Vol. 5, No. 4, pp 3436, 39.
Cancer: From Detection Through Therapy
Molecular profiling has transformed our understanding of the disease and of drug responsivelness, which may improve potentials for treatment.
In the United States, the battle against cancer has been institutionalized for more than 60 years. The National Cancer Institute (NCI) was established in 1937; in 1955, Congress authorized a national chemotherapy program. And despite the overt U.S. declaration of hostilities in 1971, when Richard Nixon inaugurated the War on Cancer under the auspices of the NCI, there has been little cause for triumph. Improvements in survival have been piecemeal, resulting from organized efforts based on prevention through diet (high fiber and antioxidants), lifestyle changes (quitting smoking, for example), early diagnosis, and a wide variety of therapeutics that prolong life and provide seemingly permanent remissionsespecially for several childhood cancers.
Our understanding of the enemy has never been profound. Until recently, the diagnosis of cancer has been crude. Biopsies have shown cellular abnormalities in tumor tissues that are considered characteristic but are little more than recognition of generalized chromosomal instabilities and morphological disarray. The location of any particular aberrant growth has defined its naturesuch as lung, liver, or bone marrow canceras much as any knowledge of physiology or genetics.
The diagnosis of cancer by these methods has proved little more sophisticated than the diagnosis of influenza based on gross symptoms. And how far would flu vaccination have gotten if scientists had remained at such a level, not realizing that there were fundamentally different forms of the virus that had different clinical effects? Last years shot is unlikely to protect against this years flu.
So perhaps the most important development in the modern understanding of cancer has come from more sophisticated attempts at identifying the disease through molecular profilinga more rational and individualized form of diagnosis. These new methods of diagnosis have helped transform our view of cancer from that of a metabolic mistake to that of a genetic disease.
Advances in histopathology, the identification of disease through tissue examination, have gone far beyond the previous stage of relying on trained observation of morphological aberrations and chromosomal chaos in dyed tissue samples. Newly available monoclonal antibodies and genetic analysis techniques tagged to fluorescent markers have transformed the field. Innovations such as DNA microarrays and sophisticated forms of the polymerase chain reaction (PCR) are providing diagnostic profiles of cancers at the level of proteins and DNA in a way never before possible. This new molecular appraisal of cancer has given rise to the concept of cancer as a much more highly individualized disease based on patient geneticsbreaking down monolithic diagnostic genera into a host of distinct disease subspecies with profound ramifications for detection and treatment.
The early word
Besides improving the prospect for early detection, such molecular techniques are making it possible to differentiate similar cancers from one anothersometimes with profound (if at times depressing) implications for treatment and prognosis.
For example, Shigeo Nakamura of the Aichi Cancer Center Hospital in Nagoya, Japan, reported at the 2001 symposium Cancer Diagnosis with the Power of Molecular Knowledge that mantle cell lymphoma occurs in at least two distinct (but hitherto indistinguishable) types. Using an immunohistochemical approach, Nakamura determined that cyclin D1-staining positive and negative lymphomas are different entities that have vastly different five-year survival prognoses: 30% for the D1 positive cancer and 86% for the D1 negative group (2).
Of course, the earliest form of detection is prediction. With new research into the genomics and proteomics of cancer, firm biological risk factors for cancer will inevitably be established. And better means of diagnosing the precancerous or cancer-prone state are likely to become routine for a wide variety of disorders. By following a molecular profiling approach, researchers in tobacco-associated cancers have begun to find indications of a definable, precancerous state based on changes in specific genes or gene products. Breast cancer is one of the most powerful, and often disturbing, examples of this trend. The discovery of genes associated with the development of breast cancer has been a boon to research progress, but their detection can be frightening to anyone diagnosed as at risk.
In an example for males, University of Michigan researchers reported that the protein made by a gene called hepsin appears to be strongly elevated in prostate tissue just before it turns cancerous, giving a potential early-warning marker (3). Similarly, a loss of the genetic heterozygosity of tumor suppressor genes due to deletions and alterations in early-stage carcinogenesis was detected in the lungs of smokers who had not yet developed lung cancer. Such changes are also seen as potential markers for the diagnosis and monitoring of bladder cancer (2).
Although knowledge of a genetic predilection to cancer can be an opportunity for advanced monitoring, preventive chemotherapy, and lifestyle changes, it can also be a tremendous psychological burden, especially if it is seen as a sentence of death. But this has always been the dark side of diagnosis, and it is likely to become more common because detection, although a necessary first step, has always outpaced the ability to cure.
Diagnosis for prognosis
It is becoming all too evident that cancer therapy, to be most effective, must be tailored to the individual patients cancer. Cancer, even a particular kind such as that of the lungs, prostate, or breast, is not a monolithic killer but an individually trained assassin, designed to destroy its particular victim.
So too with the cancers that destroy us. Because they are betrayals in our own defense systems, my cancer is not yours. Your body may well be likely to fight off whatever might destroy meand hence keep the problem from occurring in the first place. (Our bodies destroy spontaneous cancers within us all the time.) My mutant oncogenes are not yours. For example, more than 400 mutants of the p53 gene are already known, many of which cause specific types of cancer.
Additionally, my immune system may prevent one type of cancer from developing, but not another. And even if we both see terrifying spots on our chest X-rays, my form of lung cancer may be resistant to the therapeutic drugs that will save your life. Researchers are coming to believe that analytical diagnosis of the nuances, not just the existence, of individual cancers on a patient-by-patient basis may be the only way to deal with this variation to define what treatments might work and which will definitely not. It is the ultimate instance of the need for a pharmacogenomics approach.
Not only are unique differences detectable between individuals, but, as might be expected, there are larger-category differentiating factors as well. These include obvious ones such as sex (ovarian vs prostate or testicular cancer, for example). Genetic variation in cancer response also is correlated with more subtle differentiators, such as race. Africans from Ghana, Kenya, and the Sudan, as well as African-Americans, have a significantly lower incidence than the general population of a mutation of a protein pump involved in cellular detoxification. Individuals with the mutation have a better chance of being helped by chemotherapeutic drugs (4).
Diagnosing drug resistance
Differences in drug resistance can occur between one genotype of cancer and another. For example, the nonsteroidal anti-inflammatory drug sulindac is used to trigger apoptosis (programmed cell death) in precancerous polyps before they can become fully cancerous. In the November 3, 2000, issue of Science, Johns Hopkins University researchers Bert Vogelstein and colleagues reported how knocking out the BAX genes can create sulindac resistance in human colon cells. They determined that the effectiveness of the drug was determined by the ratio of proapoptotic proteins (such as the BAX gene product) to anti-apoptotic proteins. With BAX knocked out, the ratio was always dominated by those proteins preventing apoptosis, and cell death could not be triggered by the drughence resistance occurred (5).
Resistance to a particular drug can also occur if there is a deletion or disabling of an enzyme required to activate that drug. For example, for the drug cytosine arabinoside to become cytotoxic, it must first be activated by deoxycytidine kinase in the cancerous cells. Cancers with mutations or deletions in this kinase gene are resistant to the drug (6).
In addition to single drug resistance, the more dangerous multiple drug resistance (MDR) can occur when specific alleles of genes involved in cancer are present initially or through subsequent mutation. This can make a wide variety of standard chemotherapeutic agents ineffective, often dooming the patient. Obviously, then, the diagnosis of potential drug resistance (whether single or multiple) is a critical step in determining which form of chemotherapy (if any) is likely to be most effective. Choosing the wrong drug not only can expose the patient to the usually distressing side effects of such drugs to no purpose, but also foolishly wastes the most important commodity in all cancer treatmentstime.
There are several molecular mechanisms for MDR (7). One major type of resistance is induced by the overexpression of a drug efflux pump in cancerous cells, which keeps the drugs from accumulating to toxic levels. These include the MDR1/P-glycoprotein pump and the aptly named multidrug resistance associated proteins, MRP1 and MRP2.
Another surprising cause of resistance (against drugs that act to damage DNA in tumor cells) is the mutation to a more effective DNA repair enzyme in certain cancer cells. Because many new chemotherapeutics act by triggering programmed cell death in the aberrant cancer cells, mutations in oncogenes involved in apoptosis, such as p53, may also lead to MDR. If the apoptotic pathway is damaged, not only are cancers more likely to occur, but they are more likely to be resistant to such drug treatments.
Considerable research is going on, and several clinical trials are examining the potential benefits of compounds that inhibit such drug resistance for their ability to improve the effectiveness of chemotherapy.
Complexity or chaos?
Mark S. Lesney is a senior editor of Modern Drug Discovery. Send your comments or questions regarding this article to email@example.com or the Editorial Office by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.