February 24, 2006

Kennedy Center talk probes brain’s growth

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Boston Children’s Hospital’s Christopher Walsh, M.D., Ph.D., was at the Kennedy Center to speak about brain development.
Photo by Dana Johnson

Kennedy Center talk probes brain’s growth

Mounting evidence suggests that genes responsible for devastating developmental brain defects and mental retardation also may have played a role in the explosive growth in brain size from non-human primates to humans.

As part of the Vanderbilt Kennedy Center Lectures on Development and Developmental Disabilities, Christopher Walsh, M.D., Ph.D., chief of Genetics at Boston Children's Hospital, recently discussed the research leading to this realization in his talk “Human Mental Retardation: Genes That Control the Shape and Size of the Human Brain.”

“There is a tremendous genetic diversity in the way the brain can be disrupted during development because… more genes are expressed in the brain than any other organ in the body,” Walsh said. “When it comes to the development of the brain, anything that can go wrong has gone wrong.”

By finding the genes that have 'gone wrong,' Walsh and colleagues are identifying those genes that direct the development of the cerebral cortex — the elaborately folded outer layer of brain cells responsible for higher cognitive function.

To identify these genes, Walsh and colleagues have focused much of their attention on special populations, particularly in the Middle East. Marriage between distant relatives (cousins) is common in the region and the rate of birth defects is slightly higher than in other populations. These factors make it easier to identify the responsible genes.

Through these studies, Walsh and colleagues have identified a number of genes underlying rare developmental brain defects. One gene, called ASPM, appears responsible for a type of microcephaly, a condition in which the overall size of the brain is severely reduced.

Further research showed that ASPM may have played a role in the evolutionary changes in brain size across species. The protein encoded by the ASPM gene contains short segments, coincidentally called 'IQ domains' — the letters 'I' and 'Q' representing the amino acid building blocks 'isoleucine' and 'glutamine,' respectively. Researchers have found that species differ in their number of IQ domains — humans have 71, while worms have only 2 — and this difference correlates with brain size.

The evolutionary implications of these findings were surprising, Walsh said.

“These genes not only make us who we are during development, but also changes in these genes between different species seem to relate to the differences in the ways our brain is shaped between humans and other animals.”