Elizabeth B. Claus, MD, PhD
What causes brain tumors is a mystery that people are struggling to understand and scientists are trying to solve. This article by neurosurgeon and epidemiologist, Dr. Elizabeth Claus, explains where we presently are in the search for answers.
What is epidemiology? It is defined as the study of the distribution and causes of disease risk in a population. Epidemiologists are scientists who try to explain why certain diseases affect some groups of individuals more than others and why some diseases occur in certain locations or at specific times. Epidemiological studies sometimes identify specific risk factors associated with a disease. This information may be used to help prevent or treat certain health problems.
There are many types of risk factors. Some are disease-causing agents such as viruses, while others are personal habits that make a person more likely to develop a particular disease, such as cigarette or alcohol use. Frequently, risk factors are divided into two categories: environmental and genetic.
Environmental risk factors generally refer to external influences to which an individual is exposed. Some examples include diet and outdoor sunlight. Genetic risk factors refer to an individual’s hereditary characteristics found in the DNA in his or her cells.
Genetic Epidemiology
Genetic or molecular epidemiology is an important new area of research which examines whether individuals with a certain type or variation of a particular gene have a higher risk for disease. In some instances, genetic changes may be transmitted from parent to child, as is true for the NF2 gene in families with neurofibromatosis. In other instances, the change occurs in an individual, possibly due to an environmental exposure. For example, frequent exposure to sunlight may cause genetic changes in skin cells leading to skin cancer.
While changes in certain genes may be associated with the development of some brain tumors, it is likely that most brain tumors develop as a result of a combination of genetic and environmental factors. The study of such interactions is an extremely important aspect of epidemiology that requires thousands of study subjects. As epidemiologists start to discover which combinations of genes and risk factors cause health problems, they will be better able to counsel individuals regarding their risk and thus help prevent disease.
Approximately 40,000 individuals in the United States will be diagnosed with a primary brain tumor in 2005. Epidemiological studies are vital given the lack of information on causes (etiology) or risk factors associated with the development and progression of these tumors. We also need to learn more about the outcomes associated with brain tumor diagnosis and treatment, including decreased quality of life and mortality.
Three general categories of epidemiological research are: descriptive studies, analytical studies, and clinical studies (or clinical trials). Descriptive studies describe who gets the disease (by age, gender, geography, etc.), how often it occurs (the incidence rate), and how severe it is (the mortality rate). Analytic studies compare groups of people with and without disease and analyze a wide range of possible environmental and genetic risk factors. Clinical studies investigate how well treatments or interventions prevent or control disease. Clinical studies have yielded valuable information about the role of certain genes in brain tumor progression and/or sensitivity to radiation or chemotherapy treatments.
Advances
A new era is coming in brain tumor research. The United States House of Representatives and Senate Appropriations Committee have targeted brain tumor research as warranting increased attention. Funding agencies such as the National Institutes of Health are working to increase financial support for the study of this important topic. Additionally, dedicated individuals have worked to assure that data on certain brain tumors such as meningiomas are included in federal cancer registries. These and other tumors are defined as benign but are associated with significant decreases in quality of life and years of life. As a result of this change, epidemiologists can more accurately identify and enroll patients in brain tumor studies.
Brain tumor epidemiology will also benefit from recent advances in genetics. New statistical and laboratory methods are being developed, enabling scientists to identify patterns in genes that might make a person more susceptible or more resistant to a brain tumor. These new methods will allow scientists to examine thousands of genes for hundreds of individuals, and will likely improve the ability of doctors to correctly classify brain tumors. Determining molecular and genetic markers within a given brain tumor may also enable scientists to tailor treatments to individual cancer patients, based on the genetic makeup of their tumors. In some instances this information is already in use in clinical settings. For example, data exist which indicate that some types of low-grade gliomas such as oligodendrogliomas may have varied response to chemotherapeutic agents based on certain genetic changes. The loss of chromosome 1p and/or 19q has been associated with a positive response to PCV chemotherapy treatment.
The current era is a key time to pursue epidemiological studies of brain tumors. We are moving ahead with large, population-based projects to collect information on genetic and environmental risk factors and possible interactions between the two. We are also making advances in molecular research, increased federal support, and new legislation to identify and register patients with benign brain tumors. The study of brain tumors is a greatly underserved area within epidemiology and genetics. Opportunity now exists to develop a focused group effort on this important topic in public health.
Elizabeth B. Claus, M.D., Ph.D. is an Associate Professor in the Department of Epidemiology and Public Health (Biostatistics) at the Yale University School of Medicine, New Haven, CT, as well as a neurosurgeon in the Department of Neurosurgery at the Brigham and Women’s Hospital, Boston, MA.
This article was reprinted with permission from Search, Issue #61 (Fall 2004). For more information, contact the National Brain Tumor Foundation at 800.934.2873 or visit www.braintumor.org.