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The excellent recent review
article by Ni et al.(1) in your journal prompted
us to point out what we feel are some common misapprehensions about animal
models, and particularly mouse models, of atherosclerotic plaque rupture.
main point to make is that definitions of plaque rupture that are derived from
human pathology must be applied with great caution to the mouse, particularly
with regard to the presence of luminal thrombosis. There are important
differences between the conduct of human and mouse studies, and between human
and mouse anatomy, that mean that it is not reasonable to expect to observe luminal
thrombus over disrupted plaques in mice at the same frequency as it is in
Human studies are usually performed on individuals who have died as the result
of an acute cardiovascular event and are therefore likely to have had a recent
plaque rupture, because about 70% of such events are caused by rupture. Mice
are terminated at predetermined time points so there is no self-selection of
individuals through sudden cardiovascular death. The likelihood of observing an
atherosclerotic plaque soon after it has ruptured is thus intrinsically lower
in mice than in humans, even if the actual frequency of occurrence of such
events is the same. Since mouse plaque ruptures are much smaller than human
ruptures, they will also heal more quickly (2). The period during
which they are observable as frank defects in the plaque is therefore shorter,
further reducing the probability of observing an acute plaque rupture in mice.
Fibrin fibrils in mice are only 10% to 20% of the thickness of those formed in humans
(3) but the activity of
plasmin, the primary fibrinolytic enzyme, is similar across species. Murine
arteries are obviously much smaller than those of humans, so thrombi are therefore
both smaller and also easier to lyse. Even if thrombi form over ruptured plaques
in humans and mice at the same frequency, the probability of observing one
before it is lysed is accordingly 30- to 200-fold lower in mice than humans (2). Indeed, the
presence of luminal thrombus over ruptured plaques in mice is very low even
when disruption is triggered by induction of apoptosis and hypertension (4) or by combined
inflammatory and haemodynamic stress (5). When a mouse plaque
is artificially disrupted, the initial thrombus that forms disappears within 1
to 7 days (6) whereas human
thrombi can persist for many months (7).
analysis makes it clear that termination of atherosclerotic mice at
predetermined time points is rather unlikely to result in the observation of an
acute plaque rupture, and it is even less likely that the rupture will be
accompanied by luminal thrombus. The message is clear – luminal thrombus should
not be used as a diagnostic feature for murine plaque rupture. What is more, retaining
this requirement would arguably be unethical as it would involve the use of unnecessarily
large groups of animals in order to study the phenomenon with appropriate
statistical power (in excess of 700 mice per group).
are two strategies that can be adopted to resolve this problem. The first is to
target the time of termination of the mouse to the period immediately after a
plaque rupture. This may be possible through the use of telemetry-acquired
electrocardiogram and/or blood pressure analysis or by imaging, approaches we
are actively investigating in our laboratories. It has the drawback that the
time of termination is variable, complicating the analysis of intervention
second stratagem, one that we have advocated before (2,
to take account of healed plaque ruptures in the analysis of murine
intervention studies. Mouse plaque ruptures heal quickly but persist as buried
structures visible in the plaque, and their use thus circumvents the problem of
the rapid resolution of acute ruptures. Such buried structures are considered
the hallmarks of healed plaque ruptures in humans (9,
approach absolutely requires that definitions of human atherosclerotic plaque
rupture, such as the presence of a thrombus that communicates across the
fibrous cap from the necrotic core to the lumen (11), are recognised as
unsuitable for use in the mouse. To that extent, we would disagree with the
conclusions of Ni et al. (1) who say that an
animal model “…should exhibit the salient features of spontaneous human plaque
rupture.” We believe that adoption of this tight restriction would harm work in
this field, particularly in mice. Since mice offer many advantages in terms of
ease of genetic manipulation, size and cost, that would be most undesirable.
Christopher L Jackson
Jason L Johnson.
Bristol Heart Institute
Level 7, Bristol Royal Infirmary
Telephone/fax: +44 (0) 117
1. Ni M, Chen WQ, Zhang Y. Animal models and
potential mechanisms of plaque destabilization and disruption. Heart. 2009;
2. Jackson CL,
Bennett MR, Biessen EA, Johnson JL, Krams R. Assessment of unstable
atherosclerosis in mice. Arterioscler
Thromb Vasc Biol. 2007;27:714-720.
3. Pretorius E,
Humphries P, Ekpo OE, Smit E, van der Merwe CF. Comparative ultrastructural
analyses of mouse, rabbit, and human platelets and fibrin networks. Microsc Res Tech. 2007;70:823-827.
4. von der Thusen
JH, van Vlijmen BJ, Hoeben RC, Kockx MM, Havekes LM, van Berkel TJ, Biessen EA.
Induction of atherosclerotic plaque rupture in apolipoprotein E-/- mice after
adenovirus-mediated transfer of p53.
5. Ni M, Zhang M,
Ding SF, Chen WQ, Zhang Y. Micro-ultrasound imaging assessment of carotid
plaque characteristics in apolipoprotein-E knockout mice. Atherosclerosis. 2008;197:64-71.
6. Bentzon JF,
Sondergaard CS, Kassem M, Falk E. Smooth muscle cells healing atherosclerotic
plaque disruptions are of local, not blood, origin in apolipoprotein E knockout
mice. Circulation. 2007;116:2053-2061.
7. Takano M, Inami
S, Ishibashi F, Okamatsu K, Seimiya K, Ohba T, Sakai S, Mizuno K. Angioscopic
follow-up study of coronary ruptured plaques in nonculprit lesions. J Am Coll Cardiol. 2005;45:652-658.
8. Jackson CL.
Defining and defending murine models of plaque rupture. Arterioscler Thromb Vasc Biol. 2007;27:973-977.
9. Burke AP,
Kolodgie FD, Farb A, Weber DK, Malcom GT, Smialek J, Virmani R. Healed plaque
ruptures and sudden coronary death: evidence that subclinical rupture has a
role in plaque progression.
10. Mann J, Davies MJ.
Mechanisms of progression in native coronary artery disease: role of healed
plaque disruption. Heart.
11. Schaar JA, Muller
JE, Falk E, Virmani R, Fuster V, Serruys PW, Colombo A, Stefanadis C, Ward
Casscells S, Moreno PR, Maseri A, van der Steen AF. Terminology for high-risk
and vulnerable coronary artery plaques. Report of a meeting on the vulnerable
plaque, June 17 and 18, 2003, Santorini, Greece. Eur Heart J. 2004;25:1077-1082.