Pennsylvania University. US: Tens of millions of people around the world have abnormal, leak-prone
sproutings of blood vessels in the brain called cerebral cavernous
malformations (CCMs). These abnormal growths can lead to seizures,
strokes, hemorrhages, and other serious conditions, yet their precise
molecular cause has never been determined.
Now, cardiovascular
scientists at the Perelman School of Medicine at the University of Pennsylvania
have studied this pathway in heart development to discover an
important set of molecular signals, triggered by CCM-linked gene
defects, that potentially could be targeted to treat the disorder.
“We hope that these findings will lead to a better understanding of
the origins of CCM, and thus to treatment possibilities,” says Mark L. Kahn, MD, a professor of Cardiovascular Medicine, and senior author of the new study, published in Developmental Cell.
Although CCM has a relatively high prevalence of 1 in 200 people
worldwide, it typically goes undiagnosed until symptoms arise and can
only be treated by brain surgery.
Research on CCM has been slowed by the difficulty of recreating
the disease in lab animals. About 20 percent of CCM patients have a
highly aggressive, inherited form of the disorder that is caused by
inactivating one of three genes, whose protein products normally work
together in a complex. But knockout mice bred without a full set of
those genes don’t mature to have CCMs in their brains—they die in the
womb, having failed to develop a working vascular system.
“Those animals die so early in their development that you just
don’t get enough information about what the genes normally should be
doing,” Kahn says.
Studies by Kahn’s lab and others have shown that CCM gene
knockouts remain lethal to fetal mice even when they are limited to the
endothelial cells that line blood vessels and the heart.
In the new study, Kahn and colleagues used advanced techniques to
restrict CCM gene disruption to the endothelial cells of the
developing heart, leaving the mouse vascular system to develop
otherwise normally.
The resulting mice still died before birth, this time from a
failure of normal heart development, which is not seen in human CCM
patients. But they survived in the womb about a week longer than
standard CCM knockout mice. That allowed Kahn’s team to learn more
about the effects of the gene disruptions, and ultimately to find a
previously unknown CCM-related signaling pathway.
An initial clue was that the mice developed an abnormally thin
layer of heart muscle. A substance known as “cardiac jelly” that should
separate the developing heart muscle from the heart endothelium in
healthy mice was severely reduced. Analyses of gene expression changes
revealed that the CCM-disrupted heart endothelial cells were
overproducing protease enzymes that degrade cardiac jelly. Suppressing
the activity of the protease genes largely prevented the heart defects.
Kahn’s team traced the triggers of the protease gene overactivity
back to a signaling protein called MEKK3, which helps drive cell growth
and survival and has been implicated in cancers. Reducing MEKK3
activity shut down the jelly-degrading protease genes and mostly
prevented the heart defects. Experiments also showed that MEKK3 bound
to the CCM-complex proteins, whose absence causes familial CCM disease.
More than a decade ago, other researchers noted that MEKK3 somehow
associated with one of the CCM-complex proteins. “That observation
didn’t really lead anywhere because at the time our understanding of
the CCM pathway was minimal,” says Kahn. Now it appears that, at least
in the endothelial cells of the developing heart, CCM proteins normally
bind MEKK3, and when they are absent, MEKK3 becomes abnormally—and
harmfully—active.
A key question now is whether overactive MEKK3 also contributes to
the brain vascular malformations of CCM disease. The Kahn lab is
addressing that question by studying MEKK3 activity in a recently
developed CCM mouse model. In this animal model, the deletion of CCM
genes in endothelial cells shortly after birth produces vascular
lesions in the brain that closely resemble those of human CCMs.
“If we can show that MEKK3 signaling leads to CCMs in that model,
then it will be time to think about what the best therapeutic strategy
will be,” he said.
The co-lead authors on the paper, graduate student Zinan Zhou and
medical student David Rawnsley PhD, performed most of the experiments
in the study. The co-senior author was Xiangjian Zheng, a postdoctoral
researcher in the Kahn Laboratory during much the study, who recently
began his own laboratory at the Centenary Institute in Sydney,
Australia.
Funding was provided by the National Heart, Lung and Blood
Institute (R01HL094326 , R01HL102138, R01NS075168, T32HL007971), and
the American Heart Association (11SDG7430025).