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.

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.

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.

Where do you go from there?