UCLA and Finnish researchers have identified a genetic mutation for lactose intolerance, a painful digestive condition that afflicts some 30 million to 50 million North Americans, 75 percent of African Americans and 90 percent of Asian Americans. The findings are reported in the Jan. 14 issue of Nature Genetics.
Dr. Leena Peltonen, UCLAs Gordon and Virginia MacDonald Distinguished Chair in Human Genetics, and her associates at the UCLA School of Medicine, collaborated with colleagues at Finlands National Public Health Institute to identify a DNA variant outside of the gene associated with lactose intolerance.
Lactose intolerance occurs in children after weaning, when the cells that line the small intestine decrease their production of an enzyme called lactase-phlorizin. For years, scientists analyses of the human gene that encodes this enzyme revealed no mutation associated with the disorder. As a result, Peltonens team looked for a DNA variant outside of the encoding gene.
The researchers drew blood samples in order to study the DNA of a Finnish group of 196 lactose-intolerant adults of African, Asian and European descent. Each of them showed the genetic mutation for lactose intolerance in their DNA.
That we found the same DNA variant in all lactose-intolerant people across distant ethnic groups indicates to us that it is very old, Peltonen said. We believe that the variant we identified in patients is the original form of the gene which mutated to tolerate milk products when early humans adopted dairy farming.
This suggests that everyone was originally lactose intolerant, she added. Its an excellent example of a useful mutation in human history.
Lactose intolerance is the inability to digest large amounts of lactose the main sugar found in dairy products. Symptoms include nausea, cramps, bloating, gas and diarrhea, which begin about 30 minutes to 2 hours after consuming foods containing lactose.