Clinical, radiological and genetic aspects of leukodystrophies

The authors summarize the pathomechanism of the myelination process, the clinical, radiological and the genetical aspects of the leukodystrophies, as in 18q deletion syndrome, adrenoleukodysrtophy, metachromatic leukodystrophy, Pelizaeus-Merzbacher leukodystrophy, Alexander disease and olivo-ponto-cerebellar atrophy (OPCA).

Properties of astrocytes cultured from GFAP over-expressing and GFAP mutant mice

Alexander disease is a fatal leukoencephalopathy caused by dominantly-acting coding mutations in GFAP. Previous work has also implicated elevations in absolute levels of GFAP as central to the pathogenesis of the disease. However, identification of the critical astrocyte functions that are compromised by mis-expression of GFAP has not yet been possible. To provide new tools for investigating the nature of astrocyte dysfunction in Alexander disease, we have established primary astrocyte cultures from two mouse models of Alexander disease, a transgenic that over-expresses wild type human GFAP, and a knock-in at the endogenous mouse locus that mimics a common Alexander disease mutation. We find that mutant GFAP, as well as excess wild type GFAP, promotes formation of cytoplasmic inclusions, disrupts the cytoskeleton, decreases cell proliferation, increases cell death, reduces proteasomal function, and compromises astrocyte resistance to stress.

Classification of childhood white matter disorders using proton MR spectroscopic imaging

BACKGROUND AND PURPOSE

Childhood white matter disorders often show similar MR imaging signal-intensity changes, despite different underlying pathophysiologies. The purpose of this study was to determine if proton MR spectroscopic imaging ((1)H-MRSI) may help identify tissue pathophysiology in patients with leukoencephalopathies.

MATERIALS AND METHODS

Seventy patients (mean age, 6; range, 0.66-17 years) were prospectively examined by (1)H-MRSI; a diagnosis of leukoencephalopathy due to known genetic defects leading to lack of formation, breakdown of myelin, or loss of white matter tissue attenuation (rarefaction) was made in 47 patients. The diagnosis remained undefined (UL) in 23 patients. Patients with definite diagnoses were assigned (on the basis of known pathophysiology) to 3 groups corresponding to hypomyelination, white matter rarefaction, and demyelination. Choline (Cho), creatine (Cr), and N-acetylaspartate (NAA) signals from 6 white matter regions and their intra- and intervoxel (relative to gray matter) ratios were measured. Analysis of variance was performed by diagnosis and by pathophysiology group. Stepwise linear discriminant analysis was performed to construct a model to predict pathophysiology on the basis of (1)H-MRSI, and was applied to the UL group.

RESULTS

Analysis of variance by diagnosis showed 3 main metabolic patterns. Analysis of variance by pathophysiology showed significant differences for Cho/NAA (P < .001), Cho/Cr (P < .004), and NAA/Cr (P < .002). Accuracy of the linear discriminant analysis model was 75%, with Cho/Cr and NAA/Cr being the best parameters for classification. On the basis of the linear discriminant analysis model, 61% of the subjects in the UL group were classified as hypomyelinating.

CONCLUSION

(1)H-MRSI provides information on tissue pathophysiology and may, therefore, be a valuable tool in the evaluation of patients with leukoencephalopathies.

Adaptive autophagy in Alexander disease-affected astrocytes

The ubiquitin-proteasome and autophagy-lysosomal pathways are the two main routes of protein and organelle clearance in eukaryotic cells. The proteasome system is responsible for unfolded, short-lived proteins, which precludes the clearance of oligomeric and aggregated proteins, whereas macroautophagy, a process generally referred to as autophagy, mediates mainly the bulk degradation of long-lived cytoplasmic proteins, large protein complexes or organelles.(1) Recently, the autophagy-lysosomal pathway has been implicated in neurodegenerative disorders as an important pathway for the clearance of abnormally accumulated intracellular proteins, such as huntingtin, tau and mutant and modified alpha-synuclein.(1-6) Our recent study illustrated the induction of adaptive autophagy in response to mutant glial fibrillary acidic protein (GFAP) accumulation in astrocytes, in the brains of patients with Alexander disease (AxD), and in mutant GFAP knock-in mouse brains.(7) This autophagic response is negatively regulated by mammalian target of rapamycin (mTOR). The activation of p38 MAPK by GFAP accumulation is responsible for mTOR inactivation and the induction of autophagy. We also found that the accumulation of GFAP impairs proteasome activity.(8) In this commentary we discuss the potential compensatory relationship between an impaired proteasome and activated autophagy, and propose that the MLK-MAPK (mixed lineage kinase-mitogen-activated protein kinase) cascade is a regulator of this crosstalk.

Novel deletion mutation in GFAP gene in an infantile form of Alexander disease

Alexander disease is a rare, fatal neurologic disorder characterized by white-matter degeneration and cytoplasmic inclusions in astrocytes known as Rosenthal fibers, which are immunohistochemically positive to glial fibrillary acidic protein. Mutations in the glial fibrillary acidic protein gene were reported in patients with Alexander disease who had clinical and pathologic characteristics of the disease. All reported cases manifest heterozygous missense mutations, except for some insertions or deletions with no frame shift. Our patient had a heterozygous deletion of genomic sequence 1247-1249GGG>GG in exon 8 of the glial fibrillary acidic protein gene, which leads to a frame shift changing 16 amino acids and inducing a stop codon at codon 431 of 432 codons. The deletion mutation induces a structural conformation change in glial fibrillary acidic protein and their abnormal aggregation in astrocytes. This is the first report of a novel deletion mutation in the glial fibrillary acidic protein gene with a frame shift associated with Alexander disease.

GFAP mutations and polymorphisms in 13 unrelated Italian patients affected by Alexander disease

Alexander disease (AD), a rare neurodegenerative disorder of the central nervous system, is characterized by the accumulation of cytoplasmic protein aggregates (Rosenthal fibers) composed of glial fibrillary acidic protein (GFAP) and small heat-shock proteins within astrocytes. To date, more than 40 different GFAP mutations have been reported in AD. The present study is aimed at the molecular diagnosis of Italian patients suspected to be affected by AD. By analyzing the GFAP gene of 13 unrelated patients (eight with infantile form, two with juvenile form and three with adult form), we found 11 different alleles, including four new ones. Among the novel mutations, three (p.R70Q, p.R73K, and p.R79P) were identified in exon 1 and p.L359P in exon 6. The sequence analysis also detected six different single nucleotide polymorphic variants, including two previously unreported ones, spread throughout non-coding regions (introns 2, 3, 5, 6, and 3'UTR) of the gene. All patients were heterozygous for the mutations, thus confirming their dominant effect.

A case of infantile Alexander disease diagnosed by magnetic resonance imaging and genetic analysis

We encountered a male infant with infantile Alexander disease presenting with megalencephaly and hydrocephalus as a neonate and subtle seizures at 3 months of age. At 6 months of age, bulbar paralysis appeared. Brain magnetic resonance imaging (MRI) showed abnormal findings with white matter involvement and a characteristic periventricular rim, satisfying the diagnostic criteria proposed by van der Knaap, except for MRI contrast. R239H mutation of glial fibrillary acidic protein gene was identified, representing a common cause of infantile-type Alexander disease.

[Generation of mice with glial cell dysfunction]

To examine astrocytic function, we have developed model mice harboring astrocyte-specific disease causal gene and tried to examine astrocytic function in vivo. Alexander disease, megalencephalic leukodystrophy with subcortical cysts (MLC), and autistic spectrum disorder with neuroligin 3/4 mutations are known to be astrocyte-specific disease so far. First of all, we have established Alexander disease model mouse. Alexander disease is caused by coding mutation in glial fibrillary acidic protein (GFAP) and mutant GFAP forms inclusion bodies, called Rosenthal fibers, in astrocytes. Its pathophysiology is still unknown. We generated transgenic mice that express human GFAP R239H mutant under the control of mouse GFAP promoter. Lines with single copy exhibited weak human GFAP expression in astrocytes that did not produce aggregates despite the existence of mutation, whereas lines with multi copies exhibited strong expression and the formation of aggregates, starting at P14. The line with aggregates showed higher sensitivity to kainate than the line without them, whose sensitivity was not different from the wild type mouse, suggesting that the presence of GFAP aggregates but not the presence of mutant GFAP altered the sensitivity. Changes in several electrophysiological parameters, including facilitation of LTP, were also observed in this model mouse. We believe that this transgenic line is a useful tool to study astrocytic function in vivo.

Dynamics of mutated GFAP aggregates revealed by real-time imaging of an astrocyte model of Alexander disease

Alexander disease (AxD) is a rare neurodegenerative disorder characterized by large cytoplasmic aggregates in astrocytes and myelin abnormalities and caused by dominant mutations in the gene encoding glial fibrillary acidic protein (GFAP), the main intermediate filament protein in astrocytes. We tested the effects of three mutations (R236H, R76H and L232P) associated with AxD in cells transiently expressing mutated GFAP fused to green fluorescent protein (GFP). Mutated GFAP-GFP expressed in astrocytes formed networks or aggregates similar to those found in the brains of patients with the disease. Time-lapse recordings of living astrocytes showed that aggregates of mutated GFAP-GFP may either disappear, associated with cell survival, or coalesce in a huge juxtanuclear structure associated with cell death. Immunolabeling of fixed cells suggested that this gathering of aggregates forms an aggresome-like structure. Proteasome inhibition and immunoprecipitation assays revealed mutated GFAP-GFP ubiquitination, suggesting a role of the ubiquitin-proteasome system in the disaggregation process. In astrocytes from wild-type-, GFAP-, and vimentin-deficient mice, mutated GFAP-GFP aggregated or formed a network, depending on qualitative and quantitative interactions with normal intermediate filament partners. Particularly, vimentin displayed an anti-aggregation effect on mutated GFAP. Our data indicate a dynamic and reversible aggregation of mutated GFAP, suggesting that therapeutic approaches may be possible.

[Juvenile form of Alexander's disease - a case confirmed by detection of mutation in GFAP gene]

Alexander's disease is a rare and fatal disorder of the central nervous system. It may appear at any age so three forms are delineated: infantile, juvenile and adult form. Alexander's disease inescapably leads to psychomotor retardation, progressive loss of nervous functions and characteristic changes in neuroimaging studies. The authors present a case of a 6-year-old girl, who was admitted to the Neurology Department after an episode of long-term vomiting, trismus and blurred speech. Computed tomography and magnetic resonance imaging of the brain showed characteristic changes of the white matter in the frontal lobes, which enabled us to make a preliminary diagnosis of Alexander's disease. The diagnosis was subsequently confirmed by molecular genetic testing of the gene encoding glial fibrillary acidic protein (GFAP). This article also presents clinical symptoms and course of this degenerative disorder. The authors point out the important role of neuroimaging and the necessity of molecular examination as a new diagnostic tool.

Murine model of Alexander disease: analysis of GFAP aggregate formation and its pathological significance

Alexander disease is caused by a coding mutation in the glial fibrillary acidic protein (GFAP) gene. The pathological hallmark is the formation of cytoplasmic inclusions within astrocytes known as Rosenthal fibers (RFs), which primarily consist of GFAP and several heat shock proteins. The presence of mutant GFAP would appear to be involved in RF formation; however, overproduction of wild type human GFAP in mouse brain also results in RF formation. Here, we investigated the in vivo conditions leading to formation of RF-like aggregates. We used transgenic mice (mouse GFAP promoter-human GFAP cDNA with R239H mutation) in which the dosage of the GFAP transgene could be manipulated within the same genetic locus. We found that the presence of mutant GFAP per se was insufficient for aggregate formation. Instead, a 30% increase in GFAP content over that in wild type was also required. GFAP aggregates upregulated endogenous GFAP and nestin gene expression, and intermediate filament structure revealed by immunostaining was fragmented under these conditions. However, overall morphology of astrocytes, including their fine processes, was unaffected. In this transgenic animal model, mice did not show megalencephaly, leukodystrophy, or seizure characteristic of Alexander disease with R239H mutation. Nevertheless, their mortality after kainate challenge was dramatically increased, whereas transgenic mice lacking aggregates exhibited mortality similar to that of wild type mice. These results indicate that the presence of GFAP aggregates containing mutant GFAP is not sufficient to induce a major phenotype of Alexander disease, even though it causes some abnormalities in the mouse.

GFAP and its role in Alexander disease

Here we review how GFAP mutations cause Alexander disease. The current data suggest that a combination of events cause the disease. These include: (i) the accumulation of GFAP and the formation of characteristic aggregates, called Rosenthal fibers, (ii) the sequestration of the protein chaperones alpha B-crystallin and HSP27 into Rosenthal fibers, and (iii) the activation of both Jnk and the stress response. These then set in motion events that lead to Alexander disease. We discuss parallels with other intermediate filament diseases and assess potential therapies as part of this review as well as emerging trends in disease diagnosis and other aspects concerning GFAP.

The functional alteration of mutant GFAP depends on the location of the domain: morphological and functional studies using astrocytoma-derived cells

To clarify the functional effects of mutant glial fibrillary acidic protein (GFAP), we examined the expression patterns of mutant GFAPs (V87G, R88C, and R416W) in astrocytoma-derived cells and performed migration assay. The morphological change was found in mutant GFAP cells, although the number of changes was small. On migration assay, the migration rate in cells with the V87G or R88C mutation, which are located in the helical rod domain in GFAP, was significantly higher than those of wild-type and R416W. These findings suggest that the functional abnormalities of astrocytes might be induced prior to aggregation of GFAP in Alexander disease and that the functional alteration depends on the location of the domain.

Unusual diagnosis in a child suffering from juvenile Alexander disease: clinical and imaging report

Alexander disease is a rare, sporadic leukoencephalopathy characterized by white-matter abnormalities with frontal predominance and, as a rule, clinically associated with megalencephaly, seizures, spasticity, and psychomotor deterioration. We describe a boy who was diagnosed as affected by anorexia nervosa because of his refusal to eat, progressive weight loss, and psychologic disturbances. The observation of a hyperintense lesion on T(2)-weighed magnetic resonance images (MRIs) was initially explained as a pontine and extrapontine myelinolysis related to malnutrition. Following MRI and DNA analysis, we diagnosed a juvenile type of Alexander disease. Therefore, we can affirm the importance of the history and clinical examination to look for brainstem dysfunction in patients presenting with atypical anorexia nervosa.

Alexander disease-associated glial fibrillary acidic protein mutations in mice induce Rosenthal fiber formation and a white matter stress response

Mutations in the gene for the astrocyte specific intermediate filament, glial fibrillary acidic protein (GFAP), cause the rare leukodystrophy Alexander disease (AxD). To study the pathology of this primary astrocyte defect, we have generated knock-in mice with missense mutations homologous to those found in humans. In this report, we show that mice with GFAP-R76H and -R236H mutations develop Rosenthal fibers, the hallmark protein aggregates observed in astrocytes in AxD, in the hippocampus, corpus callosum, olfactory bulbs, subpial, and periventricular regions. Astrocytes in these areas appear reactive and total GFAP expression is elevated. Although general white matter architecture and myelination appear normal, when crossed with an antioxidant response element reporter line, the mutant mice show a distinct pattern of reporter-gene induction that is especially prominent in the corpus callosum, and histochemical staining reveals accumulation of iron in the same region. The mutant mice have a normal lifespan and show no overt behavioral defects, but are more susceptible to kainate-induced seizures. Although these mice demonstrate increased GFAP expression by themselves, further elevation of GFAP via crosses to GFAP transgenic animals leads to a shift in GFAP solubility, an increased stress response, and ultimately death. The mice do not display the full spectrum of pathology observed in human infantile AxD, but may more closely resemble the adult form of the disease. These studies provide formal proof linking GFAP mutations with Rosenthal fibers and oxidative stress, and correlate gliosis and GFAP protein levels to the severity of the disease.

Discrepancy between neuroimaging findings and clinical phenotype in Alexander disease

We present a case of infantile-onset Alexander disease (AD) with a novel glial fibrillary acidic protein mutation but without clinical evidence of neurologic deterioration. Brain MRI studies showed typical AD findings and increasing size of frontal cavitations. Serial proton MR spectroscopy demonstrated high levels of myo-inositol and lactic acid and decreasing levels of N-acetylaspartate. The degree of demyelination and the timing of the axonal degeneration may determine phenotypic severity of the disease. Conventional neuroimaging techniques cannot always predict the outcome.

TRH therapy in a patient with juvenile Alexander disease

Alexander disease is a rare disorder of the central nervous system caused by a de novo mutation in the glial fibrillary acidic protein (GFAP) gene. Unlike the much more common infantile form, the juvenile form is slowly progressive with bulbar, pyramidal and cerebellar signs. Herein, we report a 9-year old Japanese girl suffering from frequent vomiting, slurred speech and truncal ataxia. Juvenile Alexander disease was diagnosed by genetic analysis, which detected a novel GFAP mutation, D360V. We also describe our clinical success in treating this patient with thyrotropin releasing hormone (TRH).

Early mitochondrial dysfunction in an infant with Alexander disease

Alexander disease is a neurodegenerative disorder characterized by macrocephaly and progressive demyelination with frontal lobe preponderance. The infantile form, the most frequent variant, appears between birth and 2 years of age and involves a severe course with a rapid neurologic deterioration. Although magnetic resonance imaging is useful for diagnosis, currently diagnosis is confirmed by the finding of missense mutation in the glial fibrillary acidic protein (GFAP) gene. This case reports a female who presented at the age of 5 months with refractory epilepsy and hypotonia. Laboratory examinations, muscle biopsy examination, and energetic metabolic study in muscle indicated increased concentrations of lactate, mitochondria with structural abnormalities, and decreased cytochrome-c oxidase activity respectively. Later, both clinical course and magnetic resonance findings were compatible with Alexander disease, which was confirmed by the finding of a novel glial fibrillary acidic protein gene mutation.

The Alexander disease-causing glial fibrillary acidic protein mutant, R416W, accumulates into Rosenthal fibers by a pathway that involves filament aggregation and the association of alpha B-crystallin and HSP27

Here, we describe the early events in the disease pathogenesis of Alexander disease. This is a rare and usually fatal neurodegenerative disorder whose pathological hallmark is the abundance of protein aggregates in astrocytes. These aggregates, termed "Rosenthal fibers," contain the protein chaperones alpha B-crystallin and HSP27 as well as glial fibrillary acidic protein (GFAP), an intermediate filament (IF) protein found almost exclusively in astrocytes. Heterozygous, missense GFAP mutations that usually arise spontaneously during spermatogenesis have recently been found in the majority of patients with Alexander disease. In this study, we show that one of the more frequently observed mutations, R416W, significantly perturbs in vitro filament assembly. The filamentous structures formed resemble assembly intermediates but aggregate more strongly. Consistent with the heterozygosity of the mutation, this effect is dominant over wild-type GFAP in coassembly experiments. Transient transfection studies demonstrate that R416W GFAP induces the formation of GFAP-containing cytoplasmic aggregates in a wide range of different cell types, including astrocytes. The aggregates have several important features in common with Rosenthal fibers, including the association of alpha B-crystallin and HSP27. This association occurs simultaneously with the formation of protein aggregates containing R416W GFAP and is also specific, since HSP70 does not partition with them. Monoclonal antibodies specific for R416W GFAP reveal, for the first time for any IF-based disease, the presence of the mutant protein in the characteristic histopathological feature of the disease, namely Rosenthal fibers. Collectively, these data confirm that the effects of the R416W GFAP are dominant, changing the assembly process in a way that encourages aberrant filament-filament interactions that then lead to protein aggregation and chaperone sequestration as early events in Alexander disease.

Plectin regulates the organization of glial fibrillary acidic protein in Alexander disease

Alexander disease (AxD) is a rare but fatal neurological disorder caused by mutations in the astrocyte-specific intermediate filament protein glial fibrillary acidic protein (GFAP). Histologically, AxD is characterized by cytoplasmic inclusion bodies called Rosenthal fibers (RFs), which contain GFAP, small heat shock proteins, and other undefined components. Here, we describe the expression of the cytoskeletal linker protein plectin in the AxD brain. RFs displayed positive immunostaining for plectin and GFAP, both of which were increased in the AxD brain. Co-localization, co-immunoprecipitation, and in vitro overlay analyses demonstrated direct interaction of plectin and GFAP. GFAP with the most common AxD mutation, R239C (RC GFAP), mainly formed abnormal aggregates in human primary astrocytes and murine plectin-deficient fibroblasts. Transient transfection of full-length plectin cDNA converted these aggregates to thin filaments, which exhibited diffuse cytoplasmic distribution. Compared to wild-type GFAP expression, RC GFAP expression lowered plectin levels in astrocytoma-derived stable transfectants and plectin-positive fibroblasts. A much higher proportion of total GFAP was found in the Triton X-insoluble fraction of plectin-deficient fibroblasts than in wild-type fibroblasts. Taken together, our results suggest that insufficient amounts of plectin, due to RC GFAP expression, promote GFAP aggregation and RF formation in AxD.