By Will Boggs, MD

NEW YORK, Mar 27 (Reuters Health) - Two genes interact in a way that increases the risk of obesity in Mexican Americans, suggesting that several genes can combine to cause weight problems for susceptible individuals.

However, the gene combination does not make obesity a fait accompli--there are many factors that play a role, including exercise and diet.

"There are likely to be many genes which contribute to the development of obesity," Dr. Braxton D. Mitchell from University of Maryland in Baltimore, told Reuters Health. "Moreover, many of these are likely to interact in complex ways with each other and certainly with lifestyle behaviors."

Mitchell and colleagues looked at about 450 people from 10 Mexican-American families who were enrolled in the San Antonio Family Heart Study. The investigators specifically looked at the interaction of two genes--ADRb3 and PPARg2--already linked to the development of obesity in this group. ADRb3 is produced in fat cells and plays a role in the breakdown of fats and PPARg2 helps regulate the growth of fat cells as well as their sensitivity to insulin, the blood sugar-regulating hormone.

The researchers found that people who had specific versions of both the genes were more likely to become obese than those who had other versions of the genes.

In fact, according to the report in the April issue of Diabetes Care, those with the PPARg2 gene version were unlikely to be obese unless they also had the ADRb3 gene version. About 18% of those tested had the obesity-linked version of ADRb3 and 12% had PPARg2.

However, the good news was the gene combination did not increase the risk of diabetes, a common problem in Mexican Americans.

"Family and twin studies are remarkably consistent in demonstrating that, collectively, genes account for about 50% of the total variation in body mass index," said Mitchell. Body mass index (BMI) is a measure of height and weight. "We just don't know yet what these genes are or which of these genes are the big players."

Even though these genes were clearly linked to the development of obesity among Mexican Americans, they still accounted for less than 3% of the variation of BMI in this population, the authors note.

"The true clinical significance of this (genetic) association may be that it suggests a molecular pathway that could potentially serve as a (treatment) target," Mitchell concluded.

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