Background: Non-medullary thyroid cancer (NMTC) is the most frequent endocrine tumor with in most cases a good prognosis as a result of implementing treatment regimens consisting of thyroidectomy and 131-I radioactive iodide (RAI) ablation. In contrast, 30-40% of patients with metastatic NMTC are unresponsive to 131I radioactive iodide (RAI) treatment as a result of tumor dedifferentiation that could lead to poorly differentiated NMTC and, ultimately, anaplastic thyroid cancer (ATC). Currently, underlying molecular mechanisms of NMTC dedifferentiation still remain elusive and predictive biomarkers are lacking. In the present study we therefore aim to discover molecular biomarkers in primary tumors that predict RAI sensitivity and to elucidate underlying intracellular processes involved.

Methods: A retrospective cohort of 71 NMTC patients, including all histological subtypes, was gathered for this study consisting of proven RAI-sensitive differentiated NMTC patients (N=35), proven RAI-refractory (poorly) differentiated NMTC patients (N=28) and ATC patients (N=8). Total DNA and RNA was extracted from archived FFPE tumor tissues. Extensive intratumoral mutation profiling, gene fusions analysis, TERT promoter mutation analysis and FFPE-compatible RNA sequencing was performed in all patients.

Results: RAI-refractory NMTC and ATC patients were diagnosed at older age and displayed less favorable TNM staging as compared to RAI-sensitive NMTC patients. Genetic analyses revealed an increased mutational load in RAI-refractory NMTC, including mutations in AKT1, PTEN, TP53 and TERT promoter. Furthermore, numerous genes were observed to be differentially expressed between patient subgroups, including genes encoding transcription factors and genes involved in thyroid differentiation and the immune response.

Conclusion: Important clinical, genetic and transcriptomic differences are identified between patients with RAI-sensitive NMTC, RAI-refractory NMTC and ATC, providing molecular insights into the dedifferentiation process and identifying potential biomarkers of RAI resistance.