Alexander disease is a very rare neurodegenerative disease that generally presents in the infantile period, although other variants are occasionally seen. This disorder, first described in 1949 by W. Stewart Alexander, is part of a group of neurological disorders, which is collectively known as leukodystrophies. These disorders are a group of rare, progressive, metabolic, genetic diseases (neurogenetic diseases) that predominantly affect the white matter of the central nervous system (CNS) with or without peripheral nervous system involvement and characterized by abnormal development or destruction of the myelin sheath.
Alexander disease is caused by a mutation in the GFAP gene that codes for the fibrillar acid glial protein and represents the only known example of a genetic disorder affecting astrocyte cells. It is, thus, an example of a primary disease of astrocytes. The disease has multiple clinical forms spanning from newborn to adult, and typically, the early onset of the disease correlates with an increase in severity. Interestingly, before the genetic features were well understood, various names were used to characterize features of Alexander disease such as 'dysmyelinogenic leukodystrophy' or 'demyelinogenic leukodystrophy' or 'fibrinoid degeneration of astrocytes' or 'Leukodystrophy with Rosenthal fibers.' However, these terms are not typically used anymore.[1]
The diagnosis can be made based on clinical and imaging features. After the diagnosis is suspected, genetic testing is usually done for confirmation. The treatment of Alexander disease is largely supportive (e.g., anticonvulsants for seizures), and patients have a variable life expectancy. The neonatal type is related to severe disability or death within two years. Children affected children by the infantile form (within 2-4 years of age) survive weeks to several years. On the contrary, when the disease begins after 4-5 years (juvenile and adult forms), survival is variable and can even reach 30 years of age and more.[2][3]
This chapter is aimed at describing the etiology, clinical evaluation, and management options available for Alexander disease. Moreover, the importance of the interprofessional team to improve outcomes for patients affected by this heritable disorder is also addressed.
Alexander disease usually occurs through a mutation of the GFAP gene. The disease is inherited in an autosomal dominant manner.[4][5] The GFAP gene was identified in the region of chromosome 17q21.[6] This gene (nine exons with a length of 9.8 kb) encodes for a 432 amino acid protein that belongs to the intermediate filament proteins and appears to play a role in regulating the morphology and motility of astrocytes as well in the interaction between astrocytes and oligodendrocytes. These filaments are rapidly synthesized during reactive astrogliosis and CNS injury. The precise mechanism is not fully understood. It is thought that the GFAP mutations act as a gain of function mutation that disrupts the dimerization of intermediate filaments.[7] This results in abnormal protein aggregation and cytoskeleton collapse.[8]
There is also a suggestion of the GFAP accumulation disrupts filament assembly. The most frequent mutations affect exon 1 (54%), exon 4 (31%), exon 8 (7%), exon 6 (4%), exon 5 (3%), and consist of point mutations that bind to an alternative transcript and different mRNAs by transcription start site or by alternative RNA splicing. Of note, despite the genetic homogeneity, there are different clinical phenotypes.[9]
Alexander disease is a very rare disorder with the true prevalence not known. Since the initial description of Alexander disease, only around 550 cases have been described. A Japanese study estimated an incidence of 1/2.7 million.[10] Additionally, Alexander disease has only been found to account for approximately 1.6 percent of leukodystrophies.
The alteration and increased expression of GFAP induce the development of protein aggregates (Rosenthal fibers) in the astrocyte cytoplasm. In turn, reactive gliosis and astrocyte dysfunction induce a set of secondary changes in neurons and other types of glia.[11] Apart from the accumulation of protein aggregates (GFAP toxicity), other mechanisms such as impaired degradation and GFAPs interfering with the function of the proteasome could be responsible for the pathogenesis of the disease. Whatever the cause, the damage begins at the astrocytic level and extends beyond involving other cellular elements through probably microglial activation.
In the pathogenesis of the disease, therefore, an important role is played by the overexpression of the mutant protein and, thus, filament accumulation. Indeed, experiments conducted on GFAP-null mice have shown that the reduction of GFAP expression is associated with a low morbidity phenotype.[12] Moreover, no null variants were demonstrated in human patients. Thus, Alexander's disease is an example of a gain-of-function disease. This feature could have important therapeutic implications. For instance, Hagemann et al. conducted interesting investigations by using the antisense oligonucleotides for suppressing GFAP expression through single intracerebroventricular injections in a GFAP mutant mouse models of Alexander disease. They proved that this approach reduced the level of Rosenthal fibers and the activation of astrocytes and microglia.[11]
The white matter of the frontal lobes and, less often, of the temporal lobes, is affected by extensive demyelination, which sometimes evolves into real cavity lesions. In the subsequent stages, the pathological process can involve all the white matter of the cerebral hemispheres, cerebellum, and brainstem with dilation of the ventricular cavities.
From the histological point of view, in the areas affected by the disease process, it is shown widespread vacuolation due to the degeneration of the myelin sheaths. This degeneration is characterized by the finding, in subpial, subependymal, and perivascular sites, of eosinophilic fibers with radial arrangement with respect to adjacent surfaces, called Rosenthal fibers that increase in number and volume over the course of the disease. These fibers, composed of fibrillary acidic protein, vimentin, αβ-crystallin, and heat shock protein 27 (HSP27), are contained exclusively in the soma and the astrocytic axons, and they were initially thought to be pathognomonic of Alexander disease. Subsequently, however, they have been found in numerous other pathological processes such as central pontine myelinolysis and multiple sclerosis as well as no-neurodegenerative diseases such as cancer disease, respiratory and heart failure, and diabetes mellitus.[13]
The relief of Rosenthal fibers remains, however, a differential diagnostic criterion between Alexander disease and the other leukodystrophies.[14]
Alexander disease most often affects infants and children. Patients usually present with macrocephaly, developmental delay, progressive quadriparesis, and seizures.
Traditionally, Alexander disease has been divided into four subtypes, which include neonatal, infantile, juvenile, and adult.[15]
Alexander disease classification was revised in 2011 based on statistical analysis.[18] Although this model system predicts trends, it does not allow the classification of an individual with complete certainty.
Signs of Alexander disease can be demonstrated on CT and MRI, with MRI being the preferred imaging modality because of its sensitivity. Findings usually demonstrate cerebral white matter changes and swelling. The disease begins anteriorly and extends posteriorly. The subcortical U-fibers are generally spared early on in the course.
MRI - typical features: Demonstrates abnormal signal in the following locations (enhancement may be seen in the same areas):
Of note, there is relative sparing of the temporal and occipital lobe white matter.
MRI criteria for diagnosis: Four out of five required to make a diagnosis.[19]
MRI- atypical features: Demonstrates abnormal signal in the following locations:[20]
Magnetic resonance spectroscopy:[21]
The diagnosis of Alexander disease is primarily based on imaging features and clinical presentation. The diagnosis is typically confirmed genetically because of the variability of this disease. When children with Alexander disease present with typical clinical symptoms, the diagnosis can be made if four out of the five MRI criteria are met. The MRI findings are also invaluable when genetic testing is equivocal. But in children with atypical imaging features, the imaging findings cannot exclude the diagnosis. Thus, genetic testing is necessary to confirm the diagnosis in children with atypical imaging findings. In this regard, tests for the most frequent mutations (exons 1,4,8) are available in the clinical setting, and it is possible to resort to the sequencing of the entire gene in the case of recurrent mutations. The genetic study is also practicable in the field of prenatal diagnosis.
Treatment of Alexander disease is centered upon supportive care and utilizing a multidisciplinary team.[22] Using these methods can improve the quality of life of the affected individuals. These methods are based on the treatment of manifestations and maintaining close surveillance. Some examples include:
Overall the treatment of Alexander disease remains supportive in nature. Nevertheless, decreasing the expression of the GFAP gene can represent an interesting future therapeutic perspective.
The differential diagnosis of Alexander disease is centered on disorders with cerebral white matter changes or macrocephaly. In younger children, all conditions characterized by megalencephaly, developmental delay, and spasticity must be addressed. In older children, the differential diagnosis must be made with disorders presenting with spasticity, signs ofinvolvement of the brain stem with or without megalencephaly, and convulsions.
However, Alexander disease can be distinguished from similar diseases by utilizing the MRI criteria and the clinical presentation. The differential that should be considered includes:
Very often, the type of distribution of white matter lesions helps the diagnosis. The distribution of the cerebral white matter involvement is, in Alexander disease, pathognomonic. The alterations, in fact, are predominantly of frontal distribution with rostral-caudal progression.
The prognosis of Alexander disease is generally poor.
Traditional classification scheme:
Revised classification scheme:
The treatment of Alexander is centered upon supportive care of the long-term complications. Some of these long-term complications include seizures, spasticity, mobility problems, hypotonia, scoliosis, obstructive hydrocephalus, dysphagia, gastrointestinal symptoms (vomiting and reflux), failure to thrive, and urinary retention. Interestingly, these findings may wax and wane throughout the treatment course.
The management of patients with Alexander disease is best when using an interprofessional team that includes primary care doctors, neurologists, neurosurgeons, feeding specialists, physical therapists, geneticists, urologists, radiologists, and pathologists. The patient most often presents during the infantile period. The diagnosis of Alexander disease is made based on imaging features and clinical presentation and then typically confirmed genetically.
The treatment of Alexander disease is largely supportive. It is centered upon controlling seizures, spasticity, mobility problems, hypotonia, scoliosis, obstructive hydrocephalus, dysphagia, gastrointestinal symptoms (vomiting and reflux), failure to thrive, and urinary retention. This requires a wide range of specialties with a clear understanding of treatment goals.
Open communication between the interprofessional is essential for improving outcomes. It is necessary to schedule periodic checks to assess the state of growth, the nutritional structure, the neurological status and to prevent potential complications. In addition to the vast spectrum of physical manifestations, the psychological aspects should not be underestimated. In this regard, it may be important to design interventions for the improvement of communication skills and strategies focused on the patient and parents.
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