# Gadd45B Deficiency Drives Radio-Resistance in BRAFV600E-Mutated Differentiated Thyroid Cancer by Disrupting Iodine Metabolic Genes

## TL;DR

This study shows that low levels of Gadd45B in thyroid cancer cells make them resistant to radioactive iodine treatment, and restoring Gadd45B can improve treatment response.

## Contribution

The paper identifies Gadd45B as a novel regulator of iodine metabolism and RAI sensitivity in BRAFV600E-mutated thyroid cancer.

## Key findings

- Restoring Gadd45B in thyroid cancer models increases iodine uptake and reduces tumor growth.
- Gadd45B interacts with MAP3K4 and MYCBP to regulate iodine-handling genes and tumor differentiation.
- Low Gadd45B levels correlate with poor clinical outcomes in RAI-refractory thyroid cancer patients.

## Abstract

Radioactive iodine (RAI) is a cornerstone therapy for differentiated thyroid cancer, but many tumours—especially those with the BRAFV600E mutation—become RAI-refractory and stop taking up iodine. We analysed patient tissues and public datasets and found that Gadd45B is consistently reduced in RAI-refractory disease. Using thyroid cancer cell lines and mouse models (including patient-derived xenografts), we show that restoring Gadd45B re-sensitises tumours to RAI, increases uptake, and slows growth. Mechanistically, Gadd45B modulates two complementary axes: it interacts with MAP3K4 to dampen MAPK signalling, and it restrains MYCBP–c-Myc–TERT activity. Together, these effects upregulate iodine-handling genes (e.g., NIS, TPO, Tg) and improve tumour differentiation. Clinically, low Gadd45B correlates with poor outcomes, supporting its potential as a biomarker and therapeutic target. While intratumoural recombinant Gadd45B showed benefit in vivo, we did not directly confirm its cellular entry; future studies will test delivery strategies and validate safety in patients.

Background: Differentiated thyroid cancer (DTC) is commonly treated with radioactive iodine (RAI), but resistance to RAI remains a significant clinical challenge. The molecular mechanisms driving dedifferentiation and RAI refractoriness, particularly in BRAFV600E-mutated tumors, are not fully understood. Methods: RNA sequencing was conducted on BRAFV600E-mutated DTC and RAIR-DTC tissue samples to identify differentially expressed genes. Gadd45B was identified as significantly downregulated in RAIR-DTC. Functional studies including overexpression and knockdown experiments were performed in thyroid cancer cell lines and xenograft models. Downstream targets, including MAP3K4 and MYCBP, were evaluated through co-immunoprecipitation, luciferase assays, and Western blot. The therapeutic efficacy of recombinant Gadd45B protein in combination with BRAFV600E and TERT inhibitors was assessed in patient-derived xenograft (PDX) models. Results: Gadd45B overexpression suppressed MAPK pathway activity by interacting with MAP3K4 and downregulated c-MYC stability through competition with MYCBP. These interactions enhanced the expression of iodine-metabolism genes (NIS, TPO, Tg), increased RAI uptake, and reversed tumor dedifferentiation. In vivo, Gadd45B restoration reduced tumor burden and improved RAI uptake. Combined treatment with Gadd45B protein, PLX4720, and BIBR1532 produced synergistic therapeutic effects in PDX models. Conclusions: Gadd45B plays a pivotal role in regulating the differentiation status and RAI sensitivity of BRAFV600E-mutated thyroid cancer. These findings identify Gadd45B as a promising therapeutic target for restoring RAI responsiveness in RAIR-DTC patients.