Atypical MRI features in familial adult onset Alexander disease: case report

Analysis of GFAP variants in UK Biobank suggests underdiagnosis or incomplete penetrance of adult-onset Alexander disease

BACKGROUND

Alexander disease is an autosomal dominant leukodystrophy caused by heterozygous pathogenic variants in the glial fibrillar acidic protein (GFAP) gene. Although increasingly recognised, there is evidence that Alexander disease, particularly later-onset disease, is significantly underdiagnosed and its true prevalence is unknown (the only population-based prevalence was estimated at one in 2.7 million). Using the extensive UK Biobank dataset, we analysed the frequency of pathogenic and likely pathogenic variants, GFAP variants, within the UK population and identified clinical and radiological phenotypes linked to these variants.

METHODS

Pathogenic, likely pathogenic and GFAP variants of uncertain significance were identified in the UK Biobank whole-exome sequencing data (n=4 70 000). Demographic information, previous medical history-including symptoms associated with Alexander disease-collected from self-reported data and hospital records, family history and various MRI metrics were compared between variant carriers and controls.

RESULTS

We identified 36 unique pathogenic and likely pathogenic GFAP variants in 106 carriers, yielding a carrier frequency of approximately 1 in 4435. Modelling based on the UK population estimated a prevalence of 6.8 per 100 000. Carriers of pathogenic and likely pathogenic GFAP variants had higher odds of bladder dysfunction (OR 3.17, p<0.0001), upper airway dysfunction (OR 7.82, p=0.004) and psychiatric conditions (OR 1.51, p=0.04). Additionally, carriers were more likely to report a paternal history of dementia (OR 2.79, p<0.0001). MRI data revealed significant atrophy in brainstem regions among variant carriers.

CONCLUSION

Pathogenic and likely pathogenic GFAP variants are more prevalent in the general population than previously expected and are associated with clinical and radiological characteristics of Alexander disease. This study indicates that Alexander disease may be under-reported, misdiagnosed, or exhibit reduced penetrance.

Disorders with prominent posterior fossa involvement

Inherited white matter disorders include a wide range of disorders of various origins with distinct genetic, pathophysiologic, and metabolic backgrounds. Although most of these diseases have nonspecific clinical and radiologic features, some display distinct clinical and/or imaging (magnetic resonance imaging, MRI) characteristics that might suggest the causative gene. Recent advances in genetic testing allow assessing gene panels that include several hundred genes; however, an MRI-based diagnostic approach is important to narrow the choice of candidate genes, particularly in countries where these techniques are not available. Indeed, white matter disorders with prominent posterior fossa involvement present specific MRI (and clinical) phenotypes that can directly orient the diagnosis. This chapter describes the main genetic disorders with posterior fossa involvement and discusses diagnostic strategies.

Identification of a novel de novo pathogenic variant in GFAP in an Iranian family with Alexander disease by whole-exome sequencing

Background

Alexander disease (AxD) is a rare leukodystrophy with an autosomal dominant inheritance mode. Variants in GFAP lead to this disorder and it is classified into three distinguishable subgroups: infantile, juvenile, and adult-onset types.

Objective

The aim of this study is to report a novel variant causing AxD and collect all the associated variants with juvenile and adult-onset as well.

Methods

We report a 2-year-old female with infantile AxD. All relevant clinical and genetic data were evaluated. Search strategy for all AxD types was performed on PubMed. The extracted data include total recruited patients, number of patients carrying a GFAP variant, nucleotide and protein change, zygosity and all the clinical symptoms.

Results

A novel de novo variant c.217A > G: p. Met73Val was found in our case by whole-exome sequencing. In silico analysis categorized this variant as pathogenic. Totally 377 patients clinically diagnosed with juvenile or adult-onset forms were recruited in these articles, among them 212 patients were affected with juvenile or adult-onset form carrier of an alteration in GFAP. A total of 98 variants were collected. Among these variants c.262C > T 11/212 (5.18%), c.1246C > T 9/212 (4.24%), c.827G > T 8/212 (3.77%), c.232G > A 6/212 (2.83%) account for the majority of reported variants.

Conclusion

This study highlighted the role of genetic in AxD diagnosing. It also helps to provide more information in order to expand the genetic spectrum of Iranian patients with AxD. Our literature review is beneficial in defining a better genotype–phenotype correlation of AxD disorder.

A novel in-frame GFAP p.E138_L148del mutation in Type II Alexander disease with atypical phenotypes

Alexander disease (AxD) is a neurodegenerative astrogliopathy caused by mutation in the glial fibrillary acidic protein (GFAP) gene. A 42-year-old Korean man presented with temporary gait disturbance and psychiatric regression after a minor head trauma in the absence of bulbar symptoms and signs. Magnetic resonance images of the brain and spinal cord showed significant atrophy of the medulla oblongata and the entire spinal cord as well as contrast-enhanced T2 hypointensity in the basal ganglia. DNA sequencing revealed a novel 33-bp in-frame deletion mutation (p.Glu138_Leu148del) within the 1B rod domain of GFAP, which was predicted to be deleterious by PROVEAN analysis. To test whether the deletion mutant is disease-causing, we performed in vitro GFAP assembly and sedimentation assays, and GFAP aggregation assays in human adrenal carcinoma SW13 (Vim^-) cells and rat primary astrocytes. All the assays revealed that GFAP p.Glu138_Leu148del is aggregation prone. Based on these findings, we diagnosed the patient with Type II AxD. This is a report that demonstrates the pathogenicity of InDel mutation of GFAP through functional studies. This patient's atypical presentation as well as the discrepancy between clinical symptoms and radiologic findings may extend the scope of AxD.

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.

Aggregate formation analysis of GFAPR416W found in one case of Alexander disease

Alexander disease (AxD) is a neurodegenerative disease in astrocytes caused by a mutation in the gene encoding glial fibrillary acidic protein, GFAP. We herein present the case of a 12-year-old girl who showed intermittent exotropia at 3 years of age and central precocious puberty at 7 years of age. The periventricular and medulla oblongata showed high signal intensity on T2-weighted magnetic resonance imaging. The patient was diagnosed with AxD after direct sequencing revealing a de novo recurrent mutation, c.1246C>T (p.R416W) in GFAP. The transient expression of GFAP^R416W in cells resulted in the significant formation of aggregates, which recapitulated the hallmark of AxD. We firstly utilized In Cell analyzer to prove the tendency of aggregate formation by mutants of GFAP.

Affected astrocytes in the spinal cord of the leukodystrophy vanishing white matter

Leukodystrophies are often devastating diseases, presented with progressive clinical signs as spasticity, ataxia and cognitive decline, and lack proper treatment options. New therapy strategies for leukodystrophies mostly focus on oligodendrocyte replacement to rescue lack of myelin in the brain, even though disease pathology also often involves other glial cells and the spinal cord. In this study we investigated spinal cord pathology in a mouse model for Vanishing White Matter disease (VWM) and show that astrocytes in the white matter are severely affected. Astrocyte pathology starts postnatally in the sensory tracts, followed by changes in the astrocytic populations in the motor tracts. Studies in post-mortem tissue of two VWM patients, a 13-year-old boy and a 6-year-old girl, confirmed astrocyte abnormalities in the spinal cord. For proper development of new treatment options for VWM and, possibly, other leukodystrophies, future studies should investigate spinal cord involvement.