Infantile Alexander disease: spectrum of GFAP mutations and genotype-phenotype correlation

Abstract

Heterozygous, de novo mutations in the glial fibrillary acidic protein (GFAP) gene have recently been reported in 12 patients affected by neuropathologically proved Alexander disease. We searched for GFAP mutations in a series of patients who had heterogeneous clinical symptoms but were candidates for Alexander disease on the basis of suggestive neuroimaging abnormalities. Missense, heterozygous, de novo GFAP mutations were found in exons 1 or 4 for 14 of the 15 patients analyzed, including patients without macrocephaly. Nine patients carried arginine mutations (four had R79H; four had R239C; and one had R239H) that have been described elsewhere, whereas the other five had one of four novel mutations, of which two affect arginine (2R88C and 1R88S) and two affect nonarginine residues (1L76F and 1N77Y). All mutations were located in the rod domain of GFAP, and there is a correlation between clinical severity and the affected amino acid. These results confirm that GFAP mutations are a reliable molecular marker for the diagnosis of infantile Alexander disease, and they also form a basis for the recommendation of GFAP analysis for prenatal diagnosis to detect potential cases of germinal mosaicism.

Figure 1

A and B, Typical T₂-weighted MRI images of brains of patients with infantile Alexander disease. A, Classical form (patient 7). B, Mild form (patient 3). Note that both patients show the high signal intensity of white matter, predominately in the frontal area, and of the basal ganglia. C, Rosenthal fiber in Alexander disease viewed by electron microscopy (patient 15); original magnification ×28,500.

Figure 2

A, Identification of novel mutations in the GFAP gene in four patients with Alexander disease. Segments of the sequencing chromatograms in the region of each mutated nucleotide are shown, with the site of the heterozygous point mutation indicated by an arrow. B, Confirmation of the heterozygous 240C→T point mutation by RFLP analysis. This point mutation results in the loss of a restriction site for SacI. Digestion of a PCR-amplified fragment of genomic DNA from control subjects produced three fragments of 368, 280, and 39 bp (C1 and C2). RFLP analysis of the affected individual generated four fragments of 648, 368, 280, and 39 bp; the 648-bp fragment (368 bp + 280 bp) arose from loss of the SacI site on one of the chromosomes (case 1). The 39-bp restriction fragment is not shown in this gel. ND = nondigested PCR-amplified fragment. C, Schematic representation of the GFAP gene and the corresponding protein, showing the localization of mutations in Alexander disease. The nine exons of the GFAP gene are represented by boxes, and introns are represented by lines. Multiple occurrences of a mutation are indicated by the number shown in parentheses, and mutations described elsewhere (Brenner et al. 2001) are shown in blue. Mutations are clustered within exons encoding the rod domain of GFAP.