APOBEC SBS13 Mutational Signature—A Novel Predictor of Radioactive Iodine Refractory Papillary Thyroid Carcinoma

SIMPLE SUMMARY: Around 15% of papillary thyroid carcinoma (PTC) patients are not cured using standard surgery followed by radioactive iodine (RAI) therapy, and instead develop refractory disease. The aim of this study was to help identify RAI-refractory PTC patients early and guide precision medicine by performing a clinical and genomic characterization of RAI-refractory and avid PTCs. RAI-refractory PTCs had a more aggressive clinical presentation, a higher number of mutations, harbored more TERT promoter (TERTp) mutations, and were enriched with APOBEC-related mutations. Notably, the APOBEC single-base substitution (SBS) mutational signature, SBS13, and TERTp mutations were revealed to be independent predictors of RAI refractoriness in PTC. Although SBS13 and TERTp mutations alone highly predicted RAI refractoriness, when combined, they formed a stronger predictor of RAI refractoriness in PTC. This study highlights the APOBEC SBS13 mutational signature as a novel independent predictor of RAI refractoriness in a distinct subgroup of PTC. ABSTRACT: Standard surgery followed by radioactive iodine ((131)I, RAI) therapy are not curative for 5–20% of papillary thyroid carcinoma (PTC) patients with RAI refractory disease. Early predictors indicating therapeutic response to RAI therapy in PTC are yet to be elucidated. Whole-exome sequencing was performed (at median depth 198x) on 66 RAI-refractory and 92 RAI-avid PTCs with patient-matched germline. RAI-refractory tumors were significantly associated with distinct aggressive clinicopathological features, including positive surgical margins (p = 0.016) and the presence of lymph node metastases at primary diagnosis (p = 0.012); higher nonsilent tumor mutation burden (p = 0.011); TERT promoter (TERTp) mutation (p < 0.0001); and the enrichment of the APOBEC-related single-base substitution (SBS) COSMIC mutational signatures 2 (p = 0.030) and 13 (p < 0.001). Notably, SBS13 (odds ratio [OR] 30.4, 95% confidence intervals [CI] 1.43–647.22) and TERTp mutation (OR 41.3, 95% CI 4.35–391.60) were revealed to be independent predictors of RAI refractoriness in PTC (p = 0.029 and 0.001, respectively). Although SBS13 and TERTp mutations alone highly predicted RAI refractoriness, when combined, they significantly increased the likelihood of predicting RAI refractoriness in PTC. This study highlights the APOBEC SBS13 mutational signature as a novel independent predictor of RAI refractoriness in a distinct subgroup of PTC.