TRANSLATIONAL ISSUES
More than 15 years have passed since the first
transgenic models were derived that recapitulated
aspects of AD pathology. Somewhat surprisingly,
since that time, no new therapies for
AD have been approved and introduced into
the clinic, and the two currently approved drug
classes (acetylcholinesterase inhibitors andmemantine)
were not tested in these transgenic
mice prior to the clinic. This stark reality begs
the question of how useful have these animal
models actually been for the field? Have they
been a distraction to the real issues facing an
AD patient, or will their continued use bear
fruits over the coming years? Why have so
many therapies and interventions been successful
in these models, but have universally failed
when evaluated in clinic trials? Although an
in-depth discussion of these questions is beyond
the scope of this review, we will touch on
some of the translational issues (Box 1).
The most widely used animal models in
the field are in factmodels based on the genetics
of familial AD. Less than 1% of AD cases are due
to autosomal-dominant AD, rather than sporadic
AD, so the obvious initial question is
whether fAD and sAD are the same phenotypical
disease or are there subtle differences
between the pathologies that would allow a
treatment to work in one but not the other.
By all pathological counts, they are essentially the same disease, with abundant plaque and
tangle accumulations in the same brain areas
as well as high levels of synaptic and neuronal
loss. The differences appear to be what causes
the buildup of the pathologies in the first place.
In fAD, a mutation in APP or PS1/2 causes the
accumulation of Ab, whereas the causes of
Ab accumulation in sAD are unclear, but likely
to be a combination of genetic and environmental
factors. Both are highly influenced by
aging, with fAD manifesting at younger ages,
and being more aggressive in its progression.
A priori, one could postulate that fAD might
be harder to treat than sAD, because of the
aggressiveness of the pathology. Hence, transgenic
mice that model fAD, and do so in a
very short time frame (1–2 years) should be
the hardest to treat, and should therefore translate
to sAD very effectively. Obviously, the
plethora of numerous failed clinical trials indicates
that this is not the case, and so why do so
many treatments show success in these aggressive
mouse models of fAD and then fail in
patients with sAD?
Although there are numerous hypotheses
that may account for the discordance in results
between preclinical animal models and human
clinical trials, no doubt one of the most significant
may be that many AD models do not recapitulate
the extensive neuronal loss observed in
the human condition. Human imaging studies
and clinical–pathological studies show that
patients with mild–moderate AD already have
not only brain atrophy but also extensive neuronal
loss in several brain regions. The therapies
that are being pioneered in mouse models are
primarily targeting the pathologies modeled
and not dealing with the issue of extensive neuronal
loss. Hence, many of these therapies may
be effective at preventing or clearing the pathology,
and hence the disease, but are ineffective
in people in which the pathology has already
destroyed a huge proportion of the neurons
that they need for memories and cognition.
The acetylcholinesterase inhibitors were developed
following studies identifying cholinergic
loss as being a highly important factor in AD
cognitive decline.
This raises two pertinent questions: First,
why do mice not recapitulate the neuronal loss
seen in the disease, and, second, can we develop
models that do develop similar loss in which
therapeutics can be evaluated? We believe that
the fundamental reason why transgenic mice
do not develop extensive neuronal loss, like human
AD patients, is the amount of time needed.
In human AD, disease progresses over decades.
During this time synaptic disturbances, such
as those measured in transgenic mice, could
eventually lead to neuronal death. The two years
during which we keep most transgenic APP
mice may not be long enough for this to occur.
Other issues also clearly play a role, such as
background strains, such as the widely used
C57/Bl6, which may be more resistant to excitotoxicity
and heterogeneity of humans versus
inbred mouse strains, as well as fundamental
differences between mice and humans. For example,
it is well established that APP transgenic
mice have cognitive decline prior to plaques, and prior to any measurable neuronal loss. Furthermore,
Ab oligomers have been universally
shown to impair LTP in countless studies, and
it is easy to then connect this to the cognitive
impairments seen in these mice. Yet there is
no evidence that Ab causes cognitive decline
in humans prior to plaques (and neuronal
loss). If Ab impairs LTP in the human brain in
the same, robust, fashion that it does in the
rodent brain then presumably humans would
experience cognitive decline in the absence of
both plaques and neuronal loss whenever Ab
oligomers could first be detected (Kuo et al.
1996; Tomic et al. 2009; Woltjer et al. 2009),
and this cannot be explained by cognitive
reserve. This has not yet been shown to occur,
suggesting that it is possible that Ab may have
different modes of action in the rodent brain
compared with that of a human.
So how do we develop therapies that target
the neuronal loss and how do we then test and
validate them in vivo, prior to the clinic? Clinical
trials forADare expensive—upward of $10–
20 million for a well-powered phase II/III trial.
Hence, only the most promising compounds
can be brought into the clinic, and every failure
is costly and discouraging to alternative future
trials. It should be noted that prevention trials
have not yet been feasible, in part owing to a
lack of biomarkers and the extreme expense
associated with the numbers of people needed
and the amount of time for which they would
have to be evaluated, and therefore trials have
only used cognitive outcome measures in usuallymild–
moderateADpatients—which means
they already have extensive plaque and tangle
loads, and that these have already caused extensive
neuronal damage and loss, which in turn
causes dementia. We have developed a novel
approach to this problem by using an inducible
transgenic mouse model of neuronal loss
(Yamasaki et al. 2007). We use the tetracyclineoff
system to drive expression of diptheria toxin
A chains in neurons under control of the Calmodulin
Kinase II promoter. By withdrawing
doxycycline from the diet, we can specifically
ablate neurons in regions of the brain that are
impacted in AD, and we can titrate that loss to
levels seen in the AD brain. Mice show cognitive
impairments, as expected, and can then be used
to identify treatments that can improve cognition
in the presence of extensive neuronal loss,
such as that seen in AD patients. We are taking
the approach of combining therapy testing in
this model alongside a traditional APP/tau
transgenic mouse, such as the 3xTg-AD, to
identify treatments that can improve cognition
in the presence of Ab and tau pathologies, but
also neuronal loss. This ensures that only the
best therapies will be selected and proposed as
clinical candidates.
If 15 years of using transgenic mouse models
of AD have yielded no positive clinical
results, then have these mice been a failure or
even a distraction from the real problems with
AD? The answer is unequivocally no—many
novel approaches to reducing AD pathology
have been discovered and developed in these
transgenic mice, and will probably progress
into successful clinical trials when we find
ways to target prodromal stages of the disease
through biomarkers, or attempt prevention. It
is through these approaches that we will one
day be able to prevent the occurrence of the disease
as we age. For example, immunotherapy
was developed in APP transgenic mice and
could not have been proven to clear pathology
without them. Immunotherapy has progressed
into numerous clinical trials and has been
shown to reduce both Ab and tau levels in
patients (Boche et al. 2010), as shown in mouse
models (Schenk et al. 1999; Oddo et al. 2004).
The effects on cognition have been mixed—
benefits have been seen in patients without the
apo14 allele, but not in those with apo14—
which accounts for 60% of patients. As targeting
the plaques does nothing to address the
extensive neuronal loss that has occurred in
these patients, hints of effects on cognition are
extremely promising. We would predict, from
this, that immunization as an AD preventative
may be effective. Furthermore, encephalitis
caused by immunotherapy in a small cohort of
patients may not occur before abundant Ab
deposits are found throughout the brain.
Other potential therapeutics developed in
AD transgenic mice may yet show clinical success,
either as preventatives or as treatments.Some promising approaches include the copper/
zinc chelator PBT2 (Adlard et al. 2008),
which has shown efficacy in phase II clinical
trials (Faux et al. 2010), and scyllo-inositol
(McLaurin et al. 2006), which breaks up Ab
oligomers. Many companies are now exploring
potential cognitive enhancers in AD transgenics,
such as a7 agonists (Marighetto et al.
2008), phosphodiesterase inhibitors (Puzzo
et al. 2009; Verhoest et al. 2009), H3 antagonists
(Medhurst et al. 2007), and other approaches.
Perhaps targeting cognitive decline in the presence
of pathology will be more successful than
targeting the pathology alone, which has been
the trend of the past decade, or using a combination
approach.