Journal of Controversies in Biomedical Research 2015; 1(1):4-22.
Doi: http://dx.doi.org/10.15586/jcbmr.2015.3
Review Article
Disparity of outcomes: the limits of modeling amyotrophic lateral sclerosis in murine models and translating results clinically
Pierre Zwiegers1,3, Christopher A Shaw1,2,3
1Experimental Medicine Program; 2Graduate Program in Neuroscience; 3Department of Ophthalmology and Visual Sciences, University of British Columbia, Canada.
Abstract
Amyotrophic
lateral sclerosis (ALS) is a devastatingly progressive
neurodegenerative disorder with multiple underlying etiological factors
contributing to disease pathogenesis. Despite intensive research
efforts and therapeutic development, disease presentation in ALS
remains largely intractable to intervention. To date, the most common
rodent model used in pre-clinical drug development accounts for a small
proportion of the affected patient population and is predicated upon
the significant overexpression of a mutant form of the human
antioxidant protein, superoxide dismutase 1 (mSOD1). After more than 50
clinical trials, there is an alarming paucity of positive outcomes at
the clinical level of ALS therapeutics with strong supporting
pre-clinical data in mSOD1 models. Potential reasons for the negative
clinical results are multifactorial in nature and include an overly
reductionist model system that is heavily influenced by individual
transgene level variation, as well as attempting to widely apply
findings derived from a model of specific genetic causality to a
patient population where the majority of cases are of unknown etiology.
With such a tremendous disease burden and a lack of therapeutic
options, it is critical that the research community re-evaluate the
dependence on mSOD1 pre-clinical models as the gold standard prior to
translating findings at the clinical level. Here we briefly review both
the clinical and pre-clinical findings of select therapeutics, discuss
the limitations of pre-clinical mSOD1 models, and suggest future
stratagems that could aid in the clinical translation of efficacious
therapeutic agents.
Received: 21 July 2015; Accepted after revision: 13 August 2015; Published: 24 August 2015.
Author for correspondence: Christopher A Shaw, Department of Ophthalmology and Visual Sciences, University of British Columbia, Canada. E-mail: [email protected]
How
to cite: Zwiegers
P, Shaw CA. Disparity of outcomes: the limits of modeling
amyotrophic lateral sclerosis in murine models and translating results
clinically. Journal of Controversies in Biomedical Research 2015;
1(1):4-22. Doi: http://dx.doi.org/10.15586/jcbmr.2015.3
Introduction
Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset, multifactorial, and progressively neurodegenerative disorder of motor neurons within the brainstem and spinal cord. Due to the progressive nature of the disease, symptoms worsen temporally as motor neurons deteriorate and muscle atrophy becomes increasingly established. Severe respiratory complications typically culminate in death 2-5 years following diagnosis. In North America, the incidence rate of ALS is approximately 2 per 100,000 and is expected to increase with an aging demographic (1). To date, more than 20 genetic mutations have been identified in familial forms of ALS (fALS); accounting for approximately 5% of all cases (2, 3). The most common underlying genetic causes of fALS pathogenesis include the recently characterized C9orf72 intronic hexanucleotide repeat expansion, as well as mutations in the genetic loci for the Cu/Zn superoxide dismutase 1 enzyme (SOD1), TAR DNA binding protein (TARDBP), and Fused in Sarcoma (FUS) RNA-binding protein (Figure 1; reviewed in 3, 4). Implicated in up to 20% of all fALS cases, more than 180 toxic-gain-of-function mutations in the SOD1 gene has been described throughout all exonic sequences (Figure 2; 3, 5). The vast majority of ALS patients (95%), however, suffer from a sporadic form of the disease (sALS) in which unknown etiological factors are causal in inducing the disease (4, 6).
Figure 1. Graphical representation of the approximate frequencies of
familial and sporadic ALS cases in the overall affected patient population,
rounded to the nearest integer. Frequency of causes are expressed per 100 ALS
patients and do not account for sex differences. Infographic generated from the upper
frequency limit summarised in (3).
From
a clinical and pathological standpoint, both forms of ALS (SOD-1 fALS and sALS)
have primarily been considered to be virtually indistinguishable, since it is
thought that different initiating events ultimately converge to induce a
similar mechanistic cascade of disease progression (7, 8). This has led to the notion that if
researchers can dissect and understand the defective pathway(s) identified in
the context of genomic SOD1 abnormalities, that this knowledge can be
translated to sporadic forms of the disease and thus aid in therapeutic
development.
After more than two decades of work, various SOD1 genetic anomalies have been identified and actively studied in rodent models. An overarching pathogenic mechanism of action, however, remains elusive and the majority of therapeutic agents investigated have failed to yield positive outcomes when translated to the clinical level. Further challenging this reductionist viewpoint are neuropathological findings which suggest that the phenotypic expression of ALS may be consequent to heterogeneic pathologies affecting the central nervous system (CNS). For instance, by utilizing antibodies specific for labeling misfolded conformations of the SOD1 protein, some groups have demonstrated positive immunolabeling in fALS cases, a finding not always recapitulated in sALS (9–12). Similarly, pathological TDP-43 positive cytosolic inclusions common in sALS have not been described in mutant SOD1-linked fALS (13); a finding which is not always observed in mutant SOD1 models (14, 15). Taken together, these outcomes may signify diverse pathological cascades which ultimately present with the typical ALS phenotypic end state.
Figure 2. Distribution of hSOD1 amino acid substitutions within the monomer. A) PyMOL1 graphical visualization of the SOD1 protein identifying >150 amino acid substitutions throughout the 3D structure of the protein. B) The panel outlines the sites of substitution within the primary sequence of the polypeptide. Sites of the mutant residues depicted are derived from the ALS Online Genetic Database (5). 1The PyMOL Molecular Graphics System. Schrodinger, LLC.
Historically,
mutations within the SOD1 locus were first associated with the clinical
presentation of ALS, and were instrumental in developing the initial animal
models that have been employed in pre-clinical research (16–18). After more
than 50 randomized control trials testing various therapeutic agents which had
a positive effect in pre-clinical models, Riluzole remains the only
FDA-approved drug available to the affected patient population (19).
What
follows is a brief review of the common mutant SOD1 (mSOD1) fALS murine models,
the initial promising pre-clinical results, as well as the resultant outcomes
of select clinical trials. We will then shift our attention to a discussion
regarding the future applicability of these model systems in pre-clinical
research studies.
Murine models of familial
ALS
In the mid-nineties with the discovery that mutations in the antioxidant Cu/Zn-superoxide dismutase 1 (SOD1) gene locus was inheritable and linked to an ALS phenotype, there was renewed hope in the research community that a treatment was on the verge of being developed. To aid in therapeutic discovery, a murine model overexpressing a clinically-relevant mutant form of the human SOD1 enzyme with a glycine to alanine substitution at codon 93 (G93A) was created (17, 20–22). This result was soon followed by the development of mice overexpressing a variety of missense mutations in the SOD1 locus including G37R (18), D90A (23), G85R (24), and several other genomic abnormalities (25, 26). Due to the random genomic integration of the transgene, various lines of each mutation were established; each carrying the mutant gene to a varying degree and exhibiting a severity of disease that was highly dependent on the number of integrated transgenes (see Table 1 for the G37R and G93A mSOD1 models). These animal models were instrumental in delineating that the disease outcome was not a result of reduced dismutase activity, but due to a so-called “toxic-gain-of-function” property inherent to the mutant form of human SOD1 (22). More recent evidence, however, suggests that a loss of SOD1 activity may also contribute to disease presentation (27).
Table 1. Timing of the progressively degenerative phenotype in mSOD1
murine
models is highly dependent on the degree of mutant protein expression.
Prototypically,
the G93A mSOD1 animals experience a rapidly progressive phenotypic deterioration,
ultimately showing signs of hind limb paresis at approximately 150 days of age
(17). On account of the rapid and robust expression of the disease phenotype in
these animals, the G93A model has become the most widely used system in
assessing putative therapeutic agents (28). Neuropathologically this model
shows neuronal cell loss within the spinal cord as well as evidence for both
astrocytic and microglial proliferation; thus recapitulating some key features
inherent to clinical ALS cases (20, 29–32). Within this context, these in vivo mSOD1 models have provided a
glimpse into an aspect of the purported pathomechanism(s) underlying disease
presentation. This was thought to underscore their utility in not only studying
the inherent degenerative cascade, but also in translational therapeutic
development for all forms of ALS. However, since their inception, a plethora of
potential therapeutics have been tested in these models and similar positive
outcomes have not been recapitulated when applied to the clinical setting. The
paucity of positive clinical outcomes from therapies derived from pre-clinical
studies suggest that critically re-evaluating the sole reliance on mutant SOD1
models may identify non-productive research endeavours and accelerate therapeutic
development of efficacious agents.
Clinical applications
derived from investigations based on murine models
To
date, various therapeutics have been investigated for the ability to attenuate
and/or ameliorate the pathological disease cascade induced in ALS (19, 33).
Each of these agents addresses a unique hypothetical mechanism which is
purported to play a key role in establishing the eventual disease state.
Selected outcomes from both pre-clinical and clinical trials are discussed
below (also see Supplementary Tables S2, S3, and S4).
Riluzole
Glutamate
is a major excitatory neurotransmitter within the CNS (reviewed in 34). Once
released into the synaptic cleft, glutamate relays the action potential to the
downstream target neuron and the original signal is further propagated towards
the target site. Under normal physiological conditions, an intricate system of
neuronal and glial glutamate transporters result in the expedient endocytic
uptake of extracellular glutamate, and so remove the excitatory stimulus acting on the post-synaptic cell.
Increased levels of excess glutamate in the extracellular milieu will result
in excitotoxic activity of the post-synaptic cell due to aberrant Ca2+
homeostasis within the cytosol arising from excessive membrane depolarization
mediated through glutamate receptors (35). Increased cytosolic Ca2+
ions are compartmentalized within the mitochondrion, which can result in
alterations of mitochondrial permeability, production of reactive oxygen
species (ROS) and/or activating a plethora of enzymes involved in cell death
pathways (36, 37). Preliminary evidence invoking a role for glutamate in the
pathogenesis of ALS included an increase in peripheral levels (and a
concomitant decease in CNS tissue) in ALS patients (38, 39), reduced glutamate
uptake in patient-derived synaptosomes (40), and a reduction in levels of the
astrocytic glutamate transporter, EAAT2 (35). Riluzole, the only approved
pharmacological agent for treating ALS, has been shown to be effective in
extending survival, but only by a modest 2-3 months (41). A concrete mechanism
of action is yet to be elucidated; however, it appears to act on glutamate
transporters to facilitate the extracellular clearance of the excitotoxic
neurotransmitter (42), as well as inhibiting the presynaptic cell from
releasing glutamate (43). Synergistically, both of these actions would serve to
decrease the glutamate load in the extrasynaptic space and thus attenuate the proposed
cellular excitotoxic effects involved in disease pathogenesis.
In
contrast to the other ALS therapeutics discussed below, Riluzole treatment in
the clinical setting preceded validation in the ALS animal model, but was
explored based on evidence of anti-glutamatergic activity. The initial clinical
study at 100mg/day indicated a significant improvement in overall lifespan
after 12 months of treatment, a concomitant decrease in the rate of muscle
deterioration, and evidence of a more efficacious effect in patients exhibiting
bulbar-onset ALS (S3, S4; 44). Subsequent follow-up studies were shown to
replicate the initial therapeutic effect and investigated additional secondary
outcome measures. A dose-response study (50-200 mg/day) showed a significant improvement
in survival at higher doses. However Riluzole treatment failed to significantly
affect functional assessments (i.e. muscle testing, respiratory function, and
subjective symptom assessment), or differentially affect limb- or bulbar-onset
cases (45, 46). Population-based studies confirmed a positive therapeutic
effect of Riluzole treatment that particularly affected bulbar-onset patients.
However, the overall effect in ALS patients was transient in that the survival
curves were unaffected when examined in prolonged follow up studies (47, 48).
In ALS animal models (S2), Riluzole treatment
(100 micrograms/mL water) during the presymptomatic stage was initially shown to exert
no effect on delaying the onset of disease, but did significantly extend
survival by 13-15 days (49). A follow-up study showed a dose-dependent
preservation of motor function, while exhibiting an identical positive effect
of survival at doses of 24-44 mg riluzole/kg body weight/day of 12-13 days
(50). The positive, but modest, clinical effect combined with an extended
survival in the fALS model has solidified the notion of the clinical utility of
Riluzole as a therapeutic agent in ALS, which to date, remains the only drug
approved for treatment.
Minocycline
Minocycline
is a lipid-soluble, semi-synthetic tetracycline antibiotic that can readily
penetrate the blood-brain barrier, and has been shown to exhibit
anti-inflammatory properties apart from its expected antimicrobial activity
(51). Initial work in models of neurodegeneration had shown that treatment with
the antibiotic prevented microglial activation, inhibited release of the
pro-inflammatory cytokine IL-1β, and rescued primary neuronal cells from
glutamate cytotoxicity (51, 52). Increased neuroinflammatory processes and the
activation of microglia have long been suspected in the underlying
pathomechanism of ALS (53). The strong preliminary data in models of
neurodegeneration warranted further study in the context of ALS. Mutant SOD1
animal models (G37R or G93A) receiving minocycline either in their diet or via
intraperitoneal injections during the presymptomatic phase of the disease,
showed evidence of a dose-dependent delay in onset and an attenuated
progression of disease (S2; 54–56).
Following preliminary studies assessing the safety and tolerability of
minocycline in ALS patients (57, 58), a randomised phase 3 clinical trial set
out to test the efficacy of the treatment paradigm (S3 and 4; 59). In contrast to pre-clinical findings, oral dosage of
minocycline was found to be associated with a significantly more rapid decline
in functional outcome scores (as measured by the revised ALS functional rating
scale), while concurrently showing evidence for a reduction in lung capacity
and muscle performance. Although pre-clinical evidence showed a marked
attenuation of disease presentation, similar findings were not recapitulated
within the clinical setting, and treatment with minocycline seemed to
exacerbate ALS-related deterioration.
Creatine
Mitochondria
are crucial in generating the intracellular energy stores required to drive
biochemical processes, maintain calcium homeostasis, and play a role in
regulating apoptosis (60). Murine models of SOD1-ALS show evidence of aberrant
mitochondrial pathology; most notably mitochondrial swelling and vacuolization
(18, 20). Functionally, in vivo model
systems expressing the G93A missense mutation in the SOD1 locus showed evidence
for a decrease in mitochondrial membrane potential, an increase in the
concentration of cytosolic calcium ions, as well as an increased susceptibility
to oxidative stress (61, 62). Clinically, ALS patients show aberrant
mitochondrial pathology in that a dense aggregation of mitochondria is
localized to the presynaptic terminal (63). Such dysfunction of mitochondria
would ultimately perturb the energy balance of the system and contribute to
cellular degeneration. Therapeutics directed at enhancing and/or stabilizing
normal mitochondrial functioning was thus speculated to be of some utility in
treating ALS. Creatine, generated through a biosynthetic pathway involving
arginine and glycine, exists in non-phosphorylated and phosphorylated forms.
Together, these biomolecules serve an important function in producing ATP at
sites of high energy consumption (64, 65). Exogenously administered creatine
was shown to exert neuroprotective properties through various inter-related
pathways including attenuating glutamate-induced cytotoxicity (66), and
directly acting as an antioxidant against reactive ion species (67). Elevated
creatine levels are thought to protect mitochondrial creatine kinase from
oxidative damage which inhibits the opening of the mitochondrial transition
pore, and so protects against cell death (68, 69). Oral creatine treatment in
the G93A model showed evidence of a dose-dependent increase in overall lifespan
(26 days at 2% w/w creatine supplementation compared totransgenic controls),
improved motor performance, and exhibited an increase in neuronal viability
comparable to wild-type littermates in a cohort receiving 1% dietary creatine (S2; 70). In the context of a lower copy
number, dietary creatine (2% w/w) was shown to delay symptom onset by 12 days
and decrease the severity of disease presentation, however, this did not
correlate with a significant amelioration of motor neuron loss (71).
Combinatorial approaches showed that creatine dosage supplemented with
minocycline, but not Riluzole, resulted in an additive therapeutic effect with
a delay in phenotypic onset and a concomitant increase in overall lifespan (71,
72). Clinically at a dose of 10g/day, dietary creatine intake (for 16 months)
did not positively affect survival rates or the degree of decline for a variety
of clinically-relevant measures (65). A
concurrent study at a lower dosage (5g/day) for six months examining a different
set of outcome parameters similarly failed to establish a role for creatine in
ameliorating ALS pathogenesis (73). Following a preliminary study with positive
outcomes, Rosenfeld et al. set out with a larger cohort and followed patients
for nine months receiving creatine at 5g/day (74). As in the previous clinical trials, dietary
supplementation failed to demonstrate a therapeutic benefit in multiple
measures of relevance to the clinical presentation of ALS (S3 and S4). The lack of
positive correlation between pre-clinical models and clinical trials in
therapeutic outcomes of creatine treatment has halted further clinical
investigation of the agent.
Limitations of clinical
ALS trials and modeling disease in mouse models expressing mutant human SOD1
Having
briefly reviewed the clinical outcomes of three selected therapeutic agents, it
is likely that the discrepancy between pre-clinical results and clinical
translation are multifactorial in nature. Mounting evidence supports a
heterogeneous cascade of events that underlie the inherent pathomechanism(s) of
ALS (6). Clinically, patients present with a phenotypically heterogeneous
disease that can affect site of onset, rate of disease progression, upper
and/or lower motor neuron involvement, and whether the phenotype is strictly
behavioural or elicits some form of cognitive impairment (75). It is probable
that inadequate stratification of the patient population in terms of phenotypic
variability (and perhaps yet unknown genetic and environmental causal factors)
could undermine robust clinical outcomes, since any specific treatment effects
may only be applicable to a subset of ALS patients (19, 76).
The
theoretical utility of animal models is in their ability to mimic the
underlying disease cascade and thus act as a platform for therapeutic
development. A model system should be representative of the patient population
as a whole so that any pre-clinical drug development should be applicable when
translated clinically. In the case of ALS, there is roughly a global prevalence
rate of 4-7 per 100, 000 population; thus around half a million patients would
be affected world-wide (1). Familial ALS cases due to mutant SOD1 loci would
then account for approximately 5000-10,000 patients. Pre-clinical development
based solely on a model of mutant SOD1 thus has the potential to greatly impede
translational applicability since (i) such a small percentage of patients that
comprise the clinical trial will be carrying a mutant copy of the SOD1 gene,
and (ii) logistically enrolling SOD1-affected fALS patients in a trial of
sufficient power to properly test a developed therapy would prove daunting.
Further,
as is evidenced in this brief review, there is limited proof for translational
applicability of therapeutics developed in mSOD1 models of ALS: a caveat not
restricted to neurodegenerative disease research. Generally a positive
pre-clinical effect has less than a 40% chance of recapitulating a similar
clinical outcome (77), with some animal models not even accurately portraying
the inherent disease pathomechanism(s) (78).
Limited
independent validation studies, publication bias and deficits in good
experimental design may also account for some of the discrepancy in outcomes
between clinical and pre-clinical mSOD1 studies
(28). Furthermore, as Scott et al. poignantly discuss, failing to control for
specified biological factors in ALS models has likely resulted in the
measurement of chance variability (i.e.
“noise”), and has thus resulted in false positive results (79). To underscore
the importance of these variables, an optimized study paradigm investigating the
efficacy of Riluzole, minocycline, or creatine were unable to replicate the
positive pre-clinical outcomes previously discussed (79).
In
short, due to the complex interplay of etiological factors, effectively
modeling ALS has proven to be quite challenging. Thus, the discrepancy in
translating therapeutics clinically is likely multifactorial in nature and will
be the focus of the remaining discussion. Of primary concern here is how well
the in vivo model mimics clinical
ALS, impediments hindering replicability between pre-clinical studies, as well
as the importance of species differences in drug metabolism.
Models representative of
clinical ALS
Focusing
on the original line of G93A mSOD1 animals with the greatest number of
transgene integrations (G1), neuropathological changes are of a progressively
degenerative nature (17, 20, 21). At the outset, vacuolar degeneration is
observed within motor neurons, but with time, alongside a marked reduction of
neuronal cell bodies, pathological vacuoles are present within the surrounding
neuropil with a marked deterioration of the anterior horn. Evidence for
vacuolar degeneration in animals highly expressing the mutant G93A SOD1 locus
does not reflect the reality of end-state pathology in human cases, and has
been suggested to be a toxic artefact arising from the transgene being
significantly overexpressed (21). Thus, it is possible that the marked
overexpression of the mutant transgene results in a more severe disease
pathology which may not reliably measure a positive or negative clinical
outcome for translational purposes. Within the context of an accelerated
disease cascade, it is probable that failed therapeutic agents at the
pre-clinical stage may have had some clinical utility in the protracted form of
human disease. However, without strong pre-clinical data in these progressive
models, candidate therapeutics will not be promoted to the clinical trial
stage.
Expression
of the aberrant phenotype in SOD1 fALS models are driven by the significant
overexpression of the mutant hSOD1 locus (Table
1). In stark contrast, familial forms of the disease (SOD1-linked fALS) are
inherited mostly in an autosomal dominant manner and thus present with ALS in
the context of one aberrant copy. Transgenic lines carrying significantly fewer
copies of various gain-of-function mutations do not develop disease after
prolonged observation (17, 18), or give rise to a significantly protracted
disease course (80). For instance the SOD1A4V strain of mice carry
the most common SOD1 mutation found in North America (81), yet fail to
recapitulate the requisite phenotypic correlates of disease (17). This is
primarily thought to be a consequence of decreased transgene expression levels.
Of the two A4V lines developed, the line expressing the mutant protein to a
higher degree (20% greater than the lower expressing line) was shown to exhibit
an affected phenotype only when crossbred with a line overexpressing wild-type
hSOD1 suggesting that a threshold level of mutant SOD1 needs to be exceeded
prior to phenotypic onset (82).
An
additional impediment in utilizing the model to predict clinical efficacy is in
the timing of drug administration. Typically the timing of drug delivery in
these models is prior to phenotypic onset and thus provides a wide therapeutic
window for the effect to be realized (S2).
In lieu of biomarkers which specifically gauge the onset and progression of the
disease, a typical therapeutic intervention initiated in the clinic will only
be administered following symptom onset; most notably at a stage of the disease
when marked neuronal cell loss has already occurred. Due to the intractable and
rapidly progressive nature of ALS, it should not be surprising that many
promising pre-clinical therapeutics have not been able to attenuate and/or
ameliorate the disease when initiated at an advanced stage when translated to
the clinic.
Impediments to
replicability between pre-clinical studies
The
literature is rife with studies exemplifying the relationship between mutant
hSOD1 transgene levels incorporated into the genome and the degree of
phenotypic severity. Our recent experience with the G37R model underscores this
dilemma in that we had generated a colony of animals from commercial breeders
with an uncharacterized drop in copy number. As expected, these presented with
a concomitant increase in lifespan and delay in disease progression (83).
Transgene level variation can arise due to meiotic recombination. In the G93A
mSOD1 model, this is relatively infrequent with recombination accounting for
transgene fluctuations in approximately 3% of the progeny produced (84). Due to
random meiotic events, the original G93A G1 line has spawned two additional
sub-lines with either a 40% expansion (25 copies) or 30% retraction (13 copies)
of the mutant hSOD1 locus; each showing variations in phenotype severity on
account of altered transgene expression levels (85, 86).
Experimentally,
variations in transgene dosage are detrimental to research efforts. First,
small colonies are likely to be adversely impacted and would serve limited
applicability in replication studies. Second, undetected fluctuations in
individual copy number will give rise to animals exhibiting varied phenotypic
expression of disease severity, and thus respond differently to therapeutic
intervention. Should an experimental cohort by chance be comprised of outliers
with either higher/lower copy numbers, a tested therapeutic effect may be
primarily a consequence of disease severity linked to transgene levels. Without
dutifully reporting on the number of integrated transgenes/mSOD1 expression
levels, the issue is further compounded when independent groups attempt to
replicate previous results.
The
incongruent findings between clinical and pre-clinical studies may further be
explained by underlying physiological differences in metabolizing the
therapeutic. To the authors’ knowledge, no investigation has yet been conducted
to study whether any putative ALS pharmacological agent is processed similarly
in both mice and man, and thus whether it is bioactive to the same degree in
these different species. Although it is used to induce a form of parkinsomism,
systemic administration of the neurotoxic agent MPTP exemplifies
species-specific differences in metabolic activity (87). In humans and primate
models, intravenous delivery mediates an acute parkinsonian syndrome virtually
indistinguishable from idiopathic Parkinson’s disease. Rodent models however
have proven to be somewhat more resilient to the toxic insult accompanying
systemic administration. In rats, because of peripheral enzymatic catabolism of
MPTP by monoamine oxidase, the polar MPP+ metabolite cannot cross the
blood-brain-barrier and thus fails to selectively induce dopaminergic neuron
degeneration (87). Without dutifully assessing the possibility of cross-species
drug metabolic differences, it is possible that any pre-clinical effect may not
be fully recapitulated in the clinic.
Future considerations for
modeling disease and testing clinically-relevant ALS therapeutics
Due
to the definitive need for therapeutics targeting the neurodegenerative
cascade(s) inherent to ALS pathogenesis, it is of crucial importance that
non-productive research endeavours are expediently identified and resources
redirected to more promising avenues. To date, the reductionist approach of
understanding ALS as a mSOD1-mediated insult has not borne out an effective
treatment strategy as was initially expected. In contrast to the current
approach of studying potential therapeutics in the context of a specific
genetic causality, a disease such as ALS with a complex etiology necessitates a
multi-pronged strategy. Herein we broadly consider future strategies that could
greatly facilitate the development and testing of potential ALS therapies.
Clinically-relevant
biomarkers: assessing disease progression and therapeutic efficacy
There
is a desperate need for the development of clinically-relevant biomarkers that
will not only aid in an earlier diagnosis of disease, but also provide
information with regard to disease progression. To date, various potential
peripheral blood biomarkers have been identified, but none have been
successfully translated into clinical use (reviewed in 88). Identification of
such biomarkers would not only aid in diagnosing patients at an earlier stage
of the disease, but also provide for an alternate measure of the effectiveness
of any putative therapeutic agents.
Having
a distinct correlative marker of disease progression might also shorten
clinical trials since the effectiveness of a drug could be more efficiently assessed
(88, 89). With shorter clinical trials, patients could then participate in more
studies, thus allowing for more therapeutics to be clinically assessed and a
treatment regimen to be fast-tracked. In addition, predictive biomarkers would
allow for an earlier diagnosis of disease. This could facilitate an earlier
intervention strategy in the progressive neurodegenerative cascade and
theoretically mitigate additional CNS damage.
Blood
provides for an easily-obtained biological fluid which constitutes a viable
source for biomarker discovery, with the added caveat that blood biomarkers may
not directly correlate with motor neuron degeneration (88). As Robelin and De
Aguilar discuss, both the blood-brain barrier and the blood-cerebrospinal fluid
barrier may act as an impediment to the crossing of relevant biomarkers into
systemic circulation (88). There is thus a possibility that surrogate
peripheral biomarkers may not adequately reflect the underlying degenerative
cascade, or provide a direct measure of the intended therapeutic effect within
the CNS. On account of the complex neurodegenerative cascade at play during ALS
pathogenesis, it may be more appropriate to identify panels of biomarkers that
clearly distinguish ALS from related disorders (88).
A
recently conducted clinical trial of rasagiline, an inhibitor of monoamine
oxidase B, demonstrates the application of potential blood biomarkers in
assessing the clinical outcome of a therapeutic agent (90). Prior work had
demonstrated that the drug had specific beneficial effects on mitochondrial
function, decreased oxidative damage, and inhibited apoptosis. As previously
pointed out, mitochondrial dysfunction is a clinical hallmark of ALS
pathogenesis. Thus measuring particular biomarkers related to mitochondrial
function (e.g. changes in mitochondrial membrane potential, oxidative stress,
and the relative abundance of pro-survival/pro-apoptotic signals) may allow for
a biochemically-relevant measure of therapeutic effectiveness in ALS clinical
trials (90). The trial did not demonstrate an improvement in the functional
ALSFRS-R scores after 12 months compared to historical controls (when corrected
for symptom duration). However, peripheral lymphocytes showed evidence of an
increased mitochondrial membrane potential, decreased oxidative stress, and an
increased Bcl-2: Bax ratio; indicative of pro-survival cell signaling. A major
caveat to the use of peripheral biomarkers as a surrogate indicator for
activity within the CNS is the degree to which the therapeutic agent is able to
penetrate, and act on cells within the CNS. In the case of rasagiline, the use
of a peripheral mitochondrial biomarker is expected to be an adequate indicator
of CNS activity since the therapeutic agent has demonstrated peripheral
distribution and CNS penetration (90).
Development of alternate
models to more effectively mirror the diverse etiological factors that underlie
ALS pathogenesis
A
critical problem lies in developing animal models which are more representative
of ALS patients. To date, primarily on account of the unknown etiology, there
is a lack of models that are representative of the sporadic form of the
disease. Our group has previously investigated the dietary administration of
cycad flour or extracted sterol glucosides as causative agents in the purported
disease cascade (91-93), while others have assessed low Calcium/Magnesium, high
Aluminum diets (94). Potential therapeutics have yet to be formally
investigated within the context of these environmental model systems.
Furthermore, there needs to be a renewed effort to develop and validate
additional animal models with suspected environmental etiological factors.
On
account of our increased understanding of the underlying genetic causal factors
at play in ALS (Figure 1), there
have been multiple additional murine models developed since the advent of the
mutant SOD1 mouse. These include mice expressing mutant TDP-43 or the
C9ORF72-associated hexanucleotide repeat expansions (95-97). Characterising
these additional systems allow for a more representative model of the overall
ALS population as it takes into account the multiple underlying pathological
changes that are implicated in disease pathogenesis.
However,
a central caveat to conducting pre-clinical studies in animal models is the
inability to rapidly screen multiple therapeutic agents in a single assay. To
address this, an approach involves modeling the disease with nerve cells
derived from induced pluripotent stem cells from sALS and/or fALS patients (98,
99). Not only could this strategy allow for the screening of a plethora of
neuroprotective compounds, but it may pave the way for determining
patient-specific drug responses to ALS therapeutics. An alternate approach
employs the use of small invertebrate and vertebrate model systems expressing
mutant forms of loci associated with ALS as platforms for pre-clinical drug
development. Zebrafish and Caenorhabditis elegans animal models
that express mutant forms of human TDP-43 or FUS allow for the production of
large numbers of animals that can be housed in multi-well plates, tested with
an array of potential therapeutic compounds, and assessed whether the disease
phenotype is attenuated (100). Strategies such as these may allow for the rapid
identification of potential therapeutic agents that can ultimately be applied
at the clinical level.
Realistically,
due to the heterogeneity of the pathological mechanism(s) at play, it is quite
unlikely that any one model would be sufficient in mimicking all of the
relevant underlying pathobiology. What should be adopted instead is a
multi-pronged approach where a potential therapeutic is validated in multiple
model systems testing specific outcomes prior to clinical translation. Adopting
this strategy would enhance the likelihood of success since a treatment with
positive outcomes in multiple model systems will be of greater relevance to the
heterogeneous patient population.
Clinical trials should be
designed and stratified so that the therapeutic effect of any agent is tested
in a patient subset that is homologous for set criteria
Clinical
ALS trials should be conducted in a manner that reflects the pathobiological
heterogeneity underlying the neurodegenerative cascade. As discussed elsewhere,
current evidence suggests that the disease presents along a continuum which
ranges from “pure” ALS to frontotemporal dementia (6, 101). Employing a
heterogeneous study population in a clinical trial with undefined disease
mechanisms will not see positive outcomes unless all disease pathways converge
for the ultimate phenotypic expression. It is possible that this may not be the
case in ALS. That said, stratifying patient cohorts based on both the
underlying causal mechanism(s) and degree of disease progression (i.e. due to
validated biomarkers), would allow clinical researchers to develop targeted
strategies that address definite causal mechanisms within a specific context.
Conclusion
We
have reviewed that the most commonly used animal model in pre-clinical ALS
therapeutic development is only representative of a small proportion of
clinical cases, and has been shown to be a poor predictor of clinical success.
To date, Riluzole is the only FDA-approved therapeutic, but one which has only
a modest effect on overall lifespan and disease progression. All other
therapeutics with positive pre-clinical outcomes in the murine model have not
successfully translated to the clinical setting. The reasons for this are
multifaceted in nature and not limited to a publication bias against negative
pre-clinical data, a lack of reproducibility between research groups, a
reductionist paradigm in pre-clinical testing, and a patient population
affected by potentially multiple causal factors. As our understanding of the
complex etiological factors in ALS evolves over the coming years, we can
investigate multiple therapeutic compounds acting on diverse disease mechanisms
and perhaps more effectively intervene in a large proportion of the affected
populace.
Conflict
of Interest
CAS
is the Co-founder of Neurodyn Inc.
Acknowledgement
The
authors thank the Luther Allyn Shrouds Dean Estate for their support.
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