Abstract 770: Identification of MASTL as novel mitotic vulnerability of thyroid tumor cells

Abstract

Background: Despite thyroid tumors are generally curable, a fraction of patients develops resistance to therapy and tumors progress towards undifferentiated forms, which have a poor prognosis and whose treatment is still a demanding challenge. To identify potential novel targets for thyroid cancer treatment, especially for those aggressive forms, we faced the non-oncogene addiction paradigm, which states that tumor cells become addicted to stress support pathway genes, not required to the same degree by normal ones. To this aim, in a previous study we screened a siRNA library on normal and tumor thyroid cell lines and found several genes essential for the growth of tumor but not normal cells; among the top-ranking hits we found MASTL, a serine/threonine kinase involved in mitosis regulation. Here, we investigated the mechanisms of growth inhibition mediated by MASTL silencing in thyroid tumor cells, in order to identify whether it could represent a potential target for tumor treatment. Methods: MASTL gene expression profile was assessed in 58 normal thyroid tissue samples and 72 papillary (PTC), 17 poorly differentiated (PDTC) and 31 anaplastic (ATC) thyroid carcinoma samples, derived from a meta-analysis of different publicly available datasets. HTC/C3 and 8505C cell lines (ATC-derived) were transfected with MASTL and control siRNAs and tested for cell growth, nuclear aberrations, DNA damage and cell death. Results: By gene expression data analysis, we found that MASTL expression levels in PTC and PDTC samples were similar to those observed in control group, while the highest levels were evidenced in ATC. We next tested by different growth assays the effects of MASTL depletion on HTC/C3 and 8505C cells and found that MASTL silencing impaired their proliferation capacity. Notably, MASTL depletion increased the percentage of cells presenting nuclear anomalies, such as micronuclei, multiple nuclei and lobular nuclei, which are indicative of mitotic catastrophe. We observed that MASTL depletion was associated with increased phosphorylation of CDK1 (Y15), supportive of impairment of cell growth, and of H2AX (Ser139), indicative of increase of DNA damage. Furthermore, MASTL depletion was associated with increased expression of the apoptotic markers cleaved caspase 3 and cleaved PARP, thus suggesting that MASTL inhibition may cause thyroid tumor cell death through apoptosis. These finding was further corroborated by Annexin V assay, which showed an increase of apoptotic and necrotic cells upon MASTL silencing, compared to control. Conclusions: Our results demonstrated that MASTL inhibition is able to impair the growth of thyroid tumor cell lines and is associated with mitotic catastrophe and tumor cell death, thus suggesting that it may represent a valuable candidate for validation as therapeutic target in thyroid cancer. Citation Format: Elena Cetti, Maria Chiara Anania, Katia Todoerti, Giuseppe Mauro, Antonino Neri, Angela Greco. Identification of MASTL as novel mitotic vulnerability of thyroid tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 770.