August 14, 1986

From Gene to Oncogene

Salvador E. Luria


2

The topic of this lecture -- from Gene to

Oncogene --  is both historical and personal:


historical because I wish to recall the story

of a concept -- the gene concept -- with fifty


years since the beginning of molecular biology;

and personal,  because fifty years ago the gene


took me in its wake and molded my life, making

a naive young physician into a molecular biolo-

gist and causing me fifty years later to end my

career as a cancer research administrator.

The cancer of the gene itself was stupen-

dous.  The rise of molecular biology has been


one of those exhilarating periods in the his-

tory of science in which a whole branch of sci-

ence seems to fall into place.   For molecular

biology the process has been even faster than


                                        3


for other sciences:  what are fifty years com-


pared with the course of mechanics to Galileo

to Newton and          or that of chemistry



from John Dalton to                              --



that is,  from the first formulation of models



             ishment as a 11 embracing

to their establ

theories?

Concepts are inventions that scientists

produce in order to give body to their theo-

ries.  Galileo's acceleration, Newton's mass



and force and       waves were concepts of


accidents (in the scholastic sum of the word),

that is,  of properties of the nation of matter.


Dalton's alone was a concept of matter itself,

included to explain gas pressure and the pro-

portions of elements participating in chemical

reactions.


4

The gene concept emerged form Mendel's al-

gebra (and his rediscoverers) a concept of mat-

ter,  the substance of heredity, a concept as

naked and as perfect as Botticelli's Venus, and

equally enigmatic.   What kind of matter?

Heredity from one generation to the next was

atomized into the statistical assortment of

       of discrete units,  two copies of each


in freely        pairs.  Two copies of a


unit could be sufficient enough to change shape

or color or size of an organism but not to al-

ter the plan, that is,  the intrinsic program of

the organizer.  Organisms were born, developed,

died, disintegrated; but through the process of

reproduction genes escaped the fate of decay --

stable not as atoms are stable but because of

copied identically from one gener-


5

ation to the next and from one cell to its

daughter cells --  stable while all other sub-


stances of living cells and organisms were in a

state of chemical and structural flux.

Stable yet unstable only because of its

ability to change could the existence of any

gene be recognized (until molecular biology

made other approaches possible).   It had to ex-



ist in at least two alternative forms when

assortment followed Mendel's algebra.   From the


work of Morgan, Sturbevant, Bridges, and Miiller

it became known that genes were linked together

in        sequencing --  the chromosomes --


the sequence being itself almost perfectly sta-

ble.  Heredity was determined by thousands of


almost perfect pearls strung in a few almost

perfect necklaces -- fit to adorn the body of


Botticelli's

    Yet,  not pearls nor any other known sub-



stance.   Until the late 1930's no one could do


more than guess what genes really are.   Then


physics developed into quantum mechanics, and

Heither and London developed the theory of the

covalent bonds between atoms in molecules.   And


in 1937,  the physicist Max Delbrfick proposed


that the stability of a gene was similar to the

stability of molecules in general.   Genes were



molecules,  large molecules,  and mutations in-


cluding those induced by radiation, were

changes in the gene molecules, that is, chem-

ical changes.

I do not know how many scientists were

electrified by Delbriick's model of the gene.

It influenced the great physicist Schraedinger


to write years later a famous book, What is



Life?.  At a humbler level, Delbriick's model


influenced a young physician, myself, who was


naively dreaming of joining physics to biology


(knowing little of either) into jumping into


the fray.  I decided that I would try to test


Delbriick's model of gene mutation by studying


the action of radiation on organisms or agents


as akin to a gene as possible.   Thus I encoun-


tered bacteriophages,  the viruses that pray on



bacteria.  And soon,  with excitement and some


trepidation,  I discovered that Delbrfick, who


had migrated to the United States, had also by


a strange chance started to work on bacterio-


phage.  Only at the end of 1940 did Delbriick


and I get together and joined forces.


   The 1940's may be considered as the


8

"heroic" period of molecular biology by analogy

with the heroic periods in history that precede

the full flowering of each civilization. In

1941 Beadle and Tatum announced their first

successes in the pursuit of the one-gene-one-

enzyme theory,  correlating individual genes

with individual steps of metabolic pathways.

Delbrtick working together on bacteria and

bacteriophage provided in 1943 the first

genetic analysis of bacterial mutations,

opening up the genetics of bacterial cells. At

about the same time,  Avery,  MacLeod, and

McCarty announced the identity of a

Transforming Principle that changed the

heredity of the pneumonia bacillus as DNA.   The

dream of convergence between genetics and

biochemistry began to materialize.


9

Convergence,  but not yet fusion -- not yet

a      molecular biology,  lack of a molecular

model for the gene.  We may compare the situ-

ation to that of planetary theory after 1620,

when Kepler published his laws of planetary mo-

tion.  The planets move in planetary



around the sun --  but what drove them to it?



God?   chance?   or their own will?  Fifty years

later,  Newton provided the explanation by in-

venting the concept of universal gravitation.

In 1953 Watson and Crick discovered, or actu-

ally  "invented" the structure of DNA -- the

double helix.  The convergence of genetics and

biochemistry into molecular biology was an

accomplished fact.  The gene,  the mysterious

lump of genetic matter, the atom of Mendelian

heredity, became a sketch of DNA fiber, more


10

akin to segment of computer tape than to a

pearl in Venus' necklace.

Delbriick's hypothesis of mutations as

chemical changes of gene was soon confirmed.

Reshuffling of genes in chromosomes and even of

parts of genes of recombination could be inter-

preted in chemical        . And earlier inter-


pretations of the gene as either a unit of

function,  or a unit of mutability, or of recom-


bination were immediately reconciled.

In gaining chemical identity, genes lost

their splendid isolation.  Being part of a


       -- DNA files in which all genetic

information was stored in the four-letter

alphabet of nucleotides -- a gene had to be

"read" in order to be translated into the

alphabet of proteins -- 20 amino acids.   And


                                        11

the reading of gene had to be responsible to


the needs of the cell, that is, to the environ-


ment which the cell had to cope with, whether


the external milieu or an internal one for


cells of complex organisms.  Thus,  new lives of


discovery grew,  which together marked the tri-


umph of Escherichia coli, an humble bacterium,


which I take some irrational pride in having


pushed into a career in molecular biology


Escherichia coli,  and its bacteriophages, were


the protagonists of the major advances:   the


transcription and translation of the informa-

tion of genes from DNA into RNA into proteins;


and the regulation of the processes by classes


of DNA-combining proteins -- repressors or


activators  -- controlled by enogenous sub-



stances.


12

By the middle 1965 the molecular biology

of Escherichia coli, however, still incomplete

in detail, had reached a satisfying state.


Soon, however,  it became clear that the dictum


"What holds for Escherichia coli held, also for

elephants" was somewhat optimistic.  Elephants,


or fruitflies,  or humans have all that


Escherichia coli does, plus something more.

This something was what fifty years earlier

geneticists and molecular biologists had

deliberately put on a shelf for further atten-

tion -- embryology.  Complex organisms have

more than just many cells.  They have a pro-

gram,  that is, a plan that makes it possible

for cells with identical genetic information to

become functionally and structurally different

in different parts of an organism.   Molecular


13

biologists tackling such complex organisms had

to go back to the shelf and reexamine what they

had ignored.  They had to reverse the so-called

Morgan derivation, that is, the choice by

Morgan and his followers after 1910 to

concentrate of genetics at the expenses of

embryology and differentiation.

The molecular biology of the cells of com-

plex organisms has provided a number of sur-

prises.  Some of these have been relatively

easy to  fit as extensions of the molecular

biology of Escherichia coli, adding to it new

complications.  Thus the discovery of reverse

transcription --  from RNA to DNA -- and that of

enhancers of glue transcription did not change

our view of the gene.   Another line of dis-

covery -- the presence within genes of


14

whose information was not used but

"spliced out"      before translating RNA


      into protein sequences -- cut deeper

into the gene concept itself.  The gene that

makes a protein is an expurgated region of the

gene in the chromosome, freed of nonsense

" j unk It whose evolutionary significance remains

a matter of speculation.

These complexities of molecular biology

turn out to be of little relevance to the prob-

lem of development and differentiation, how-

ever.  Contrary to what might have been hoped,

gene splicing turned out not to be organ spe-

cific:   it does not account for the differen-

tial expression of a given gene in different

sets of cells.  It throws no light on the se-

cret of embryology:  the preparing for differ-


15

ential gene expression is different cells of a

plant or an animal.

The central problem of embryology may be

posed as follows.  At some points in the devel-


opment of an organisms each line of cells be-

comes programmed (by evens intrinsic in the

heredity of the organism) so that its cells

will finally express certain genes and not to

express others once the organism grows into its

final form.  For example,  the nuclei of cells


that will give rise to muscle fibers become

programmed so that once in a muscle they will

express genes that make contractible proteins

and to lose expression of genes that function

in cell division.   Cells of the mammalian liver


become programmed so that in the normal liver

they express genes for liver proteins.  They


16

still can make proteins needed for cell

division but are prevented to do so by a signal

that is made by the normal liver itself and

that gears the total amount of liver cells to

the normal requirement of the adult organism.

   More generally,  in a complex organism each


cell is exposed to a more or less precisely

regulated flow of chemical signals coming from

other cells.  Some of these signals are





            I  many of which have already been



identified.  Other are "factors" that is, pro-


teins or peptides whose presence is revealed by

their effects on specific cells; many factors

may turn out to be short-lived chemical sub-

stances appropriate for communication between

adjacent cells;           I  some signals may be

transmitted by physical contact between cells


17

without intervention of soluble substances.

Any given cell will respond to certain signals

if it has been programmed to express appro-

priate  "receptors.1'  The receptor protein


convey signals to certain sets of genes turning

them on a oft or modularity the level of their

activity.

   In this schematic view, the         of


cellular differentiation becomes less mys-

terious:  it consists of "marking" different


clones of cells with the capacity to express

specific sets of receptors that will determine

specific sets of responses.  The prescribed


responses will consist of the activation of

specific cell functions through regulation of

gene activity.  In some instances, the response

can be a growth response.  That is,  cells are


                                       18

stimulated to enter the "growth" cycle, dupli-


cating their DNA and undergoing cell division.


Alternatively,  they may be restrained from


growing and dividing.


   Of these controls over cell responses,


especially over cell growth and division, lit-


tle was known until recently.   The molecular


biology of cancer may throw light in this area.



Cancer cells differ from their normal counter-


parts both because of differences in function


and because of altered growth capacity.   Cancer


cells have lost the discipline of the organism;


they fail to respond to the signals that keep


normal cell functionally and reproductivally in


their places within the plan of the organisms.


This failure turns out to be due to the pre-


sence in cancer cells of certain altered genes


19

called oncogenes.

Evidence for the existence of oncogenes

came first from the study of tumor-producing

viruses,  each of the viruses that causes


leukemia or lymphoma turned out to be one gene

whose presence was sufficient to induce the

transformation of normal bone marrow cells into

cancer cells.  Loss of the oncogene made a tu-


mor virus non-oncogenic.   Other tumor viruses


were found to have not one but two oncogenes

which contributed to transformation into cancer

cells.  The presence of each of the viral onco-


genes in tumor cells could be recognized by the

presence of corresponding proteins detectable

by serological tests.

It soon was clear that the transforming

activity of viral oncogenes had no necessary


20

relation to their being part of viruses.   Onco-

genes were close relatives of normal cell genes

(or proto-oncogene) whose presence could be

discovered by the homology of their DNA se-

quence to that of oncogenes.  From proto-onco-

genes, the oncogenes had originated by mutation


and had then been picked up by viruses, which

transferred them from cell to cell or between

organisms.

Next the activities of oncogene proteins

were explored.  Some turned out to be enzymes


with altered specifications, for example, the

ability to add phosphate residing to certain

sites of cellular proteins.  Others were enzyme


proteins that had lost their catalytic acti-

vity.  How did these altered functions explain

the transformation of normal cells into cancer


21

cells?

More clues emerged when oncogenes were

found not only in viruses but also in the cells

of spontaneous cancers, especially human can-

cers,  simply by extracting their DNA and trans-


ferring parts of it to normal cells in culture.

Some of the recipient cells became cancer cells

and if introduced into appropriate animals gave

rise to cancers.  Thus many different oncogenes


were discovered,  a few of them related to the



viral ones.  Among them are oncogenes that


bring us closer to the central problem, how

cell growth and cell division are regulated,

and how oncogene alter that regulation.

Among the signals to which cells respond

is a set of substances called growth factors.

These are produced in different body organs and


22

their growth stimulating activity is specifi-

cally exerted on one or another class of cells.

Their activity is to stimulate the specific

target cells -- for example, epidermal cells --

to grow,  enter the growth cycle, duplicate


their DNA,  and divide.   Growth factors act on


cells by combining on their surface with spe-

cific receptors for that factor.  A receptor


thus activated conveys to the cell nucleus a

signal that triggers directly or indirectly the

activity of genes that regulate cell growth.

Such an activation can be recognized by the ap-

pearance of increased RNA or protein from the

strain labeled genes.   If a cell is a normal




one,  it's response to a growth factor remains


within the limits of the needs of the organism.

For example,  epidermal growth factor may cause


                                        23

regeneration of wounded skin epidermis until


the wound is repaired.   In cancer cells, how-




ever,  growth responses are disorderly.


   Certain oncogenes produce abnormal pro-


teins related either to growth factors or


growth factor receptors.   If secreted, an ab-


normal growth factor can stimulate the recep-



tors on the cell surface.  Other oncogenes are


altered forms of genes for receptors and make


membrane proteins that behave as if they were


permanently activated receptors, pushing the


cell to grow and divide without responding to



external controls:   the cell becomes a tumor



cell.


   The role of oncogenes is upsetting the


pattern of cell response to growth factors is


one part of the story.   There remains a second


24

part,  the identification of the genes that in

the cell nucleus respond to the signals and

triggered cell growth and division.   How is the


expression of these genes regulated in normal

cells and not regulated in cancer cells?

Here it is appropriate to recall that the

problem of control of the cell cycle is still a

biochemical mystery.   Even for Escherichia coli


we only know that initiation of a cycle of DNA

replication is regulated by the nature and

amount of nutrients;  the set of genes that re-


spond to this regulation is poorly known.  Only


for a few DNA elements such as plasmids we

probably know all the component of the replica-

tive response.

Oncogenes have begun to give us clues

about growth control mechanisms of the mam-


                                       25


malian cell.  Among the oncogenes isolated from


human cancer or from tumor viruses, there are


some whose protein products are found within



the nucleus itself.  That is,  after being made


like all cell proteins in the cytoplasm, these


oncogene proteins return to the nucleus.   This


is what we would expect of regulatory proteins,



that  "talk to genes" by combining directly with


specific DNA sequences.


   A further development has been the dis-


covery that oncogenes with nuclear proteins can


act in collaboration may with oncogenes with


cytoplasmic proteins.  Most oncogenes isolated


from cancer cells were first recognized by


their ability to transfer cells from


established cell lines, but failed to transform


newly cultured embryonal cells.   The


26

established cultured cell were already capable

of indefinite reproduction, potentially

immortal.   Transformation made them insensitive


to growth inhibition from medicine components

or given cell-to-cell contacts.  Embryonal

cells could be transformed only if two

oncogenes were introduced into them. A

successful pair had to consist of one nuclear-

protein oncogene and one cytoplasmic-protein

oncogene.  The nuclear acting protein provides


one immortalizing function:   it makes the


cellular response to the other oncogene a

permanent growth response.   It weakens the


capacity for uncontrolled multiplication

characteristic of the cancer cell.

The two-step mechanism of transformation

of cells to cancer cells has several implica-


                                        27


tions.  It puts a solid genetic basis under the


large body of observations that suggest a dual-

step mechanism in the origin of cancer.   And it

opens the possibility of detecting the activity

of single oncogenes within body cells before

the second step of cancerous transformation has

occurred.

As the standpoint of cell biology the two-

step mechanism promises new insights.   The

failure of embryonal cells to become trans-

formed by a single oncogene suggest a possible

step in cell differentiation in the course of

embryonal development.   Cells that become com-

mittee to differentiation may lose

"immortality" which can be restarted by the ac-


tion of an exogenous oncogene.   Differentiation


may then imply limitation in cell lineage: in


                                       28

culture,  limited number of generations; in the


body, dependence on specific growth factors.

The two classes of oncogenes suggest a

model of cell regulation consisting of two sep-

arate but interactive levels.   One of cellular


responses to external stimuli, the other level

dealing with responses within the cell nucleus.

We may envision a double network, a cytoplasmic

one modularity and transmitting signals from

the outside,  and an nuclear one regulating the


extent of gene response to the signals. An

oncogene expressing a cytoplasmic or membrane

protein will disturb the first network and may

cause among other effects, an excessive stimu-

lation of a subset of genes concerned with cell

growth.  But the stimulation may remain within

normal limits and fall short of cancerous


29

transformation unless the nuclear regulatory

network has itself been altered by another

oncogene.

Transformation may in a sense be seen as

the opposite of differentiation, a reversal of

the original "marking" of cells for orderly re-


sponse to external signals.

The above model, however, speculative, is

interesting for one additional reason.   It sug-


gests that oncogenes may become a major source

of information concerning all genetics, partic-

ularly human cell genetics.  Oncogenes are not


infinite in number:  among the first score or



so,  several repeats have been found.  Clearly,


only a small subset of the thousands of human

genes can generate oncogenes.  This may be the

subset of genes that      at relevant nodes of


                                       30

the networks,  either as conveyors of signals or


as controllers of responses.  Through the anal-


ysis of many oncogenes of the altered cell


functions they encode t should be possible to


identify the components of the networks.   Then


we may also discover new sets of signals, such


as short-lived soluble factors,  from the re-


sponse they evoke (or fail to evoke) in onco-


gene carrying cells.  We may find in cancer an


unexpected source of insight into human


genetics.