Global Health Nexus, Spring 1999
Genes, Mutations, and Cancer:
Possible Applications to the Prevention of Cancer of the Oral Cavity
Joseph B. Guttenplan, PH.D., Professor of Basic Sciences (Biochemistry)
DNA, 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.
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.
Mutations, 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.
Tumor Suppressor Genes
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 NYU Dentistry 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.
Carcinogenesis, the 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 identified there.
Causes of Cancer: 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.
"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."
Although inherited 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.
Cancer 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 hazards.
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. NYU Dentistry 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 medical follow-up.
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 NYU Dentistry and faculty at medical centers in the area toward initiating such a project.
It is also noteworthy that researchers at NYU Dentistry 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 their effects.
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 NYU Dentistry will play an increasingly important role in prevention. Significant efforts in all of these directions are underway.
Joseph B. Guttenplan, Ph.D., is professor of basic sciences (biochemistry) and director of Research, Training, and Program Development at NYU Dentistry. 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.
Antimutagens- protective 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.