David Mann Ph.D.

Position Department / Business Unit
Senior Lecturer Division of Cell & Molecular Biology
Institution Disciplines
Imperial College, London Nanobiology
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Dr Mann's lab focuses on the elucidation of the mechanisms regulating cell division in mammalian cells and how they are subverted in cancer. Specificially we are interested in the role of protein phosphorylation in these processes, as outlined below.

At one level, cancer can be thought of simply as unrestrained cell division. A thorough understanding of the mechanisms that operate to control cell division is therefore essential if we are to identify cellular targets for therapeutic intervention in cancer patients.

The process of cell division can be regarded as a cyclical event. In the initial growth phase (G1), the cell assesses whether conditions are favourable for division. If a cell commits to division then it duplicates its DNA during S phase (DNA Synthesis phase). The cell then undergoes a second growth period (G2) in which it prepares for the subsequent mitosis (M phase) in which the parental cell divides equally in two, with each daughter cell receiving one copy of the duplicated DNA. The key period for the regulation of this process is late in the G1 phase.

At the molecular level, entry into S phase is known to be controlled by a negative regulatory cascade (see figure above). The E2F transcription factor promotes entry into the cell cycle but is inhibited by pRb, the retinoblastoma gene product. pRb is itself negatively regulated through phosphorylation catalysed by the G1-specific cyclin/cdk complexes (D-type cyclin/cdk4 or cdk6 and cyclin E/cdk2). The cyclin/cdks are in turn inhibited by two families of proteins, the p16 family and the p21/p27 family. Perturbation of the levels of any of these key cell cycle regulatory proteins can lead to aberrant cell division and cancer. Our understanding of the cell cycle has been greatly strengthened by the study of virus/host interactions. Viruses typically possess small genomes that are not large enough to encode all of the proteins required for viral replication. Therefore viruses are dependent on the host cell to provide the functions that they lack. Most host cells infected by viruses are quiescent i.e. they are in a resting state and have withdrawn from the cell cycle. Viruses have evolved mechanisms to subvert the quiescent state and force the host cell to re-enter the cell cycle so that the enzymes required for viral DNA replication are produced as the host enters S phase.

We are studying the potential of the cyclins encoded by a number of g-Herpesviruses to contribute to cell cycle deregulation and cancer. Human Herpesvirus 8, also known as Kaposi's sarcoma-associated Herpesvirus, infects lymphoid cells and viral infection is linked to the development of a number of lymphoproliferative disorders which are particularly prevalent in immunosuppressed individuals such as AIDS patients. The viral cyclin (known as K cyclin) complexes with cdk6, the major D-type cyclin partner in lymphocytes. We have shown that K cyclin can confer a remarkable property on the associated cdk6 subunit, namely resistance to the two families of cdk inhibitors. Thus, K cyclin/cdk6 complexes are completely resistant to physiological levels of p16, p21 and p27. Since these inhibitor proteins are largely responsible for maintaining cells in growth-arrested states such as quiescence, expression of K cyclin provides a powerful means of enforcing cell cycle progression. At present we are studying additional properties of the viral cyclin which may aid in its ability to deregulate cell cycle control. The viral cyclins are proving useful tools with which to dissect the mechanisms through which normal cell cycle control is exerted.

In addition, we are pursuing chemical genetic approaches to understand protein kinase function. Most protein kinases utilise ATP as the phosphate donor and have highly conserved catalytic sites. This makes many protein kinase inhibitors promiscuous in their specificity. We are using site directed mutagenesis to subtly alter the catalytic site of specific protein kinases so that they can accept analogues of ATP that are not normally used. This approach allows the molecular construction of unique kinase/ATP analogue pairs and can be used for the identification of the substrates of the mutant kinase.

By this Researcher

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