Direct conversion of human fibroblasts to liver cancer cells

Cancer is the most complex genetic disease known, with mutations in more than 250 genes contributing to different forms of the disease1,2,3. Most human driver mutations are specific to particular types of cancer, at least in part due to differences in expression pattern between cell types, and the diversity of mutational mechanisms across different human tissues4. However, the fact that many apparently oncogenic mutations fail to transform fibroblastic cells in culture suggests that different cell types could be susceptible to transformation by different sets of oncogenes. Here we show that reprogramming human fibroblasts to induced hepatocytes (iHeps) makes the cells sensitive to transformation by a combination of oncogenes that is characteristic of liver cancer (CTNNB1, TERT and MYC). The transformed iHeps are highly proliferative, tumorigenic in nude mice, and bear gene expression signatures of liver cancer. Temporal analysis of the tumorigenic program using single-cell RNA-seq and RNA velocity analysis revealed that the cells progress along a common path to transformation, invariably acquiring liver cell identity prior to expressing markers characteristic of liver tumor cells. These results, together with analysis of chromatin accessibility using ATAC-seq and NaNoMe-seq indicate that lineage-determining factors act by defining a chromatin state that is permissive for transformation. Taken together, our results indicate that cell identity is a key determinant in transformation, and establish a paradigm for defining the molecular states of distinct types of human cancer.