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Early BC is potentially curable, but the mortality rates still observed among BC patients demonstrate the urgent need of novel and more effective diagnostic and therapeutic options. Limitless self-renewal is a hallmark of cancer, governed by telomere maintenance. In around 95% of BC cases, this process is achieved by telomerase reactivation through upregulation of the human telomerase reverse transcriptase (h<jats:italic>TERT<\/jats:italic>). The hypermethylation of a specific region within the h<jats:italic>TERT<\/jats:italic> promoter, termed <jats:italic>TERT<\/jats:italic> hypermethylated oncological region (THOR) has been associated with increased h<jats:italic>TERT<\/jats:italic> expression in cancer. However, its biological role and clinical potential in BC have never been studied to the best of our knowledge. Therefore, we aimed to investigate the role of THOR as a biomarker and explore the functional impact of THOR methylation status in h<jats:italic>TERT<\/jats:italic> upregulation in BC.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Results<\/jats:title>\n                <jats:p>THOR methylation status in BC was assessed by pyrosequencing on discovery and validation cohorts. We found that THOR is significantly hypermethylated in malignant breast tissue when compared to benign tissue (40.23% vs. 12.81%, <jats:italic>P<\/jats:italic>\u2009&lt;\u20090.0001), differentiating malignant tumor from normal tissue from the earliest stage of disease. Using a reporter assay, the addition of unmethylated THOR significantly reduced luciferase activity by an average 1.8-fold when compared to the h<jats:italic>TERT<\/jats:italic> core promoter alone (<jats:italic>P<\/jats:italic>\u2009&lt;\u20090.01). To further investigate its biological impact on h<jats:italic>TERT<\/jats:italic> transcription, targeted THOR demethylation was performed using novel technology based on CRISPR-dCas9 system and significant THOR demethylation was achieved. Cells previously demethylated on THOR region did not develop a histologic cancer phenotype in in vivo assays. Additional studies are required to validate these observations and to unravel the causality between THOR hypermethylation and h<jats:italic>TERT<\/jats:italic> upregulation in BC.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Conclusions<\/jats:title>\n                <jats:p>THOR hypermethylation is an important epigenetic mark in breast tumorigenesis, representing a promising biomarker and therapeutic target in BC. We revealed that THOR acts as a repressive regulatory element of h<jats:italic>TERT<\/jats:italic> and that its hypermethylation is a relevant mechanism for h<jats:italic>TERT<\/jats:italic> upregulation in BC.<\/jats:p>\n              <\/jats:sec>","DOI":"10.1186\/s13148-022-01396-3","type":"journal-article","created":{"date-parts":[[2022,12,18]],"date-time":"2022-12-18T08:02:21Z","timestamp":1671350541000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":16,"title":["THOR is a targetable epigenetic biomarker with clinical implications in breast cancer"],"prefix":"10.1186","volume":"14","author":[{"given":"Joana Dias","family":"Apol\u00f3nio","sequence":"first","affiliation":[]},{"given":"Jo\u00e3o S.","family":"Dias","sequence":"additional","affiliation":[]},{"given":"M\u00f3nica Teot\u00f3nio","family":"Fernandes","sequence":"additional","affiliation":[]},{"given":"Martin","family":"Komosa","sequence":"additional","affiliation":[]},{"given":"Tatiana","family":"Lipman","sequence":"additional","affiliation":[]},{"given":"Cindy H.","family":"Zhang","sequence":"additional","affiliation":[]},{"given":"Ricardo","family":"Le\u00e3o","sequence":"additional","affiliation":[]},{"given":"Donghyun","family":"Lee","sequence":"additional","affiliation":[]},{"given":"Nuno Miguel","family":"Nunes","sequence":"additional","affiliation":[]},{"given":"Ana-Teresa","family":"Maia","sequence":"additional","affiliation":[]},{"given":"Jos\u00e9 L.","family":"Morera","sequence":"additional","affiliation":[]},{"given":"Luis","family":"Vicioso","sequence":"additional","affiliation":[]},{"given":"Uri","family":"Tabori","sequence":"additional","affiliation":[]},{"given":"Pedro","family":"Castelo-Branco","sequence":"additional","affiliation":[]}],"member":"297","published-online":{"date-parts":[[2022,12,18]]},"reference":[{"issue":"6","key":"1396_CR1","doi-asserted-by":"publisher","first-page":"394","DOI":"10.3322\/caac.21492","volume":"68","author":"F Bray","year":"2018","unstructured":"Bray F, Ferlay J, Soerjomataram I, Siegel RL, Torre LA, Jemal A. 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