Amyotrophic Lateral Sclerosis (ALS) is a debilitating neurodegenerative disease that affects neurons in the brain, brain stem, and spinal cord. When these neurons die, it affects the connections that they have to muscles throughout the body resulting in muscle weakness that affects speaking, swallowing and breathing leading to paralysis and eventual dead. The cause of ALS isn’t known in 90-95% of cases. However, recently scientists have identified a mutation in a gene nondescriptly called “chromosome 9 open reading frame 72,” which is abbreviated to C9ORF72. This mutation results in six nucleotides, GGGGCC, being repeating up to 1000 times within this gene in ALS patients. Even though this is the most common mutation found in ALS patients, it’s still unclear exactly how these repeats affect neurons or the progression of the disease. The two hypotheses that are most studied are that the mutated C9ORF72 makes mutated RNA transcripts (RNA is the molecule that usually helps DNA be translated into proteins) or makes unusual proteins called dipeptide repeat proteins (DRPs), each of which can aggregate (clump) together and disrupt the normal activity of the neurons leading to neurodegeneration.
In two recent publications in the journal Neuron, researchers have used mice as a model system to look at how these large GGGGCC repeats in C9ORF72 affect the mouse nervous system. Why use a mouse? First, scientists know how to experimentally change the mouse genome in order to add hundreds or thousands of repeats to a single gene, like C9ORF72. Second, mouse and human genes are about 85% identical, so if scientists can understand how a gene like the mutated C9ORF72 affects neurons in mice, it may also help scientists understand how it works in humans. Third, by creating a mouse “model” of ALS, scientists can use this model to better understand ALS and to test potential future therapies (it’s worth noting that only one drug currently exists for ALS and it typically extends life only by a few months).
In each article, the mouse model was slightly different – one had a mutated C9ORF72 with 500 GGGGCC repeats and the other varied between 100-1000 repeats. However, both sets of researchers found the same results. The mice had aggregated RNA transcripts and DRPs in their neurons just like what are found in human patients with ALS, but none of the mice had behavioral changes or neurodegeneration that are seen in human ALS patients. So why didn’t the clumped up RNA and proteins cause neurodegeneration in mice like they do in humans? There are lots of potential reasons – including the fact that even though mice and humans are similar, there are still lots of differences and mice may respond to these aggregated RNA and proteins differently than humans. However, the authors of these papers suggest that other environmental and/or genetic factors along with the aggregates caused by the C9ORF72 mutation must be involved in developing the neurodegeneration. It may also mean that getting rid of these aggregates before neurodegeneration occurs may prevent development of ALS. Now that these mice are available to study, they should help in identifying the other factors involved in ALS development along with developing possible treatments for this debilitating disease.
Want to read the articles? Unfortunately, they are behind a paywall, but you can see the abstracts here:
Peters et al. (2015) Human C9OFR72 Hexanucleotide Expansion Reproduces RNA Foci and Dipeptide Repeat Proteins but Not Neurodegeneration in BAC Transgenic Mice. Neuron. Volume 88, Issue 5, p902–909, 2 December 2015 Article
Read the Article from PN News that I contributed to here