Oral Presentation ESA-SRB-AOTA 2019

Targeting TERT activation with mTOR inhibitors in thyroid cancer (#156)

Althea Bastian 1 2 , Saskia Fung 1 3 , Farhana Mollah 1 4 , Hilda Pickett 5 , Eugene Choi 5 , Roger Reddel 6 , Deborah Marsh 2 7 , Roderick Clifton-Bligh 1 7 8 , Martyn Bullock 1 7
  1. Cancer Genetic Laboratory, Kolling Institute, St Leonards, NSW, Australia
  2. School of Life Sciences, University of Technology Sydney, Sydney, NSW, Australia
  3. University of New South Wales, Sydney, NSW, Australia
  4. Western Sydney University, Sydney, NSW, Australia
  5. Telomere Length Regulation Unit, Children's Medical Research Institute, Sydney, NSW, Australia
  6. Cancer Research Unit, Children's Medical Research Institute, Sydney, NSW, Australia
  7. University of Sydney, Sydney, NSW, Australia
  8. Department of Endocrinology, Royal North Shore Hospital, Sydney, NSW, Australia

New therapies to treat advanced forms of thyroid cancer (TC) are urgently needed. Healthy somatic cells cannot divide beyond a finite number of cell-divisions, as progressive loss of their chromosomal end-structures called telomeres, ultimately triggers replicative senescence or apoptosis. In contrast, cancer cells possess an unlimited replicative capacity, afforded to them by Telomerase activation, which functions to sustain telomere length. Activating mutations within the promoter of TERT (TERTp), which encodes the catalytic component of Telomerase, occur at high frequencies in TC. Synergistic interaction between the TERTp and BRAF or RAS driver mutations promotes tumor progression and aggressiveness. The activation mTOR by upstream BRAF or RAS signaling drives several thyroid tumorigenic processes and the kinase is a well-established chemotherapeutic target. In this study we investigated whether mTOR plays a role in the regulation of TERTp activity.

 

Dose-dependent changes in TERTp activity following 24 hr treatment with Everolimus, AZD8055 or Omipalisib were determined using CRISPR-edited SW1736 cells in which the endogenous mutated TERTp controls expression of a luciferase gene. Then, SW1736, C643, 8505C, TPC1 cell-lines were treated for 24-96 hrs with IC50-dose mTOR inhibitor versus DMSO control, and TERT expression and telomerase activity were measured by qRT-PCR and qTRAP respectively. Phosphoblots were performed to confirm mTOR inhibition.

 

The inhibitors Everolimus, AZD8055 or Omipalisib potently suppressed TERTp activity with IC50 doses of 20, 50 and 50 nM respectively. Correspondingly, a 20-50% reduction in TERT expression was observed at 24 hrs post-treatment, which conferred a similar down-regulation in telomerase activity. Interestingly, at later time-points there appeared to a compensatory upregulation in TERT expression, although the inhibition of telomerase activity was sustained.

 

This is the first report implicating mTOR in the regulation of the mutated TERTp, and highlights the therapeutic potential of mTOR inhibitors for TERTp harbouring thyroid cancers.