The role of 5-hydroxymethylcytosine as a potential epigenetic biomarker in a large series of thyroid neoplasm

Abstract

Cytosine modifications at the 5-carbon position play an important role in the regulation of gene expression, and its deregulation is considered a hallmark of cancer. Recent studies demonstrate that 5-hydroxymethylcytosine (5-hmC) generated through 5-methylcytosine (5-mC) oxidation is significantly depleted in several human cancers. Although its role in tumour progression is still unclear, 5-hmC loss has been proposed as a marker of tumour malignancy. Concerning thyroid tumours, the literature is scarce, and the studies are sparse with a low number of cases and limited diversity of histotypes, not allowing robust conclusions. In this work, we evaluated the levels of 5-hmC, by immunohistochemistry, in a retrospective series of 318 thyroid tumours, including benign, low-risk, and malignant, classified according to the 4th edition of WHO, and we correlate its expression with demographic and clinicopathological features of the patients and tumours, aiming to verify whether 5-hmC levels can be used as a diagnostic or prognostic marker. Our data show a significant association between loss of expression of 5-hmC and extrathyroidal extension, invasive/infiltrative capsule status, lymphovascular invasion, bilaterality, multifocality, tumour malignancy, and an unprecedented link with oncocytic morphology. Additionally, in a subgroup of 183 papillary thyroid carcinoma (PTC) cases, we also observed a statistically significant loss of 5-hmC in cases with TERT promoter mutations and distant metastasis. Our study evidences an important role for 5-hmC in thyroid tumourigenesis and indicates that 5-hmC levels have the potential to be used as a diagnostic and prognostic marker.