Common and specific roles of the related CDK inhibitors p27 and p57 revealed by a knock-in mouse model.

Although p27 and p57 are structurally related cyclin-dependent kinase inhibitors (CKIs), and are thought to perform similar functions, p27 knockout (p27(KO)) and p57(KO) mice show distinct phenotypes. To elucidate the in vivo functions of these CKIs, we have now generated a knock-in mouse model (p57(p27KI)), in which the p57 gene has been replaced with the p27 gene. The p57(p27KI) mice are viable and appear healthy, with most of the developmental defects characteristic of p57(KO) mice having been corrected by p27 knock-in.

Modeling cell cycle control and cancer with pRB tumor suppressor.

Cancer is a complex syndrome of diseases characterized by the increased abundance of cells that disrupts the normal tissue architecture within an organism. Defining one universal mechanism underlying cancer with the hope of designing a magic bullet against cancer is impossible, largely because there is so much variation between various types of cancer and different individuals. However, we have learned much in past decades about different journeys that a normal cell takes to become cancerous, and that the delicate balance between oncogenes and tumor suppressor is upset, favoring growth and survival of the tumor cell.

Visualizing dynamic E2F-mediated repression in vivo.

Although many E2F target genes have been identified recently, very little is known about how any single E2F site controls the expression of an E2F target gene in vivo. To test the requirement for a single E2F site in vivo and to learn how E2F-mediated repression is regulated during development and tumorigenesis, we have constructed a novel series of wild-type and mutant Rb promoter-LacZ transgenic reporter lines that allow us to visualize the activity of a crucial E2F target in vivo, the retinoblastoma tumor suppressor gene (Rb). Two mutant Rb promoter-LacZ constructs were used to evaluate the importance of a single E2F site or a nearby activator (Sp1/Ets) site that is found mutated in low-penetrance retinoblastomas.

Cell cycle, proteolysis and cancer.

Research in the past 15 years has shown that the mammalian cell cycle is controlled by the action of cyclin-dependent kinases (CDKs). A crucial substrate of the CDKs in G1-phase is the retinoblastoma tumor suppressor (pRB), which restrains proliferation largely by repressing the activity of the E2F transcription factors. More recent work has shown that the cell cycle is also a tale of two classes of ubiquitin ligases, referred to as SCF and APC/C ligases.

Role of F-box protein betaTrcp1 in mammary gland development and tumorigenesis.

The F-box protein betaTrcp1 controls the stability of several crucial regulators of proliferation and apoptosis, including certain inhibitors of the NF-kappaB family of transcription factors. Here we show that mammary glands of betaTrcp1(-/-) female mice display a hypoplastic phenotype, whereas no effects on cell proliferation are observed in other somatic cells. To investigate further the role of betaTrcp1 in mammary gland development, we generated transgenic mice expressing human betaTrcp1 targeted to epithelial cells under the control of the mouse mammary tumor virus (MMTV) long terminal repeat promoter.

Dp1 is largely dispensable for embryonic development.

E2F/DP complexes activate or repress the transcription of E2F target genes, depending on the association of a pRB family member, thereby regulating cell cycle progression. Whereas the E2F family consists of seven members, the DP family contains only two (Dp1 and Dp2), Dp1 being the more highly expressed member. In contrast to the inactivation of individual E2F family members, we have recently demonstrated that loss of Dp1 results in embryonic lethality by embryonic day 12.

A dynamic switch in Rb+/- mediated neuroendocrine tumorigenesis.

Rb+/- mice develop a complex spectrum of neuroendocrine tumors on a mixed genetic (129Sv x C57BL/6) background. To understand how the 129Sv and C57BL/6 contributions affect Rb+/- tumorigenesis, we serially backcrossed Rb+/- animals to the 129Sv or C57BL/6 strain, and analysed their pathological profiles. Strikingly, the length of survival and the penetrance, severity and multiplicity of neuroendocrine tumors switch dramatically between Rb+/- animals from the two genetic backgrounds.

Control of meiotic and mitotic progression by the F box protein beta-Trcp1 in vivo.

SCF ubiquitin ligases, composed of three major subunits, Skp1, Cul1, and one of many F box proteins (Fbps), control the proteolysis of important cellular regulators. We have inactivated the gene encoding the Fbp beta-Trcp1 in mice. beta-Trcp1(-/-) males show reduced fertility correlating with an accumulation of methaphase I spermatocytes.

Role of the RB tumor suppressor in cancer.

Apart from their coordinated inactivation by DNA tumor viral oncoproteins, the pRB and p53 tumor suppressor pathways were not known to be connected ten years ago. Within the last decade, our appreciation of how these pathways are interconnected has grown substantially. The checks and balances that exist between pRB and p53 involve the regulation of the G1/S transition and its checkpoints, and much of this is under the control of the E2F transcription factor family.

Dp1 is required for extra-embryonic development.

Release of E2F1/DP1 heterodimers from repression mediated by the retinoblastoma tumor suppressor (pRB) triggers cell cycle entry into S phase, suggesting that E2F1 and DP1 proteins must act in unison, either to facilitate or to suppress cell-cycle progression. In stark contrast to the milder phenotypes that result from inactivation of E2Fs, we report that loss of Dp1 leads to death in utero because of the failure of extra-embryonic development. Loss of Dp1 compromises the trophectoderm-derived tissues - specifically, the expansion of the ectoplacental cone and chorion, and endoreduplication in trophoblast giant cells.