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miRNAs that translocate from the nucleus to mitochondria are referred to as mitochondrial microRNAs (mitomiR). mitomiRs have been shown to modulate the translational activity of the mitochondrial genome, yet their role in mitochondrial DNA (mtDNA) transcription remains to be determined. Here we report that the mitomiR-2392 regulates chemoresistance in tongue squamous cell carcinoma (TSCC) cells by reprogramming metabolism via downregulation of oxidative phosphorylation and upregulation of glycolysis. These effects were mediated through partial inhibition of mtDNA transcription by mitomiR-2392 rather than through translational regulation. This repression required specific miRNA-mtDNA base pairing and Argonaute 2. mitomiR-2392 recognized target sequences in the H-strand and partially inhibited polycistronic mtDNA transcription in a cell-specific manner. A retrospective analysis of TSCC patient tumors revealed a significant association of miR-2392 and regulated mitochondrial gene expression with chemosensitivity and overall survival. The clinical relevance of targeted mitochondrial genes was consistently validated by The Cancer Genome Atlas RNA sequencing in multiple types of cancer. Our study revealed for the first time the role of mitomiR in mtDNA transcription and its contribution to the molecular basis of tumor cell metabolism and chemoresistance. Significance: These findings uncover a novel mechanism by which mitomiRNA regulates mitochondrial transcription and provide rationale for use of mitomiRNA and mtDNA-encoded genes to predict chemosensitivity and patient clinical prognosis.

Mitochondrial miRNA Determines Chemoresistance by Reprogramming Metabolism and Regulating Mitochondrial Transcription