Abstract 3660: ATF4 is a putative tumor suppressor gene in medullary thyroid cancer

Abstract

Abstract Medullary thyroid carcinoma (MTC) is derived from the calcitonin-producing C-cells of the thyroid gland. Oncogenic mutations of the RET proto-oncogene are found in all hereditary MTC and ∼45% of the sporadic cases. However, it is clear that additional genetic lesions contribute to the development of both RET-dependent and RET independent MTC. We analyzed 30 primary medullary thyroid cancer (MTC) tumors by comparative genomic hybridization (CGH) arrays (Agilent 244K) and a subset of those tumors by ultra-high-resolution SNP arrays (Affymetrix SNP6.0). A consistent finding with both arrays was a loss localized to 22q13.1 in 40% of tumors. This region contains the ATF4 gene and loss of this gene was validated by real time PCR. ATF4 is a central mediator of integrated stress response pathway and activates expression of a cohort of downstream target genes known to be involved in cell survival, apoptosis, autophagy and senescence. We hypothesize that the loss of ATF4 expression contributes to MTC progression through mechanisms involving aberrant proliferation and escape from apoptosis. We present several findings to support this. First, ATF4 protein levels were found downregulated in 50% MTC tumor tissues (n = 43) and low ATF4 expression was associated with poor overall survival in MTC patients (HR = 5.015, 95% CI: 1.38-10.64), P = 0.013). A negative correlation was observed between RET and ATF4 protein expression in MTC tumors (r = -0.89, R square = 0.799, p<0.0001). Second, we showed that targeted heterozygote and homozygous deletion of Atf4 in mice causes C cell hyperplasia, a precancerous lesion preceding MTC. Third, ATF4 overexpression inhibits proliferation and survival of MTC derived cell lines. ATF4 decreases RET protein levels by inducing ubiquitination and degradation of activated RET. In contrast, ATF4 depletion upregulates RET protein levels as well as RET autophosphorylation. Finally, combination treatment of tyrosine kinase inhibitor (sunitinib) and eeyarestatin that stimulates ATF4 transcription, induces apoptosis of the MTC cells in a synergistic manner through activation of ATF4 and its target genes NOXA and PUMA as well. These results suggest ATF4 as a potential tumor suppressor gene in MTC. We postulate that inactivation of ATF4 is a central impediment to induction of apoptosis in MTC tumors. Therefore, strategies to upregulate ATF4 during TKI therapy would render tumor cells exquisitely sensitive to stress induced apoptosis, offering a novel synergistic treatment. Citation Format: Rozita Bagheri-Yarmand, Gilbert J. Cote, Elizabeth G. Grubbs, Michelle D. Williams, Steven I. Sherman, Robert F. Gagel. ATF4 is a putative tumor suppressor gene in medullary thyroid cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3660.