Alexander Disease
https://www.sciencedirect.com/science/article/pii/S109671922500681X?via%3Dihub#ab0005
Alexander disease is a rare, progressive neurological condition caused by changes, known as variants, in the GFAP gene. This gene provides instructions for making glial fibrillary acidic protein (GFAP), a key structural protein in astrocytes, which are support cells in the brain. When GFAP is altered, it can form abnormal clumps called Rosenthal fibres, disrupting brain development and white matter function. This study reviewed data from 74 children with Type I Alexander disease, a form that begins early in life but shows wide differences in severity. To better capture this variation, Type I disease is divided into four subtypes (Ia, Ib, Ic & Id), ranging from very severe, rapidly progressive disease to milder forms with slower progression. The analysis examined whether specific GFAP variants were linked to these different clinical patterns. Clear genotype–phenotype correlations were identified. Variants located in exon 1 of the GFAP gene were more often associated with milder subtypes, while variants in exon 4 were mainly linked to more severe disease. Changes affecting certain amino acids were particularly important. Variants at position R79 were commonly found in children who achieved independent walking, whereas variants at R239 were associated with severe disease and were not seen in the mildest subtype. Laboratory experiments (such as in vitro assays) supported these findings, showing that GFAP variants linked to severe disease were less able to form normal filament networks inside cells, instead forming damaging aggregates. Computer-based CADD scores, which predict the likely functional impact of single-nucleotide and small insertion/deletion variants on the human genome, were generally lower in children with milder disease. This suggests that milder disease may result from variants with less disruptive effects on the GFAP protein, although CADD scores should be interpreted cautiously for complex or regulatory variants. Overall, this work suggests that the location and nature of GFAP variants strongly influence disease severity in Type I Alexander disease, with important implications for prognosis, family counselling, and future therapies.