Two genes that influence brain size, and accordingly intelligence and possibly susceptibility to Alzheimer's disease, have been discovered by scientists.
The first of these, HGMA2, affects the overall size of the brain and links to intelligence. People with a small change in this gene had larger brains and performed slightly better on IQ (intelligence quotient) tests in studies. The other gene, TESC, is linked to the size of a brain region called the hippocampus, which is involved in memory and is often smaller in Alzheimer's patients.
DNA - and therefore individual genes - is made up of four chemicals called bases. People whose HMGA2 gene contained a cytosine base (referred to by the letter 'C') a instead of thymine (or 'T') at a specific location, had larger brains. The effect of this variation on intelligence was small but significant - on average a 1.3-point increase in IQ test scores.
'A single letter change leads to a bigger brain', confirmed Professor Paul Thompson, of the University of California, the lead researcher on the study.
Despite the minimal effect on IQ, Professor Thompson says that his team 'found fairly unequivocal proof supporting a genetic link to brain function and intelligence. For the first time, we have watertight evidence of how these genes affect the brain. This supplies us with new leads on how to mediate their impact'.
The researchers also found that people with a genetic variant on the TESC gene had shrinkage in the hippocampus equivalent to almost five years ageing. The brain naturally shrinks with age but this variant sped up the process and this could make people more vulnerable to developing Alzheimer's.
The research is the result of an international collaboration of over 200 scientists to map genes in the brain that increase the risk of developing brain disorders and mental illness. Brain images from over 20,000 healthy people were analysed to measure the size of the whole brain and its memory centres while at the same time screening for variations in the DNA.
'Our individual centres couldn't review enough brain scans to obtain definitive results', Professor Thompson said, 'by sharing our data [...] we created a sample large enough to reveal clear patterns in genetic variation and show how these changes physically alter the brain'.
Discussing the HGMA2 findings, Dr Tom Hartley from the University of York in the UK, who was not involved in the study, told AFP that he was 'a little wary of thinking in terms of a gene for intelligence. There are undoubtedly a lot of things that have to work properly in order to get a good score on an IQ test, if any of these go wrong the score will be worse'.
And Professor Thompson also warned against strict genetic determinism. He told AFP: 'If people wanted to change their genetic destiny they could either increase their exercise or improve their diet and education. Most other ways we know of improving brain function more than outweigh this gene'.
The study is published in Nature Genetics.