October 17, 2005
Discovery Provides New Clues about Causes of Rett Syndrome
Researchers studying the childhood neurological disorder Rett
syndrome have discovered a new clue about how the disorder can cause a
devastating range of symptoms. They found that MeCP2, the protein that
is altered in patients with the syndrome, plays a critical role in
snipping and rearranging messenger RNA molecules that carry the genetic
code for the construction of other proteins that are important for
brain function.
This newly discovered function of MeCP2 offers an additional hint at
why the protein is so crucial for development. The finding is helping
to round out the picture of MeCP2's complete range of activities, which
also include serving as a regulator to repress activity of target genes
in the brain.
“Once we discover the primary molecular changes that underlie the disorder, we can begin to explore pharmacologic targets to modulate those changes, to alleviate the symptoms of the disease.”
Huda Y. Zoghbi
The researchers said their discovery offers a promising pathway for
understanding the broad and variable symptoms of Rett syndrome, which
include language and growth retardation, breathing problems, seizures,
motor dysfunction, hand-wringing and social impairment. Furthermore,
the discovery may yield insight into a broader range of disorders,
including mental retardation and autism, which may also be linked to
abnormal MeCP2. Rett syndrome is an X-chromosome-linked disorder that
affects about 1 in 10,000 females and is the leading cause of mental
retardation in girls.
The researchers, led by Howard Hughes Medical Institute investigator
Huda Y. Zoghbi, published their findings in the online Early Edition of
the Proceedings of the National Academy of Sciences, posted the
week of October 17, 2005. Zoghbi and her colleagues are at the Baylor
College of Medicine.
In 1999, Zoghbi's research group identified MeCP2, or
methyl-CpG binding protein 2, as the causative gene in Rett
syndrome. Zoghbi's team subsequently created a mouse model of Rett
syndrome, in which an aberrant MeCP2 gene produces many of the
symptoms seen in humans with the disorder.
According to Zoghbi, MeCP2 was known to represses the transcription
of target genes by binding to them and preventing them from
transcribing their information into mRNA. However, few MeCP2 target
genes had been identified, she said.
“We decided to take a fresh approach to find out more about
what the protein implicated in this very complex disorder actually
does,” said Zoghbi. “Our hypothesis was that MeCP2 might
serve additional functions besides being a repressor.”
Thus, the researchers used a technique called co-immunoprecipitation
to identify proteins in the cell that interact with MeCP2. To ensure
that any proteins they found were universal MeCP2 partners, they
performed their experiments on different types of cells in culture,
including neuronal cells, and used different types of
“tags” for the MecP2 protein.
“Whichever way we did this identification, a protein called
YB-1 always came up as being associated with MeCP2,” said Zoghbi.
Since YB-1, or Y box-binding protein 1, is known to be involved in RNA
splicing, the researchers proceeded to explore whether MeCP2 regulates
splicing.
RNA splicing occurs when RNA copied from a DNA gene is snipped and
rearranged. A process called alternative splicing is critical to the
cell's ability to generate an assortment of proteins from the same
genes by snipping together different portions of the DNA, and is
especially important in development of the brain.
The researchers' experiments revealed that MeCP2 affects RNA
splicing through its interaction with YB-1. They also found that mutant
MeCP2 interacted less efficiently with YB-1 than did normal MeCP2. In
particular, the researchers found that MeCP2 affected splicing of the
RNA for a major neuronal receptor, called NR1.
“The finding with NR1 is important, because this receptor
seems to be modulated by activity, and we know that MeCP2 level
increases in neurons throughout childhood — implying a link to
neuronal activity,” said Zoghbi. Thus, she said, the gradual
onset of Rett syndrome symptoms in children, who begin developing
normally as infants, might be explained by the progressive pathology
caused by abnormalities in proteins such as NR1.
Using the mouse model of Rett syndrome, the researchers next
searched for splicing alterations in genes that were active in the
cerebral cortex. For this experiment, they used a microarray of
thousands of such genes supplied by co-author Jason Johnson of Merck.
The researchers found that splicing was altered in a significant number
of genes from the mutant mice as compared to normal mice.
“So, we now know that in this mouse model of Rett syndrome,
there is quite a bit of RNA splicing alteration in the brain,”
said Zoghbi. “The next big question is exactly how these splicing
alterations relate to changes in gene expression that might occur in
this syndrome. For this disorder, it gets us thinking that regulation
of gene transcription and splicing are perhaps coupled and functionally
integrated through MeCP2.”
Zoghbi said the next step is to understand how the multitude of
abnormalities in RNA splicing caused by mutant MECP2 contributes
to the complex, variable symptoms of Rett syndrome. “What is
important is to begin to tie particular molecular changes to a
particular clinical feature,” she said. “Once we discover
the primary molecular changes that underlie the disorder, we can begin
to explore pharmacologic targets to modulate those changes, to
alleviate the symptoms of the disease.”
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Jennifer Michalowski
