The major risk factor for both cancer and Alzheimer's disease is aging. Changes in our biology that happen as we age contribute to the development of both diseases. Better understanding the biology of aging will help us develop drugs to treat Alzheimer's. It will also help us identify drugs currently used to treat cancer that could also be "repurposed" to treat Alzheimer's patients.
Latin Americas and Scandinavians are more likely to be chipper than, say, the Chinese or the Iraqis (of course, there are very real non-genetic reasons for their respective unhappiness too). Is it possible, then, that as more Hispanics intermarry and intermingle and inter-you-know-what, they will spread their happiness genes among more and more Americans?