Neuropsychological features of adult form of Alexander disease

GFAP mutation and astrocyte dysfunction lead to a neurodegenerative profile with impaired synaptic plasticity and cognitive deficits in a rat model of Alexander disease

Alexander disease (AxD) is a rare neurological disorder caused by dominant gain-of-function mutations in the gene for glial acidic fibrillary protein (GFAP). Expression of mutant protein results in astrocyte dysfunction that ultimately leads to developmental delay, failure to thrive, and intellectual and motor impairment. The disease is typically fatal, and at present there are no preventative or effective treatments. To gain a better understanding of the link between astrocyte dysfunction and behavioral deficits in AxD we recently developed a rat model that recapitulates many of the clinical features of the disease, including failure to thrive, motor impairment, and white matter deficits. In the present study, we show that both male and female AxD model rats exhibit a neurodegenerative profile with a progressive neuroinflammatory response combined with reduced expression of synaptic and mitochondrial proteins. Consistent with these results AxD rats show reduced hippocampal long-term potentiation and are cognitively impaired, as demonstrated by poor performance in the Barnes maze and novel object recognition tests. The AxD rat provides a novel model in which to investigate the impact of astrocyte pathology on central nervous system function and provides an essential platform for further development of effective treatments for AxD and potentially other neurodegenerative diseases with astrocyte pathology.Significance Statement Alexander disease (AxD) is a fatal neurodegenerative disorder caused by gain-of-function GFAP mutations. We recently developed a Gfap +/R237H rat model which demonstrates hallmark astrocyte pathology, myelin deficits, and motor impairment. Here, we show that Gfap +/R237H rats also exhibit reduced synaptic plasticity and cognitive deficits as additional clinically relevant phenotypes, further demonstrating its utility as a model. Hippocampal transcriptomic analysis in young adult animals reveals a neurodegenerative signature with an innate immune response and loss of synaptic and metabolic gene expression, features that are typically associated with chronic diseases of aging. These results reveal mechanisms by which astrocyte dysfunction leads to learning and memory deficits in AxD and perhaps contributes to other diseases such as Alzheimer's and Parkinson's.

Large-scale gene expression changes in APP/PSEN1 and GFAP mutation models exhibit high congruence with Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder with both genetic and non-genetic causes. Animal research models are available for a multitude of diseases and conditions affecting the central nervous system (CNS), and large-scale CNS gene expression data exist for many of these. Although there are several models specifically for AD, each recapitulates different aspects of the human disease. In this study we evaluate over 500 animal models to identify those with CNS gene expression patterns matching human AD datasets. Approaches included a hypergeometric based scoring system that rewards congruent gene expression patterns but penalizes discordant gene expression patterns. The top two models identified were APP/PS1 transgenic mice expressing mutant APP and PSEN1, and mice carrying a GFAP mutation that is causative of Alexander disease, a primary disorder of astrocytes in the CNS. The APP/PS1 and GFAP models both matched over 500 genes moving in the same direction as in human AD, and both had elevated GFAP expression and were highly congruent with one another. Also scoring highly were the 5XFAD model (with five mutations in APP and PSEN1) and mice carrying CK-p25, APP, and MAPT mutations. Animals with the APOE3 and 4 mutations combined with traumatic brain injury ranked highly. Bulbectomized rats scored high, suggesting anosmia could be causative of AD-like gene expression. Other matching models included the SOD1G93A strain and knockouts for SNORD116 (Prader-Willi mutation), GRID2, INSM1, XBP1, and CSTB. Many top models demonstrated increased expression of GFAP, and results were similar across multiple human AD datasets. Heatmap and Uniform Manifold Approximation Plot results were consistent with hypergeometric ranking. Finally, some gene manipulation models, including for TYROBP and ATG7, were identified with reversed AD patterns, suggesting possible neuroprotective effects. This study provides insight for the pathobiology of AD and the potential utility of available animal models.

Clinical and radiological characteristics of older-adult-onset Alexander disease

BACKGROUND

Alexander disease (ALXDRD) affects a wide range of ages from infancy to adulthood. However, only a few cases involving patients with older-adult onset over 65 years of age have been reported. In contrast, regarding in-house data, 10.6% of 85 cases with the identification of GFAP mutations demonstrated older-adult onset. This discrepancy may be due to poor awareness of such cases.

METHODS

The subjects included 9 older-adult-onset cases, with an onset age of 65 years or older. We characterized older-adult-onset ALXDRD by assessing neurological findings and several magnetic resonance imaging (MRI) parameters.

RESULTS

The age at onset, mean age at diagnosis, and mean period from onset to diagnosis were 68.2 years, 70.4 years, and 2.2 years, respectively. The main neurological features at diagnosis included pyramidal signs with muscle weakness and/or cerebellar ataxia. Two-thirds of cases were dependent, and the dependence was significantly correlated with a longer period from onset to diagnosis. Quantitative MRI evaluation for brainstem atrophy demonstrated distinctive morphological features of bulbospinal ALXDRD. The corpus callosum index tended to be negatively correlated with the period from onset to diagnosis.

CONCLUSIONS

Although neurological and MRI findings of older-adult-onset ALXDRD patients showed typical features of bulbospinal ALXDRD, their disease progression was more severe than that in younger-adult-onset ALXDRD, and patients developed dependence within 2 years from onset. Cerebral white matter damage tended to progress in proportion to the duration of illness. Our case study may help to advance understanding of the clinical spectrum of ALXDRD.

Does genetic anticipation occur in familial Alexander disease?

Alexander Disease (AxD) is a rare leukodystrophy caused by missense mutations of glial fibrillary acidic protein (GFAP). Primarily seen in infants and juveniles, it can present in adulthood. We report a family with inherited AxD in which the mother presented with symptoms many years after her daughter. We reviewed the age of onset in all published cases of familial AxD and found that 32 of 34 instances of parent–offspring pairs demonstrated an earlier age of onset in offspring compared to the parent. We suggest that genetic anticipation occurs in familial AxD and speculate that genetic mosaicism could explain this phenomenon.

Clinical characteristics of Alexander disease

Alexander disease (ALXDRD) is a primary astrocyte disease caused by GFAP gene mutation. The clinical features of ALXDRD vary from infantile-onset cerebral white matter involvement to adult-onset brainstem involvement. Several studies revealed that the level of GFAP overexpression is correlated with disease severity, and basic research on therapies to reduce abnormal GFAP accumulation has recently been published. Therefore, the accumulation of clinical data to advance understanding of the natural history is essential for clinical trials expected in the future. This review focuses on the clinical characteristics of ALXDRD including the clinical symptoms, imaging findings and genetics to provide diagnostic information useful in daily clinical practice.

[Clinical characteristics and diagnostic criteria on Alexander disease]

Alexander disease (ALXDRD) is a primary astrocyte disease caused by glial fibrillary acidic protein (GFAP) gene mutation. ALXDRD had been clinically regarded as a cerebral white matter disease that affects only children for about 50 years since the initial report in 1949; however, in the early part of the 21st century, case reports of adult-onset ALXDRD with medulla and spinal cord lesions increased. Basic research on therapies to reduce abnormal GFAP accumulation, such as drug-repositioning and antisense oligonucleotide suppression, has recently been published. The accumulation of clinical data to advance understanding of natural history is essential for clinical trials expected in the future. In this review, I classified ALXDRD into two subtypes: early-onset and late-onset, and detail the clinical symptoms, imaging findings, and genetic characteristics as well as the epidemiology and historical changes in the clinical classification described in the literature. The diagnostic criteria based on Japanese ALXDRD patients that are useful in daily clinical practice are also mentioned.