Background: Alexander disease (AxD) is a progressive and fatal neurological disorder characterized by white matter degeneration and Rosenthal fibers inclusions, which was caused by GFAP mutations. Since the first description of AxD, more than 550 cases have been reported. We identified two patients with de novo mutations in GFAP gene causing bulbospinal AxD, and further investigated the pathogenicity of identified variants.
Results: Two de novo mutations in GFAP gene were identified (c.214G>A, p.E72K and c.1235C>T, p.T412I) by whole exome sequencing, of which c.1235C>T was the first reported. Clinical assessment revealed that both probands similarly presented bulbar symptoms, pyramidal signs and white matter atrophy in periventricular regions. We conducted a novel data-driven method to analyze fMRI characteristics. Increased brain functional connectivity in occipital and posterior parietal cortex was first discovered in AxD. Grey matter atrophy was also observed in one patient. Functional studies showed that the solubility of p.T412I GFAP was lower than wild type, while p.E72K and p.T412I were at a similar level to wild type, suggesting that mutations located in the tail domain could decrease the solubility of GFAP. Abnormal inclusions of mutant GFAP were colocalized with ubiquitin, 20S proteasome, protein 1 light chain 3-II (LC3-II) and lysosome. Western blotting indicated the ubiquitin-proteasome pathway was blocked while autophagy was activated as a mechanism for degrading aggregates.
Conclusions: GFAP mutations identified in our study were associated with AxD and accompanied by protein functional alterations. Our findings further expand the clinical and genetic spectrum of AxD.