Antisense suppression of glial fibrillary acidic protein as a treatment for Alexander disease
Human GFAP contains 432 amino acids, and disease‐causing mutations occur throughout the rod and tail domains, involving >70 of these amino acids. Nearly all are point mutations, or short in‐frame insertions or deletions, and occur in the heterozygous state (http://waisman.wisc.edu/alexander/mutation-table.pdf). In contrast to most mutations of other intermediate filaments, which lead to loss‐of‐function,3 no null mutations for GFAP have ever been found in human disease, and the Gfap‐null mouse has a relatively mild phenotype.4 Based on mouse models and cell cultures, GFAP mutations associated with AxD appear to produce a gain‐of‐function,2 and a key step in pathogenesis is elevation of GFAP to a threshold that causes toxicity.7 In both mouse8 and human,11 the levels of GFAP roughly correlate with disease severity (with severity defined in humans by age of onset).
Given the large number of disease‐causing mutations, and the apparent minimal consequence of the null state in the mouse, we previously proposed general suppression of GFAP as an approach for therapy.13 We have conducted screens to identify existing drugs capable of general suppression, using primary cultures of mouse astrocytes.14 The resulting hits (such as clomipramine) were active in wild‐type (WT) mice but subsequently failed in mutant mice. Other known drugs under study such as ceftriaxone (for glutamate transporters) and lithium (to increase autophagy) either failed completely or were only modestly effective with unacceptable side effects.15
As an alternative, antisense oligonucleotides (ASOs) are rapidly becoming a realistic option for manipulating gene expression in the CNS.17 In mouse models of Huntington disease, the use of intracerebroventricular (ICV) infusion to bypass the blood–brain barrier led to long‐lasting suppression by relatively short‐term treatments.19 ASOs for the treatment of spinal muscular atrophy (SMA) recently passed through phase III clinical trials and have been approved by the U.S. Food and Drug Administration.20 However, little is known about the accessibility of astrocytes to ASO delivery and the amenability of a highly expressed astrocyte target to ASO suppression.
In this report, we use ASOs to suppress GFAP in mouse models of AxD with remarkable results. Although rare, AxD is one of only a few primary disorders of astrocytes, and as such, specifically demonstrates astrocyte responsiveness to ASO treatment. More importantly, this is the first significant advance toward therapy for this devastating disease.