Although the majority of colorectal cancers exhibit chromosome instability (CIN), only a few genes that might cause this phenotype have been identified and no general mechanism underlying their function has emerged. To systematically identify somatic mutations in potential CIN genes in colorectal cancers, we determined the sequence of 102 human homologues of 96 yeast CIN genes known to function in various aspects of chromosome transmission fidelity. We identified 11 somatic mutations distributed among five genes in a panel that included 132 colorectal cancers. Remarkably, all but one of these 11 mutations were in the homologs of yeast genes that regulate sister chromatid cohesion. We then demonstrated that down-regulation of such homologs resulted in chromosomal instability and chromatid cohesion defects in human cells. Finally, we showed that down-regulation or genetic disruption of the two major candidate CIN genes identified in previous studies (MRE11A and CDC4) also resulted in abnormal sister chromatid cohesion in human cells. These results suggest that defective sister chromatid cohesion as a result of somatic mutations may represent a major cause of chromosome instability in human cancers.CDC4 ͉ MRE11A ͉ somatic mutation

H2AX is a core histone H2A variant that contains an absolutely conserved serine/glutamine (SQ) motif within an extended carboxy-terminal tail. H2AX phosphorylation at the SQ motif (␥-H2AX) has been shown to increase dramatically upon exogenously introduced DNA double-strand breaks (DSBs). In this study, we use quantitative in situ approaches to investigate the spatial patterning and cell cycle dynamics of ␥-H2AX in a panel of normally growing (unirradiated) mammalian cell lines and cultures. We provide the first evidence for the existence of two distinct yet highly discernible ␥-H2AX focal populations: a small population of large amorphous foci that colocalize with numerous DNA DSB repair proteins and previously undescribed but much more abundant small foci. These small foci do not recruit proteins involved in DNA DSB repair. Cell cycle analyses reveal unexpected dynamics for ␥-H2AX in unirradiated mammalian cells that include an ATM-dependent phosphorylation that is maximal during M phase. Based upon similarities drawn from other histone posttranslational modifications and previous observations in haploinsufficient (H2AX Ϫ/ϩ ) and null mice (H2AX Ϫ/Ϫ ), ␥-H2AX may contribute to the fidelity of the mitotic process, even in the absence of DNA damage, thereby ensuring the faithful transmission of genetic information from one generation to the next.

Histone-modifying enzymes play a critical role in modulating chromatin dynamics. In this report we demonstrate that one of these enzymes, PR-Set7, and its corresponding histone modification, the monomethylation of histone H4 lysine 20 (H4K20), display a distinct cell cycle profile in mammalian cells: low at G 1 , increased during late S phase and G 2 , and maximal from prometaphase to anaphase. The lack of PR-Set7 and monomethylated H4K20 resulted in a number of aberrant phenotypes in several different mammalian cell types. These include the inability of cells to progress past G 2 , global chromosome condensation failure, aberrant centrosome amplification, and substantial DNA damage. By employing a catalytically dead dominant negative PR-Set7 mutant, we discovered that its mono-methyltransferase activity was required to prevent these phenotypes. Importantly, we demonstrate that all of the aberrant phenotypes associated with the loss of PR-Set7 enzymatic function occur independently of p53. Collectively, our findings demonstrate that PR-Set7 enzymatic activity is essential for mammalian cell cycle progression and for the maintenance of genomic stability, most likely by monomethylating histone H4K20. Our results predict that alterations of this pathway could result in gross chromosomal aberrations and aneuploidy.Dynamic alterations in chromatin structure are modulated, in part, by the post-translational modifications of the DNAassociated histone proteins. Specialized chromatin-modifying enzymes can phosphorylate, acetylate, ubiquitylate, or methylate specific amino acids within certain histones, and each of these modifications are associated with distinct biological events (1). One of the first histone modifications to be identified nearly forty-five years ago was the methylation of histone H4 lysine 20 (H4K20) 4 (2). Earlier biochemical studies linked H4K20 methylation to diverse biological events including transcriptional regulation, chromatin compaction, cell division, and the formation of heterochromatin (3-9). Importantly, it was also found that H4K20 is differentially methylated in vivo and therefore can be either mono-, di-, or trimethylated (10). Together, these findings strongly suggest that different methylated states of H4K20 may be involved in distinct biological processes, similar to what is observed for the various methylated states of histone H3 lysine 4 and 9 methylation (11, 12).Increasing evidence indicates that certain enzymes are responsible for the specific degree of histone lysine methylation (13). For example, the mono-and dimethylation of histone H3 lysine 9 in humans is mediated by the G9a enzyme, whereas trimethylation is mediated by the SUV39H1 enzyme (14,15). Similarly, the Suv4 -20 enzymes are responsible for di-and trimethylation in mammals (16,17). Trimethylated H4K20 is associated with repressed chromatin because it is targeted to constitutive heterochromatin, various repetitive elements, and imprinting control regions (16,18,19). Dimethylated H4K40 is more widely distributed within...

Mutations that cause chromosome instability (CIN) in cancer cells produce ''sublethal'' deficiencies in an essential process (chromosome segregation) and, therefore, may represent a major untapped resource that could be exploited for therapeutic benefit in the treatment of cancer. If second-site unlinked genes can be identified, that when knocked down, cause a synthetic lethal (SL) phenotype in combination with a somatic mutation in a CIN gene, novel candidate therapeutic targets will be identified. To test this idea, we took a cross species SL candidate gene approach by recapitulating a SL interaction observed between rad54 and rad27 mutations in yeast, via knockdown of the highly sequence-and functionally-related proteins RAD54B and FEN1 in a cancer cell line. We show that knockdown of RAD54B, a gene known to be somatically mutated in cancer, causes CIN in mammalian cells. Using high-content microscopy techniques, we demonstrate that RAD54B-deficient human colorectal cancer cells are sensitive to SL killing by reduced FEN1 expression, while isogenic RAD54B proficient cells are not. This conserved SL interaction suggests that extrapolating SL interactions observed in model organisms for homologous genes mutated in human cancers will aid in the identification of novel therapeutic targets for specific killing of cancerous cells exhibiting CIN.cancer therapeutics ͉ chromosome instability ͉ synthetic lethality