Brain size and IQ secret may lie in genes: Study

By Dr. Ananya Mandal, MDApr 18 2012

Researchers, in one of the world’s largest collaborative studies, have uncovered specific genes that are linked to brain size and intelligence.

The study, conducted by a team of more than 200 scientists from 100 institutions worldwide, measured the size of the brain and its memory centers in thousands of MRI images from 21,151 healthy people while simultaneously screening their DNA. According to the researchers, a variant in a gene called HMGA2 affected the brain size, as well as a person's intelligence.

People whose HMGA2 gene held a letter "C" instead of a "T" at a specific location on the gene possessed larger brains and scored more highly on standardized IQ tests, noted the researchers. Every gene contains a unique sequence of four bases namely, adenine (A), cytosine (C), guanine (G) and thymine (T).

Since reduced brain size is a biological marker for disorders like schizophrenia, bipolar disorder, depression, Alzheimer's disease and dementia, if we identify the gene variants that deplete brain tissue beyond normal in a healthy person, it can be targeted with a drug to reduce the risk of those diseases, said the researchers.

Commenting on the study findings, lead researcher Paul Thompson, a neurologist at the University of California, Los Angeles, School of Medicine, said, “This is a really exciting discovery, that a single letter change leads to a bigger brain. For the first time, we have watertight evidence of how these genes affect the brain.” The study was published April 15 in the advance online edition of the journal Nature Genetics.

Thompson dubbed it “an intelligence gene” and said it was likely that many more such genes were yet to be discovered. “It is a strange result, you wouldn't think that something as simple as one small change in the genetic code could explain differences in intelligence worldwide,” said Thompson.

People who received two Cs from their parents, a quarter of the population, scored on average 1.3 points higher than the next group -- half of the population with only one C in this section of the gene. The last quarter of people, with no Cs, scored another 1.3 points lower. “The effect is small,” said Thompson, but “would be noticeable on a (IQ) test ... (it) may mean you get a couple more questions correct. It wouldn't be an enormous change. Even so, it would help our brain resist cognitive decline later in life.”

It is generally accepted that genes, a good education and environmental factors combine to determine our intelligence. “If people wanted to change their genetic destiny they could either increase their exercise or improve their diet and education,” said Thompson. “Most other ways we know of improving brain function more than outweigh this gene.” He added there were ethical safeguards and laws in place to guard against the abuse of genetic information.

Asked to comment on the research, Tom Hartley, a psychologist at Britain's University of York said 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,” he said. But he said it was “fascinating” to find that such small genetic changes could affect the size of critical structures such as the hippocampus, the brain's memory centre. “Given the importance of the hippocampus in disorders such as Alzheimer's disease these could turn out to be very significant findings,” said Hartley.

John Williams, head of neuroscience and mental health at the Wellcome Trust, said the findings paved the way for further research into “structural changes” which occur in disorders such as dementia, autism and schizophrenia.

“It's important they've found this gene, but it took a sample of 20,000 people to find it, precisely because the effect is so small,” says Robert Plomin at the Institute of Psychiatry in London, and lead author of a ground-breaking study in 2007 which failed to find any single genes of disproportionate importance in intelligence. “If it's this hard to find an effect of just 1 per cent, what you're really showing is that the 'cup' is 99-per-cent empty,” he says.

Steven Pinker, an author and professor of neuropsychology at Harvard University, agrees. “It's an important finding, assuming it holds up,” he says. Pinker says that the findings are a first step in demonstrating that intelligence relies on large numbers of genes, each with a tiny effect, rather than on single genes that have moderate or large effects, but which are so rare that none has yet been identified.