Mitochondrial fusion drives oxidative metabolism to immortalize neural stem cells during tumorigenesis

Metabolic reprogramming is a key feature of many cancers, but how and when it contributes to tumorigenesis remains unclear. Here, we demonstrate that metabolic reprogramming induced by mitochondrial fusion can be rate-limiting for immortalization of tumor initiating cells (TICs) and trigger their irreversible dedication to tumorigenesis. Using single-cell transcriptomics, we find that Drosophila brain tumors contain a rapidly dividing stem cell population defined by upregulation of oxidative phosphorylation (OxPhos). We combine targeted metabolomics and in-vivo genetic screening to demonstrate that OxPhos is required for tumor cell immortalization but is dispensable in the neural stem cells (NSCs) giving rise to the tumors. Employing an in-vivo NADH/NAD+ sensor, we show that NSCs increase OxPhos precisely during immortalization. Blocking OxPhos or mitochondrial fusion stalls the TICs in quiescence and prevents tumorigenesis through impaired NAD+ regeneration. Our work establishes a unique connection between cellular metabolism and the immortalization of tumor initiating cells. SMART-Seq2 single-cell RNA-seq of control neuroblasts and bratRNAi tumor neuroblasts in Drosophila melanogaster third instar larvae