COMMEMORATIVE LECTURE
Cancer research in the 21st century
Arnold J. Levine
President, The Rockefeller University, New York, NY, USA(Received for publication on April 17, 2001)

Key words: viruses, chemical carcinogens, gene-mutations, aging, p53

In 1961, I went to the University of Pennsylvania in
Philadelphia, in the United States, to begin my ®rst
year as a graduate student and to train in science. I had
two goals at that time which I've kept with me for my
40 years in research: The ®rst goal was to work withviruses, because viruses were the simplest of all livingorganisms. The second was to work with viruses thatcause cancer in animals. I was hopeful that this researchwould elucidate the origins of cancer in animals. We
would then be able to take that knowledge and apply it
to the origins of cancer in human beings.
Re¯ecting back, we see that in 1961, we did notknow the right questions to ask. We couldn't even
begin to phrase the questions that would elucidate theorigins of cancer in human beings. Even if we had theright questions, we did not have the technological ability which would allow us to arrive at these insights. We
didn't have the experimental tools or the information.
Developing both novel techniques and new information
was a great quest and the right path for the past 40
years ± from 1961 to the year 2000.
Now, we live in extraordinary ± even revolutionary ±times in science. And today, as we start our new century, we understand for the ®rst time what the causes ofcancer are in human beings. We have identi®ed manyof the genes that humans have, and the changes in thosegenes that can heighten the risk of developing cancer.
So I want to tell you the story of how that happened. I
want to review with you the experiments that havecontributed to shedding light on the causes of cancer inhumans ± from the beginning of the 20th century until
today.
This story begins in the year 1911 in New York City,
at The Rockefeller University, which at the time wascalled the Rockefeller Medical Research Institute. Thisis the same institute which I joined two and a half years
ago, as a professor and president. A young man who
went to Johns Hopkins Medical School in Baltimore,

and who trained as a microbiologist in 1908 and 1909,
came to The Rockefeller Medical Research Institute as
an assistant professor. His name was Peyton Rous.
Upon his arrival, the director of the institute, Simon
Flexner, said to Peyton Rous: ``I want you to work on
cancer, and what causes cancer.'' Peyton Rous was terri®ed by this idea, because no one knew in 1911 whatcancer was ± the cause of it, how it forms in humans.
But Peyton Rous was a good scientist and started his
research on cancer.
Within the ®rst year of working at The Rockefeller
Medical Research Institute, a farmer from New Jersey
walked into his laboratory carrying a chicken. This
farmer's chicken had a tumor in its breast ± in themuscles of the breast, to be more speci®c. The chicken
had what we now call a sarcoma, a tumor of the muscle
tissue of the breast. Peyton Rous listened carefully as
the farmer told him the story of how this chicken got atumor, along with other chickens in the ¯ock. The possibility that whatever caused cancer in chickens was infectious in nature occurred to Peyton Rous for the ®rst
time. Maybe because chickens live so close together inchicken coops, one chicken might give the next chicken
the agent that carries the information that causes atumor. It also occurred to Peyton Rous that the agent
could be a virus.
Peyton Rous decided to test this hypothesis: could avirus cause the chicken to develop a tumor? To test his
theory, he chose one of the more innovative methods
available at the time ± the experiment aided by thework of Dr. Kitasato, the ®rst dean of Keio Medical
School. This is an experiment where the tumor is removed from the chicken and the tumor tissues are
ground up so that no cells survive. All of the debris isthen removed to leave a clear supernatant, which isthen put it through a Kitasato ®lter or a Chamberland®lter. If the agent that causes the tumor passes throughthe ®lter, it must be smaller than any bacteria and, byPresented at the commemorative lecture of the award ceremony of the 2000 Keio Medical Science Prize, November 29, 2000.
Reprint requests to: Dr. Arnold J. Levine, President, The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA
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de®nition, must be a virus. So Peyton Rous took theclear, ®ltered ¯uid and re-inoculated a second chicken.
The second chicken developed a tumor. He repeatedthis same experiment numerous times over two years.
And every time, in the end, he was able to transmit thetumor in the clear ®ltrates that penetrated the ®lter.
Therefore, he concluded that some cancers ± a sarcoma
in chickens, at least ± could be caused by a virus. In
1911 he published his paper and for the ®rst time the®eld understood that, in chickens, cancer could becaused by viruses.
Peyton Rous' discovery was one of the cornerstonesof the foundation upon which we now understand cancer, and we did not have to wait very long for the second
cornerstone to be laid. We now move from New York
City in 1911 when Peyton Rous published his paper,to Tokyo, Japan in 1914, when Yamagiwa and Ichikawa
did a very important experiment at the University ofTokyo.
Drs. Yamagiwa and Ichikawa took the chemicalcomponents from coal tar ± found in the soot fromchimney smoke or in places with concentrated cigarettesmoke ± extracted the chemical components, andpainted those chemicals on the backs of mice. And rightwhere they painted the chemicals, the mice developed
tumors. Through this experiment, they realized for the®rst time that there were certain chemicals that could
cause tumors. They had published their results by 1914
± that chemicals could cause cancer as well as viruses.
What the relationship was between the viruses and thechemicals, no one yet understood.
By the 1930's, at the University of Wisconsin, Jim
and Betty Miller had been able to repeat Ichikawa
and Yamagiwa's experiments. The Millers' experiments
were repeated with a larger number of chemicals thatwere shown to be able to cause cancer. These chemicals were called carcinogens ± or chemicals that cause
cancer.
Also in the 1930's, scientists found inherited cancers
in mice. Shortly after that, human geneticists began toappreciate that cancer could occur in some families.
In these families with high occurrences of cancer, they
were able to track the genetic information that could
cause the cancer. This focus allowed them to narrow infor the ®rst time on the fact that certain altered genes
or mutant genes that travel in some families could be
the cause of cancer in human beings. By the year 1950,
we had many examples of an inherited basis of cancer.
And now we had three parts to the cornerstone of thefoundation that's being built here:
1) Cancer can be caused by viruses;
2) Cancer can be caused by chemicals; and
3) Cancer can be caused by genes.
Of course, from the perspective of the 1950's, we didn't
know how these three elements related to each other,

Levine AJ: Cancer research in the 21st century

and we still could not begin to understand how cancer
formed in human beings.
By 1960, a fourth group of people came to play inthe cancer ®eld: epidemiologists. Epidemiologists are
scientists who study how relationships between various
factors in a person's life relate to his susceptibility to
speci®c disease: whether gender is important, whether
age is important, whether your environment is important, whether your socioeconomic group is important,or whether the job you have is important. The epidemiologists found, by the 1960's, that cancer was, by andlarge, a disease of the elderly. Very few young people
got cancer.
If one looks at the rate or the incidence of cancer ±how many people acquire cancer at a particular age ±very few 20-year-olds acquire cancer. At 30, still very
few people acquire cancer. At 45, just a few more people acquire cancer. But at 55, the curve starts rising very
rapidly. By 65, it rises very, very rapidly. And by 75, theincidence is very high. That is an exponential curve. It
takes a lifetime to accumulate something ± whatever it
is ± that gives rise to cancer. So an individual becomes
more likely to develop cancer as he ages.
In 1961, as I started my graduate career, I knew
those four facts ± the four cornerstones of the structure
upon which we now understand cancer.
1) Cancer can be caused by viruses.
2) Cancer can be caused by chemicals.
3) Cancer can be caused by genes.
4) Cancer can be caused by aging.
In fact, there is a thousand-fold increase in the rate orincidence of cancer when comparing a 20-year-old toa 75-year-old. If you smoke, there is a hundred-foldincrease. If you have the viruses hepatitis B or hepatitis C, there is an eighty-fold increase. But aging is a
thousand-fold increase, so age is clearly one of the four
cornerstones for understanding cancer.
As a ®rst-year graduate student who wanted to knowwhat caused cancer in human beings, I had these four,
disparate observations with which to work. And everybody in science was arguing with each other, convincedthat one or the other of these observations was the mostimportant in understanding the origins of cancer inhuman beings. But no one could put them together.
With that as a background, we enter the decade of
the 1970's with the extraordinary development of technology that has come to be called the recombinant
DNA revolution. This is the ability, for the ®rst time,of scientists and laboratories to go into the chromosomes of any animal or human being, and to pull out
from those chromosomes genes ± genetic elements ±which contain information about how to build the wholeorganism.
In the 1970's, we enter the laboratories of Harold
Varmus and Mike Bishop, at the University of Califor-

Keio J Med 2001; 50 (3): 129±133

nia at San Francisco. Drs. Varmus and Bishop, who
subsequently won the Nobel Prize for their observations, began their research with, of all things, the Rous
sarcoma virus: the same virus found in chickens that
Rous had isolated in 1911. Upon examining the Rous
sarcoma virus, they found that the virus contained agene ± genetic information ± which caused the cancer inchickens. They called this the ``sarcoma gene'' because
the tumor was a tumor of the muscle tissue, or a sarcoma(src), and the src gene caused the tumor. They named
the gene an ``oncogene'' ± or cancer-causing gene.
This discovery united two of the cornerstones of ourunderstanding of cancer for the ®rst time. Viruses could
cause cancer, and genes could cause cancer. And now
we knew that viruses carried genes that could cause
cancer. It related these two elements for the ®rst time.
But perhaps much more remarkably ± and why Varmus
and Bishop were awarded the Nobel Prize ± was thatthey found that there was a very close relative of theoncogene in normal chickens. Chickens possessed agene which was almost identical to the src gene in
the virus. The only difference was that the viral src
gene carried some mutations. It carried some genetic
changes or mistakes that made the normal gene (c-src),
an oncogene (v-src) ± a cancer-causing gene that wasin the virus. Mutations converted the normal gene toa cancer-causing gene, with the virus picking up thecancer-causing gene, resulting in cancers in thesechickens. Mutations are also caused by chemical carcinogens, just like the coal tar and the cigarette smokethat were found to be carcinogens in 1914. This related
three of the four early observations: genes, chemicals,
and viruses were all brought together. By 1980, we
understood for the ®rst time the relationship between
the cornerstones:
1) Chemicals can cause genetic mutations;2) Mutated genes cause cancer;
3) Some viruses can acquire mutated cellular
genes, like src, and cause cancer.
This did not yet explain, however, why it was thataging was important in cancer. If we really were tounderstand that these oncogenes caused cancer, and thatthey could be picked up by a virus and be mutated, why
is it that cancer is a disease of the elderly? And thereason for that is that there isn't just one oncogene thatcauses cancer; there are whole groups of other genesthat have to suffer mutations when cancer arises, andthese genes were discovered in 1979, when p53 was ®rst
discovered. My laboratory at Princeton University was
one of the research groups that discovered p53 andshowed it to be not an oncogene at all, but somethingcalled a tumor suppressor gene ± a gene that prevents
cancer. Oncogenes cause cancer when they're mutated.
Tumor suppressor genes normally prevent cancer and,
when mutant, they fail to prevent cancer.

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So here, we had a situation of two different kindsof genes: the oncogenes, which cause cancer, are verymuch like the accelerator of an automobile. If the accelerator of an automobile breaks and is stuck in the``on'' position so that the automobile keeps going, thenyou have a mutated oncogene. The tumor suppressor
gene, on the other hand, prevents cancer by acting likea brake of an automobile. If the brake is broken by amutation and you have an accelerator that's stuck in the``on'' position, then you have two mutations ± one in atumor suppressor gene and one in an oncogene: andtogether, they greatly increase your susceptibility tocancer. It's not suf®cient to have just a faulty oncogene.
You have to have a bad accelerator and you have tohave a faulty brake, and then cancer begins to develop
in human beings.
Now, what kind of ``brake'' is the p53 tumor suppressor gene that we found in Princeton in 1979? It isa kind of brake that helps us make corrections when
we make a mistake. For example, when we get on our
computer in the morning and type a few paragraphs,
only to see that we've made some spelling mistakes, wepress the ``spell check'' key. The spell check key islinked to a dictionary and it replaces misspelled words
with correct ones. p53 is our spell check. If our chromosomes make a mistake when they replicate, they
carry a mutation from that point on, which can create
and oncogene that helps cause cancer. p53 notices when
the DNA is damaged and a genetic mistake ± or mutation ± is going to be made, and activates a very, verystrong correction device: it kills the mutant cell. So it
wipes out the entire paragraph that contains a spelling
mistake. And that's how a tumor suppressor geneworks ± it acts as a genetic spell check which eliminates
cells that have damaged genetic information and carrymutations in oncogenes, and helps to prevent the development of cancer.
But what happens when your spell check in thecomputer breaks because it has a mutation in the p53
gene? The error frequency increases greatly. We makemistakes that can't be corrected. Every time an oncogene has a mutation, there is no correctional device,and other mutations occur in other oncogenes andother tumor suppressor genes. The combination ofthese numerous mutations eventually leads to cancer.
This understanding, for the ®rst time, explained
why aging was important in cancer. We have in our
body some 60 or 70 genes that, if they suffer mutations,become oncogenes. And there are perhaps 25 or so tumor suppressor genes, and if they suffer mutations
these can increase one's susceptibility to various kindsof cancer. And you have to have multiple mutations in
oncogenes and tumor suppressor genes for cancer todevelop.
So what happens over a lifetime is that, at the age of132

20, you may suffer a mutation in an oncogene for the®rst time. Although one mutation is not suf®cient tocause the cancer, it sits there for life. And at the age of
35, you may suffer another mutation in an oncogene ±your second. And at the age of 45 or 50, one of yourtumor suppressor genes may get a mutation. And if you
have a tumor suppressor gene with a mutation and two
oncogenes with a mutation, and they happen to be all inthe same cell, you're on your way to developing cancer.
Then, at 60 or 65, the fourth mutation may occur in thesame cell, in a tumor suppressor gene, and now the
combination of these mutations is suf®cient to initiate acancer. Suddenly, we are able to understand how genes
and chemicals and viruses and aging all come together
to elucidate the understanding of the origins of cancer
in human beings.
As we complete the 20th century, we now have aclearer picture of the causes of cancer. We now haveidenti®ed some 100 genes in our chromosomes which,when altered via mutation can, in certain combinations,contribute to cancer formation.
We can now fairly ask the question, understanding
the origins of cancer in human beings, ``What can be
done?'' And in response to this question, I can end with
an optimistic story that takes us into the 21st century.
But in order to tell you that story, I must go back intime again, and focus upon a speci®c cancer. We'regoing back to 1961 ± my ®rst year as a graduate student, at the University of Pennsylvania.
Just down the hall from where I was working on
viruses and cancer in my own laboratory, there was apathologist named Peter Nowell. Peter was particularlyinterested in a disease called chronic myelogenous leukemia, a disease of white blood cells termed CML. And
what Peter had found, in every leukemic cell, wereparts of each of two chromosomes came together, fusing to make one. This creates a translocation, or a fusion of two chromosomes ± a type of mutation in thegenes. So he discovered a very close association of thischromosome fusion and the disease, chronic myelogenous leukemia. Since a translocation was found in every
patient with chronic myelogenous leukemia, most people thought that Peter had found that this particularmutation caused the disease. This type of chromosometranslocation came to be called the Philadelphia chromosome, because it was found in Philadelphia. This
happened in 1965.
In the 1970's, as additional oncogenes were discovered and it was shown that an oncogene called abl
was localized at the junction of the translocation on the
Philadelphia chromosome. The abl oncogene suffered amutation because of the chromosome translocation, just
as Varmus and Bishop had found that the src oncogene
suffered a mutation and caused the cancer. In David
Baltimore's laboratory at MIT, they took the trans-

Levine AJ: Cancer research in the 21st century

located and mutated abl oncogene and put it into amouse. The mouse developed chronic myelogenous
leukemia, proving that the translocation was the causeof the disease.
In a very short time, Owen Witte and David Baltimore were able to show that the mutated oncogene
called abl produced the protein ± an enzyme called aprotein kinase. This protein kinase was important inkeeping the cancer cells alive and causing the disease.
The 1980's ended with our understanding that thetranslocated gene caused cancer, and that this enzyme ±protein kinase ± that sustains mutations or changes, wasdriving the disease in some way. The protein kinase wascausing the survival of the cancer cells.
Now, as we entered the 1990's, we began to realizethat oncogenes caused cancer. A large number of people began to appreciate that if we could develop drugs,
if we could ®nd small molecules, chemicals, that would
inhibit these abnormal oncogene enzymes that werecalled protein kinases, that for the ®rst time we would
have a rational basis for trying to cure cancer. A young
man in Basel, Switzerland named Alex Matter, who was
the head of oncology at a pharmaceutical company
called Ciba Geigy (now Novartis), appreciated that if
it were possible to inhibit the abl oncogene protein
kinase, you could stop chronic myelogenous leukemia.
His research group spent a number of years developing a small chemical that ®t into the site in the protein
kinase, and blocked its ability to function. And they put
it into their ®rst patient which was a mouse; the mousethat David Baltimore had made, that contained the abl
oncogene and developed chronic myelogenous leukemia. Within a month they were able to cure the mouseby inhibiting the abl oncogene product on which thecancer depended for its survival.
Brian Drucker then started clinical trials withhumans and was astonished to see that the ®rst group ofpeople, who had chronic myelogenous leukemia andwho received this drug, almost all went into remission.
We now realize that from the bene®t of 90 years ofscience ± from 1911, as Peyton Rous isolated the sarcoma virus, all the way to our understanding of oncogenes that cause cancer ± we really do understand theorigins of cancer in human beings. We have the ability,
for the ®rst time, to rationally develop drugs against theseries of oncogenes, or to reactivate the series of tumor
suppressor genes, to turn off the accelerator and ®x thebrakes.
So we now have one example of a drug that wasdeveloped as a rational response to our knowledge
about the origins of cancer. It's a small triumph. In
the United States, 4,000 people a year develop chronic
myelogenous leukemia. Soon, every one of them will
receive four pills a day that they will take for life, which
will reverse the disease without harsh side effects. The

Keio J Med 2001; 50 (3): 129±133horri®c side-effects of chemotherapy and radiation become obsolete if we are able to develop drugs that haverational targets. The name of this particular drug, which
I'm sure will change as soon as it becomes commerciallyavailable, is STI-571. What is most important about
STI-571 is not that it cures a leukemia that 4,000 people

133

have in the United States each year, but that it is,
in fact, the ®rst generation of many drugs that will be
designed to inactivate oncogenes and to activate tumor
suppressor genes. As we understand the origins ofcancer in humans, we can, for the ®rst time, design
drugs to ®ght this disease.