The development of cancer involves a complex series of genetic changes by which normal cells evolve into transformed cells. Understanding the consequences of these genetic alterations will be central for developing novel cancer therapies, which must exploit differences between tumor cells and the normal cells from which they are derived. Our laboratory studies the normal mechanisms of cell division and cell division abnormalities in cancer cells. We are particularly interested in the causes and consequences of whole-chromosome aneuploidy, a prevalent but poorly understood feature of human cancers. We use a range of experimental approaches, including genetics, functional genomics, biochemistry, and live-cell imaging. Our ongoing projects use yeast, mammalian tissue culture, and genetically engineered mice.
Normal Cell Division Mechanisms
Our laboratory studies cytoskeletal dynamics to understand normal cell division, addressing the following questions: How does the actin cytoskeleton control the position of mitotic spindles during asymmetric cell division? How is actin assembly in cells controlled in space and time? How are microtubule filament lengths controlled so that the size of cellular microtubule structures, such as the mitotic spindle, can be matched to cell size? We identified proteins that track the ends of microtubules and defined their roles in connecting microtubules to the actin cytoskeleton and positioning the mitotic spindle during asymmetric cell division. This work defined a molecular mechanism linking the ends of microtubules to polarized actin structures in the cell. These findings on spindle positioning in yeast have influenced models of cortical-microtubule interactions in animal cells. Current work in the laboratory has defined an analogous mechanism in animal cells that controls centrosome position in normal cells and the organization of multiple centrosomes in cancer cells.
Our work on spindle positioning led to the discovery that formin proteins mediate the assembly of actin structures necessary for polarized cell growth in yeast. In all eucaryotes, formins are now known to mediate the assembly of the cytokinetic actin contractile ring. Work from our lab and others revealed that formins nucleate actin and promote the assembly of linear actin filaments by a unique mechanism. Our work demonstrated that different actin nucleators, by generating differently shaped actin filaments, enable the assembly of differently shaped actin cytoskeletal structures in cells.
We have recently defined new regulatory mechanisms for formins. The actin and septin cytoskeletons are known to interact in eukaryotic cells, but the underlying molecule mechanisms are poorly defined. We recently gained molecular insight into this problem by identifying molecules that link one of the yeast formins to the septin scaffold at the division site, controlling its actin assembly activity. We also uncovered regulatory mechanisms that reorganize formins to enable the cellular wound-healing response after membrane damage.
We also study the mechanisms that enable the size of microtubule structures to be appropriately scaled with cell size and chromosome content. An important molecular handle on this problem is the kinesin-8, Kip3, which we found walks to the plus ends of microtubules, where it promotes microtubule disassembly. Recently, we have made progress in understanding the mechanism by which Kip3 controls microtubule dynamics, allowing it to "measure" microtubule lengths. We found that genome duplications increase cell size and cause marked distortions in the geometry of spindles. We are taking evolutionary genetic approaches to understand how normal spindle function is restored after genome duplication.
Polyploidy and Aneuploidy
Abnormal numbers of chromosomes (aneuploidy) and centrosomes (microtubule-organizing centers) are hallmarks of most human cancers. Both the mechanisms leading to aneuploidy and the consequences of aneuploidy for tumorigenesis have been controversial. Much of the controversy about aneuploidy and tumorigenesis is attributable to the paucity of mechanisms by which aneuploidy might be linked to tumor development. A major thrust of our current work aims to reveal such mechanisms.
Prior work from our group used a mouse breast cancer model to show that cytokinesis failure leads to tumorigenesis. These tetraploid-derived tumors manifested striking chromosomal aberrations—both whole-chromosome aneuploidy and chromosomal rearrangements. This finding is likely to have clinical relevance because many oncogenic mutations are now known to predispose to cytokinesis failure. In addition, genome doublings occur commonly during evolution, and extra sets of chromosomes have been proposed to facilitate adaptation by providing fodder for evolutionary experimentation.
These findings raised several questions being pursued in my laboratory: How does a genome doubling affect cell physiology? Are the extra centrosomes or chromosomes (or both) important to promote tumorigenesis? Can whole-chromosome aneuploidy affect the structural integrity of chromosomes, potentially leading to cancer-causing mutations?
We recently found that the genetic instability that accompanies cytokinesis failure is mainly attributable to the accompanying increase in centrosome number (centrosome amplification). It was widely assumed that extra centrosomes in tumor cells lead to disorganized (multipolar) mitoses that cause cells to fragment into more than two daughters. Our comprehensive live-cell imaging experiments demonstrated that this is uncommon. Fragmented divisions usually lead to cell death; to avoid this, cancer cells bundle extra centrosomes to assemble near-normal (pseudobipolar) spindles. Although this enables cancer cells to survive with extra centrosomes, there is a trade-off: this division mechanism produces abnormal attachment of chromosomes to spindles. These abnormal attachments lead to chromosome mis-segregation and, in extreme cases, to physical isolation of chromosomes into separate nuclear structures called micronuclei.
Because micronuclei are common features of human cancer cells, we tested the idea that they could be a source of DNA damage. Indeed, partitioning of chromosomes into micronuclei can result in massive DNA damage, potentially providing a mechanism by which whole-chromosome aneuploidy could produce cancer-causing mutations. Micronulcei display defects in nuclear envelope assembly and undergo abnormal DNA replication. Interestingly, because of the physical isolation of the chromosome in the micronucleus, this mutagenesis is highly local—restricted to one or two chromosomes. Because of this local effect, we propose that micronuclei could be a source of chromothripsis, a phenomenon discovered by cancer genome sequencing that involves massive genomic rearrangement, restricted usually to a single chromosome. We are evaluating the relationship between micronucelei and chromothripsis; we are also characterizing the origin of the defects in micronuclei. These experiments address the fundamental question of how nuclear architecture affects the integrity of the genome.
Grants from the National Institutes of Health provided partial support for these projects.
As of March 10, 2016