From an interview with Science Watch® - July/August 1999 Vol. 10, No. 4, a publication of the Institute of Scienctific Information®.
Curtis C. Harris
on Carcinogenesis and the Molecular Epidemiolgy of Cancer



Curtis C. Harris
"Molecular epidemiology is an emerging field in cancer research," says Curtis C. Harris, Chief of the Laboratory of Human Carcinogenesis, National Cancer Institute. "One of its challenging goals is to identify individuals at high cancer risk."

Photo: Manuello Paganelli

The Interviews of Science Watch®What happens at the molecular level to convert a normal cell into a cancer cell? That fundamental question lies at the root of an entire subdiscipline of medical research–molecular carcinogenesis. Increasingly, cancer researchers are focusing their efforts on the genetic alterations that have led to specific cancers. Understanding the molecular signatures of cancers, they reason, will lead to the development of better therapies.

   Curtis C. Harris, Chief of the Laboratory of Human Carcinogenesis at the National Cancer Institute (NCI) in Bethesda, Maryland, has made major contributions to the understanding of this and related topics. He pioneered the development of in vitro models using human tissues and cells to compare metabolic pathways for the activation of chemical carcinogens and detoxification in humans and laboratory animals. He and Andres Klein-Szanto were the first to show that chemical carcinogens in tobacco smoke induce neoplastic transformation of human bronchial epithelial cells in the laboratory. He has gained international recognition for his cellular and molecular studies of asbestos-induced human pleural mesothelioma and lung cancer. More recently, Harris has made significant contributions to the discovery that mutation of the p53 tumor suppressor gene is one of the most common genetic lesions in human cancers. Understanding of the p53 gene in human tumors has revealed critical molecular links between environmental carcinogens and specific human cancers. In all, Harris's published papers have had a large impact on his scientific peers: last year in these pages, Harris was featured among the 50 most-cited biomedical scientists of the 1990s (see Science Watch, 9[3]:1-2, May/June 1998).

   Harris is a child of the space age. Aged 14 when the Soviet Union orbited Sputnik, the Earth's first artificial satellite, he benefited from the U.S. reaction to that event, which took the form of financial and philosophical encouragement of science education. Harris started doing research for high school science fairs. As an undergraduate majoring in zoology at the University of Kansas (B.A., 1965), he received a research participation award from the National Science Foundation to work on radiobiological immunology. Those studies led to his first paper, which he published while still an undergraduate. During his last undergraduate year he boosted his CV by working as an instructor and helping to teach an honors biology course and a graduate course in quantitative immunochemistry.

   Harris then earned his M.D. in 1969 at the University of Kansas School of Medicine. During medical school, his interest quickly turned to carcinogenesis. In collaboration with Donald Svoboda and Jan Reddy (now chair of the pathology department at Northwestern University), he published eight reports on nuclear structure and function and on chemical carcinogenesis.

   In his junior year of medical school, he accepted what was essentially a post-doctorate at NCI. He earned a certificate in internal medicine at the University of California Los Angeles Hospital, and then, in 1971, moved to NCI, where he finished his clinical training and continued his research. He has remained there since, moving up the institute's research ladder. Harris talked to Science Watch correspondent Peter Gwynne about his research and its implications.

SW: What persuaded you to go into laboratory, rather than clinical, research?

   Harris: I always wanted to go into academic medicine. For a number of years I kept the balance between clinical and laboratory research until 1981, when I became chief of the Laboratory of Human Carcinogenesis. Because of the wave of molecular biology coming into cancer research, I thought that the greatest opportunities were in the laboratory. I believed that if I was going to get serious about that area, I would have to spend essentially full time in the lab. Now, I consider myself primarily a laboratory researcher–a physician-scientist.

SW: How did you decide on subject matter for your research?

   Harris: When I joined NCI in the Laboratory of Experimental Pathology, my strategy–which I still have–was to use clinical and epidemiological observations to generate hypotheses, and test them in the clinic or in the laboratory using animal models and in vitro systems. Early in my career, long before it became fashionable, I developed in vitro models using human tissues and cells, including tissue implant and cell cultures from human donors. The idea was to use these in vitro models to study the effects of negative and positive growth factors, to introduce genes, and to expose the cells to chemical carcinogens.
   Human studies have gone largely in three directions: molecular carcinogenesis, improving molecular diagnosis, and molecular epidemiology. The two major facets of molecular epidemiology are the dosimetry of carcinogen exposure, and the inherited predisposition. These studies focus on the gene-environment interactions in determining an individual’s cancer risk. I spend about a third of my time now in molecular epidemiology, and two thirds in the areas of molecular diagnosis and carcinogenesis.

SW: How did the p53 gene become part of your research?

   Harris: Two groups in Europe and two in the U.S. discovered p53 in 1979. It is a cellular protein frequently overexpressed in mouse and human tumor cell lines. It was cloned a few years later by Ed Harlow, Moshe Oren, and Arnold Levine. They and others found that the mouse and human p53 genes which they were investigating were nuclear oncogenes. In retrospect, they were studying missense mutants of p53 and not the normal gene.
   In the late 1980s, we were searching for candidate tumor-suppressor genes. One site frequently deleted in lung cancer was on chromosome 17p13; the location of the p53 tumor-suppressor gene. Bert Vogelstein's lab at Johns Hopkins and my lab got together to ask two simple questions: Is p53 frequently mutated in human cancer, and is it mutated in several types of human cancer? In both cases, the answer was yes. That led to a paper in Nature in 1989 (See table below, paper #3). These results further stimulated our interest in p53, because of its high mutation frequency in human cancer.

SW: How did you proceed then?

   Harris: The work went in two different directions. The first was to extend the observations to many different kinds of cancers and to determine the timing of p53 mutations in cancer development. This led to the finding that the mutational spectrum in the p53 gene is quite different from that in other tumor-suppressor genes, in that the majority are missense mutations. That can lead to a loss of tumor-suppressor function but, in certain mutants, a gain in oncogene function.
   In 1990, Monica Hollstein, who was a visiting scientist in my lab, and I started a p53 mutational database. It is now the world's largest database of mutations, with over 10,000 entries. In the mid-1990s, we transferred it to the International Agency for Research on Cancer in Lyon, France, where it is used by scientists around the world to generate hypotheses, especially about molecular links between the causes of cancer and cancer itself. The database has also been useful in generating studies that relate to prognosis and responses to therapy.
   That same year–1990–Monica, David Sidransky, Bert Vogelstein and I wrote an article in Science, titled "p53 Mutations in Human Cancer." That set the stage for analyzing the mutational spectrum, and for using that information to generate hypotheses about various functional domains within the p53 gene. This has become one of the more heavily cited papers of the last decade, with more than 3,000 citations. (See table below, paper #1.) Nowadays, probably 3,000 to 4,000 papers per year are related to p53.


High-Impact Papers by Curtis C. Harris
Published Since 1989
(Ranked byaverage citations per year)

Rank Paper Total
Citations
Avg.
cites
per
year
1 M. Hollstein, D. Sidransky, B. Vogelstein, C.C. Harris, "p53 mutations in human cancers," Science, 253(5015):49-53, 1991. 3,259 407
2 M.S. Greenblatt, W.P. Bennett, M. Hollstein, C.C. Harris, "Mutations in the p53 tumor suppressor gene: Clues to cancer etiology and molecular pathogenesis," Cancer Res., 54(18):4855-78, 1994. 1,060 212
3 J.M. Nigro, et al., "Mutations in the p53 gene occur in diverse human tumor types", Nature, 342(6250):705-8, 1989. 1,893 210
4 C.C. Harris, M. Hollstein, "Clinical implications of the p53 tumor-suppressor gene," New Engl. J. Med., 329(18):1318-27, 1993. 693 139
5 I.C. Hsu, R.A. Metcalf, T. Sun, J.A. Welsh, N.J. Wang, C.C. Harris, "Mutational hotspot in the p53 gene in human hepatocellular carcinomas," Nature, 350(6317):427-8, 1991. 789 99
SOURCE: ISI's Personal Citation Report, 1981 - June 1998

SW: What other direction have studies gone in?

   Harris: The other major area is p53 function, particularly in apoptosis. p53 has multiple functions. It upregulates certain genes and suppresses others, including those involved in apoptosis. It's clearly involved in one or more of the apoptotic pathways. p53 is also involved in cell-cycle checkpoints, DNA repair, and chromosomal segregation. In fact, we have a long laundry list of p53 functions. Different domains of the protein have different functional activities. The research community has shown a great interest in this: in defining what portions of the protein are important for its activity; in investigating interaction with other proteins that might modify their function; and seeing what portion of p53 is involved with modification of apoptotic function. Because most cancers are clonally derived, these mutations have affected p53 functions in such a way as to give clues regarding how the mutations are involved in carcinogenesis.
   p53 may also play a role in viral carcinogenesis. For example, p53 binds to the X protein of the hepatitis B virus. This may be important in liver cancer associated with hepatitis B virus. The E6 protein of certain human papillomaviruses also targets p53 for proteolytic digestion, and, as Peter Howley and Harold zur Hausen have shown, this inactivation of p53 contributes to the development of cervical cancers.

SW: You have recently started to study nitric oxide (NO) in connection with p53. Why?

   Harris: Is it possible that NO and its oxyradical derivatives might cause damage in cells and might in fact be an endogenous carcinogen? That's been an interesting question. It's important to know that NO is produced by three different isoforms of nitric oxide synthase. Two produce very small bursts and concentrations of NO. The third form, which is inducible, is found mainly in macrophages and epithelial cells. When activated, this produces much more NO over a more prolonged time and is thought to be a defense against pathogenic organisms, such as bacteria. This could have some pathological consequences for the host, including chronic inflammation leading to cell damage. There is accumulating evidence that chronic inflammation is a cancer-prone condition.
   That led us to be interested in NO in two ways: first, investigating whether it might cause mutations in cells that could lead to cancer; and second, asking whether there might be a feedback loop between NO production and p53. If the inducible NO synthase produces high levels of NO resulting in DNA damage–which we know leads to activation of p53–perhaps p53 in a feedback loop might suppress expression of inducible NO synthase. We found that was indeed the case in mouse and human cells in culture. In p53 knockout mice, there's an increased basal level of NO compared with genetically normal animals. That provides in vivo evidence that p53 is part of a negative feedback loop in which it can reduce NO. In cells that have a mutant p53 or lack the gene, such as knockout mice, you would have disregulation of NO, and hence higher levels; that could lead to a cancerous state. We're investigating this hypothesis now.

SW: What does research on molecular epidemiology promise at the level of individual patients?

   Harris: Molecular epidemiology is an emerging field in cancer research. Interesting leads have been generated from studies of a few hundred subjects. We want to take this back to the clinic to see if one can extend this information to much larger population-based studies and contribute to improved cancer risk assessment, particularly at the level of the individual. One of the challenging goals of molecular epidemiology is to identify individuals at high cancer risk.
   One of the exciting things about the current state of cancer research is that there is more specific definition of the molecular events that occur as the normal cell becomes a cancer cell. There's much greater understanding of the whole area of molecular carcinogenesis. Also, we're starting to define cancer not in terms of lung or breast cancer, but in terms of the genetic alterations that have occurred in them. We anticipate that knowing the molecular signature of cancer cells will help us to develop more rational therapies. Our research offers hope of going one level down in the specificity of treatment. It should help us to use our armamentarium more effectively.

SW: Where do you go from there?

   Harris: p53 keeps surprising all of us, because of its involvement in multiple pathways and its multiple functions. p53 is clearly at the crossroads of the pathways responding to cellular stress, including DNA damage, hypoxia, and oncogene activation. Understanding these pathways should pay dividends in the prevention and treatment of cancer. I'm still prepared to be surprised by p53.end

From an interview with Science Watch® - July/August 1999 Vol. 10, No. 4, a publication of the Institute of Scienctific Information®.
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