September 30, 1999
Rett Syndrome Traced to Defective Gene "Silencer"
A 14-year search for the cause of a rare genetic disease that
strikes young girls has uncovered the first example of a human disease
that is linked to a defective gene silencing mechanism.
Rett syndrome (RTT) is a neurodevelopmental disorder seen in young
girls that causes a sudden and permanent decline in mental
capabilities. In 1985, HHMI investigator Huda
Zoghbi, who was then a neurology fellow, published a clinical
research report on Rett syndrome. Her initial encounter with the
disorder had a lasting impact on her career. Shortly after seeing her
first RTT patients, Zoghbi decided to change her career plans,
switching from clinical medicine to a research position. Shortly after
switching to research, she started the long, tedious search for the
genetic causes of RTT.
“Finding that gene is the hardest thing I've ever worked on.”
Huda Y. Zoghbi
"Finding that gene is the hardest thing I've ever worked on," Zoghbi
said in an interview. "It's the best case for the rewards of
perseverance that I can think of."
The gene, called MECP2, resides on the far end of the longest
arm of the X chromosome. MECP2 encodes a protein named
methyl-CpG-binding protein 2, or MeCP2, that binds to genes that a cell
has methylated. During development certain genes that are to be
silenced, or prevented from being expressed, are physically marked by
the addition of methyl groups. When bound to a methylated gene, MeCP2
facilitates gene silencing by attracting other proteins that prevent
access to the cell's DNA transcription machinery.
Previous work by other researchers had shown that inactivating the
mouse version of Mecp2 caused a lethal defect in male mouse
embryos. Zoghbi's group at Baylor College of Medicine, in collaboration
with researchers led by Uta Francke, an HHMI investigator at Stanford
University School of Medicine, found several mutations in this gene in
patients with Rett syndrome. The researchers report their discovery in
the October 1999 issue of the journal Nature Genetics.
Girls with RTT are born without any sign of the disease and appear
to develop normally for the first six to 18 months of life. Then they
begin to regress, sometimes abruptly, losing their burgeoning skills in
muscle control and communication. Eventually the disease stabilizes and
the girls grow up with extensive mental retardation and a propensity to
sit and wring their hands.
"I saw my first RTT patient as a pediatric neurology fellow in
October 1983, though I didn't yet know what it was," said Zoghbi. "I
was amazed by two things. First, her history of normal development
followed by a period of regression. Second, I can't think of another
neurological disease in which patients will sit there and endlessly
wring their hands."
A week later, Zoghbi was scheduled to see a young girl who had been
diagnosed with cerebral palsy. This patient too was wringing her hands.
"I thought that since I saw two in one week, there must be more,"
Zoghbi said. Examination of patient records revealed six more girls
with similar signs: hand-wringing, ataxia and spasticity.
RTT has been her biggest challenge, Zoghbi said, largely because
only one percent of recorded cases are inherited. Researchers have a
much easier time homing in on a defective gene when they are working
with large families with numerous affected members. Francke, who has
also been searching for the RTT gene independent of Zoghbi's efforts,
agrees. "We couldn't use any of the standard techniques available for
finding such genes. We basically had to pick candidate genes and see if
they were the gene involved in Rett syndrome," she explained.
While 99 percent of RTT cases occur randomly in the population, both
Zoghbi and Francke – who began collaborating instead of competing
five years ago – had a logical starting hypothesis: If the
syndrome strikes only girls, whose sex-determining chromosomes are XX,
and not boys, whose sex is determined by the X and Y chromosomes, then
the defect must reside in a gene carried on the X chromosome.
Furthermore, the two investigators reasoned that the gene was both
dominant – meaning one mutant copy of the gene could trigger the
disease – and lethal.
Such properties would explain why only girls develop RTT. A female
fetus could survive the effects of the otherwise lethal mutation
because it has two X chromosomes — one that harbors a normal copy of
the gene and the other a defective copy. The normal gene can compensate
partially for the defective gene, allowing the fetus to survive. In a
male fetus, however, there would be no opportunity to inherit a
compensating, functional copy of the gene since the male fetus has but
one X chromosome. As a result, development is so out of kilter that the
male fetus dies either before or shortly after birth.
Zoghbi and Francke, along with members of their laboratories,
narrowed the search for the RTT gene by analyzing shared and unshared
DNA sequences in a small number of Rett families. By 1998, other
scientists helped to further narrow the search to about 200 candidate
genes. Postdoctoral fellows Ruthie Amir of Zoghbi's laboratory and Mimi
Wan of Francke's laboratory split up the work of analyzing those 200
genes.
A quarter of the way through a set of 50 candidate genes, Igna Van
den Veyver, a member of Zoghbi's lab, suggested to Amir that
MECP2 would be an excellent candidate gene based on some
unexpected experimental results from an unrelated project. Sure enough,
Amir found mutations in MECP2 that were similar in about 30
percent of their RTT patients.
"This was an absolute surprise, the last gene I would have ever
expected to be involved in Rett syndrome," said Francke. "The fact that
knocking out the gene in mice led to developmental arrest and death of
the fetus seemed to rule it out as a candidate for RTT. We also thought
that MeCP2 may be involved in regulating a great many genes in
different tissues and therefore would be unlikely to be involved in a
disease that primarily affected the nervous system."
Zoghbi said that it is not yet clear exactly how MECP2
defects cause RTT. One hypothesis, she said, is that the defective
MeCP2 protein allows genes to remain active that should have been
silenced at points along the precisely timed process of nervous system
development.
"The exciting part of this discovery is not just what it may mean
for RTT patients," she explained. "Now we know about a whole genetic
pathway involved in neural development, and that human disease can
result from its destruction. Many other disorders could be caused by
defects in other components in this pathway—perhaps autism, for
example."
Though the scientists have identified a fraction of the mutations
affecting their RTT patients, Zoghbi believes that they will find the
other mutations in the DNA sequences of the MECP2 gene that have
yet to be analyzed. And while the discovery of the mutations may
someday suggest possible therapies for this tragic disorder, the more
immediate result should be a test for the early diagnosis and prenatal
detection of RTT in the rare families where the mutation is
inherited.
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