Production of poly(GA) in C9ORF72 patient motor neurons derived from induced pluripotent stem cells
GGGGCC (G4C2) repeat expansion in the first intron of C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) [3, 9]. A key pathological hallmark of C9ORF72-related ALS/FTD is the accumulation of dipeptide repeat (DPR) proteins synthesized from both sense and antisense repeat RNAs in affected neurons [2, 8, 14]. The toxicity of poly(GA), poly(GR) and some other DPR proteins in cell and animal models has been well documented, including in mouse models. However, it is poly(GR) whose distribution in patient brains seems to best correlate with neurodegeneration [10, 11].
How DPR proteins are synthesized in human C9ORF72 neurons is unknown. One widely cited hypothesis is repeat-associated non-AUG (RAN) translation, which posits a novel mechanism by which ribosomes directly initiate translation on expanded repeats . Over time, the term “RAN translation” has evolved to also refer to the phenomenon, that various expanded repeat sequences such as G4C2 repeats can be translated in different frames. However, whether this truly represents a novel translation mechanism has not yet been vigorously tested in human neurons.
To determine whether this deletion affects DPR production, in addition to the intron deletion and the parental iPSC lines described above, an iPSC line derived from a control subject  and an isogenic line in which expanded G4C2 repeats have been deleted by CRISPR-Cas9  (Fig. 1h) were differentiated three times into motor neurons. After 1.5 months, motor neuron cultures were collected and analyzed for poly(GA) content, in a blinded manner, with a new Meso Scale Discovery (MSD) immunoassay developed at Biogen Inc (see Suppl. Information). We found that poly(GA) production was abolished in neurons containing the intronic deletion (Fig. 1i). The absence of poly(GA) production was not due to the absence of the mRNA translation template, as repeat-containing introns were still spliced (Fig. 1c, e) and the level of the pre-mRNA was not significantly decreased (Fig. 1g). More importantly, both poly(GR) and poly(GP) were still produced (Fig. 1j, k). To confirm these results, we differentiated again the no-repeats, the C9ORF72 parental and the two intronic deletion lines 6 or 3 more times into 3-month-old motor neuron cultures. Once more, blinded poly(GA) measurements confirmed the absence of poly(GA) in intronic deletion lines (Fig. 1l), while poly(GR) and poly(GP) production was not decreased (Fig. 1m, n).
Next, we used this model to investigate the contribution of poly(GA) to molecular phenotypes found in C9ORF72 iPSC-derived neurons or patient brain tissues. Upregulation of the expression of heat shock proteins and TAR DNA binding protein 43 (TDP-43) have been reported in ALS/FTD patient brain tissues [6, 7]. Here, we show that both HSPA1B and TARDBP mRNA are upregulated in 4-month-old motor neuron cultures (Fig. 1o). However, in the absence of poly(GA), these molecular phenotypes were not rescued, suggesting that, at least for these phenotypes in this experimental system, poly(GA) was not a key toxic DPR protein.
Our results show that in C9ORF72 patient neurons, ~ 1000 copies of G4C2 repeats alone are not sufficient to direct ribosome entry to initiate translation of poly(GA) frame. What remains to be determined is whether other cis elements within the first intron initiate the synthesis of poly(GR) and other DPR proteins or whether their production results from bona fide “RAN translation” in C9ORF72 patient neurons. These mechanistic investigations will help in the design of therapeutic approaches that aim to decrease DPR protein production.
This work was supported by grants from the NIH (R37NS057553, R01NS101986, and R01NS093097 to FBG and R21NS112766 to SA), the MDA Foundation and the Target ALS Foundation (F-BG), as well as the Frick Foundation for ALS Research, Alzheimer's Association (2016-NIRG-396129) and ALS Association (SA).
- 10.Saberi S, Stauffer JE, Jiang J, Garcia SD, Taylor AE, Schulte D et al (2018) Sense-encoded poly-GR dipeptide repeat proteins correlate to neurodegeneration and uniquely co-localize with TDP-43 in dendrites of repeat-expanded C9orf72 amyotrophic lateral sclerosis. Acta Neuropathol 135:459–474CrossRefGoogle Scholar
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