A sad story unfolds in a mountain village in Columbia.

A teenager watches his father slowly lose his mind.

What's worse is that the boy may be seeing his own future.

The boy and his father are part of a large extended Columbian family totaling 5,000 members who have an inherited form of Alzheimer's Disease. A third of them begin showing symptoms at age 43 and dementia sets in at 53.

Because the family is so large, they offer scientists a rare chance to understand this largely mysterious illness which is a form of the most common dementia in America.

Already the findings are surprising. Changes in the brain begin much earlier and in more ways than we presumed. Of the studies done on the family, one involved 44 members between ages 18 and 26, and about half carry the mutation.

Brain scans show carriers' brains possess several differences. The grey matter in memory regions are smaller, the same parts that shrink with Alzheimer's. During memory tests, parts of their brain seemed to be overactive compared with non-carriers.

What's provocative is finding these differences even before the accumulation of something called amyloid deposits in the brain, which scientists had believed is the cause of Alzheimer's. Amyloid is a protein that isn't properly cleared away which is linked to neuronal death and brain shrinkage.

Scientists now wonder whether affected brain areas were already impaired, perhaps even reaching back to early brain development? Their next step is to study family members as young as seven.

Finding ways to diagnose Alzheimer's as early as possible is key to developing treatments to slow its progression.