B. Guttenplan, PH.D., Professor of Basic Sciences (Biochemistry)
Genes, and Cancer
It has been known for
centuries that cancer is a disease characterized by uncontrolled growth
in certain tissues of the organism. As growth occurs by cell divisions,
it is apparent that the growth abnormality must be transmitted to daughter
cells and the abnormality must be heritable among future cellular progeny.
Thus, the cellular blueprint must contain aberrant growth instructions for
these future generations of cancer cells.
Although these characteristics
of cancer have been long known, the mysteries leading to uncontrolled
cell growth are only recently being unraveled. Indeed, until 1953 when
the structure of DNA (the material into which the cellular blueprint is
engraved) was first elucidated, the general mechanism by which genetic
information is transmitted was speculative.The
elucidation of the double stranded structure of the DNA immediately suggested
a mechanism during cell division by which a single cell could transmit
its genetic information to two daughter cells; one strand was allotted
to each of the progeny. It was found that information encoded in DNA was
arranged linearly, much like that on an audio- or videocassette. Some
10 billion bits of information (nucleotides) are present in each strand
of the DNA.
these characteristics of cancer have been long known, the mysteries
leading to uncontrolled cell growth are only recently being unraveled."
Shortly afterward, the genetic alphabet (genetic code) was decrypted.
The DNA alphabet enables the cell to accurately specify the sequence of
"letters" necessary to instruct the cellular machinery to produce individual
proteins. Proteins are the main cellular building blocks and perform most
of the essential cellular roles. The segment of the DNA encoding a protein
is referred to as a gene. The genetic characteristics of the outward manifestations
(or phenotypes) of certain of these proteins were also known for many
years before the structure of DNA was elucidated.
As observed in plants by Mendel nearly 150 years ago, the phenotype from
one generation was passed down to future generations in a predictable pattern
that was dependent on the characteristics of the genes of the two parents.
Thus, it seemed obvious that on a cellular level abnormal (mutant) growth
control proteins encoded by abnormal DNA would be responsible for the abnormal
growth pattern seen in cancerous cells.
Oncogenes, and Cancer
About 25 years after
the deduction of the structure of DNA, the first genes directly involved
in the abnormal cell growth in certain cancers were identified. Cells
are normally programmed to grow and, indeed, under the right conditions
almost any type of cell isolated from a human source can be induced to
grow. However, except during youth, most cells grow slowly and have a
finite life span. The cellular growth and death rates are tightly regulated
by a complex circuitry involving growth control genes. One major class
of such genes has the ability to turn on cell growth. Such genes are normally
in a near quiescent state (somewhat analogous to "idling") under most
conditions in adults, but if the sequence of the DNA letters is altered
(often only minutely) the structure and function of the encoded protein
is changed so that an active, rather than a quiescent, protein is produced.
Any alteration in the DNA sequence is known as a mutation; the normal
growth-directing protein gene is called a proto-oncogene and the decontrolled
protein gene, an oncogene (sometimes the terms oncogene and activated
oncogene, respectively, are employed).
By now tens of oncogenes are known. This is a case of a good gene gone
bad. Instead of guarding the growth coop from harm, it has joined the
foxes. One of these, known as ras, is occasionally mutated in tumors of
the oral cavity. It is of interest that the percentage of oral tumors
carrying this mutation varies in different geographical locations, suggesting
a strong environmental or lifestyle component.
An important characteristic of oncogenes is that they are dominant mutations;
that is, only a single copy of the two normally inherited genes (one from
each parent) need be mutated, since it only requires one mutant to produce
the overactive oncogene-encoded protein. Although oncogenes can be generated
by minute changes in DNA sequences, the mutations that result in enhanced
growth rates must arise from a very fortuitous combination of effects;
it is difficult to create a more effective protein than the one nature
evolved. Therefore only a very limited number of sites along the DNA can
be mutated to produce an oncogene from its precursor proto-oncogene.
A second general class
of genetic targets can be mutated, resulting in a cancerous phenotype
in affected cells. Mutations in this class of target gene lead to uncontrolled
growth via inactivation. This is a case of a good gene gone out to lunch,
permanently. In contrast to proto-oncogenes, which are usually near-quiescent
(except for periods of rapid growth), this second class of genes (known
as tumor suppressor genes) are thought to be actively controlling and
limiting growth rates. This class of growth control gene was discovered
about a decade after oncogenes, but already a similarly large number of
such genes is known.
One such gene, designated p53, has been shown to be mutated in about 50
percent of human tumors and is frequently mutated in tumors of the oral
cavity. Normal individuals have two copies of this tumor suppressor gene
in each cell and inactivation of one copy of this gene by mutation or
other genetic alteration still leaves a second active copy. However, those
individuals who have lost one functional copy of the gene are at increased
cancer risk not only because they have no spares, but because the genomic
machinery facilitates the subsequent inactivation of a second copy after
loss of the first. In several rare genetic disorders (Li-Fraumini syndrome,
LiF; familial retinoblastoma, FAP) where one copy of a tumor suppressor
gene is nonfunctional, the affected individuals have a much higher risk
for cancer for the same reason.
Since it is much easier to inactivate than to activate a protein, there
is a whole spectrum of mutations that can inactivate p53 and other tumor
suppressor genes. (This is in contrast to the activation of proto-oncogenes;
see above.) It is also of interest that the mutational spectrum is characteristic
of the mutagen and thus reflects a "genetic fingerprint" of the mutagen.
Scientists are currently applying this knowledge to track down the culprit
carcinogens in several cancers, and at NYUCD research on the identification
of oral carcinogens by this method is being actively pursued. Although
further in the future, if genetic therapies for cancer can be perfected,
it may prove more effective to replace tumor suppressor genes than proto-oncogenes
because most relevant experiments involving cell fusions have shown that
when oncogene meets tumor suppressor gene, the latter reigns; that is,
the cell remains normal.
process of cancer formation, involves a number of stages, certain of which
involve genetic alterations. In several cases such as colon and skin cancer,
individual abnormal cell types can be isolated and studied throughout
the progression from normalcy to malignancy. In colon carcinogenesis an
initial genetic alteration often involves the loss of a tumor suppressor
gene resulting in the formation of a polyp. Another mutation in a proto-oncogene
may then lead to the formation of a nonmalignant tumor, and further mutations
may lead to increased aggressiveness of the tumor until a malignant phenotype
emerges, spreads, and outgrows normal tissues. The tumor cells may grow
more rapidly than normal tissue, may not die as normal cells do (a process
known as apoptosis), or may grow in an inappropriate environment in the
organism. Whatever the mechanism, the mutated cells eventually overwhelm
the normal tissue; the mutant cells are naturally selected because of
their propensity toward growth.
Although some investigators argue that as few as two or three mutations
are sufficient to allow the transition from normal to nonmalignant to
malignant growth, in the case of colon cancer a pattern of at least half
a dozen mutations in proto-oncogenes and tumor suppressor genes is observed,
often in a fairly specific sequence. Thus, mutagenesis drives the cancer
process. There is a need for additional tumor models, however, and cancer
of the oral cavity appears to be a promising candidate because 1) the
oral cavity is accessible and 2) certain precancerous conditions can be
Nature Versus Nurture
Mutations can arise
from a variety of sources. About five percent of human cancer is believed
to result from inherited mutations (germline mutations) in tumor suppressor
genes. Perhaps the most well known of these in the popular press are the
breast cancer mutations (BRCA1 and 2), which confer a very high probability
of developing breast cancer in individuals carrying this mutation. Such
individuals contain this mutation in all of their cells. Other inherited
tumor suppressor mutations confer enhanced susceptibility in the retina,
colon, kidney, ovaries, neural tissues, and other organs. Absent from
this list however, are tumors of the oral cavity.
is known that certain genes can predispose an individual toward acquiring
mutations in tumor suppressor or proto-oncogenes. For instance, many
components of tobacco smoke would be harmless
if the organism did not convert them to more toxic metabolites."
mutations account for a relatively small fraction of human cancer, mutations
in the same genes and others (including proto-oncogenes) are frequently
acquired as a result of environmental (including lifestyle) exposure to
mutagenic agents. Thus, the identification of the genes responsible for
the heritable forms of cancer has had applications far beyond the original
disease. The nongermline somatic mutations are believed responsible for
most human cancer, and many such mutagenic agents are avoidable. The most
obvious example results from cigarette smoking, which is responsible for
about half the cancer deaths in the United States. The attribution of
a large fraction of human cancer to environmental etiology (nurture over
nature) has become increasingly accepted in recent years with advances
in epidemiology and genotyping. Individuals with acquired mutations will
exhibit mutations in certain growth control genes within tumor tissue,
but the surrounding normal tissue is devoid of these mutations (in contrast
to the heritable mutations). A striking epidemiological demonstration
of the importance of environment is the observation that second generation
Japanese women born in the United States do not retain the low breast
cancer incidence characteristic of native Japanese, but acquire the much
higher Western incidence, despite the fact they never intermarry.
A more complex, but more prevalent etiology probably underlies much of
the individual differences in susceptibility to environmental agents.
For instance, many individuals smoke the same amount and the same brands,
but not all will not develop lung or oral cancer in a normal lifetime.
Why? One major origin of these idiosyncratic effects is thought to be
the existence of common inherited genetic variants.
It is known that certain genes can predispose an individual toward acquiring
mutations in tumor suppressor or proto-oncogenes. For instance, many components
of tobacco smoke would be harmless if the organism did not convert them
to more toxic metabolites. The enzymes that carry out these conversions
(the cytochrome P-450 or CYP family) probably evolved to help detoxify
potentially toxic ingested substances. An example of the beneficial functioning
of this enzyme system is the ability of the body to break down and excrete
alkaloid narcotics such as morphine. Of course the system can be overloaded,
and the unchanged narcotic is then deadly, but without this system even
very low doses would be fatal.
These enzymes are also inducible, that is, their levels increase after
exposure to their substrates (witness the higher tolerance in addicts).
On the one hand, there are considerable individual variations in the levels
and efficiency of these enzymes, as well as their responses to inducers,
resulting in different susceptibilities to the toxic effects of cigarette
smoke. In specific populations exposed to cigarette smoke or related environmental
pollutants, individuals with higher levels of a particular variant of
CYPIA (levels in lung and certain other tissues) exhibited higher incidences
of cancer in these organs.
On the other hand, there are a number of enzymes that can detoxify many
carcinogens and their potentially carcinogenic metabolites. A substantial
fraction of the population is deficient in one of these enzymes, glutathione
S-transferase M (GSTM), and several studies have associated this deficiency
with increased cancer incidence in certain organs.
A final example of a genetic factor that becomes apparent in conjunction
with environmental insults is DNA repair. There are many layers of DNA
repair enzymes that serve to restore damaged DNA to its original form.
Defects in the genes encoding these enzymes lead to several rare but devastating
syndromes that predispose affected individuals to very high incidences
of cancer. For instance, individuals with xeroderma pigmentosum suffer
multiple skin cancers after exposure to only minimal amounts of ultraviolet
light. Less severe defects in other DNA repair genes lead to elevated
levels of colon cancer. Thus, certain inherited genetic variants or defective
genes only become cancer-predisposing when they interact with an environmental
agent and lead to mutations in growth control genes.
the identification of a gene that can be specific for oral cancer
can be of great importance as a marker for early detection in susceptible
of the Oral Cavity
Oral cancer poses a
significant public health problem. It is estimated that 30,000 new cases
of the most common form of oral cancer (squamous cell carcinoma) are diagnosed
each year in the United States. This number represents about three percent
of the total cancer incidence in the United States and places it in a
similar numerical category as ovarian cancer and leukemia. Worldwide the
number exceeds 350,000. Importantly, approximately half of these patients
will die within five years of the day they were diagnosed. Even successfully
treated patients often undergo extensive surgical treatment that can cause
disfigurement and adversely affect their quality of life. Despite these
sobering statistics, cancer of the oral cavity is underrepresented in
terms of research, and many people are unaware of its potential health
The major risk factors for cancer of the oral cavity are tobacco smoking,
smokeless tobacco usage (chewing tobacco and snuff), and alcohol consumption.
A puzzling observation is that women are at about double the risk as men
for developing tobacco-related cancers when exposed to the same levels
of tobacco smoke. Hormonal influences have been suggested to account for
this effect. African Americans appear to have an increased risk even when
factors such as smoking and access to health care are taken into account.
A particularly dangerous combination is smoking and alcohol consumption.
While tobacco smoking (cigarettes, cigars, or pipes) increases the risk
of oral cancer several- to 10-fold (depending on whose study one reads),
and alcohol increases it severalfold, the combination of the two results
in increases of 10- to 30-fold. This is a striking example of synergism;
that is, the two risk factors are near-multiplicative rather than additive.
The use of smokeless tobacco (chewing tobacco and snuff) can elevate risk
3- to 30-fold, depending on length and frequency of exposure. It is clear
that smokeless tobacco usage is not a low risk alternative to smoking,
at least with respect to cancer of the oral cavity.
Unlike many of the heritable cancers referred to above, there are no known
heritable oral cancer syndromes. The p53 tumor suppressor gene is frequently
mutated in oral tumors, consistent with an environmental etiology for
many cases. Recently, however, a candidate tumor suppressor gene for oral
cancers (doc-1) was identified in experimental animals. For many years,
researchers have been studying the mechanisms of oral cancer development
in hamsters. Only three years ago, it was discovered that one difference
between normal epithelial cells and cancer cells derived from these tumors
is that the cancer cells do not express the doc-1 gene. Last year, similar
findings were reported when researchers compared normal cells and cells
derived from oral cancers in humans.
At this point, it is not known what causes the lack of expression of doc-1
in oral cancer, but probably it is environmental. Nevertheless, the identification
of a gene that can be specific for oral cancer can be of great importance
as a marker for early detection in susceptible individuals, especially
those who drink and smoke. The further combination of such data with information
about inherited genetic variants, such as those described above, could
suggest a profile for exceptionally high-risk individuals.
However, genetic testing on an individual basis may be ethically problematic,
and family history may have to provide a guide until ethical considerations
are resolved. NYUCD is planning a new clinical facility with the capacity
to serve as a storage bank for oral tissues, with associated demographic
and lifestyle information kept in computerized records. Such tissues and
associated data may be extremely useful in matching outcomes to exposure,
genotype, and other factors in large populations. The results of such
analyses may help determine which genetic and/or lifestyle factors increase
or decrease the risk for oral cancer. In the future, if ethical issues
can be resolved, testing for genetic markers may provide susceptible individuals
with valuable information on prudent lifestyle choices and frequency of
The oral cavity may also prove to be the next important model to study
the individual stages of carcinogenesis. As indicated above, its accessibility
and the known clinical appearance of precancerous lesions there strongly
support its use as a model for squamous cell carcinoma. Preliminary discussions
have been held between faculty at NYUCD and faculty at medical centers
in the area toward initiating such a project.
It is also noteworthy that researchers at NYUCD have established the first
experimental animal model for detecting mutations in the oral cavity.
This model is currently providing leads that may lead to the discovery
of protective agents (antimutagens). The model may also prove valuable
in identifying the individual mutagenic components in tobacco smoke and
smokeless tobacco. This knowledge may prove useful in designing tobacco-specific
antimutagens and less harmful tobacco products. It is also established
that some nonusers of alcohol or tobacco will eventually develop oral
cancer. A likely cause in such individuals is diet (including drink).
The model system for mutagenesis can also be applied to the detection
of mutagenic agents in diet and the discovery of antimutagens to counteract
In conclusion, cancer of the oral cavity remains a serious, largely underestimated
problem. The most common causes (tobacco smoking, smokeless tobacco consumption,
and alcohol usage) have been known for many years. Although smoking has
declined, its use, as well as that of smokeless tobacco and alcohol in
the younger population, may actually be increasing. There is also relatively
little known about the etiology of oral cancer in nonusers. Through screening,
education, and research, faculty at NYUCD will play an increasingly important
role in prevention. Significant efforts in all of these directions are
Joseph B. Guttenplan, Ph.D., is professor of basic sciences (biochemistry)
and director of Research, Training, and Program Development at NYUCD.
Petros D. Damoulis, D.D.S., D.M.S., assistant professor of basic sciences
(oral medicine and pathology) and surgical sciences (periodontics), assisted
with this article.
at NYUCD have established the first experimental
animal model for detecting mutations in the oral cavity."
agents against mutagenesis.
Apoptosis-programmed cell death.
Carcinogenesis-the process of cancer formation.
DNA repair enzymes-enzymes that serve to restore damaged DNA
to its original form.
Gene-the segment of the DNA encoding a protein.
Genetic code-the sequences of deoxynucleotides that specify
the individual building blocks (amino acids) of proteins. Three deoxynucleotides
specifie (code for) one amino acid.
Genotype-the sequence of the genetic information encoding the
protein responsible for the phenotype.
Germline mutations-inherited mutations.
Inducible proteins-those whose levels are increased after exposure
of the organism to specific agents.
Mutation-any alteration in the DNA sequence.
Mutational spectrum-a usually unique pattern of mutations produced
by a particular mutagen.
Oncogene-a decontrolled (mutated) proto-oncogene that leads
to permanently active growth stimulation.
Phenotype-the genetic characteristics of the outward manifestations
of a gene. These are expressed through the proteins encoded by the
genes. When two different variants of the same gene are present, the
phenotype of the dominant form will be expressed.
Proto-oncogene-a normal growth-directing gene.
Somatic mutations-nongermline or acquired mutations.
Synergism-two factors which, taken together, result in a greater
than additive effect.
Tumor suppressor gene-a gene that actively controls and limits
cellular growth rates.
* These definitions
refer to the terms in the context of this article.