The process of inducing GFAP aggregates in astrocytoma-derived cells is different between R239C and R416W mutant GFAP. A time-lapse recording study

A report of two cases of bulbospinal form Alexander disease and preliminary exploration of the disease

Alexander disease (AxD) is a cerebral white matter disease affecting a wide range of ages, from infants to adults. In the present study, two cases of bulbospinal form AxD were reported, and a preliminary exploration of AxD was conducted thorough clinical, functional magnetic resonance imaging (fMRI) and functional analyses. In total, two de novo mutations in the glial fibrillary acidic protein (GFAP) gene (c.214G>A and c.1235C>T) were identified in unrelated patients (one in each patient). Both patients showed increased regional neural activity and functional connectivity in the cerebellum and posterior parietal cortex according to fMRI analysis. Notably, grey matter atrophy was discovered in the patient with c.214G>A variant. Functional experiments revealed aberrant accumulation of mutant GFAP and decreased solubility of c.1235C>T variant. Under pathological conditions, autophagic flux was activated for GFAP aggregate degradation. Moreover, transcriptional data of AxD and healthy human brain samples were obtained from the Gene Expression Omnibus database. Gene set enrichment analysis revealed an upregulation of immune‑related responses and downregulation of ion transport, synaptic transmission and neurotransmitter homeostasis. Enrichment analysis of cell‑specific differentially expressed genes also indicated a marked inflammatory environment in AxD. Overall, the clinical features of the two patients with bulbospinal form AxD were thoroughly described. To the best of our knowledge, the brain atrophy pattern and spontaneous brain functional network activity of patients with AxD were explored for the first time. Cytological experiments provided evidence of the pathogenicity of the identified variants. Furthermore, bioinformatics analysis found that inflammatory immune‑related reactions may play a critical role in AxD, which may be conducive to the understanding of this disease.

A novel mutation in the GFAP gene expands the phenotype of Alexander disease

BACKGROUND

Alexander disease, an autosomal dominant leukodystrophy, is caused by missense mutations in GFAP. Although mostly diagnosed in children, associated with severe leukoencephalopathy, milder adult forms also exist.

METHODS

A family affected by adult-onset spastic paraplegia underwent neurological examination and cerebral MRI. Two patients were sequenced by whole exome sequencing (WES). A candidate variant was functionally tested in an astrocytoma cell line.

RESULTS

The novel variant in GFAP (Glial Fibrillary Acidic Protein) N-terminal head domain (p.Gly18Val) cosegregated in multiple relatives (LOD score: 2.7). All patients, even those with the mildest forms, showed characteristic signal changes or atrophy in the brainstem and spinal cord MRIs, and abnormal MRS. In vitro, this variant did not cause significant protein aggregation, in contrast to most Alexander disease mutations characterised so far. However, cell area analysis showed larger size, a feature previously described in patients and mouse models.

CONCLUSION

We suggest that this variant causes variable expressivity and an attenuated phenotype of Alexander disease type II, probably associated with alternative pathogenic mechanisms, that is, astrocyte enlargement. GFAP analysis should be considered in adult-onset neurological presentations with pyramidal and bulbar symptoms, in particular when characteristic findings, such as the tadpole sign, are present in MRI. WES is a powerful tool to diagnose atypical cases.

A case of severe Alexander disease with de novo c. 239 T > C, p.(F80S), in GFAP

Alexander disease (AxD) is a progressive neurodegenerative disease caused by a mutation in the glial fibrillary acid protein (GFAP) gene. A 4-year-old boy presented several times with hemiclonic seizures with eye deviation for a few minutes at 28 days after birth. Electroencephalogram showed independent sharp waves in the right and left temporal area. Magnetic resonance imaging showed high intensity T1-weighted images in the white matter of the frontal lobe and basal ganglia. He showed no head control at 4 years of age, and his weight gain was insufficient. He did not show macrocephaly. At 4 years of age, he died of bacterial pneumonia and septic shock. He was diagnosed with AxD, and direct sequencing revealed a de novo known mutation, c. 239 T > C, p.(F80S), in GFAP. Hela and U2-OS cells transfected with GFAP cDNA with c. 239 T > C showed dot-like cytoplasmic aggregation, similar to R239C, a common mutation found in severe infantile AxD. Aggregation in the cytoplasm caused by a GFAP mutation is a hallmark of AxD. Although there is only one previous report of a patient with an F80S mutation, our data support that F80S can cause the severe, infantile form of AxD.

Alexander Disease Mutations Produce Cells with Coexpression of Glial Fibrillary Acidic Protein and NG2 in Neurosphere Cultures and Inhibit Differentiation into Mature Oligodendrocytes

BACKGROUND

Alexander disease (AxD) is a rare disease caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). The disease is characterized by presence of GFAP aggregates in the cytoplasm of astrocytes and loss of myelin.

OBJECTIVES

Determine the effect of AxD-related mutations on adult neurogenesis.

METHODS

We transfected different types of mutant GFAP into neurospheres using the nucleofection technique.

RESULTS

We find that mutations may cause coexpression of GFAP and NG2 in neurosphere cultures, which would inhibit the differentiation of precursors into oligodendrocytes and thus explain the myelin loss occurring in the disease. Transfection produces cells that differentiate into new cells marked simultaneously by GFAP and NG2 and whose percentage increased over days of differentiation. Increased expression of GFAP is due to a protein with an anomalous structure that forms aggregates throughout the cytoplasm of new cells. These cells display down-expression of vimentin and nestin. Up-expression of cathepsin D and caspase-3 in the first days of differentiation suggest that apoptosis as a lysosomal response may be at work. HSP27, a protein found in Rosenthal bodies, is expressed less at the beginning of the process although its presence increases in later stages.

CONCLUSION

Our findings seem to suggest that the mechanism of development of AxD may not be due to a function gain due to increase of GFAP, but to failure in the differentiation process may occur at the stage in which precursor cells transform into oligodendrocytes, and that possibility may provide the best explanation for the clinical and radiological images described in AxD.

GFAP isoforms control intermediate filament network dynamics, cell morphology, and focal adhesions

Glial fibrillary acidic protein (GFAP) is the characteristic intermediate filament (IF) protein in astrocytes. Expression of its main isoforms, GFAPα and GFAPδ, varies in astrocytes and astrocytoma implying a potential regulatory role in astrocyte physiology and pathology. An IF-network is a dynamic structure and has been functionally linked to cell motility, proliferation, and morphology. There is a constant exchange of IF-proteins with the network. To study differences in the dynamic properties of GFAPα and GFAPδ, we performed fluorescence recovery after photobleaching experiments on astrocytoma cells with fluorescently tagged GFAPs. Here, we show for the first time that the exchange of GFP-GFAPδ was significantly slower than the exchange of GFP-GFAPα with the IF-network. Furthermore, a collapsed IF-network, induced by GFAPδ expression, led to a further decrease in fluorescence recovery of both GFP-GFAPα and GFP-GFAPδ. This altered IF-network also changed cell morphology and the focal adhesion size, but did not alter cell migration or proliferation. Our study provides further insight into the modulation of the dynamic properties and functional consequences of the IF-network composition.