Imagine learning just a few years into your young marriage that your husband has a very high risk of developing Alzheimer's disease — as early as in his 40s — and all you can do is wait for symptoms to appear, never knowing when (or if) they will come.

Welcome to Ione Whitney's life. "He'd forget where his keys were and our hearts would just stop," says Ione, 65, of Port Orchard, Washington.

But to this day, 46 years into their marriage, her husband Doug Whitney, 66, has not shown signs of the irreversible brain disease, which is the most common form of dementia and begins with symptoms that include forgetting names, misplacing things or experiencing changes in mood.

To say that he has beaten the odds is an understatement. Doug has a rare genetic mutation that his doctor said gave him a 95 to 98 percent chance of developing what's known as younger-onset or early-onset Alzheimer's, meaning it develops before age 65.

Those chances weren't all that surprising to Doug. Younger-onset Alzheimer's has killed 18 of Doug's relatives, including his mother, older brother, six cousins and 10 aunts and uncles. He also has two cousins, one cousin's child and one cousin's grandchild who are currently living with the disease, meaning at least 22 members of Doug's family have been affected by Alzheimer's. (The family suspects that Doug's grandfather also had the disease, but there was no diagnosis at that time.) Doug could still develop Alzheimer's, but it would no longer be considered younger-onset because he is 66.

Alzheimer's, unfortunately, is all too common, although younger-onset is relatively rare. More than 5 million people in the United States have Alzheimer's and about 200,000 have younger-onset Alzheimer's, according to the Alzheimer's Association, a nonprofit based in Chicago, Illinois, that's dedicated to eradicating the disease.

The Letter That Changed Everything

In 1971, only a couple years into their twenties and their marriage — and pregnant with their first child, Brian — Ione and Doug were not prepared for the letter they received from Doug's aunt Ester.

Doug wasn't close with his aunt, so the letter was unexpected. In it, Ester matter-of-factly explained what was known about the family's harrowing medical history. Ester's husband and brothers-in-law were acting "loopy," and she had been trying to figure out why for years.

"It took a couple of days to get over the anger. But you just have to deal with it," Ione says. They started paying more attention to newspapers, TV and radio to learn more about the disease.

Doug's mother died in 1976, five years after Doug and Ione received the letter. Although Alzheimer's was discovered in 1906 and named in 1910, little else was known about the disease until the mid-1980s.

Genetic Risk: Nobody Wants to Hear 50-50

Over the next two decades, researchers discovered three genetic mutations that increase the risk for younger-onset Alzheimer's. Inherited younger-onset Alzheimer's is called familial Alzheimer's disease (FAD). Researchers also learned that if your parent has one of these mutations, you have a 50-50 chance of inheriting the mutation yourself. And if you do inherit the mutation, you will "almost surely develop FAD," according to the National Institute on Aging.

Over dinner one night in 2011, Doug's cousin Gary Reiswig encouraged him to take part in a study called DIAN (for Dominantly Inherited Alzheimer Network) at the Washington University School of Medicine in St. Louis. Participating meant Doug could get genetically tested for free.

Ione and Doug's children, Brian and Karen, were in their late 40s at the time — around the age that Doug's mother, older brother and other family members with younger-onset Alzheimer's first began showing symptoms. Brian also had a young daughter. Doug felt that it was important to know his genetic status so that his kids could think about whether they wanted to get tested, too.

"When I found out that I had the genetic mutation, I was a bit surprised, as everyone else was," Doug says. "I felt the same that day as I had the day before."

Ione was shocked, too. "We are pretty blessed that we had so many wonderful years. I don't know if it's luck or what, but it's an amazing thing," she says.

As soon as Doug found out that he had the mutated gene, Brian and Karen called DIAN researchers to get tested.

"When Karen found out she did not have the mutation, it was like a rock was lifted," Ione says. "But then there were [Brian's test results] hanging there, like, what's it going to be?"

Like his father, Brian has the genetic mutation.

"It was heartbreaking to learn that Brian did have the mutation, especially when he was standing there with his daughter Emily, who was barely 2 years old at the time," Ione says.

There is a 50 percent chance that Emily also has the mutation, but the family will let her decide if she wants to get tested when she turns 18.

Since learning of her family's genetic test results, Ione has tried her best to process the devastating news and focus on supporting her husband, children and grandchild.

"She's the rock. Brian is always calling, and they talk about it. She always has words of encouragement," Doug says. "I don't talk enough for her to be sure of what I'm thinking, but she's there."

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Three generations: (left to right) Andrea, Emily, Brian, Doug, Ione and Karen Whitney.

The Future of Alzheimer's Disease

What's frightening: "Alzheimer's is on the top ten list of fatal diseases, and it's the only one on the list that doesn't have a way to prevent it, cure it or slow it down," says Beth A. Kallmyer, MSW, vice president of constituent services for the Alzheimer's Association.

"By mid century, if nothing changes in terms of treatments, the number of people with Alzheimer's could nearly triple — from 5 million to almost 14 million," she says. "We're living longer, and age is still the biggest risk factor."

But the good news is that some research is showing promise. The findings from the DIAN observational study that Doug participated in have made it possible for a second study called the DIAN-TU Trial. This trial is testing two drugs that work to clear beta-amyloid plaques — clumps of protein that are the primary sign of Alzheimer's — from the brain to in the hopes it can prevent or at least delay the start of the disease.

"Now, coming all the way back around, these drugs are being used for the same family members that have led to the discovery of the mutations in the first place," says Randall Bateman, MD, director of the DIAN-TU Trial and professor of neurology at Washington University School of Medicine.

The first results of the drug trial won't be available until 2016. But Dr. Bateman says it's possible one or both of the drugs could become available as early as five years from now and start to bring down the number of Alzheimer's diagnoses.

"We have every reason to believe that these drugs could be helpful," he says. "And for that reason, I'm hopeful. But we have to prove it. And you can only do that when you run a study and you determine: 'Are these drugs helpful?'"

That's why Ione, Doug and the rest of his family are so passionate about supporting and participating in Alzheimer's research. Without studies like DIAN, there is no way to find a cure.

"I want to do whatever I can to end this disease," Doug says.

But Doug has already made a tremendous difference, Bateman says. "If it wasn't for people like Doug Whitney participating, nothing would ever change... When people participate and they help that way, they're literally changing the future of humanity and what we're all going to face."

If Doug does eventually start showing symptoms, Ione knows she'll have a reliable support system: "We both come from big families. We know there will be someone who can help if we need them." she says. "If something comes up, you help the next person."

If you think someone you know might be suffering from Alzheimer's disease, contact the Alzheimer's Association's free, 24/7 helpline at 1-800-272-3900 or visit www.alz.org to find resources and support. And if you're interested in getting genetic testing or participating in, learning more about or supporting the DIAN trial, visit www.dianxr.org.