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Volume 62(2), February 2005, p 192-195
Defining Molecular Targets to Prevent Alzheimer Disease
[Neurological Review]
Selkoe, Dennis J. MD
Center for Neurologic Diseases, Department of Neurology, Brigham and Women's Hospital and
Harvard Medical School, Boston, Mass.
Correspondence: Dennis J. Selkoe, MD, Center for Neurologic Diseases, Department of
Neurology, Brigham and Women's Hospital and Harvard Medical School,
77 Louis Pasteur Ave, Seventh Floor, Boston, MA 02115.
Financial Disclosure: Dr Selkoe is a founding scientist of Athena Neurosciences and a consultant
to Elan Corp.
Accepted for Publication: August 17, 2004.
---------------------------------------------An explosion of new techniques to explore DNA and protein biology during the last 2
decades has illuminated one of the most enigmatic and intractable subjects in
biomedicine-neurodegeneration. Eponymic diseases of the nervous system that were
until recently characterized by mechanistic ignorance and therapeutic nihilism are falling
steadily to the power of molecular genetics, cell biology, biochemistry, and animal
modeling. Alzheimer, Huntington, Creutzfeld-Jacob, and Parkinson diseases, as well as
amyotrophic lateral sclerosis, spinocerebellar atrophies, frontotemporal dementia, and
other previously obscure diseases, have all yielded rapid progress in the deciphering of
their biochemical pathology and genetic underpinnings. This sea change in our
understanding of a group of incurable diseases that confer enormous personal and
societal burdens has brought us to the verge of rationally designed therapies and, in
some cases, into actual human trials.
Perhaps the foremost example, both in terms of its impact on society and how far we
have moved toward clinical application, is that of Alzheimer disease (AD).
This most common of the late-life dementias is rising in prevalence with the aging of
populations in developed countries and may now affect 20 to 30 million people
worldwide. I will review the classic neuropathological lesions of AD that, despite some
doubts along the way, turned out to provide a road map to the etiology and pathogenesis
of the disease. Then, I will discuss how elucidating the genotype-to-phenotype
relationships of certain genetic alterations linked to familial forms of AD has pinpointed
molecular targets for treatment and prevention, some of which are now being addressed
in clinical trials.
A WELL-DEFINED NEUROPATHOLOGY SPURRED THE QUEST FOR THERAPY
The successful attempts during the mid-1980s to develop methods for purifying
microvascular and plaque amyloid deposits and neurofibrillary tangles from postmortem
brains have enabled much of the further progress in this field. In particular, the isolation
and partial sequencing of the amyloid [beta]-protein
(A[beta]) by Glenner and Wong, 1 followed by the recognition that amyloid plaques (but
not neurofibrillary tangles) were composed of the same protein, led to the cloning of the
[beta]-amyloid precursor protein (APP) and the localization of its gene to human
chromosome 21. 2 The latter finding offered an explanation for the invariant
development of the neuropathology of AD (often accompanied by cognitive decline) in
patients with trisomy 21 (Down syndrome). It soon became clear that patients with Down
syndrome begin to show immature A[beta] deposits, known as diffuse plaques, as early
as the second decade of life and later develop mature neuritic (amyloid) plaques and
neurofibrillary tangles indistinguishable from those found in patients with AD. This
information provided the first genetic evidence that A[beta] build-up can precede and
apparently initiate the typical neuropathology of AD. It was subsequently discovered that
A[beta] is naturally produced by all cells throughout life, 3 namely through the sequential
cleavage of APP by 2 proteases nicknamed [beta]-secretase and [gamma]-secretase
Separate studies established the nature of the neurofibrillary tangles, namely that they
consist of paired helical and straight cytoplasmic filaments composed of highly
phosphorylated forms of the microtubule-associated protein tau. 4 As in the case of
identifying A[beta] in plaques, the discovery of tau as the principal component of tangles
made it possible to link these protein abnormalities to the genetics of familial forms of AD.
GENETIC DEFECTS THAT PREDISPOSE TO AD STRONGLY IMPLICATE A[beta]
The hypothesis that AD represents an amyloidosis of the brain (ie, that the disease is
caused by the gradual build-up and aggregation of A[beta]) preceded the first genetic
discoveries in the disease and helped direct the search for causative genes. In accord,
the first AD-causing gene to be identified was APP, in which dominantly inherited
mutations lead to an early and aggressive form of the disease. 5 As many as 20 APP
missense mutations are now known, and they all cluster at or near the sites in the
precursor that are cleaved by the [beta]-,[gamma]- or [alpha]-secretases. 6 Next, the
[varepsilon]4 allele of apolipoprotein E was identified as a major genetic risk factor for
conventional (late-onset) AD, after the apolipoprotein E protein (apoE) in human
cerebrospinal fluid was found to bind specifically to immobilized A[beta] peptide. 7
Genetic epidemiology has robustly confirmed an enhanced risk for developing AD in
patients with 1 or 2 [varepsilon]4 alleles who are in their sixth and seventh decade of life.
Subsequently, elegant experiments in apoE knockout and transgenic mouse models
showed that the presence of the apoE4 protein markedly increases the amount of fibrillar
A[beta] deposits in the brain compared with the effects of the apoE3
protein. 8
The next 2 AD genes discovered were presenilin (PS) 1 and PS2. 9,10 More than 140
known missense mutations in PS1 and roughly 10 in PS2 confer an aggressive, earlyonset phenotype. In cell culture, transgenic mouse models, and the sera and brains of
affected patients, it has been shown that PS1 and PS2 missense mutations increase the
production of A[beta]42, the highly self-aggregating 42-residue form of A[beta], often at
the expense of the more commonly produced 40-residue form. 11 The mutations do so
because PS is the catalytic subunit of [gamma]-secretase, an unusual intramembranecleaving aspartyl protease. 12
Mutations causing AD alter the conformation of PS, apparently enhancing coordination
between the 2 catalytic aspartates and the A[beta] 42-43 peptide bond in the
transmembrane domain of APP. The identification of PS as the active site of [gamma]secretase provides a linchpin for the amyloid hypothesis of AD: all of the mutations
currently known to cause autosomal dominant AD occur either in the substrate (APP) or
the protease (PS) of the reaction that produces A[beta].
Several other candidate genes are in the process of being definitively confirmed as
predisposing to late-onset AD. Ultimately, a variety of genes will probably be determined
to confer risk, each one responsible for only a relatively small fraction of all cases.
TAU MUTATIONS LEAD TO FRONTOTEMPORAL DEMENTIA AND RELATED
DISORDERS WITHOUT INDUCING A[beta] DEPOSITION
The long-standing debate about the primacy of tangles vs plaques in AD pathogenesis
has been largely resolved by the exciting discovery of missense and splicing mutations
in tau, the subunit of the neurofibrillary tangles.
The
phenotypes associated with the inheritance of tau mutations to date include
frontotemporal dementia with parkinsonism, progressive supranuclear palsy, and Pick
disease, but not AD. This knowledge indicates that a primary alteration of tau can cause
severe neuronal degeneration, profound dementia, and the death of the host in the
absence of A[beta] deposition. Alzheimer disease is defined neuropathologically by the
presence of both plaques and tangles, but the accumulation of tangles cannot by itself
explain the development of A[beta] deposits in this disease. Indeed, studies in double
transgenic mice suggest that the presence of A[beta]-elevating APP mutations augments
the formation of tau deposits in neurons, rather than the converse. 13 The precise way in
which A[beta] accumulation induces a cascade of neuronal metabolic changes that
includes the hyperphosphorylation of wild-type tau molecules in AD is a subject of active
study, and several kinases have been proposed as culprits.
ELUCIDATION OF THE PATHOGENIC PATHWAY PROVIDES MOLECULAR
TARGETS AMENABLE TO BIOTECHNOLOGY
Although numerous aspects of the detailed mechanism of neuronal dysfunction in AD
remain unsettled, a broad consensus about some of the principal pathogenic events has
been reached by many investigators in the field. Evidence from genetic,
neuropathological, cell biology, and animal modeling studies suggests that the gradual
accumulation of A[beta]42 in limbic and association cortices leads to its aggregation into
oligomers, polymers, and amyloid fibrils. These various assemblies-in particular, small
diffusible oligomers-appear to be able to induce synaptic and dendritic dysfunction and
to activate microglia and astrocytes (representing a local inflammatory reaction). Such
changes, first subtle and then increasingly robust, are accompanied by an array of
further cellular and biochemical alterations, including altered ionic homeostasis, free
radical formation, oxidative injury, neuritic dystrophy, and, ultimately, neuronal death.
Because the potential cellular responses to A[beta] are myriad and complex, it has been
argued that it would be more efficient to decrease the levels of A[beta]42 in the brain
than to attempt to interfere with 1 or more of these secondary effects of A[beta].
While intensive research into AD pathogenesis has occurred in large part in academic
laboratories and enabled the field to move toward therapy, the actual discovery,
preclinical validation, and clinical development of agents that lower A[beta] are being
conducted primarily by biotechnology and pharmaceutical companies. Indeed, the
movement from basic research to the identification of treatments represents an
important cooperative interaction between these2 sectors. Many companies are now
vigorously pursuing several well-defined molecular targets, the inhibition of which could
allow not only treatment of active AD but perhaps also its prevention.
Based on the information reviewed herein, there is great interest in identifying smallmolecule inhibitors of the [beta]- and [gamma]-secretases. The availability of a crystal
structure for the active site of [beta]-secretase (also known as BACE-1) 14 is enabling
medicinal chemists to systematically modify inhibitors arising from initial compound
screens in ways that should enhance potency and still ensure specificity. Because
genetic deletion of the BACE-1 gene in mice appears to have little or no adverse
consequences, [beta]-secretase inhibitors would be expected to have few mechanismbased adverse effects. Therefore, [beta]-secretase inhibition represents a particularly
attractive therapeutic target. On the other hand, the size and shape of the active site of
this aspartyl protease present considerable challenges for the design of potent smallmolecule inhibitors that can also achieve good brain penetration.
In the case of [gamma]-secretase, the situation is more complex. Because its active site
is intramembranous, and because PS requires 3 additional membrane proteins (nicastrin,
Aph-1, and Pen-2) for proteolytic activity, the possibility of crystallizing [gamma]secretase is highly remote. Moreover, this unusual enzyme has numerous substrates
besides APP, perhaps the most important of which is Notch. Notch comprises a family of
cell-surface receptors that must undergo intramembranous cleavage by PS/[gamma]secretase to mediate cellular differentiation in multicellular organisms. Notch processing
is thus required for life.
It remains to be seen whether [gamma]-secretase inhibitors can be found that lower the
cleavage of APP to hinder sufficiently the release of A[beta] but do not interfere with
Notch cleavage to an extent that would produce adverse effects.
Intriguingly, there is evidence that certain nonsteroidal anti-inflammatory drugs such as
ibuprofen may subtly modulate [gamma]-secretase cleavage to lower A[beta]42
production without impairing A[beta]40 production and Notch cleavage.
15 Derivatives of certain anti-inflammatory drugs that can no longer inhibit
cyclooxygenase activity but still lower A[beta]42 production are currently being tested in
patients with AD.
A different treatment approach to the A[beta] problem in AD is to prevent the
aggregation of the peptide and/or promote its clearance. One novel paradigm that
appears to work by the latter mechanism is anti-A[beta] immunotherapy.
16 Two general approaches have been studied in AD mouse models: active vaccination
with A[beta] or fragments thereof and passive infusions of monoclonal antibodies to
A[beta]. Both have worked well to clear plaques and lower A[beta] levels in mice, and
they have decreased neuritic dystrophy and even ameliorated learning deficits in these
models. A clinical trial of active vaccination withA[beta]42 peptide plus an adjuvant led to
the development of meningoencephalitis (apparently mediated by anti-A[beta] T cells) in
18 (6%) of 300 study patients after just 2 vaccinations. Administration of the vaccine was
halted, but all patients were followed up closely, and the 18 patients recovered to
baseline status. Neuropsychological evaluations after 1 year suggested that some of the
vaccine recipients had experienced stabilization of their cognitive decline and perhaps
even some improvement on some measures of cognition regardless of whether they had
experienced the adverse T-cell reaction-and this was correlated with their titers of antiA[beta] antibodies. 17 Moreover, when study patients died of causes other than AD, a
few autopsies revealed an apparent partial clearing of A[beta] deposits in some cortical
regions. All of this new knowledge has led to the continuation of the immunotherapy
approach in a new phase 1 trial, this time in the form of passive administration of a
humanized monoclonal antibody to A[beta]. An alternative active approach that may
carry a lower risk of adverse events is mucosal (nasal or oral) vaccination. This method
has been tried in APP transgenic mice (but not in humans) and has led to significant
decreases in plaque burden, A[beta] levels, neuritic dystrophy, and gliosis. 18
WHERE DO WE GO FROM HERE?
There are, of course, numerous other prospective strategies to treat or prevent AD,
including some directed against the amyloid problem (eg, antiaggregation compounds
and metal chelators) and some directed against other features of pathogenesis (eg,
neurotrophins, antioxidants, and free radical scavengers). But anti-A[beta]
immunotherapy represents the most advanced disease-modifying attempt so far, in
terms of experience in humans. All of the various thrusts toward treatment and
prevention should be pursued vigorously by both academic and biopharmaceutical
investigators so that more than 1 approach ultimately becomes available for clinical use.
In the future, it is likely that middle-aged adults will commonly undergo a formal risk
assessment for AD that will include detailed family history taking, brief
neuropsychological assessment, screening for known genetic risk factors,
measurements of plasma A[beta] (and perhaps cerebrospinal fluid A[beta] and tau), and
quantitative assessment of brain [beta]-amyloid burden by imaging procedures. 19
Based on the results, patients would be offered 1 of the potentially disease-slowing
therapies contemplated herein or others not discussed here. One can only hope that
such a scenario comes into practice well before another generation of older humans
succumbs to this devastating disease.
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Alzheimer Disease; Amyloid beta-Protein; Mutation; NEUROLOGICAL REVIEWS
(Pleasure DE, ed); tau Proteins
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