# NOX4-derived oxidative DNA damage impairs thyroid differentiation through an epigenetic mechanism in BRAF-mutated radioactive iodine refractory papillary thyroid cancer cells

## TL;DR

This study shows how NOX4-generated oxidative damage disrupts thyroid cell differentiation in BRAF-mutated thyroid cancers, reducing radioiodine therapy effectiveness.

## Contribution

The paper reveals a novel epigenetic mechanism by which NOX4-derived DNA damage silences NIS expression in BRAFV600E-mutated thyroid cancer cells.

## Key findings

- NOX4-generated ROS cause oxidative DNA damage that recruits repair proteins and DNMT1, blocking thyroid differentiation factors.
- Inhibiting MAPK and TGF-β1 pathways restores chromatin accessibility for PAX8 and NKX2.1 in BRAF-mutated cells.
- RAI-refractory tumors show elevated NOX4, OGG1, MSH2/MSH6, and phospho-Smad3 expression compared to normal tissue.

## Abstract

Radioiodine (RAI) therapy, used for treating differentiated thyroid cancers (DTCs), hinges on the functional expression of the sodium-iodide symporter (NIS). However, up to 60% of papillary thyroid carcinomas, the most common DTC subtype, harbor BRAFV600E mutations, which are strongly associated with reduced NIS expression, impaired RAI uptake, and poor differentiation scores. For patients with RAI-refractory, a promising therapeutic strategy is to restore RAI sensitivity by inducing tumor redifferentiation. Here, we demonstrate that NOX4-derived reactive oxygen species (ROS) contribute to NIS repression in BRAFV600E-mutated thyroid cancer cells. Particularly, NOX4-generated oxidative DNA damage recruits DNA repair proteins, including OGG1 and MSH2/MSH6 proteins, which in cooperation with DNMT1, convert these lesions into transcription-blocking events. This mechanism prevents key thyroid differentiation transcription factors, PAX8 and NKX2.1, from accessing their chromatin binding sites, thereby silencing NIS expression. Importantly, combining inhibition of the MAPK pathway, which regulates MSH2/MSH6 and DNMT1 expression, and the TGF-β1 pathway, which controls NOX4 expression, restores PAX8 and NKX2.1 chromatin occupancy. Compared to normal tissue an increased expression of NOX4, OGG1, MSH2/MSH6 proteins and phospho-Smad3 was found in RAI Refractory BRAFV600E mutated tumors. Collectively, our findings reveal a mechanistic basis for NOX4's role in thyroid dedifferentiation.