# NSUN2 mediates intestinal stem cell expansion and colorectal tumour initiation via MAPK/ERK signalling

## TL;DR

This study shows that NSUN2, an enzyme that modifies RNA, helps intestinal stem cells grow and start colorectal tumors by affecting a key signaling pathway.

## Contribution

The study identifies NSUN2 as a novel regulator of intestinal stem cell expansion and tumor initiation through m5C methylation and MAPK/ERK signaling.

## Key findings

- NSUN2 is upregulated in colorectal cancer models and human tumors, and its depletion reduces tumor initiation.
- NSUN2 mediates m5C methylation on mRNAs of ISC regulators and MAPK/ERK pathway components.
- Loss of NSUN2 reduces ERK phosphorylation, but oncogenic KrasG12D can restore ERK signaling and ISC expansion.

## Abstract

Colorectal cancer is initiated by loss of APC, which drives expansion of LGR5+ intestinal stem cell (ISC) populations. Whilst LGR5 + ISC expansion is a critical step for tumour initiation and progression, its regulation is poorly understood. Emerging evidence suggests post-transcriptional RNA modifications play a key role in cancer biology, but their role in CRC initiation has not been explored. Here, we identify the m5C methyltransferase NSUN2 as a key regulator of ISC expansion and intestinal tumourigenesis. NSUN2 is upregulated in multiple CRC mouse models and human tumours, and its depletion impairs ISC expansion and hyperproliferation, leading to reduced tumour initiation. Transcriptome-wide bisulfite sequencing revealed that NSUN2 mediates m5C methylation on mRNAs encoding key ISC regulators and components of the MAPK/ERK pathway. Mechanistically, loss of NSUN2 reduces ERK phosphorylation in Apc-deficient models, and oncogenic KrasG12D expression is sufficient to restore ERK signalling and rescue ISC expansion. Together, this establishes a novel role for NSUN2 as a key regulator of ISC-driven CRC initiation and describes a critical molecular mechanism linking m5C methylation to MAPK-driven stem cell transformation.