Cosmos Online, January 19, 2012
by Daniel Cossins
PHILADELPHIA: Differences in our genes contribute to how well we maintain our cognitive abilities over the course of our lives, according to a new study.
Comparing DNA from almost 2,000 people who were tested for intelligence as children and again in old age, researchers estimate that genetic factors account for around a quarter of the changes in cognitive performance that people experience over a lifetime. The research indicates that we should now search for the specific gene variations that govern this change if we want to understand why some people age better than others.
"For the first time we've been able to quantify how much genetic variants influence how people age cognitively," said Peter Visscher, a quantitative geneticist at the University of Queensland in Australia and a co-author of the study published in Nature today. "So now we can try to find specific genetic factors that influence this trait. That may tell us why some people decline cognitively and may give leads into a better understanding of the onset of dementia."
The search for influence
General cognitive ability (also known as general intelligence) - a measure of our ability to solve problems and perform everyday tasks - changes with age. Some people retain their cognitive abilities better than others. Since the amount and the rate of change in intelligence has such a large effect on mental health and well being in later life, researchers have long been interested in the causes, both genetic and environmental.
Up to now, no population-based genetic study of age-related intelligence had looked at people as children and followed them into old age. So there was no way to know how much genetic variation contributes to the fate of our cognitive abilities.
In the new study, however, Visscher and colleagues, including researchers from the Queensland Brain Institute, report the results of their analysis of data on a full lifetime of cognitive change in a group of Scottish people. They found that genetic factors account for about 24% of changes in intelligence between childhood and old age. In short, then, the study shows that there is a strong genetic basis for how we retain our cognitive abilities as we age - though it also indicates that the biggest influence is probably environmental.
Studying a lifetime of change
The researchers looked at data from three groups: people from the Aberdeen Birth Cohort (all born in 1936) and people from two separate Lothian birth cohorts (born in 1921 and 1936). In total 1,940 individuals who had been tested for general intelligence levels at the age of 11 were tracked down and re-tested in old age (age 65, 70 or 79). That provided the team with a unique dataset on the change or stability of cognitive abilities over a period of 54 years or more.
In an attempt to tease out variations in the genes of these people that correlate with variations in the fate of their cognitive abilities, the researchers analysed their DNA. They looked at 'genome-wide single nucleotide polymorphism (SNP)' data, which means they examined over half a million tiny variations in the DNA sequence. Comparing the two sets of data, they were able to calculate whether these genetic markers were associated with variations in cognitive ability.
"We could only do this because my colleagues did a fantastic detective job in finding a group of people for whom we had measures of intelligence over an entire lifetime," said Visscher. "We realised that we if we could take snapshots of the genomes we could compare the similarities and link that with the similarities in their intelligence. So we were able to tease out how much the change over time was due to genetic factors."
Time to track down the genes
The researchers are careful to point out that their estimate - that genetic factors account for roughly a quarter of changes in intelligence between childhood and old age - does not have conventional statistical significance because the dataset is too small. Nevertheless, the results are useful because such estimates have not previously been available. What's more, the results show that it is worthwhile to search for the specific gene variations that have an influence on how intelligence changes over time.
"I wouldn't take the exact figure as all that important," said David Goldstein, a geneticist at Duke University Medical Centre in Durham, North Carolina, who was not involved with the study. "It's a minimum estimate, so it may be an incomplete representation of the effect of the genetic variants that are there. What's more important is that this study has shown that there is a strong genetic contribution to the stability of cognitive performance over a lifetime. It tells us that there is a genetic trait there to be studied, so now we know it's worth trying to find the specific variants."
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http://www.cosmosmagazine.com/news/5191/cognitive-ageing-genes-partly
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