Abstract 400: Noncanonical role of SEC23B in thyroid carcinogenesis: Anemia gene meets cancer

Abstract

Abstract Hereditary cancer syndromes serve as powerful models to uncover cancer-relevant genes. Cowden syndrome (CS), typically caused by germline PTEN mutations, is a heritable cancer syndrome with high risks of breast and thyroid cancers. However, there is a growing subset of CS without PTEN mutations. By exome sequencing cum family-studies, we identified heterozygous SEC23B variants in CS-associated and apparently sporadic thyroid cancer. Functional characterization revealed mutant CS-associated SEC23B led to endoplasmic reticulum (ER)-stress-mediated cell-colony formation and survival, growth and invasion, reflecting major hallmarks of cancer. Classically, SEC23B is a component of coat protein complex II (COPII) vesicles that transport secretory proteins from the ER to the Golgi apparatus. Germline homozygous or compound heterozygous loss-of-function SEC23B mutations cause Congenital Dyserythropoietic Anemia Type II (CDA II), resulting in decreased SEC23B levels. Intriguingly, our in vitro data does not recapitulate the CDA II-like decreased SEC23B protein expression, suggesting possible change-of-function effects in a cancer context. Here, we show mutant SEC23B exists within nucleoli, besides classical distribution at the ER/Golgi. This occurs in the absence of nucleolar localization of other COPII protein components and does not compromise canonical secretory function. Global transcriptomic and polysome profiling analyses reveal increased expression of ribosomal protein and translation-related genes, accompanied by enhanced translational capacity in mutant cells in response to ER stress. We also show mutant SEC23B directly binds to Upstream Binding Transcription Factor (UBF), concomitantly with increased UBF binding at the ribosomal DNA (rDNA) promoter region with ER stress. Unbiased proteomic analyses further show SEC23B, both wildtype and mutant, can interact with proteins in the ER stress and EIF2 signaling pathways. We validate through immunoprecipitation the binding interaction between SEC23B and UBA52, with notably increased binding with mutant SEC23B. UBA52 is ribosomal protein RPL40, known to regulate selective translation of stress-related transcripts, protein synthesis, and the cell cycle. Knockdown of SEC23B in vitro resulted in a concomitant decrease in protein levels of UBA52, further supporting that SEC23B and UBA52 are potentially co-regulated. All in all, our data suggest that SEC23B has non-canonical COPII-independent function, particularly within the ribosome biogenesis pathway, that may contribute to the pathogenesis of cancer-predisposition. Citation Format: Lamis Yehia, Ying Ni, Supriya Jindal, Ata Abbas, Anton A. Komar, Charis Eng. Noncanonical role of SEC23B in thyroid carcinogenesis: Anemia gene meets cancer [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 400.