiments involved taking DNA from transformed cells, and compar- ing the efficiency of this in transforming NIH 3T3 cells with DNA from untransformed cells. It was found that normal DNA cut into small fragments would not transform NIH 3T3 cells very effectively but DNA from transformed cells would do so." The discovery was then made that DNA from many cell lines derived from tumours could also transform NIH 3T3 cells although they had not been treated with a virus. Some of the tumours which had this property included not only well- known experimental tumours in animals but also human spontaneous tumours.

These findings stimulated studies aimed at identifying the nature of the DNA which produced the transformation. By the application of genetic engineering methods the responsible genes were isolated and analysed. It was first discovered by Southern blotting that the transforming sequences in many cases were related to v-oncs, particularly to the v-oncs, Ki-ras and Ha-ras. These are closely related, the Ki-v-ras and Ha-v-ras being the oncogenes of viruses which produce sarcomas in rodents. When some of the transforming genes were isolated and the DNA sequences established, it became clear that they were not genes which had originated as v-oncs but were 'activated' c-oncs. The NIH 3T3 cell seems to be particularly sensitive to transformation by activated c-ras genes although i t does also reveal the presence of some other transforming genes.

CHANGES ASSOCIATED WITH ACTIVA- TION OF c-oncs

The nature of the gene alteration which produces activation has obviously become a matter of intense study, particularly from two points of view. The first of these is: what is the normal function of the proto-oncogene? The second is: what alteration of the gene and of the gene product accounts for the cellular transformation?

Several different mechanisms have been unco- vered which are associated with activation of proto-oncogenes (Fig. 5 ) . Activation by an LTR has already been discussed and some very interesting experiments have been done in which a viral LTR has been chemically linked to Ha-c-ras. Whereas Ha-c-ras by itself will not normally transform NIH 3T3 cells, this complex will. However, it has to be mentioned that the efficiency of transformation with this combination is still considerably less than that of DNA containing Ha-v-ras and, moreover, most similar combina- tions between LTRs and other proto-oncogenes do not lead to cellular transformation. So this is not the whole story.

In some instances, amplification of proto- oncogenes, that is, an increase in the number of copies in the genome, has been correlated with transformation and in yet others point mutations would seem to be necessary and sufficient for oncogene activation. A particularly interesting case of the latter is a point mutation affecting the twelfth amino acid of the ras gene product, which results in activation of the Ha-c-ras gene. The same

8 J . PAUL

amino acid is altered in the product of Ha-v-ras and Ki-v-ras, although the rest of this peptide is very rigorously conserved.

CHROMOSOME TRANSLOCATION'2-'4

Perhaps the most interesting correlation has been found to be with certain chromosomal translocations. In Burkitt's lymphoma, chromo- some 8 is very commonly involved in a reciprocal translocation with chromosome 2, chromosome 14 or chromosome 22 of which the 8 : 14 translocation is by far the commonest (Fig. 6). The myc

8

proto-oncogene has been located on chromosome 8 near the point of translocation and chromosomes 2, 14 and 22 each are known to code for immunoglobulin chains, again in the region of the translocation. It has now been found that in many cases of Burkitt's lymphoma, this translocation involves the reorganization of the myc gene and indeed it has been found that frequently the immunoglobulin gene is fused (usually back-to- back) with the myc gene. Very recently irnmunog- lobulin genes have been demonstrated to contain an enhancer element which acts over a consider- able distance and it has, therefore, been postulated that this accounts for the activation of the myc gene

Heavy Chain

X Light Chain

K Light Chain

Fig. &Diagram of human chromosomes 8, 2, 14 and 22 which are involved in translocations as seen in Burkitt's lymphoma. The position of c-myc on chromosome 8 is approximately in the region of q24. Reciprocal translocations can occur with fragments of the other chromosomes: these involve breaks at the positions of the immunoglobulin genes shown

ONCOGENES 9

in this condition. However, it has to be mentioned that there are recent observations which conflict with this otherwise attractive hypothesis. Other chromosomal translocations have been shown to be regularly correlated with certain tumours and in many instances the translocation in one chromo- some occurs in the region of a proto-oncogene.

HOW MANY EVENTS ARE NEEDED FOR TRANSFORMATION?

These and other observations have, therefore, led to the rather strong speculation that proto- oncogenes are genes concerned with cellular growth and that their activation by one of the means outlined gives rise to a partial relaxation of the control of growth of the cells in which the change has occurred. Many of the experimental findings still require to be pursued further to harden up the speculations and certain important questions remain outstanding. One of these relates to the number of events necessary to cause transformation of a normal cell to a malignant cell. In the natural evolution of human cancers there is evidence for a requirement for at least two and possibly several events. How then can this be reconciled with the observations on transforma- tion of NIH 3T3 cells by a single event. Some experiments which have been done recently suggested that NIH 3T3 cells are unusual and that, indeed, more than one event is involved in transformation of normal cells. For example, although in the bursa1 lymphoma of chickens there is evidence for activation of the c-myc gene, this is not the gene which is identified when DNA from these cells is used to transform NIH 3T3 cells. Instead, a different transforming sequence, B-lym, has been isolated and purified. Moreover, in other instances involving malignancies arising from normal cells i t has been claimed that a single activated oncogene may not be sufficient for transformation. Since the NIH 3T3 cell seems to be particularly sensitive to transformation by c-ras, the possibility has to be considered that it is already partly transformed.

The current feeling is that to transform normal cells at least two events are needed.’ One is an alteration which ‘immortalizes’ cells, i.e. makes them capable of indefinite survival in culture although they are not capable of forming tumours in susceotible animals. If this is combined with a

activation of a cellular oncogene then a transplant- able tumour results. It is too early to say whether this is a scenario which applies to a significant number of ‘spontaneous’ naturally-occurring tumours and there are some experiments from this Institute which suggest it is an oversimplification but .it provides a strong hypothesis for further investigations.

Not all questions about the molecular abnorma- lities in malignancy have therefore been answered by any means. Indeed we have a long way to go before we have anything like a complete under- standing of the molecular nature of cancer but these findings have influenced our thinking dramatically and have provided both a theoretical framework and a wide range of experimental possibilities for further investigation. The field of cancer research has been altered for all time and there can be little doubt that within the next few years we will have a detailed understanding of at least some of the mechanisms concerned with the transformation of a normal cell to a malignant one.

ACKNOWLEDGEMENT

The Beatson Institute is supported by the Cancer Research Campaign.

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