ROS control in human iPS cells reveals early events in spontaneous carcinogenesis

Reactive oxygen species (ROS) generated during cellular respiration oxidize various cellular constituents, which cause carcinogenesis. Because most studies on the role of ROS in carcinogenesis have mainly been performed using tumor-derived cell lines, which harbor various types of mutation, it has been difficult to determine the molecular details that lead to cancer formation. To overcome this difficulty, we established human-induced pluripotent stem cell lines in which the intracellular ROS levels are controlled at various differentiation stages by manipulating the ROS-yielding mitochondria. By introducing a specific amino acid substitution (I69E) into the succinate dehydrogenase complex, subunit C protein, a component of mitochondrial respiratory chain complex II, the ROS level increased considerably. When ROS-overproducing cells at the early stage of endoderm differentiation were subcutaneously inoculated into the backs of nude mice, we observed tumor formation. These tumor-initiating cells were subjected to a comprehensive analysis by RNA sequencing. It was revealed that tumor-initiating cells showed 27 upregulated transcripts compared with control cells. The newly identified genes include those coding for PAX8 and FOSB (transcription factors) as well as FGF22, whose expressions are known to increase in developing embryos. These results suggest that these genes may play a pivotal role in cancer formation at the very early stages of cell differentiation.