I_C9orf72_landscape

C9orf72 mutation in ALS: How the cause of neurodegeneration is bound to repeat

By Hannah Ralph
University of Oxford, UK

This summary was highly commended by the judges for Access to Understanding 2015

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Introduction

Amyotrophic Lateral Sclerosis (ALS), a neurodegenerative disorder also known as motor neurone disease, received considerable media attention in 2014 thanks to the viral fundraising efforts of the Ice Bucket Challenge and The Theory of Everything, a film chronicling Stephen Hawking’s life after his ALS diagnosis. ALS is caused by degeneration of motor neurones, eventually leading to loss of muscle control1. Frontotemporal Degeneration (FTD), another neurodegenerative disorder, is a form of dementia characterised by progressive damage to the frontal and temporal lobe of the brain2. Both have been linked to a mutation in the gene C9orf72. Despite much research, the pathological basis of the mutation has remained unclear. Now, Cooper-Knock and colleagues at the University of Sheffield provide new insights that begin to explain how this mutation causes degenerative disease.

The key mutation

The mutation reported in C9orf72 is a repeat expansion. This means that the sequence “GGGGCC”, normally present only a few times, repeats many hundreds of times. Think of this as the cell holding down the ctrl + v keys for too long when copying that stretch of DNA. Interestingly, this is the same kind of mutation already linked to other neurological disorders, including Huntington’s and Myotonic Dystrophy, the most common form of adult-onset muscular dystrophy3.

FISHing for the answer

The researchers began by looking at C9orf72 RNA. Our DNA is very valuable – would you lend out the only copy of your favourite book? – and so a copy (RNA) is made and processed to ensure the correct RNA is translated into a fully functional protein. RNA FISH (Fluorescence in situ Hybridisation) is a technique more like fishing than you might expect: fluorescent bait (complementary RNA) is added to your ‘pool’ and will bind your prey (C9orf72 RNA), showing where in the cell it is located. This revealed concentrated deposits (known as foci) of RNA in the nucleus, where DNA is stored and copied into RNA, and to a lesser extent in the surrounding cytoplasm, where RNA is translated into protein. Strikingly, RNA foci were observed only in the patients carrying a repeat mutation in C9orf72 and were absent from ALS patients with different causative mutations and neurologically normal controls.

Bound to repeat

The researchers next sought to discover which proteins interacted with the RNA by extracting any proteins bound to the GGGGCC repeat sequence. By doing this, they were able to identify more than 100 proteins known to be involved in various stages of RNA processing. To confirm these potential ‘protein-RNA’ interactions, they looked at the localisation of a subset of the proteins in cerebellar granule cells and motor neurones – the cells that are characteristically affected by degeneration in ALS.   They saw a tendency for the proteins to associate with the same position in the cell as the RNA foci in patients with C9orf72 mutation but not in those patients without this mutation. One protein associating with the RNA deposits was ALYREF, which is known to play a role in exporting RNA from the nucleus.

When hoarding becomes toxic

How do these findings explain the observed neurodegeneration in ALS and FTD? The researchers propose two potential explanations.

Disease may be a result of RNA processing proteins being hoarded at the site of mutation, causing cell wide disruption in copying genes into RNA. However, this does not completely explain the late age of onset of ALS and FTD, as the repeat expansion would be hoarding RNA processing proteins throughout life. Additionally, the researchers suggest hoarding the proteins is reversible.

A second explanation is that accumulation of these proteins allows C9orf72 RNA to override the normal cellular signals preventing it from leaving the nucleus. Once outside, it can be translated into a protein that has a toxic effect on neurones.

Why is this research important?

The previous lack of understanding about the molecular basis of ALS and FTD has severely hampered our ability to develop effective treatments. The work of Cooper-Knock et al provides important information about how this particular mutation may lead to neurodegeneration.

For example, if the underlying cause of disease in these patients is mutant RNA exiting the nucleus and being translated into a toxic protein, blocking this exit is an attractive therapeutic route. Alternatively, if the disease-causing step is hoarding RNA processing proteins, then supplementing the patient with these proteins may alleviate symptoms of neurodegeneration.

As repeat mutations in C9orf72 is the cause of ALS and FTD in a large number of patients, developing a treatment targeted to this particular genetic variant will benefit many. It may even prove valuable in our understanding of other neurodegenerative conditions caused by repeat expansion mutations.

References

1 http://www.alsa.org/about-als/what-is-als.html

2 http://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=167

3 http://ghr.nlm.nih.gov/condition/myotonic-dystrophy

This article describes the research published in:

Sequestration of multiple RNA recognition motif-containing proteins by C9orf72 repeat expansions
J. Cooper-Knock, M. J. Walsh, A. Higginbottom, J. R. Highley, M. J. Dickman, D. Edbauer, P. G. Ince, S. B. Wharton, S. A. Wilson, J. Kirby, G. M. Hautbergue, P. J. Shaw Brain (2014) 137(7), 2040–2051
http://EuropePMC.org/articles/PMC4065024

This article was selected for inclusion in the competition by the Motor Neurone Disease Association.

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