20 September 2012

Gene mutation behind African migration also responsible for modern health woes

by Will Parker

The brain-boosting genetic mutation that allowed primitive man to migrate across the African continent is over-represented in modern-day African Americans and is believed to be responsible for the higher rates of hypertension, diabetes, coronary heart disease and cancer in that racial group. The findings are the result of a genetic analysis of different racial and ethnic populations around the world conducted by scientists from Wake Forest Baptist Medical Center, Johns Hopkins University and the University of Washington School of Medicine.

The new findings may explain why homo sapiens, who appeared approximately 180,000 years ago, stayed predominately around bodies of water in central Africa for almost 100,000 years. Previous studies have hypothesized that such locations were critical as early humans needed large amounts of docosahexaenoic acid (DHA) to support complex brain function. DHA is a polyunsaturated fatty acid and is found in shellfish and fish.

"This may have kept early humans tethered to the water in central Africa where there was a constant food source of DHA," explained Wake Forest's Floyd H. Chilton, one of the study's senior authors. "There has been considerable debate on how early humans were able to obtain sufficient DHA necessary to maintain brain size and complexity. It's amazing to think we may have uncovered the region of genetic variation that arose about the time that early humans moved out of this central region in what has been called the 'great expansion.'"

In the new study, Chilton and his co-researchers demonstrate that a critical genetic variant arose in a gene cluster on chromosome 11 more than 85,000 years ago. This variation, in the fatty acid desaturase cluster, would have allowed early humans to convert plant-based polyunsaturated fatty acids (PUFAs) to brain PUFAs necessary for increased brain size, complexity and function.

The fatty acid desaturase (FADS) cluster plays a critical role in determining how effectively medium-chain PUFAs found in plants are converted to the long-chain PUFAs found in the brain. Once this trait arose, says Chilton, selective pressure ensured it spread rapidly throughout the population of the entire African continent.

This conversion meant that early humans didn't have to rely on just one food source - fish - for brain growth and development. "This may have been particularly important because the genetic variant arose before organized hunting and fishing could have provided more reliable sources of long-chain PUFAs," explains co-researcher Joshua M. Akey, from the University of Washington.

The findings also add to our knowledge of how the pressures of human evolution contribute to modern-day diseases and disorders. Chilton notes that compared to Caucasians, African Americans have much higher rates of hypertension, type 2 diabetes, stroke, coronary heart disease and certain types of cancer. This he says, is because African Americans have a much higher frequency of the gene variants that convert plant-based medium-chain omega-6 PUFAs (found in cooking oils and processed foods) to long-chain PUFAs (that can cause inflammation). "The current observation provides another important clue as to why diverse racial and ethnic populations likely respond differently to the modern western diet," he said.

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Source: Wake Forest Baptist Medical Center