Double-strand breaks activate the ataxia telangiectasia mutated (ATM) kinase, which promotes the accumulation of DNA damage factors in the chromatin surrounding the break. The functional significance of the resulting DNA damage foci is poorly understood. Here we show that 53BP1 (also known as TRP53BP1), a component of DNA damage foci, changes the dynamic behaviour of chromatin to promote DNA repair. We used conditional deletion of the shelterin component TRF2 (also known as TERF2) from mouse cells (TRF2 fl/2 ) to deprotect telomeres, which, like doublestrand breaks, activate the ATM kinase, accumulate 53BP1 and are processed by non-homologous end joining (NHEJ) 1,2 . Deletion of TRF2 from 53BP1-deficient cells established that NHEJ of dysfunctional telomeres is strongly dependent on the binding of 53BP1 to damaged chromosome ends. To address the mechanism by which 53BP1 promotes NHEJ, we used time-lapse microscopy to measure telomere dynamics before and after their deprotection. Imaging showed that deprotected telomeres are more mobile and sample larger territories within the nucleus. This change in chromatin dynamics was dependent on 53BP1 and ATM but did not require a functional NHEJ pathway. We propose that the binding of 53BP1 near DNA breaks changes the dynamic behaviour of the local chromatin, thereby facilitating NHEJ repair reactions that involve distant sites, including joining of dysfunctional telomeres and AID (also known as AICDA)-induced breaks in immunoglobulin class-switch recombination.Previous work has shown that mouse telomeres lacking TRF2 are processed by a KU70-and DNA-ligase-IV-dependent NHEJ reaction 1,2 that requires ATM kinase signalling and is stimulated by the ATM targets H2AX (also known as H2AFX) and MDC1 (refs 3 and 4). Here we focus on 53BP1, a third ATM target, which accumulates at double-strand breaks (DSBs) and deprotected telomeres [5][6][7][8] . The interaction of 53BP1 with chromatin involves the binding of its tudor domains to H4K20me2 and an MDC1-dependent interaction with c-H2AX 9-14 . Although 53BP1 is not strictly required for DNA damage signalling, homology-directed repair or NHEJ in the context of V(D)J recombination, NHEJ of DSBs in class-switch recombination (CSR) is severely affected by 53BP1 deficiency 15,16 . In the absence of 53BP1, DSBs in different switch regions fail to join successfully, resulting in a predominance of intra-switch recombination events 17 . It has been proposed that 53BP1 might either facilitate synapsis of DNA ends 17,18 or 'shepherd' NHEJ factors to the break 19 .We generated SV40 LT immortalized TRF2 fl/2 53BP1 2/2 and TRF2 fl/2 53BP1 1/2 mouse embryonic fibroblasts (MEFs) 1,20 ( Supplementary Fig. 1a, b) and assayed the frequency of telomere fusions in metaphase spreads collected 120 h after Cre-mediated deletion of TRF2 (Fig. 1a). Whereas 53BP1-proficient cells showed the expected level of telomere fusions (33% of telomeres fused after four population doublings), the rate of NHEJ in 53BP1 2/2 cells was at least 50-fold lower, nearly as lo...

Septins assemble on the cortex and restore normal cell shape by retracting aberrantly protruding membranes and promoting cortical contraction during amoeboid motility.

BackgroundHepatitis C virus (HCV) infection and chronic kidney disease (CKD) have high prevalences in Taiwan and worldwide, but the role of HCV infection in causing CKD remains uncertain. This cohort study aimed to explore this association.MethodsThis nationwide cohort study examined the association of HCV with CKD by analysis of sampled claims data from Taiwan National Health Insurance Research Database from 1998 to 2004. ICD-9 diagnosis codes were used to identify diseases. We extracted data of 3182 subjects who had newly identified HCV infection and no traditional CKD risk factors and data of randomly selected 12728 matched HCV-uninfected control subjects. Each subject was tracked for 6 years from the index date to identify incident CKD cases. Cox proportional hazard regression was used to determine the risk of CKD in the HCV-infected and control groups.ResultsThe mean follow-up durations were 5.88 years and 5.92 years for the HCV-infected and control groups, respectively. Among the sample of 15910 subjects, 251 subjects (1.6%) developed CKD during the 6-year follow-up period, 64 subjects (2.0%) from the HCV-infected group and 187 subjects (1.5%) from the control group. The incidence rate of CKD was significantly higher in the HCV-infected group than in the control group (3.42 vs. 2.48 per 1000 person-years, p = 0.02). Multivariate analysis indicated that the HCV-infected group had significantly greater risk for CKD (adjusted hazard ratio: 1.75, 95% CI: 1.25-2.43, p = 0.0009). This relationship also held for a comparison of HCV-infected and HCV-uninfected subjects who were younger than 70 years and had none of traditional CKD risk factors.ConclusionsHCV infection is associated with increased risk for CKD beyond the well-known traditional CKD risk factors. HCV patients should be informed of their increased risk for development of CKD and should be more closely monitored.

The observed motion of subcellular particles in fluorescence microscopy image sequences of live cells is generally a superposition of the motion and deformation of the cell and the motion of the particles. Decoupling the two types of movements to enable accurate classification of the particle motion requires the application of registration algorithms. We have developed an intensity-based approach for nonrigid registration of multi-channel microscopy image sequences of cell nuclei. First, based on 3-D synthetic images we demonstrate that cell nucleus deformations change the observed motion types of particles and that our approach allows to recover the original motion. Second, we have successfully applied our approach to register 2-D and 3-D real microscopy image sequences. A quantitative experimental comparison with previous approaches for nonrigid registration of cell microscopy has also been performed.