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

**To:** Principal Investigator  
**From:** Research Assistant  
**Date:** 4 May 2026  
**Subject:** State of Evidence (2023–2025) for C9orf72–NCT Pathogenesis in ALS

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## 1. Summary
A hexanucleotide (GGGGCC)n repeat expansion in *C9orf72* is the most common genetic cause of familial ALS/FTD, accounting for ~40% of familial cases (McGoldrick & Robertson, 2023). A prominent pathogenic mechanism is impairment of nucleocytoplasmic transport (NCT) via three convergent routes: (i) direct interference by arginine-rich dipeptide repeat proteins (DPRs) with nuclear transport receptors and the Ran-GTPase system; (ii) loss of C9orf72 protein itself, which disrupts the Ran gradient; and (iii) physical injury to nuclear pore complex (NPC) architecture driven by mechanical stress on the nuclear envelope. Recent work has strengthened the causal role of NCT disruption while also highlighting unresolved debates about the relative contribution of each mechanism.

## 2. Recent Mechanistic Findings (2023–2025)

### 2.1 Direct Molecular Interference by DPRs
- **PolyPR binds multiple transport components in silico:** Molecular dynamics simulations reveal that polyPR, the most toxic DPR, directly interacts with several Importin-α family members, CAS, RanGAP, and, at lower salt concentrations, RanGEF and NTF2 (Jafarinia et al., 2024, *eLife*, doi:10.7554/eLife.89694). The same study predicts that polyPR can interfere with RanGTP/RanGDP binding, cargo-NLS release from Importin-α, and nuclear export of Importin-α itself—offering a molecular rationale for broad-spectrum NCT deficits.
- **Membrane trafficking route targeted:** Expanded *C9orf72* RNA repeats and DPRs specifically affect retrograde Golgi-to-ER vesicle transport by inhibiting ArfGAP-1 function, implicating a disruption of intra-cellular trafficking that extends beyond classical NCT (Rossi et al., 2023, *Cells*, doi:10.3390/cells12152007).

### 2.2 Loss of C9orf72 Protein Disrupts the Ran Gradient
- McGoldrick and colleagues demonstrated that **loss of the C9orf72 protein alone is sufficient to perturb the Ran-GTPase gradient and generate compositionally diverse cytoplasmic Importin β-1 granules** (McGoldrick et al., 2023, *Cell Reports*, doi:10.1016/j.celrep.2023.112134). These granules associate with the nuclear envelope, are co-immunoreactive for G3BP1 and K63-ubiquitin, and disrupt interactions with NPC proteins. This finding links haploinsufficiency directly to the same NCT defects caused by toxic DPRs, suggesting loss-of-function and gain-of-function pathways converge.

### 2.3 Altered Nuclear Envelope and NPC Architecture
- **LINC complex and NPC injury:** In iPSC-derived motor neurons, C9-ALS postmortem tissue, and spinal cord organoids, extensive alterations to the LINC complex are evident, correlating with nuclear morphological abnormalities **independently of TDP-43 mislocalization** (Sirtori et al., 2024, *Acta Neuropathol Commun*, doi:10.1186/s40478-024-01778-z). A related preprint showed that mechanical stresses transmitted from the cytoskeleton to the nucleus via the LINC complex cause NPC injury in C9orf72 mutant neurons; disconnecting the nucleus from the cytoskeleton rescues NPC integrity and reduces DNA damage (Sirtori et al., 2024, bioRxiv).
- **CHMP7 nuclear surveillance pathway:** NPC injury may be amplified by dysregulation of the ESCRT-III nuclear surveillance pathway. CHMP7 nuclear accumulation—triggered by SMN complex/SmD1 dysregulation and splicing defects—causes NPC damage and TDP-43 dysfunction (Al-Azzam et al., 2024, *Neuron*, doi:10.1016/j.neuron.2024.10.007; Keeley et al., 2024, *Acta Neuropathol Commun*).

### 2.4 Therapeutic-Targeted NCT Rescue
- **Blocking repeat RNA nuclear export:** A cell-penetrant peptide that disrupts the SRSF1–NXF1 interaction prevents nuclear export of *C9orf72*-repeat transcripts, reduces toxic DPR translation, increases motor neuron survival in iPSC-derived co-cultures, and rescues locomotor deficits in Drosophila (Castelli et al., 2023, *Sci Transl Med*, doi:10.1126/scitranslmed.abo3823). This directly implicates **export of repeat-containing RNA as a druggable NCT node**.
- **SIGMAR1–POM121–TFEB axis:** C9 HRE disrupts TFEB nuclear import by interfering with the interaction between the molecular chaperone SIGMAR1 and the nucleoporin POM121, which normally recruits KPNB1/importin-β1. Restoring this axis (e.g., via SIGMAR1 agonist pridopidine) rescues TFEB nuclear entry and autophagy in NSC34 cells (Wang et al., 2023, *Autophagy*, doi:10.1080/15548627.2022.2063003).

## 3. Downstream Consequences for Motor Neuron Survival
1. **TDP-43 Nuclear Depletion / Cytoplasmic Aggregation:** Impaired NCT causes TDP-43 mislocalization into the cytoplasm, the near-universal pathological hallmark of ALS. Loss of nuclear TDP-43 disrupts splicing (e.g., STMN2 cryptic exon inclusion), while cytoplasmic TDP-43 forms toxic inclusions (McGoldrick & Robertson, 2023, *Front Cell Neurosci*).
2. **Autophagy Failure:** TFEB remains cytoplasmic when NCT is damaged, shutting down lysosomal and autophagic programs necessary for clearance of DPRs and aggregated proteins (Wang et al., 2023).
3. **DNA Damage:** NPC injury breaches the nuclear envelope barrier, permitting cytoskeletal-mediated mechanical damage and accumulation of DNA damage in motor neurons (Sirtori et al., 2024, bioRxiv).
4. **Bulk mRNA Export Defects:** Expanded repeats cause marked accumulation of poly(A) mRNAs within nuclei, globally perturbing gene expression (Rossi et al., 2023; foundational work by Freibaum & Taylor, 2017, *Front Mol Neurosci*).

## 4. Open / Contested Questions
- **Relative contribution of loss-of-function versus gain-of-function:** The *C9orf72* expansion reduces gene expression and simultaneously produces toxic RNA foci and DPRs. Whether NCT deficits are driven primarily by loss of the C9orf72 protein (McGoldrick et al., 2023), direct DPR–NPC binding (Jafarinia et al., 2024), or RNAs that sequester transport factors remains unsettled. The field increasingly favors a **convergent model**, but quantitative human data are lacking.
- **Which DPR drives NCT toxicity?** Poly-GR and poly-PR are arginine-rich and the most toxic to transport components; however, poly-GA is the most abundant DPR in patient brain. Whether abundance or intrinsic biochemical potency is more relevant to human motor neuron death is contested.
- **Cause versus consequence:** It remains debated whether NCT disruption is an **upstream initiating event** or a downstream consequence of cellular stress/aggregate load. That blocking repeat RNA export or restoring the Ran gradient can rescue disease phenotypes in model systems supports a causal role, but temporal resolution in human neurons is lacking.
- **Direct NPC obstruction versus soluble transport-factor sequestration:** Poly-PR binds FG-repeat nucleoporins and transport receptors in model systems, yet whether physical obstruction of the NPC channel or soluble sequestration of RanGAP/importins dominates the pathology is unresolved.

## 5. Conclusion
Evidence from the last three years has tightened the causal link between the *C9orf72* expansion and NCT failure, implicating DPR-mediated binding to transport factors, C9orf72 loss-of-function perturbation of the Ran gradient, and physical NPC injury driven by nuclear envelope–cytoskeletal mechanics. Therapeutically, both **upstream blockade of repeat RNA export** and **downstream restoration of transport-factor/nucleoporin function** have shown preclinical rescue. The key unresolved issues are the relative weight of loss-of-function versus gain-of-function pathways and whether restoring NCT in human motor neurons can materially alter disease progression.

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**Key References**
- Jafarinia, H., van der Giessen, E., & Onck, P. (2024). eLife. doi:10.7554/eLife.89694
- McGoldrick, P., Lau, A., You, Z., Durcan, T. M., & Robertson, J. (2023). Cell Reports. doi:10.1016/j.celrep.2023.112134
- Sirtori, R., et al. (2024). Acta Neuropathol Commun. doi:10.1186/s40478-024-01778-z
- Castelli, L. M., et al. (2023). Sci Transl Med. doi:10.1126/scitranslmed.abo3823
- Al-Azzam, N., et al. (2024). Neuron. doi:10.1016/j.neuron.2024.10.007
- Rossi, S., et al. (2023). Cells. doi:10.3390/cells12152007
- Wang, S.-M., et al. (2023). Autophagy. doi:10.1080/15548627.2022.2063003
- Lee, K.-H., et al. (2016). Cell. doi:10.1016/j.cell.2016.10.002
- McGoldrick, P., & Robertson, J. (2023). Front Cell Neurosci. doi:10.3389/fncel.2023.1247297