Opening New Horizons in the Treatment of Childhood Onset Leukodystrophies

Opening New Horizons in the Treatment of Childhood Onset Leukodystrophies Stina Schiller1 Marco Henneke1 Jutta Gärtner1
1Department of Paediatrics and Adolescent Medicine, University Medical Centre Göttingen, Georg August University Göttingen, Germany
Neuropediatrics
Introduction
Leukodystrophies (LDs) comprise a group of raremonogenetic neurologicaldisorders that primarilyaffect thewhitematterof the central nervous system (CNS). Single LDs are associated with an additional involvement of the peripheral nervous system (PNS). Besides myelin, the major component of white matter, other structural components, andmetabolic pathways canbeaffected.1–3 LDs canmanifest at all ages,whichmeansas early as in the fetal and neonatal period or as late as in adolescence and adulthood. The clinical manifestations are highly variable. Besidesprogressive loss ofmotor and cognitive function, LDsoftengo alongwithvisual orhearing impairment and epilepsy. Brain imaging, especially magnetic resonance imaging (MRI) pattern recognition, plays a pivotal role in the diagnostic process andclassificationof LDs.3Classical LDshave defects in neurometabolism including lysosomal storage dis- orders (LSDs) like metachromatic leukodystrophy (MLD), per-
oxisomal disorders like X-linked adrenoleukodystrophy (X- ALD), or defects in myelin protein like Pelizaeus-Merzbacher disease (PMD). Over the past decade, the impressive advances in next-generation sequencing technologies led to continuous expansion of the spectrumofdefinedwhitematter disorders.4
The knowledge of the underlying disease gene, the corre- sponding protein, and/or the affected metabolic pathways facilitates the establishment of new therapeutic approaches. Thus, an early diagnosis is crucial, especially in those disorders where treatment or treatment approaches are possible.
Innovative Treatment Options for Leukodystrophies
In general, the treatment of LDs is hampered by the restricted access of substances to the CNS. Therapeutic approaches have to implicate the need for overcoming the blood–brain barrier. Thus, potential therapies are based on small molecules with
Keywords
leukodystrophy myelin neurometabolism enzyme replacement gene therapy novel therapies
Abstract Leukodystrophies (LDs) predominantly affect the white matter of the central nervous system and its main component, the myelin. The majority of LDsmanifests in infancy with progressive neurodegeneration. Main clinical signs are intellectual and motor function losses of already attained developmental skills. Classical LDs include lysosomal storage disorders like metachromatic leukodystrophy (MLD), peroxisomal disorders like X-linked adrenoleukodystrophy (X-ALD), disorders ofmitochondrial dysfunction, andmyelin protein defects like Pelizaeus-Merzbacher disease. So far, there are only single LD disorders with effective treatment options in an early stage of disease. The increasing number of patients diagnosed with LDs emphasizes the need for novel therapeutic options. Impressive advances in biotechnology have not only led to the continuous identification of new disease genes for so far unknown LDs but also led to new effective neuroprotective and disease-modifying therapeutic approaches. This review summarizes ongoing and novel innovative treatment options for LD patients and their challenges. It includes in vitro and in vivo approaches with focus on stem cell and gene therapies, intrathecal substrate or enzyme replacement, and genome editing.
received August 16, 2018 accepted after revision March 4, 2019
© Georg Thieme Verlag KG Stuttgart · New York
DOI https://doi.org/ 10.1055/s-0039-1685529. ISSN 0174-304X.
Review Article
D ow
nl oa
de d
by : K
in g'
s C
ol le
ge L
on do
n. C
op yr
ig ht
ed m
at er
ia l.
high lipophilicity and lowmolecular weight or on specific cells like macrophages or microglia. Another therapeutic approach is bypassing the blood–brain barrier by applying lacking substances including enzymes directly into the cerebrospinal fluid (CSF). Thismode of CNS application is extensively used in the treatmentofbrain cancers, in cerebral folatedeficiencyand also with only limited success in the treatment of MLD.
The increasing number of patients diagnosed with LDs emphasizes the need for novel therapeutic options. Besides widespread and complex problems that have to be solved in the development and application of new compounds, the proof of treatment efficacy is further hindered by the rarity of LDs and their highly variable clinical onset and course. Wide-ranging insights into the pathogenic mechanisms of neurodegeneration are of uppermost importance and set the basis for developing effective future neuroprotective and disease modifying therapies. Furthermore, monitoring dis- ease progression and treatment efficacy is crucial for eval- uating novel therapies. Thus, searching for white matter biomarkers is needed. MRI as the follow-up examination of choice mainly detects macroscopic lesions. Neurofilament light chain (NfL) as an unspecific marker of neuroaxonal injury is a potential biomarker for disease activity and axonal damage.5 Elevated NfL levels have been shown in children with white matter abnormalities as in newborns with hypoxic–ischemic encephalopathy6 and in children with acquired demyelinating syndromes7
Intrathecal Enzyme Replacement Therapy The beginning of enzyme replacement therapy (ERT) goes back to the early 1960s.8 In 1991, ERT was first applied as a pharmaceutical for the treatment of patients with Gaucher disease, a LSD.9 At present, ERT is efficiently used for several metabolic diseases, which are characterized by mostly non- cerebral manifestations. The impermeability of the blood– brain barrier for enzymes mandates to circumvent the blood–brain barrier and to administer the lacking enzyme directly into the CSF.
This procedure is currently being tested in clinical studies, for example, in children with MLD using recombinant human arylsulfatase Avia a surgically placed intrathecal drug delivery device. A phase½ studywas completed in2017 (Clinical Trials. gov: NCT01510028, a long-term extension study started recently (ClinicalTrials.gov:NCT01887938). Thisfirst attempts to bypass the blood–brain barrier appear promising,10 but further long-term efficacy and safety data are needed.
Stem Cells Allogeneic hematopoietic stem cell transplantation (HSCT) has shown to be beneficial for single LDs such as late infantile and juvenile MLD, cerebral form of X-ALD, and globoid cell leukodystrophy (GLD/Krabbe disease). The ability of donor- derived microglial cells to circumvent the blood–brain bar- rier and settle in the CNS enables the production and secretion of functional proteins and thereby the replacement of nonfunctional or lacking enzymes by “cross-correction” similar to a local ERT.11 However, the engraftment and functioningof the transplanted cells needmonths. Therefore,
allogeneic HSCT shows low or no efficiency in the treatment of patients with late stages of disease or rapidly progressing disease courses.12 Furthermore, allogeneic HSCT can be complicated by an impeded and delayed identification of matched donors and potential transplantation-related risks like treatment-related toxicity or graft-versus-host disease.
The transplantation of unrelated donor umbilical cord blood (UCB) represents a more accessible source of hema- topoietic stem cells. Some cautiously optimistic results have been reported in presymptomatic patients with late-infan- tile MLD and patients with juvenile MLD with only minimal symptoms.13 Currently, HSCT should be considered as the therapy of choice for early stages of cerebral X-ALD and juvenile or adult onset MLD. Furthermore, HSCT is a possible treatment approach to attenuate the disease course in early stages of early onset GLD.11 Accordingly, HSCT is a suitable treatment option for selected LDs but is, even then, limited to certain subtypes and early onset diseases. Although the general risk of HSCT including mortality significantly declined within the last decade, one has still to take into consideration that HSCT can be accompanied by severe side effects such as graft versus host disease and severe life- threatening infections due to immunosuppression.
Gene Therapy The general aim of gene therapy is to restore the function of the deficient protein, especially the enzymatic activity of a protein through the insertion of the wild-type form of the disease gene. Possible techniques comprise in vivo methods being directly applied to the patient or ex vivo methods isolating, modifying, and reinfusing target cells, which have the capability to migrate into the CNS.14 An important obstacle for in vivo methods is the blood–brain barrier. Moreover, immune responses to vectors or transgene pro- ducts represent major problems. However, the direct and fast transfer of the wild-type gene into the CSF constitutes a beneficial option. “After promising results in MLD mice2 and non-human primates15 an open label study of direct intra- cerebral administration of a replication deficient adeno- associated virus vector expressing ARSA (ClinicalTrials.gov Identifier: NCT01801709) has been discontinued as without effect (personal communication).” Another in vivo clinical trial evaluates the self-inactivating lentiviral vector TYF- ABCD1 in X-ALD patients via intracerebral injection (Clin- icalTrials.gov Identifier: NCT03727555).
Examples for ex vivo methods are the reinfusion of bone marrow-derived CD34þ hematopoietic stem cells transduced with lentiviral vectors carrying human wild-type ARSA com- plementary deoxyribonucleic acid (cDNA) in MLD16 (Clinical- Trials.gov Identifier: NCT01560182) orwild-type ABCD1 cDNA in cerebral X-ALD17 (ClinicalTrials.gov Identifier: NCT018 96102). In particular, children with early-onset, late-infantile, or early juvenile MLD or cerebral X-ALD seems to benefit from lentiviral HSCTs gene therapy.18,19
Gene therapy seems to be an especially beneficial thera- peutic option for patients without matching donors for HSCT. In adult X-ALD patients, one also has to take into account that these patients have higher risks for fatal complications in
Neuropediatrics
D ow
nl oa
de d
by : K
in g'
s C
ol le
ge L
on do
n. C
op yr
ig ht
ed m
at er
ia l.
allogeneic HSCT when compared with children and adoles- cents.19 However, further studies to evaluate the long-term efficacy and safety for each LD are still needed for general recommendations.
Drug Repurposing In viewof LDs being rare diseases, the costs of developing new drugs are disproportionately highwith only few patients to be treatedafterapproval.Moreover, thereareonlysmallnumbers of patients that can be recruited for a clinical trial in a given time range to demonstrate efficacy and safety.
The search for overlaps between common and rare diseases might be a solution for these problems. Single genemutations in rare diseases may disturb well-studied cellular proteins or metabolic pathways overlapping with common diseases for which effective therapies and drugs with proven safety and pharmacokinetic properties have already been established. Such situations open the possibility of identifying new indica- tions for approved drugs and significantly shorten the risky and costly process of drug development in rare LDs.
Examples for successful drug repurposing for patients with LDs are the substitution of folate in cerebral folate deficiency20 and the administration of luteolin and N-acetyl cysteine in NRF2 activation.21 For both examples, the success is evident. It underlines that drug repurposing is disease specific and depends largely on the availability of safe and active substances as well as their accessibility to the brain.
Innovative Treatment Options—Examples of Diseases
Metachromatic Leukodystrophy LSDs are rare, genetically determined diseases that are pro- voked by deficiencies or malfunctions of lysosomal enzymes or transporters.MLD, a characteristic example for an LSD, is an autosomal recessive disorder caused by a deficiency of aryl- sulfatase A due to mutation in the ARSA gene or, in extremely rare cases, mutations in the PSAP gene encoding for the ARSA activator protein saposin B.22 MLD is characterized by an accumulation of sulfatides in microglia, oligodendrocytes, and Schwann cells leading to inflammatory demyelination in the CNS and PNS.23 The clinical course is heterogeneous depending on the age at onset and rate of progression.
The spectrum of clinical symptoms comprises progressive psychomotor retardation, spastic tetraparesis, hearing loss, and blindness.24 There is some evidence for an at least partial genotype–phenotype correlation with different residual enzyme activities, leading to distinct clinical courses.25 A late-infantile form (onset before 3 years of age), a juvenile form (onset before 16 years of age), and an adult form with mainly cognitive and behavioral problems are known.26 The incidence is estimated around 1/100.000 live births in the European population.26
MLD is one of the more common LDs, for which several novel therapeutic approaches are ongoing. Clinical trials regardingERTswith recombinanthumanarylsulfataseAeither by intravenous (ClinicalTrials.gov Identifier: NCT00418561, ClinicalTrials.gov Identifier: NCT00633139) or intrathecal
administration (ClinicalTrials.gov Identifier: NCT01510028, ClinicalTrials.gov Identifier: NCT01887938) are ongoing. The treatment results for conventionalHSCT are controversial.27,28
Other approaches refer to the use of UCB13 or HSCT gene therapy16,18 (ClinicalTrials.gov Identifier: NCT01560182, Clin- icalTrials.gov Identifier: NCT03392987). Currently, HSCT is an available therapy and has proven beneficial for patients with early stage juvenile MLD.27 However, the results of the gene therapy study with lentivirally modified stem cells are pro- mising and should be available soon as a regular therapy. Therefore, presymptomaticearly juvenileMLDpatients should at least be considered to undergo gene therapy before treating them with allogenic HCST.
Krabbe Disease (Globoid Cell Leukodystrophy) GLD represents another characteristic example of an LSD. This autosomal recessive disorder is caused by a deficiency of galactocerebrosidase, which catalyzes the hydrolytic degrada- tion of galactocerebroside accumulating in cerebral macro- phages that transform to multinucleated globoid cells. An alternative pathway leads to the formation of toxic psychosine (galactosphingosine) inoligodendrocytes destructing themye- lin sheaths.29 Galactocerebrosidase is encoded by the GALC gene. Inextremely rare cases,GLD is causedbymutations in the saposinAgene encoding for a cerebrosidase activatorprotein.30
The clinical symptoms of GLD depend on the clinical course. A classification can be made according to the age at onset as infantile, late infantile, juvenile, and adult formwith the latter two being often combined as late forms. Severe phenotypes with early onset show progressive psychomotor impairment with blindness, deafness, ataxia, spastic tetra- paresis, and seizures followedby deathwithin few years. Late onset GLD shows more variable symptoms with progressive neurological disturbances and longer survival.31
HSCT is considered in later32 or infantile forms33,34 but has proven beneficial for patientswith early disease stages. There- fore, some States in the United States implemented newborn screens forGLD. First results indicate that theprompt initiation of HSCT in specialized centers shortly after diagnosis is a particular challenge.35 There is also a clinical trial comprising the transplantation of human placental-derived stem cells (ClinicalTrials.gov Identifier: NCT01586455).
Currently, HSCT is the only available therapy. Neverthe- less, approximately only 10% of enzyme activity is needed to prevent the neurological implications of GLD. Therefore, interventions leading to a small increase of residual activity could attenuate GLD symptoms.36 Experimental therapies with this focus will be discussed further below.
Pelizaeus-Merzbacher Disease PMD is an X-linked LD that is caused by duplications, point mutations, or deletions in the proteolipid protein gene 1 (PLP1) encoding the major protein component of the myelinating oligodendroglia.37 PLP1-related disorders show a genotypic and phenotypic heterogeneity with a clinical continuum ran- ging from severe forms without psychomotor development to purespastic paraparesis. A connatal, a classic, anda transitional subtype can be distinguished. Spastic paraplegia type 2 (SPG2)
Neuropediatrics
D ow
nl oa
de d
by : K
in g'
s C
ol le
ge L
on do
n. C
op yr
ig ht
ed m
at er
ia l.
and PLP1 null disease are mild variants.38 Additionally, auto- somal-recessive mutations in further genes can lead to com- parable phenotypes called PMD-like disease (PMLD), of which PMLD-1 due tomutations of the gap junctionprotein gamma 2 (GJC2) gene is the most common.37
The complete lack of PLP1 leads to milder forms of PMD/ SPG2 than misfolded PLP1 proteins due to certain missense mutations. Accumulation of misfolded protein in the endo- plasmic reticulum and the subsequent triggering of unfolded protein response enforce apoptosis and play a decisive role in the pathogenic mechanisms of PMD.39,40
For PMD there is currently no available efficient therapy. Transplantation of UCB in two affected children showed slow neurocognitive improvements and stable or improved mye- lination,41 but the effect is controversially discussed.42 Allo- geneic hematopoietic stem cell and umbilical cord-derived cell transplantation can be beneficial in myelin disorders with single enzyme defects. Nevertheless, a convincing pathomechanical basis for their benefit in myelin disorders with structural protein defects like PMD is still missing. A clinical trial with adjacent intrathecal administration of UCB cells in patients undergoing standard transplantation of unrelated UCB (ClinicalTrials.gov Identifier: NCT02254863) is underway. Another trial is the intracerebral transplanta- tion of neuronal stem cells (ClinicalTrials.gov Identifier: NCT01005004). Even though a clinical benefit could not be shown yet,43 this innovative treatment proved to be safe and the further course after transplantation will be explored in these patients (ClinicalTrials.gov Identifier: NCT01391637).
Another approach refers to a cholesterol-enriched diet in a mouse model for PMD, which improved the clinical phe- notype and prevented disease progression.44 Unfortunately, compassionate use in single patients showed no effect (per- sonal communication).
X-Linked Adrenoleukodystrophy X-ALD is characterized by an impaired peroxisomal β-oxida- tion of very long-chain fatty acids (VLCFAs; C22) accumu- lating in all body fluids and tissues, especially in the cerebral white matter and the adrenal cortex. Primary cause is mutations in the ABCD1 gene encoding a peroxisomal pro- tein, which is involved in the transport of VLCFAs.
There are at least six distinct clinical phenotypes ranging from the severe childhood cerebral form to asymptomatic individuals, which can vary among affected members within the same family. Most common phenotypes are the child- hood onset cerebral form and the adult onset adrenomyelo- neuropathy. The clinical symptoms for the childhood onset cerebral form include behavioral changes, visual and hearing impairment, dementia, polyneuropathy, spastic paraplegia, and Addison disease. Due to the X-linked mode of inheri- tance severe courses predominantly affect males even though a majority of heterozygous females develop symp- toms later in life. The individual course of disease is unpre- dictable; there is no genotype–phenotype correlation.
Cerebral X-ALD shows good responses to treatment with HSCTwhen administered at first signs of cerebral demyelina- tion. In later disease stages, the success of HSCT is limited45
underlining the significance of early diagnoses and immediate identifications of appropriated allogenic stemcell donors. Best outcomes can be expected with cells from related unaffected andHLA (human leukocyte antigen)-identical donors,who are not always available.45 Thus, alternative approaches are on the way, for example, using autologous hematopoietic stem cells transduced ex vivo with a lentiviral vector including ABCD1 cDNA. After individual applications19 a phase study showed first promising results17 (Clinical Trials.gov: NCT01896102).
As withMLD, the only therapy currently available is HSCT. The results of the ongoing gene therapy trial with lentivirally modified stem cells are promising and should also be avail- able as a regular therapy soon.
Canavan Disease Canavan disease is a classical autosomal recessive LD. Pri- mary cause is mutations in the ASPA gene encoding aspar- toacylase that hydrolyzes N-acetyl-L-aspartic acid to L- aspartic acid and acetate. The resulting highly elevated urinary level of N-acetyl-L-aspartic acid is the diagnostic biomarker.46 Canavan disease is characterized by diffuse spongiform white matter degeneration and intramyelinic edema. The severity of the disease course seems to correlate with the type of mutation and the resulting residual enzyme activity.47 The spectrum of symptoms includes macroce- phaly, severe developmental delay, hypotonia, blindness, and seizures. So far, there is no treatment. The lack of available successful therapies reflects the lack of knowledge of the exact disease mechanisms.46 Therapeutic approaches comprise gene therapies and substances to reduce the pro- duction of L-aspartic acid and acetate to prevent N-acetyl-L- aspartic acid accumulation46 (ClinicalTrials.gov Identifier: NCT00278707, ClinicalTrials.gov Identifier: NCT00724802).
Leukodystrophies Due to Mitochondrial Defects Mitochondria have a key function in the energy metabolism of cells. A wide variety of defects in either mitochondrial- or nuclear-encoded proteins can cause mitochondrial LDs. Examples are Kearns–Sayre syndrome, mitochondrial ence- phalomyopathywith lactic acidosis, and stroke-like episodes and Leigh’s disease.48Mitochondrial disorders show a highly heterogeneous spectrum of clinical phenotypes and parti- cularly affect tissues with high energy demand like the CNS or skeletal and heart muscles. In severe courses, mitochon- drial disorders show congenital manifestations but disease onset in middle age is also possible.49 The underlying com- plex etiopathology of mitochondrial disorders complicates targeted therapies. Currently a multitude of different ther- apeutic strategies is under development. A detailed review has been published recently.50
Aicardi-Goutières Syndrome Aicardi-Goutières syndrome (AGS) is the prototype of a novel group of monogenetic autoinflammatory and autoimmune disorders termed type I interferonopathies comprising all phenotypes associated with a pathological upregulation of type I interferon signaling. AGS is caused by autosomal recessive or, in rare cases, autosomal dominant/de novo
Neuropediatrics
D ow
nl oa
de d
by : K
in g'
s C
ol le
ge L
on do
n. C
op yr
ig ht
ed m
at er
ia l.
arisingheterozygousmutations in sevendistinct genes encod- ing proteins involved in nucleotide metabolism or sensing (TREX1, SAMHD1, ADAR1, RNASEH2A, RNASEH2B, RNASEH2C, or IFIH1).51 The clinical phenotype of AGS resembles conge- nital viral infection and is highly variable even…