24th International Symposium on ALS/MND
Dario Magnani, post-doc in Siddharthan Chandran’s lab
One of the most exciting findings of the last few years, in the ALS field, is the discovery of
the C9ORF72 mutation. This mutation consists of a hexanucleotide expansion (G4C2) within
the regulatory region of the C9ORF gene, the function of which is still unknown. The G4C2
expansion contributes to 50% of familial cases of ALS and up to 10% of sporadic cases. In
addition, this mutation is also the second most important cause of frontotemporal dementia.
Several mechanisms have been proposed to affect the motor neurones carrying the
C9ORF72 expansion, and all were discussed at the Symposium, by leading groups in the
ALS field, such as J Rothstein, R Baloh, J Ravits, DW Cleveland, C Shaw and C Haass.
First, this expansion results in a down-regulation of the C9ORF gene, which in turn may
affect motor neurone survival. Secondly, G4C2 is transcribed producing junk RNA (RNA foci)
in the cells and sequestering several RNA binding proteins important for neuronal RNA
homeostasis and function. Thirdly, this junk RNA is translated into di-peptide repeats (in a
process known as ATG independent translation), which in turn may affect basic proprieties
of the cells. Altogether this may compromise neuronal health and resistance to stress by
interfering with cellular metabolism and functions, such as ER function or autophagy.
A number of scientific reports have been recently published suggesting that RNA toxicity is
perhaps the major contributor to MND pathology and this idea was further discussed at the
conference. R Baloh, C Lagier-Tourenne and CJ Donnelly - post docs in Cleveland and
Rothstein labs respectively - discussed their recent papers and showed how using antisense
therapeutics targeting the C9ORF mRNA or the repeat expansion (work done in a
collaboration with Isis Pharmaceuticals) mitigate the pathogenic characteristic of iPSCs
derived neurons obtained from patients with C9ORF72 ALS/MND. Moreover, C9ORF downregulation, also induced by antisense treatment, does not seem to affect neuronal health.
However, further study will be needed to clarify what are the consequences of C9ORF
protein loss of function in different cell types, at different stages of MN maturation and in
more complex systems such as in vivo. It would also be of interest to study the effects of
C9ORF loss of function in the absence of RNA toxicity caused by RNA foci, in a C9ORF loss
of function system, as several groups are now investigating (e.g. V Belzil group).
What the contribution is of RAN translation in C9ORF cellular models and how this
contributes to MN pathology is still controversial. Work from T Zu et al has shown that the
C9ORF72 expansion is also expressed in the antisense direction and production of antisense RAN proteins also occurs. Therefore sense and antisense therapeutics may be
necessary for C9ORF mutation treatment. Moreover, forced expression of RAN proteins in
cells results in increased cell death. However, as discussed by C Haass in his talk, further
work will be necessary in order to understand if these proteins, in physiological conditions,
are just a by-product of the G4C2 expansion or whether they are actively contributing to the
neuronal and/or glial pathology in MND.
Importantly, these findings are now leading to clinical trials of sense- and antisense therapy
in human patients carrying the hexanucleotide expansion, as described in the conclusive talk
by TM Miller. The G4C2 expansion, together with other ALS genes such as SOD1 and
miRNAs, such as miR-155 are the next targets in clinical trials on MND patients. We are all
looking forward to seeing the outcomes of these clinical trials selectively targeted at genes
shown to be responsible for ALS.
(left) Dario presenting his poster on oligodendrocyte
dysfunction in ALS