Understanding how a normal cell, sometimes carrying mutations in oncogenes or tumor suppressor genes, switches to malignancy and gives rise to a tumor remains a hotly debated question. Although many cells can harbor oncogenic mutations, only a few ultimately develop into a tumor, in an unpredictable and rare manner. Several theories have been proposed to explain this transition, but none could be tested in vivo at the single-cell scale: the malignant transformation of an isolated cell is too improbable and too unpredictable an event to be followed in a statistically meaningful way. This difficulty precludes characterizing the state of the cell of origin and its environment at the very onset of tumorigenesis.
To overcome this obstacle, the team developed an optogenetic approach enabling activation of oncogene expression in a single cell within the brain of a living zebrafish. The model relies on transgenic lines in which the oncogenic form of KRAS (KRASG12V) can be triggered in a targeted manner by photoactivation, using localized illumination defining a region of approximately 80 µm in diameter. In half of the cases, a single cell was thereby activated. To this permanent oncogene activation, the authors coupled the transient co-activation of a reprogramming factor — VENTX, NANOG, or OCT4 — genes linked to embryonic development and pluripotency, whose abnormal reactivation is known in the late stages of cancers where they endow cells with cancer stem cell properties. The role of these "epigenetic drivers" in the early phases of malignant transformation nevertheless remained unknown.
The results reveal a marked synergy between these two factors. KRASG12V, the most frequent oncogenic driver of several human cancers, only rarely leads to transformation when activated alone; the same holds for the reprogramming factor in isolation. By contrast, their combination drives the single cell through a deterministic malignant transition, increasing the probability of carcinogenesis by several orders of magnitude. Under these conditions, a fully formed tumor develops robustly and reproducibly within six days from the cell of origin. The accompanying analyses — quantification of gene expression, detection of ERK signaling, and cell transplantation experiments — support the characterization of this process.
The controlled and reproducible nature of this transformation lends support to the "ground-state theory of cancer initiation," according to which the first malignant cells disseminate over short distances prior to tumor growth. By making possible a targeted, specific, and reproducible malignant transformation at the single-cell scale in vivo, this approach paves the way for a quantitative study — tracking and characterization — of the initial steps of carcinogenesis.