ERK Signaling in Thyroid Cancer: Lineage Suppression, Epigenetic Reprogramming, and Theranostic Reversal

Differentiated thyroid carcinoma is a morphologic diagnosis, not a direct measure of preserved thyroid function. Although most papillary, follicular, and oncocytic thyroid carcinomas retain architectural or cytologic features of follicular-cell derivation, a clinically important subset loses the molecular machinery required for effective iodine transport, organification, and radioiodine therapy. This functional divergence is central to the biology of radioiodine-indifferent and radioiodine-refractory thyroid cancer. This review examines the role of oncogenic MAPK/ERK signaling as a principal regulator of thyroid lineage suppression and therapeutic failure. Aberrant activation of the RAF-MEK-ERK axis, most prominently through BRAF^V600E, RAS alterations, and receptor tyrosine kinase fusions, does more than promote proliferation. It reshapes follicular-cell identity by repressing thyroid-lineage transcriptional programs and silencing iodine-handling genes, including SLC5A5/NIS, TPO, TG, TSHR, SLC26A4, and related components of the iodine metabolic transcriptome. This repression is mediated through coordinated transcriptional, epigenetic, and post-transcriptional mechanisms involving lineage transcription factors such as PAX8, NKX2-1, and FOXE1; chromatin-modifying programs including histone deacetylation and PRC2/EZH2-associated repression; DNA methylation; and non-coding RNA networks. Importantly, ERK-driven functional dedifferentiation is not always a fixed terminal state. Preclinical models and early clinical trials demonstrate that pharmacologic inhibition of the MAPK pathway can restore iodine avidity in selected radioiodine-refractory tumors, creating the theranostic basis for redifferentiation therapy. Iodine-124 PET/CT and lesion-level dosimetry have shown that restored iodine uptake can be measured diagnostically and then exploited therapeutically with iodine-131. However, clinical translation remains limited by inter- and intratumoral heterogeneity, incomplete durability of response, adaptive pathway reactivation, persistent epigenetic repression, tumor microenvironmental influences, and the absence of standardized dosimetric thresholds. We propose that advanced follicular-cell-derived thyroid cancers should be understood along a functional differentiation continuum rather than through morphology alone. Within this framework, radioiodine refractoriness reflects not merely treatment failure, but a therapeutically interrogable state of lineage suppression. Refinement of redifferentiation therapy will require genotype-informed patient selection, functional imaging, standardized dosimetry, rational combination strategies, and prospective trials capable of distinguishing true restoration of radioiodine therapeutic efficacy from the antiproliferative effects of kinase inhibition alone.