Results from three recent studies — two supported in part by MDA — shed light on how the chromosome 9 open reading frame 72 (C9ORF72) mutation causes nerve cell death in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).
Study findings also describe promising early-stage results with antisense-based therapies in cellular models. Antisense compounds are pieces of genetic information that keep other genetic information from being processed.
Taken together, the findings support continued investigations into antisense therapies as potential treatments for ALS.
About the C9ORF72 mutation
Identified in 2011, the C9ORF72 mutation is the most common genetic cause of ALS and FTD, accounting for approximately 8 percent of sporadic ALS and FTD, and more than 40 percent of familial cases of the two disorders.
In healthy people, a "six-letter" stretch of DNA coded GGGGCC in the C9ORF72 gene is repeated up to 30 times in a row. In people with ALS or FTD, the same six-letter string is repeated anywhere from 30 to several thousand times.
When cellular machinery reads the DNA "blueprint" for protein production, the extra repeats are copied into RNA molecules. There they stick together, forming into clumps (called RNA foci) in the nuclei of nerve cells. These RNA clumps containing the GGGGCC repeats are a hallmark of C9ORF72 ALS and FTD.
Toxic RNA plays a key role in C9ORF72 ALS and FTD
In three separate studies, investigators observed the effects of the C9ORF72 mutation in nerve cells that were generated by reprogramming skin cells taken from people with ALS or ALS+FTD into induced pluripotent stem cells (iPSCs) and then coaxing them to develop into motor neurons (the muscle-controlling nerve cells that are lost in ALS).
In all three studies, clumps of RNA containing the expanded repeats were found in the nuclei of cells affected by the C9ORF72 mutation. This RNA buildup was not found in unaffected cells or in cells taken from people with ALS caused by other ALS-causing mutations.
In addition, findings showed:
Ravits and colleagues demonstrated that two distinct sets of toxic RNA buildup were associated with the C9ORF72 mutation. They noted that different therapies will be needed to degrade both sets.
Results from the three studies demonstrate that generation of toxic RNA plays a major role in C9ORF72 ALS, while loss of normal function of the C9ORF72 gene does not.
Targeting RNA with antisense
All three research teams found that treatment with antisense compounds called antisense oligonucleotides (ASOs) reduced the effects of the C9ORF72 mutation.
Each team tested two types of ASOs in cells containing C9ORF72 repeat expansion — one designed to reduce activity of the entire C9ORF72 gene, and the other designed to target only the repeat expansion mutation.
Treatment with both types of ASO:
Findings from the three groups provide a basis for further investigation into the potential of ASO therapy as a treatment for C9ORF72 ALS.
For more information
Baloh's team published its findings online Oct. 23, 2013, in Science Translational Medicine. See Targeting RNA Foci in iPSC-Derived Motor Neurons from Patients with a C9ORF72 Repeat Expansion.
The team led by John Ravits published its findings online Oct. 29, 2013, in Proceedings of the National Academy of Sciences. See Targeted Degradation of Sense and Antisense C9ORF72 RNA Foci as Therapy for ALS and Frontotemporal Degeneration. MDA supported Clotilde Lagier-Tourenne at the University of California, San Diego, for her work on the project.
Rothstein's team published its findings online Oct. 16, 2013, in Neuron. See RNA Toxicity from the ALS/FTD C9ORF72 Expansion is Mitigated by Antisense Intervention. MDA supported Rothstein for his work on the project.