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Chunk of protein explains our big brains, study proposes

Aug. 18, 2012
Courtesy of the University of Colorado Denver
and World Science staff

Re­search­ers have found what they think is the key to un­der­stand­ing why the hu­man brain is larg­er and more com­plex than that of oth­er an­i­mals.

The hu­man brain, with its un­equaled cog­ni­tive ca­pa­city, has evolved rap­idly and dra­mat­ic­ally. “We wanted to know why,” said James Sike­la of the Uni­vers­ity of Col­o­rad­o School of Med­i­cine, who head­ed the team of sci­en­tists in the new stu­dy. “The size and cog­ni­tive ca­pa­city of the hu­man brain sets us apart. But how did that hap­pen?”

“This re­search in­di­cates that what drove the ev­o­lu­tion­ary ex­pan­sion of the hu­man brain may well be a spe­cif­ic un­it with­in a pro­tein — called a pro­tein do­main — that is far more nu­mer­ous in hu­mans than oth­er spe­cies.”

A pro­tein do­main is a part of a pro­tein that can re­ap­pear in iden­ti­cal form in many dif­fer­ent pro­teins. Pro­teins are the mo­le­cules that do much of the work that keep us func­tion­ing, and they are pro­duced in the body us­ing a blue­print set by genes.

The pro­tein do­main in ques­tion is called DUF1220. Hu­mans have more than 270 cop­ies of DUF1220 en­cod­ed in the ge­nome, far more than oth­er spe­cies. The clos­er a spe­cies is to hu­mans, the more cop­ies of DUF1220 show up. Chimps have the next high­est num­ber, 125. Go­ril­las have 99, mar­mosets 30 and mice just one. 

“The one over-rid­ing theme that we saw re­peat­edly was that the more cop­ies of DUF1220 in the ge­nome, the big­ger the brain. And this held true wheth­er we looked at dif­fer­ent spe­cies or with­in the hu­man popula­t­ion,” Sikela said.

Sikela and his team al­so linked DUF1220 to brain dis­or­ders. They as­so­ci­at­ed low­er num­bers of DUF1220 with mi­cro­ceph­a­ly, in which the brain is too small; larg­er num­bers of the pro­tein do­main were as­so­ci­at­ed with mac­ro­ceph­a­ly, in which the brain is too large.

The find­ings were re­ported Aug. 16 in the on­line edi­tion of The Amer­i­can Jour­nal of Hu­man Ge­net­ics. The re­search­ers drew their con­clu­sions by com­par­ing ge­nome se­quences from hu­mans and oth­er an­i­mals as well as check­ing the DNA of peo­ple with mi­cro­cephaly and macro­cephaly and of a healthy group.

“The take-home mes­sage was that brain size may be to a large de­gree a mat­ter of pro­tein do­main dosage,” Sikela said. “This dis­cov­ery opens many new doors. It pro­vides new tools to di­ag­nose dis­eases re­lat­ed to brain size. And more broad­ly, it points to a new way to study the hu­man brain and its dra­mat­ic in­crease in size and abil­ity over what, in ev­o­lu­tion­ary terms, is a short amount of time.”


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Researchers have found what they believe is the key to understanding why the human brain is larger and more complex than that of other animals. The human brain, with its unequaled cognitive capacity, has evolved rapidly and dramatically. “We wanted to know why,” said James Sikelao of the University of Colorado School of Medicine who headed the team of scientists in the new study. “The size and cognitive capacity of the human brain sets us apart. But how did that happen?” “This research indicates that what drove the evolutionary expansion of the human brain may well be a specific unit within a protein — called a protein domain — that is far more numerous in humans than other species.” A protein domain is a part of a protein that can reappear in identical form in many different proteins. Proteins are the molecules that do much of the work that keep us functioning, and they are produced in the body using a blueprint set by genes. The protein domain in question is called DUF1220. Humans have more than 270 copies of DUF1220 encoded in the genome, far more than other species. The closer a species is to humans, the more copies of DUF1220 show up. Chimps have the next highest number, 125. Gorillas have 99, marmosets 30 and mice just one. “The one over-riding theme that we saw repeatedly was that the more copies of DUF1220 in the genome, the bigger the brain. And this held true whether we looked at different species or within the human population,” Sikela said. Sikela and his team also linked DUF1220 to brain disorders. They associated lower numbers of DUF1220 with microcephaly, when the brain is too small; larger numbers of the protein domain were associated with macrocephaly, when the brain is too large. The findings were reported Aug. 16 in the online edition of The American Journal of Human Genetics. The researchers drew their conclusions by comparing genome sequences from humans and other animals as well as checking the DNA of people with microcephaly and macrocephaly and of a healthy group. “The take-home message was that brain size may be to a large degree a matter of protein domain dosage,” Sikela said. “This discovery opens many new doors. It provides new tools to diagnose diseases related to brain size. And more broadly, it points to a new way to study the human brain and its dramatic increase in size and ability over what, in evolutionary terms, is a short amount of time.”