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Genetics’ Role in Health Disparities

Charles Rotimi explains why a more complete snapshot of genetic variation is important.
March 21, 2008

The last year has seen an explosion in studies linking specific genetic variations to common illnesses, such as diabetes and heart disease. But how common are these variations in different groups, and do they play the same role in different populations? Those are just two of the questions that genetic epidemiologist Charles Rotimi aims to answer as head of a new center devoted to the study of genetics, lifestyle, and disease in minority groups, at the National Institutes of Health, in Bethesda, MD.

Race and medicine: Charles Rotimi, a genetic epidemiologist, will head a new center at the National Institutes of Health to research diseases disproportionately affecting minority groups.

Rotimi’s research has focused on obesity, hypertension, and diabetes–three disorders that disproportionately affect African Americans; together, the high rates for these diseases account for more than 80 percent of the health disparities between African Americans and European Americans. The new Intramural Center for Genomics and Health Disparities will attempt to uncover the reasons for the differences by exploring the interactions between genetics and environment in African and African-American populations. While many disparities are clearly linked to socioeconomic factors and a lack of access to medical care, genetics may also play a vital role. A genetic vulnerability to hypertension or diabetes, for example, may only be realized in an environment with easy access to high-salt, high-fat foods. Genetic variations can also impact how well a drug works, or whether it will induce harmful side effects in the patient taking it. The variations can occur at different frequencies in different populations–something that needs to be taken into account when studying and prescribing new medicines.

Technology Review recently asked Rotimi to explain his work and its importance.

Technology Review: Why was the center created?

Charles Rotimi: We are right at the point where genomics is beginning to yield interesting fruits, and we want to see those fruits shared by all populations across the world.We want to take advantage of the fact that we are making considerable progress in understanding genetic variation and how it impacts the disease distribution we see across different populations. Only by including all populations can we truly understand human genetic variation and its importance for disease and response to drugs.

The center is set up to take advantage of all of these genomic tools, as well as to try to understand things like culture and lifestyle, and how they interact with genetics in terms of human disease. We want to look specifically at diseases that disproportionately affect minority groups in the United States, including obesity, hypertension, and diabetes.

We are trying to advance research into the role of culture, lifestyle, and genomics–not just genes, but the interactions between genes and environment–to help us understand common complex diseases. Because Africa is the original source of all human migration, whatever we find will be informative for the general population, not just the African people.

TR: Do you think the general public has a misconception about genetics and race?

CR: Yes. The misconception is that genetics can be used to unequivocally identify all members of a particular “race,” compared to others. While it is true that, with enough genetic markers, it is possible to draw imprecise boundaries such as “Africans,” “Europeans,” and “Asians,” there is no set of genetic markers that can be used to identify all persons that self-identify as belonging to a “racial” group without error.

Despite this, scientists have been unable to move beyond racial categorization in science, medicine, and society. Partially responsible for our continued obsession with race is the fact that, although we do not have distinct biological types of “races,” we do have differences in the frequencies of genetic markers across human ancestral groups. These differences, which for the most part describe geographically distant populations, are believed to harbor the answers to why some individuals and groups may be more susceptible or resistant to diseases, and may also hold the key to understanding why certain groups respond differently to medications.

TR: Are existing genomic studies biased?

CR: Yes. The overwhelming number of major genetic studies done have been done in Europeans. But a center like this will help us change that dynamic by helping to set up a large cohort of African-American and African populations.

TR: Why is it so important to study genetic variations and disease in different populations?

CR: In the context of common complex diseases such as hypertension, diabetes, and heart disease, genes are usually not enough, and environmental factors [are] also not enough to cause disease. It is the combination of both. Gene-environment interaction is particularly important when making comparisons between populations. The underlying genetic variant might be the same in both groups, but environmental factors [may change its impact]. For example, let’s say gene A is related to hypertension because it responds to a high-salt diet. If the frequency of this genetic variant is about the same in both African Americans and European Americans, but African Americans eat more salt, they will be more likely to develop hypertension and therefore have higher rates of the disease.

TR: One area of great controversy in race-based medicine was over Bidil, a heart-failure drug and the first pharmaceutical targeted exclusively at a specific racial group–in this case, African Americans. What do you see as the problem with Bidil?

CR: The irony of Bidil is that it was not based on genetic studies. The study did not look at any specific gene in this group called African Americans. It’s important for us to be able to understand the genetics behind these issues. Then we can ask, who among the African-American and the European populations has this variation, and thus the ability to respond to a particular drug, or the potential to suffer an adverse side effect? It would no longer be an issue of group identity.

Part of the point I have been trying to make is that we can get jaded in our own thinking and in the ways we design scientific studies. For example, it’s important when comparing African Americans to their European-American counterparts that we take a critical look at what is going on in West Africa–the ancestral source populations of African Americans. Most African Americans share a considerable proportion of their gene pool with West Africans–greater than 80 percent. So before ascribing a genetic explanation to group differences, it is important to understand what is going on in the source population. For example, the rate of hypertension is about 7 percent in most of rural West Africa, about 16 percent in the urban centers, about 26 percent in the black nations of the Caribbean, and 35 percent among African Americans. This data clearly shows the importance of current environment in the prevalence of hypertension in these populations that share very recent ancestry.

TR: What are you working on now?

CR: One of the most exciting things we’re working on is using the Affymetrix 6.0 chip [a gene chip that can simultaneously detect close to one million specific genetic variations] to study more than 2,000 African Americans from the Washington, DC, area. We’ve collected the data and will begin the analysis soon; if no one beats us to it, this will be the first genome-wide association study in an African-American cohort. We are searching for genes linked to hypertension and obesity, and may look for diabetes-linked genes in the future.

We have also been studying the genetics of diabetes in Nigeria, Kenya, and Ghana, and are planning to expand that cohort to have a sufficient number of participants to do genome-wide association studies.

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