Investigating the Effects of C9orf72 Haploinsufficiency on TDP-43 Pathology in ALS

In amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), transactive response DNA-binding protein 43 (TDP-43), a mainly nuclear DNA/RNA binding protein, is mislocalized to the cytoplasm of diseased neurons, where it forms abnormally phosphorylated and ubiquitinated inclusions, known as TDP-43 proteinopathy. TDP-43 proteinopathy is characteristic of patients with chromosome 9 open reading frame 72 (C9orf72) G4C2 repeat expansions, the most common genetic cause of ALS/FTD. C9orf72 is a differentially expressed in normal and neoplastic cells (DENN) domain containing protein and is reported to have roles in autophagy. The mechanisms in which TDP-43 mislocalizes and how C9orf72 mutations contribute to TDP-43 proteinopathy remain unclear. I hypothesize that loss of C9orf72 could lead to autophagic deficits promoting TDP-43 pathology. To test this, a mouse model of TDP-43 proteinopathy was generated using AAV9-mediated intracerebroventricular (i.c.v) injections of EGFP-tagged pathological isoforms of TDP-43, TDP-35 and TDP-25. TDP-35 and TDP-25 mice exhibited differential expression patterns and behavioral deficits, with TDP-35 mice displaying FTD-like phenotypes and TDP-25 mice showing ALS-like phenotypes with TDP-25 forming neuronal cytoplasmic abnormally phosphorylated and ubiquitinated aggregates, recapitulating central hallmarks of TDP-43 proteinopathy. When expressed in C9orf72 deficient mice, loss of C9orf72 exacerbated neuropathological and motor phenotypes caused by TDP-25 aggregation. Changes in common autophagic markers and accumulation of p62 puncta were found in TDP-25-expressing C9orf72 deficient mice, implicating autophagy as a major pathway for a two-hit system in which TDP-43 proteinopathy synergizes with loss of C9orf72 function in ALS. These results were replicated in a C9orf72 haploinsufficient mutant TDP-43A315T transgenic mouse model. These findings demonstrate a combined effect of EGFP-TDP-25 expression and C9orf72 deficiency that leads to neuronal loss and motor behavioral deficits, suggesting a two-hit model causing neurodegeneration in ALS.

Department dc:contributor.department

Laboratory Medicine and Pathobiology

Year dc:date.issued

2023

Author dc:creator

• Lin, Lilian Tsai Wei

Advisor dc:contributor.advisor

• Robertson, Janice

• Amyotrophic Lateral Sclerosis
• Autophagy
• C9orf72
• Mouse Model
• Neurodegenerative Diseases
• TDP-43

Handle dc:identifier.uri

https://hdl.handle.net/1807/149794

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/149794