Diagnostic and prognostic markers in thyroid carcinoma with focus on TERT activation

Well-differentiated follicular cell-derived thyroid carcinomas are usually categorized into papillary thyroid carcinoma (PTC) and follicular thyroid carcinoma (FTC), which represent the most common and second most common types of thyroid carcinoma, respectively. PTC patients generally have a good prognosis, although a subset of patients progress in their disease with poor clinical outcome. Also, it remains a diagnostic challenge to differentiate FTC pre-operatively from follicular thyroid adenoma (FTA) and follicular tumor of uncertain malignant potential (FT-UMP). For both PTC and FTC, promoter mutations in the telomerase reverse transcriptase (TERT) gene have been identified as an important underlying genetic event. However, other TERT-activating alterations are less studied in thyroid carcinoma. Thus, this thesis aimed to investigate potential diagnostic and prognostic biomarkers, and TERT-activating alterations in FTC and PTC, respectively.

In Paper I, we identified high cytological Ki-67 proliferation index and larger tumor size as predictors of FTC in follicular thyroid tumors using univariate and multivariate analyses. In the FTC subgroup, extrathyroidal extension and the widely invasive sub-type were associated with higher cytological Ki-67 index. Taken together, cytological Ki-67 index could potentially add pre-operative diagnostic information for a subset of patients.

In Paper II, we found that TERT expression was coupled with co-existing TERT copy number gain, promoter mutation or hypermethylation in follicular thyroid tumors. Several of these alterations were observed in higher frequencies in FTC/FT-UMP and associated with poor clinical outcome in FTC. Thus, they are promising as biomarkers for diagnosis and prognostication.

In Paper III, the E26 transformation-specific (ETS) transcription factor GABPA was shown to exhibit mutant TERT promoter-activating properties in thyroid carcinoma cells. In contrast, GABPA also exhibited tumor-suppressive properties in vitro and in vivo, highlighting its dual roles. In PTC tumors, low expression of GABPA was associated with poor clinical outcome. Moreover, DICER1, involved in the microRNA machinery, was identified as a potential downstream target for the GABPA-mediated tumor suppressive effect. We conclude that loss of tumor suppressive functions of GABPA is coupled to PTC progression.

In Paper IV, the 28 known ETS transcription factors were analyzed in relation to clinical phenotype in PTC. EHF was found to be upregulated in PTC and associated with aggressive clinical features, BRAFV600E and TERT promoter mutation or expression. In PTC cells with concurrent BRAFV600E and TERT promoter mutations, over-expression of EHF induced an increase in TERT expression. Collectively, EHF is a potential oncogenic ETS factor in PTC.

In summary, the findings in this thesis add insights into the underlying TERT-activating alterations in follicular cell-derived thyroid carcinomas. Further, several potentially clinically relevant biomarkers for diagnosis and prognostication for this patient group were identified.

List of scientific papers

I. Mu N, Juhlin CC, Tani E, Sofiadis A, Reihnér E, Zedenius J, Larsson C, Nilsson IL. High Ki-67 index in fine needle aspiration cytology of follicular thyroid tumors is associated with increased risk of carcinoma. Endocrine. 2018 Aug;61(2):293-302.
https://doi.org/10.1007/s12020-018-1627-z

II. Paulsson JO, Mu N, Shabo I, Wang N, Zedenius J, Larsson C, Juhlin CC. TERT aberrancies: a screening tool for malignancy in follicular thyroid tumours. Endocr Relat Cancer. 2018 Jul;25(7):723-733.
https://doi.org/10.1530/ERC-18-0050

III. Yuan X*, Mu N*, Wang N*, Strååt K, Sofiadis A, Guo Y, Stenman A, Li K, Cheng G, Zhang L, Kong F, Ekblad L, Wennerberg J, Nilsson IL, Juhlin CC, Larsson C, Xu D. GABPA inhibits invasion/metastasis in papillary thyroid carcinoma by regulating DICER1 expression. Oncogene. 2019 Feb;38(7):965-979. *Authors contributed equally.
https://doi.org/10.1038/s41388-018-0483-x

IV. Mu N, Gao J, Wang N, Zedenius J, Nilsson IL, Juhlin CC, Xu D, Larsson C. The ETS transcription factor EHF is upregulated in papillary thyroid carcinoma carrying BRAFV600E and TERT promoter mutations with impact on poor patient outcome. [Manuscript]