June Ward (right) — pictured with her sisters Susie Gilliam (center) and Karen Douthitt (left) — carries a rare gene mutation that virtually guarantees she will get Alzheimer's in the next few years. She is part of a network of more than 200 families with these kinds of gene mutations who have volunteered as research subjects over the past two decades. Juan Diego Reyes for NPR hide caption toggle caption Juan Diego Reyes for NPR Some of the most important studies of potential treatments for Alzheimer's disease rely on a group of participants who know they may never fully reap the benefits. "It's not for us," says June Ward, 64, who carries a rare gene mutation that virtually guarantees she will get Alzheimer's in the next few years.…