# Aberrant NSUN1 activity connects m5C-RNA modification to TDP-43 neurotoxicity in ALS/FTD

## TL;DR

This study reveals that NSUN1 activity and m5C-RNA modification are linked to TDP-43 neurotoxicity in ALS/FTD, suggesting a new pathway for disease progression.

## Contribution

The paper identifies NSUN1-mediated m5C-RNA methylation as a novel driver of TDP-43 cytoplasmic accumulation and neurotoxicity in ALS/FTD.

## Key findings

- NSUN1 methyltransferase activity causes TDP-43-induced m5C-RNA hypermethylation and cytoplasmic accumulation.
- Reducing Nsun1 activity in a Drosophila model alleviates TDP-43 pathology and degeneration.
- NSUN1 isoform 3 is associated with pathological TDP-43 interactions in ALS/FTD postmortem tissue.

## Abstract

The study shows that NSUN1-mediated m5C-RNA methylation drives TDP-43 cytoplasmic accumulation and neurotoxicity, revealing a new pathway contributing to ALS/FTD pathogenesis.

In amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), the nuclear RNA-binding protein TDP-43 mislocalises to the cytoplasm and forms insoluble aggregates, but the mechanisms controlling this remain unclear. We define a native TDP-43 interactome in human SH-SY5Y cells and identify proteins linked to the 5-methylcytosine (m5C) RNA modification as highly enriched. Using a Drosophila model of TDP-43 pathology, we show that aberrant activity of m5C-RNA methyltransferases Nsun1 drives TDP-43–induced m5C-RNA hypermethylation, whereas Nsun1 down-regulation alleviates TDP-43–induced degeneration, lifespan deficits, and cytoplasmic accumulation. In human cells, TDP-43 selectively interacts with NSUN1 isoform 3 independently of RNA. Furthermore, NSUN1 is nucleolar and TDP-43 is largely nucleoplasmic, yet they interact in both compartments, suggesting functional roles beyond their predominant localisations. In ALS/FTD postmortem frontal cortex, NSUN1 isoform 3 persists, whereas the shorter isoform is reduced, suggesting that a pool of NSUN1 capable of contributing to pathological TDP-43 interactions remains in disease. These findings suggest that TDP-43 neurotoxicity is coupled to NSUN1 activation and m5C-RNA methylation, revealing a potential therapeutic axis in ALS/FTD.