Poor Competition Hypothesis: understanding the mechanism leading to increased cancer predisposition on the background of impaired cell cycle progression

Andriy Marusyk*, Ganna Bilousova and James DeGregori

Department of Biochemistry and Molecular Genetics, Program in Molecular Biology, University of Colorado Health Sciences Center,  4200 East Ninth   Avenue, Denver, CO 80262, USA     andriy.marusyk@uchsc.edu

The E2F family of transcriptional factors is a key regulator of cell cycle progression. Deregulations of the E2F pathway are observed in almost all human tumors. Previous work in our lab has demonstrated that the loss of E2F1 and E2F2 results in defective hematopoiesis due to reduced S-phase progression in blood cell progenitors (1). However, mice lacking E2F2, and to a lesser extent E2F1, develop lymphomas and leukemias at high frequencies. To explain the apparent discrepancy of increased tumor predisposition on the background of impaired cell cycle progression we have proposed the following Poor Competition hypothesis: a background of high homeostatic demand and impaired cell cycle progression of hematopoietic progenitor cells selects for mutations that improve cell cycle progression and these mutations are frequently oncogenic. 

To test the Poor Competition hypothesis, we have developed the following experimental strategy. Bone marrow (BM) progenitor cells derived from either E2F1/E2F2 mutant (DKO) or control donor mice are transduced with either a vector control or an oncogene using an MSCV-based vector co-expressing GFP as a marker. The transduced hematopoietic progenitors are used to reconstitute the hematopoietic systems of lethally irradiated recipient mice and the percentage of GFP positive cells in the peripheral blood or hematopoietic organs of the reconstituted animals is monitored over time.

As the first “oncogene” for testing the Poor Competition hypothesis we chose dominant-negative mutant forms of p53 (DN p53). Following the experimental protocol outlined above, two weeks post reconstitution samples of peripheral blood were taken from the recipient mice, and the percentage of GFP positive cells was determined for multiple hematopoietic lineages. All of the analyzed lineages exhibited increased percentage of cells expressing DN p53 in animals reconstituted with DKO, but not WT, BM, while no differences were observed for vector only controls. Furthermore, the expression of DN p53 was found to markedly reduce (from 20 hrs to 5 hrs) the length of  S-phase in DKO red blood cell progenitors, but not in WT progenitors (which remained at about 5 hrs). Thus, the expression of DN p53 confers a competitive advantage to DKO blood progenitors (but not WT), apparently by restoring S-phase progression. 

To analyze the mechanisms allowing DN p53 to bypass defects in S-phase progression we analyzed the impact of DN p53 expression on the effect of drugs which inhibit replication  under concentrations that show growth-inhibitory effects but still allow at least several cycles of replication. Expression of DN p53 was found to allow continuous proliferation of REF52 cells in 100 μM hydroxyurea (HU) and 200 nM aphidicolin, while under these conditions the control cell cultures showed cumulative inhibition of growth rates and eventual collapse. The effect of DN p53 was not due to direct antagonism of drug action since both HU and aphidicolin exhibited equal perturbations in dNTP pools both in DN p53 expression and control cells.

The idea of considering principles of Darwinian evolution to understand cancer formation is not new to the field. However, that poor cell cycle progression resulting in poor competition can promote tumor initiation diverges from the conventional picture. In fact, conditions where increased tumor rates are observed on the background of impaired cell cycle progression are not uncommon. Besides relatively rare genetic disorders, these conditions may result from the far more common setting of malnutrition, such as deficient folate intake.  Establishing the Poor Competition Model is expected to provide an additional perspective to understand the principles underlying the evolution of cancer.

1. Li, F. X., J. W. Zhu, C. J. Hogan, and J. DeGregori. 2003. Mol. Cell. Biol. 23:3607-3622.