Novel functions for atypical E2Fs, E2F7 and E2F8, in polyploidization and liver cancer

Abstract

Atypical E2F transcription factors, E2F7 and E2F8, function as transcriptional repressors of E2F target genes and are crucial for controlling the cell proliferation. In this thesis, we reveal that these two factors are crucial for liver cell polyploidization, embryonic development and prevention of liver cancer in mice. In chapter 2, we show that atypical E2Fs, especially E2F8, play an important role in promoting polyploidization in the liver. Loss of E2F7/8 resulted in dramatic reduction in binucleation and polyploidization of hepatocytes. In contrast, single loss of E2F1 enhanced polyploidization in hepatocytes. Interestingly, loss of E2f1 in E2f7/8-deficient liver led to partial rescue of polyploidization defect observed upon loss of E2f7/8. Moreover, we identified a transcriptional network program regulated by repressor E2F8 and activator E2F1 that controls polyploidization in liver. Together, these data suggest opposing functions for classical activator E2Fs and atypical repressor E2Fs in regulating polyploidization in liver. Surprisingly, contrary to long-term theory, we discovered that loss of polyploidization has no impact on liver differentiation and regeneration. We demonstrate in chapter 4 that E2F7 and E2F8 are required for murine embryonic development. While mice mutant for either E2F7 or E2F8 developed normally with no obvious developmental defects, global deletion of both E2F7 and 8 resulted in embryonic lethality owing to vascular and cell survival defects such as dilated blood vessels associated with multifocal hemorrhages and massive apoptosis. Interestingly, loss of either E2F1 or p53 suppressed the massive apoptosis observed in E2f7/8 double knockout embryos, however, the triple knockout embryos still carried the vascular defects and died around same embryonic age, suggesting complex mechanisms underlying the embryonic lethality. As described in chapter 5, we have revealed that E2f7/8 function as tumor suppressor genes and that they can cooperate with the known tumor suppressor gene Rb in preventing liver cancer. Using liver-specific knockout mice, we found that deletion of E2f7/8 results in spontaneous development of hepatocellular carcinoma in mice. Furthermore, additional deletion of Rb along with E2f7/8 resulted in development of tumors earlier than in E2f7/8 deficient livers, indicating that loss of Rb accelerates tumorigenesis.