Cancer incidence and mortality rates vary among ethnic and racial groups. These differences offer clues to the causes, risk factors, and forces that influence the development and progression of cancer. Although some authors have attributed varying rates to genetic differences, epigenetic influences on cancer are becoming more appreciated. Still, as cancer research seeks to explain why cancer statistics differ among various groups of people, practical details concerning research on racial and ethnic groups remain neglected.
Two questions are critical: (1) When is genetic research on a population appropriate? (2) How should researchers define a given population being studied? All research on ethnic groups must, in fact, address these two questions, even if investigators do not explicitly take them into consideration. Studies of Ashkenazi Jews and BRCA1/2, for instance, represent fairly concrete genetic analyses on a specific population. As such, an examination of these studies offers a simplified vantage point from which to dissect the practicalities of ethnic research as well as clarify what is necessary for research concerning all racial and ethnic groups.
The concept of "race" implies a genetic homogeneity among a group of people that correlates with skin hue or other physical characteristics. Yet modern genetic techniques have repeatedly demonstrated that the amount of variation within a racial group is greater than the difference between two groups. Such findings are not surprising, given the arbitrary nature of definitions used to categorize people. For instance, in Virginia in 1750, a person was considered a member of the "Negro" race if born to a "Negro" mother. Later, the definition was changed, and a person was designated as Negro if he or she had "one drop of Negro blood." Moreover, at varying times in the 20th century, immigrants from India have been categorized as white, Asian, and Indian. Clearly then, such definitions are political and unscientific. Dividing people into racial groups is more a consequence of social constructs than a reflection of biological reality.
"Ethnicity" is another term that is sometimes used interchangeably with "race." Yet, unlike race, ethnicity includes a variety of social factors and is not strictly a biological term. As Bhopal defines it, ethnicity denotes "a group that people belong to because of shared characteristics, including ancestral and geographical origins, cultural traditions, and language." Like race, however, ethnicity is a socially constructed phenomenon. Ethnic boundaries are based primarily on self-assessment, and are thus inherently imprecise and can change over time or depending on the specific context.
Thus, it is not surprising that racial and ethnic groups do not represent a genetically homogeneous population. In some instances, however, a correlation exists between an ethnic group and a specific mutation, as can occur in situations of enforced segregation. For example, approximately 20% of Ashkenazi Jewish women with breast cancer carry mutations in BRCA1 or BRCA2a finding that is 10- to 15-fold higher than in breast cancer patients who are not Jewish. Specific mutations in both BRCA1 and BRCA2 are present at least 10 times more often in unaffected Ashkenazi Jewish women than in the general population. Nevertheless, the Ashkenazi Jewish population does not monopolize the gene. The genetic mutations associated with cancer have been found in people of several races and ethnicities.
Familial vs Racial Correlations
Although they have been associated with the entire Ashkenazi population, such genetic abnormalities should be considered familial and not racial. This is particularly true given the high rates of intermarriage among the American Jewish population; any genetic homogeneity that may have existed under the conditions of segregated ghettos in Eastern Europe has been and continues to be diluted. This point is confirmed by looking at the prevalence of the 185delAG BRCA1 mutation among groups of Ashkenazi Jewish breast cancer patients, as was determined in 14 separate studies.