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The cancer-associated microprotein CASIMO1 controls cell proliferation and interacts with squalene epoxidase modulating lipid droplet formation

Abstract

Breast cancer is a leading cause of cancer-related death in women. Small open reading frame (sORF)-encoded proteins or microproteins constitute a new class of molecules often transcribed from presumed long non-coding RNA transcripts (lncRNAs). The translation of some of these sORFs has been confirmed, but their cellular function and importance remains largely unknown. Here, we report the identification and characterization of a novel microprotein of 10 kDa, which we named Cancer-Associated Small Integral Membrane Open reading frame 1 (CASIMO1). CASIMO1 RNA is overexpressed predominantly in hormone receptor-positive breast tumors. Its knockdown leads to decreased proliferation in multiple breast cancer cell lines. Its loss disturbs the organization of the actin cytoskeleton, leads to inhibition of cell motility, and causes a G0/G1 cell cycle arrest. The proliferation phenotype upon overexpression is observed only with CASIMO1 protein expression, but not with a non-translatable mutant attributing the effects to the sORF-derived protein rather than a lncRNA function. CASIMO1 microprotein interacts with squalene epoxidase (SQLE), a key enzyme in cholesterol synthesis and a known oncogene in breast cancer. Overexpression of CASIMO1 leads to SQLE protein accumulation without affecting its RNA levels and increased lipid droplet clustering, while knockdown of CASIMO1 decreased SQLE protein abundance and ERK phosphorylation downstream of SQLE. Importantly, SQLE knockdown mimicked the CASIMO1 knockdown phenotype and in turn SQLE overexpression fully rescued the effect of CASIMO1 knockdown. These findings establish CASIMO1 as the first functional microprotein that plays a role in carcinogenesis and is implicated in the cell lipid homeostasis.

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Acknowledgements

We thank Drs. K. Boulay and A. Roth for helpful discussions and suggestions and for critical reading of the manuscript, S. Wiemann for helpful discussions, L. Chatel-Chaix (University of Heidelberg) and R. Pepperkok (EMBL Heidelberg) for antibodies and suggestions, V. Benes, T. Ivacevic, and S. Schmidt (EMBL Heidelberg, Genome Core Facility) for support with microarray performance and the DKFZ Light Microscopy and Flow Cytometry Core Facilities and the ZMBH Flow Cytometry & FACS Core Facility (University of Heidelberg) for technical support. Research in the Diederichs lab is supported by the German Research Foundation (DFG Di 1421/7-1) and the RNA@DKFZ Cross Program Topic. This work is part of the Ph.D. thesis of M.P.-S.

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Correspondence to Sven Diederichs.

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S.D. is a co-owner of siTOOLs Biotech GmbH, Martinsried, Germany. The remaining authors declare no conflict of interest.

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Polycarpou-Schwarz, M., Groß, M., Mestdagh, P. et al. The cancer-associated microprotein CASIMO1 controls cell proliferation and interacts with squalene epoxidase modulating lipid droplet formation. Oncogene 37, 4750–4768 (2018). https://doi.org/10.1038/s41388-018-0281-5

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