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Epidemiology
AAA is most
common in Caucasians, with men affected four times more than women. AAA is
most prevalent between the ages of sixty and seventy. First-degree relatives of
persons with AAA develop dilation at earlier ages. If AAA is untreated its
natural course is to expand and rupture. Surgery is recommended if
patients’ aneurysm is greater than five cm. There is a twenty percent chance of
rupture within a year, for an aneurysm that is greater than six cm.
Our Clinical Research
In 1997, our work published in Circulation
was the first to link larger, more explosion-prone aneurysms to high levels of
an enzyme, (MMP-9, released by inflammatory cells) that digests the connective
tissue that helps hold arteries together. A better understanding of the
biochemistry behind aneurysm growth could potentially pave the way for drugs to
slow the swelling, head off bursting, or even shrink an aneurysm.
We previously performed plasma estimations
implicating metalloproteinase (MMP-9) enzymes as possibly contributing to
aneurysm development. MMP-9 and its relatives (MMP-2, etc.), for example, tear
apart elastin, which provides strength and elasticity to arterial walls and may
cause the initial swelling of an aneurysm. To see whether direct evidence
existed for MMP-9's involvement, we collected aortic tissue from 19 patients
whose aneurysms were surgically repaired.
Estimations of the amount of MMP-9 present in the
arteries were performed by using competitive polymerase chain reaction to gauge
the number of copies of MMP-9 messenger RNA (mRNA) in each tissue sample. We
also determined the size of each patient's aneurysm from computed tomography
scans taken before the surgeries, since size is a measure of the aneurysm's
severity. The likelihood that an aneurysm will burst increases exponentially
once it reaches 5 centimeters. Controls were aortic tissue samples taken from
four organ donors without aneurysms.
The findings showed that MMP-9 expression correlated
with aneurysm size, up to a point. The smallest aneurysms (less than 5
centimeters across) expressed nearly six times as much of the enzyme as normal
tissue. Medium-sized aneurysms (5 to 6.9 centimeters) expressed five times as
much MMP-9 as the small aneurysms. However, MMP-9 expression levels in aneurysms
7 centimeters and larger were 70% lower than in medium-sized aneurysms. We
theorized, that by the time aneurysms get that large, the connective tissue is
already seriously degraded and MMP-9 production from the inflammatory cells has
come to a halt. Currently we are
continuing plasma collections from our large population of aneurysm patients to
assess if this relationship between aortic size and MMP-9 levels persists.
We also would like to determine if these levels might provide a manner to
gauge the effectiveness of minimally invasive aneurysm repair.
Our
Basic Research
Our principal goal is to understand the molecular events
leading to the formation of aneurysm. MMP-9 has been implicated in aneurysm
pathogenesis, presumably by the gradual thinning of the blood vessel, through the
degradation of extracellular matrix. Although
the upstream trigger of increased MMP-9 activity is yet to be rigorously
identified, a more immediate approach would be to confirm the requirement of
this metalloprotease in aneurysm formation. We therefore utilize various animal
models to study the hypothesis that elevated MMP-9 activity directly
leads to aneurysm formation.
Since the smooth muscle layer is an important underlying
component of all major arteries, we have generated transgenic mice in which
MMP-9 over-expression is driven by a smooth muscle-specific promoter. This
powerful experimental tool thus provides a means to not only decipher the
pathophysiological effects of MMP-9 overexpression, but more importantly allows us
to dissect the molecular interplay between MMP-9 and other key players
in aneurysm such as TIMPs, LDL, and ApoE.
Aneurysm can be artificially induced by aortic elastase perfusion, as outlined
in the diagram below. After
intraperitoneal anesthetic injection, mice
undergo
midline laparotomy and isolation and control of the infrarenal aorta.
A tapered PE-10 polyethelene catheter is inserted into the aorta and
0.049 ml of enzyme (elastase or MMP-9) is perfused into the aorta over 5
minutes. The catheter is then
removed,the aortotomy repaired and arterial clamps removed restoring aortic
blood flow. The abdomen is then
closed and the mice are allowed to recover and sacrificed 14 days later after
measurement of the infrarenal aorta. Measurement
of preperfusion, postperfusion, and final aortic diameter are recorded for each
animal. In our hands, elastase
perfusion results in > 100% dilation (our experimental definition of AAA) in
aortas who had a postperfusion diameter of at least 50%. Our reproducibility has
been very good with this model with nearly 100% of animals developing AAA if the
proper postperfusion diameter is obtained.
Confirmatory evidence further implicating MMP-9 as a key factor in
AAA formation has been shown in our latest study. No AAA development can
be induced by aortic elastase perfusion in MMP-9 knock-out mice, clearly
defining the requisite of MMP-9 in AAA pathogenesis. In addition,
infusion of rMMP-9 protein resulted in AAA formation in MMP-9 knockout
mice. To further confirm that our observation is the direct results of
MMP-9 activity, aortic infusion of denatured rMMP-9 failed to induce AAA
in these k/o animals. These xperiments lend convincing evidence that
MMP-9 induced extracellular matrix degradation resulting in AAA
formation.
Page
author - Albert Sam
Project
investigators - Amy Flores, Vera Shively, Lin He, Albert Sam, Nick Garcia, Mark
Eskandari, Andy Chiou, and Bill Oppat
February
18, 2002
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