Igor Splawski of Children's Hospital, Harvard Medical School and the Howard Hughes Medical Institute in Boston and his colleagues studied DNA samples from patients suffering from cardiac arrhythmia. The team identified a single nucleotide substitution in the SCN5A gene (which acts on sodium channels in the heart) of an African-American woman. Because the patient did not possess any of the previously known mutations linked to arrhythmias, the researchers posited that the substitution, dubbed Y1102, could play a role in her heartbeat irregularity. In a normal heart, the flow of ions through these channels causes the heart muscle to contract. In cultured cells, the variant gene form caused the channels to remain open too long or open when they were supposed to be closed. Such activity in a living person could contribute to the development of an uneven heartbeat. When the scientists screened a sample representative of the general population, they found that Y1102 is not distributed evenly: 19 percent of West Africans and Caribbeans studied had it, as did 13 percent of African-Americans. They did not detect Y1102 in any of the Caucasians or Asians who participated in the study. Carrying Y1102 confers an eightfold risk for arrhythmia, the team reports, although the variant alone probably will not cause an abnormal heart rhythm. "We estimate that 46 million African-Americans carry Y1102," the authors write. "Most of these individuals will never have an arrhythmia because the effect of Y1102 is subtle."
But for people who do have Y1102 extra precautions--such as avoiding drugs known to affect heart rhythm and ensuring that electrolyte levels remain at safe levels--couldn¿t hurt, the researchers say. "What's often required for a dangerous arrhythmia is that several things go wrong at the same time," study co-author Mark T. Keating notes. But, he says, widespread testing is a long way off. "This is among the first pieces of a big puzzle of genetic effects on arrhythmia. We need to have many more pieces before we can begin large-scale genetic testing of people for such variants."