Keywords
X-linked adrenoleukodystrophy, X-ALD, ABCD1, Very long chain fatty
acids, VLCFA, Neurodegeneration, Demyelination, Myelopathy
Introduction
X-linked adrenoleukodystrophy (X-ALD) is a neurodegenerative disease
that mostly affects males. There are three main forms with varying
degrees of severity: cerebral adrenoleukodystrophy (cALD),
Adrenomyeloneuropathy (AMN), and Addison’s disease.(Berger,
Forss-Petter, & Eichler, 2014; Köhler, Curiel, & Vanderver, 2018;
Kemp, Huffnagel, Linthorst, Wanders, & Engelen, 2016) The gene involved
in X-ALD pathology, ABCD1 , encodes adenosine triphosphate (ATP)-
binding cassette subfamily D member 1 (ABCD1) protein, a peroxisomal
transmembrane protein. Defective ABCD1 protein fails to transport fatty
acyl-CoAs, including very long chain fatty acid (VLCFA)-CoAs into
peroxisomes from the cytoplasm for β-oxidation with consequent
accumulation of VLCFAs in the cells,(Wiesinger, Kunze, Regelsberger,
Forss-Petter, & Berger, 2013) leading to neuron damages. To date, the
underlying pathology of X-ALD is poorly understood. A lack of reliable
cALD animal models is a hurdle of X-ALD studies. In this review, we will
summarize current knowledge of X-ALD, as well as pre-clinical and
clinical studies. We will also introduce a novelABCD1- rabbit model created by our lab and
discuss potential development of future X-ALD investigation.
Clinical and Biochemical Features of X-ALD
The diagnostic markers of X-ALD include an increase in VLCFAs in blood
plasma, especially lignoceric acid (C24:0) and hexacosanoic acid (C26:0)
in tissues and body fluid. Since the amount of shorter fatty acid, such
as docosanoic acid (C22:0) or even shorter (carbon chain length
<22), is little affected by dysfunctional ABCD1 proteins,
C26:0/C22:0 is particularly an important index of X-ALD diagnosis.
However, this index does not necessarily reflect disease severity.
Recently, levels of
1-hexacosanoyl-2-hydroxy-sn -glycero-3-phosphocholine (C26:0
LysoPC) and C26:0-carnitine in dry blood spot are proposed to be more
sensitive biomarkers that correlate with VLCFA levels in brain and
spinal cord in X-ALD mice.(van de Beek et al., 2016) More work is needed
to investigate the relationship between the levels of these two markers
and disease progression.
Cerebral demyelination is a hallmark of cALD and can be diagnosed on
magnetic resonance imaging (MRI).(Liberato et al., 2019; Paik, Kim,
Yoon, & Kim, 2001) Demyelination may be present in the brain of cALD
patients, commonly in the corpus callosum.(Nowak, Löbel, Wölfl,
Schlegel, & Warmuth-Metz, 2015; Santosh Rai, Suresh, Bhat, Sekhar, &
Chakraborti, 2013) Such lesions can lead to motor impairment, memory
loss and seizures. Patients often have rapid spread of demyelination in
the brain with death occurring within a few years.
In AMN, no such damage is evident in the brain although axonopathy is
common in the spinal cord and peripheral nerves.(Castellano et al.,
2016) Compared with cALD, symptoms of AMN are less severe. Patients
usually experience weak muscle control and poor control of urination.
Patients will experience difficulty with activities of daily living but
will not necessarily progress to a vegetative state or succumb to the
disease on disease onset. However, the chance of developing cerebral
demyelination after AMN onset is not neglectable.(de Beer, Engelen, &
van Geel, 2014)
Myelopathy and adrenal insufficiency are usually defined as common early
symptoms of X-ALD.(Huffnagel, Laheji, et al., 2019) Most cALD cases,
especially those occur in adulthood, already have myelopathy with gait
disorder before cerebral demyelination.(Engelen, Kemp, & Poll-The,
2014) This comes to a thought that monitoring myelopathy level during
early phase of the disease can somehow estimate the time or the
probability of cALD onset. Recently, there are studies developing method
to quantify progression of myelopathy in X-ALD.(Huffnagel, van
Ballegoij, et al., 2019; van de Stadt et al., 2020) These studies aim to
provide new markers in detecting disease progression in future clinical
studies.
For adrenal insufficiency, elevation of plasma adrenocorticotropic
hormone and hyperpigmentation are the characteristics. Rarely, it was
reported that generalized skin hyperpigmentation could be the only sign
of X-ALD. In that case, a boy that had hyperpigmentation at gums, elbows
and knees was diagnosed with X-ALD subsequently, although he appeared to
be healthy when this manifestation was observed.(Lee, Ko, & Lee, 2020)
These reports have raised the awareness that patients who have signs of
adrenal insufficiency, especially in young boys, should also be screened
for X-ALD, in order to prevent delayed therapeutic treatments. Early
diagnosis is also important for their siblings to have early DNA
screening for X-ALD.
The main damaged site in cALD is the central nervous system (CNS).
Microglia and macrophages play an important role in the progress of
demyelination in cALD. Aberrant activation of microglia is considered to
be involved in myelin degradation process,(Bergner et al., 2019; F. S.
Eichler et al., 2008) and macrophages were found to have lost the
plasticity in X-ALD patients.(Weinhofer et al., 2018) cALD macrophages
lost the ability to switch to anti-inflammatory activation state to
phagocytose destructive myelin, which may lead to progressive myelin
damage. The blood-brain barrier permeability was also found to be
altered at the demyelinating edge of the brain.(Musolino et al., 2015)
These findings provide new insights for the development of novel
therapeutic targets.
On cellular level, oxidative stress is proven to be another hallmark of
X-ALD.(Deon, Marchetti, Donida, Wajner, & Vargas, 2016; Galea et al.,
2012) Impaired mitochondrial oxidative phosphorylation and mitochondrial
depletion have been demonstrated in X-ALD mice and cells.(Baarine,
Beeson, Singh, & Singh, 2015; Fourcade, Ferrer, & Pujol, 2015) One
example marker is the significant increase of oxidized glutathione, a
strong antioxidant, in lymphocytes.(Petrillo et al., 2013) Heat shock
proteins, which would response with cellular stress, was also found to
be upregulated in X-ALD astrocyte before demyelination process.(Görtz et
al., 2018)
X-ALD is a complex disease affecting different cell types and involving
different pathological pathways. The level of ABCD1 gene mutation
is not necessarily correlated with the severity of the
disease,(Wiesinger, Eichler, & Berger, 2015) even a single mutation can
lead to cALD, AMN, or Addison’s disease only.(Ozdemir Kutbay, Ozbek,
Sarer Yurekli, & Demirbilek, 2019) It has also been reported that even
in genetically identical twins with similar environmental impacts,
different forms of X-ALD may occur.(Di Rocco, Doria-Lamba, & Caruso,
2001; Korenke et al., 1996) All these prove that genetic background is
not the sole determinant of the phenotypic heterogeneity of X-ALD. There
may be some underlying pathology that has not been clearly identified.
In 2010, Singh and Pujol suggested a “Three-Hit Hypothesis” that
described an ABCD1 mutation that was not the sole cause of
X-ALD.(Singh & Pujol, 2010) The complex nature of the disease makes
fully understanding its pathophysiology more difficult.
Current Clinical Treatments and Limitations
The first clinical case of X-ALD was reported in the early
1990s.(Aubourg et al., 1990) A 7.5-year-old boy diagnosed with cALD
received a bone marrow transplant (BMT). The therapy was successful and
favorable results persisted for at least eight years. BMT and allogeneic
hematopoietic stem cell transplantation (HSCT) were shown to have
successfully reduced levels of VLCFAs as well as oxidative stress and,
more importantly, to have improved the patients’ performance in later
trials performed during the early stage of cALD.(Kühl et al., 2017;
Rockenbach et al., 2012; van Geel et al., 2015) Nonetheless although
transplantation is an effective therapy for cALD, there remains a risk
of mortality if performed during late-stage disease.(Bladowska et al.,
2015; Jia et al., 2019; Kühl et al., 2018) Serious fatal complications
such as graft-verse-host disease (GVHD) and graft failure can occur.
Transplantation involves stressful surgery that may not be advisable
during the middle and advanced stages of the disease. The availability
of an HLA-matched donor is another barrier to transplantation. Patients
may need to wait a long time for a suitable donor by which time the
disease has advanced beyond early stage.
Another clinical trial with lentiviral hematopoietic stem cell (HSC)
gene therapy.(Cartier et al., 2012; F. Eichler et al., 2017) The first
reported case in 2009 involved two boys (7 and 7.5 years old) for whom
no suitable bone marrow donor was available.(Cartier et al., 2009) Both
patients enjoyed favorable therapeutic effects for up to 30 months. The
advantage of HSC gene therapy over BMT is that the patient’s own stem
cells are used, obviating the need to wait for a suitable donor and
avoiding the risk of GVHD. This approach is currently in phase II/ III
of clinical research. Nonetheless safety is a main concern of lentiviral
HSC gene therapy. Due to the mechanism of lentiviral infection, there is
a risk of developing insertional mutagenesis following gene therapy,
which can trigger tumor formation.(Luis, 2020; Marcucci et al., 2018)
Some modified viral vectors, such as Lenti-D lentivirus, were shown to
reduce the risk of mutagenesis.(F. Eichler et al., 2017) More
evaluations are required to further assess the safeness in clinical
trials.
Dietary therapy is another approach to lower VLCFA level in patients.
Lorenzo’s oil (LO), which is a mixture of glyceryl trioleate oil and
glyceryl trierucate oil in a ratio of 4:1, has been found to reduce
accumulation of VLCFAs by inhibiting endogenous synthesis.(Sassa,
Wakashima, Ohno, & Kihara, 2014) Administration of LO can help
significantly decrease plasma VLCFA levels.(Ahmed et al., 2016; Moser,
Moser, Hollandsworth, Brereton, & Raymond, 2007; Stradomska, Drabko,
Moszczyńska, & Tylki-Szymańska, 2014) It is a comparably safe
treatment, but its limitation is that while it may be able to reduce
VLCFA levels during the early stage of X-ALD and slow disease
progression, it has not been proven effective for cALD. The effect of
arresting disease progression, especially in patients who already have
neurological involvement, is still questionable.(Deon et al., 2008;
Majori et al., 2014)
Current Animal Models of X-ALD
The first X-ALD mouse model was created in 1997 in different
laboratories by disrupting the ABCD1 gene.(Forss-Petter et al., 1997;
Kobayashi, Shinnoh, Kondo, & Yamada, 1997; Lu et al., 1997) All the
three ABCD1- mouse models showed some
biochemical changes and late onset resembling AMN. VLCFA level was
increased in various tissues compared with wild type (WT) mice.
Accumulation of VLCFAs was evident in the brain with an approximate
5-fold increase in C26:0/C22:0 ratio in the model compared with WT mice.
In a human sample, the ratio in a X-ALD brain demonstrated a 3- to 15-
fold increase compared with normal samples, depending on the level of
demyelination in different brain regions.(Kobayashi et al., 1997) These
results were exciting because it showed that the animal model was
moderately successful. Nonetheless surprisingly, theABCD1- mouse model showed no significant
changes in blood plasma C26:0/C22:0 level. In contrast, in human
patients, accumulated VLCFA levels can be easily measured in blood
plasma, with an average 5-fold increase.(Moser, Moser, Singh, &
O’Neill, 1984) This may imply that the accumulation of VLCFAs in mice is
not as severe as that in humans.
No ABCD1- mice showed any behavioral changes at
a young age. On the contrary, in humans, the most severe form of X-ALD,
cALD, often has an onset in childhood, certainly below the age of 10
years. AMN often has an onset at ages 20 to 30 years. InABCD1 - mice, some neurological phenotypes in
the spinal cord started to appear at 16-months of age, and clinical
presentations such as decreased muscle strength were evident in
20-month-old mice,(Pujol et al., 2002) equating to over 50 years of age
in humans. No demyelination was found in the brain.ABCD1- mice can be said to mimic AMN according
to its onset age and the site of neuron degeneration.
Scientists have also endeavored to knockout both ABCD1 andABCD2 genes in mice.(Pujol et al., 2004) These double knockout
mice have dysfunctional ABCD1 proteins, as well as its close homolog,
ABCD2 proteins.(Kawaguchi & Morita, 2016; Morita & Imanaka, 2012) It
was expected that this genetic modification would lead to more severe
symptoms, similar to the characteristics of cALD. Excitingly, compared
with single knockout ABCD1- mice, the double
knockout mouse model exhibited earlier disease onset at 12-months of
age. In addition, the phenotypes, such as enhanced VLCFA level in spinal
cord and level of oxidative damage, were more obvious. Nonetheless
disappointingly, and similar to ABCD1- mice, no
demyelination was found in the brain. Both models developed AMN-like
pathology (Table 1).
In 2017, a new zebrafish model of X-ALD was developed. Absence of ABCD1
proteins in zebrafish results in an elevated level of C26:0 by 1.5 to
2-fold in whole animal extracts.(Strachan et al., 2017) Altered myelin
development was found in this new model. Excitingly, this model displays
impaired motor function very early, within a week of life. In 2020, the
deletion of the pmp-4 gene in worm, which is an orthologue ofABCD1, also demonstrated some AMN phenotypes, such as a 1.25-fold
increase in C26:0 accumulated in lysophosphatidylcholine from whole body
extract, axonal dysregulation, and impaired locomotion.(Coppa et al.,
2020) Nonetheless further investigations with these two novel models of
X-ALD are required. They may be a potential model for drug screening and
molecular study of X-ALD but are not appropriate for assessing effect of
therapy at this stage.
Apart from ABCD1 and ABCD2, studies suggested that double
mutation of bubblegum (bgm) and double bubble (dbb) that
cause acyl-CoA synthetases (ACS) dysfunction in Drosophila showed
some X-ALD phenotypes, such as accumulation of VLCFAs and loss of
neurons and neuron supporting cells in the brain.(Sivachenko et al.,
2016) ACS is an enzyme that activates free fatty acids to become the
substrates for ABCD1 transporter. It is particularly exciting that this
model showed neuron impairment in the brain, which is potentially
important for studying the relationship between evaluated VLCFA levels
and neuron damages in the brain. More interestingly, due to the fact
that X-ALD is not genotype-phenotype correlated, scientists used this
fly model to study the gene-environment interactions by exposing the
flies in different light patterns.(Gordon, Valdez, & Letsou, 2018) The
results demonstrated that environmental stress can modulate the
phenotype expressions in the same genetic background.
Pre-clinical studies of X-ALD for New Therapeutic Development
To understand the pathogenesis of X-ALD, reliable cell and animal models
are essential to find out the causes of the disease. Apart from patient
fibroblasts and patient-derived cell models such as a patient’s induced
pluripotent stem cells (iPSCs),(Baarine, Khan, Singh, & Singh, 2015;
Son et al., 2017; Yeon et al., 2019) researchers started to rely onABCD1- andABCD1-/ABCD2 -/- mouse models
and their cells to study pathologies and develop promising therapeutic
strategies for X-ALD.(Morita, Shinbo, Asahi, & Imanaka, 2012; Muneer et
al., 2014) To date, different underlying pathological pathways have been
proposed, and corresponding drug treatments and therapies investigated
using X-ALD models. The ABCD1- mouse model is
often used to assess biochemical signs, but theABCD1-/ABCD2- /-mouse model is commonly used to determine a therapeutic effect by
studying the clinical signs. We summarize some examples of in
vivo therapeutic studies that have targeted different pathways over the
past few years [See Additional file 1].
Gene therapy is a direct approach for treating X-ALD. Studies showed
that overexpression of the ABCD1 gene using adeno-associated virus (AAV)
serotype 9 in an ABCD1- mouse model is
promising. In these studies, level of VLCFAs inABCD1- mice in multiple tissues was lowered
after injecting AAV intravenously.(Gong et al., 2015) Nonetheless blood
plasma level of VLCFAs were unchanged. This may be a limitation of theABCD1- mouse model where there is no elevation
in plasma VLCFAs compared with WT mice and hampers its reliability to
evaluate the effect of the gene therapy. Studies also showed that
injecting AAV intrathecally into ABCD1 - mice
can lower 20% of VLCFAs in spinal cord, which is the main lesion site
of AMN.(Gong et al., 2019) It is believed that this injection method
will be more effective in the treatment for AMN.
Reducing VLCFA level is a simple concept for halting symptom
development. Overexpression of ABCD2 has been an approach to compensate
the function loss of ABCD1. In 2017 and 2020, studies showed that
upregulation of ABCD2 by thyromimetics, such as sobetirome and its
prodrug in ABCD1- mice significantly reduced
VLCFA level in various tissues,(Hartley, Kirkemo, Banerji, & Scanlan,
2017; Hartley et al., 2020) up to 50% in peripheral tissues and 15-20%
in the brain. In their studies, the ABCD1-mouse model did not show serious impairment of motor functions, so it
was difficult to access its effectiveness in curing X-ALD. Nonetheless
it is a promising approach that should be further studied in reliable
X-ALD disease models.
Oxidative stress was suggested as a key factor of neurodegenerative and
metabolic diseases inducing X-ALD. Drugs that target antioxidant
pathways are thought to be effective in arresting X-ALD disease
progression.(López-Erauskin et al., 2012; Marchetti et al., 2015) In the
last few years, dimethyl fumarate (DMF) has been reported to
successfully prevent oxidative damage to proteins and
lipids,(Ranea-Robles et al., 2018) and more importantly, arrest axonal
degeneration and restore normal motor activities inABCD1-/ABCD2 -/- mice.
Novel therapeutic strategies have also tried to target mitochondria
related pathways to cure X-ALD. For example, a common drug for type 2
diabetes, Pioglitazone, was found to be a potential drug for
X-ALD.(Morató et al., 2013) Pioglitazone works by regulating
PPARγ/PGC-1α-dependent-pathways and thereby stimulating mitochondrial
biogenesis in ABCD1- deficient cells. The mouse
models showed therapeutic effects upon Pioglitazone treatment.
Researchers continue to search for methods to normalize redox balance in
X-ALD mouse models. Most recently, high-dose biotin has been reported to
help stimulate mitochondrial biogenesis and restore redox
balance.(Fourcade et al., 2020; Sghaier et al., 2019)ABCD1-/ABCD2 -/- mice clinical
presentations were improved upon such treatment. These successful trials
support the concept that restoring mitochondrial functions are one of
the key approaches in X-ALD treatment.
Apart from mitochondria, the endoplasmic reticulum (ER) has been proven
to be another important organelle that is associated with VLCFA-induced
oxidative stress.(van de Beek et al., 2017) Recent findings suggest that
administration of a bile acid tauroursodeoxycholate (TUDCA), which is an
ER stress inhibitor, can halt axonal degeneration and locomotor
impairment inABCD1-/ABCD2-/ -mice.(Launay et al., 2017)
Autophagy dysfunction has been shown to be related to some
neurodegenerative diseases such as Alzheimer’s and Parkinson’s.(Nixon,
2013; Orr & Oddo, 2013; Sarkar, 2013) Autophagy was shown to provide
protection to VLCFA enriched cells.(Doria et al., 2019) Researchers have
tried to target this mechanism to cure X-ALD. One example of attempts to
restore autophagy in X-ALD mice is the use of Temsirolimus, an mTOR
inhibitor. Results showed an ability to stop disease progression in
double knockout mice models.(Launay et al., 2015)
Novel Clinical Trials and Development
The ABCD1- andABCD1-/ABCD2-/- murine models
pave the way to new therapeutic development. From the pre-clinical
studies, we learn that oxidative stress plays an important role in X-ALD
pathogenesis. Recently, a phase II study of AMN suggested that
administration of a combination of antioxidants, α-tocopherol,
N-acetylcysteine and α-lipoic acid to patients improved their motor
performance,(Casasnovas et al., 2019) which agreed with the pre-clinical
trial in murine model.(López-Erauskin et al., 2011)
To date, BMT, HSCT and HSC gene therapies remain the only effective
treatments for cALD. A recent successful case of HSCT reported in 2020
was of a 31-year-old man diagnosed with X-ALD at the early stage of the
disease.(Ciftciler et al., 2020) For patients who do not have suitable
donors, haploidentical allogenetic HSCT, together with post-transplant
cyclophosphamide treatment or infusion of umbilical cord blood, are
currently proposed and under investigation.(Chen et al., 2019; Fernandes
et al., 2018; Jiang et al., 2015) Transplantation is undoubtedly
beneficial, but a post-transplantation follow-up study showed that
worsening in neurocognitive function can still occur in cALD survivors
who have undergone HSCT at early stage. This observation is more likely
to happen in younger boys, and patients with more advanced disease prior
the time of transplantation.(Pierpont et al., 2017; Raymond et al.,
2019)
There were reports that patients with advanced stage of cALD were
treated by cord blood transplantation with reduced-intensity
conditioning regimen.(Awaya et al., 2011; Niizuma et al., 2012) These
trials not only demonstrated promising treatments for advanced stage of
the disease with favorable effects in the follow-up period, but also
provide an alternative therapy for patients who lack suitable donor for
HSCT. More large-scale and robust studies should be carried out to
further assess its efficacy.
On the other hand, there was a new therapeutic approach for patients who
were in advanced stage of cALD in 2020. Disappointingly it was not
successful. This study included three cALD patients who received
Vorinostat orally, a drug that could stimulate ABCD2 expression. In this
study, the drug failed to stop disease progression and all patients
developed thrombocytopenia.(Zierfuss et al., 2020) Until now, treatments
for late stage of cALD is still lacking.
In view of this, screening of X-ALD in babies is thought to be
beneficial for timely treatment. In 2013, New York implemented newborn
screening (NBS) to screen for newborn X-ALD patients, using dry blood
spots as the testing samples.(Vogel et al., 2015) In 2016, X-ALD was
added to the federal Recommended Uniform Screening Panel (RUSP), more
states began NBS for X-ALD. An NBS pilot study was carried out in North
Carolina and reported in 2020.(S. Lee et al., 2020) With the experience
of NBS in the US, other countries such as Japan and Netherland also
started to develop reliable protocols for screening X-ALD
newborns.(Barendsen et al., 2020; Wu et al., 2017) With careful
considerations, such as ethical and feasibility, this trend may spread
to more countries in the future.
Challenges in the development of new therapies for X-ALD
Given that most current treatments are effective only at the very early
stage of X-ALD, they are clearly not ideal for such a rapidly
progressing disease. For cALD, the brain is the major affected site.
This makes treatment more difficult because of the inability of drugs to
penetrate the blood-brain barrier.(Banks, 2016) Drug distribution within
the CNS is also another attribute that needed to be considered.(Warren,
2018)
Study of X-ALD is currently hampered by the lack of a suitable animal
disease model. As mentioned before, the ABCD1-mouse model shows only symptoms that mimic AMN.ABCD1-/ABCD2-/ -double knockout mice show earlier onset and more severe biochemical
alterations and are thought to be a better model for AMN studies.
Nonetheless it is not perfect. There is no direct evidence that ABCD2
genotyping is associated with X-ALD phenotypes.(Maier et al., 2008) The
absence of ABCD2 may cause alterations to other pathways that may cause
phenotypes that are irrelevant to X-ALD.
Apart from animal models, scientists recently created a novelABCD1-/ABCD2-/- microglial
model that successfully showed biochemical changes similar to those in
X-ALD cells, such as accumulation of VLCFAs and mitochondrial
modifications.(Raas et al., 2019) This would be a useful cell line to
further investigate the pathology of X-ALD. Nevertheless, the accuracy
of mimicking tissue cells in vivo is limited by 2D cell culture
models.
Current studies tend to focus on the upstream pathology pathways, such
as ABCD1 gene defect, VLCFA accumulation, mitochondria depletion and
oxidative stress. All these biochemical characteristics can be studied
in both X-ALD cells and animal models. The downstream pathologies that
lead to cerebral demyelination in cALD, the life-threatening form of
X-ALD, remain unknown. Although favorable therapeutic effects can be
seen in AMN mouse models, we still cannot estimate the effectiveness in
curing cALD. A reliable animal model of cALD is vital to study the
underlying pathology and to develop novel therapies.
Perspectives
The difficulties in creating an ideal animal model for a complex disease
such as X-ALD are understandable. Many factors apart from genetic
involvement need to be considered when efforts to create a cALD animal
model. It is possible that living environment, lifestyle and eating
patterns should also be taken into account when developing new animal
models of X-ALD. Much work remains to be done. The failure ofABCD1- andABCD1-/ABCD2-/- mouse models
to develop cALD might imply some compensation mechanisms in response to
the absence of ABCD1 proteins in rodents that differ to human beings.
In this regard, our lab tried to use rabbits (Oryctolagus
cuniculus ) to create X-ALD model by knocking out ABCD1 using
CRISPR/Cas9 technologies. Three male rabbits with mutations inABCD1 gene exon 1, namely Rabbit 1, 2 and 3, were successfully
created. All of them have 4 to 4.5-fold increase of C26:0/C22:0 in blood
plasma, which is more similar to human X-ALD compared with bothABCD1 - andABCD1 -/ABCD2 -/- mouse
models. Nonetheless, no rabbits showed cALD phenotypes in the 2-year
observational period. No behavioral and locomotion disability were
observed, and body weights were steady. No abnormalities were found in
brain MRI (Fig. 1 ). More examination is needed to evaluate the
pathophysiological changes in this novel model.
Recently there was a case reporting a chimpanzee with an ABCD1mutation developed cALD.(Curiel et al., 2017) This fact gives hope that
chimpanzee could be a suitable model for cALD. Despite the increasing
ethical concerns, few numbers of offspring in long term and high cost,
creating an X-ALD model using ABCD1-absent non-human primates may still
be a direction for better disease studies.
Before the establishment of reliable X-ALD, particularly cALD animal
models, 3D cell culture models may play a contributive role in the
disease studies. Human PSC-derived brain organoid may serve as a disease
model that mimics in vivo environment, offering an opportunity to
better understand cell-to-cell interactions or metabolic profile in a
diseased brain, and to develop new therapeutic interventions in near
future.
Conclusions
There is currently no effective treatment for severe cALD that remains a
rapidly progressing fatal disease. The full pathophysiology of X-ALD is
poorly understood. The mechanism that drives ABCD1 mutation into
different types of X-ALD remains a mystery. The absence of a reliable
cALD animal model is the biggest hurdle to cALD pathological study and
therapeutic development. Researchers have relied onABCD1- andABCD1-/ABCD2-/- mouse models
in X-ALD studies but these fail to simulate cALD phenotypes in human.
Although there are a number of promising therapeutic strategies being
tested in mouse models, these novel interventions can only be applied to
AMN phenotypes. There remains a lack of investigations for cALD
(Fig. 2 ). Hopefully, the emerging new cell and animal models
will provide potential means to investigate the disease pathophysiology
and develop new therapies in the future.