RALEIGH, N.C. — Although the exact cause of Alzheimer's disease is still unknown, two new studies could help lead scientists to some answers.

Researchers believe one major culprit is the buildup of plaque in the brain. It can disrupt activity and memories. Like heart disease, a poor diet and lack of exercise can cause the buildup.

Dr. Michael Rapp, of the Mt. Sinai School of Medicine, may have found another reason. He examined a part of the brain called the Hippocampus. He discovered people who battled major depression during their lives were more likely to have the plaque.

"We specifically looked at the hippocampus because it's the area of the brain that we know consolidates and forms memories," he said.

Researchers will try to figure out how depression may cause plaque and see if treating major depression can actually lower the risk of the disease.

Like any disease, family history is also connected to Alzheimer's disease. But another new study has found that genetics may be a larger factor than first thought.

Dr. Margaret Gatz, of the University of Southern California, studied almost 12,000 pairs of identical and fraternal twins. She found that identical twins, who share more genes, were much more likely to both get the disease.

"We can say genetics plays a strong role," she said.

That information could help researchers eventually discover what genes may influence the disease.

"I think it's most likely that what we're looking for is a large number of genes each with very small accumulative effects," Gatz said.

Six states could see their Alzhelmer's cases more than double by 2025. The most dramatic increases are expected in the west and southeast.