The researchers found that normal brain development requires 14-3-3 epsilon, a member of a protein family universally found in everything from yeast to mammals. In studies with humans and mice, the researchers determined that death or severe brain abnormalities occurred when the protein was defective or missing.
"This is the first demonstration in mammals that 14-3-3 epsilon is essential for brain development," said the paper's senior author, Anthony Wynshaw-Boris, M.D., Ph.D., UCSD associate professor of pediatrics and medicine. "We also showed that the gene is always deleted on one chromosome in patients with a rare, but severe brain disorder called Miller-Dieker syndrome (MDS), a form of lissencephaly, which means "smooth brain".
In normal brain development, neurons migrate to various areas of the brain. Lissencephaly is a severe developmental defect of the brain caused when neurons born deep within the brain are unable to migrate normally to areas such as the hippocampus and cortex. In addition to smoothness of the brain surface, there is a thickening of the cortex, with four rather than six layers. Children with MDS, in addition to a severely damaged brain from lissencephaly, have characteristic facial features, such as a prominent forehead, short nose, and malformed ears and eyes. Patients with MDS have the severest form of a lissencephaly, which renders individuals with profound mental retardation, increasingly severe epilepsy and early death.
LIS1 (which stands for Lissencephaly-1) is a gene that is deleted or mutated on a chromosomal region of chromosome 17 (called 17p13.3) in many children with lissencephaly. LIS1 resides fairly close to the 14-3-3 epsilon gene on this chromosome. Patients with lissencephaly only have deletions of one copy of LIS1, but never 14-3-3 epsilon, while patients with MDS have deletions of both genes on the same chromosome.
One of the genes that directly interacts with LIS1 and is involved in that migration, called NUDEL, was discovered and named by the Wynshaw-Boris team, which published their findings December 20, 2000 in the journal Neuron. In new studies with mice lacking 14-3-3 epsilon, the Wynshaw-Boris teams determined that 14-3-3 epsilon binds to NUDEL, which may explain its essential role in neuronal migration.
Wynshaw-Boris explained that NUDEL becomes phosphorylated (combines with phosphoric acid or a phosphorus-containing group) by interaction with another molecular compound called Cdk5/p35. By binding to the phosphorylated form of NUDEL, 14-3-3 epsilon protects NUDEL from losing its phosphorylation through interactions with other molecules, thus stabilizing it for its role in neuronal migration.
"Not only does out study determine a novel function for 14-3-3 epsilon, but it also provides a molecular explanation for why MDS patients have more severe lissencephaly," Wynshaw-Boris said.
The study was supported by grants from the National Institutes of Neurological Diseases and Stroke, an institutional grant from the Howard Hughes Medical Institute, and UCSD School of Medicine funds.
Additional authors were first author Kazuhito Toyo-oka, Ph.D., UCSD Departments of Pediatrics, Medicine and the UCSD Cancer Center; Aki Shionoya and Shinji Hirotsune, M.D., Ph.D. from Saitama Medical School PRESTO, and Japan Science and Technology Corporation; Michael J. Gambello, M.D., Ph.D., UCSD and the University of Texas Health Science Center, Houston; Carolos Cardoso, Ph.D., Richard Leventer, M.D., Heather L. Ward, William Dobyns, M.D., David Ledbetter, Ph.D., all from the University of Chicago; and Ramses Ayala and Li-Huei Tsai, Ph.D., Howard Hughes Medical Institute and Harvard Medical School.
Journal
Nature Genetics