c9orf72 ALS/FTD pathogenesis - glutamate toxicity, RNA-expansion traps ADARB2
Epub published 2 days ago in Neuron.
The innovation of this paper comes from ALS patients' iPS cell line-derived neurons and astrocytes. Two of the three patients were Finnish (Hannu Laaksovirta's patients) and the iPS cells were generated in collaboration with the Biomedicum stem cell center.
The iPS neurons/astrocytes and further experiments were made in Johns Hopkins Univ in Jeff Rothstein's lab.
c9orf72 mutation is the most common genetic cause of ALS and frontotemporal dementia (FTD), and especially common in Finland.
The findings provide an interesting model of ALS and FTD pathogenesis:
- c9orf72 intronic ggggcc-repeat expansion forms pre-mRNA foci in iPS neurons nuclei.
- c9orf72 mutation alters the regulation of several genes (that can be used to monitor treatment effects).
- The ggggcc-repeat RNA foci bind several proteins, one of which is ADARB2, a protein involved in glutamate receptor RNA editing.
- A phenotypic consequence of the c9orf72 ggggcc-repeat expansion in iPS neuron is the cells' sensitivity to glutamate.
- inhibition of ADARB2 expression makes wildtype iPS neurons sensitive to glutamate, too.
- Therapy with antisense oligonucleotides (coupled with RNAse-H) in iPS neurons inhibits the formation of RNA foci (by 50%) and improves the iPS neurons tolerance to 30uM glutamate (by 30%).
The pathogenetic mechanism, via toxic RNA gain-of-function (trapping of nuclear proteins) is analogous to what has been previously described in myotonic dystrophies (reviewed by Udd and Krahe Lancet Neurol 2012). Glutamate toxicity is probably a general mechanism in ALS, not restricted to the c9orf72 mutation.
Neuron 80, 415-428, October 16, 2013
RNA Toxicity from the ALS/FTD C9ORF72 Expansion Is Mitigated by Antisense Intervention
Donnelly CJ, Zhang P, Pham JT, Heusler A, Mistry NA, Vidensly S, Daley EL, Poth EM, Hoover B, Fines DM, Maragakis N,
Tienari PJ, Petruccelli L, Traynor BJ, Wang J, Rigo F, Bennett F, Blackshaw S, Sattler R, Rothstein JD
A hexanucleotide GGGGCC repeat expansion in the noncoding region of the C9ORF72 gene is the most common genetic abnormality in familial and sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The function of the C9ORF72 protein is unknown, as is the mechanism by which the repeat expansion could cause disease. Induced pluripotent stem cell (iPSC)-differentiated neurons from C9ORF72 ALS patients revealed disease-specific (1) intranuclear GGGGCCexp RNA foci, (2) dysregulated gene expression, (3) sequestration of GGGGCCexp RNA binding protein ADARB2, and (4) susceptibility to excitotoxicity. These pathological and pathogenic characteristics were confirmed in ALS brain and were mitigated with antisense oligonucleotide (ASO) therapeutics to the C9ORF72 transcript
or repeat expansion despite the presence of repeat-associated non-ATG translation (RAN) products. These data indicate a toxic RNA gain-of-function mechanism as a cause of C9ORF72 ALS and provide candidate antisense therapeutics and candidate human pharmacodynamic markers for therapy.