Accurate duplication and segregation of the genome is critical to normal development and mutations that lead to genomic instability promote a wide range of cancers. The importance of high fidelity genome transmission is reflected in the elaborate pathways cells employ to limit genetic damage. DNA damage and replication checkpoints help maintain genetic stability by delaying cell division cycle progression to allow time for damage repair or to complete DNA synthesis. An alternative response to DNA damage termed apoptosis actively kills cells carrying genetic lesions, eliminating mutant cells from the population. Checkpoint and apoptotic responses thus represent distinct strategies for maintaining genome integrity in an organism, and defects in checkpoint function or apoptosis lead to genetic predisposition to cancer.

These well established DNA damage responses function during interphase, the portion of the cell cycle when growth and DNA replication takes place. Much less is known about the response to genetic damage as the replicated chromosomes are aligned and segregated to produce identical daughter cells during mitosis. If DNA damage or replication defect persist into mitosis, chromosome segregation will produce genetically non-identical or mutant cells, increasing the likelihood that critical tumor suppressor genes will be compromised and that a cancer will develop.

Recent studies on fruit fly embryo have revealed a previously unrecognized pathway that links assembly of the mitotic cell division machinery to genomic integrity. This pathway blocks segregation of damaged chromosomes by actively disrupting the centrosomes. The mitotic spindle is a bipolar cellular machine that physically partitions the replicated chromosomes, and the centrosomes define the spindle poles. Work in the Theurkauf lab has demonstrated that a wide range of DNA damaging agents and DNA replication inhibitors trigger centrosome defects and a complete block in damaged chromosome segregation (Sibon et al., Nature Cell Biology, 2000; Takada et al., Cell, 2003). Recent studies show that the fly Checkpoint kinase 2 (Chk2) gene is essential to this response. In embryos lacking Chk2, genetic damage fails to trigger centrosome defects and the compromised chromosomes are segregated to produce defective daughter nuclei. Chk2 mutations therefore lead directly to DNA damage induced genetic instability in the early embryo. Work is currently underway to determine if human cells also disrupt centrosome function and chromosome segregation in response to genetic damage. Consistent with this hypothesis, mutations in human Chk2 increase genetic instability and are associated with hereditary predisposition to cancer.

Work in the Doxsey lab also support a link between defects in centrosome regulation and cancer (reviewed by Doxsey 2002, Molecular Cell). When more than 2 centrosomes are present during mitosis, spindles with extra poles can form, leading to chromosome segregation errors, division failures, and genomic instability. These are hallmarks of human tumors, and recent studies show that defects in centrosome number are found in almost all aggressive human cancers. Significantly, these defects are often found early precancerous lesions, suggesting that they may directly contribute to tumor progression. Furthermore, some centrosome-associated proteins are elevated in human tumors and over-expression of these proteins in cultured cells can induce tumor-like features. By increasing errors in chromosome segregation, centrosome defects in precancerous cells could act synergistically with mutations tumor suppressor genes to dramatically increase genomic instability and cancer progression.

Cytokinesis is the final stage of the cell cycle, in which one cell divides to make two genetically identical and physically separate daughter cells. To ensure proper segregation of chromosomes, cell division must initiate at an appropriate time, and there must be some mechanism to delay cytokinesis if the chromosomes have not been properly segregated. Failure to do so could lead to abnormal chromosome segregation, anueploidy, and cancer. The basic mechanisms of cell cycle control are highly conserved between yeast and humans, and a conserved signaling network called the SIN in the fission yeast S. pombe functions to trigger cytokinesis at the end of anaphase. Work in the McCollum lab has shown that components of this network localize to spindle pole bodies, which are functionally equivalent to centrosomes in higher organisms. They have also found that this network is crucial for coordinating cell division and chromosome segregation, which is required to maintain genomic stability. In work recently published in Developmental Cell (Guertin et al., 2002), the McCollum group show that the Dma1 protein functions to inhibit the SIN and cytokinesis in response to defects in the mitotic spindle. Cells that lack Dma1 undergo cell division when the spindle is damaged and chromosomes have not been properly segregated. The key function of Dma1 seems to be to inhibit Polo kinase, a highly conserved cell cycle regulator involved in mitotic control in systems ranging from yeast to man. Interestingly, a human homolog of Dma1 called Chfr is deleted in a broad range of tumors, suggesting that it has a key role in preventing tumor formation. Given the conservation of basic cell division proteins, studies on Dma1 in yeast may well yield important insights into the function of Chfr in human cancer.

William Theurkauf
Stephen Doxsey
Dannel McCollum

Cell Dynamics

• Doxsey S. (2002). Duplicating dangerously: linking centrosome duplication and aneuploidy. Mol Cell. 10:439-40
• Sibon, O. C. M., A. Kelkar, W. Lemstra and W. E. Theurkauf. (2000). DNA replication/damage-dependent centrosome inactivation in Drosophila embryos. Nature Cell Biology 2, 90-95.
• Takada, S., Kelkar, A., and Theurkauf, W. E. (2003). Drosophila Chk2 kinase couples centrosome function and spindle assembly to genomic integrity. Cell, 113, 1-13.*
• Guertin, D.A., Venkatram, S., Gould, K.L., and McCollum D. (2002). Dma1p prevents mitotic exit and cytokinesis by inhibiting the Septation Initiation Network (SIN). Developmental Cell. 3:779-790.

*Subject of a News and Views by Sibon in the May Nature Genetics, Volume 34.