A new mouse model of a CHMP2B mutation exhibits symptoms and pathology reminiscent of both ALS and frontotemporal dementia (FTD), according to a study published June 21 in Human Molecular Genetics. The findings strengthen the case for involvement of the lysosomal pathway in the pathogenesis of these two disorders.
“Lysosomal function has become a common theme in FTD,” commented Adrian Isaacs of the University College London Institute of Neurology in the U.K., who was not involved in the study, “and it will be interesting to see how this links to ALS as well.”
Mutations in charged multivesicular body protein 2B (CHMP2B) have been identified as a rare cause of FTD (Skibinski et al., 2005, Lindquist et al., 2008) and ALS (Cox et al., 2010, van Blitterswijk et al., 2012). They were also the first identified mutations associated with a combined ALS and FTD syndrome (Parkinson et al., 2006). The normal protein is part of the endosomal sorting complex required for transport III (ESCRTIII), which is involved in vesicle transport and autophagy, as well as dendritic spine development and function. Disease-causing mutations are found throughout the gene, but one of the best characterized is a missense mutation in the splice acceptor site for exon 6 (CHMP2B-intron5) that leads to a truncated C-terminus of the protein.
“It seems that modification in this part of the protein can lead to either FTD or ALS”, pointed out senior author Frédérique René of the University of Strasbourg, France. “That’s why we decided to use the truncation, to see whether expression of this form of the mutation is able to induce either FTD symptoms or ALS symptoms.”
To generate a mouse model of CHMP2B mutations, René, first author Aurélia Vernay, and colleagues linked the CHMP2B-intron5 gene to the Thy1.2 promoter, a neuron-specific promoter that strongly drives expression in both motor and cortical neurons, and tagged it with hemagglutinin sequences. Mice homozygous for the transgene expressed 12 copies of the CHMP2B-intron 5 gene. The mutant RNA was specifically expressed in the brain and spinal cord, and expression of wild-type protein was unaffected.
By six months of age, protein aggregates positive for p62, ubiquitin, and the hemagglutinin tag, but negative for TDP-43 and FUS, formed in multiple brain regions, including frontal cortex, thalamus, brainstem, and spinal cord, with larger and more extensive aggregates forming in the homozygous relative to hemizygous mice. Homozygous mice exhibited a significantly reduced lifespan averaging 7-8 months as compared to 2 years for their non-transgenic littermates.
Interestingly, these homozygous transgenic mice also develop motor deficits not observed in other mouse models of CHMP2B mutations, state the authors. Transgenic mice developed a progressive gait abnormality, with reduced muscle strength and decreased coordination culminating in generalized paralysis. These motor features were accompanied by denervation of neuromuscular junctions but without motor neuron death. Mice also developed behavioral disturbances, including disinhibition, decreased social interaction, and compulsive behaviors.
“Altogether,” the authors concluded, “these data indicate that the neuronal expression of human CHMP2B-intron5 in areas involved in motor and cognitive functions induces progressive motor alterations associated with dementia symptoms and with histopathological hallmarks reminiscent of both ALS and FTD.”
The cellular mechanisms behind the effects seen in the model remain to be worked out, René said. Her lab is currently developing a conditional knock-in model in order to more precisely understand the neuronal populations responsible for cognitive and motor disturbances, and to further examine the pathways involved.
“This is definitely an interesting addition to the models we have,” commented Adrian Isaacs, especially with the neuronal specificity conferred by the promoter. The appearance of both ALS- and FTD-like phenotypes is gratifying, “but we will have to await more functional work to get the next step in terms of insight.” Previous CHMP2B models, including ones from Isaacs’s lab (see Sep 2015 news, Ghazi-Noori et al., 2012, Clayton et al., 2015), have shown lysosomal pathology, in keeping with loss of function of the normal protein, and it will be important in future studies to examine those effects in this model, he said.
As to how mutations in a single gene can cause both an ALS and an FTD phenotype, he said, “that is a major question in the whole field,” even more so for C9ORF72 than CHMP2B.
Could the key lie in the autophagy-lysosomal pathway? Robert Baloh of Cedars-Sinai Medical Center in Los Angeles pointed out, “what little we know about the normal role of the C9 protein is that it is also involved in autophagy and vesicular trafficking. We aren’t sure yet how much the effects of the two mutations overlap, but there is plenty of reason to suspect they do. To some degree, we haven’t quite done all the right experiments to flesh that out yet in either gene.” However, he cautioned, the major driver of disease in the case of C9ORF72 mutations appears to be a toxic gain of function unrelated to loss of its normal role in the cell.
Nonetheless, he said, it is exciting to see more genes turning up with common functions. “Up to this point, we haven’t been able to connect the known genes into common pathways. But now we have a series of genes,” including CHMP2B, optineurin, and p62, “falling into the autophagy pathway,” suggesting it is likely playing an important role in ALS/FTD pathogenesis.
Vernay A, Therreau L, Blot B, Risson V, Dirrig-Grosch S, Waegaert R, Lequeu T, Sellal F, Schaeffer L, Sadoul R, Loeffler JP, René F. A transgenic mouse expressing CHMP2Bintron5 mutant in neurons develops histological and behavioural features of amyotrophic lateral sclerosis and frontotemporal dementia. Hum Mol Genet. 2016 Jun 21. [Pubmed].