Philipp Kaldis received his PhD from the Institute for Cell Biology, Swiss Federal Institute of Technology (ETH), Zürich, Switzerland, in 1994 where he worked on the mitochondrial creatine kinase with Dr. Theo Wallimann and Dr. Hans Eppenberger. In 1995, he joined Dr. Mark Solomon’s laboratory at Yale University, Department of Molecular Biophysics and Biochemistry, New Haven, Connecticut, as a postdoctoral fellow/associate research scientist where he investigated the activation of cyclin-dependent kinases (Cdks). In 2000, Dr. Kaldis joined the NCI-Frederick as tenure-track investigator and was promoted to senior investigator with tenure in 2006. In 2007, he joined the IMCB as principal investigator.

Cell division is a highly regulated process that can differ in specific types of cells or under conditions that lead to cancer. Our laboratory investigates the regulation of the cell cycle in vivo by combining mouse genetics with cell biology and biochemistry. We are focusing on the cyclin dependent kinases (Cdks) and all pathways that control or are controlled by Cdks. In addition to the functions of the Cdks under normal conditions, we are actively studying the involvement of Cdks in cancer. Ultimately, we want to define drugable targets in vivo that would lead to improved cancer therapy.

The cell cycle is regulated by cyclin dependent kinases (Cdks). In mammals, approximately 20 potential Cdks have been identified and the most important ones are Cdk1, Cdk2, Cdk4, and Cdk6. The activity of Cdks is stimulated by binding to cyclin subunits, phosphorylation of the activating threonine, dephosphorylation of inhibitory sites, and dissociation from inhibitors (Cip/Kip or Ink4 type). Originally, it was thought that each Cdk/cyclin complex executes specific functions, which would render Cdks essential gene products. Surprisingly, knockout mice for Cdk2, Cdk4, and Cdk6 are viable and display rather minor phenotypes.

Cdk2 knockout mice generated in our laboratory have been shown to be completely sterile but no other major phenotype was found. This indicated that Cdk2 is essential in meiosis but not in mitosis. In order to study the functional overlap between the Cdks, we generated Cdk2^-/-Cdk4^-/- and Cdk2^-/-p27^-/- mice. Cdk2^-/-Cdk4^-/- double knockouts (DKO) die during embryogenesis around E15 as a result of heart defects. We observed a gradual decrease of Rb phosphorylation and reduced expression of E2F target genes, like Cdk1 and cyclin A2, during embryogenesis and in embryonic fibroblasts (MEFs). DKO MEFs are characterized by a decreased proliferation rate, impaired S phase entry, reduced immortalization rate, and premature senescence. HPV E7 mediated inactivation of Rb restored normal expression of E2F inducible genes, senescence, and proliferation in DKO MEFs. Our results demonstrate that Cdk2 and Cdk4 cooperate to phosphorylate Rb in vivo and to couple the G1/S phase transition to mitosis via E2F-dependent regulation of gene expression.

The cyclin-dependent kinase inhibitor p27^Kip1 is known as a negative regulator of cell cycle progression and as a tumor suppressor. p27^-/- mice are large, develop pituitary tumors, retinal dysplasia, thymic hyperplasia, female sterility, and increased Cdk2 activity. Cdk2 is the major target of p27 and therefore we hypothesized that loss of Cdk2 activity should modify some of the p27^-/- mouse phenotypes. Although Cdk2^-/-p27^-/-mice developed ovary tumors and tumors in the anterior lobe of the pituitary, we failed to detect any functional complementation in Cdk2^-/-p27^-/-double knockout mice indicating a parallel pathway regulated by p27. We observed elevated levels of S phase and mitosis in tissues of Cdk2^-/-p27^-/-mice concomitantly with elevated Cdk1 activity in Cdk2^-/-p27^-/-extracts. p27 binds to Cdk1, cyclin B1, cyclin A2, or suc1 complexes in wild-type and Cdk2^-/- extracts, indicating that p27 regulates Cdk1 in vivo. In addition, we detected cyclin E associated activity in Cdk2^-/-p27^-/-mice and found that cyclin E forms complexes with Cdk1. These active Cdk1/cyclin E complexes are able to promote the G1/S transition in the absence of Cdk2. Our in vivo results provide strong evidence that Cdk1 may compensate the loss of Cdk2 function. Therefore, Cdk1 not only is important for mitosis but also for the G1/S transition.

Our work demonstrates the plasticity on how the cell cycle is regulated and the central importance of Cdk1. We are now exploring the reasons why Cdk1 is unique and whether Cdk1 is essential in all tissues in vivo. In addition, we are investigating the functions of Cdks during tumorigenesis using mouse genetics.