Neurodegeneration in frontotemporal lobar degeneration and motor neurone disease associated with expansions in C9orf72 is linked to TDP‐43 pathology and not associated with aggregated forms of dipeptide repeat proteins

Abstract

AimsA hexanucleotide expansion in C9orf72 is the major genetic cause of inherited behavioural variant Frontotemporal dementia (bvFTD) and motor neurone disease (MND), although the pathological mechanism(s) underlying disease remains uncertain.MethodsUsing antibodies to poly‐GA, poly‐GP, poly‐GR, poly‐AP and poly‐PR proteins, we examined sections of cerebral cortex, hippocampus, thalamus, cerebellum and spinal cord, from 20 patients with bvFTD and/or MND bearing an expansion in C9orf72 for aggregated deposits of dipeptide repeat proteins (DPR).ResultsAntibodies to poly‐GA, poly‐GP and poly‐GR detected numerous rounded cytoplasmic inclusions (NCI) within granule cells of hippocampal dentate gyrus and those of the cerebellum, as well as ‘star‐burst’ shaped NCI in pyramidal neurones of CA3/4 region of hippocampus. NCI were uncommon in Purkinje cells, and only very rarely seen in anterior horn cells. Poly‐PA antibody detected occasional NCI within CA3/4 neurones alone, whereas poly‐PR antibody did not identify any NCI but immunostained the nucleus of anterior horn cells, CA3/4 neurones and Purkinje cells, in patients with or without expansion in C9orf72, as well as in normal controls. Poly‐GA antibody generally detected more DPR than poly‐GP, which in turn was greater than poly‐GR. All patients with bvFTD + MND or MND showed plentiful p62/TDP‐43 positive inclusions in remaining anterior horn cells.ConclusionDegeneration and loss of anterior horn cells associated with expansions in C9orf72 occurs in the absence of DPR, and implies that changes involving loss of nuclear staining for and a cytoplasmic aggregation of TDP‐43 are more likely to be the cause of this.

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

“…Inclusions of unconventionally translated DPRs from both sense (poly-GP, poly-GA, and poly-GR) and antisense (poly-GP, poly-PA and poly-PR) strand RNA repeats are well-known neuropathological hallmarks of C9 ALS [3, 9, 14, 30, 31, 36, 45, 60]. We assessed the five unique DPRs in C9 ALS nervous system using DPR-specific antibodies (Fig.…”

“…Aggregations of all five DPR proteins have been observed in C9 ALS nervous systems [3, 14, 36, 60]. However, several studies have shown that TDP-43 pathology rather than DPR pathology correlates to neurodegeneration [9, 30, 31, 45]. Neuropathological evidence of intra-nuclear accumulation such as dot-like aggregates of arginine-containing poly-GR or poly-PR DPRs that would support their nuclear toxicity has been controversial, with some reports supporting their presence [32, 45, 52], and others suggesting non-specificity [9, 31, 36].…”

“…However, several studies have shown that TDP-43 pathology rather than DPR pathology correlates to neurodegeneration [9, 30, 31, 45]. Neuropathological evidence of intra-nuclear accumulation such as dot-like aggregates of arginine-containing poly-GR or poly-PR DPRs that would support their nuclear toxicity has been controversial, with some reports supporting their presence [32, 45, 52], and others suggesting non-specificity [9, 31, 36]. Using RanGap and Nup205, one study reported abnormalities that consisted of discontinuous nuclear membranes and diffuse nuclear protein deposition consistent with defective nuclear cytoplasmic transport [57].…”

C9ORF72 poly(GA) aggregates sequester and impair HR23 and nucleocytoplasmic transport proteins

“…Thus, our data not only indicate that expression of poly(GA) is sufficient to induce neurodegeneration, but also that insoluble forms of the protein (i.e., oligomers or aggregates) are the toxic culprit. Given that the frequency of poly(GA) inclusions associates with cortical neurodegeneration only moderately 27 if at all 28,46 , it seems likely that oligomers are the more likely effector, as is suspected to be the case for other aggregation-prone proteins in neurodegenerative disorders 47 .…”

RAN translation—What makes it run?

“…Several additional studies have similarly shown that DRP burden is not well-correlated with degeneration (Mackenzie et al , 2013; Davidson et al , 2015; Schludi et al , 2015). This is in contrast to TDP-43-positive inclusions, which are most abundant in the most severely affected brain regions (Mackenzie et al , 2013; Davidson et al , 2015; Mackenzie et al , 2015). …”