Mechanics of Cell Division

This program sub-group is focused on understanding the molecular basis of chromosome segregation as it relates to the cancer cell cycle, growth and programmed death.

Investigator: Cynthia McMurray, Ph.D.

Dr. McMurray's research involves investigating the genetic basis and somatic effects of DNA repair deficits. The response to DNA damage determines whether a cell lives normally, lives at the expense of mutation and cancer, or ages and dies. Her laboratory's current focus is on three projects that explore the molecular mechanisms of DNA amplification and their somatic consequences with respect to cancer, aging, and neuronal death. Studies support the novel paradigm that base oxidation and its impaired removal, and faulty mismatch repair are key factors in progressive genomic instability associated with cancer and neurological disease. Initial findings have been published (Nature. 447(7143):447-52, 2007; Nature Structural & Molecular Biology. 12(8):663-70, 2005), which show that CAG trinucleotide repeats generating hairpin DNA bind the mismatch repair protein MSH2/MSH3 and change the properties of mismatch recognition. As uncorrected DNA lesions give rise to mutation and disease, understanding the structure/function mechanism(s) provides fundamental information to cancer initiation/progression.

Repair deficits such as the one mediated by MSH2/MSH3 in amplifying CAG repeats are reminiscent of a less well-studied process in which the normal MSH2/MSH6 complex dissociates normal coupling and signals cell death at some lesions. MSH2/MSH6 recognizes a variety of damaged DNA substrates including the common alkylation lesion mismatches O6-methylguanine-C and O6-methylguanine-T as well as mismatches containing 8-oxoguanine. Chemical exposures can directly signal apoptoic pathways, as well as induce strand breaks in futile attempts at repair. Thus, the mechanism by which the MSH2/MSH3-CAG hairpin complex leads to disease and cell death may shed light on general mechanisms relevant to repair at other lesions. The similarities raise interesting questions as to whether there were common and measurable states by which a DNA-induced conformation change, in an otherwise normal MMR complex, might uncouple lesion binding from ATP hydrolysis to cause cell death.

Investigator: Evette Radisky, Ph.D.

Proteinases are enzymes that digest other proteins. While they fulfill many important and essential functions in normal physiology, the increased activity of certain proteinases contributes to the development of cancer. Drugs that specifically target these oncogenic proteinases present a promising approach to cancer therapy.

The Radisky Lab is focused on identifying and studying proteinases involved in the progression of several epithelial cancers, with a current focus on breast cancer. Dr. Radisky's team employs approaches ranging from 3-dimensional cell culture models of breast cancer progression, to proteomic analysis of the spectrum of proteinases contributing to cancer growth, invasion, and metastasis, to crystallographic and biochemical analysis of the structures and mechanisms of specific oncogenic proteinases that present intriguing and novel drug targets.

Investigator: Jeffrey Salisbury, Ph.D.

Dr. Salisbury's team is focused on providing a comprehensive assessment of the molecular mechanism for centrin-based regulation of centriole duplication and its coordination with the cell cycle in human cells. Their studies test the hypothesis that centrin exerts both spatial and temporal control over centriole duplication through its role as a structural component of centriole precursor structures and through periodic cell cycle-specific changes in centrin phosphorylation and abundance.

The research takes advantage of the team's recent development of HeLa cells that conditionally express tagged centrin with specific mutations in the carboxyl-terminal regulatory domain of the protein. Two of these mutations target the known phosphorylation site on centrin and show opposing phenotypes: one arrests centriole duplication at the mother/bud stage and promotes primary cilia formation, and the second promotes centriole duplication.

These results lead to the exciting possibility that phosphorylation of centrin may be a key to the control of both centriole duplication and primary cilia formation. The goals of Dr. Salisbury's research are to:

• Determine dynamics of centrin synthesis, phosphorylation, de-phosphorylation, and destruction during the cell cycle and to determine the role of key cell cycle regulators and mitotic kinases in these processes;
• Undertake a thorough analysis of the role of centrin-based precursor structures in centriole duplication, and to determine the molecular basis for centrin's terminal-terminal regulatory domain in the control of this process; and
• Define the mechanism of centrin exchange between the centrosome and cytoplasmic pools.

This work has direct relevance to human health because cells must exercise strict control over centriole number to avoid chromosomal instability and the development of aneuploidy in cancer; and because centrioles underlie the formation and function of primary cilia – the cell's sensory organelle – defects in which lead to a broad of range of important human diseases including polycystic kidney disease, retinal degeneration and the laterality abnormality situs inversus.

Investigator: Jan van Deursen, Ph.D.

Dr. van Deursen has directed most of his efforts toward addressing the role of chromosome number instability in cancer development. Given the high incidence of numerical chromosome aberrations in cancers, it has been proposed that aneuploidy initiates or promotes tumor development. However, whether chromosome number instability is a cause or a consequence of the malignant phenotype is a complex and hotly debated issue.

Ever since the discovery of the spindle assembly checkpoint apparatus, there has been wide speculation that mutations in mitotic checkpoint genes are instrumental in the development of aneuploidy in human cancers. Such mutations have since indeed been found in a variety of human tumors with chromosome number instability.

In an attempt to rigorously address the role of chromosome number instability in tumorigenesis, the van Deursen laboratory has generated a series of mutant mouse strains that segregate chromosomes inaccurately and, as a result, accumulate substantial numbers of aneuploid cells. One class includes components of the spindle assembly checkpoint, a molecular network that ensures proper separation of duplicated chromosomes by preventing anaphase onset until all kinetochores have attached to spindle microtubules.

By generating mice expressing small amounts of the mitotic checkpoint protein BubR1, Dr. van Deursen's team has previously shown that these mice age much faster than normal mice. However, it was unknown whether other mitotic checkpoint genes function to prevent the early onset of aging. To address this question, double haplo-insufficient mice for the mitotic checkpoint genes Bub3 and Rae1 have been generated. The findings suggest that early onset of aging-associated phenotypes in mice with mitotic checkpoint gene defects is linked to cellular senescence and activation of the p53 and p16 pathways rather than to aneuploidy (The Journal of Cell Biology. 172:529-40, 2006). These studies provide fundamental information addressing the relation between aging, aneuploidy, and the development of cancer.