The endocycle is a developmentally programmed variant cell cycle consisting of successive S (DNA synthesis) and G (Gap) phases without an intervening M phase or cytokinesis. As a consequence of the regulated “decoupling” of DNA replication and mitosis, which are two central processes of the traditional cell division program, endocycling cells acquire highly polyploid genomes after having undergone multiple rounds of whole genome reduplication. Although essential for metazoan development, relatively little is known about the regulation of endocycle or its physiologic role in higher vertebrates such as the mammal.
A substantial body of work in the model organism Drosophila melanogaster has demonstrated an important function for dE2Fs in the control of endocycle. Genetic studies showed that both endocycle initiation and progression is severely disrupted by altering the expression of the fly E2F activator (dE2F1) or repressor (dE2F2). In mammals, the E2F family is comprised of nine structurally related proteins, encoded by eight distinct genes, that can be classified into transcriptional activators (E2f1, E2f2, E2f3a and E2f3b) or repressors (E2f4, E2f5, E2f6, E2f7 and E2f8). The repressor subclass may then be further divided into canonical (E2f4, E2f5 and E2f6) or atypical E2fs (E2f7 and E2f8). Until this study, there has been sparse knowledge regarding the role of E2fs in the control of mammalian endocycle, or what the physiologic consequences might be of perturbed endocycles in tissues composed of polyploid cells such as the liver.
Now using a combination of novel and established lineage-specific cre mice we identified that two opposing arms of the E2F program, one driven by canonical transcription activation (E2F1, E2F2 and E2F3) and the other by atypical repression (E2F7 and E2F8), converge on the regulation endocycles in vivo. Ablation of canonical activators in the two endocycling tissues of mammals, trophoblast giant cells (TGCs) in the placenta and hepatocytes in the liver, augmented genome ploidy, whereas ablation of atypical repressors diminished ploidy. Surprisingly, the severe reduction of ploidy caused by E2f7/E2f8 deficiency had no apparent adverse impact on placental and hepatic physiology. However, the sole inactivation of E2f8 in hepatocytes, which biases toward a diploid state, resulted in the development of spontaneous hepatocellular carcinoma (HCC). Loss of E2f8 also potentiated the development of carcinogen-induced HCC, suggesting that E2F8 functions to suppress tumor initiation and impede tumor progression. Taken together, the results presented within provide the first in vivo evidence for a direct role of E2Fs in regulating non-traditional cell cycles in mammals and genetically link endocycle control with cancer.