his hands on her money. david harris and hazel allinson did have happy times, but his obsession with another woman, a0 years younger, led him to push three men to kill, to satisfy his lust, greed and distorted fantasies. duncan kennedy, bbc news. dementia in old age is the biggest cause of death in the uk. but in some families — extremely rare gene mutations can cause alzheimer's disease in middle—age. now experts believe that studying the way the disease develops in such families could hold the key to treatment in the future. 0ur medical correspondent — fergus walsh — spoke to two families with a history of alzheimer's — both of whom are taking part in medical trials. i'm almostjust waiting for the first sign, really. the minute you forget something, the minute you can't find your car keys... sophie leggett from suffolk has a 50—50 chance of having inherited a rare gene for alzheimer's.