# Memo: C9orf72 Repeat Expansion and Nucleocytoplasmic Transport Defects in ALS

**To:** Research Team  
**From:** Scientific Review  
**Date:** 2026-05-04  
**Re:** Mechanistic findings, downstream consequences, and contested questions

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## Summary

Hexanucleotide repeat expansion in *C9orf72* is the most common genetic cause of familial and sporadic amyotrophic lateral sclerosis (ALS). A prominent pathogenic mechanism involves disruption of nucleocytoplasmic transport (NCT), driven by both gain-of-function toxic RNA and dipeptide repeat proteins (DPRs) and loss-of-function of the C9ORF72 protein. Recent publications (2023–2025) have moved the field from phenomenological description to molecular pathway dissection, revealing specific protein–protein interactions and identifying candidate therapeutic targets.

## Most Recent Mechanistic Findings (Last 3 Years)

**1. Loss-of-function disrupts the Ran-GTPase gradient and forms importin granules.**  
McGoldrick et al. (2023) demonstrated that loss of C9orf72 protein perturbs the Ran-GTPase gradient in vitro and in vivo. In *C9orf72*-deficient neurons, cytoplasmic Importin-β1 assembles into compositionally diverse granules enriched for K63-ubiquitin and the stress granule marker G3BP1. These granules associate with the nuclear envelope and compromise interactions with nucleoporins (Nups), directly linking *C9orf72* haploinsufficiency to NCT disruption.[PMID: 36821445]

**2. Arginine-rich DPRs attack nuclear pore complex (NPC) components; poly-PR suppresses Pom121.**  
Arginine-containing DPRs (poly-PR and poly-GR) are the most potent disruptors of NPC integrity. Lin et al. (2025) found that poly-PR specifically downregulates the transmembrane nucleoporin Pom121 in NSC-34 cells and an AAV-based mouse model. Pom121 depletion causes mislocalization of the neuroprotective transcription factor ATF3 from the nucleus and damages the nuclear envelope. Overexpression of either Pom121 or ATF3 rescues poly-PR-induced toxicity, defining a protectable signaling axis (Σ-1R–Pom121–ATF3).[PMID: 40480424]

**3. CHMP7-mediated NPC quality control is an upstream initiating event.**  
Coyne et al. (2021) showed that the endosomal sorting complex protein CHMP7 accumulates prematurely in the nuclei of *C9orf72* and sporadic ALS iPSC-derived motor neurons, prior to Nup loss. Nuclear CHMP7 accumulation triggers Nup depletion, Ran GTPase mislocalization, and subsequent TDP-43 dysfunction. CHMP7 knockdown restores Nup homeostasis and reverses downstream disease-associated mRNA expression deficits and glutamate-induced neuronal death, positioning CHMP7 as an initiating node shared across familial and sporadic ALS.[PMID: 34321318]

**4. Karyopherin-α4 (KPNA4) pathology as a molecular signature of TDP-43 proteinopathies.**  
Atwal et al. (2025) demonstrated that cytoplasmic accumulation of TDP-43—induced by *C9orf72* or other causes—selectively depletes KPNA4 in patient spinal cord tissue and in *Drosophila* models. Structural analyses confirmed preferential interaction of KPNA4 with the nuclear localization signal of TDP-43. Genetic restoration of the KPNA4 homolog (Impα3) partially rescues nuclear TDP-43, establishing karyopherin pathology as a convergent feature of ALS/FTD rather than a late secondary event.[PMID: 40772263]

**5. Interference with RNA export machinery drives DPR production.**  
Castelli et al. (2023) developed a cell-penetrant peptide that blocks nuclear export of repeat-containing transcripts by competitively inhibiting the SRSF1–NXF1 interaction. The peptide suppressed DPR translation in C9orf72-ALS iPSC motor neurons and improved survival in co-culture with astrocytes; in a fly model, oral administration reduced DPR levels and rescued locomotor deficits.[PMID: 36857431]

## Proposed Downstream Consequences for Motor Neuron Survival

- **TDP-43 Mislocalisation:** Disrupted import/export leads to loss of nuclear TDP-43 and accumulation of phosphorylated, cytoplasmic TDP-43, which is the defining pathology in ~97% of all ALS cases.[PMID: 40772263; PMID: 34321318]
- **Translation-to-NMD shift:** Nucleocytoplasmic mislocalization of nonsense-mediated decay (NMD) factors such as eRF1 shifts cellular proteostasis toward destructive RNA decay pathways in patient neurons.[PMID: 32059759]
- **Loss of neuroprotective transcription factors:** Defects in NPC integrity cause cytoplasmic retention of ATF3 and other stress-responsive transcription factors, stripping motor neurons of adaptive responses to oxidative and excitotoxic stress.[PMID: 40480424]
- **DNA damage persistence:** Inhibition of nuclear import impairs access of DNA repair proteins to the nucleus, resulting in persistent double-strand breaks—a phenotype rescued by HDAC inhibitors in patient-derived models.[PMID: 34827542]
- **ArfGAP-1/Golgi trafficking disruption:** C9orf72 RNA repeats and DPRs specifically impair ArfGAP-1-mediated retrograde Golgi-to-ER vesicle transport, suggesting membrane trafficking as an additional downstream vulnerability beyond NCT.[PMID: 37566088]

## Strongest Open Questions Where Evidence Is Contested

1. **Which toxic species dominates?** While poly-PR is widely regarded as the most cytotoxic DPR, neuropathological studies have not consistently found arginine-containing DPRs correlating with TDP-43 pathology in patient motor cortex. Some quantitative neuropathology studies report that sense-encoded poly-GR, not poly-PR, preferentially localizes to clinically affected motor regions.[PMID: 29196813]

2. **Haploinsufficiency vs. DPR gain-of-function:** Whether the Ran-GTPase disruption reflects a pure loss-of-function phenomenon or is amplified by DPR co-expression remains debated. Ran gradient defects are observed in both *C9orf72*-null models and DPR-expressing neurons, and the relative contribution of each to sporadic disease is unresolved.[PMID: 36821445]

3. **Global nuclear lamina changes are disputed:** While NPC dysfunction is widely accepted, claims of widespread nuclear morphological changes (e.g., nuclear lamina invaginations) have been challenged. Work in iPSC patient neurons and postmortem motor cortex found no increase in lamin invaginations or gross nuclear shape alterations attributable to the *C9orf72* repeat expansion, suggesting some overexpression artifact may confound earlier reports.[PMID: 33741069]

4. **CHMP7 as a shared or specific trigger:** Whether CHMP7-mediated Nup homeostasis disruption is the universal initiating event in sporadic ALS, or predominantly relevant to repeat-expansion cases, requires validation in larger sporadic patient cohorts and distinct genetic backgrounds.

5. **Therapeutic translatability:** Drugs that restore NCT (e.g., HDAC inhibitors, karyopherin stabilizers, Nrf2 activators) show efficacy in cellular and fly models, but it remains unclear whether transport rescue must occur before TDP-43 mislocalization becomes entrenched, and whether peripheral delivery can target vulnerable motor neurons effectively.

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*Sources cited via PubMed identifiers (PMID). Full references available upon request.*