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Gene May Affect IQ in Males, Scientists Say

Posted on: Friday, 2 December 2005, 18:00 CST

DALLAS _ Scientists in North Carolina say they have identified a gene that affects IQ, a finding that, if confirmed, would be a significant step toward understanding the genetic basis for intelligence.

The new research could also have ethical implications because the effect of the gene appears to be quite dramatic: The scientists say that males who inherit a particular version of the gene have, on average, an IQ that is 20 points lower than males who don't.

"I have to admit, the ramifications of it are great," said Randy Jirtle, the Duke University biologist who led the new research, noting that current genetic testing techniques can easily determine which males have the gene version and which ones do not.

However, he stressed that the IQ results in his research were based on a group average; individual males carrying the gene version had a wide range of IQ scores. While females also can carry the gene variation, it does not appear to affect their IQ, he said.

Jirtle reported the new findings last month at a scientific conference in Durham, N.C.

As early as the 1920s, research suggested that genetics play a key role in determining a person's mental capabilities.

But so far, connections between IQ and specific genes have been just correlations, with little supporting evidence. The new research, Jirtle and other experts said, will need to be replicated before it is considered definitive.

Jirtle's research centers on a gene identified by the abbreviation IGF2R, for type 2 insulin-like growth factor receptor. The gene governs the production of a protein that, among other jobs, affects cell growth. All people carry the gene, but some have a version with a slightly different code, or variation, Jirtle said. This gene variation, he and his colleagues found, correlates with a lower IQ.

The researchers studied about 300 children with an average age of 10. The children, all Caucasian, came from six counties in the Cleveland area. As a group, males _ but not females _ who had the variant gene had IQ scores about 20 points lower than males who didn't.

Jirtle cautioned that inheriting the different version of the gene did not guarantee a lower IQ. Although as a group average IQ scores were lower, there were still males who had the variant gene and a higher IQ. (Two boys with the variant version of the gene had very high IQs of 160, Jirtle said.) And males with the more common form of the gene can also have a lower IQ.

Jirtle said his assertion that the IGF2R gene affects IQ is bolstered by experiments in mice. When he and his colleagues disabled a copy of the gene in lab mice _ an experiment intended to mimic humans who inherit the variant copy of the gene _ they noticed that the male mice were slow learners on a maze test. Electrical recordings of the mice's brain tissue were also altered in a way that is consistent with slow learning.

Also, Jirtle said, what scientists already know about the protein produced from the IGF2R gene fits with a role in brain function. Research has suggested the protein regulates cell growth as well as the speed at which signals travel between nerve cells.

In 1998, scientists from England reported a connection between a portion of the IGF2R gene and IQ, but later retracted their work when they couldn't replicate the results. Jirtle's research concerns a different, but nearby, area of the same gene.

The new research has not been published in a journal, so it is not yet widely known in the scientific community. And many researchers question the usefulness of IQ as a measure of mental ability, saying it's a broad measurement that encompasses many brain functions.

"I think they are probably looking at something much smaller than IQ, like reaction time or working memory," said Elena Grigorenko, a geneticist and psychologist at Yale University School of Medicine in New Haven.

On learning of the results, Grigorenko said the study would need to be replicated with another set of research subjects. Assuming the results are confirmed, she said, it would be interesting to find out what effect the gene variation has on the brain.

The new research is likely to spark interest among parents and those considering having children, said Arthur Caplan, a bioethicist at the University of Pennsylvania in Philadelphia. He said he could imagine people wanting to test newborns _ or even embryos created through in vitro fertilization _ for the gene.

"Parents are very concerned about intelligence in their kids," Caplan said. "I could imagine people saying we want to sort out our embryos to get the smartest kids possible."

Currently, some genetic testing is done with in vitro fertilization or in higher-risk pregnancies, but it's done to avoid genetic disease. Using genetic testing to assess positive traits, such as higher IQ, takes ethical discussions to a new level, he said.

If the new results connecting IQ with the IGF2R gene are confirmed, Caplan said it will be important to remember that IQ tests measure only certain types of mental abilities, including analytical thinking and mathematical ability. The tests don't capture emotional intelligence, social skills, motivation or artistic and musical talent.

Also, Jirtle said, the version of the IGF2R gene linked to lower IQ probably does some good too, because it is present at fairly high levels in the population.

Jirtle predicts that about 25 percent of Caucasians carry the variant gene. An international database that catalogs genetic variation states that the variant gene was found in 28 percent of Japanese living in Tokyo and 24 percent of Han Chinese living in Beijing. The variant gene was not found in a population of Yoruba people living in Ibadan, Nigeria.

It's still unclear why the gene has an effect on IQ only in males, Jirtle said. The variant gene may confer some health advantage, he said, such as resistance to certain diseases.

"There must be an advantage," Jirtle said. "It was selected for something, but I can't believe it was to make males dumber."

___

(c) 2005, The Dallas Morning News.

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Source: The Dallas Morning News

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