Characterizing TOP1MT variants provides novel mechanistic insights into TOP1MT functions
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2022-01
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Abstract
Mitochondria are organelles that have their own genome (mtDNA), which encodes 37 genes crucial for oxidative phosphorylation. Proper regulation of the mtDNA is critical for cellular functions, while impairment of its regulation can lead to disease. Mitochondrial topoisomerase 1 (TOP1MT) is a protein involved in mtDNA maintenance through its ability to bind mtDNA and relax supercoiled structures. In addition, TOP1MT is involved in the translation of mtDNA encoded proteins. To begin to understand how TOP1MT mechanistically performs these various roles, we characterized three candidate pathogenic variants (R198C, V338L and P193L) in the TOP1MT protein. We investigated how these variants affect the topoisomerase activity of TOP1MT, influencing mtDNA transcription and general mitochondrial health and function. We also examined the involvement of TOP1MT in replication and mtDNA protein translation. Combined with structural modelling, our functional characterization suggests an unexpected role for the nucleic acid binding domain of TOP1MT in controlling translation. Further, we detailed how these variants differ from each other and from wild-type protein. We demonstrated that the TOP1MT variants we investigated are not fully functional and are thus potential candidates to contribute to disease. During these studies, we also identified a novel role for TOP1MT, as its loss triggers mtDNA release into the cytosol and activation of cGAS-STING innate immune signaling. This novel finding was exciting, as the P193L variant was found in a family with autoimmune disorders. Interestingly, P193L could not rescue cGAS-STING activation in cells lacking TOP1MT, consistent with a role in autoimmune dysfunction. Overall, the molecular analysis we performed on these TOP1MT variants broadens our understanding of the different functions performed by TOP1MT, how its different roles are related, and the likelihood that TOP1MT is a disease gene.
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Al Khatib, I. (2022). Characterizing TOP1MT variants provides novel mechanistic insights into TOP1MT functions (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.