Unraveling the mysteries of the aging brain is a major goal for brain science, especially given the exploding population of senior citizens and the obvious desire to preserve brain function as long as possible. Now, researchers at Children's Hospital Boston
and Harvard Medical School have uncovered a kind of genetic signature associated with the aging human brain that may contribute to cognitive decline associated with aging. The study appears June 9 as an advance on-line publication of the journal Nature.
One of the study's more surprising results was that these gene changes start in the 40s for some individuals. The results raise intriguing questions about when and why the brain begins to age and the possibility of developing strategies to protect critical genes early in life in an attempt to preserve brain function and delay the onset of age-related conditions such as Alzheimer's disease.
To investigate age-associated molecular changes in the human brain, Dr. Bruce A. Yankner, professor in the Department of Neurology and Division of Neuroscience at Children's Hospital Boston and Harvard Medical School, and colleagues examined patterns of gene expression in postmortem samples collected from thirty individuals ranging in age from 26 to 106 years. Using a sophisticated screening technique called transcriptional profiling that evaluates thousands of genes at a time, the researchers identified two groups of genes with significantly altered expression levels in the brains of older individuals. A gene's expression level is an indicator of whether or not the gene is functioning properly.
"We found that genes that play a role in learning and memory were among those most significantly reduced in the aging human cortex," said Yankner. "These include genes that are required for communication between neurons."
In addition to a reduction in genes important for cognitive function, there was an elevated expression of genes that are associated with stress and repair mechanisms and genes linked to inflammation and immune responses. This is evidence that pathological events may be occurring in the aging brain, possibly related to gene damage.
The researchers then went on to show that many of the genes with altered expression in the brain were badly damaged and could not function properly. They showed that these genes also could be selectively damaged in brain cells grown in the laboratory, thereby mimicking some of the changes of the aging brain.
"Our findings suggest that these genes are unusually vulnerable to damage from agents such as free radicals and toxins in the environment," said Yankner. "The brain's ability to cope with these toxic insults and repair these genes declines with age, leading to their reduced expression. It will now be important to learn how to prevent this damage, and to understand precisely how it impacts brain function in the elderly."
According to Yankner, "If you examine brain gene patterns among young adults, they are quite similar. In very old adults, there is some increased variability, but there is still similarity between individuals. In contrast, individuals in the middle age population between 40 and 70 years of age are much more variable. Some middle-aged individuals exhibit gene patterns that look more like the young group, whereas others show gene patterns that look more like the old group."
This is evidence that people may age differently during middle age. It will now be of great interest to understand what it is that makes some people age more rapidly than others.
These findings raise the exciting possibility that treatments or lifestyles that reduce gene damage in young adults may delay cognitive decline and the onset of brain diseases in later years. However, more research is needed.
"We can repair these aging genes in the laboratory, but that is a far cry from the human brain. This is only a first step," cautions Yankner.
Future directions of Dr. Yankner's lab include investigating whether these changes of normal aging are responsible for age-related conditions such as Alzheimer's and Parkinson's disease in some individuals. A major goal is the development of therapeutic approaches to preventing gene damage in the brain that could conceivably preserve cognitive function and prevent or delay the onset of neurodegenerative diseases. Along these lines, his lab is also interested in determining whether gene damage in the brain can be reversed after it occurs.