'Most of my family had dementia. I knew I should tell someone'

Suddenly, Alzheimer's disease is the new frontier. Like cancer and HIV/Aids when they were thrust into the public consciousness, it presents a massive research challenge. At present there is no treatment for it: some drugs slow its progress, but there are no cures and no way of preventing it. So how can victims be identified before symptoms appear? One answer is that some forms of Alzheimer's are inherited, and by testing members of families in which it has been passed down from generation to generation, and identifying what distinguishes those who get the disease from those who do not, researchers can close in on the causes.

In the late 1980s, a team of neurologists at St Mary's Hospital in west London was hunting the gene which caused familial Alzheimer's when a letter arrived from a teacher in Nottingham. Carol Jennings' decision to contact them about her family's history was to prove momentous in advancing our understanding of how and why Alzheimer's strikes.

Carol has vivid memories of her father Walter in his prime: the way he sang all the time, his attention to detail, his cheeriness. "He was always really chatty," she said. "He'd come in and talk to anybody."

Walter had been in the Navy in the war, and was meticulous in his job, collecting monthly dues from members of the Nottingham Co-op.

But then in his late 50s things started to go wrong. "Mum was finding that he was doing things that were out of character," Carol recalls. "It wasn't him all of a sudden."

"He was always the life and soul of the party. But all of a sudden he was a different person. If he went in a shop with her he'd wander down the wrong aisle, pick up the wrong things and put them in the trolley or in somebody else's trolley; he didn't know what he was doing half the time..."

Over the years, word arrived that others in her father's family were behaving similarly as they neared 60. "I was thinking, what's going on here?" Carols says "There's this one, and this one and this one – and I thought, somebody ought to know about this."

In 1987 she decided to act. Her GP had told her about the team at St Mary's Hospital that was researching the causes of Alzheimer's.

At the time Carol's letter arrived, "dementia was a real Cinderella subject", says Martin Rossor, today a senior academic at University College London. "It was thought to be bizarre to go into it." A German psychiatrist called Alois Alzheimer had identified the disease that bears his name in 1901. When one of his patients died and her brain was examined, it was found to be shrivelled and dotted with particles. Those particles were deposits of a protein fragment called beta amyloid, the accumulation of which creates the toxic environment in which the degeneration of the brain begins.

But what caused this cognitive disaster? Scientists believed it was a genetic mutation, but nobody had a clue which gene was responsible. "Hunting disease genes," says Dr Rossor, "was like looking for a needle in a haystack." Underfunded and out of fashion, Alzheimer's research was struggling to make progress until the arrival of Carol's letter. When Carol learned that the St Mary's researchers wanted to know more, she persuaded her relatives to co-operate.

Dr Rossor recalls: "Carol was very perceptive in seeing the importance of her relatives for research." As this variant of the disease was inherited, with every child having a 50:50 chance of succumbing to it, by screening all family members the researchers could log the genetic differences between those with the disease and those without it. The team was able to establish beyond doubt that the family's disease was caused by a mutated gene on chromosome 21 – "the same gene", Dr Rossor points out, "that codes for the amyloid protein that was observed by Alzheimer." Those with the gene got the disease; those without it did not. There was no room for doubt.

The discovery was made in 1991 by a young researcher called Alison Goate, now a professor at Washington University. Recently she told The New York Times: "Sometimes in science you generate the data gradually. But this was like, boom, a eureka moment."

The knowledge researchers gained from studying Carol's family gave them the incentive to develop drugs aimed either at stopping the plaques from developing, or clearing them away once they were found.

The work on Carol's family was the forerunner to the biggest research effort into familial Alzheimer's ever undertaken. Called DIAN, for "Dominantly Inherited Alzheimer Network", it involves families in the US, Australia, Germany and Spain as well as Britain.

Will it produce a cure? Dr Rossor is cautious. "There aren't many cures in medicine," he says, "but amelioration can be dramatic. Recovery of the brain's deficit in patients who already have symptoms may not happen, but by getting in at a very early stage of the disease we may be able to delay its onset and progression."

Meanwhile, Carol and her family must deal with the frightening fact that half of them will develop the disease. Carol is now 58, the same age as her father when his symptoms appeared, but she is not tempted by the idea of learning whether she too carries the mutated gene on chromosome 21.

"Having the test has always been an option: I could find out now. But I think I would just collapse in a heap if I thought, 'This was it.'"