Expanded repeats in the C9orf72 gene are the most common genetic cause of ALS identified to date. Why this repeat expansion leads to C9orf72 ALS remains hotly debated (see June 2017 news; for review, see Moens et al., 2017). However, the disease is increasingly thought to be mediated by both gain-of-function and loss of function mechanisms (for review, see Gitler and Tsuiji, 2016).
Short proteins, synthesized from these expanded repeats, may contribute to motor neuron toxicity in C9orf72 ALS (see August 2015, March 2017, June 2017 news). Therefore, a growing number of scientists are developing treatment strategies that aim to reduce the levels of these dipeptide repeat proteins (DPRs) in hopes to help protect motor neurons against the disease (see October 2017 news). But there are five DPRs produced in motor neurons and glia in people with C9orf72 ALS according to post-mortem analysis (Mori et al., 2013; Mackenzie et al., 2015). And, researchers remain unsure which, if any of these short proteins plays a key role in the disease (for review, see Freibaum and Taylor, 2017).
Now, a research team, led by King’s College London’s Christopher Shaw in England reports that the glycine-alanine dipeptide repeat protein “poly(GA)” may be a key contributor of neurotoxicity – at least in the developing chick spinal cord.
The study is published on September 13 in Human Molecular Genetics.
Shaw’s team expressed each of the 5 DPRs in the spinal cord in the chick embryo. The team found, using TUNEL analysis, that poly(GA) exhibited the highest toxicity – leading to the loss of over 50% more neurons than arginine-rich DPRs.
The study builds on previous work, led by Mayo Clinic’s Leonard Petrucelli in Florida, which found that the aggregation of poly(GA)50 in the mouse brain led to neuronal loss in key regions affected by ALS and ALS/FTD including the motor cortex and the hippocampus (see March 2016 news; Zhang et al., 2016).
Key questions remain. Does C9orf72 poly(GA) contribute to motor neuron toxicity, including in the spinal cord (see Schludi et al., 2017)? And, is this short protein the most toxic DRP when produced at the same levels in people with the disease?
In the meantime, a growing number of research teams are turning to CRISPR-Cas9 editing technologies to tackle C9orf72 ALS (see October 2017 news). One of these strategies, being developed by CRISPR Therapeutics in Cambridge, Massachusetts in collaboration with University of Florida’s Laura Ranum and Eric Wang, aims to “silence” these expanded repeats or simply delete them from the genome. The approach is at the preclinical stage. Stay tuned.
To learn more about potential therapies for C9orf72 ALS and ALS/FTD in the pipeline, check out our recent feature: A New CRISPR Technique Fries C9orf72 RNAs.
Lee YB, Baskaran P, Gomez J, Chen HJ, Nishimura A, Smith B, Troakes C, Adachi Y, Stepto A, Petrucelli L, Gallo JM, Hirth F, Rogelj B, Guthrie S, Shaw CE. C9orf72 poly GA RAN-translated protein plays a key role in Amyotrophic Lateral Sclerosis via aggregation and toxicity. Hum Mol Genet. 2017 Sep 13. [PubMed].
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Strongest poison? The dipeptide repeat protein poly GA appears to be most toxic, at least when expressed in the developing chick spinal cord [Courtesy of Lee et al., 2017, Human Molecular Genetics under a CC-BY 4.0 license].
C9orf72 ALS/FTD: motor neurons gone GAGA? Arginine-rich dipeptide proteins are highly toxic to neurons (for review, see Freibaum and Taylor, 2017). But according to some studies, poly GA is more abundant and more stable (Kwon et al., 2014; Mackenzie et al., 2015). [Courtesy of Zhang et al., 2014, Acta Neuropathologica under a CC-BY 4.0 license].