Formation of cancerous translocations requires the illegitimate joining of chromosomes containing double-strand breaks (DSBs). It is unknown how broken chromosome ends find their translocation partners within the cell nucleus. Here, we have visualized and quantitatively analysed the dynamics of single DSBs in living mammalian cells. We demonstrate that broken ends are positionally stable and unable to roam the cell nucleus. Immobilization of broken chromosome ends requires the DNAend binding protein Ku80, but is independent of DNA repair factors, H2AX, the MRN complex and the cohesion complex. DSBs preferentially undergo translocations with neighbouring chromosomes and loss of local positional constraint correlates with elevated genomic instability. These results support a contact-first model in which chromosome translocations predominantly form among spatially proximal DSBs.DSBs occur frequently in the genome through the action of DNA-damaging agents or during genome replication 1,2 . DSBs are hazardous to the cell because failure to properly repair them may lead to tumorigenic trans-locations 3 . How broken ends of different chromosomes meet in the cell nucleus to eventually form a translocation is poorly understood 4 . Two hypotheses have been put forth: the 'contact-first' model proposes that interactions between breaks on distinct chromosomes can only take place when the breaks are created in chromatid fibres that colocalize at the time of DNA damage 5 . In contrast, the 'breakage-first' hypothesis postulates that breaks formed at distant locations are able to scan the nuclear space for potential partners and come together to produce translocations 6 . The two models make divergent predictions as to the dynamic behaviour of broken chromosome ends. In the breakage-first model, single DSBs are required to undergo large-scale motions within the cell nucleus and must be able to roam the nuclear space in search of appropriate interaction partners. In the contact-first model, only limited local positional motion of DSBs is expected. The available experimental data is contradictory: in mammalian cells, induction of extensive chromosome damage using ultrasoft X-rays 7 , laser microirradiation 8 or γ-irradiation 8 indicates that damaged DNA is largely stationary. In contrast, α-particle irradiation leads to large-scale motion and clustering of 6Correspondence should be addressed to T.M. (e-mail: mistelit@mail.nih.gov). AUTHOR CONTRIBUTIONS E.S. and T.M. designed the study, E.S., J.F.D. and K.S. performed the experiments, J.M., A.N. and T.R. provided reagents and advice, and E.S. and T.M. wrote the manuscript. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests.Note: Supplementary Information is available on the Nature Cell Biology website. NIH Public Access Author ManuscriptNat Cell Biol. Author manuscript; available in PMC 2008 July 3. Published in final edited form as:Nat Cell Biol. 2007 June ; 9(6): 675-682. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manus...

An automated, quantitative 4D image analysis method is used to track kinetochore dynamics in metaphase cells.

Epigenetic mechanisms regulate genome activation in diverse events, including normal development and cancerous transformation. Centromeres are epigenetically designated chromosomal regions that maintain genomic stability by directing chromosome segregation during cell division. The histone H3 variant CENP-A resides specifically at centromeres, is fundamental to centromere function and is thought to act as the epigenetic mark defining centromere loci. Mechanisms directing assembly of CENP-A nucleosomes have recently emerged, but how CENP-A is maintained after assembly is unknown. Here, we show that a small GTPase switch functions to maintain newly assembled CENP-A nucleosomes. Using functional proteomics, we found that MgcRacGAP (a Rho family GTPase activating protein) interacts with the CENP-A licensing factor HsKNL2. High-resolution live-cell imaging assays, designed in this study, demonstrated that MgcRacGAP, the Rho family guanine nucleotide exchange factor (GEF) Ect2, and the small GTPases Cdc42 and Rac, are required for stability of newly incorporated CENP-A at centromeres. Thus, a small GTPase switch ensures epigenetic centromere maintenance after loading of new CENP-A.