5hmC immunohistochemistry: a predictor of TERT promoter mutational status in follicular thyroid carcinoma?

Abstract

Abstract Purpose TERT promoter mutations and TERT gene expression correlate to adverse prognosis in follicular thyroid carcinoma (FTC), identifying cases at risk of poor outcome. As loss of 5-hydroxymethylcytosine (5hmC) immunoreactivity has been associated with TERT promoter mutations in papillary thyroid carcinoma, this study sought to analyze the levels of 5hmC in a well-characterized cohort of follicular thyroid tumors with available TERT data. Methods 29 tumors (26 FTCs, 2 follicular thyroid tumors of uncertain malignant potential (FT-UMPs), and one oncocytic thyroid carcinoma) with known TERT promoter mutational status and TERT gene expression levels were assessed for 5hmC immunoreactivity using two monoclonal antibodies (clones RM236 and 4D9.) Slides were analyzed using a semiquantitative scoring system. Results Of the 10 tumor cases with a TERT promoter mutation and TERT expression, only one was scored as negative with both antibodies (1/10; 10%), while the remaining 9 cases (9/10; 90%) exhibited various degrees of positivity for at least one antibody. Of the 19 TERT wild-type tumors, no case (0/19; 0%) was scored as negative using the RM236 clone, and two cases (2/19; 11%) using the 4D9 clone. The differences between TERT promoter mutated and wildtype groups were non-significant (Fisher’s Exact test P = 0.35 and 0.59 respectively). The sensitivity and specificity for 5hmC IHC to detect mutated cases were 10% and 100% for RM236 and 20% and 89% for 4D9 respectively. Conclusion 5hmC IHC is not a highly sensitive marker for the detection of TERT promoter mutations in follicular thyroid tumors. Further analyses in larger cohorts are warranted.