Sizing up the brain: mutations that produce small brains may reveal how human intelligence evolved

Science News, Nov 16, 2002 by John Travis

In the 1960s, Pakistan built a mammoth dam on the river Jhelum to generate electric power and store water for irrigation. Known as Mangla, the dam created an upstream lake that displaced about 20,000 families from the district of Mirpur. Around the same time, England's textile industry was facing a major shortage of skilled laborers, especially in the county of Yorkshire. Many of the people from Mirpur who were displaced by Mangla traveled to Bradford and other Yorkshire districts.

The coincidental timing of the dam's construction and Yorkshire's need for workers has, nearly 4 decades later, provided scientists with insight into how the human brain develops and, possibly, into how it evolved from the smaller brains of our hominid ancestors. A few years ago, a physician from St. James' University Hospital in Leeds, England, noticed something unusual among the Pakistani families he examined at a Bradford clinic. "I was seeing a lot of children who had microcephaly with moderate mental retardation but no other disease features," recalls clinical geneticist C. Geoffrey Woods.

Microcephaly is a rare condition characterized by an abnormally small head, the result of an undersized brain. In particular, the cerebral cortex--the layers of nerve cells that cover the brain's surface and are the seat of higher reasoning--is shrunken. "The cerebral cortex is the part of the brain that, for better or worse, makes us human," notes Christopher A. Walsh, a Howard Hughes Medical Institute (HHMI) investigator at Harvard Medical School in Boston. "Children who have abnormal development of the cerebral cortex fail to achieve the kind of talents we pride ourselves on, such as language."

Intrigued by his patients with microcephaly, Woods began studying the DNA of Pakistanis with the condition and their unaffected relatives. He and his colleagues gradually realized that there isn't just one gene that causes microcephaly when mutated; there are at least six genes. This year, the researchers identified two of the genes, including one responsible for microcephaly in about half of the nearly 60 Pakistani families studied to date.

These two genes, as well as another one studied by Walsh's group (see box, page 314), are shedding light on how the cerebral cortex forms. Moreover, by pinpointing genes that seem to regulate the size of the cerebral cortex, the scientists have set the stage for studies into what genetic changes produced the rapid expansion of the cerebral cortex as primates, including humans, evolved. Indeed, one of the human-microcephaly genes is dramatically different from its counterparts in much simpler animals such as worms and mice.

"I suspect there may be many things that contribute to the increased size of the primate cortex, but the microcephaly genes are a fascinating beginning," says Martin Raff of University College London, who studies regulation of nerve cell division.

BRAIN'S BLACK BOX Containing billions of nerve cells, the cerebral cortex is the largest structure of the human brain. Essentially a flat sheet not much thicker than an orange peel, the cortex folds and refolds into the familiar deep creases of the brain's surface. The cerebral cortex varies in size dramatically among species. It "mostly grows by becoming a larger sheet rather than a thicker sheet," says Walsh.

The human cortical surface area is about 1,000 times greater than that of the mouse, for example. And compared with the cortex of the chimpanzee, our closest living relative, the human cerebral cortex has three to four times more surface area. Some scientists attribute the greater intelligence of modern humans to the rapid expansion of the cerebral cortex as hominids evolved.

In microcephaly, the cerebral cortex grows unusually slowly and reaches a size no bigger than that of early hominids. Various circumstances, such as prenatal infections or a mother's alcohol abuse, can produce microcephaly, but they usually also generate other physical abnormalities. In so-called primary microcephaly, the small brain and head are the only obvious physical defects. The brain's basic architecture is preserved, albeit in a smaller form. In such cases, the child or adult is mentally retarded but has no other apparent neurological problems, such as seizures.

Primary microcephaly typically occurs when a mother and a father each pass on a mutated copy of a gene that controls brain size. Since among the families that migrated to England from Mirpur, cousins often married each other, so the chances that a baby would have two mutated copies of a gene increased. That explains the abundance of microcephaly cases that Woods saw.

In work led by Andrew P. Jackson of St. James' University Hospital in Leeds, the investigators tracked down one microcephaly gene by focusing on two Pakistani families that had an unusually large number, seven in total, of members with microcephaly. In the July American Journal of Human Genetics, Jackson, Woods, and their colleagues report finding mutations in a novel gene in affected family members but not in unaffected ones. They named the gene microcephalin and, by studying human and mouse fetal tissue, showed that the gene is active in the cerebral cortex as it develops before birth.